The Efficacy of LED Light Polymerization Units in Private ......University of Toronto 2018 Abstract...

The Efficacy of LED Light Polymerization Units in Private

Dental Offices in Toronto, Canada

by

Dave Darko Kojic

A thesis submitted in conformity with the requirements

for the degree of Doctor of Philosophy

Faculty of Dentistry

University of Toronto

© Copyright by Dave Darko Kojic (2018)

ii

The Efficacy of LED Light Polymerization Units in Private Dental Offices in

Toronto, Canada

Dave Darko Kojic

Doctor of Philosophy

Faculty of Dentistry

University of Toronto

2018

Abstract

Light-emitting-diode (LED) light-polymerization units (LPUs) are currently routinely

used for resin-based composites (RBC) photopolymerization. This study aimed to determine

the effectiveness of LED-LPUs used in participating private dental practices in Toronto,

Canada. One-hundred dental practices were recruited, and 113 LED-LPUs were examined.

Part 1 evaluated radiant exposure (RE) of participants’ LED-LPUs. Majority of LPUs (n=87,

77%) passed the study-required criterion (delivering equivalent to 16 J/cm2 in 20s). A strong,

negative correlation between LPUs' age, light-probe damage, and RE was found. Part 2

evaluated operator's technique to deliver 8 J/cm2 of RE in 10s to simulated restorations

before and post-instructional. Before instructions, 73.6% of participants delivered required-

energy to anterior restoration, and 32.2% of participants delivered required-energy to

posterior restoration. Following instructions, 94.3% of participants delivered required-energy

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to anterior restoration, and 73.6% of participants delivered required-energy to posterior

restoration. Part 3 examined relative-hardness (RH) of two RBCs polymerized at 1mm and

4mm, tip-to-target distance. Both RBCs polymerized at 1mm had average RH (ratio of mean

lower/upper surface hardness) greater than 0.80, but not at 4mm tip-to-target distance. Part 4

evaluated the diametral tensile strength (DTS) of two RBCs polymerized with participating

LED-LPUs. No significant difference between mean DTS values of two RBCs was observed.

Mean DTS values were highly correlated among the LED-LPUs tested. In conclusion, 23%

of LED-LPUs tested were deemed as incompetent for sufficient RBC polymerization. A high

percentage of dental practitioners failed to deliver 8 J/cm2 in 10s exposure cycle, even after

receiving specific instructions (5.7% to anterior restoration and 26.4% to posterior

restoration). The outcome was due to a combination of LPUs performance and operator's

technique. This finding is alarming as it relates well to early failure of Class-2 RBC

restorations due to insufficient polymerization.

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Dedication

This thesis is dedicated to my beloved Father and Mother, who are the source of love, inspiration,

and wholehearted support since the beginning of my life journey, and without whom, none of this would

be possible.

Also, this thesis is devoted to my sons, Petar and Lazar for their endeavor of bringing so much

love into my life, unshakable faith in the righteousness of my life's decisions and for making my whole

existence relevant.

Finally, this thesis is devoted to my sister and my brother-in-law who have supported and

encouraged me in this venture since the early beginning.

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Acknowledgements

First and foremost, my genuine gratitude goes to my supervisor, Dr. Omar El-

Mowafy, who has supported me throughout my academic journey with his patience and

knowledge, while permitting me to work in my way. His expertise, understanding, time and

helpfulness, added considerably to my extraordinary pleasant graduate experience. I attribute

the level of my doctorate degree to his encouragement and effort, especially with his support

of my international exposures at the IADR conferences.

Special thanks to my co-supervisor, Dr. Wafa El-Badrawy, for her expertise,

understanding, patience, professionalism, and mentorship during my educational venture at

the Faculty. Her insight and guidance contributed significantly to this project.

Also, I wish to thank my other thesis advisory committee members, Dr. Richard Price

and Dr. Gildo Santos for the time they provided throughout this project. Dr. Price's valuable

and thorough comments, expertise, incredible support and suggestions helped significantly in

improving the quality of this project. Dr. Santo's ideas and dedication have been invaluable

for the completion of this project, for which I am incredibly grateful.

Additionally, I would like to thank my sons: Petar and Lazar for their support and

great patience at all times. My parents and my sister's family have provided me their help

unconditionally, as always, for which my mere appearance of thanks furthermore does not

satisfy. Without their love, encouragement, on-going support and sacrifices throughout this

process, I would not be able to complete this graduate program.

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Furthermore, I am very grateful to my research assistants: Ms. Ana Burilo and Ms.

Ivana Orlovic whose contribution to this project was significant for the duration of this

project. I would like to express my sincere gratitude to Heather Sanguins from the Health

Sciences Writing Centre University of Toronto for her enormous patience and the

tremendous help with my thesis writing.

In conclusion, I realize that this research would not have been possible without the

generous research material donations from Shashikant Singhal (Ivoclar-Vivadent, Amherst

NY) and Sridhar Janyavula (Dentsply, York PA), as well as technical support and

CheckMARC loan from Colin Deacon (BlueLight Analytics, Halifax NS). Their support is

duly acknowledged. Furthermore, I am very appreciative to the University of Toronto for the

financial assistance during the entire course of my graduate training and the funding of this

project.

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Table of Contents:

Dedication ………………………………………………………………………… iv

Acknowledgements ………………………………………………………………. v

Table of Contents ………………………………………………………………… vii

List of Tables ……………………………………………………………………. x

List of Figures …………………………………………………………………… xi

Chapter 1 Introduction and Literature Review……….………………….……. 1

1.1 Introduction ………………………………………………….………………... 1

1.2 History of Resin Based Composites (RBC) …………………………………… 4

1.3 Classification of RBC …………………………………………………………. 11

1.4 Classification of Bonding Agents……………………………………………… 18

1.5 Evaluation of Photopolymerization Efficiency ………………………………. 23

1.5.1 Degree of conversion………………………………………………………… 25

1.5.2 Degree of crosslinking………………………………………………………. 27

1.5.3 Mechanical properties of RBCs .……………………………………………. 30

1.5.4 Shrinkage and shrinkage stress………………………………………………. 33

1.5.5 Depth of polymerization……………………………………………………... 36

1.5.6 Biocompatibility……………………………………………………………... 37

1.6 Evolution of LPUs……………………………………………………………. 40

viii

1.7 Intrinsic Factors Affecting Photopolymerization Efficiency…………………. 50

1.7.1 Photoinitiator systems………………………………………………………. 52

1.7.2 Viscosity, monomers, and fillers……………………………………………. 58

1.7.3 Optical properties……………………………………………………………. 62

1.8 Extrinsic Factors Affecting Photopolymerization Efficiency………………… 67

1.8.1 Light polymerization units (LPUs) and emission spectrum ……………….. 67

1.8.2 Radiant exposure, irradiance, and irradiation time ………………………… 70

1.8.3 Light exposure ………………………………………………………………. 73

1.8.4 The effect of temperature on photopolymerization …………………………. 74

1.8.5 Light guide tip positioning…………………………………………………… 75

1.9 Review of studies on RBCs Longevity and Effectiveness of LPUs………….....77

1.9.1 Longevity of RBCs direct restorative materials……………………………….77

1.9.2 Effectiveness of LPUs in dental practices………………………………….… 83

Chapter 2 Objective and Aims………………………….……………………….. 91

Chapter 3 Manuscript 1 ……………………………………………………….... 93

Assessment of LED Light-Polymerization Units Used in Toronto Private

Practices…………………………………………………………………………….. 94

Chapter 4 Manuscript 2………………………………………………………….. 125

Assessment of photopolymerization application effectiveness among private

dental practices in Toronto, Canada ….…….…………………………............. 126

ix

Chapter 5 Manuscript 3…………………………………………………………. 159

Relative Hardness of Resin Based Composites polymerized by LED Light-

Polymerization Units in Toronto Private Practices………………………………. 160

Chapter 6 Manuscript 4………………………………………………………….. 190

Assessment of Biaxial Tensile Strength of Resin Based Composites Polymerized by

LED Light-Polymerization Units in Private Dental Practices in Toronto, Canada

………………………………………………………………………...…...……...191

Chapter 7 Findings, Analysis, Discussion, Limitations and Future

Directions, and Conclusion …………………………………………………… 212

7.1 Research Findings ………………………………………………………….. 213

7.2 Analysis ……………………………………………………………….…….. 218

7.3 Discussion ………………………………………………………………..…. 220

7.4 Limitations and Future Directions ….………………………..………………221

7.5 Conclusions …………………………………………………………..…….223

Appendices ……………………………………………………………………….225

Appendix I ………………………………………………………………………..226

Studies on survival of RBC restorations …………………………………………227

Appendix II ………………………………………………………………………229

Studies of Effectiveness of LPUs in dental practices ……………………………230

References……………………………………………………………….………232

x

List of Tables

(Table 3.1) Listed LED LPUs that failed to reach a minimum of 16 J/cm2 …… 117

(Table 4.1) Radiant exposure delivered by 113 dental practitioners ….………….147

(Table 5.1) Descriptive statistics for RH levels for two RBCs…..….….……….. 179

(Table 6.1) Average BTS values of two RBC brands, polymerized by

monowave (type 0) and multiwave (type 1) LPUs ……………………………... 207

xi

List of Figures

(Figure 1.6.1) Ultralume 5® (Ultradent Products, South Jordan, UT) …….…..….... 48

(Figure 3.1) The CheckMARC® device ….…………………………………..…... 114

(Figure 3.2) Schematic representation of calculated damage levels observed on

light probe surfaces of 113 LPUs tested ….………………………………………. 114

(Figure 3.3) Calculated 20s radiant exposure of 113 LPUs tested …………….. 116

(Figure 3.4) Box and Whisker plots: LED LPUs which met/did not meet the

study-required criterion ……………………………………………………….….. 117

(Figure 3.5) Q-Q plot: multiwave LPUs incident irradiance distribution …………119

(Figure 3.6) Q-Q plot: monowave LPUs incident irradiance distribution…………119

(Figure 4.1) The MARC® Patient Simulator system …………..………………….146

(Figure 4.2) The MARC® PS radiant exposure of each professional group..….….146

(Figure 4.3) 10s RE delivered to the MARC®PS (Anterior before instructions).... 148

(Figure 4.4) 10s RE delivered to the MARC®PS (Anterior post-instructional)…. 148

(Figure 4.5) 10s RE delivered to the MARC®PS (Posterior before instructions)…149

(Figure 4.6) 10s RE delivered to the MARC®PS (Posterior post-instructional)…. 149

(Figure 4.7) Q-Q plot: Anterior restoration before instructions.………………… 150

(Figure 4.8) Q-Q plot: Posterior restoration before instructions…………………. 150

(Figure 4.9) Q-Q plot: Anterior restoration post instructional.….…………..…… 151

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(Figure 4.10) Q-Q plot: Posterior restoration post instructional ………………….151

(Figure 4.11) 10s RE of monowave vs. multiwave LPUs….…………..…………152

(Figure 5.1) Plastic mold used for the specimens’ production .…………….…. 178

(Figure 5.2) Specimen ready for Knoop microhardness testing ………….…… 178

(Figure 5.3) Percentage of LPUs that reached the .80 level of RH …………. …. 179

(Figure 5.4) Average RH by LPU type ..………………….…………………….180

(Figure 5.5) Average RH at 1mm/4mm Tetric EvoCeram ………………………180

(Figure 5.6) Average RH at 1mm/4mm TPH Spectra …………………………...181

(Figure 5.7) Q-Q plot: RH of Tetric EvoCeram at 1 mm distance ………….….181

(Figure 5.8) Q-Q plot: RH of Tetric EvoCeram at 4 mm distance ………….….182

(Figure 5.9) Q-Q plot: RH of TPH Spectra at 1 mm distance ….…………….….182

(Figure 5.10) Q-Q plot: RH of TPH Spectra at 4 mm distance …………………183

(Figure 6.1) The BTS Specimen ……..…………………………………………. 206

(Figure 6.2) The BTS testing in a universal testing machine …………………..206

(Figure 6.3) BTS values polymerized by multiwave /monowave LPUs ….……. 207

(Figure 6.4) Q-Q plot: BTS values of TPH Spectra …………….…..…………...208

(Figure 6.5) Q-Q plot: BTS values of Tetric EvoCeram …………..……………208

1

Chapter 1

Introduction and Literature Review

Introduction

Direct esthetic restorative materials have experienced significant changes since the

1980s. Up to this time, amalgam was the material of choice for direct posterior restorations.

However, over the past decade, extensive research into, and implementation of, innovative

technologies have significantly improved the dependability and predictability of amalgam-

alternative dental materials, especially direct resin-based composite (RBC) materials. At the

same time, restorative dentistry has experienced a dynamic shift to adhesive dentistry. During

the past decade, the use of RBC for direct restorations has increased exponentially, due to the

high esthetic demands and increasing public awareness of mercury in dental amalgam, in

addition to environmental concerns about the industrial use of mercury 1, 2. In 2013, the

Minamata Convention on Mercury committed to the reduction and ultimate dismissal of the

production and use of mercury-containing products globally. The use of RBC continues to

increase worldwide. In some countries, however, RBCs have entirely replaced amalgam,

such as Norway where amalgam elimination started in 2008 3. According to the United States

insurance data claims for the 2010 year, RBC has become the first-choice restorative material

accounting for more than 65% of all private practice placements 4. The RBC has three

2

distinct components, each with its role in influencing material properties: the organic resin

matrix, the inorganic fillers, and the filler-resin interface (coupling agent). The resin matrix

phase is composed of polymerizable monomers that convert to the highly crosslinked

polymers upon photopolymerization, which catalyzes the development of active centers,

which induce polymerization. The filler phase has several roles, including improving the

mechanical properties, reducing thermal expansion, and decreasing polymerization shrinkage

by reducing the resin content. Finally, silane at the filler-resin interface creates a bond

between the filler particles and the resin interface that is important for their bonding

capabilities 5.

Polymerization of RBC materials is initiated by the appropriate spectra and sufficient

quantities of light in the blue region of spectrum from the LPU that activates photoinitiators

with subsequent release of free radicals that initiate the polymerization reaction. During this

process, the terminal aliphatic C=C bonds are severed and converted to the primary C-C

covalent bonds. During the polymerization reaction, the monomer is converted into a

polymer, nonetheless, full conversion is never achieved. Although the majority of monomers

are transformed, however, a certain number of monomers remains unreacted or trapped in the

polymeric matrix, and is the cause of the free monomer elution 6. The percentage of

unconverted aliphatic C=C bonds in the polymeric matrix represents the degree of conversion

(DC), the most important feature that affects the clinical performance of RBC materials 7, 8.

The literature shows that the development mechanism of free radicals in the monomer differs

according to the photoinitiator system used 9. Therefore, for adequate RBC

photopolymerization, light-activated resins must not only receive sufficient radiant exposure

3

(J/cm2) but also must be given this energy within the appropriate wavelength range to match

the spectral range of the photoinitiator system of the resin 10.

The first law of photochemistry, the Grotthuss-Draper law, states that photon has to

be absorbed by a chemical material for a photochemical reaction to take place. Therefore, the

presence of light alone is not enough to trigger a photochemical reaction; the light must

exhibit the correct wavelength match to be absorbed by the reactant used 11. The second law

of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed by a

chemical system, only one molecule can be activated for the photochemical reaction to occur.

However, this fundamental principle applies to the photoinitiators with a low free radical

production efficiency, such as the camphorquinone system (CQ) 11. The ability to accurately

determine the number of photons, leading to a reaction, enables the efficiency or quantum

yield of the photochemical reaction 11. Thus, the use of LED-LPUs that can generate

adequate radiant power (mW) and spectral irradiance (mW/cm2/nm), which produces

sufficient polymerization, is a crucial factor for the clinical success of RBC direct restorative

materials 12.

The factors affecting the photopolymerization efficiency of RBCs are intrinsic,

related to the RBC composition, and extrinsic, related to the LPUs. The intrinsic factors are

co-monomer composition and filler-content ratio, photoinitiator type, and concentration.

While the extrinsic factors are the light spectrum, irradiation protocols, temperature, and light

guide tip positioning. It is imperative for clinicians to comprehend and optimize the RBC

photopolymerization process to maximize material properties. According to the results of a

recent study (Kopperud et al., 2017) the majority of Norwegian dental practitioners did not

demonstrate sufficient knowledge of this matter 13. Furthermore, the majority of the study

4

participants (78.3%) were unaware of the irradiance value of their LPUs. It was reported that

78.3% of dentists failed to perform the regular maintenance of their LPUs. The research

findings provide the foundation for many clinical practice guidelines developed to improve

dental care procedures 14. Although contemporary dental practice exhibits a substantial

increase in RBC restoration placements, the literature demonstrated that the longevity of

RBCs has fallen short in comparison to that of amalgams 15. Consequently, the practitioners'

selection of LPUs and RBC materials, as well as the latest guidelines to be followed, are

important detrimental factors. The literature demonstrated a substantial effect of LPUs

performance and clinician's polymerizations technique on the longevity of RBC restorations,

and this project attempted to characterize and quantify findings from a group of private

dental practices in the Greater Toronto Area and how these results correlate with published

studies.

1.2 History of Resin-Based Composites (RBC)

The introduction of silicate cement as a direct restorative material in 1871 was the

beginning of esthetic restorative dentistry 16. Silicates were noted for their anti-cariogenic

effects and remained the material of choice for restoring anterior teeth until the early 1950s

17. However, silicates had some undesirable physical properties, especially color instability

and quite a high rate of dissolution in saliva, which led to their discontinuation in the early

1960s. Acrylic resin (unfilled resin), another type of direct restorative material, was

5

developed in 1936, but was not used as a direct esthetic material until the late 1940s 17. It was

composed of Poly methyl methacrylate (PMMA) and has also been used in dentistry in

another form as a denture base material replacing vulcanite. Although unfilled resins offered

benefits over silicates, bonding was not as yet discovered 18. In the mid-1950s, Buonocore

was the first to introduce an orthophosphoric acid etching technique, which enabled the

formation of a stable bond between enamel and acrylic resin 18. Knock and Glenn patented

the first particulate-filled composite in 1951 20. However, their product lacked bonding

between the resin matrix and the filler particles.

Composite dental materials were developed in responses to the shortcomings of

silicate cement and unfilled resins. The breakthrough came in 1962 when Rafael Bowen

patented a new resin, Bisphenol glycidyl di-methacrylate (Bis-GMA). This material

resembled an epoxy resin except the epoxy groups were replaced by methacrylate groups 21.

Bis-GMA formulations could polymerize rapidly under oral conditions, and they had less

polymerization shrinkage than methyl methacrylate. Although Dr. Buonocore continued his

acid etching research with various resins, however, he achieved the best bonding results at

that time using the Bis-GMA resin 22, 23. In addition to Buonocore findings, Regenos 24 and

others further developed the acid-etching concept using Bis-GMA resin to repair fractured

incisors without drilling the dentin 25, 26. The birth of adhesive dentistry was recognised as a

consequence of Bowen's 1963 publication. Bowen successfully demonstrated that chemical

treatment of silica particles' surfaces with vinyl silane, within the Bis–GMA resin matrix

could produce a new direct restorative material with consistent mechanical properties and

clinical effectiveness. Bis–GMA has become the basis for all RBC materials used in

dentistry. Bowen worked at the U.S. National Bureau of Standards. Thus, he was in a

6

position to conduct testing in the areas of polymerization setting times, shrinkage, solubility,

degradation, water sorption, color stability, mechanical testing, and toxicity 27, 28, 29, 30, 31.

The early two-paste chemically-polymerized RBCs required manual mixing. The mixing

process was a technique-sensitive one. During the mixing, the mixture usually trapped air

bubbles that had unfavorable effects on mechanical properties after RBC’s polymerization.

Furthermore, these materials demonstrated excessive shrinkage, leading to sensitivity, and

recurrent decay. Due to their relatively large size filler particles, RBCs from that era

exhibited high wear rates 32. Over the course of 4 decades, the Bis-GMA chemistry has

improved considerably, yielding clinically acceptable RBCs. The particle size of the fillers

has been considerably reduced, and the resin chemistry further developed, allowing RBCs to

exhibit superior physical and mechanical properties, which extended their clinical longevity.

As time progressed, photopolymerized RBC materials replaced the early two-paste self-

polymerized ones in the 1970s. This development was regarded as a revolutionary step in

dentistry which allowed quick chairside polymerization.

The photopolymerization technology was adapted from the chemical industry and

modified for use in dentistry 33, 34, 35. The first LPUs were designed to emit ultraviolet light

(UV) 36. A 40s polymerization exposure was advocated, but 60s exposure provided enhanced

results 37. The common issues associated with UV polymerization of RBCs were: a limited

light penetration throughout the material (limited depth) and health hazards related to UV

light exposure on the operators' eyes 38, 39, 40. However, the rise of UV photopolymerization

was short lived when in 1976 Bassoiuny, at the University of Manchester, School of

Dentistry, placed the first visible light-polymerized RBC restoration. Again, the technology

7

was adopted from the Imperial Chemical Industries of England and modified for dental use

41, 42, 43.

The new visible light polymerization technology initially used a photoinitiation

system based on camphorquinone with a tertiary amine co-initiator, a system that was

enhanced over the years and became a template for new RBCs 44, 45. The light source used to

photopolymerize the newly developed RBCs was quartz–tungsten–halogen (QTH),

developed by the Johnson and Johnson Company. The new venture launched the FotoFil

restorative system and QTH light source unit 46. This technology enabled

photopolymerization of a 2 mm thick increment of RBC 47, 48. Since that time, efforts have

been directed to modify the filler particles of RBC to improve their mechanical properties.

The macrofill RBC had filler particles of about 10 µm that exhibited excessive wear 49. In the

1980s, microfilled RBCs were developed for the first time 50. Due to microscopic filler

particles (0.02 µm to 0.04 µm), microfilled RBCs demonstrated excellent polishing

characteristics. Furthermore, they were translucent, however, due to their inferior mechanical

properties, bulk fractures were common 51.

The concept of combining high strength and esthetic properties was accomplished

with the launch of the first hybrid RBCs in the late 1980s 49, 52. These materials had improved

strength and better wear resistance, however, the esthetic properties were inferior to those of

the microfilled RBCs 53. The next development in the search for an ideal direct restorative

material led to the creation of microhybrid RBCs in the mid-1990s. These materials had filler

particles that were smaller than those of hybrid formulation, with better multi-modal

distribution 54. The properties of these RBCs exhibited the strength of the hybrid RBCs with

significantly improved esthetic attributes 54, 55. Further improvements in the filler technology

8

have allowed nanofillers to formulate nanohybrid RBCs, which became available in the early

2000s. Nanohybrid RBCs contain nanometer-sized filler particles (0.005–0.01µm), along

with conventional fillers. They became the first genuinely universal RBCs with the handling

and esthetic properties of micro filled RBCs and the strength and wear resistance of

traditional hybrid RBCs 56.

The first generation of flowable composites, introduced in 1996, belonged to the

hybrid category of RBCs. A notable reduction in the percentage of fillers and a particular

modification of the resin matrix resulted in the low viscosity of the flowable RBCs 52.

Flowable RBCs were used as luting materials for indirect restorations for a long time.

However, they were reintroduced for direct restorative RBC materials in the late 2010s.

Following the success of nanotechnology and the incorporation of pre-polymerized

resin fillers, a new epoch for nanofilled RBCs began 57. These materials have been produced

with nanofiller and formulated with a nanomer® (Nanocor, Arlington Heights, IL) and

nanocluster filler particles 57. Nanomers® are discrete nano-agglomerated particles with a size

of 20 to 75 nm, while the nanoclusters represent loosely bound agglomerates of nano-sized

particles 58. The combination of nanomer® and nanocluster formations reduces the interstitial

spacing of the filler particles, which creates the ability to increase filler load with excellent

polishing capability 58.

Since the mid-2000s, there has been considerable research for an alternative resin

with less polymerization shrinkage and a higher degree of conversion 58. A newly discovered

resin based on dimer chemistry, which exhibited higher than 70% conversion, was launched

in the late 2000s 59, 60.

9

Following the Dimer based RBCs’ commercialization, a novel organically modified

ceramic (ORMOCER) was launched. Materials based on ORMOCER technology are

inorganic-organic hybrid polymers that form a siloxane network selectively modified through

the incorporation of organic groups 61. A distinctive feature of these RBCs is a blend of

polysiloxane groups with photo-polymerizable methacrylate groups covalently bonded to

silica. Because of its structural relation to silicate ceramic or glass, such materials are also

known as organo-ceramics. These materials have exhibited up to 50 % reduction in

polymerization shrinkage when compared to traditional RBCs 62, 63. The DC was reported to

be in a range from 95% to 99%, which is virtually identical to that of indirect restorative

RBCs 63. The new material exhibited a higher level of resistance to stain and hydrolysis.

Thus, it was able to maintain a high degree of luster for a considerable long time 64, 65. The

latest innovation that has happened with ORMOCER materials was the fusion of nanohybrid

and ORMOCER technology, which exhibited further improvements regarding initial

shrinkage, shrinkage stress, esthetics, and biocompatibility of the nano ORMOCER materials

66.

However, in the late 2000s, a different resin synthesized from oxiranes and siloxane

molecules, which became known as a Silorane RBC material, was developed 67. This new

RBC material was introduced as an alternative to conventional RBC because of its

hydrophobicity and lower polymerization shrinkage 68, 69, 70. When compared to

methacrylate-based resins, the cyclo-aliphatic oxirane functional groups exhibit less

shrinkage 71. Oxiranes are cyclic ethers that polymerize via a cationic ring-opening

mechanism, while methacrylate polymerization is achieved via a free-radical polymerization

process 70, 71. The distinctive feature of Silorane RBCs arises from the ability of the

10

cyclosiloxane backbone to impart hydrophobicity, while the cyclo-aliphatic oxirane sites

have high reactivity and less shrinkage than methacrylate RBCs 72, 73. Silorane RBCs

demonstrated improvements with delayed gel and vitrification points and reduced shrinkage

strain 74, 75. However, these materials tend to produce more heat during the cationic ring-

opening polymerization 76. Furthermore, Silorane RBCs exhibit the high initial flexural

modulus, which may explain its high polymerization stress values, despite the low volumetric

shrinkage (both post-gel and total) 77, 78.

Recently, a new class of RBC materials has been introduced to the dental profession

under the name of bulk-fill restorative materials, with the aim of simplifying and expediting

the restoration process. It was available due to the use of new proprietary resins, individual

modulators, and different filler particles 79. To reduce some of the disadvantages of

incremental layering techniques and to save some chairside time, the innovative bulk-fill

RBCs can be placed in a 4 mm single increment 80. The manufacturers’ claim that these

materials can be adequately photopolymerized in bulk has been confirmed by in vitro

infrared spectroscopy studies 81, 82, 83. More recently, flowable bulk-fill composites have been

developed 84. They have been used along with conventional RBCs in a two-layer

arrangement for esthetic considerations in posterior teeth. It was proven their use could lower

the polymerization stress, enhance the flow and handling, and improve adaptation to the

cavity walls, which resulted in substantial stress reduction at the cavity walls 84, 85, 86. The

bulk-fill flowable RBCs can be used to restore posterior teeth using 4 mm maximum

increments because this procedure does not reduce the mechanical resistance of the restored

teeth while rendering the entire procedure less technique sensitive and more efficient 87.

11

The most recent development in contemporary dentistry was the introduction of

flowable RBCs containing adhesive monomers. This new technology is based on enhanced

traditional methacrylate resin that incorporates acidic-monomer chemistry from bonding

agents, making them capable of interacting mechanically and chemically with tooth surfaces

in an adhesive manner 89, 90. Despite their limited use for direct restorations, they are

considered to be futuristic materials in the continued search for universal, self-adhesive RBC

direct restorative materials.

1.3 Classification of RBC

RBCs emerged during the past 50 years and with different formulations that have

been successfully used as restorative material luting agents, sealants, provisional materials,

and much more. The RBC material consists of three components: resin matrix (organic

content), fillers (mineral content), and a coupling agent. The features of the filler particle and

filler particle loading, are the most critical factors that influence the mechanical properties of

restorative RBCs 91, 92. The classification introduced by Lutz and Philips in 1983 divided the

RBCs into macrofills; microfills; and hybrids, which are mostly a blend of ground glass and

the microfill particles 93.

• Type I: Macrofilled RBCs

Type I, macrofilled RBCs, which were also known as conventional or traditional

RBCs, provide some historical perspective. The fillers were composed of amorphous silica or

quartz with relatively large particles from 1μm to 50 μm. Due to the relatively large filler

particles, however, they exhibited inferior wear resistance as a consequence of differential

12

wear, which led to rapid loss of resin compared with the filler. In addition, plucking of the

large filler particles from the restoration surface resulted in surface roughness and was more

receptive to stain and plaque accumulation. Because of these limitations, the use of these

materials by dental practitioners has rapidly declined.

• Type II: Microfilled RBCs

For type II, microfilled RBCs, traditional microfillers were made from fumed silica

prepared by a pyrogenic process, with an average particle size of 40 nm. The filler load in

these materials was low, however, it was increased by adding highly filled, pre-polymerized

resin fillers (PPRF) 94. Due to the increased surface area of these relatively smaller filler

particles, overall fillers percentage was 35 to 67 by weight 94. The microfilled RBCs

exhibited superior esthetic properties and for that reason, they were recommended for

restoring the anterior teeth and cervical abfraction lesions. However, they were contra-

indicated in heavy-stress-bearing restorations because of an elevated risk of bulk-fractures

and marginal chippings 94. These materials were truly nanocomposites, with an average

particle size of 40 nm. Although nanotechnology was not revealed in the 1980s, earliest

microfilled RBCs would be recognized as nanofilled RBCs today.

• Type III: Hybrid RBCs

Hybrid RBCs were developed to retain the advantages of both traditional macrofilled

and microfilled RBCs by combining the fillers of different particle sizes and multi-modal

particle distributions. Thus, the hybrid RBCs were typically filled with a blend of 40 nm and

1µm to 5 µm particles. These materials exhibited much better wear resistance compared to

13

traditional macro filled RBCs, but surface properties remained less than ideal because of the

natural wear pattern due to the presence of larger filler particles 95. In current terms, nearly all

RBCs are hybrids, with particle distribution of two or more size ranges. In addition, the

presence of amorphous silica has been used to improve handling properties of RBC

restorative materials.

A classification introduced by Willems and Lambrechts in 1992, which is

based on a filler volume fraction and filler size, categorized RBCs as: densified, microfine,

miscellaneous, traditional, and fiber-reinforced 96. Based on Young's modulus value for

dentin, a computed value of 60% filler load was established that further classified density

filled RBCs as either midway-filled (<60 volume percentage) and compact-filled (>60

volume percentage). Furthermore, each category has been further divided into ultrafine (<3

µm) and fine (>3 µm) RBCs, based on the mean filler particle size. The average particle size

of hybrids, microhybrids, and nanohybrids, is typically less than one μm, but more than 0.2

μm. The larger particle sizes can extend to well over one μm. The nanohybrids have some

particles in the nanofiller size range less than 100 nm, but they also contain particles in the

range (0.2 to 1 μm). When the surface of the RBCs is subjected to wear, the resin between

and around the particles is lost, leading to the creation of bumps and craters with subsequent

loss of polish at the surface of restorations 97, 98, 99. Innovations in the sintering process

enabled manufacturers to produce loosely agglomerated nanoparticles, i.e., nanoclusters. The

nanoclusters exhibited a significant structural difference in comparison with densified

particles 99. However, nanoclusters behaved similarly to the densified particles found in other

RBCs that enable a high filler loading. The new nanohybrid RBCs were formulated using

both engineered nanoparticle and nanocluster fillers. The nanocluster filler particles consist

14

of loosely bound aggregates of engineered nanofiller particles. The addition of engineered

nanoparticles to formulations containing nanoclusters, the reduction of the filler particles

interstitial space, results in higher filler loadings 99. The filled matrix (resin plus engineered

nanoparticles) is harder and more wear resistant than the resin alone. The increased filler

loading resulted in better mechanical properties and wear resistance 97-99. Therefore,

nanohybrids are considered to be the first genuinely universal RBC materials, with the

handling properties and polishability of microfilled RBC materials and the strength and wear

resistance of traditional hybrid RBCs 97.

Since their introduction, flowable RBCs have been universally accepted among the

clinicians, due to their remarkable versatility in a wide variety of clinical applications. From

a materials science standpoint, the flowable RBCs represent a subclass of microfilled RBCs.

The filler loading was reduced to 30% to 55% by volume, compared to 57% to 72% by

volume as in traditional RBCs 99. The amount of fluidity or flowability varied considerably

among the brands. Because of their physical properties, these materials are used in low-stress

areas, such as the small Class I occlusal restorations, Class V restorations, and for pit and

fissure sealants or conservative restorations 100. The rationale for their application in the

Class V carious lesions and abrasion/abfraction lesions is because these materials exhibit a

low modulus of elasticity and low viscosity. The use of flowables for an initial increment in

the proximal box portion of a Class II restoration is somewhat controversial 101. Despite the

many controversies associated with the use of flowables, these materials are often used as

liners under hybrid materials for an initial increment in the proximal boxes of the Class II

restorations 101, 102.

15

For many years, an incremental layering technique was followed when placing

posterior composite restorations. This procedure was deemed to offer the optimum

polymerization of the restorations. In the late 1990s, the packable RBCs were developed to

enhance incremental material placement 103. These products had high viscosity and contained

a much higher filler content. Manufacturers claimed that their handling is amalgam-like and

that their stiffness aided in forming good proximal contacts. However, the high viscosity of

these materials made the cavity adaptation quite challenging 103, 104. Furthermore, these

materials exhibited a lower degree of monomer conversion and reduced polymerization

depth. Even if the polymerization was acceptable, the clinical outcomes of high shrinkage

and polymerization stress became more prominent with thicker layers of RBCs 105. These

materials did not have high clinical success, and currently, they are no longer considered to

be the mainstream of the RBCs.

The concept of bulk-fill materials gained momentum as a result of further RBC

development. Through the use of innovative proprietary resins, alternative photoinitiator

systems, different fillers, bulk-fill materials were designed to photopolymerize up to 5 mm

thick increments 106. They exhibited low volumetric polymerization shrinkage and low-stress

build-up upon polymerization, when compared with incrementally placed conventional RBCs

106, 107, 108. Adequate polymerization was achieved by efficiently adding novel photoinitiators,

with higher quantum yield and polymerization modulators, which were responsible for

decreased shrinkage stress 109. Furthermore, increased translucency and the incorporation of

mixed oxide fillers with a refractive index matching that of the resin matrix or glass fibers

enhanced the light penetration, which resulted in improved photopolymerization 110, 111.

16

A further development in bulk-fill flowables has occurred by combining an

enhanced material formulation with a sonic delivery system 112. The sonic energy generated

by a particular handpiece causes a temporary reduction in RBCs viscosity, thus considerably

improving the ability of the material to adapt to the internal surfaces of the cavity 88. By

ending the sonic energy from the handpiece, the RBC gradually returns to its original

viscosity before it is photopolymerized and finished using traditional techniques 88.

According to the manufacturers’ claims, the material exhibits polymerization shrinkage of

approximately 1.6% and can be adequately photo-polymerized to a depth of 6 mm in a single

increment 88.

The critical breakthrough in the field of restorative materials occurred with the launch

of advanced giomer restoratives. This class of restorative materials has the distinguishing

feature of a stable glass ionomer core in a protective resin matrix 113. The different filler,

Surface Pre-Reacted Glass (S-PRG), gives the material a composite-like feature in addition

to a significant release of fluoride ions 114, 115. Studies have shown that these materials, in

both low and high viscosity consistencies, were able to achieve adequate polymerization, had

excellent mechanical properties and demonstrated low polymerization shrinkage when

compared with conventional RBCs 116, 117.

Recently, increased use of bioactive restorative materials has been noticed as these

products promise untapped benefits for long-lasting restorations. The idea of bioactivity of

dental materials was proposed by Hench 118 in the late 1960s, with the following definition:

“A bioactive material is one that elicits a specific biological response at the interface of the

material which results in the formation of a bond between the tissues and the material.” The

concept of bioactive restorative dental material with the capacity of adhesion to tooth

17

structure and the fluoride-releasing ability has been widely used in dentistry for decades.

However, the current concept of bioactivity is defined as the ability of the restorative

material to develop a surface deposition of mineral apatite in the proximity of the restoration

margin in the presence of an inorganic ions solution 119. In 2014, the Activa™ Bioactive

(Pulpdent, Watertown, MA) was launched, and that event changed a paradigm in the arena of

bioactive types of cement and restoratives. Activa represents the first bioactive dental

material with an ionic resin matrix (Embrace resin), a shock-absorbing resin component, and

bioactive fillers that mimic the physical and chemical properties of natural teeth 120. The

ionic resin contains phosphate acid groups. Through an ionization process that is dependent

upon water, hydrogen ions break off from these groups and are replaced by calcium in tooth

structure 120. This ionic interaction binds the resin to the minerals in the tooth, forming a

robust resin-hydroxyapatite complex and a positive seal against microleakage, hence, the

need for a separate bonding system is eliminated 120. The Activa exhibits higher resistance to

fracture and chipping than any other dental restorative material due to a patented rubberized-

resin component 120, 121. In the oral environment, this product responds to pH cycles and plays

an active role in release and recharge of substantial amounts of calcium, phosphate, and

fluoride 120. The presence of these beneficial ions that supersaturate dental pellicle and saliva,

enables hydroxyapatite or fluorapatite precipitation at favorable pH, mimicking the natural

tooth behavior 120. Due to its patented resin formulation, Activa contains no Bisphenol A

(BPA), Bis-GMA, or BPA derivatives 120. At the present moment, this class of bioactive

materials is poised to offer substantial benefits to high-caries-risk patients and those with

proven allergies to BPA derivatives in conventional RBC products, at a reasonable cost.

Although results of in vitro and short-term clinical studies presented favorable results, there

18

is an urgent need for quality long-term clinical studies to confirm the advantages of bioactive

restoratives over currently used restorative materials.

1.4 Classification of Bonding Agents

Since Dr. Michael Bounocore's work has revolutionized the world of adhesive

dentistry, significant developments have taken place, which has contributed to an improved

understanding of factors affecting the durability of adhesive resin restorations 22, 23. The

complexity of dentin-resin interfaces demands in-depth knowledge of all mechanical,

physical, and biochemical aspects that collectively play a pivotal role in the adhesion of

RBCs to the tooth structures. The ionic and hydrophilic nature of modern dental adhesives

yields porous, unstable hybrid layers that are susceptible to water sorption, hydrolytic and

bacterial degradation, and the resin by-products leakage 122, 123. The hydrolytic activity of

host-derived proteases also contributes to the degradation of resin-dentin bonds. Preservation

of the collagen matrix is critical for the improvement of resin-dentin bond durability 124.

Based on experimental results and clinical verification, enamel bonding is very predictable

and reliable 125. However, dentin-bonding procedures are not that predictable. This situation

is not surprising since dentin contains 17 % collagen by volume encapsulated into the

hydroxyapatite structure. Thus, the micromechanical retention can be achieved by utilizing

the pores of dentinal tubules 126. These tubules contain fluid, which may prevent adequate

bonding. Numerous factors influence the bonding mechanism: the number and direction of

19

tubules, the presence of cementum, the age of the tooth, the type of dentin, and the pulp

approximation 127.

Nakabyashi proposed the removal of the mineral phase of the surface layers of dentin

by acid etching, which exposes the dentinal collagen matrix as a bonding substrate 128. The

subsequent penetration and interaction of the hydrophilic monomer within the porous tissue

substrate creates a dentin-resin hybrid layer that represents a significant milestone in dentin–

adhesive bonding 129. The hydrophilic nature of this matrix prompted early researchers to use

monomers with both hydrophilic and hydrophobic groups for improved adhesion. The

rationale was that hydrophilic functionality would help promote penetration of the monomer

into the collagen matrix, leading to the formation of a collagen-resin hybrid layer, whereas

the hydrophobic groups would enhance bonding to the hydrophobic resin matrix of the RBC

restoration 129.

The presence of a smear layer was another hurdle in the dentin bonding process. The

smear layer represents debris that is left untouched on the dentin surface after cavity

preparation. Etching the dentin surface removes the smear layer and provides a predictable

substrate for bonding 130. The enamel conditioning by the acid enhances the removal of the

smear layer and facilitates the creation of a honeycomb structure receptive to bonding 131.

The flow of adhesive into this porous substructure improves the creation of the resin

tags at the enamel-restoration interface for improved micromechanical bonding. In contrast,

the etchant on the dentin surface removes the mineral phase and smear layer, which exposes

the collagen fibril network 132. This network appears to be a very complicated structure,

wherein the naturally wet condition keeps it from collapsing, thereby allowing adhesive

permeation into the substrate for adequate bonding. Research findings indicated that a

20

slightly moist environment at the collagen fibril network enhances the dentin bonding, which

was referred to as wet bonding 131, 132, 133.

Bonding systems have evolved through several generations with improvements in

their chemistry, reduction in the number of steps, simplified application techniques, and

enhanced clinical effectiveness. New bonding agents were developed to combine one or

more steps of etching, priming, and bonding, to reduce standard errors in an already

technique-sensitive process. Thus, current bonding agents represent three categories:

• Three-step systems, whereby the etching (and rinsing), priming and bonding steps are

carried out separately, are also known as total-etch systems.

• Two-step systems that include two different steps:

A. Etching and rinsing step followed by application of a self-bonding primer.

B. Application of a self-etching primer followed by a bonding step.

• Single-step systems, which are composed of self-priming, self-bonding adhesives

where the etching, priming, and bonding are merged into a single step.

In the late 1980s, a new generation of bonding systems was introduced, with the

premise of mildly acid etching of the dentin to partially remove and modify the smear layer

129. These systems utilized a phosphate primer to modify and to soften the smear layer. Due

to limited penetration of the primer, bonding to smear layer did not reach a clinically

accepted level 130.

In the early 1990s, complete removal of the smear layer was accomplished with a

new generation of bonding agents. These systems use the total-etch technique, which

21

involved the simultaneous etching of enamel and dentin with phosphoric acid. The system

facilitated the establishment of resin tags into tubules and adhesive lateral branches for an

enhanced bonding process, while the resin tags sealed the tubule orifices firmly 134. The total-

etch systems exhibited excellent bonding results, which have become a gold-standard for

dental bonding.

The next generation of bonding agents was introduced in the mid-1990s. The

bonding systems from this generation met demands of the profession to simplify the clinical

procedures and to prevent the collapse of the collagen after the acid-etching of the dentin

surface. It consisted of two distinct types of adhesive systems. A single bottle system

combined the primer and adhesive into one solution to be applied after etching the enamel

and dentin. This system exhibited high bond strength to enamel and dentin by

micromechanical interlocking with the formation of the hybrid layer, resin tags and adhesive

lateral branches 133, 134, 135. The next generation of bonding employed another type of bonding

agent, known as a self-etching primer, which was the combination of etching and priming

steps in one solution (phenyl-P and HEMA). However, in comparison to the one-bottle

system from the same generation of bonding agents, the bond strength of this method seemed

to be less predictable 135. Moreover, the self-etching primer system resulted in clinically-

relevant higher microleakage than what is performed with the one-bottle system 136. The

bonding agents of the next two generations eliminated the need for etching with phosphoric

acid by incorporating an acidic primer.

In the mid-1990s, two ingredients: an acidic primer and wetting agent and an unfilled

bonding agent were combined. Self-etch adhesives, which comprised an aqueous mixture of

acidic functional monomers that were phosphoric acid esters, did not require a separate acid-

22

etch step and subsequent rinsing. This generation of bonding agents had two unique types of

specific two-bottle systems. Type I consisted of self-etching primer and adhesive. This

system required the application of self-etching primer first, which is followed by an adhesive.

Type II consisted of two bottles or unit doses containing acidic primer and adhesive. In this

case, a drop of each liquid was mixed and applied to the tooth. Since the etching and bonding

procedures were almost instantaneous, there was minimal potential for postoperative

sensitivity. However, these bonding systems showed lower enamel bond strength, which

prompted some manufacturers to advocate separate enamel acid conditioning before the

application of self-etching adhesive 136.

The next generation of bonding systems was introduced in the mid-2000s. This

generation of bonding agents, simplified bonding procedures, increased bond strength, and

significantly reduced microleakage. Using a single-step technique lessens the clinical

application time, but also notably reduces technique sensitivity. Adhesion is consequently

obtained micro-mechanically through shallow hybridization and by the supplementary

chemical interaction of particular carboxyl/phosphate groups of functional monomers with

remaining hydroxyapatite 137.

The latest advances in dental bonding systems were launched in 2012 in the form of

universal bonding systems. These single-bottle, multi-mode bonding systems are designed to

bond to tooth structures in either the etch-and-rinse or the self-etch mode, depending on the

clinician's preference. According to the adhesion-decalcification concept proposed for self-

etching adhesives, aggressive demineralization of hard tissues by strong acids will result in

the dissolution of apatite crystallites. Demineralization reduces the opportunity to establish a

chemical bond between functional adhesive resin monomers and hydroxyapatite and also

23

reduces the potential for creating nano-layers of calcium precipitates with phosphate resin

monomers with extended spacer groups 138, 139. Research has shown that universal adhesives

with the acidic resin monomer 10-MDP (10-Methacryloyloxydecyl dihydrogen phosphate),

which has been acknowledged as a gold-standard functional monomer, responsible for

chemical bonding to hydroxyapatite, demonstrated improved bond strength, increased

resistance to hydrolytic degradation, and reduced nano leakage 140, 141.

The ideal bonding system should be biocompatible, have an excellent bond strength

to dentin, be resistant to degradation and be simple to use. Unfortunately, even though

extensive research has been achieved in the past 50 years of adhesive dentistry, the ideal

bonding system has not been realized as yet. However, it is crucial to understand the nature

of bonding to see the clinical implications of bond failure, which ultimately lead to failure of

RBC restorations. In a typical case with weak bonding to dentin, the low bond strength is not

sufficient to resist the forces of RBC polymerization shrinkage (about 24 MPa in Class I

cavities). Thus, one or more of the bonded walls could exhibit some level of separation and

gap formation, which can create bacterial leakage through the permeable dentin, resulting in

the pulp irritation 142.

1.5 Evaluation of Photopolymerization Efficiency

Light-polymerization units (LPU), introduced into dental practice in the 1970s,

paved the way to simplified restoration procedure. A large number of studies has been

conducted on several types of light sources, including quartz–tungsten–halogen (QTH) and

24

light-emitting-diodes (LED) 305-317. In today's contemporary dental office, LED LPUs are the

primary source of photopolymerization. In the majority of LED LPU light-spectrum ranges

from 400 nm to 515 nm, with a high peak around 468 nm, referred to as monowave LED

LPU, the light spectrum which is the most efficiently absorbed by a combination of

camphorquinone (CQ) and a tertiary amine (Type II system) 97, 143. On the other hand, LED

LPUs that are capable of generating more than one peak, are referred as multiwave

(Polywave®) LED LPUs, capable of activating the alternative photoinitiators, in addition to

the CQ systems. The Polywave® technology is registered trademark of Ivoclar-Vivadent. The

light spectrum of multiwave LED LPUs has a high peak around 470 nm and secondary peak

around 400 nm depending on the manufacturer's specification for the coverage of alternative

photoinitiators (Type I systems) 97, 143. For example, phosphine oxide derivatives diphenyl (2,

4, 6-trimethylbenzoyl) phosphine oxide (TPO) and phenylbis (2, 4, 6-trimethylbenzoyl)

phosphine oxide (BAPO), like any other Type II photoinitiator system, do not require a

tertiary amine for generating free radicals 143. The TPO high peak absorption is around 400

nm and features much higher absorption efficiency and quantum yield 143, 144. It has been

reported that TPO is very efficient visible light photo-sensitizer, comparable to CQ for the

initiation of RBC photopolymerization 145. However, there are other commercially available

photoinitiators such as phenylpropanedione (PPD) and Ivocerin®, incorporated into various

formulations of commercially available RBC materials. The physical properties of

polymerized RBCs are affected by the generation of primary radicals during the initial stage

of polymerization 143. Thus, free radicals created in this process, attack the carbon double

bonds of the monomers, and rapidly convert the resin to a cross-linked three-dimensional

polymer network 143. Ideally, during the polymerization process, all monomers should be

25

converted into polymers. However, in clinical settings as well as under laboratory conditions,

not all monomers are polymerized. Also, the monomer molecules exhibit loosely bonded

configuration by weak Van der Waals force. Following the polymerization, the monomers

are tightly linked by covalent bonds into a polymer with a smaller distance between the

molecules, which is the principal factor for polymerization shrinkage 97. Inadequately photo-

polymerized RBCs exhibit inferior mechanical and physical properties, which ultimately lead

to compromised longevity of the restoration. Research has shown that degree of conversion

(DC) is a co-determinant of mechanical properties of restorative RBC materials 146.

Furthermore, the literature has also shown a significant correlation between DC and

hardness, modulus of elasticity and flexural strength of RBCs 147. A large number of factors

can influence the effects of photopolymerization. The filler particle size, geometry, and filler

load, polymeric matrix, refractory indices, radiant exposure, and a distance from the light

probe to the RBC surface. In addition, the polymerization protocol can also influence the DC

of RBCs and thereby their mechanical properties 148, 149. The recent photopolymerization

studies support the assumption that LPUs irradiance and exposure time independently affect

the mechanical and physical properties of RBCs 150, 152, 153. Adequate polymerization of

RBCs is needed for desired physical and mechanical properties, biocompatibility, and

longevity 154, 155.

1.5.1 Degree of conversion

During the RBC photopolymerization reaction, the radiant exposure (RE) and

irradiance are principal factors that influence the degree of conversion (DC) and ultimate

clinical success of the restorations 156-160. The exitant irradiance (mW/cm2) represents the

26

flux incident over the total surface area of the light probe, while RE (J/cm2) is the product of

the exposure time and the incidence irradiance at the RBC surface 280. During the free-

radicals photopolymerization, double bonds in the resin of the RBCs are converted into

single covalent bonds, enabling converting monomers into the linear or cross-linked

polymers 196. Thus, the DC represents the ratio of converted single bonds to a total number of

remaining double bonds. Aforementioned is an important determining factor for

polymerization kinetics, which is highly correlated to mechanical, physical, and chemical

properties of RBCs 159,160. Fourier transform infrared (FTIR) and Raman spectroscopy are

broadly used for characterizing the DC of polymerizable RBC materials 161. Both quite

similar techniques produce molecular changes in vibrational energy state by subjecting them

to excitation radiation in selected spectral regions 161. However, FTIR and Raman

spectroscopy differ in the means by which photon energy is transported to the molecules.

Thus, FTIR is an absorption technique, while Raman spectroscopy represents a scattering

method. Since the bands in FTIR spectra are due to polar functional groups while the bands

in Raman spectra are due to nonpolar functional groups, FTIR and Raman spectroscopy are

complementary techniques 161. The process of measuring DC (methacrylate-based RBCs) is

mainly based on comparing the frequency band (1634 cm−1), which corresponds to C=C

vinyl group stretching vibration and those of an internal standard frequency band (1608

cm−1). That frequency band corresponding to very stable aromatic rings during the

polymerization process, to those frequencies of an unpolymerized RBC material 161. Even

though FTIR and Raman spectra are parent techniques, some studies are more efficiently

conducted in FTIR than in Raman spectroscopy, depending on the chemical formulation of

tested materials 161.

27

The literature showed that differential thermal analysis (DTA) using a split fiber-optic

light source was a convenient and a valuable method for investigating polymerization

performance of light-activated RBCs 162. However, this approach introduced by McCabe

showed some drawbacks with different composition of commercial RBCs, empowered with

nano-technology 162. The literature revealed that the most meaningful evaluation of DC of

dental resin materials is obtained by utilizing FTIR and Raman spectroscopy. However, the

relative microhardness (bottom/top ratio) can provide reasonably accurate estimates 163.

Rueggeberg and Craig reported that microhardness values exhibit more sensitivity in

identifying minuscule changes in the DC after the network is cross-linked, in comparison to

FTIR 164. The literature has shown that radiant exposure (J/cm2) and irradiance (mW/cm2) of

the LPUs are highly correlated to DC of RBC materials 164. However, the study conducted by

Selig showed that high irradiance values had a significantly high effect on DC, compared to

time increase of RE 165. The high irradiance could enable a more substantial number of free-

radicals due to a higher rise in exothermic heat. The subsequent temporal viscosity of a

polymeric matrix decreases, allowing the polymer chains increase mobility, consequently

resulting in a much higher DC 165, 166, 167. Thus, the concept of exposure reciprocity law,

which proposes the reciprocity between the incident irradiance and exposure time, suggests

that observed DC values will be similar if the same amount of radiant exposure is delivered

to the RBC surface, however, it is not applicable to the majority of contemporary RBCs 152,

155.

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1.5.2 Degree of crosslinking

Dimethacrylate monomers produce a highly crosslinked network after induced

photopolymerization. Glass transition temperature can indirectly measure the degree of

crosslinking, while the frequently used softening test can also determine the degree of

crosslinking in a relatively simple way 168. The degree of crosslinking, which is closely

related to the function of the DC, is an influential determinant of photopolymerization

efficiency, which has clinically significant implications for the mechanical properties and

ultimately the longevity of RBC restorations 168. During the photopolymerization reaction,

the conversion of C=C double bonds into primary covalent C-C bonds, which allow high

mobility and free rotation of polymer chains results in a highly crosslinked structure.

However, the double bonds conversion is never complete and always contains considerable

quantities of unreacted double bonds 169. Therefore, these residual double bonds can

influence the degree of crosslinking, by reacting with propagating radicals to form a cross-

linked, primary, or secondary cycle 169. The primary cycle is created when a radical reacts

with an unreacted double bond on its kinetic chain, while the reaction of the free radical on a

different kinetic chain, which becomes crosslinked, is called a secondary cycle. The primary

cycles create microgels and lead to heterogeneity in the polymer network, with co-existence

of loosely crosslinked and more highly crosslinked microgel regions. The formation of the

cycles enhances a higher local conversion rate, with almost no influence on the chain

mobility 169.

The literature reports a strong correlation between the degree of crosslinking and

fracture toughness, elastic modulus, solvent resistance, and glass transition temperature of

RBCs 168, 171. Studies have shown that polymerization modes had an essential influence on

29

the DC and mechanical properties of RBCs. Thus, at the same time, polymerization modes

have an impact on a degree of crosslinking which subsequently lead to the formation of more

linear polymer networks 168-171. The pulse delay or slow start mode has often been correlated

with the creation of a polymer structure with a lower degree of crosslinking, which is more

affected by an organic solvent attack during the softening tests 172. Furthermore, a slow start

polymerization technique generates a few cycles per area, which creates a more linear

polymer structure with relatively few crosslinks. In contrast, a rapid start continuous

polymerization technique creates a multitude of growth cycles per area, resulting in a

polymer with a higher degree of crosslinking 172, 173.

These results supported the sentiment, previously seen in DC that incident irradiance

and not radiant exposure has a tremendous influence on the degree of crosslinking 154,155. The

increased concentration of crosslinks has been correlated with improved physical properties

and stability of polymers 168. It has been established that DC does not provide a complete

characterization of polymer structures, due to polymers with similar DC may exhibit a

different degree of crosslinking 168, 171. The assessment of the degree of crosslinking can be

performed by measuring a resin glass transition temperature, using the Arrhenius plot 175.

Within the dimethacrylate monomers, a degree of crosslinking influences the reduction of

molecular mobility, which increases the transition temperature 175.

The characterization of the crosslinked network structure presents a challenge due to

the structural heterogeneity that develops in the polymer during photopolymerization.

Although some studies have used the ethanol solution softening test, a hardness measurement

is taken before and after ethanol immersion, as an alternative method of evaluation of the

degree of crosslinking. Furthermore, some researchers advocate acetone solutions for

30

softening test 176-178. It has been recognized that crosslinked methacrylate networks swell

when exposed to organic solvents. Apparently, the covalent bonds forces between the

polymer chains are exceeded by the forces of attraction between solvent molecules and

components of the chains. Thus, a higher degree of crosslinking is fundamental for better

mechanical properties and enhanced resistant to solvents 178.

1.5.3 Mechanical properties of RBC materials

The relationship between mechanical properties of RBCs and their intraoral

performance has been a subject of numerous studies. For instance, Ferracane showed that

mechanical properties such as Fracture Toughness (FT) and Flexural Strength (FS) of RBCs

are strongly correlated to the clinical performance of RBC restorations 179. Likewise, a strong

inverse correlation exists, between wear and FT, and wear and FS, and a weaker inverse

correlation between wear and hardness 180. Moreover, for the quicker and meaningful

characterization of mechanical properties, the identification of one parameter can lead to the

discovery of other properties, based on these correlations 180, 181, 182. For example, by

obtaining hardness data and retrieving the correlation between hardness and elastic modulus

(EM), it could be possible to estimate the EM of any RBC material 183. However, comparing

results of different studies and the variability among them makes it difficult to make a

meaningful conclusion, due to the differences in their experimental methodology. For

example, the FS values are usually enhanced when a higher cross-head speed of the universal

testing machine is applied 184.

31

The effect of filler (load, composition, size, etc.) on mechanical properties of RBCs

has been widely demonstrated. It has been observed that as a result of increased filler

content, the modulus of elasticity and surface hardness increase accordingly, while

volumetric shrinkage decreases 183,184. Some researchers are considering the more biomimetic

approach, formulating RBCs with the closest possible mechanical properties of dentin, in

which Young's modulus (20–25 GPa) and ultimate tensile strength (UTS) values in the range

of 52–105 MPa 185. According to ISO 4049 international standards requirements, FS and EM

are to be measured by a three-point bending test on 25mm×2mm ×2mm sample 186.

However, these standardized dimensions are not clinically realistic, considering the teeth

diameter and length average. Due to the overlapping irradiation required to polymerize the

entire portion of the specimen, may render the reliability of this testing procedure somewhat

questionable 187. The FT is an important mechanical property, which characterizes the

relative resistance to crack propagation from the surface or inherent flaws in the RBC

material 188. Innovative technologies currently employed, such as fiber or whisker

reinforcements have produced very significant enhancements in FT of RBCs, however, not to

the level of high toughness ceramics or casting alloys, and this may be what is required to

render the materials substantially fracture resistant in the clinical settings 189.

The testing procedure of FT and FS are similar, with the exception that FT specimens

have a notch, and the same overlapping irradiation protocol is applied with this testing

method. Diametral tensile strength test (DTS) has been designed to test brittle materials by

utilizing a cylinder-type specimen that is subject to compressive load across its diameter. It

has been observed that materials which exhibit plastic deformation would produce false DTS

values 16. Although DTS test is not a part of the ISO 4049:2009 standards, it was recognized

32

as technically reliable and a straightforward test for the assessment of RBC mechanical

properties 189. However, DTS test can provide useful information about the effectiveness of

RBC polymerization and in conjunction with other measuring methods can provide

meaningful data on the mechanical properties of given materials 189. Vickers and Knoop

microhardness tests are routinely employed for testing of RBCs' mechanical properties.

Hardness (H), represents an important parameter in the characterization of the surface

properties of the material on a microscopic scale, which is established as the ratio of the

applied load (P), to the contact area (A), which caused subsequent indentation impression:

H = 𝑃

𝐴 = β

𝑃

𝑑2 (1)

Where d represents the size of indentation, and β represents a constant related to the

geometry of a given indenter 318, 319. Vickers and Knoop are the most frequently employed

microhardness tests for characterization of RBC materials by utilizing a pyramid-based

indenter. The Vickers hardness number (VHN) is determined by using the contact area of the

indentation impression and the parameter d in Eq. (1), which is described as an average of the

two diagonals of the resultant square-shaped impression 320. Thus, the β constant for VHN

calculation is 1.8544. The Knoop microhardness test uses a pyramid-based indenter with the

length to width ratio of 7:1 and with corresponding face angles of 172.5 degrees for the long

edge and 130 degrees for the short edge. The Knoop hardness number (KHN) is calculated as

applied load (measured in kilograms-force) divided by the square length of the long diagonal

of the indentation in mm, and the constant of indenter associating the projected area of the

indentation to the square of the length of the long diagonal, equal to 0.07028 321.

33

Although the measurement of a residual surface plastic impression is the basis for the

hardness measurement, however, this surface property test represents an elastic-plastic

surface deformation process. Thus, the relative importance of reversible (elastic) and

irreversible (plastic) components of surface deformation induced by the hardness indenter

could be evident in the elastic recovery during unloading of the indenter. Therefore, this

elastic depth recovery following the maximum indentation should be taken into

microhardness calculation. The literature has shown, especially concerning RBC materials,

the inequality between the plastic zone beneath the indentation and the surrounding elastic

matrix exists. The elastic recovery is highly related to the surface geometry of an indenter,

which differentiates the elastic recovery of Vickers indentation in comparison to Knoop

indentation. The elastic recovery attributed to Vickers indentation usually develops along the

depth of the indentation, while the diagonal length of the surface impression remains

unchanged, in contrast to the Knoop indentation, where the elastic recovery appears as a

result of a minor diagonal contraction of the indenter 319, 320. The Knoop microhardness

computation takes the length of the longer diagonal only into the calculation of KHN, thus

making this method preferred for its reliability over other methods 318, 319, 321. A linear

correlation exists between Knoop microhardness and the mechanical properties of the RBC;

and Knoop microhardness and the irradiance of LPUs 180, 183, 190.

34

1.5.4 Shrinkage and shrinkage stress

Despite significant developments in the field of adhesive dentistry, the occurrence of

RBCs’ polymerization shrinkage and shrinkage stress remains a clinically relevant issue 192.

During the polymerization reaction, the distance between monomer molecules becomes

reduced by the transitional changes from loosely bounded Van der Waals forces, into tightly

linked covalent bonds of the polymer network, which produces the shrinkage stress. Cavity-

restoration interface exhibits the maximum shrinkage stress force, with degradation of that

interface resulting in subsequent microleakage and gap formation 193. The consequences of

gap formation may increase the possibility of tooth hypersensitivity and an adverse pulpal

reaction, with a high probability of detrimental effects on restoration longevity.

Stress represents a complex interaction among the following: the resin viscosity, the

volume shrinkage, the polymerization rate, the DC, modulus development, and network

structural evolution 193. However, each of these properties cannot be individually

manipulated and studied without having a significant impact on the others. Thus,

polymerization shrinkage stress is multifactorial, being related to cavity configuration and

RBCs properties.

The cavity configuration properties are defined by the geometry and size of the

preparation and its C-factor. The C-factor, or cavity configuration factor, represents the ratio

of bonded to non-bonded preparation wall areas, which have also been a subject of intense

debate in the literature 193. Empirically, the higher C-factor, the higher the stress level

generated. According to the literature, when a rigid (non-compliant) testing system is

utilized, a direct relationship between the contraction stress and C-factor value was reported,

while results of the studies using a less rigid, more compliant set-up, showed an inverse

35

relationship 194,195. However, from a clinical standpoint, the C-factor seemed to be a valid

parameter for the comparison of restorations with identical shapes and volumes, as reported

in experimental settings 196.

The material properties associated with RBCs’ polymerization shrinkage stress

include filler content, resin-matrix composition volumetric shrinkage, and elastic modulus

and viscous flow 197. The literature has shown that RBCs with a high filler content exhibit

low volumetric shrinkage and high stiffness 195-197. Furthermore, strain capacity is inversely

correlated to the filler content of RBCs 198. For instance, microfilled RBCs usually

experience lower elastic modulus, a higher strain capacity and similar volumetric shrinkage

properties in comparison with hybrids, due to their relatively lower filler volume percentage,

which is in a range from 35% to 50% 199.

Viscoelastic properties define another essential factor in contraction stress

development of RBCs during photopolymerization, the flow capacity at initial stages of the

polymerization, and the elastic modulus generated during the polymerization reaction 200.

Composite flow is influenced by the structure of individual molecules, crosslinking, the

filler/matrix interfacial characteristics, reaction kinetics, and the C-factor. Flow can happen

quickly when the RBC is allowed to shrink, which means that flow can be increased when

the preparation C-factor is low 201. Polymerization shrinkage is positively correlated to the

DC. Thus, an increase in the DC leads to higher polymerization strains because more

covalent bonds and more highly cross-linked networks are formed 202. The higher DC values

of RBCs correlate with improved mechanical and chemical properties, color stability and

biocompatibility. Therefore, the stress reduction cannot be accomplished at the expense of

adequate DC.

36

Various methods are aimed to measure polymerization shrinkage stress. Shrinkage

strain-rate and stress are measured during the polymerization reaction with a tensiometer,

based on the cantilever beam deflection theory 203. Various devices and methods have been

used for the measurement of polymerization shrinkage regarding volumetric and linear

shrinkage and the cuspal displacements. The reasonably straightforward, indirect techniques

to measure shrinkage include the microleakage assessment test and finite element analysis,

which utilize a three-dimensional micro-CT data 203. For maximum accuracy and clinical

relevance, the instrument compliance should be as close as possible to the features of a

natural tooth 203. Clinical performance of Bis-GMA based RBCs could be enhanced by

maximizing the DC and elastic modulus, decreasing volumetric shrinkage (maximizing

viscous flow), and by minimizing the polymerization stress 203. Another stress reduction

mechanism readily available is to increase the compliance of the preparation walls by

applying an intermediate, low-modulus layer, using a low-shrinkage flowable RBC material

203.

1.5.5 Depth of polymerization

Achieving sufficient photopolymerization of RBC materials is a critical parameter to

maximize mechanical properties of the restoration and to obtain a desirable clinical outcome.

The depth of polymerization (DP) of light-activated RBCs depends on the filler composition

and resin formulation, resin shade and translucency, photoinitiators content, the intensity and

spectral distribution of the LPU, and finally on the irradiation protocol. There are two DP

assessment techniques: direct and indirect. The indirect method for DP assessment includes

scraping tests 204, the dye uptake tests 205, and the surface microhardness tests 206. The direct

37

method measures the polymerization efficacy by utilizing spectroscopy, as a part of the DC

assessment, which has not been routinely used due to the cost associated with this

complicated methodology.

The simple scrape test has been modified and became a part of ISO 4049 standardized

testing procedures 186. The test utilizes a 4 mm diameter stainless steel mold, filled with the

RBC, and polymerized with the LPU from a single side only. At the non-irradiated side of

the specimen, a plastic spatula is used to scrape any material that is soft (inadequately

polymerized), leaving remaining hard and apparently polymerized material. The measured

remaining thickness of the RBC specimen is divided by 2 and is referred to as the DP scrap

test value, expressed in mm. This technique, while simple, it has several disadvantages. The

method does not provide an accurate estimate of the polymerization quality in any region of a

given material, especially in the deeper layers adjacent to the soft, slightly polymerized resin

which has been removed. The results of some studies found that scraping methods severely

over-estimated DP as compared to relative hardness testing or DC analysis 180. The results

verified that DC appeared to be the most sensitive test for DP. A good correlation exists

between the Knoop microhardness testing and infra-red spectroscopy 158-163. Microhardness

testing is the most popular method for investigating factors that influence DP, because of its

relative simplicity and accuracy.

38

1.5.6 Biocompatibility

Although RBC products are known to be highly stable structures, they are susceptible

to degradation, because of insufficient polymerization. Several components could be released

from RBC restorations into the oral environment 207. Thus, biocompatibility is not a direct

way to assess the photopolymerization efficiency. However, it is an essential prerequisite of

all materials that are placed into a biological system to be bio-friendly to the environment.

RBCs can affect the oral cavity in at least two ways: through the elution of bioactive

substances and by a quick and sharp temperature increase during polymerization. Sufficient

polymerization renders superior mechanical, physical, and chemical properties of RBCs,

which prevents and limits leaching of potentially harmful components from the materials 207.

The cytotoxicity of RBCs has been attributed to the leaching of unpolymerized

monomers, such as products of an incomplete polymerization and consequently the by-

products of the resin matrix degradation processes in the oral cavity 208. The most commonly

used monomers in RBCs compositions include Bis-GMA, TEGDMA, and UDMA, all of

which have been identified as cytotoxic molecules 209, 210. Further, literature has shown

particular concern with TEGDMA, which seems to be a mutagenic compound in mammalian

cell assays. It appears that these cytotoxic molecules induce gene mutations, by covalent

binding to the host DNA 211, 212. Several factors influence the release of unbound substances

from polymerized RBC materials 213. The literature shows that DC values are highly

correlated with the amount of leachable components from any given RBC material 213. The

solvent composition, which is applied for the extraction, affects kinetics and mechanism of

the elution process 213. The chemical characteristics, as well as the size of leachable

39

substances, moderate diffusion through the polymer network. The current concept presumes

that smaller molecules have enhanced mobility and exhibit a faster elution rate 213.

Chemical degradation of RBCs can be attributed to hydrolysis and attack of salivary

enzymes in the oral cavity 213. Studies showed that percolated monomers enhance bacterial

growth and are responsible for glutathione depletion, which is a critical factor in pulp or

gingival cell apoptosis and production of reactive oxygen species (ROS) 214. Additionally,

leached substances are associated with a variety of allergic reactions 214. Therefore, the

toxicology results from the cell cultures have revealed that monomers released from the

RBCs generate ROS and, therefore, affect the redox balance in the oral tissues cells 208-212.

The occurrence of diffusion of small molecule monomers throughout the polymer network is

more easily accomplished, due to their small and flexible structures, polar ends, and flexible

and smaller suspended groups 215. TEGDMA is considered the most frequently eluted

monomer among RBC materials, because of its monomer molecular structure and higher

solubility 213,214, 215. Compared with TEGDMA, however, Bis-GMA monomer is reported to

be much less soluble because of its hydrophobic nature and reduced diffusion in liquid

environment 216. Furthermore, the hydrophilic monomers such as HEMA, which is very

abundant in bonding agents and TEGDMA, predominantly found in RBCs, are the monomers

capable of diffusion through the dentin, endangering the pulp vitality 213, 215.

The magnitude of monomer diffusion is displayed in a millimolar/Liter (mmol/L)

range, which is the clinically significant concentration that could cause detrimental effects on

pulpal homeostasis 217. The literature showed that diffusion rate increases when the

remaining dentinal pulpal coverage thickness is less than 1mm, and the cavity has been

conditioned with acid before the bonding procedures 218. In addition, experimental research

40

showed that HEMA pulpal concentrations could be in the range of 1.5–8 mmol/L, whereas

TEGDMA concentrations could be in the range of 4 mmol/L, respectively 219, 220. These

concentrations have been found to cause significant cytotoxicity through mechanisms

associated with oxidative stress, ROS production, and depletion of intracellular glutathione

with the consequences of cell death, mainly via apoptosis. Long-term exposures to nontoxic

concentrations of HEMA (0.05–0.5 mmol/L) and TEGDMA (0.05–0.25 mmol/L)

significantly delay physiological migration, odontogenic differentiation, and mineralization

process of human deciduous teeth pulp stem/progenitor cells in a concentration-dependent

manner 221. Furthermore, just a single exposure to higher HEMA concentrations of at least of

2 mmol/L and TEGDMA at least of 1 mmol/L have caused inhibition of the formation of

reparative dentin entirely 220, 221.

The methodology used to evaluate eluted compounds from RBC materials into

biological fluids is based on chromatography, which is an analytical procedure that isolates

molecules related to differences in their structure and composition 221. High-performance

liquid chromatography (HPLC) is now one of the most influential tools in analytical

chemistry, capable of separating, identifying, and quantifying the compounds that are present

in any sample, dissolved in a solvent 221. However, reversed-phase liquid chromatography

appears to be the most preferred method in a dental materials analysis 222. Improvements in

the properties of RBCs to reduce and prevent some leachable compounds are continually

being studied. Thus, improvements in the photopolymerization quality could minimize all

detrimental consequences to the pulp health.

41

1.6 Evolution of Light Polymerization Units (LPUs)

The LPUs have become the essential instruments in the contemporary dental practice

because a large number of dental procedures requires some photopolymerization. These units

emit a beam of light having a specific spectral output that activates photoinitiators in a wide

range of photopolymerizable materials. Since their introduction into the dental profession in

the 1970's, the evolution of LPUs started from UV polymerization and developed to visible

light polymerization. During this time, LPUs became smaller, battery-powered, and capable

of delivering a high amount of energy in a brief time.

From the historical perspective, polymerization technology was adapted from the

chemical industry and modified for the use in dentistry 33, 34, 35. The first LPUs were designed

to emit ultraviolet light 36. The typical polymerization time was the 40s, but a 60s exposure

provided enhanced results 37. Initially, the technology was far from perfect, but it was

revolutionary for that era. The limitations associated with UV polymerization included

limited light penetration through the RBC material (limited depth) and health hazards related

to UV lights exposure on the eyes 38, 39, 40. However, UV polymerization was short lived

when Dr. Bassoiuny at the University of Manchester School of Dentistry, in 1976, placed the

first visible light polymerized RBC restoration. The Imperial Chemical Industries of England

developed the novel technology and modified for dental use 41, 42, 43. The new visible light

photoinitiation system was based on the photoinitiator camphorquinone along with a tertiary

amine co-initiator. This system underwent through several modifications over the years.

However, it is still in use with today's modern RBC materials 44, 45.

The initial light sources used to photopolymerize the new RBC were based on quartz–

tungsten–halogen (QTH) lamps developed with the Johnson and Johnson partnership. The

42

new venture launched the FotoFil restoration system and QTH dental light source 46.

Although this new technology had the capability to adequately photopolymerize a 2 mm

thick increment of RBC material, the QTH technology could not eliminate the hazard for

ocular damage without an appropriate eye protection 47, 48. The QTH LPU used an

incandescent quartz light bulb consisting of a tungsten filament surrounded by a halogen gas,

which emitted a beam of filtered white light which provided a broad spectrum between 400

and 500 nm. Although these light sources were manufactured with relatively low-cost

technology, the short lifespan of the QTH lightbulb with a gradual degradation of the filter,

compromised their light output consistency. The QTH LPU exhibited a low energy

performance because the filter system wasted a considerable amount of radiation, which

transferred the majority of the unwanted radiation into heat. Therefore, a cooling mechanism

was needed with these light sources. The QTH became the mainstream LPUs, for many

years. Even after the introduction of LED LPUs in the 1990s, the QTH LPUs underwent a

series of modifications and improvements, in such a way that the bulb power increased from

35W up to 100W. The QTH LPUs have been typically used for the 40s to 60s polymerization

exposure, for a 2 mm thick increment of the RBC 223. The inconvenience for the cooling

timeout was a disadvantage. Furthermore, the excessive heat produced by these LPUs

reduced their clinical effectiveness and impacted their lifespan significantly.

In the late 1990s, the next generation of LPUs was developed, with the most

promising technology at the time: the argon laser units and plasma-arc (PAC). The principal

reason for their introduction to dental practice was to enhance the effectiveness, and to

increase the magnitude of the light output and to shorten the exposure time 223.

43

The argon laser (Light Amplification by Stimulated Emission of Radiation), with an

active medium of argon gas, unlike QTH LPUs, does not employ filters. It produces a

collimated (focused, non-divergent) beam of coherent energy, when applied to a particular

target, resulting in a more consistent radiant exitance over the tip-to-target distance 224, 225.

Unfortunately, the major output wavelengths were out of the absorption range of CQ, and

thus, this type of laser was nowhere near as effective as it could have been, if the output had

been a slightly shorter wavelengths. Although this novel technology showed some

exceptional improvements (coherent beam of light energy), it experienced some drawbacks.

For instance, the light beam should not be allowed to be directed to the eyes, as the eye

exposure would result in an immediate retinal damage 226. Although the size, weight, and

portability of newer argon laser LPUs have been improved dramatically in recent years, they

often take considerably more space than QTH LPUs. Laser LPU generate a substantial

amount of heat, the cooling fan tends to be noisy, and these LPUs were very expensive. Thus,

these LPUs never gained popularity. However, laser polymerization demonstrated an

immediate higher DC than that obtained with the QTH LPU. The laser LPUs enhanced RBCs

polymerization, which resulted in improved physical properties and bond strengths 227.

The Plasma arc (PAC) LPUs, use technology that uses plasma for the light source.

The PAC, initially designed in France, consisted of two tungsten electrodes separated by a

small distance, embedded in a high-pressure xenon gas chamber. When electric current

passes through the plasma, it generated the emission of the UV light with a broad spectral

range 228. The PAC LPUs uses sophisticated bandpass filters to curb UV and IR radiation,

while the light is delivered by utilizing a three-foot-long liquid light guide 229. Early PAC

LPUs produced spectral output wavelengths between 370 to 450 nm (low spectrum units) or

44

between 430 to 500 nm (high spectrum units), and a typical radiant exitance was around

2,000 mW/cm2. The early PAC light (the Apollo) produced broad, banded, white light. The

different tip ends had filters that only passed light of shorter or longer wavelengths to pass

either wavelengths more near violet or more near blue. The clear tip passed all wavelengths,

emitting white light that was used to enhance tooth bleaching

The PAC LPUs were designed to produce a 3s exposure, while an upgraded version

of these LPUs was launched with four adjustable polymerization modes with exposure up to

10s 229. Although PAC LPUs exhibited an adequate RBCs' polymerization, faster than other

LPUs, they were frequently associated with a higher risk of pulpal damage 229, as result of

their increased heat production compared to the QTH LPUs. Furthermore, due to a relatively

high cost, this technology had limited utilization in dental practices.

The LED LPUs are the most popular and the most current source of

photopolymerization in dental practices. The light-emitting diode technology uses two

semiconductor crystals, type "n" and type "p" with different electron density 230. When

electrical current passes through these crystals, light is generated at the "np" junction, with

the wavelength determined by composition difference between the semiconductor substitutes.

The LED LPUs produce high efficiency and medium-intensity light, which does not require

filters nor fans. Unlike QTH light sources, LED LPUs generate light within a narrow spectral

range. For instance, gallium nitride diodes produce light with wavelengths between 450 and

490 nm with a peak at 460 nm. LED light sources often require less power to operate, which

gave them the ability to be smaller in size, portable, and battery operated. The LED LPUs

have shown another advantage: extended service life compared to any other light types. For

instance, the halogen bulbs' life expectancy, under best condition, can last a hundred hours,

45

whereas the LEDs can last thousands of hours 230, which could be a reason behind the

enormous LED LPUs popularity among dental practitioners. The spectral output of the first

developed LED LPUs was not tailored to match the maximal absorption of CQ

photoinitiator. However, it happened to be a close match. The evolution of LED LPUs can be

observed as a function of the power increase, the ability to polymerize alternative

photoinitiators, and the LPUs beam profile homogeneity 243. The first generation of LED

LPUs started in the early 2000s, with the launch of the LuxøMax (Akeda Dental A/S,

Lystrup, Denmark), a product based on prototype patented in the US 231. It was a first ever,

large, pen-like, NiCad battery- powered LPU, having seven discrete LEDs. The concept of

the first generation of LED LPU design was to use multiple single-emitting LEDs, with each

LED being able to produce 30 to 60mW of radiant power, embedded into an axial array to

provide sufficient spectral power output to start activation and subsequent

photopolymerization 232. Other manufacturers followed the lead and launched their versions

of LED LPUs.

Performance of the first LED LPUs was not in line with the high expectation, as they

were relatively low-powered, compared to the traditional QTH LCUs at that time. The early

results of LED LPUs performance, in comparison with QTH LCUs, as well as issues with

NiCad batteries, led to some unanticipated, negative predictions for LED LPUs future 233.

However, a high success achieved in the blue LED semi-conductor research boosted the

efficiency of the first generation commercially available LED LPUs. Therefore, in late 2001,

a launch of the EliparTM FreeLight (3M/ESPE, St. Paul, MN) dramatically changed the

outlook of LED LPUs market, by increasing the efficiency and the performance of LED light

sources 234. The new EliparTM FreeLight had an innovative design, with a 10 mm light probe,

46

incorporating 19 discrete LEDs in an array and was capable of generating a radiant exitance

of 400 mW/cm2. The success achieved by the incident irradiance level increase was partly

contributed to the use of a tapered, fused glass fiber light guide. According to the

manufacturer's claim, the radiant exitance level produced by new LED LPUs was equivalent

to the 800 mW/cm2 value generated by the conventional QTH LPUs.

The second generation of LED LPUs started in the early 2000s, due to advances in

the LED chip industry and incorporation of the new 1-W chips into the LED arrays, which

enabled the production of more powerful, single LED devices 235. In addition, the chip

manufacturers started fabricating chips specifically within the wavelength required for the

polymerization of RBC materials. Examples of these single more powerful LED emitters

were the Luxeon LXHL-BRD1 in 2004 235, which produced 140 mW (1 W chip) or 600 mW

(5 W chip) of blue light. Moreover, the more recent LED Engin Inc., LZ4-00B200 in 2012,

which produced 4,200 mW or 5,600 mW of radiant power depending on the chip 236. The

launch of the 2nd generation LED LPUs officially started with EliparTM FreeLight

2(3M/ESPE, St. Paul, MN), an improved version of EliparTM FreeLight. The new LED LPUs

successfully increased 2.5 times radiant exitance 237, by utilizing the tapered waveguide (the

so-called “turbo-tip”) to boost emitted irradiance 237. Nickel metal hydride batteries were the

power source that enabled the breakthrough. Other manufacturers started launching different

designs: a first in this group, L.E. Demetron II® (Kerr Dental, Orange, CA) began in 2005 238.

This unit had a conventional pistol grip design, with a built-in cooling fan, which was a

compromise to heat sink design. However, like other manufacturers, the light guide exhibited

a tapered turbo-tip to boost the incident irradiance. Later, manufacturers started to

experiment with light probe design, which led to the launch of the SmartLite PS® (Dentsply,

47

Milford DE), an ergonomic-pen-style light, which was small (24cm long), lightweight

(100g), and silent (no fan) 239. The manufacturer claimed that device could generate a radiant

exitance up to 950 mW/cm2. The advantage of this design allowed LPUs to produce a more

unidirectional light, with a substantial reduction in light loss, without a light guide between

the light source and the material to be polymerized. In addition to an increased irradiance

level, this innovation led to lower power consumption, longer battery life, and most

importantly, the significant reduction in heat production. This innovative technology helped

the industry to resolve the disadvantage associated with tapered light guides: more divergent

light beam and a smaller light tip.

In late 2010, this problem seemed to be resolved with the innovative design of

EliparTM S10 (3M ESPE. St. Paul MN). This light featured a 10 mm diameter, and a parallel

glass fiber light guide 240. The manufacturer claimed that the LPU could generate the radiant

exitance up to 1,200 mW/cm2. The second generation of LED LPUs, have experienced

tremendous improvements, a substantial radiant power increase, which enabled sufficient

photopolymerization in shorter exposure time. All previously mentioned LED LPUs were

designed to produce a single peak of spectral irradiance to match the maximum absorption

spectra of CQ photoinitiator system, they are referred as single-peak, or monowave LED

LPUs 243.

The third generation of LED LPUs provided an extended light spectrum in the violet

range, to match the range of alternative photoinitiators, as well as the CQ photoinitiator

spectra. Thus, the third generation of LED LPUs have the blue-light chips with a spectrum

from 420 nm to 510 nm and the violet-light chips with a spectrum from 380 to 420 nm. The

new LED generation was marked by significant improvements in power conversion, which

48

drastically reduced the internal but not external heat generation. Also, the LPUs showed a

substantial power increase, ability to control their exposure time, radiant exitance values and

the polymerization protocol. This new LED technology referred to as "multiwave," was

produced by some manufacturers, although Ultradent initially applied for the patent, Ivoclar-

Vivadent was allowed to patent this broad-spectrum technology as Polywave® (Dual-peak)

LED LPU. The majority of the manufacturers continued development of the second

generation of LED LPUs. The first multi-peak (multiwave) LED LPU became available in

2003 when the Ultralume 5® (Ultradent Products, South Jordan, UT) came to the market 241,

(Figure 1.6.1).

Figure 1.6.1 Ultralume 5® (Ultradent Products, South Jordan, UT) source:

http://www.airforcemedicine.af.mil/Portals/1/Documents/DECS/Product_Evaluations

/Equip/Lights/Curing/UltraLume_LED%205.pdf

This unit was a corded, pen-shaped LPU, powered with a 5W blue LED central chip

surrounded by 4, low power violet LEDs, capable of producing a broad spectrum, with high

peaks around 403 nm and 453 nm and the manufacturer claimed a radiant exitance of 1,200

mW/cm2.The LPU featured no fiber optic light guide. The results from a study evaluating

polymerization performance of 4 LPUs, by conducting a microhardness test, showed that the

49

Ultralume 5 achieved the highest KHN of all RBC materials tested, outperforming all of the

2nd generation LEDs and QTH LPUs 242. In 2004, the first product of the Bluephase® family

(Ivoclar-Vivadent, Amherst, NY), with a radiant exitance of 1,100 mW/cm2 came to the

market 243.

An improved product, the Bluephase 20i, exhibited a more traditional gun-shape with

a pistol grip, and standard fused-glass fiber light guide. The new multiwave LPUs from the

Bluephase family could generate radiant exitance up to 2,200 mW/cm2, which brought back

the more advanced cooling features to their design. Furthermore, these LPUs featured highly

sophisticated, programmable polymerization modes for the variety of clinical applications, an

extended spectrum with two spectral peaks at approximately 410 nm and 470 nm 244.

The Valo® (Ultradent Products, South Jordan, UT), represented the hallmark of the

multiwave light sources approach 245. A wand-shape design of the Valo featured two blue

emitters (around 460 nm), and one violet (about 445 nm), capable of producing a spectral

output from 395 nm to 480 nm 245. The Valo is made from a solid bar of tempered, high-

grade aerospace aluminum, which provides this light source with unsurpassed durability and

superior heat dissipation. The ergonomic and streamlined design enables this unit to easily

access intraoral locations to provide optimal light for polymerization, without sacrificing

patient comfort. The unit featured an optimally collimated, almost homogenous beam profile,

which had not been seen by any LPU before. The LPU exhibited programmable

polymerization options: Standard Power (1,000 mW/cm2), High Power (1,400 mW/cm2), and

Xtra Power (3,200 mW/cm2).

The launch of a multiwave LPU, the ScanWave® (Acteon, Mettmann, Germany),

which has not gained much popularity in North America, showed an entirely innovative and

50

sophisticated product 246. The LPU used 4 different wavelength LED emitters (405nm /

440nm / 460nm and 480nm), capable of producing spectral output coverage from 390 nm to

505 nm. The ScanWave® incorporated a laser aiming system using a wavelength of 655nm,

which allowed a precise light delivery for the light probe. According to the manufacturer's

claim, the LPU is capable of generating a radiant exitance of 2,200 mW/cm2 over a 5.5mm

light probe diameter. The unit featured very sophisticated programming options, where the

manufacturer claims that LED LPU can optimize polymerization while minimizing the heat

generation within the target 246. The ScanWave® is equipped with a system for the detection

of possible unit operation anomalies. Thus, the third generation LED LPUs represented the

examples of state-of-the-art light sources, able to generate low or high power in a variety of

clinical scenarios that had never been seen before.

While the LED LPUs have become the gold standard for the polymerization in the

contemporary dental practice, they have some inherent drawbacks, particularly concerning

the non-homogeneity of their irradiance beam profiles. This drawback is because irradiance

does not represent a single value, but rather the mean of all irradiated light intensities. Thus,

with a non-uniform beam profile of LED LPUs, the irradiance value can range from 300

mW/cm2 to 3,000 mW/cm2, which can make a significant difference in polymerization

efficiency and the RBC's surface hardness profiles 242. Spectral non-homogeneity of

multiwave LPUs beam profiles relates to different wavelengths of embedded LEDs within

the LPUs light beam profile. Newly developed LED LPUs with much more uniform beam

profile have been designed to overcome the shortcomings of the previous generation of light

sources could be identified as the fourth generation of the LED LPUs. The examples are

51

Bluephase Style (Ivoclar-Vivadent, Amherst, NY) and DeepCure-S (3M ESPE, St. Paul,

MN).

1.7 Intrinsic Factors Affecting Photopolymerization Efficiency

RBC restorative materials represent one of many success stories in advanced

biomaterials research, as evidenced by their use increasing exponentially in the last decade.

However, simplifying numerous factors that affect their polymerization efficiency is not an

easy task because commercial RBCs represent a variety of chemical formulations that are not

readily revealed, due to the trade secrets. The following factors were researched extensively:

the photoinitiator systems, alternative monomer chemistry, and a novel formulation of fillers

and their coupling agents. The fundamental understanding of photochemical reactions is

critical to establish correlations among the RBCs, to modulate desired photopolymerization

outcome.

The photochemistry represents the analysis of chemical reactions and physical

behavior that occur under the influence of visible and ultraviolet light. Two fundamental

principles are the foundation for understanding photochemical transformations. The first law

of photochemistry, the Grotthuss-Draper law, states that a photon has to be absorbed by a

chemical material for a photochemical reaction to take place 11. Therefore, the presence of

light alone is not enough to trigger a photochemical reaction; the light must exhibit the

correct wavelength match to be absorbed by the reactant species 11. The second law of

photochemistry, the Stark-Einstein law, states that, for each photon of light absorbed by a

chemical system, only one molecule can be activated for the photochemical reaction to occur

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11. Light must be present, and it must then be absorbed. In other words, there is a one-to-one

correspondence between the number of absorbed photons and the number of excited species.

However, not all absorbed photons can produce the same number of reactive species. The

ability to accurately determine the number of photons leading to a reaction enables the

efficiency or quantum yield of the photochemical reaction 11. The practical implication of

photochemistry is of immense importance, as it is the basis of photopolymerization of RBC

restorative materials. LED LPUs deliver light energy to the surface of RBC material. The

capacity of LPUs to deliver a sufficient quantity of photons within the appropriate spectra is

the critical step in a photochemical process. In regard to the previously mentioned, the

photoinitiator system is elevated to a state of higher energy, which represents the

photoexcitation state, a fundamental requirement for photopolymerization. For instance, if a

system does not absorb the light of a particular wavelength, no photochemistry will occur,

regardless of the duration of the light exposure. Thus, the prime objective of successful

photopolymerization would be an adequate quantity of light to be delivered to the surface of

any given RBC material, and the closest match between the spectra of incoming light and a

corresponding RBC photoinitiator system.

1.7.1 Photoinitiator systems

The dimethacrylate-based RBC material is the most frequently employed chemistry

formulation among the commercially available RBCs, and it has been a focus of extensive

studies since its introduction into general practice 154. Due to the RBCs' exponential clinical

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acceptance, clinicians focused their expectation for fast polymerization, superior mechanical

properties, and greater longevity of the restorations on the photopolymerization concept.

A photopolymerization reaction is a complicated process induced by the LPU’s

irradiated light over the RBC surface, which in essence the photo-reactive systems absorb

light irradiation, and the monomers in the molecular structure become excited resulting in the

formation of a polymer structure. The free radicals that trigger the polymerization process are

formed by exposing a photo-unstable compound, the photoinitiator, to the appropriate light

source's quantities and wavelengths, causing the photolysis 5. The radicals have unstable

(reactive) groups of atoms with at least one unpaired electron in their methacrylate-based

radical groups.

The classification of photoinitiators is based on the wavelength range of light used for

activation or according to the mechanism employed for the photolysis. The light sources used

in dentistry for the photopolymerization produce a spectral emission in a range from 380 nm

to 515 nm. This spectrum is apportioned into the violet light range, from 380 nm to 420 nm,

and the blue light, from 420 nm to 515 nm 247. Because the photon energy is inversely

proportional to its wavelength, the photons in the violet light spectrum exhibit more energy

than those in the blue light spectrum 247. Radicals that initiate a polymerization reaction are

either formed by using the bond fusion or by contacting the hydrogen atom (H transfer) to a

second compound, the so-called “co-initiator” 247. Regardless of the mechanism used, it is of

the greatest importance that the photoinitiator reaches an excited state. Many photoinitiators

contain carbonyl groups as light-absorbing species. The majority of RBCs contain

methacrylic acid esters, which are used as radically polymerizable monomers. Depending on

their number of polymerizable methacrylate groups, they are classified into monofunctional

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(n = 1; e.g. methyl methacrylate), difunctional (n = 2) and multifunctional (n > 2)

methacrylates 247. During the radical photopolymerization of monofunctional monomers,

linear polymers are developed. The rate of the radical formation is dependent on irradiance of

the LPU, the photoinitiator quantum efficiency, the extinction coefficient of the

photoinitiator and its concentration, and the thickness of the layer through which light passes

through 257. Useful photoinitiators are defined by high quantum efficiency and a high

extinction coefficient, i.e., the capacity to show a rapid absorption of the light in the

wavelength range applied 257.

There are two types of photoinitiator systems currently employed among commercial

RBC materials: Norrish Type I and Norrish Type II systems. Camphorquinone (CQ), a

Norrish Type II system developed by Dart and Nemcekin in 1972, became the most common

photoinitiator system for dental applications 1. CQ has a visible light absorption range from

360 to 510 nm, with a maximum absorption peak at 468 nm. This Type II system of

photoinitiator reacts by bimolecular hydrogen abstraction from suitable co-initiators,

preferentially aromatic amines, such as ethyl-4-dimethylaminobenzoate (EDMAB), aliphatic

amines, such as 2-(dimethylamino) ethyl methacrylate (DMAEMA), or a common

piperidines derivate, such as 1,2,2,6,6-Pentamethylpiperidine 247, 248. The first step in this

photopolymerization reaction is high-speed electron transfer radical. During a much slower

proton transfer process, two radicals are created, of which only an amine-based radical

initiates a photopolymerization reaction 196. The photochemical radical formation is a

diffusion-controlled process. In the case of high viscosity formulations, the efficiency of type

II photoinitiator is considerably decreased. Several attempts have been made to combine a

photoinitiator with the co-initiator in one molecule, either in a low molecular dimer or a

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copolymer 248. The study results which utilized a low molecular dimer showed an increase of

up to a factor of 2 in the rate of a photopolymerization reaction. However, on the contrary, in

the study where the copolymer has been used, a decrease in reactivity was observed 248. The

free radicals generated in this process attack the C=C bonds of monomers, resulting in the

formation of new radicals with a much longer chain. This process propagates through a

cascade of chain reactions, which includes an auto-acceleration and, auto-deceleration,

coupled with high polymerization rates, along with diffusion-controlled termination,

followed by the free radical entrapment 248. The Type I photoinitiators exhibit greater

absorption efficiency and quantum yield compared with Type II photoinitiators 248.

Although CQ systems exhibit a very high acceptance among the RBCs

manufacturers, there are some drawbacks associated with their usage. Small amounts (ppm)

of yellow-colored CQ have a profound influence on color properties of RBC materials. It has

frequently been observed, that, under the effect of LPU's irradiation leachable by-products

could be generated, with the capacity to produce visible discoloration under restorations 249.

Also, the α-diketone group, derived from CQ, has a peak absorption in a visible range, which

could initiate RBCs' polymerization under ambient light, significantly reducing a working

time of the materials 250. Furthermore, some concerns regarding the biocompatibility and

toxicity of used amine compounds have been proposed 213. As a possible solution for

previously mentioned concerns, adding a small amount of specific polymeric or

macromolecular photoinitiator aromatic tertiary amine, with a bisphenol-A skeleton to CQ

chemistry, has been proposed 251. In addition, the initiator CQ can be substituted by

polymeric radical photoinitiators bearing the photo-sensitive CQ groups in the side chain 253.

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The Type I photoinitiator system, is based on the photochemical cleavage of

aldehydes and ketones into two free radical intermediates, which initiate polymerization

without the need for an additional amine 248. Type I photoinitiators exhibit much faster

photopolymerization kinetics under short irradiation times, improved functional group

conversion, reduced monomer elution, and improved mechanical properties 180. The second

law of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed

by a chemical system, only one molecule can be activated for the photochemical reaction to

occur 11. However, this fundamental principle applies to photoinitiators with a low free

radical production efficiency, as seen with camphorquinone (CQ) 11. For instance, as much as

four reactive radicals could be produced from one molecule of Irgacure 819, compared to

only one obtained from CQ 253, 255. Thus, photon absorption efficiency (PAE) is a very useful

parameter to determine the most efficient photoinitiator with corresponding LPU’s spectral

irradiance 254. Therefore, photoinitiators absorption spectra should be highly correlated with

the spectral emission profiles of LPUs. The type I systems, currently used with commercially

available RBC include:

• Phenylpropanedione (PPD) (1-phenyl-1, 2-propanedione), with a maximum

absorption spectrum in the range of 398 nm 249.

• Lucirin TPO Diphenyl (2, 4, 6-trimethylbenzoyl) phosphine oxide, with a maximum

absorption spectrum in the range of 381 nm 254.

• Irgacure 819 (bis-(2, 4, 6-trimethylbenzoyl) phenyl phosphine oxide), whose

maximum absorption spectrum is in the range of 370 nm 255.

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• Ivocerin ® represents a blend of germanium-based compounds such as benzoyl

trimethyl germane (Ge-1) or dibenzoyl diethyl germane (Ge-2), whose maximum

absorption spectra are 411 nm and 418 nm respectively 256. The bis-(4-

methoxybenzoyl) diethyl germane Ge-3, with the broad absorption bandwidth to

460nm, which exhibits a maximum absorption spectrum at 408 nm 256.

The literature reports a quantum efficiency of the light-induced cleavage for Ge-2 is 0.85,

while that for Irgacure 819 is 0.59 256. In addition to the photoinitiator concentration and the

irradiation protocol, specific characteristics related to the chemistry of the molecule itself can

also affect polymerization initiation. The photoinitiator molecule must have a high molar

extinction coefficient, which is defined as the absorption per unit length divided by the molar

photoinitiator concentration of the solution 257.

Efficiently absorbed energy has enormous influence on a photoinitiation process.

Therefore, absorbed irradiance should be reported as well, to make a meaningful comparison

among the photoinitiators 257. Type I photoinitiator systems exhibit a higher molar extinction

coefficient when compared with Type II systems 257. The probability for CQ to absorb

photons at its maximum absorption peak is significantly lower than for any Type I system,

excluding Ivocerin, which has a much higher order of magnitude than any other Type I

photoinitiator system 257. The Type I systems do not need an amine co-initiator. Furthermore,

they also exhibit exceptional color stability, and they are employed in many commercially

available RBCs' bleach shades 258. Several studies report good results with mixtures of CQ

and other photoinitiators, resulting in more dependable polymerization 259.

Besides other factors, viscosity alone may significantly affect the polymerization

efficiency of a given photoinitiator during the evaluation process. The result of a well-

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controlled study validated the assumption that the initiation system exhibited the significant

influence on the conversion and a degree of network crosslinking of highly-filled RBCs 258-

260. Those studies highlighted the advantages of using MAPO photoinitiator systems (Type I)

in a highly-filled dimethacrylate-RBCs with the assumption that a sufficient Bis-GMA

content (>40 mol %) and an adequate light spectrum were applied 259. Moreover, the radiant

exposure values for the proper polymerization of tested systems ranged from 3 J/cm2 for

MAPO systems, having values around 20 J/cm2 were commonly reported for CQ systems 259.

The MAPO photoinitiator systems, with Bis-GMA fractions of 40 mol% or more, have

experienced an increased trapped radical concentration, a reduced monomer elution, and

superior mechanical properties, in comparison to the CQ photoinitiator systems 259.

1.7.2 Viscosity, monomers, and fillers

Abundant evidence exists to support the assumption that initial RBC viscosity plays a

significant role in the reaction kinetics and final DC by affecting the mobility of each

monomer, and its ability to react 197. It is universally recognized that the initial viscosity of

RBC and its composition, pre-determine the extent of polymerization, through mobility

restrictions, with the onset of auto-acceleration and the beginning of diffusion-controlled

termination being changed to earlier stages of the conversion 197, 261.

The local viscosity is primarily influenced by the monomer composition and a filler

load. The polymerization efficiency is profoundly affected by the monomer molecular

structure. The viscosity exponentially increases as the percentage of a filler volume increases

but decreases significantly with a temperature increase 288. Although the majority of

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commercial RBCs are very sophisticated in their chemical formulation, they are primarily

composed Bis-GMA (2, 2-bis-[4-(methacryloxy-2-hydroxy-propoxy)-phenyl]-propane), or

its derivates blended with TEGDMA (Triethylene glycol dimethacrylate) as a diluent. The

function of inorganic fillers is to improve mechanical properties and to reduce

polymerization shrinkage and to minimize the thermal expansion coefficient 197-200.

The viscosity of RBCs depends on the composition and ratio of the resin matrix, the

content and shape, size distribution and silane treatment of the inorganic filler, interlocking

between the filler particles, and interfacial interaction between the filler particles and the

matrix resin 171. Bis-GMA is the main monomer that is frequently used for the production of

commercial RBCs. Bis-GMA shows a relatively low polymerization shrinkage (around 6%),

rapid hardening by free-radical polymerization, low volatility, and acceptable mechanical

properties after the polymerization 260. Bis-GMA has a very high viscosity at ambient

temperature, which requires its dilution with a low viscosity monomer, to obtain an organic

matrix that can be mixed with an adequate amount of inorganic fillers 260. However, dilution

with low viscosity monomers (mostly TEGMDA) leads to a significant deficiency. Because

viscosity is correlated with the molecular weight of monomers, and because low molecular

weight monomers have higher polymerization shrinkage, it can be assumed those different

formulations of Bis-GMA/TEGMA could have a significant influence on polymerization

efficiency 199.

Although RBCs that have been analyzed exhibited the same total inorganic filler

weight content, their rheological behavior is very different from one to another 199. For

instance, 60 wt% of macrofillers alone do not have a considerable influence on the

viscoelastic properties of the material, except that they increase the viscosity of the material

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in the way, which a slight shear-thinning behavior appears. However, by increasing the

microfiller content, a more complex viscosity increases 261. For instance, adding 60% of

macro-filler to a 50/50 Bis-GMA/TEGDMA mixture changes the viscosity from 0.2 Pa s to

1.4 Pa s at 1 rad/s 261. Regardless of RBCs formulation, RBCs will experience deformation

when applied to the cavity walls, and that phenomenon is called thixotropy. Thixotropy is

always anticipated from shear-thinning mechanism 261. Thixotropy occurrence is always

observed because of the finite time required for flow-induced changes in the microstructure,

and the process is entirely reversible. Therefore, in clinical situations, RBCs are

photopolymerized in a state which is very far from their rheological state of rest. It could

further indicate that RBCs should ideally be polymerized after the restructuring phase. From

that point of view, and considering the present results, it has been recognized that the time

needed for the RBCs to reach equilibrium after their deformation is exceptionally long. It has

been shown that RBCs need approximately 3,600 s to reach 80% of their initial viscosity, and

from 7,200s to 14,400s to return to their state of equilibrium 261. However, that would be

impossible to implement in clinical practice. Fillers have a significant impact on

polymerization efficiency, by increasing a filler volume fraction, the maximized DC has been

acknowledged 160.

The RBCs viscosity change is related to an increase in filler volume percentage, filler

size and, morphology 261. The viscosity of RBCs is substantially influenced by a resin matrix

chemistry, the shape, and size, as well as the content of a filler particle. Furthermore, the

viscosity is dependent on interlocking between the filler particles and the interfacial

interactions between the filler and a resin matrix 262. It has been shown that the resin matrix

exhibits major influence on polymerization shrinkage stress than the filler content. Therefore,

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further research has been conducted with the aim of reducing stress by modifying the resin

chemistry, without the reduction of a filler content 263.

The incorporation of a novel class of mono vinyl (meth) acrylate monomers that

contain secondary and tertiary functionalities such as urethanes, carbonates, cyclic acetyl,

morpholine, cyclic carbonates, hydroxy/carboxy, oxazolidones, and aromatic rings,

significantly enhance the polymerization kinetics and significantly improve mechanical

properties of conventional RBCs 263. The incorporation of reactive organic nano gel-

polymers into the standard dimethacrylate matrices have been reported to delay vitrification

and elastic modulus development 264. Nanogels provide a stable, transparent solution of

swollen, dispersed or overlapped particles. During the polymerization reaction, nano gels

enhance a physical entanglement and potential chemical crosslinking between nano gel

structures and the resin network, which consequently reinforce mechanical properties of the

final network structure 264.

Temperature has a significant, inverse correlation to viscoelasticity changes of RBCs,

in the way that temperature increase will lead to an exponential decrease of RBCs viscosity

265. This phenomenon was explained by the Arrhenius equation, which represents the

relationship between the temperature and the viscosity of the fluid 265. The research showed

that a relatively weak molecular bond exists in a high-concentration suspension of the filler

particles in a resin matrix. For example, if a shear force is generated, the resultant shear flow

can induce the rolling of the fillers, which can increase the interstitial distance between the

filler particles, or between the filler and a resin matrix, which results in individual bonds

between molecules to become broken. Further increase of shear rate, an array of particles

becomes directional, which leads to diminished interactions between particles. As a result,

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viscosity decreases with the shear rate increase. This phenomenon represents ‘shear thinning,'

which can further explain why a rapid, slight tapping after the RBC placement, can enhance

RBC flow and better adaptation to the preparation walls 263.

Although mechanical properties of RBCs, which exhibit viscoelastic behavior, have

often been characterized using static tests, these tests were primarily focused on the elastic

component of RBCs, without the valuable information about the viscous domain of the

material tested. Therefore, static tests are considered appropriate for characterization of the

ultimate strength of RBCs 266. Additionally, the results of commonly used tests with static

loading, including the creep test, stress relaxation test, and steady shear test, are in essence

with a low level of clinical relevance, because masticatory loading conditions are dynamic.

There are two test methodologies for RBCs viscoelastic properties characterization exist:

irreversible (destructive) and reversible (non-destructive). Although static and dynamic

destructive tests such as tensile and dynamic mechanical analysis (DMA) were extensively

conducted to measure viscoelastic properties of RBC materials, due to their irreversible

nature and problems associated with specimens' standardization, have been replaced with

non-destructive tests. A non-destructive methodology has the ability for a quick and accurate

estimate of RBC’s viscoelastic properties, without any specimens' damage. Therefore, the

majority of DMA tests that are accepted for characterization of RBC's viscoelastic properties

employ this methodology. For example, a dynamic oscillatory shear test is the most often

used test to characterize the linear viscoelastic properties of RBC restorative materials 261, 264.

The analysis employs the resonant vibration, which primarily generates specimens’

mechanical vibration in any of selected modes: torsional, transverse, or longitudinal, over a

specific range of frequencies 266.

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1.7.3 Optical properties

The optical properties of RBCs and their photopolymerization reaction are

interdependent, in the way that material constituents affect light transmission, which further

influences the polymerization depth. Light penetration of RBC materials is defined by the use

of the Beer–Lambert's law 287. According to this law, if a beam of monochromatic radiation

passes through a medium, its power attenuates based on the following equation:

P=P0 (1-RF) exp (-αd) (1)

Where P0 is initial optical power, RF is the total Fresnel reflectance coefficient, α is

the attenuation coefficient, and d is the thickness of the sample. However, in a function of the

RBC sample, the attenuation of LPUs' irradiance in the RBC sample follows Lambert's Law,

expressed in differential formats:

−𝑑𝐼

𝑑𝑥 = µI (2)

Where ‘ I’ represents the irradiance at depth ‘x’ in the RBC specimen, and ‘µ’ is the

linear attenuation coefficient (dimension L−1). Furthermore, the RBC material exhibits

various refractive indices and interfaces, including trapped air bubbles, and thus µ needs to

be further divided into two components, due to a true absorption ‘Ʈ’ and scattering ‘δ’. It

was discovered that the scattering coefficient is relatively consistent among the tested volume

fractions. Based on Eq. (2) the further rearrangements to show the transmittance (T) of the

RBC specimen, is often expressed as the optical density D of the RBC specimen:

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D=µx log 10(e) (3)

It can be recognized, based on Eq. (3) that optical density is a mere linear function of

the depth of the RBC sample (assuming uniformity). However, taking into the consideration

that most frequently used LED LPUs, do not generate a monochromatic light spectrum, some

further adjustments should be made, however, the basic principle of the law is very relevant

to RBCs polymerization. Thus, the law describes the loss in optical power due to reflections,

absorption and scattering upon LPUs light transmission through the RBC materials 267.

As previously mentioned, LPUs do not produce a monochromatic light beam, as their

spectral irradiance depending on the type of LPUs, which can vary from 380 nm to 515 nm.

The portion of that LPUs spectrum referred to as a violet spectrum, usually interacts with a

particular type of RBC materials by producing Rayleigh scattering. The Rayleigh scattering

law states that the percentage of the light that will be scattered is inversely equivalent to the

fourth power of the wavelength 268. Small particles will scatter in a much higher percentage

of short wavelength (violet spectrum) than long (blue light) wavelength. Because the

mathematical association comprises the fourth power of the wavelength, even a small

wavelength difference can mean a substantial difference in scattering coefficients.

The following contributing factors are correlated with RBCs light attenuation: the

surface gloss, the resin matrix composition (resin volume fraction), filler composition (filler

volume fraction), the content of pigments, the presence of some chromatic by-products, and

refractive indices of monomers, silane, and the filler particles 269. The surface light reflection

has two functional components, the diffuse component, which results from a light penetrating

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surface, following the multiple reflections and refractions and also re-emerging at the

surface. The spectral element is a surface phenomenon, which can be represented as a

function of the incidence angle and the refractive index (RI) of the material, the surface

roughness, and a geometrical shadowing function 270. The color coordinates of RBCs (value,

chroma, and hue) are related to the scattering and absorption characteristics, light reflectivity,

and translucency of the material 270. Translucency is one of several factors that determine the

optical features of the material and refers to the partial passage of light through a particular

structure. Therefore, the presence of different degrees of translucency in any RBC material is

a determining factor in the quality of esthetic restoration.

The RI of RBCs plays a significant role in their optical properties. Therefore, studies

have confirmed that transparency, opacity, and surface gloss values were profoundly

correlated with the RI value 271. Furthermore, a decreased transparency value will be

correlated to increased value for the opacity and surface gloss, while at the same time, the RI

value will be increased 271. Concerning polymerization depth, several factors can interfere

with the light transmission throughout the RBC material. The pigments and photoinitiators

can absorb the light photons 271. The light interference with pigments and a variety of shade

additives has been documented. The darker shades would experience a lower DC at the same

irradiance, in comparison to lighter shades 272. The inverted relationship between the

photoinitiator’s molar absorptivity of the incident light and polymerization depth has been

established. Thus, the higher molar absorptivity will present the lower polymerization 257.

The most of RBCs are formulated with camphorquinone (CQ) as a profoundly yellow

photoinitiator, which has a profound influence on the color of RBC materials. During the

photopolymerization process, depending on quantity and spectral emission of the LPU, the

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color transformation of CQ changes to the transparent under favorable polymerization

conditions. However, a certain degree of yellow remains within the material insufficiently

polymerized. Thus, the further conversion of CQ continues under the influence of ambient

light. Therefore, the “bleaching” represents the conversion of CQ from yellow color to

almost transparent as a result of “adequate” polymerization 271.

The relationship of filler particle size and attenuation of light has been a subject of

extensive research. The following features of filler particles such as filler size, geometry,

density, and silane treatment, have an enormous effect on reduction of light intensity.

Therefore, an increase in filler content will accelerate light absorbance 267. Furthermore, light

scattering will be increased, with a significant difference in refractive indices of the matrix

and the filler particles 267. The RBCs light intensity reduction has been explained by the

Rayleigh equation, which demonstrates relationships between the following factors: the

thickness of a specimen, filler volume content, particle radius, refractive indices of the

particles, refractive indices of the matrix, and wavelength of incidental light 271. The

following relationships can be observed:

• In the case when the refractive index of the matrix is smaller than the particle's

refractive index, light transmittance reduction will occur with the increase of a filler

particle radius and filler volume fraction, while an increase in transmittance will

happen with an increase of incidental light wavelength;

• In the case where the matrix refractive index is higher than the particle's refractive

index, an increase in transmittance will occur with an increase of a filler particle

radius and filler volume fraction, and a reduction in transmittance will happen with an

increase of wavelength of incidental light;

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Furthermore, the effect of each of the factors is differentiated, because a filler particle

radius is elevated to the third power, the wavelength of an incidental light to the fourth power

and a multiplier factor of 3 is attained to the filler volume fraction 272, 273. However, this

complex theoretical model has some meaningful, practical explanation. For instance,

Rayleigh scattering is contributed to the higher attenuation of LPUs' incident light in a

particularly short wavelength spectrum (under 420 nm), in comparison to the longer

wavelengths spectrum (above 420 nm). Furthermore, an incident light wavelength hitting the

equal or a smaller filler particle size, produces scattering of a higher magnitude. A

considerable extent of scattering would be expected when a filler particle size approaches to

the half of LPUs' wavelength, resulting in that the extent of light scattering is inversely

proportional to the fourth power of an incidental light wavelength 273. Based on these

findings, the LPUs with a violet spectrum should not be used for the bulk-fill composite

polymerization, because the violet spectrum has induced a substantial scattering effect and

reduced polymerization efficiency. Furthermore, the thickness of a specimen plays a

significant role in light attenuation.

1.8 Extrinsic Factors Affecting Photopolymerization Efficiency

RBCs' polymerization quality is determined by a variety of factors, which could be

classified into the two groups: intrinsic and extrinsic factors. The intrinsic factors are

frequently associated with RBCs properties. However, extrinsic factors are related to features

of LPUs: the radiant power and irradiance, the spectral emission, light beam profile, distance

from the light probe, the exposure time, and irradiation modes.

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1.8.1 Light polymerization units (LPUs) and emission spectrum

LED LPUs are the most common photopolymerization light sources in a

contemporary dental practice because a large number of dental procedures require

photopolymerization. The LED LPUs offer several advantages over other light sources: a

relatively high radiant exitance, enhanced mobility, a low operational cost, and an affordable

price structure. The two main designs are a gun-looking shape with a traditional light probe

or a turbo-probe, and a wand-type style, which has an LED source at its distal end sealed

with a fixed lens. However, the Bluephase Style is a wand-style unit having its emitting chips

still within the body of the wand, and not at the tip end. The advantage of the wand type

LPUs is easy to access to posterior teeth, especially in pediatric patients or in patients with

limited mouth opening. Furthermore, these types of LPU’s have a well-collimated light

guide, contrary to a turbo light guide that provides a significant irradiance decrease as a result

of an increased distance between the light tip and the RBCs 242.

The first and foremost property of LED LPUs is their ability to deliver adequate

quantities of energy to the RBCs, to achieve maximum polymerization. Thus, LPUs need to

have an appropriate radiant power output (mW) as well as spectral irradiance (mW/cm2/nm),

with as close as a possible match to the maximum absorption spectrum of the photoinitiators

present in the RBCs. It is well- recognized that monowave LED LPUs can deliver irradiance

spectra in the range to the maximum absorption spectrum of CQ (468 nm), while multiwave

LPUs deliver a broad range, with usually two peaks of spectral irradiance, suitable for the

activation of CQ and the alternative photoinitiators 287. However, a reduced polymerization

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depth has been observed with bulk-fill RBCs, polymerized with multiwave LED LPUs,

proposedly attributed to the significant Rayleigh scattering effect between a filler particle

size and shortwave visible light 273, 274.

Despite conflicting views in the literature, some manufacturers have been working on

Norrish Type I photoinitiators, particularly with the PPD, Lucirin-TPO and the latest one,

Ivocerin® a germanium-based photoinitiator. Absorption spectra of those photoinitiators are

predominantly in the violet spectrum range: between 380 to 420 nm. Furthermore, due to

their absorption quantum yield and superior polymerization kinetics, these photoinitiators

have been attributed to the significant DC improvements and increased the degree of

crosslinking, resulting in much better mechanical properties of RBCs 273, 275. The

combination of Type I and Type II photoinitiators in commercial RBC formulations is

associated with significantly better mechanical properties that of CQ when used by itself 276.

The results were further validated by successful use of various blends of the CQ with of

Lucirin-TPO and Ivocerin® in Ivoclar-Vivadent commercially available RBCs 276, 277, 278.

The uniformity of the LPUs beam profile has been a subject of much research lately.

The spectral analysis of the LPUs irradiance beam profile showed enormous differences in

their light distribution across the light tips. Those LPUs without a uniform irradiance

distribution are associated with the production of hot spots (high irradiance) and cold spots

(low irradiance) on the RBCs surface 279. As a result, materials polymerized with these LPUs

exhibit areas with adequate and inadequate polymerization. Furthermore, a strong positive

correlation has been found between microhardness and elastic modulus values and beam

profile irradiance values 279. This issue could potentially have more detrimental clinical

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results with the use of multiwave LPUs when using a non-uniform spectral irradiance beam

profile of the violet and blue light spectra.

1.8.2 Radiant exposure, irradiance, and irradiation time

There is much of a debate about the accuracy of reported values for LPUs' output in

the literature, as different studies report their findings using their terminology, which has

created a controversy 280. Because LPUs used for photopolymerization of RBCs produce

electromagnetic radiation of a specific wavelength range, primarily from 360 nm to 515 nm,

sufficient radiometric quantities should always be stated.

The radiant power is the correct term to describe the output from the LPU and is

expressed in units of mW. The next important feature of the LPU is the radiant exitance,

which characterizes the radiant power output from an LPU emitted thought out the area of

the light probe tip, expressed in mW/cm2. However, the most critical term regarding the light

source, which is often incorrectly used in the literature, is the incident irradiance. This

parameter represents a radiant power incident on a surface of the RBC material, expressed in

mW/cm2. The incident irradiance can be used to characterize the radiant exitance at 0 mm

target distance. The radiant exposure is the product of the irradiance and the exposure

duration: expressed in J/cm2. Due to a nonhomogeneous beam profile, the incident irradiance

represents a mean of all values irradiated throughout the tip surface, which is substantially

affected by the change of a tip diameter size. The tip diameter has been a center of a

controversy. There is a substantial difference, between an optical tip diameter and an

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effective tip diameter, which the former represents the actual, light-emitting area of the light

probe end.

A data set founded on the erroneous measurement of an optical instead of an active

tip diameter would potentially compromise the results of any study 279. In addition, a correct

tip diameter measurement should be comprehended to any LPU's operator. The clinically

relevant issue occurs with the Class II restorations, whether the LPUs' diameter of the light

probe irradiating the surface of Class II proximal box portion has enough of the beam

diameter coverage to irradiate the restoration. In this situation, an optical diameter of the

LPU tip should be at least 2 mm greater than the assumed surface of the restoration so that

the polymerization could be achieved in a single exposure 279, 280.

The International Organization for Standardization (ISO) has also adopted a set of

standards, mainly for testing LPUs performances, known as 10650-1 (ISO 2004) and 10650-

2(ISO 2007, updated September 2015) standards 281, 282. The ISO 10650 standard for

measuring the LPUs’ output, uses a laboratory grade power meter to measure the radiant

power output, however, that device is not readily available to dentists. The most common of

all available devices to measure the light emission from LPUs, frequently employed in dental

offices, are hand-held dental radiometers. However, it has been published that dental

radiometers were not reliable sources for the accurate measurements of LPUs' performances.

The irradiance measurements from hand-held dental radiometers are affected by a correct tip

diameter, the light beam profile and the LPUs emission spectrum 283. Furthermore, these

devices are calibrated for a particular brand and a model of LPU. Thus, measurements of

other brands of LPUs with it may not render accurate results, which would cause a significant

discrepancy 283.

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The radiant exposure, a product of incident irradiance and an exposure time is a

critical factor that controls mechanical properties of polymerized RBCs. The radiant

exposure required for any given RBC depends on two factors: a material's formulations and

the LPU's properties. Thus, these variations could be in a range from 6 J/cm2 to 48 J/cm2 as

reported in the literature 284.

Another issue that caused considerable debate over the last decade was the reciprocity

law. In photography, reciprocity has the inverse relationship between the intensity and

duration of light that determines the reaction of a light-sensitive material 285. Thus, the

exposure reciprocity law (adopted from photography) has been proposed based on the

concept that a given quality is directly subordinate on its exposure dose, but independent of

the individual values of the irradiation and exposure time 151, 152. In the wake of some

advanced LPUs that can generate more than 4,000 mW/cm2, ultra-fast polymerization

becomes an attractive feature for a significant clinician’s chairside reduction, while

researchers are trying to answer a question of the validity of the exposure reciprocity law in

dentistry.

Recent studies demonstrated that the majority of RBCs should not be expected to

follow the reciprocity law 152. As per increased irradiation, mostly above 1,500 mW/cm2, the

overall dose required to achieve an adequate conversion is also increased, and this

polymerization behavior is resultant from the dominant bimolecular termination mechanism

for the free radical polymerization. It has been confirmed that final conversion of RBCs, as

well as their ultimate shrinkage stress values, were not dependent upon a dose of light energy

delivered, but instead were dependent on the irradiance and a corresponding polymerization

rate 151, 285.

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1.8.3 Light exposure

Contemporary LPUs are usually equipped with programmable polymerization modes

that can deliver radiant energy in different forms: continuous, pulse-delay, or stepped mode.

The continuous mode delivers the same amount of exitant irradiance from the start to the end

of the polymerization cycle. The pulse-delay mode initiates light emission by a short flash of

emitted light, followed by a programmable time delay before the final exposure is performed.

In a step-polymerization mode, an initial cycle starts with a low exitant irradiance, for a

precise period, then is promptly followed by a much higher exitant irradiance. The ramp-

mode starts with zero amount of exitant irradiance, and then the amount of exitant irradiance

reaches the maximum almost immediately, continuing the same level for the rest of the cycle.

Polymerization with a pulse delay and a stepped mode are ordinarily regarded as "soft-start"

modalities. Soft start exposures typically do not start at zero, but instead initiate at a very low

level and apply that low level for a specified time period (step), or they gradually increase

output over time, until full output is generated (ramp). The soft-start polymerization modes

have been associated with a significant reduction in gap formation, due to a lower level of a

polymerization stress formation at the cavity margins because of the contraction stress 286.

Research has shown, however, that this polymerization type has been associated with

a reduction of DC, in addition to a significantly lower degree of crosslinking, which caused

substantial mechanical properties decrease 150, 168. Literature reports that a 40s radiant

exposure has not produced any statistically significant differences among the polymerization

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modes used, however, by using a ramped soft-start polymerization mode, a substantial

reduction in a contraction strain and a corresponding polymerization temperature has been

recognized 286. Consequently, with the latest RBCs' developments, in particular, with the

introduction of low-shrinkage materials and the alternative photoinitiator systems, the

relevance of this issue will probably become less important.

1.8.4 The effect of temperature on photopolymerization

Temperature has a significant effect on RBCs polymerization. It has been reported

that for each one 0C temperature increase, the corresponding polymerization reaction

increases by 1.9%. However, an initial RBCs' temperature increase from 25 0C to 35 0C,

duplicates the corresponding rate of the DC, which also correlates to an elevated and swift

polymerization stress build-up, compared to the RBCs at ambient temperature 287, 289.

Photopolymerization generates an elevated temperature due to a chemical exothermal

reaction and the incidental radiant energy from the LPUs source. The extent of generated

heat depends on the incident irradiance values, the thickness of the RBC sample, and the

corresponding polymerization rate. The LED LPUs with an extraordinary high radiant power

can generate a sudden temperature spike, which can cause a sharp temperature increase near

the pulp, as much as 5.50C, the value that is considered harmful for the pulp 290, 291. Further, a

recent in-vivo study revealed, a high, positive correlation (r2= 0.92) between the radiant

exposure and the pulpal temperature increase 291. This correlation was observed, especially in

the cases where the radiant exposure reached 80 J/cm2 which increased a 5.5 0C pulpal

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temperature 291. Depending on the tooth and patient age, the level of 5.50C pulpal

temperature increase can lead to the irreversible pulpal damage. It is interesting to note that

some RBCs, due to their much higher monomer content, exhibit an elevated exothermal

polymerization reaction, as seen in flowable RBCs 291. Thus, clinicians should avoid using

LPUs with high radiant power output, particularly with flowable RBC, due to a possible

pulpal damage.

Preheating RBC just before its placement into the preparation has recently gained

popularity among dental practitioners 292. This procedure results in viscosity reduction with

the following potential clinical outcomes: enhanced marginal adaptation, improved handling,

an increase in polymerization rate, an increase in the DC values, and improved mechanical

and physical properties. However, the exact relationship depends on the chemical

formulation and the intrinsic properties of RBCs 292. It would be recommended for the

refrigerator-stored RBCs to be preheated to at least to 450C, before the placement into the

oral cavities 292.

1.8.5 Light guide tip positioning

Adequate LPU’s light probe positioning has a crucial influence on the quantities of

light energy received by the restoration during the radiant exposure. All types of LPUs, but

the laser, deliver a non-coherent light beam profile. Furthermore, there is loss of incident

irradiance, with an increased tip-to-object distance. Manufacturers always report the

irradiance of their LPUs at 0 mm distance, without any consideration to a clinically relevant

distance in the molar region, which can be more than 7 mm 293.

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The Class II restorations, regarding location accessibility, exhibit challenging

locations for proper light energy delivery. The challenges associated with

photopolymerization in the posterior region are attributed to the rather small inter-occlusal

distance, resulting in a significant degree of a light probe positioning difficulty, in addition to

lack of visualization, rendering inadequately polymerized RBCs. An increased distance,

between the tip of the LPU light guide and the surface of the RBC restoration from 0 to 10

mm has caused a 23% reduction in incident irradiance, which was associated with a

substantial decrease in the DC at the bottom of a 2 mm thick RBC specimen 293. Good

clinical practice fosters the closest possible positioning of the LPU light probe tip to the

material being polymerized. Practitioners should always pay full attention and make sure that

the LPU's tip is positioned at the right angle to the material surface. The light probe should

be stabilized to prevent any movements during the entire polymerization cycle. In addition,

practitioners need to have a clear view of the light probe to correct any misalignment of the

light probe. The constant monitoring of the polymerization process involves a two-hand

polymerization technique. The one-hand polymerization technique usually results in poor

stabilization of the light probe tip or its misalignment, which subsequently renders an

unsatisfactory clinical outcome.

It is well-known that LED LPUs produce electromagnetic radiation in a range from

violet to blue light wavelengths. LPUs exhibit the enormous potential for ocular damage in

the range from 435 nm to 440 nm, because these blue light wavelengths are efficiently

transmitted to the retina, where they can express local harmful effects 294. Thus, clinicians

should be encouraged to use appropriate orange blue-blocker eye protection, which renders

an adequate level of safety for watching for the appropriate light probe position during the

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entire photopolymerization cycle. Furthermore, dental practitioners are required to protect

their patients and employees from ocular hazards associated with LPUs 324.

1.9 Review of studies on RBCs Longevity and Effectiveness of LPUs

Although RBC placement is a technique-sensitive procedure that requires clinicians’

full attention, polymerization by LPUs may not receive the same consideration as it should 4.

Many factors are affecting the longevity of RBCs. The main factors are the RBCs' property

and the clinical skills of the operators, as well as LPUs' performances. Those factors are

elaborated in selected studies on the RBCs’ longevity and LPUs’ effectiveness.

1.9.1 Longevity of RBCs direct restorative materials

Due to an increased demand for esthetics, as well as environmental concerns

regarding mercury in amalgam restorations, an exponential use of RBCs, has been observed,

as either a primary RBC placement or as amalgam replacement 1-4. Although amalgam has

been a direct restorative material in continuous service for more than 150 years, presently it

is not considered a material of choice for posterior restorations, due to an exponential

increase in RBC placements that was reported in many industrialized countries 4. However,

in developing countries, amalgam remains the restorative material of choice, at least for

posterior restorations, perhaps because of economic issues.

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The abundance of published studies on the clinical performance of RBC restorations

exists, however, a direct longevity comparison of different types of restorations among

different studies is almost impossible for numerous reasons 296. The following studies

published in the last 13 years are about to present more knowledge on performances and

longevity of posterior RBC restorations.

The first comprehensive study, (Manhart et al., 2004) 295, which has evaluated

longevity and annual failure rates of Class I /II restorations, restored with amalgam, direct

RBC, compomers, glass ionomers and derivative products, composite and ceramic inlays,

and cast gold, since the 1990s to 2003. The analysis has shown that an average annual failure

rate (AFR) for posterior restorations were: 3.0% for amalgam restorations, 2.2% for direct

RBCs, 3.6% for direct RBC with inserts, 1.1% for compomers, 7.2% for regular glass

ionomer (GI) restorations, 7.1% (for tunnel GIs, 6.0% for ART GIs), 2.9% for composite

inlays, 1.9% for ceramic restorations, 1.7% for CAD/CAM ceramic restorations, and 1.4%

for cast gold inlays /onlays. The factors that were associated with amalgam restorations

failure included secondary caries, a high incidence of bulk and tooth fractures, in addition to

cervical overhangings and marginal discrepancies. The principal mode of RBC restoration

failure included secondary caries, bulk fractures, marginal deterioration, and wear. This study

showed that amalgam and RBC restorations exhibited similar annual failure rate.

An international study that evaluated retrospective longitudinal data for posterior

RBC restorations, which covered a period from 1996 to 2011, with a minimum observation

period of five years included a total of 34 Class I/II studies (Demarco et al., 2012) 297. The

results revealed AFR for vital teeth ranged from 1% to 3%, while at the same time AFR of

non-vital teeth 2% to 12.4%. Interestingly, the study was the first to indicate that differences

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in RBCs’ filler characteristic may have some influence on restoration longevity. The

principal factors accredited to failure were secondary caries and restoration/tooth fracture,

however, patient caries-risk factor, the clinical setting of the study, and the socioeconomic

characteristics of the study population expressed the effect to a certain degree.

A Systematic Review and Meta-analysis (Opdam et al., 2014) investigated nine

prospective and three retrospective studies on direct Class II or Classes I and II RBC

posterior restorations in the permanent dentition 298. The analysis consisted of a total of 2,816

restorations (2,585 Class II and 231 Class I). It was concluded that the overall RBCs annual

failure rate at five years and ten years was 1.8% and 2.4%, respectively. It was reported that

secondary caries and bulk fractures were frequently associated with the restoration failure.

The authors observed that a yearly failure rate was significantly higher in high-caries-risk

patients (4.6%) compared to low-caries-risk patients (1.6%) 298. The findings revealed that a

patient caries-risk status has a substantial influence on long-term survival of posterior RBC

restorations.

A 10-year retrospective study investigated longevity of 2-surface/3-surface Class II

restorations (molar/premolar), using 4 microhybrid RBCs (Lempel et al., 2014) 299. A total of

225 adult patients (86 males, 139 females) with 701 RBC restorations were evaluated by two

operators using the modified USPHS criteria. The average AFR of RBCs tested was 0.52%,

with the range from 0.08% to 0.71%. According to the study authors, a 10-year survival rate

of 97.86% could be associated to a well-motivated patient group (university clinical practice

affiliation), which exhibited high socio-economic standards and were subjected to periodical

examinations, oral hygiene, and instructions regarding dietary habits. Similarly, the main

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reasons for the failures in this study were secondary caries, fracture, and endodontic failure.

Furthermore, similar survival rates were observed between molar and premolar restorations.

Results of a randomized controlled 27-year follow-up study (Pallesen et al., 2015) of

three RBCs in the Class II restorations, showed acceptable longevity over the 27-year

evaluation period 300. Thirty participants, 25 females and five males (mean age 38.2 years,

range 25–63), received at least three Class II restorations of moderate size. The evaluation

was carried out with modified USPHS criteria at baseline, 2, 3, 10 and 27 years. The 27-year

overall success rate was 56.5%, with an annual failure rate of 1.6%. The failed RBC

restorations were analyzed for the restoration failure cause. It was discovered that secondary

caries (54.1%), occlusal wear (21.6%), and bulk fracture (18.9%) were reasons for RBCs

failure. In addition, extended longevity was recognized with the 3-surface restorations, in

comparison to 2-surface restorations, however, the difference was not statistically significant.

A 2014 Cochrane systematic review of longevity of direct RBC restorations versus

amalgam restorations in permanent posterior teeth, presented a different picture 301. The

authors included seven clinical trials, separated into two groups (a two and a five split-mouth

trials group) in the systematic review. The analysis of a two-trial group included 1,645 RBC

restorations and 1,365 amalgam restorations (921 in children), while the five split-mouth

trials analysis included 1,620 RBC restorations and 570 amalgam restorations (unspecified

number of children). The results demonstrated that in the 5-7 years’ follow-up of RBCs

exhibited a significantly higher risk of failure, compared to amalgams, with a risk ratio (RR)

1.89, and an increased risk of secondary caries RR 2.14. However, without any substantial

evidence of restoration fracture risk, which was recorded to be RR 0.87. Nevertheless, the

authors reported some problems associated with the analysis: all seven trials were

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experiencing a high-risk of bias, and there were problems with data recording in the five

split-mouth trials. Moreover, analyzed data were perceived as "low-quality evidence." The

authors concluded that posterior RBC restorations exhibited a decreased longevity and a

higher incidence of secondary caries than the amalgam restorations. However, the frequency

of bulk fractures is similar for both materials, during the observed period of 5 to 7 years. It

was concluded that RBC restorations have a shorter lifespan in comparison to amalgams,

however, long-term, and superior quality randomized clinical trials should be conducted to

elucidate the current state of the RBC restorations longevity.

An Austrian study (Beck et al., 2017) 302, analyzed RBC longevity of the Class I/II

prospective studies and clinical trials that occurred from 1996 to 2015, which included a total

of 88 studies. Studies were classified based on the observation period (all studies vs. short-

term vs. long-term studies). The average AFR was 1.46% for short-term studies, and 1.97%

for long-term studies. The difference in failure rates between materials tested was not

significant. Further analysis showed the influence of operators, isolation techniques and

bonding systems on the overall failure rate to be not significant, while the filler-size effect

was statistically significant. The results of short-term observation studies (>5 years), the most

common reasons for failure were fractured restorations, followed by marginal defects and

secondary caries. In more prolonged study periods (<5 years) secondary caries and tooth-

fracture were the predominant reasons for failure.

A recent international study (Borgia et al., 2017) 303, examined clinical performance

of RBC restorations placed in Class I/II molars/premolars over the course of 5 to 20-year

observation period. The study analyzed 105 restorations, from which 101 RBCs were placed

on vital teeth, and only 4 restorations were placed in endodontically treated teeth, restored

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without a post. The subjects consisted of 40 women and 21 men. The evaluation used

modified Ryge criteria (marginal adaptation and discoloration, and surface morphology, with

a 3–level assessment criteria). The observed mean survival time of restorations that remained

functional was 11 years and seven months. At the time of the examination, 103 (98%) RBCs

were still in function: 62 premolars and 41 molars. Two (2%) RBCs had failed. It was

observed that 98 RBC restorations (95.1%) were rated as clinically successful. It was noted

that there was a significant difference between RBC clinical performances of premolars and

molars (60.2% vs. 39.8%), and corresponding restoration size and location. Clinical

performances among hybrids, microhybrids, nanohybrids, and microfilled or midfilled RBCs

were evaluated. The analysis revealed no statistically significant difference among all RBCs

used. However, Heliomolar (Ivoclar-Vivadent), an inhomogeneous microfilled RBC,

exhibited the longest mean survival time.

A Finish study (Palotie et al., 2017) 304, analyzed the longevity of 2-surface and 3-

surface posterior restorations according to the tooth type, restorations’ size, and the

restorative material used in Public Dental Services in Helsinki City. A total of 5,542

restorations were placed in 40% premolars and 60% molars at the baseline. These 2-surface

and 3-surface posterior RBC and amalgam restorations were followed indirectly from 2002

to 2015. Out of a total number of restorations, 93% were RBC, and 7% were amalgam. Sixty-

one percent of the restorations were placed in female patients, and 39% were placed in male

patients, and recorded patient age was between 25-30 years. AFRs of restorations were

calculated separately by tooth type, restoration size, and the material used. The most

extended median survival time and the smallest failure rates were found for teeth in the

maxilla, for premolars, and for 2-surface restorations. The median survival time of all

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restorations was 9.9 years. It was noted that restoration replacements happened less often in

the maxilla (AFR 4.0%) than in the mandible (AFR 4.7%). The median survival time of

RBCs was greater for 2-surface than for 3-surface restorations. The overall AFR across 13

years was 4.3%, as observed to be smallest for 2-surface RBC restorations in premolars and

largest for 3-surface RBC restorations in molars. Furthermore, the mean AFR in 13 years

showed that the values of 2-surface amalgams and 2-surface RBCs in molar teeth were

similar (4.7 vs. 5.0). However, the mean 13-year AFR for 3-surface amalgams was

statistically significant in comparison with 3-surface RBCs in molar teeth (5.2 vs. 7.1). The

authors concluded that longevity of posterior RBC multi-surface restorations is comparable

to the longevity of amalgam restorations.

1.9.2 Effectiveness of LPUs in dental practices

Although there was a plethora of laboratory studies on LPUs' performance, however,

a small number of studies that evaluated LPUs in dental practices have been published. The

first ever published study that characterized LPUs' performance in Britain's dental practices

was (Dunne et al., 1996) 305, a survey of QTH LPUs effectiveness and a comparative

evaluation of light testing devices. The authors characterized the light output from 49 LPUs

in clinical use. Also, they measured the effect on depth of polymerization (DOP) of RBCs

caused by a range of light outputs, and they assessed the relationship between radiometer

meter readings and DOP of RBC in a human tooth model and a Heliotest. The evaluation of

LPUs was carried out by an analog “Lamp Checker” radiometer. Although the manufacturer

of the radiometer considered optimal light output to provide a meter reading to be within the

range 5.0 to 7.0, the mean meter reading produced by the 49 LPUs using a lamp Checker

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radiometer was 4.4 (+/-2.4 SD), range 0.3 to 10.0. The cumulative study results (DOP and

Lamp Checker) revealed that more than 50% of tested LPUs have been underperforming.

Following the previous study findings, another study (Burke et al., 1997) 306 brought

new light about the QTH LPUs effectiveness in dental practices in Great Britain. These

findings were based on a survey that tested LPUs effectiveness used in general dental

practices by conducting the Heliotest (Ivoclar-Vivadent) and Z100 RBC (3M ESPE).

Heliotest is, in essence, a modification of a simple scrape test. The test utilizes a 3 mm

diameter split plastic mold, which is filled with the RBC, and polymerized with the LPU

from the top side only. At the bottom side of the specimen, any soft material is removed. The

height of the remaining hard, polymerized material is measured top-to-bottom and expressed

in mm, which represents the Heliotest result. The LPUs characterization was in a function of

DOP assessment (Heliotest and Z100). The mean incremental depth stated as being used by

the respondents was 2.6 mm. It was found that 43.5% of the participants' LPUs (n = 63)

produced DOP below that value. Although reported results could exhibit inaccuracies to a

certain degree, the findings were in line with the previous study.

An Australian study, (Martin et al., 1998) 307 assessed the adequacy of QTH LPUs

output, their pattern of usage and the maintenance, based on the survey conducted by 3M

Health Care representatives who paid visits to approximately 4% of dental practices in

Australia. All 3M representatives were trained so that data collection could be standardized.

The incident irradiance of LPUs was tested using an analog radiometer (Demetron Research,

Danbury, CT, USA). The irradiance recordings were assigned into three categories: 400

mW/cm2 and above (sufficient intensity), 201-399 mW/cm2 (marginal intensity; where

additional exposure time would be required), and 200 mW/cm2 and less (low intensity). A

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total number of units assessed was 214. Based on the minimum irradiance passing criterion

of 400 mW/cm2 that was established for this study, it was found that 47.7% of LPUs tested

showed substandard performances. Moreover, the age of LPUs tested, ranging from 6 months

to 19 years. The participating dental practitioners exhibited a general lack of awareness of the

need for maintenance of their LPUs.

Another international study (Pilo et al., 1999) 308 published results from the Middle

East with similar findings. The incident irradiance of 130 LPUs from 107 dental offices was

assessed with analog (P/N 10503) and heat (P/N 10517) radiometers (Demetron Research

Corporation, Danbury, CT, USA) according to manufacturer's instructions. Also, Knoop

hardness values of 50 randomly selected LPUs were taken to calculate relative hardness

(bottom/top ratio). The irradiance of tested LPUs ranged from 25–825 mW cm2. The study

design followed manufacturer’s recommendation that incident irradiance value of 300

mW/cm2 was recognized as a passing standard. The results of this study revealed that 55% of

LPUs tested were underperforming. However, data obtained by the heat radiometer showed

that only 24% of tested LPUs were underperforming. When hardness data applied, 46% of

LPUs tested were found to be inadequate, which was in line with previous findings.

A South African study (Solomon et al., 1999) 309 conducted assessment of QTH

LPUs used in private dental practices, and in state and, trade union dental clinics. The

irradiance of each LPU was measured using an analog radiometer Cure Rite (Efos,

Mississauga, ON). The mean reported values ranged from 22 to 448 mW/cm2. The authors

reported that 45.7% of LPUs tested functioned below the proposed standard (300 mW/cm2).

The following studies conducted in Canada and Brazil exhibited similar results,

regardless of the study design. The Toronto study (El-Mowafy et al., 2005) 310, which

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subsequently was succeeded by the Brazilian study (Santos et al., 2005) 311, evaluated a large

sample size that measured incident irradiance of QTH LPUs, using a dental radiometer in

conjunction with a Knoop microhardness test. The Toronto study evaluated incident

irradiance values of 214 QTH LPUs with an analog radiometer. The number of the

participating private dental offices was 100. In this study, an irradiance level of 400 mW/cm2

and above was deemed sufficient. It has been shown that 65 LPUs had irradiance values

below 400 mW/cm2, and 14 LPUs had irradiance values lower than 233 mW/cm2. Therefore,

a total of 30.3% of LPUs tested generated less than the sufficient irradiance values, which

was a significant improvement in comparison with the 1999 study net results, regardless the

difference in methodologies employed in those studies. The Brazilian researchers 311 visited

100 private dental offices in El-Salvador, Brazil and evaluated 120 QTH LPUs, using an

analog radiometer (Optilux 100, Kerr). The recorded exitant irradiance values ranged from

10 to 1,000 mW/cm2, with an average mean value of 255.8 mW/cm2. Based on a passing

criterion that was set at 300 mW/cm2, 56% of LPUs tested were considered underperforming.

The age of LPUs tested varied significantly, with a range from 1 to 21 years.

Other international studies evaluated several QTH and LED LPUs, with interesting

findings. First, an Indian study (Hedge, 2009) 312 investigated a pool of 200 LPUs, consisted

of 81 LED and 119 QTH LPUs. An L.E.D Radiometer (Kerr, Orange CA) was used. They

set up 400 mW/cm2 as a passing irradiance criterion. The results showed that only 10% of

LED LPUs and 2% of QTH LPUs were able to meet this criterion. These results were very

discouraging and alarming, since most of LPUs tested could not sufficiently polymerize

RBCs.

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Studies conducted in Saudi Arabia (Al-Shaafi et al., 2012) 313 followed by Jordanian

study (Maghaireh et al., 2013) 314 shed some new light on LPUs' effectiveness in private

dental practices in the Middle East. It is critical to note that the Saudi study had a mixed pool

of 140 LPUs (67 LED LPUs and 73 QTH LPUs), which was examined by a laboratory-grade

spectroradiometer, in selected urban and rural areas of Saudi Arabia. The passing criteria

were 300 mW/cm2 for QTH, and 600 mW/cm2 for LED LPUs. The results showed that urban

areas had a significantly higher number of LPUs that performed according to the passing

criteria. The passing scores reported in rural versus urban areas were 32% and 49%,

respectively, for QTH LPUs, and 66.7% and 70.5%, respectively, for LED LPUs. The results

showed that LED LPUs outperformed QTH LPUs by a large margin.

The Jordanian study 314 examined a total of 295 LPUs (154 LED and 141 QTH) using

the L.E.D Radiometer (Kerr, Orange CA) in 295 dental clinics. The researchers found that

79.2% of LED LPUs and 26.2% of QTH LPUs could generate incident irradiance at least of

300 mW/cm2. Again, these results showed that QTH LPUs exhibited severe performance

issues and that they should be replaced with LED LPUs.

Furthermore, the results from a Swiss study (Sadiku et al., 2010) 315, that used an

integrating sphere to evaluate a total of 220 LPUs, which more than 65% were LED LPUs,

showed that the total amount of insufficient and damaged LPUs was as high as 40%.

A Kenyan study (Kassim et al., 2013) 316 examined (irradiance, depth of

polymerization (DOP) and Vickers hardness) of 83 LPUs in private and public dental health

facilities in Nairobi, Kenya. The irradiance assessment was conducted using a Cure Rite

radiometer (Efos, Mississauga ON). LPUs tested included a total of 43 (51.8%) LED LPUs,

39 (47.0%) QTH LPUs, and only one (1.2%) Plasma Arc Curing (PAC) LPU. Thirty-five

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(42.17%) LPUs generated the irradiance levels in the range from 0 to 300mW/cm2, while

forty-three (51.8%) of LPUs tested produced the irradiance between 300 and 1,200 mW/cm2.

The authors did not differentiate the irradiance value differences among LED, QTH, and

PAC LPUs. However, they concluded that generally, any irradiance level above 600

mW/cm2 yielded acceptable surface microhardness but only intensities above 900 mW/cm2

resulted in adequate DOP (2 mm or more).

The latest published international study (Hao et al., 2015) 317 investigated the

incidence irradiance values, and LPUs` service information used in private dental offices in

Changchun City, China. The irradiance recordings were grouped into two categories: 300

mW/cm2 and higher (adequate), and 299 mW/cm2 and less (inadequate). A digital Cure Rite

radiometer (Dentsply Caulk, Milford, DE,) with a range of 0–2,000 mW/cm2 was used to

characterize the irradiance of LPUs tested. A total of 196 LPUs from 156 private dental

offices in Changchun City were tested. These units included 132 QTH LPUs and 64 LED

LPUs. Survey samples covered 31 brands of LPUs, 14 of which were foreign brands, and 17,

domestic. The mean irradiance of the 196 light units was 453.1 mW/cm2, and the distribution

of the irradiances varied widely from 0 (2 units) to 1,730 mW/cm2. The average irradiance

value of the QTH LPUs was 340.5 mW/cm2, while the LED LPUs had an average irradiance

of 682.5 mW/cm2. The results showed that 31.6% of all tested LPUs were performing below

the 300 mW/cm2 passing criterion. Also, 76 (57.6%) of QTH LPUs and 58 (90.6%) of LED

LPUs exhibited irradiance of 300 mW/cm2 or more. The study researchers concluded that

LED LPUs have a more extended clinical life in comparison to QTH LPUs.

A Norwegian study (Kopperud et al., 2017) 13, which examined light polymerization

procedures: LPUs` performance, clinicians’ knowledge, and safety awareness among

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participants in Norwegian Public Dental Service in 2015, presented some interesting

findings. The survey investigated dentists’ knowledge on practical use and technical features

of their light sources as well as their safety awareness. The authors invited a total of 1,313

dentists to participate in the study. However, only 748 dentists responded and agreed to

participate. In addition, 35 dentists from those who responded, stated they were not proficient

in restorative procedures and were excluded from statistical analyses. It is important to notice

that out of 713 participants in the study, 69.6% were females and 30.4% males. The results

showed that participants on a daily basis used the average of 57.5% of their time for RBC

placements, which ranged from 1 to 30 (mean 7.7, SD 3.6). The average radiant exposure per

a single increment of RBC was 27s. The longest individual mean polymerization cycle per

day was about 100 cycles longer than the shortest one. Furthermore, the majority of

participants (78.3%) were unaware of the irradiance value of their LPUs, while a

significantly higher percentage of dentists in this group had not performed regular

maintenance of their LPUs compared with all participants (17.1% vs. 3.3%). Moreover,

almost one-third of participants used inadequate eye protection during the light

polymerization. The findings revealed considerable variation among Norwegian dentists in

the Public Dental Service concerning participants' knowledge about light polymerization, the

LPU common clinical daily usage, the maintenance protocol implemented in their practices,

and safety protection during the polymerization. Although two-thirds of participants showed

adequate knowledge and practice in regard to photopolymerization, the number of dentists

that have not presented sufficient awareness about personal protection during the

polymerization, or lack of knowledge about their LPUs, is still very high. Furthermore, the

majority of the study participants (78.3%) were unaware of the irradiance value of their

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LPUs, thus rendering the radiant exposures uncertain. Based on these findings, the authors

concluded that a substantial number of practitioners in Norwegian Public Dental Service had

exhibited inadequate knowledge and technical skills for sufficient RBC polymerization. The

findings are somehow alarming because in Scandinavian countries amalgam placement is

almost eliminated and has been replaced with RBCs.

Although all of these studies reported that LPUs employed in dental practices include

a relatively large number that produce inadequate irradiance levels, it was evident that LED

LPUs have shown much better results than QTH LPUs. Although, geographic variations,

different methodologies, and differences in study designs, present challenges in making

meaningful comparisons, needless to mention that effectiveness of LED LPUs require further

investigation because the percentage of inefficient LED LPUs remains relatively high.

All the previously mentioned studies concluded that there was a persisting problem

associated with LPUs used in dental practices. In addition, the effectiveness of LPUs used in

dental practice depends on the LPUs performance as well as the operator's ability to deliver

light energy to the restoration. Therefore, there was a need to conduct a new study to

determine the combined effects of both, LPU efficiency and the adequacy of operator's light-

application technique, collectively.

To the authors’ best knowledge, this would be an essential and pertinent study, which

would shed some light on those two factors, closely related to the proper polymerization of

RBC restorations. For that reason, we focused our research on the LED LPUs used in

Toronto private dental practices, to characterize crucial factors that have been shown to have

profound effects on light polymerization of RBCs. Furthermore, the data obtained in this

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study will be compared with the QTH LPU survey conducted in Toronto, Canada, ten years

prior.

Chapter 2

Objective and Aims

92

The literature review made it clear that adequate RBC polymerization is highly

dependent on the capacity of LPUs to generate sufficient quantities of energy at appropriate

wavelengths and the ability of dental practitioners to deliver the right amount of energy

during the polymerization.

With state-of-the-art equipment, the goal of this project was to measure the effectiveness of

and characterize LED LPUs in collaboration with 100 dental practices in the Greater Toronto

Area.

The aims of this study were:

1. Quantify exitant irradiance and the 10s radiant exposure of LED LPUs in the

participating dental practices using the CheckMARC® system,

2. Assess LPU operators' technique and consistency of a10s radiant exposure with LED

LPUs using the MARC® Patient Simulator, and

3. Evaluate the mechanical properties of RBC samples polymerized with previously

tested LED LPU in the participating dental practices by conducting the Biaxial

Tensile Strength (BTS) test and Knoop b/t ratio hardness (RH) test.

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Chapter 3

Manuscript 1

Assessment of LED Light-Polymerization Units Used in

Toronto’s Dental Private Practices

94

Assessment of LED Light-Polymerization Units Used in Toronto’s Dental Private

Practices

Dave D. Kojic DDS, MSc. PhD-candidate

Department of Restorative Dentistry, Faculty of Dentistry, University of Toronto, Toronto,

ON, Canada

*Omar El-Mowafy, BDS, PhD, FADM

Professor and Head, Department of Restorative Dentistry, Faculty of Dentistry, University of

Toronto, Toronto, ON, Canada

Richard Price DDS, MS, PhD

Professor, Department of Clinical Dental Sciences, Faculty of Dentistry

Dalhousie University, Halifax, NS, Canada

Wafa El-Badrawy, BDS, MSc.

Associate Professor Department of Restorative Dentistry, Faculty of Dentistry, University of


Gildo Santos, DDS, MSc, PhD

Associate Professor Chair - Restorative Dentistry, Schulich School of Medicine & Dentistry,

Western University, London, ON, Canada

*Corresponding author Omar El-Mowafy, BDS, PhD, FADM

Department of Restorative Dentistry

Faculty of Dentistry, University of Toronto

124 Edward Street, Toronto, Ontario M5G 1G6, Canada

95

Abstract

Objectives: To characterize the radiant exposure of LED light-polymerization units

(LPUs) in 100 private dental practices in Toronto and evaluate the effect of age on their

performance. Materials and Methods: 100 dental practices from the Greater Toronto Area

were recruited. Each LPU was assessed for age and light-guide condition, and its 10s radiant

exposure (RE) measured by the CheckMARC® (BlueLight Analytics, Halifax NS). A

Bluephase Style (Ivoclar-Vivadent, Amherst, NY, USA) unit served as a reference LPU. A

10s RE was multiplied by 2 to represent a 20s RE and compared to an arbitrary passing

criterion of 16 J/cm2. Data were statistically analyzed using Pearson correlation and

independent-samples t-tests at the 95% Confidence level. Results: 113 LPUs were tested.

LPUs age ranged from 1 to 5.5 years, and damage levels to light-guide were between 0 and 4

(scale 0/8). RE ranged from 3.8 to 59.6 J/cm2 (M=21.7, SD=8.6). A considerable number of

LPUs (26 LPUs) did not pass the 16 J/cm2 criterion. The independent-samples t-tests showed

that LED LPUs that did not produce the minimum energy level, were older, t (113) =5.16,

p<0.001, and their light-guide had damage, t (113) =7.17, p<.001. Conclusions: A wide range

of RE (3.8-59.6 J/cm2) was recorded among 113 LPUs tested. Seventy-seven percent of

LPUs were able to generate 16 J/cm2 RE. LPUs with accumulative damage above 2.5(0/8),

and at least 3.5 years in service could not produce a study-required RE. A strong, (r=-0.67,

p<0.001) negative correlation between LPUs' age and light-guide damage and LPUs' incident

irradiance was found. Clinical significance: 23% of LPUs tested failed to meet an arbitrary

RE value of 16 J/cm2, while 47.8% of LPUs tested performed below the reference LPU RE

value of 21 J/cm2.

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Keywords: Light Polymerization Unit (LPU); Irradiance; Radiant exposure;

CheckMARC® device.

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Introduction

Due to an increased demand for natural-looking restorations, as well as health and

environmental concerns regarding the mercury in amalgam restorations 1, a rapid increase in

use of resin-based composite (RBC) materials have been observed, as either a primary RBC

placement or amalgam restoration replacement 2, 3. However, the literature has shown that

longevity of RBCs has been significantly shorter than that of amalgams 4. The most

commonly reported consequences associated with RBC failure are secondary caries and bulk

fractures 5, 6. The failure of RBC materials could be attributed to insufficient polymerization,

in particular in the Class II proximal boxes 7. Although RBC placement is a technique-

sensitive procedure that requires clinicians’ full attention, however, polymerization by Light

Polymerization Units (LPU) may not receive the attention it merits 7. Adequate RBC

polymerization is much more than a simple point-and-click procedure 8.

In contemporary dental offices, light-emitting diode (LED) LPUs have become the

standard units for RBC materials polymerization 9. The LED LPUs have not been exclusively

used for the polymerization of RBC materials, rather their use has been extended to a wide

range of clinical procedures, from fissure sealants to veneer cements. The LED light sources

offer several advantages over other light sources, such as a relatively high radiant exitance,

enhanced mobility, and affordable price structure 10. Currently, two main types of LED LPUs

are available. Single peak LED LPUs, which deliver their spectral irradiance in the closest

range to the maximum absorption spectrum of camphorquinone (CQ) photo-initiator

(ƛmax=468 nm), referred to as monowave LPUs 11. Multiwave LED LPUs (also known as

Polywave®, a trade name of the Ivoclar-Vivadent company), which are suitable for activation

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of CQ and the alternative photoinitiators (primarily PPD, Lucirin TPO, and Ivocerin), usually

deliver two peaks of spectral irradiance 11.

Because LPUs produce electromagnetic radiation of specific wavelengths, mainly

from 360 nm to 515 nm, radiometric nomenclature should be used 12. The output from LPUs,

which is accurately termed the radiant power, is expressed in mW. Another essential feature

of any LPU is the radiant exitance, which characterizes the LPU`s radiant power output

emitted throughout the area of the light probe tip, expressed in units of mW/cm2. The most

important feature of LPUs, often incorrectly reported, represents the incident irradiance,

which is the radiant light power delivered to the RBC material surface, expressed in

mW/cm2. The term irradiance can also be used to characterize the radiant exitance at 0 mm

light source distance 13. The radiant exposure is the product of irradiance applied to the RBC

surface and exposure time, and its value is expressed in J/cm2. Due to a non-uniform

irradiance beam profile 13, the value represents a mean of all values irradiated throughout the

tip of the light probe surface, which is considerably affected by a change to the diameter size

at the tip of the light probe 14.

Dental radiometers usually are used for measurement of the LED LPUs' irradiance in

most dental practices. However, some studies have warned about erroneous readings

associated with dental radiometers 15, 16. However, they can be used to rank LPUs on a

relative basis.

Adequate polymerization of RBCs is critical for their long-term clinical success. A

commonly held assumption that an equivalent of 16 J/cm2 of delivered radiant exposure to a

single 2 mm thick increment of RBC material is considered sufficient for adequate RBC

polymerization 17, 18, 19. However, the manufacturers' recommended minimum radiant

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exposures for RBC polymerization can vary to a great extent, depending on intrinsic

properties and chemical formulation, as well as the shade of the material and the radiant

exitance of the LPUs 19, 20, 21. Dental practitioners are not able to effectively measure the

radiant exposure at the RBC restoration. As a result, dental practitioners can either deliver

too much energy to the restoration, which can locally generate heat, or deliver too little

energy, which can cause inadequate polymerization 22, 23. Recent research has found that

contemporary LED LPUs with a radiant exitance of more than 1,000 mW/cm2, designed for a

10s exposure, often were habitually extended to 20s exposure 24. There is a plethora of

studies on performance of LED LPUs in private dental practices. However, based on earlier

studies, LPU assessments were conducted by using dental radiometers and with the

microhardness measurements for calculation of relative hardness 25- 30. A previous study

conducted to determine the state of LPUs in Toronto's private dental practices which was

based on quartz-tungsten-halogen (QTH) LPUs' assessment, was published in 2005 when

QTH LPUs were the primary source for photopolymerization 26. Current literature indicates

that LED LPUs have become the gold standard for the polymerization of RBC materials in

experimental research settings as well as the most frequently used light sources in private

dental offices 31. The percentage of clinically unacceptable LPUs found in different dental

practices varied considerably 25-30, from 10% to as much as 70%. However, these data should

be interpreted with caution, since the study designs differed significantly, and all of these

studies employed dental radiometers that had proven to exhibit inaccuracies 32.

This study aimed to examine the incident irradiance of LED LPUs in 100 private

dental practices in the Greater Toronto Area, and to evaluate the effect of age and damage of

light-guide of these LPUs' measured incident irradiance as recorded by a CheckMARC®

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device. The Check MARC® has a built-in laboratory grade UV-VIS spectrometer USB4000

(Ocean Optics, Dunedin, FL), with a spectral range from 365 to 515 nm and a laboratory-

grade cosine-corrected sensor. This device can efficiently measure incident irradiance up to

10,000 mW/cm2 (integration times are used to enable changes in intensity and prevent

saturation). A 16 mm diameter sensor is individually calibrated using a NIST-referenced

light source (HL-2000 Tungsten Halogen, Ocean Optics, Dunedin, FL). The objective was to

test the following research hypotheses. The first research hypothesis was that LED LPUs

tested can deliver 16 J/cm2 radiant exposure in a 20s exposure. The second research

hypothesis was that neither the age of the tested LPUs nor the damage to the light probe

would affect the 16 J/cm2 radiant exposure in a 20s exposure cycle.

Materials and methods

Upon receiving the University of Toronto Research Ethics Board approval, private

dental practices were recruited to participate in the study. The 2012 listing of dentists

published by the Royal College of Dental Surgeons of Ontario was used to identify dental

practices in the Greater Toronto area. Over 350 potentially qualified practices were initially

contacted via telephone to determine their eligibility to participate in the study. However,

only 250 qualified dental practices were provided with the study protocol, along with a letter

of consent. Only 100 dental practices were retained for the study.

The recruitment criteria for the study participants included the following: a general

practice physically located within the Metropolitan Toronto area, with proficiency in RBCs

placement. Testing of LED LPUs was conducted in participating dental practices using a

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commercial, laboratory grade light measuring device (CheckMARC®, BlueLight Analytics,

Halifax, NS), (Figure 3.1). The device was attached to a laptop computer. One test

administrator conducted all the testing in each of 100 participating dental practices. The

following LED LPUs information was obtained and entered into the CheckMARC® V2

software: the make and model of the LED, visual and tactile inspection of the light probe for

cracks, chips, or debris, use of a physical barrier, and sterilization method. The

CheckMARC® uses the planar active area of the light tip to calculate incident irradiance

measurements 33. The planar active area is defined by the area of the light tip from which

light exits. When a light tip has a curved lens or no flat surface, the area equivalent to the

opening plane is measured such is the case for the Valo LPU (Ultradent, Utah), which has a

9.7 mm diameter opening for the lens 33. Technically the curved lens has a more extensive

area but the plane at the tangential point of the lens to define the area has been used. The

terms internal or external diameter is not adequate to explain the area measurements for all

light types. Thus, to adequately use this device, proper identification of an external diameter

size was required. Several LPUs have more than one size of a light probe diameter. The

external diameter was measured using Mitutoyo digital caliper (Mitutoyo Canada,

Mississauga ON), in which the corresponding light probe diameter from the CheckMARC®

V2 software was selected.

The light probe level of damage and resin build-up was appraised by using an

evaluation scale (0-8), where 0 represents no damage, and 8 represents the maximum

involvement of the light emitting surface of LPUs. The damage on the probe surface was

typically observed as a localized crescent along the margins and represented the glass

chipping at the marginal area, due to a fall. The resin build-up was also observed at the

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periphery of the light probe, very close to the margin. In many instances, resin build-up was

localized at the top of damaged surface, overlapping each other. The light probe of each LPU

tested was wiped off with disinfecting wipes to remove any dirt, before commencing the

incident irradiance test. The image of the light probe was imported into the photo-editing

software (Adobe XD™, Adobe Systems, San Jose, CA). The circle represents the light

emitting area of the majority LPUs. The damage location resembled a crescent, and it was

always noticed at the margin of the LPU light-guide tip, not passing more than 5 mm towards

the center of the probe. The software has an option to cut the imported image circle into eight

equal sectors and to calculate the area that represented the damage. Furthermore, the software

has a tool which measures and calculates the area of damage along the edge of the light

probe, as shown in Figure 3.2. The grey elliptical areas in the figure represent the captured

physical damage on the surface of the light probe. However, this pattern did not work with

the SmartLite Max (Dentsply), which has an elongated ellipsoid light probe surface. In this

situation, the damage level was assessed by dividing the light probe surface into eight equal

sectors and then applying the previous methodology. The LPUs have been tested “as-is” to

obtain an accurate indication of their photopolymerization potential.

After entering the required information, the CheckMARC® device was attached to

the computer. The test administrator conducted the test by applying the LED LPUs light tip

directly to the Check MARC® sensor and completing a 10-second test cycle. This was

repeated 3 times. The mean of 3 measurements was displayed on the computer screen. The

measurements were taken in the standard mode of the LPU and without any protective

barrier. The Check MARC® software generated a LED LPU report, which included

irradiance (mW/cm2). A 10s radiant exposure was multiplied by 2 to represent a 20s radiant

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exposure. The rationale for using the 10s, and not 20s radiant exposure, was only for

efficiency purpose in a busy dental office. However, it is well-known that LPUs emit a

particular quantity of light after the termination of the process. Thus, the CheckMARC®

device could capture the amount of light energy beyond a 10s-recording cycle. Thus, when a

report was generated, it often showed the radiant exposure in 10.1s.

A computed 20s average radiant exposure of a reference LED LPU (Bluephase Style,

Ivoclar-Vivadent), which generated 21 J/cm2, was used as a positive control. The rationale to

use a positive control was to determine how the top of the line LPU (at the start of the

project) would be scored when comparing the tested LPUs.

• Statistical analysis:

Descriptive statistics were used to evaluate the irradiance of the LED LPUs tested. To

assess the association between the LPU light probe damage and age of the LPU effect on

radiant exposure, values in the function of the required energy of 16 J/cm2, the data were

statistically analyzed using the Pearson correlation (alpha of 0.001) and independent-samples

t-tests. Furthermore, monowave and multiwave LPUs were compared for the level of damage

and their age, using an independent-samples t-test and chi-square test. All statistical tests

were conducted using a pre-set alpha of 0.05. Windows-based statistical software (SPSS

version 22, SPSS Inc., IBM, Somers, NY) was used for statistical analyses.

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Results

The Q-Q plots were used for normality assessment, and the results of the incident

irradiance values of multiwave and monowave LPUs data distributions are presented in

Figures 3.5 and 3.6.

The Levene's test was employed for testing of homogeneity of variance across the

comparison groups. The test was not significant, (p=0.298), thus, homogeneity of variance

across the comparison groups was assumed. The power analysis using the G Power program

for the independent and paired t-tests revealed that the sample size of 113 was capable of

detecting a medium effect size with the independent samples t-tests (Cohen's d=.53), and a

small effect size with the paired-samples t-tests (Cohen's d=.23). Additionally, for the Chi-

square test of independence, guidelines from an article (Oyeyemi et al., 2010) 34 was used for

the power analysis. According to these guidelines, the study sample size of 113 can detect

only large effects with the power of .80.

The pool of tested LED LPUs consisted of 32 multiwave and 81 monowave LPUs. In

the multiwave group, the Valo (Ultradent, South Jordan, UT) was the most common (57.6%),

while Demi Plus (Kerr, Orange, CA) was the most common (26.3%) in the monowave group.

The interpolated radiant exposure values ranged between 3.8 and 59.6 J/cm2 (M=21.7,

SD=8.6), using a 20s radiant exposure. The average computed 20s radiant exposure was 22.3

J/cm2 (SD=9.7) for monowave LPUs and 20.4 J/cm2 (SD=4.5) for multiwave LPUs. The

minimum criterion (16 J/cm2) was reached by the majority of LED LPUs (n=87, 77.0%).

Figure 3.3 shows the relationship among all LPUs tested with a cut-off value of 16 J/cm2.

105

The LED LPUs tested were between 1.0 and 5.5 years of age (M=2.6, SD=1.2) and

had damage levels (scale 0 to 8) ranging from 0 to 4 (M=1.6, SD=1.2)

The results of the independent-samples t-tests showed that the LED LPUs that were unable to

produce the minimum acceptable level of radiant exposure, were older (t (111) =5.2, p<0.001,

95% CI for MD: .78; 1.75), and suffered more damage to the light-guide (t (111) =7.2,

p<0.001, 95% CI for MD: 1.11; 1.96).

The average age of LED LPUs that were unable to meet the minimum level of radiant

exposure was 3.6 years (SD=1.0), while the average age of the LED LPUs that met this

criterion was 2.3 years (SD=1.1). Similarly, the average damage level of the LED LPUs that

failed to meet the minimum level of radiant exposure was 2.8 (SD=0.9) as compared to 1.3

(SD=1.0) for those LED LPUs that met this criterion. The age distribution of LED LPUs that

were able/unable to meet the minimum radiant exposure criterion, depending on their

damage levels is presented in Figure 3.4.

A Pearson correlation analysis was used to explore the relationship between the

radiant exposure versus LPU's age and light probe damage level. The results revealed a

strong, r=-0.67 (p<.001) negative correlation between the LPUs' radiant exposure and age

and damage of LPUs.

Monowave and multiwave LPUs were analyzed according to their age and level of

damage. The average age of monowave LPUs in the sample was 2.7 years (SD = 1.2), and

the average age of multiwave LPUs was 2.6 years (SD = 1.2). This difference was not

statistically significant, (t (111) = 0.15, p = 0.884, CI 95% for MD: -0.46; 0.53), Cohen’s

d=0.03 indicating a small effect size. Similarly, the two types of LPUs were not significantly

different concerning the level of damage, χ2 (4) = 5.43, p = 0.246, Cramer’s V=.22 indicating

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a small effect size. Specifically, 17.7% of monowave LPUs and 17.6% of multiwave LPUs

had no damage, and 6.3% of monowave and 5.9% of multiwave LPUs had the highest level

of damage (level 4).

Discussion

Parametric statistical tests used in this study relied on the assumptions of normality

and equal variances among the comparison groups. Following these assumptions is especially

critical when the sample size used in the study is small. Considering that the sample used in

this study consists of 113 cases, parametric statistical tests were employed. However, to

investigate the potential impact of the violation of the normality assumption on the results of

statistical tests, nonparametric tests were also conducted. Should the results of the parametric

and non-parametric tests deviate, then the results of the non-parametric tests should be

reported. However, it was discovered that the results of non-parametric tests and parametric

tests were always the same. Therefore, for simplicity of the presentation, parametric tests

were used.

Three relevant parameters express crucial influence on a clinically acceptable level of

RBC photopolymerization: the chemical formulation, the operator's clinical skills, and

the LPU's characteristics. The literature shows that the longevity of RBCs has been

significantly shorter than that of amalgams 4, while RBC's failure is often associated with

insufficient polymerization 7. Although the operator's clinical skills have the most critical

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influence on RBC's survival, LPU's ability to produce sufficient quantities of light energy at

appropriate light spectra is fundamental for the RBC restoration's clinical success.

The first research hypothesis, which stated that LED LPUs in the participating dental

practices would be able to deliver 16 J/cm2 of light energy, was rejected. The minimum

criterion (16 J/cm2) was reached by the majority of LED LPUs (n=87, 77.0%), but not by all.

The relationship among all LPUs, tested with a cut-off value of 16 J/cm2, can be seen in

Figure 3.3. The pool of tested LED LPUs consisted of 81 monowave and 32 multiwave

LPUs. The average computed 20s radiant exposure was 22.3 J/cm2 (SD=9.7) for monowave

LPUs and 20.4 J/cm2 (SD=4.5) for multiwave LPUs.

The second research hypothesis, which stated that LPUs` age and its light-probe

damage would not affect radiant exposure, was also rejected. It was determined that the LED

LPUs that were unable to deliver at least of 16 J/cm2 were older (t (111) =5.2, p<0.001, 95%

CI for MD: .78; 1.75), and exhibited more light-probe damage (t (111) =7.2, p<0.001, 95% CI

for MD: 1.11; 1.96). The age distribution of the LED LPUs that were able/unable to meet the

minimum radiant exposure criterion, depending on their damage levels, is presented in

the Figure 3.4. Furthermore, a strong, r=-0.67 (p<.001) negative correlation between the

LPUs' radiant exposure and its age and level of damage was recognized. The study revealed

that 23% of LED LPUs used in the participating private dental practices performed below the

study-required 16 J/cm2 passing criterion, which was in accord with previously published

results 25-31.

Some comparisons can be drawn between the 2005 Toronto study 26 and the current

study concerning performance differences between QTH and LED LPUs. Back in 2005,

QTH LPUs were deemed as a gold standard for photopolymerization and LED LPUs just

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started to gain popularity among dental professionals. The 2005 Toronto study used an

analog radiometer, the best tool available at that time, to quantify irradiance of QTH LPUs.

However, ten years later, the current study used more accurate laboratory-grade

spectroradiometer to complete the same task. Although 100 dental practices were selected,

the sample size consisted of 113 LED LPUs. The goal was to evaluate only the LPUs that

were used daily, rather than all LED LPUs that may have been available in the participating

offices. Furthermore, the current study intended to characterize the performance of LED

LPUs in the same geographic region and to see the real differences in use among dental

practices ten years after the previous study. Instead of incident irradiance measurement, a

calculated radiant exposure of all LED LPUs tested was conducted, following the

recommendations in the literature, for a minimum requirement of 16 J/cm2 that is considered

as an adequate radiant exposure for the majority of RBC materials 17. The study results were

quite surprising compared to those of the previous studies 25-31. The current study revealed

that 23% of LPUs tested were below the study-required passing criterion, much lower than

previously reported the study average of 46% (range 12‑95%) 25-31. From

the underperforming LPUs group, only three (11.5%) multiwave LED LPUs were unable to

meet the minimum requirement, while 23 (88.5%) of the underperforming LPUs were

monowave. It was observed that multiwave LPUs outperformed monowave LPUs by a large

margin. Furthermore, by calculating the ratio of substandard LPUs to the total number of

LPUs tested (multiwave LPUs [3/32] versus monowave LPUs [23/81]), it was discovered

that multiwave LPUs were three times less likely to perform below the study-required

standard than monowave LPUs. Although the average computed 20s radiant exposure was

similar for both LPU groups, 22.3 J/cm2 (SD = 9.7) for monowave LPUs and 20.4 J/cm2

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(SD = 4.5) for multiwave LPUs, double values of SD for monowave LPUs were observed.

The double values of SD for monowave LPUs suggest that a significantly higher range of

extreme radiant exposure values contributed to values in this group. A possible explanation is

that multiwave LPUs represented 3rd-generation LED LPUs, which were introduced

relatively recently in dental practice. Thus, the LPUs' age at the time of measurement was

significantly lower.

In comparison with a 20s radiant exposure produced by a reference LED LPU

(Bluephase Style, Ivoclar-Vivadent), which generated 21 J/cm2, 47.8% of LED LPUs tested

exhibited lower radiant exposure values. The substantial reduction in reported rate of

inadequate LPUs could be attributed to technical advancements in LED light source

technology, to the significantly higher accuracy of the spectroradiometer, and to

standardization in testing by a single administrator. It is possible that QTH LPUs tested in

previous studies could have exhibited some technical issues with the equipment used in the

study, which may influence the accuracy of the data reported. Furthermore, the accuracy of

irradiance readings depends on the adequate measurement of the light probe. The external

diameter was measured using Mitutoyo digital caliper (Mitutoyo Canada, Mississauga ON),

in which the corresponding light probe diameter from the CheckMARC® V2 software was

selected. Thus, the accuracy to assess the radiant exposure of tested LPUs was at the highest

level possible. Using the integrating sphere to measure the radiant power and then to divide

this by the tip area is prone to errors because a small error in measuring the active tip

diameter will lead to a substantial error in the irradiance value. For example, if the radiant

power is 800 mW and the active tip diameter is 7.5 mm, the area is 44.2 mm2. The incident

irradiance is thus 1,811 mW/cm2. If the tip diameter is 7.0 mm, the area is 38.5 mm2. Thus,

110

for just a 0.5 mm error (0.25mm on each side), the incident irradiance is now 2,079 mW/cm2,

or approximately 10% more, even though the radiant power is the same.

Other factors may have contributed to the degradation of LED LPUs besides age and

damage of the light probe. Some additional factors could be related to the condition of the

battery or the diminished power of the LED elements. Not only external, but the internal light

probe damage was examined. During the assessment of the light probe for possible damage,

the light probe was removed (when possible, depending on the type of LPU) and carefully

inspected for any internal cracks, which might affect the light transmission through the light

probe. When possible the light patency of the probe was examined for an unobstructed

reading of any printed material, from the light probe tip to its end. Thus, internal shatter

inside the light probe, although not easily detected, could cause a justified concern for the

capacity of the LPU to deliver expected quantities of light energy. Furthermore, it is

important to note that more than 80% of the participants had no practical knowledge of how

to inspect the light probe for the light transmission patency properly.

In this study, the damage level and age of the LPUs have been included in the

function of the study-required radiant exposure of 16 J/cm2, which differs from the design of

the previous studies 25-31. Figure 3.4 shows that the age distribution of the LED LPUs had not

met the passing criterion that depended on the damage level. As can be seen from this figure,

the minimum level of radiant exposure can only be guaranteed for new LPUs. Even one-

year-old LED LPUs might not meet this criterion if their damage level is 2 or above. None of

the LPUs that had a damage level of 4 met the minimum criterion. Overall, the results

suggested that other factors might also contribute to the radiant exposure of LED LPUs as

both groups of LPUs (those that met and those that did not meet the minimum criterion)

111

exhibited overlapping age and damage level intervals. Further analysis of LED LPUs that

were not able to generate the required 16 J/cm2 of radiant exposure is shown in Table 3.1. As

can be seen from the table, LED LPUs that failed to generate the required energy exhibited a

mean age of 3.6 years and light probe damage of 2.8/8 together with their average radiant

exposure of 12.03 J/cm2.

The limitations of the current project were associated with the LED LPUs' assessment

study design, which used the CheckMARC® device for a 10s radiant exposure and

subsequently converted recorded values for a 20s radiant exposure. It would have been better

if the study design had included a 20s radiant exposure measurement. The primary reason for

choosing 10s versus 20s exposure time was based on the fact that choosing the latter would

double the experimental time spent in the participating dental practices as this project was a

part of an extensive study that required several test procedures in the same dental practice.

Future research using the same methodology should focus on different types of dental

practices. For example, it would be interesting to know the performance of LED LPUs used

in Toronto's public dental clinics and compare this data with the results of the current study.

Furthermore, the assessment of LPUs used by dental professionals in rural areas would shed

more light on the possible difference in the quality of dental care between urban and rural

private dental practices.

Regardless of the current study's results, the LED LPUs tested in the participating

dental practices in the Greater Toronto Area continued to exhibit a relatively high percentage

of substandard LPUs, which is a cause for justifiable concern regarding sufficient RBC

polymerization.

112

Conclusions

Within the limitations of this study that characterized the LED LPUs in the

participating dental practices in the Greater Toronto Area regarding their capacity to deliver

at least 16 J/cm2 of radiant exposure in the 20s, the following conclusions can be reached:

1. 23% of LED LPUs tested did not meet the 16 J/cm2 criterion.

2. Multiwave LPUs showed two and a half times better performance than

monowave LPUs.

3. A strong, r=-0.67 (p<.001) negative correlation between the LPUs' radiant

exposure and age and level of damage of LPUs was observed.

4. It was found that LPUs older than 4 years tended to exhibit a higher level of

damage.

5. Rigorous inspection of LPUs is required for early detection of any problems

associated with routine use.

6. Failure to meet the study criterion of 16 J/cm2 in the 20s implies that 23% of

the LED LPUs tested will not allow the practitioners to produce adequate

RBC polymerization.

113

Acknowledgements

The current study is part of a Ph.D. thesis submitted to the Faculty of Dentistry,

University of Toronto. This project was financially supported by grants from the Faculty of

Dentistry Research Institute of the University of Toronto, and the American Academy of

Esthetic Dentistry. The CheckMARC® device was loaned and technically supported by

BlueLight Analytics for which the authors are grateful. The Bluephase Style was graciously

provided by Ivoclar-Vivadent. There are no words to express immense appreciation to the

project research assistants, Ms. Ana Burilo and Ms. Ivana Orlovic for their extensive

contribution to the study.

114

Figure 3.1 The CheckMARC® spectroradiometer

Figure 3.2. Schematic illustration of light-tip damage index (physical damage and/or

irremovable resin build-up) used to assess on the light probe surfaces of LPUs tested. Note

that observed damage levels during the LPUs’ assessments were in the range from 0/8 to 4/8.

116

Figure 3.3 Calculated 20s radiant exposure of 113 LPUs with a minimum passing

criterion cut-off of 16 J/cm2. Note that 26 LPUs failed to reach the passing criterion.

Figure 3.4 Box and Whisker plots: LED LPUs which did not meet (above) and did

meet (below) a passing criterion. Circles and stars represent the outlier data points.

The circles are the outliers that are closer to the center of the distribution, and the

stars are the outliers that are farther from the center of the distribution. It’s interesting

to note a wide range of values at zero damage in respect to the age of LPU, only

found with LPUs that met the passing criterion.

117

Table 3.1 Listed LED LPUs that failed to reach a minimum of 16 J/cm2 of

radiant exposure in the 20s. The listing includes the LED LPUs’ age, light

probe’s damage level, and 20s radiant exposure results. The LPUs were

classified by the type of LPU: where d: dual-peak (multiwave), s: single-peak

(monowave). The LPUs listing is presented in descendent order according to

RE values.

118

LPU brands Age Damage J/cm2 Type

Demi Plus 3 2 15.6 s

SmartLite IQ2 4 4 15.4 s

Flashlight 1401 4 2 15.2 s

Valo Cordless 3.5 3 15 d


D-Lux 1.5 2 14.8 s

SmartLite MAX 2.5 2 14.6 d

Flashlight 1401 4 2 14.6 s

SmartLite MAX 2 2 13.2 d

LED Turbo Victor 4 3 13.2 s

LE Demetron 4 3 12.8 s

Elipar FreeLight 4 3 12.6 s

Demi Plus 4.8 3 12.4 s

Bluephase 16i 3.5 4 12.2 s

Coltolux LED 4 3 12.2 s

SmartLite IQ2 5.5 4 12.2 s



Coltolux LED 3 2 10.2 s


D-Lux 2.5 1 9.8 s

Demi Plus 4.5 3 9.8 s

Coltolux LED 4.5 4 9.8 s

Ultra LED 4 4 8.6 s

Coltolux LED 3.5 3 8.4 s

ART L5 3.5 4 3.8 s

119

Figure 3.5 Q-Q plot showed the multiwave LPUs incident irradiance observed

values against normally distributed data (represented by the line).

Figure 3.6 Q-Q plot showed the monowave LPUs incident irradiance observed

values against normally distributed data (represented by the line).

120

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Chapter 4

Manuscript 2

Photopolymerization application effectiveness

among private dental practices in Toronto, Canada

126

Photopolymerization application effectiveness among private dental practices in

Toronto, Canada



ON, Canada


Professor and Head, Department of Restorative Dentistry

Faculty of Dentistry, University of Toronto, Toronto, ON, Canada


Professor, Department of Clinical Dental Sciences

Faculty of Dentistry Dalhousie University, Halifax, NS, Canada


Associate Professor Department of Restorative Dentistry

Faculty of Dentistry, University of Toronto, Toronto, ON, Canada


Associate Professor Chair - Restorative Dentistry

Schulich School of Medicine & Dentistry, Western University, London, ON, Canada

*Corresponding author

Omar El-Mowafy, BDS, PhD, FADM




127

Abstract

Objectives: To determine the ability of dental practitioners to deliver 8 J/cm2 radiant

exposure (RE) in 10s to the MARC® Patient Simulator’s (PS) (BlueLight Analytics, Halifax

NS) simulated restorations. Methods: Dental practices from the Greater Toronto Area were

recruited. Light-emitting-diode (LED) Light Polymerization Units (LPU) were assessed for

their ability to deliver the study-required energy. Dental practitioners used their office's LED-

LPUs to deliver light-energy to anterior and posterior PS restorations. Following

photopolymerization instructions, participants were retested using the same LED-LPUs. Data

were statistically analyzed using mixed ANOVA, Chi-square tests, and paired-samples t-tests

with a pre-set alpha of 0.05. Results: Before instructions, 73.6% of participants delivered

required-energy to anterior restorations, and 32.2% of participants delivered required-energy

to posterior restorations. Following instructions, 94.3% of participants delivered required-

energy to anterior restorations, and 73.6% of participants delivered required-energy to

posterior restorations. Conclusions: Although LPUs could generate 8.0 J/cm2 in 10s, the

percentage of dental practitioners who failed to deliver the study-required energy, even after

specific instructions, was 5.7% for anterior restoration and 26.4% for posterior restoration.

Clinical significance: It was revealed that a large number of dental practitioners failed to

deliver the study-required 8 J/cm2 in 10s to posterior restoration. This finding is alarming as

it relates well to early failure of Class-2 RBC restorations due to insufficient polymerization.

Keywords: Light Polymerization Unit (LPU); Irradiance; Radiant exposure; Dental

spectroradiometer; the MARC® Patient Simulator.

128

Introduction

Given the demand for natural-looking restorations, in combination with health and

environmental concerns regarding mercury in amalgam restorations 1, 2, resin-based

composites (RBC), have become the material of choice for many restorative procedures 3.

However, sufficient RBC polymerization requires application of adequate quantities and

appropriate spectra of light energy delivered to the surface of RBCs 4. Although RBC

placement is a technique-sensitive procedure that requires clinicians’ full attention,

polymerization by Light Polymerization Units (LPU) may not receive the same consideration

as it should 5, 6. It is well-known that an inadequate polymerization has a profound impact on

RBCs longevity, as those unpolymerized residues exhibit health hazards due to their nature to

leach into oral fluids, which has been reported in many toxicology studies 7, 8, 9.

Dental practitioners are facing many challenges in their clinical practices when it

comes to the polymerization of RBCs, especially in the posterior areas, where correct light

probe positioning and alignment involves a significant degree of difficulty, due to limited

access. It has been confirmed that an angular change in the light probe positioning would

cause a significant impact on energy delivered to the restoration, with a corresponding

reduction in RBCs polymerization efficiency 10 – 13. Dental practitioners usually do not have

the appropriate resources to evaluate their LPUs, but to use dental radiometers. However,

recent studies have warned about erroneous readings associated with dental radiometers 14, 15,

16. Furthermore, dental radiometers measure incident irradiance (mW/cm2) at the tip of the

surface of the light probe, and thus a small change in the tip diameter will dramatically

change the actual incident irradiance values.

129

Although LPUs irradiance measured with spectroradiometers are considered the most

accurate measuring devices, this information cannot provide clinicians with the answer, of

how well their RBC restorations were polymerized 17, 18. Manufacturers of commercially

available LPUs report that irradiance values were taken at 0 mm distance between the light

probe and the measuring device, which is not a clinically relevant distance. For instance, the

measured distance in the molar region between the molar cusp tip and the bottom of Class II

proximal box could exceed 8 mm, a distance that would significantly reduce the light energy

received by the RBC 19-23.

Sufficient polymerization of RBCs depends on several key factors: the capacity of

LPUs to generate light energy (appropriate quantities and spectra), intrinsic properties of

RBCs, and the technique followed by the dental practitioners to deliver the light-energy to

the restorations 24, 25. The goal for any dental practitioner is to efficiently and adequately

polymerize RBC restorations within a clinically acceptable time frame. However, the exact

quantity of required light energy for adequate RBCs' polymerization remains unanswered. A

commonly held assumption is that an equivalent of 16 J/cm2 of received radiant exposure

delivered to a single 2 mm thick increment of RBC material was considered sufficient for a

satisfactory RBC polymerization 26. Nonetheless, in other research reports, a range between 6

and 24 J/cm2 was suggested as a minimum energy requirement for a sufficient

polymerization of majority commercially available RBC materials 27, 28.

Manufacturers` recommended minimum radiant exposure values for the

polymerization of their RBCs can vary to a great extent, depending on the intrinsic properties

of the RBC and its chemical formulations, and shade, as well as, LPU’s irradiance 24, 25.

Dental practitioners are not able to efficiently measure the radiant exposure of their LPUs. As

130

a result, dental practitioners can either deliver too much energy to the restoration, which can

locally generate heat, or deliver too little energy, which can result in insufficient

polymerization 25, 26. Because LPUs produce electromagnetic radiation of specific

wavelengths, sufficient radiometric quantities should be used in reporting the features of

LPUs 29. The radiant power is the correct term used to describe the output from LPUs, and its

measured value is denoted in units of mW. The radiant power emitted throughout the area of

the light probe tip represents the radiant exitance of the LPU, expressed in units of mW/cm2.

The incident irradiance, an essential term of the LPU performance, often incorrectly reported,

accounts for the radiant power incident (energy flux) delivered to the RBC surface, expressed

in units of mW/cm2. A radiant exposure, which represents a product of irradiance and

exposure time, is expressed in units of J/cm2.

With the introduction of the MARC® Patient Simulator (PS) (BlueLight Analytics,

Halifax NS), researchers have received a new device to accurately measure radiant exposure

received by simulated restorations in a mannequin head. The MARC® PS device incorporates

a laboratory-grade UV-VIS spectroradiometer (USB4000, Ocean Optics, Dunedin, FL) and

two, laboratory-grade cosine-corrected sensors 44. The sensors are 4 mm in diameter, which

is the diameter of the ISO depth-of-cure mold. Light captured by the sensors is forwarded to

the spectrometer through a bifurcated fiber-optic cable. The MARC® PS system simulates the

patient using the mannequin head and typodont. The maxillary typodont is equipped with two

light sensors: an anterior one, located between central incisors, 1mm below the buccal

surfaces of central incisors (Class III simulated restoration), and a recessed posterior sensor

(Class I simulated restoration), located in the maxillary left second molar. The light detector

131

is located at the base of a Class I preparation, 2 mm lower than the cavosurface margin and 4

mm below the cusp tip 44.

Studies assessing the photopolymerization techniques of dental practitioners utilizing

the MARC®PS have shown a massive difference among the participants to deliver the light

energy to the simulated restorations 30-35. The idea to explore the efficacy of LPUs in dental

practices was a subject of several studies. However, to date, no study was able to measure

and compare polymerization techniques of dental practitioners in the participating dental

practices 36-43.

The purpose of this study was to evaluate the operator's technique in delivering light

energy to the MARC® PS simulated restorations, using LED LPUs in the participating

private dental practices in the Greater Toronto Area. The objectives were to test two research

hypotheses. The first hypothesis was that a 10s radiant exposure delivered by dental

practitioners to the MARC®PS (anterior and posterior restorations) would produce energy

equal to or greater than 8.0 J/cm2, which was reported to be half the minimum requirement,

quoted in the literature 26. The second research hypothesis was that, following the

individualized photopolymerization instructions, dental practitioners would deliver more

radiant energy to the MARC®PS simulated restorations, and will improve the consistency of

energy delivered in a 10s radiant exposure cycle.


Following the University of Toronto Research Ethics Board approval, the private

dental practices were recruited. The 2012 listing of dentists published by the Royal College

132

of Dental Surgeons of Ontario was used to identify dental practices in the Greater Toronto

area. Over 350 targeted practices were initially contacted via telephone to determine their

eligibility to participate in the study. A total of 250 qualified dental practices were presented

with a letter explaining the study purpose, along with a consent form. Only 100 dental

practices met the study criteria and agreed to participate, with 113 dental practitioners that

took part in the study. The recruitment criteria for the study participants included a general

dental practice with proficiency in RBCs placement located within the Greater Toronto Area.

Study acceptance was granted exclusively for participants who had never experienced

MARC®PS testing before. The assessment of photopolymerization techniques of participants

to deliver radiant energy was measured using a commercial testing device (MARC®PS) and

was completed in the participating dental practices. Software (MARC®PS version 3.4)

provided real-time radiant exposure data display and a calculation of energy delivery within

user-defined spectral ranges and exposure time to reach 8.0 J/cm2.

The study was a part of a broad project, which examined a total number of 113 LPUs

for their ability to deliver at least of 8 J/cm2 of radiant exposure during a 10s exposure. The

assessment was completed by using a commercial, laboratory-grade light measuring device

(CheckMARC®, BlueLight Analytics, Halifax, NS). A total number of 87 LPUs was found

capable of delivering required energy. Therefore, 87 participating dental practitioners with

their LPUs were assessed for their ability to deliver the study-required 8 J/cm2 of radiant

exposure in 10s to the MARC®PS.

However, to acquire all relevant information for this study, the participating dental

practitioners (n=113) (female and male dentists, and dental assistants) were asked to deliver a

10s radiant exposure, using their LPUs, to the MARC®PS simulated restorations. The

133

practitioners were asked to use their LED LPUs in the way they perform the polymerization

on a patient. The participants were tested using the MARC®PS by choosing to use eye

protection (hand-held screens, protective glasses, or no protection at all). Participants applied

LED LPU’s light tip directly to the MARC®PS anterior sensor (Class III) and completed a

10-second test cycle, performing this test three times. The same procedure was performed on

the posterior sensor (Class I). After providing individualized instructions for proper

photopolymerization, participants were retested, using the same protocol and the same LPU.

The following instructions were given to participants after their initial testing: demonstration

of proper hand positioning during light exposure, mandatory use of protective glasses, proper

positioning and stabilization of the light probe, and visual inspection at each stage of the

process. The test results were shared with the participants after the test procedure. A positive

control group consisted of three graduate dental students who were tested in the same way as

the participating dental professionals using a reference LPU (Bluephase Stylus, Ivoclar-

Vivadent).

Statistical analyses

Mixed-design analysis of variance (Mixed ANOVA) was used to test for differences

between three independent groups (male dentists, female dentists, and dental assistants)

while subjecting participants to repeated measures (before and after instructions). Following

this, Fisher's post hoc multiple comparison tests were conducted using a pre-set alpha of

0.05. Chi-square tests of independence were conducted using a pre-set alpha of 0.05 to

investigate whether delivering the minimum acceptable level of radiant energy depends on

the dental practitioner (male dentist, female dentist, or dental assistants). Paired-samples t-

134

tests were conducted to investigate the influence of individualized instructions (before/after)

on light energy delivered to simulated restorations by all participating dental professionals

(n=113), using a pre-set alpha of 0.05. The SPSS software for Windows, version 22 (SPSS

Inc., IBM, Somers, NY), was used for all statistical analyses.

Parametric statistical tests used in this study (such as paired t-tests, chi-square tests, and

ANOVA) rely on the assumptions of normality and equal variances among the comparison

groups. According to the central limit theorem, when the sample size is greater than 30 cases

per group, normal distribution of the sample means can be assumed if the sample data are not

normally distributed. Considering that the sample used in this study consisted of 87 (113)

cases, parametric statistical tests were used.

Results

The Q-Q plots were used for normality assessment. The average 10s radiant exposure

delivered by dental practitioners (n=113) to the MARC®PS anterior and posterior

restorations, before and post-instructional, are presented in Figures 4.7 to 4.10.

Equality of variances assumption across the comparison groups was tested with the

Levene's test. The test was not significant, (p=0.816), thus, the equality of variances was

assumed. A power analysis using the G Power program for independent and paired t-tests

revealed that the sample size of 113 could detect a medium effect size with the independent

samples t-tests (Cohen's d=.53), and a small effect size with the paired-samples t-tests

(Cohen's d=.23). A power analysis for Mixed ANOVA used a reference (Green et al., 2010)

45, which states that each cell in design should have at least 15 cases to express the minimum

135

power of .80, and that criterion was met for the current study. Additionally, for the Chi-

square test of independence, a reference (Oyeyemi et al., 2010) 46 was used for the power

analysis. According to the guidelines, a study sample size of 113 can detect only large effects

with the power of .80.

The majority (73.6 percent) dental practitioners delivered the study-required energy

to the anterior restoration before instructions. Eighteen participants (20.7 percent) did not

deliver required energy before instructions but were able to deliver it post -instructional.

Only three participants (3.4 percent) were not able to deliver this energy at all. The change in

the number of dental practitioners able to deliver (before and after instructions) the required

energy minimum to anterior restoration, as measured by McNemar test, was significant

(p<.001).

Only twenty-eight dental practitioners (32.2 percent) were able to deliver the study-

required energy to the posterior restoration before instructions. An additional thirty-six more

participants (41.4 percent) were able to deliver required energy post-instructional. The

change in the number of dental practitioners able to deliver (before and after instructions) the

study-required energy to the posterior restoration, as measured by McNemar test, was

significant (p<.001). The average levels of radiant exposure delivered by dental practitioners

to the MARC® PS anterior and posterior restorations, before and post-instructional, are

presented in Table 4.1, Figure 4.2, and Figures 4.3 to 4.6.

To investigate whether delivering the acceptable level of radiant energy depends on a

dental practitioner (n=113) (male and female dentist, or dental assistants), chi-square tests of

independence were conducted. The tests disclosed that the experience of dental practitioners

to deliver the study-required energy is not related to the restoration location. Thus, for

136

anterior restoration before instructions (2(2) =2.57, p=0.277, Cramer’s V=0.17), post-

instructional (2 (2) =4.95, p=0.084, Cramer’s V=0.24), or posterior restoration before

instructions (2(2) =1.15, p=0.562, Cramer’s V=0.12), or post-instructions (2 (2) =3.65,

p=0.162, Cramer’s V=0.21) were recorded.

The average levels of radiant energy delivered to anterior and posterior restorations

before and after instructions for all dental practitioners (n=113) regardless the ability of their

LPUs to deliver the study-required energy are presented in Table 4.1. Paired-samples t-tests

were conducted to investigate whether the average energy level delivered by dental

practitioners was changed post-instructional. The results of these tests were significant for

both anterior (t (112) = -10.26, p < .001, 95%CI for MD: -3.14; -2.13) and posterior

(t (112) = -12.26, p < .001, 95%CI for MD: -3.17; -2.29) restorations.

Mixed ANOVA was used to analyze whether practitioners' consistency of energy

delivery was changed post-instructional. The results of this analysis showed a significant

effect of time (baseline or post-instructional), Wilk's Ʌ=0.91, F (1,110) =10.42, p=0.002. On

average, regardless of location (anterior/posterior), and type of dental practitioner, the

consistency of energy delivery improved after the instructions: M (before) =1.2 compared to

M (after) =0.9 (95%CI for MD: 0.115; 0.480). However, the effect size of this difference, as

measured by partial ɳ2, was small (.09).

The average light-energy that the positive control group participants delivered to the

MARC®PS restorations was: 9.8 J/cm2 to the anterior restoration, and 9.9 J/cm2 to the

posterior restoration before instructions, and 12.6 J/cm2 to the anterior restoration, and 11.1

J/cm2 to the posterior restoration, post-instructional.

137

The average 10s radiant exposure (J/cm2) that dental practitioners (n=113) delivered

to simulated MARC® PS restorations, using monowave (n=81) and multiwave (n=32) LED

LPUs is presented in Figure 4.11. No significant difference in performances between

monowave and multiwave LPUs was observed, except in only one cluster, at posterior

restoration, post-instructional (p=.022).

Discussion

The first research hypothesis was that the light energy delivered by dental

practitioners (n=87) in the Greater Toronto Area private dental practices, during the 10s

radiant exposure would be equal or higher than 8.0 J/cm2, was rejected. The majority, (73.6

%) of participants delivered the study-required energy to anterior restoration and only 32.2%

of participants delivered required energy to posterior restoration, before instructions.

Following instructions, 20.7 % of participants deliver the study-required energy to anterior

restoration, and 41.4 % of participants delivered required energy to posterior restoration.

Based on the MARC®PS results, it is clear that photopolymerization skills of some dental

practitioners at the baseline, before they received individualized instructions, were not at the

level that would allow them to pass the study-required criterion. Thus, the percentage of

participants that delivered required energy to anterior restoration was 70.2%, while only

32.2% participants were able to deliver the study-required energy to posterior restoration.

Consequently, the percentage of participants that failed to deliver required energy to the

MARC® PS simulated restorations was 29.8% to the anterior and 67.8% to the posterior

restoration, respectively.

138

The second research hypothesis was that, following individualized instructions on

proper photopolymerization, participants (n=113) would deliver more radiant energy to

simulated restorations, and they would improve the consistency of energy delivered during a

10s radiant exposure, was accepted. Because of individualized instructions, participants’

average for energy delivered to the anterior restoration increased by 22.6% and to the

posterior restoration by 30.4%. Furthermore, they improved the consistency of delivered

energy (smaller SD) during a 10s radiant exposure, 25.9% on average, regardless of a

restoration location or type of dental professional: M (before) =1.2 compared to M (after)

=0.9.

Dental practitioners (n=113) were asked to perform photopolymerization on the

MARC® PS simulated restorations, using their LPUs most frequently used for all daily

clinical procedures. The rationale for using all participating LPUs regardless of their capacity

to deliver the study-required energy was to discover how two different location of simulated

restoration changed the magnitude of delivered light energy. Further, to strengthen the data

on the advantage of individualized polymerization instructions on improved consistency and

quantity of energy delivered by dental practitioners, the participants had three attempts to

deliver light energy in a 10s radiant exposure cycle, first to anterior and then posterior

restoration using their choice of eye protection (or not at all). Participants were given

individualized instructions on proper photopolymerization, before they were asked to repeat

the test using mandatory eye protective glasses and the same LPUs, the anterior restoration

first, followed by the posterior restoration. The LPU in each dental practice was

characterized by the CheckMARC® spectroradiometer device to ensure the capacity of the

LPU to generate at least 8.0 J/cm2 of radiant exposure during a 10s-time interval.

139

Although results acquired by the CheckMARC® were accurate, the smaller diameter

size of the sensor at the MARC® PS (4 mm) increased the irradiance values depending the

differences between the LED LPUs' light probe diameter and the MARC® PS sensor. Due to

the inhomogeneity of the light-beam profile, where the LPU’s central beam exhibits higher

exitance irradiance values compared to the periphery, yielding a 4 mm the MARC® PS

sensor to capture inflated irradiance values, resulting in an increase of the MARC® PS

readings values by 10% to 20% on average. That could explain the CheckMARC® results

that twenty-six LED LPUs could not produce 8.0 J/cm2 of radiant exposure, while the

MARC® PS results showed that eight LED LPU's could deliver the study-required energy to

at least one of the MARC® PS restorations.

A positive control group consisted of three graduate dental students who were tested

in the same way as the participating dental professionals using a reference LPU (Bluephase

Stylus, Ivoclar-Vivadent). The control LPU's ability to produce a minimum energy

requirement was validated by the CheckMARC® (incident irradiance of 1,077 mW/cm2). In

addition, the graduate students that represented a control group had no prior experience with

the MARC® PS before were tested. The rationale to use a control LPU for the assessment of

photopolymerization skills of graduate students was that a single LPU would exhibit fewer

variations during the light energy production cycle, therefore, the focus was on the light-

delivering technique, not on the LPU per se. Thus, the quantity of energy delivered to the

MARC® PS represented precisely photopolymerization skills of subjects tested. The average

energy levels delivered by the control group in the 10s radiant exposure were 9.8 J/cm2 to the

anterior restoration, and 9.9 J/cm2 to the posterior restoration before instructions, and 12.6

140

J/cm2 to the anterior restoration, and 11.1 J/cm2 to the posterior restoration, post

instructional.

The average radiant exposure that dental practitioners delivered to the MARC® PS

truly represented the state of affairs in the participating dental practices in the Greater

Toronto Area. The average baseline radiant exposure values recorded using the MARC® PS

were lower in comparison with post instructional values because participants tried to avoid a

direct eye exposure to the LPU’s light source, which prevented them from correctly

positioning and stabilizing the LPUs on top of the simulated restoration. The participants that

were using the hand-held screens for eye protection exhibited insufficient stabilization of the

light probe over the simulated restoration, which resulted in substantially lower radiant

exposure values. In addition, some participants experienced jitters because they were

somewhat apprehensive. However, during the second and third attempt to deliver light

energy they gradually increased confidence, which was shown by improved values of

delivered energy. After individualized instructions on proper photopolymerization, which

also included mandatory use of protective glasses, the participants on average, delivered a

significantly higher energy level to the MARC® PS. The research conducted in the

participating dental practices measured polymerization techniques of practitioners who were

not entrusting the photopolymerization to their assistants. All LPUs were tested without

infection control because a variety of infection control methods were used among the

participating practices, and some practices were not using any infection control at all. For

that reason, the LPUs were tested without any protective barriers, to make meaningful

comparison among LPUs tested in all participating dental practices.

141

Post-instructional data showed a significant increase in delivered light energy, dental

practitioners on average, increased the energy delivered to anterior restoration by 22.6% and

posterior restoration by 30.4%. At baseline testing, most of dental practitioners employed a

single-hand polymerization, using the hand-held protective screens or no eye protection.

However, following the instructions and the mandatory use of eye protective glasses, dental

practitioners using the two-hand polymerization technique improved delivered energy to

simulated restorations by 26.5% on average.

A significant difference in delivered light-energy that was observed between the

anterior restoration and the posterior restoration may be related to the light probe design and

the limited practitioners’ ability to access the posterior restorations. There are many

difficulties in the posterior area access. A light probe design, which shows a notable

curvature on its end, usually stands for a substantial source of challenges in limited

interocclusal space, during the light application. Furthermore, the absence of visual control

during proper light probe positioning creates a struggle in keeping immovable light probe tip,

as well as keeping it at the right angle to the axis of the tooth. The MARC® PS results

revealed that the LPUs with a more curved light probe delivered, on average, less light

energy to the posterior restoration, in comparison to the LPUs with a less curved light probe.

The explanation for this phenomenon could be in the fact that a curved light probe usually

needs more space for its proper positioning.

This study tested the ability of dental practitioners to deliver light energy to simulated

restorations (anterior and posterior) using the MARC® PS. It was unveiled that the average

amount of energy delivered to each simulated restoration, was similar among the three

professional groups. However, female dentists were the most consistent in energy delivery

142

(expressed in lowered standard deviation values), followed by male dentists and dental

assistants, respectively. Although the pool of dental assistants was indisputably larger than

any other group, it can be safely assumed that the highest standard deviation values could be

associated with the larger pool size (Figure 4.2). However, the difference among the energy

values delivered by dental practitioners was not significant.

The current results presented in this study that proper light polymerization

instructions have significantly increased energy delivered to the simulated restorations on the

MARC® PS were in accord with the findings from the earlier studies 30-35. The unique

perspective of the current study was the ability to characterize incident irradiance of LPUs

tested, to assess the radiant exposure that dental practitioners delivered to the MARC® PS

restorations, as well as to make a comparison between the two LPU types, monowave, and

multiwave. The study results overwhelmingly refute the view that significant improvement

has occurred as a result of individualized instructions that dental practitioners received and

use of protective goggles and a two-hand polymerization technique.

Bias can occur in any phase of a research project, from planning, data collection, and

analysis to the publication phase. This study exhibited selection bias to some extent because

those dental practices with the possible lower quality of LPUs or questionable proficiency in

RBC placements refused to participate in this study. As mentioned before, over 350 targeted

practices were initially contacted via telephone/email to determine their eligibility to

participate in the study. Although it was concluded that 250 practices satisfy the criteria, 100

dental practices were only recruited with a total of 113 dental practitioners. Many factors led

to low study recruitment outcome.

143

The study exhibited standardized protocols for data collection and data entry. A

single administrator completed the entire study, which substantially reduced any possible

inter-observer variability and performance bias while, at the same time, enabling the test

administrator to evaluate internal validity during the entire research process efficiently.

Another potential limitation of this study was a possible conceptual problem with a

10s radiant exposure using the MARC® PS. If the passing criterion of 16 J/cm2 radiant

exposure during a 20s exposure was performed, the results might be more pronounced and

probably could bring a much more comprehensive margin of passed/failed polymerization

skills. Further, the 16 J/cm2 radiant exposure represented an arbitrary value that was more

commonly cited in the literature 26, 27.

The direction of future research could include a change in the research population,

thus to see how rural dental practitioners differ from the urban practitioners and possible

include the assessment of Dental Public Health professionals. The study findings will be

most useful to researchers, as well as to dental practitioners, as well as possibly to regulatory

agencies.

Conclusions

Within the limitations of this study, which used the MARC®PS to quantify energy

delivered to simulated restorations by dental practitioners, the following conclusions can be

reached:

144

1. Before instructions, 73.6% of participants delivered required-energy to the anterior

restoration, and 32.2% of participants delivered required-energy to the posterior

restoration.

2. Following instructions, 94.3% of participants delivered required-energy to the

anterior restoration, and 73.6% of participants delivered required-energy to the

posterior restoration

3. Participants on average increased energy delivered to the anterior restoration by

22.6% and the posterior restoration by 30.4% because of instructions.

4. Specific instructions and the mandatory use of eye-protective glasses demonstrated

that two-hand polymerization technique (post-instructional) is significantly better

than one-hand technique engaged at baseline.

5. Following specific instructions, and because of the two-hand polymerization

technique, dental practitioners improved the consistency of delivered energy to the

simulated restorations on average by 25.9%.

6. A significant percentage of dental practitioners (26.4%) were unable to deliver the

study-required energy to the posterior restorations, even when given instructions.

7. No significant difference in performances between monowave and multiwave LPUs

was observed, except in only one cluster, at posterior restoration, post-instructional.

145

Acknowledgements


University of Toronto. This project was financially supported by grants from the Faculty of

Dentistry Research Institute of the University of Toronto, and an American Academy of

Esthetic Dentistry. The CheckMARC® device was loaned and technically supported by

BlueLight Analytics, for which the authors are grateful. The Bluephase Style was kindly

provided by Ivoclar-Vivadent. There are no words to express immense gratitude to the

project research assistants, Ms. Ana Burilo and Ms. Ivana Orlovic for their contribution to

the study.

146

Figure 4.1 The MARC® Patient Simulator system with a light probe

positioned to deliver light energy to the posterior (Class I) restoration. The

front sensor, representing the anterior restoration (Class III), between the

upper central incisors, is depicted in the picture

Figure 4.2 A 10s radiant exposure (Mean and SD) that all participants

(n=113) delivered to the MARC® PS restorations, separated by each

professional group, (Male and Female Dentists, and Assistants). No

statistically significant difference among 3 professional groups was found.

147

Table 4.1 Descriptive Statistics for a10s radiant exposure (J/cm2) delivered by

113 dental practitioners using their LPUs to the MARC® PS restorations, in 4

sequences: anterior and posterior restorations before instructions, and anterior

and posterior restorations post-instructional. The results were recorded by the

MARC® PS v.3.4 software.

J/cm2

Min

Max

Mean

SD

Anterior before 1.78 32.00 9.02 4.42

Anterior after 3.03 36.16 11.65 4.89

Posterior before 0.81 15.96 6.25 3.06

Posterior after 1.59 27.99 8.98 3.95

-

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

Ant_before Post_Before Ant_After Post_After

J/cm

2

M_Dentists F_Dentists Assistants

148

Figure 4.3 10s Radiant Exposure delivered by dental practitioners (n=113) to the

MARC® PS anterior restoration, before instructions, with respect to 8 J/cm2 cut-off

value.


MARC® PS anterior restoration, post-instructional, with respect to 8 J/cm2 cut-off

value.

149


MARC® PS posterior restoration, before instructions, with respect to 8 J/cm2 cut-off

value.


MARC® PS posterior restoration, post-instructional, with respect to 8 J/cm2 cut-off

value.

150

Figure 4.7 Q-Q plot showing the average energy quantities that dental practitioners

(n=113) delivered to the MARC®PS anterior restoration before instructions. The

observed values are plotted against normally distributed data (represented by the

line).


(n=113) delivered to the MARC®PS posterior restoration before instructions. The


line).

151


(n=113) delivered to the MARC®PS anterior restoration, post instructional. The


line).

Figure 4.10 Q-Q plot showing the average energy quantities that dental

practitioners (n=113) delivered to the MARC®PS posterior restoration, post-

instructional. The observed values are plotted against normally distributed data

(represented by the line).

152

Figure 4.11 a 10s radiant exposure (J/cm2) (Mean and SD) that dental practitioners

(n=113) delivered to simulated MARC® PS restorations, using monowave (n=81) and

multiwave (n=32) LED LPUs. *A significant difference was observed between two

LPU types in only one group, at posterior restoration, post-instructional (p=.022).

*

153

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159

Chapter 5

Manuscript 3

Relative Hardness of Resin Based Composites Polymerized by LED Light-Polymerization Units in

Toronto’s Private Dental Practices

160

Relative Hardness of Resin Based Composites polymerized by LED Light-

Polymerization Units in Toronto’s Private Dental Practices

Dave D. Kojic DDS, MSc. PhD Candidate


ON, Canada





Professor, Department of Clinical Dental Sciences, Faculty of Dentistry Dalhousie

University, Halifax, NS, Canada











161

Abstract

Objectives: To assess relative-hardness (RH) of two brands of resin-based

composites (RBC), polymerized using Light-emitting-diode (LED) light-polymerization units

(LPUs) in private dental practices, Toronto, Canada. Methods: 100 dental practices were

recruited. In participating dental practices, disc-specimens (2mm-thick/4mm-diameter) (n=6)

were fabricated from TPH Spectra (Dentsply) (camphorquinone (CQ) photoinitiator) and

Tetric EvoCeram (Ivoclar-Vivadent) (CQ/Lucirin-TPO photoinitiators). Using the office's

LPU, specimens were polymerized for the 20s from the top surface, at 1mm (n=3) and 4mm

tip-to-target distance (n=3). Specimens were stored (distilled-water/370C/24h), dried for 1h,

before microhardness-testing. Knoop-hardness number (KHN) for each specimen was

recorded from 8 measurements (4lower/4upper), using microhardness tester with Knoop

indenter (Tukon 300, Bridgeport, CT), and applying 50g load/30s dwell-time. Calculated RH

ratios data were statistically analyzed using Pearson correlations, Chi-square tests, and

independent-samples t-tests with a pre-set alpha of 0.05. Results: Both RBCs polymerized at

1mm had average ratios greater than the .80 level of RH (ratio of mean lower/upper surface

hardness), but not at 4mm tip-to-target distance. A significantly higher percentage of LED

LPUs achieved the 0.80 ratio of hardness with TPH Spectra, compared to Tetric EvoCeram

RBC with both, 1mm (χ2(1) =27.32, p<.001) and 4mm tip-to-target distance (χ2(1) =15.48,

p<.001). Conclusions: The RH profiles of RBCs tested demonstrated a strong, inverse

correlation between the tip-to-target distance and average RH values. The incident irradiance

and LPUs’ beam dispersion, had a significant effect on Knoop hardness profile and

mechanical properties of RBCs.

162

Keywords: Light Polymerization Unit; Irradiance; Knoop hardness, Degree of

conversion; polymerization; Resin-Based Composite; CheckMARC®;

163

Introduction

There are several testing procedures available to characterize RBCs polymerization

efficiency. During a free-radical photopolymerization reaction, double bonds in the vinyl

groups of the RBCs are converted into single covalent bonds, thus further enhancing the

conversion of monomers into the linear and cross-linked polymers. The degree of conversion

(DC), a significant determining factor of the polymerization kinetics, is highly associated

with mechanical, physical, and chemical properties of direct restorative RBC materials 1, and

represents the ratio of converted single bonds to the total number of starting double bonds.

Several analytical methods are available for measurement of RBC conversion. For instance,

differential scanning calorimetry provides a measure of methacrylate conversion based on the

enthalpy of the exothermic polymerization process 2. The majority of analyses done to

evaluate conversion of RBCs have been based on the use of spectroscopy, which gives a

direct measure of reacted and unreacted methacrylate groups 2. Although these methods

exhibited an elevated level of accuracy and sophistication, they are always related with high-

costs 3-6.

The most efficient methods to determine a RBC polymerization level include DC,

depth of polymerization (DoP), the degree of crosslinking (DoC), and microhardness 7-10.

Researchers prefer to use reliable and easy to use microhardness tests, rather than more

elaborate tests, such as FTIR or Raman spectroscopy preferentially used for DC.

Furthermore, Knoop microhardness testing has proven to be a reliable and a relatively simple

method to characterize RBCs' mechanical properties after polymerization. The Knoop test

gained popularity among researchers, primarily due to its good correlation with DC, which

has been confirmed in numerous studies 11-16. Monomer conversion could not provide a

164

complete characterization of the polymer structure, because polymers with similar DC may

exhibit a different degree of crosslinking and consequently different mechanical properties 17.

The commonly held assumption before the introduction of bulk-fill RBC materials, was that

an equivalent of 16 J/cm2 of received radiant exposure to a single 2 mm tick increment of

RBC material is considered satisfactory for adequate RBCs polymerization 18. However,

manufacturers' recommended minimal radiant exposures for sufficient RBC polymerization

can vary to a great extent, depending on intrinsic properties and chemical formulation, as

well as on the material shade and the radiant exitance of the LPUs 19-21.

Hardness (H), represents an important parameter in the characterization of the surface

properties of a material on a microscopic scale, which is defined as the ratio of the applied

load (P), to the contact area (A), which resulted in the subsequent indentation impression:

H = 𝑃

𝐴 = β

𝑃

𝑑2 (1)

Where d represents the size of attributed resultant indentation, and β represents the

constant related to the geometry of a given indenter 22. Vickers and Knoop are the most

frequently employed micro-hardness tests for characterization of RBC materials by utilizing

a pyramid-based indenter. The Vickers hardness number (VHN) is defined by using the

contact area of the indentation impression and the parameter d in Eq. (1), is described as an

average of the two diagonals of the resultant square-shaped impression. Thus, the β constant

for VHN calculation is set to be 1.8544. Knoop microhardness test uses a pyramid-based

indenter with the length to width ratio of 7:1 and with corresponding face angles of 172.5

degrees for the long edge and 130 degrees for the short edge, as shown in Figure 3.1.

Furthermore, Knoop hardness number (KHN) calculation considers the projected area of the

165

indentation impression, and the parameter d, which is determined by the length of the long

diagonal of the resultant rhombic impression and β constant set to be 14.229. Based on Eq.

(1), it can be implied that microhardness represents an indentation area corresponding

variable. The literature has shown, especially with RBC materials that the inequality between

a plastic zone beneath the indentation and a surrounding elastic matrix exists. Therefore, that

phenomenon is described as the elastic recovery of the given material after the removal of the

indenter. The elastic recovery is highly related to the surface geometry of an indenter, which

differentiates the elastic recovery of Vickers indentation in comparison to Knoop indentation.

The elastic recovery attributed to Vickers indentation usually develops along the depth of the

indentation, while the diagonal length of the surface impression remains unchanged, contrary

to the Knoop indentation, where the elastic recovery appears as a result of a minor diagonal

contraction of the indenter 23. The Knoop microhardness computation only considers the

length of the longer diagonal into the calculation of KHN, and thus makes this method

preferential for its reliability over other methods 24, 25. A linear correlation between Knoop

microhardness and mechanical properties of RBC and Knoop microhardness and irradiance

of LPUs have been shown to exist 28.

Although the scraping test has been considered as a depth of polymerization measure

in the ISO 4049 standardization for RBCs, microhardness tests were perceived to be more

reliable 7. The test is based on top and bottom hardness measurements; however, it is

common to calculate the ratio of the bottom/top hardness, and the arbitrary minimal values of

0.80 would represent a ratio of sufficient RBCs polymerization 24, 27, 28. "Relative hardness"

term represents the average bottom-to-top hardness ratio 29.

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The project objective was to evaluate relative hardness (RH) of RBCs polymerized

using LED LPUs, which were in use in participating private dental practices in the Greater

Toronto Area. The test parameter for RH was the bottom-to-top Knoop microhardness

values. The study objectives were to test the following research hypotheses: the first

hypothesis was that the LED LPUs, from the participating dental practices, capable of

producing the radiant exposure of 16 J/cm2, would be positively correlated with the RH

values of RBCs tested. The second hypothesis was that 1mm and 4mm material-to-probe

distance, would not affect the average 0.80 RH values of 113 LED LPUs tested.


From all participating private dental practices in the Greater Toronto Area, a total of

number of 113 LED LPUs were included. Specimens were fabricated using an opaque plastic

mold (Figure 5.1) from the A2 shade of two conventional RBC materials: TPH Spectra HV

(Dentsply, Milford DE) and Tetric EvoCeram (Ivoclar-Vivadent, Amherst NY). TPH Spectra

exhibits a camphorquinone (CQ) photoinitiator system, while Tetric EvoCeram has a mix of

CQ and a small amount of Lucirin TPO photoinitiators 30.

With each LPU, a total number of 12 specimens were prepared (6 per material) with 4

mm diameter and 2 mm thickness. The specimens were covered with celluloid matrices on

both sides and with a glass slide on the top. A 20s exposure was administered to the top

surface of the specimen (n=3) by applying the LPU available in the visited dental practice

LED LPU directly through a 1mm glass slide. A 2 mm-metal spacer with a 20 mm diameter

was placed on the top of the glass slide that covered the specimen, then 1mm glass slide

167

covered the metal spacer. This setup created a 4mm material-to-probe distance. A 20s

exposure of the top specimens ‘surface (n=3) by the same LED LPU was conducted. The

same protocol was implemented to the other brand of RBC. Following the light exposure,

specimens were stored in dark containers in distilled water medium at 370C for 24h before

microhardness testing. The next day, all specimens were dried for 1h, before commencing the

tests. Knoop Hardness Numbers (KHN) were obtained at 10X magnification from eight

surface measurements, as shown in Figure 5.2. Thus, each side of the specimens received

four indentations in North-South and East-West direction, using a microhardness tester

(Tukon 300, Wilson Instrument Division, Bridgeport, CT), and an applied load of 50g and,

during a 30s dwell time. RH ratio for each specimen was calculated from the specimen’s

bottom mean KHN values divided by the specimen’s top mean KHN values. A reference,

multiwave LPU (Bluephase Style, Ivoclar-Vivadent, Amherst NY) was used as a positive

control. The assessment of 113 LPUs in the participating dental practices was completed by

using a commercial, laboratory-grade light measuring device (CheckMARC®, BlueLight

Analytics, Halifax, NS).

• Statistical analysis

Descriptive statistics were used to evaluate irradiance values of LED LPUs tested.

The 10s radiant exposure data obtained by the CheckMARC® were multiplied by 2 to

produce 20s radiant exposure values, which were correlated to the minimum passing criterion

of 16 J/cm2. To evaluate whether the association between the RH values and LED LPU

radiant exposure values, concerning the minimum required the energy of 16 J/cm2 exists, the

Pearson correlation analysis was used, significant at the 0.001 level. Furthermore, the same

168

analysis was used to evaluate the relationship between the irradiance values of LPUs (n=113)

and obtained Knoop hardness b/t ratio of two RBC materials, polymerized at 1 mm and 4

mm material-to-probe distance. Chi-square tests of independence were used to examine a

possible relationship between LED LPUs with the capacity to deliver 16 J/cm2 energy and

the minimum level of RH (0.80) for each type of specimen. The results of these tests for all

four testing groups (two brands of RBCs and two tip-to-target distance levels) at the 0.05

level of significance were plotted. All statistical analyses were conducted using the Statistical

software (SPSS for Windows, version 22, SPSS Inc., IBM, Somers, NY).

Parametric statistical tests used in this study relied on assumption of normality of data

and equal variances among the comparison groups. Following these assumptions is especially

critical when the sample size used in the study is small. Considering that the sample used in

this study consists of 113 LED LPUs, parametric statistical tests were used. However, to

investigate the potential impact on the normality assumption violations, nonparametric tests

were also conducted. In case of discrepancies that arise by using non-parametric tests, the

results of the non-parametric tests would be reported. However, it was discovered that the

results of non-parametric tests and parametric tests were identical.

Results

The Q-Q plots were used for normality assessment, and the results of the average RH

values in four data distributions are presented in Figures 5.7 to 5.10. Equality of the variances

assumption across the comparison groups was tested with the Levene’s test. The tests were

169

not significant, Tetric EvoCeram at 1 mm (p=0.351), and 4 mm (p=0.829) material-to-probe

distance, and TPH Spectra at 1 mm (p=0.114), and 4 mm (p=0.067) material-to-probe

distance, respectively. Thus, the equality of the variances was assumed. A power analysis

using the G Power program for the independent and paired t-tests revealed that the sample

size of 113 was capable of detecting a medium effect size with the independent samples t-

tests (Cohen's d=0.53). Additionally, for the Chi-square test of independence, a reference

(Oyeyemi et al., 2010) 39 was used for the power analysis. According to the reference, the

study sample size of 113 can detect only large effects with the power of 0.80.

The descriptive statistics for RH levels of two RBC's brands obtained from the

specimens polymerized at 1 mm and 4 mm material-to-probe distance are presented in Table

5. 1. The average 0.80 level of RH (ratio of the mean bottom/top surface) was accomplished

for both RBC brands, polymerized at 1 mm material-to-probe distance. However, the

specimens polymerized at 4 mm material-to-probe distance were not able to produce an

average 0.80 level of RH regardless of the RBC brand. The percentage of dental practices

that reached the 0.80 level of RH for each type of specimens is displayed in Figure 5.3.

Significant differences were observed between 1mm and 4mm tip-to-target distances for the

two RBC brands, and their RH results (inter-brand and intra-brand).

The pool of LPUs tested consisted of 32 multiwave and 81 monowave units. The

average RH values of two LPU types and two RBC brands polymerized at 1 mm and 4 mm

material-to-probe distance are presented in Figure 5.4. The results of the independent-

samples t-tests did not show any statistically significant difference between the average RH

values of two RBC brands and a type of LPU used at material-to-probe (1 mm and 4 mm)

distance. The following results were observed for Tetric EvoCeram at 1mm material-to-probe

170

distance t (113) = -0.94, p= 0.351 (95%CI for MD: -0.03; 0.01), and at 4 mm material-to-probe

distance t (113) = 0.22, p= 0.829 (95%CI for MD: -0.02; 0.03), while the results for TPH

Spectra at 1mm material-to-probe distance were t (113) = -1.59, p= 0.114 (95%CI for MD: -

0.04; 0.00), and at 4 mm material-to-probe distance were t (113) = -1.85, p= 0.067 (95%CI for

MD: -0.06; 0.00).

The average RH ratios produced by the reference LPU (Bluephase Style, Ivoclar-

Vivadent) were 0.85 at 1 mm, and 0.80 at 4 mm material-to-probe distance for Tetric

EvoCeram, and 0.84 at 1mm and 0.83 at 4 mm material-to-probe distance for TPH Spectra.

Discussion

The first research hypothesis, which stated that the participating dental practices LED

LPUs, capable of producing a 20s radiant exposure of 16 J/cm2, would be positively

correlated with the Knoop hardness b/t ratio values of RBCs tested, was accepted. A positive,

weak-to-moderate, but significant (an average r=0.36, p<.001), correlation was found

between the average 0.80 RH level of the two RBC brands and a 20s radiant exposure of

LED LPUs capable of delivering at least of 16 J/cm2 radiant energy.

During the course of this project, wide variations of LED LPUs were tested, along

with possible temperature differences (different weather conditions), and other contributing

factors that may happen during the specimens' preparation in the participating dental

practices that could cause these results. For instance, the correlation factor is not consistent

among RBC materials and the material-to-probe polymerization distance. Although TPH

Spectra and Tetric EvoCeram are popular brands of the Bis-GMA based nanohybrid RBCs,

171

which exhibit the presence of pre-polymerized filler particles in their compositions, they

differ in a total filler load values, being 60-62 volume percentage for TPH Spectra and 53–55

volume percentage for Tetric EvoCeram. Furthermore, TPH Spectra exhibits a

camphorquinone (CQ) photoinitiator system, while Tetric EvoCeram has a mix of CQ and

Lucirin TPO photoinitiators 30. Thus, the effect of polymerization distance in combination

with chemical properties of RBCs tested resulted in a wide range of correlation values. Tetric

EvoCeram exhibited a moderate correlation factor of r= 0.43 at 1 mm material-to-probe

distance, but at 4 mm material-to-probe distance the correlation factor was reduced to r=0.33,

where in an opposite direction with TPH Spectra (r=0.44 at 4 mm material-to-probe distance

and r=0.26 at 1 mm material-to-probe distance) was detected. The magnitude of these

relationships suggests that other factors are contributing to RBC microhardness, such as

LPU’s spectral irradiance output (monowave vs. multiwave), as well as a beam profile

differences, apart from the confirmed LED LPUs capacity to deliver at least 16.0 J/cm2 of

energy.

The second hypothesis, which anticipated that 1mm and 4mm material-to-probe

distance, would not affect the average 0.80 RH values of the two brands RBC tested, was

rejected. The average 0.80 level of RH polymerized at 1mm material-to-probe distance was

obtained by 69.0% of Tetric EvoCeram specimens and 82.3% of TPH Spectra specimens,

respectively. However, at 4 mm material-to-probe distance, the average 0.80 level of RH

values was not achieved by any RBC material (0.77 for both materials). Furthermore, it was

discovered that at 4 mm material-to-probe distance, only 28.3% of Tetric EvoCeram

specimens and 37.2% of TPH Spectra specimens be able to obtain the minimum RH ratios.

Therefore, a recorded RH ratio drop over the 3 mm material-to-probe distance was 59% for

172

Tetric EvoCeram and 54.8% for TPH Spectra, respectively. The results are in line with

previous finding that light energy diminishes considerably with an increased material-to-

probe distance, which is observed as reduced microhardness profile values 35, 36, 37. The

results could be associated with the interactions of light polymerization energy with the

microscopic slides, Mylar strips, and a 2 mm span of plastic mold lateral walls. Furthermore,

a metal spacer which was used to produce 4 mm distance between the light probe and the

RBC specimens, could also influence the RH values. However, the current test set up design

was employed for its simplicity, quick set up time, and its ability to use in any participating

dental practice.

The participating dental practice LED LPUs capable of delivering at least of 16 J/cm2

radiant exposure, produced the 0.80 level of RH only by 61.1% Tetric EvoCeram specimens

polymerized at 1 mm material-to-probe distance and by 25.7% polymerized at 4 mm

material-to-probe distance. A similar pattern was observed with TPH Spectra, where the

requirement was achieved by 70.8% specimens, polymerized at 1mm material-to-probe

distance and with only 34.5% specimens polymerized at 4 mm material-to-probe distance. It

was evident that specimens at 4mm material-to-probe distance, regardless of RBC brand

performed notably below what was found when the light was held at an only 1mm material-

to-probe distance. Therefore, the percentage of LPUs that exhibited insufficient

polymerization at 4 mm material-to-probe distance was greater than 60%. The material-to-

probe distance of 4 mm or more is clinically relevant because most of the polymerization

locations in posterior teeth can exceed 7 mm material-to-probe distance 37.

173

The current study tested RBC specimens of A2 shade, because of the following facts:

A2 shade is the most commonly employed shade in clinical practice 30, and the composition

of pigments and other colorants has the minimum effect on photopolymerization 31.

The pool of tested LED LPUs consisted of 32 multiwave and 81 monowave LPUs. In

the multiwave group, the Valo® (Ultradent, South Jordan, UT) was the most common (38.7

%), while the Demi Plus® (Kerr, Orange, CA) was the most common (26.2 %) in the

monowave group. Due to the possible variations among participating light sources, the

assessment was carried on all LPUs by measuring their irradiance and radiant exposure with

a laboratory-grade spectrometer, the CheckMARC® system. The rationale was that all tested

LPUs had to be examined whether they had the capacity of delivering at least 16 J/cm 2

radiant exposure, to correlate the produced energy data with averaged 0.80 RH ratios of two

RBC materials.

The results of an analysis which compared Tetric EvoCeram (an alternative

photoinitiator system) and TPH Spectra (a conventional CQ photoinitiator system) with

multiwave and monowave LPUs, showed no statistically significant difference among those

four variables (see Figure 5.4). The multiwave LPUs outperformed monowave lights in all

tests except with the Tetric EvoCeram 4 mm material-to-probe distance, where the average

RH values of both LPUs type were identical. Interestingly, multiwave LPUs performed better

on TPH Spectra specimens, regardless of the material-to-probe distance. Although TPH

Spectra exhibited a higher filler load content and a conventional CQ photoinitiator system,

apparently these parameters did not produce significant Rayleigh scattering, often seen with

the multiwave LPUs violet light spectrum.

174

This project was the first study that correlated the minimum-required level of radiant

exposure of LED LPUs in the participating dental practices using the average 0.80 level of

RH to test two RBC materials. The majority of published studies test a selected number of

RBCs using yet a variety of LPUs 11-16. This study design differed significantly from all

previous studies. Microhardness values were taken 24h after exposure and stored in 37ºC

water to mimic the oral environment. One hour before the hardness testing, the specimens

were dried using a paper towel and left in the ambient temperature, after which KHN

measurements were obtained. The RBC materials complied with the Water Sorption (ISO

4049) standards 52. Based on results from a pilot study which used the same two brands of

RBCs, no statistically significant difference in microhardness was seen when each of the two

RBC materials during the 24h storage in either distilled water or dry containers.

A reference LPU (Bluephase Style, Ivoclar-Vivadent) produced sufficient hardness,

regardless of the polymerization material-to-probe distance or the RBC tested. In comparison

with RH values for Tetric EvoCeram obtained by the reference LPU (0.85 at 1 mm and 0.80

at 4 mm material-to-probe distance), 108 participating LPUs (95.6%) were below that RH

level at 1 mm material-to-probe distance, and 81 LPUs (71.7%) did not produce that RH

level at 4 mm material-to-probe distance. Similarly, in comparison with RH values for

Spectra TPH obtained by a reference LPU (0.84 at 1 mm and 0.83 at 4 mm material-to-probe

distance), 97 LPUs (85.8%) were below that RH value at 1 mm material-to-probe distance,

while 101 LPUs (89.4%) did not reach that RH level at 4 mm material-to-probe distance.

Furthermore, the reference LPU exhibits relatively homogeneous beam profile distribution of

its spectral irradiance, which is the part of the latest LED generation development. Thus, it

175

was observed that the reference LPU exhibited more uniform Knoop hardness profile with all

materials and polymerization material-to-probe distances.

Limitations of the current project were associated with the current set-up that the

polymerization light is passing through glass slides, Mylar strips, and bouncing from the

lateral walls of the metal spacer, which probably affect the RH values to a certain degree.

However, metal sectional matrices that are used in clinical settings may similarly influence

RBC polymerization. It would be better if the current set-up had included an opaque plastic

spacer, preferably from the same material as the mold used for the specimens’ production.

Future research should be focused on bulk-fill RBC evaluation, using a similar

method as was used in this project. Furthermore, to create a clinically relevant scenario,

using the same methodology, the irradiation distance could be increased to 6 mm, because

most of the polymerization locations in posterior teeth can exceed 7 mm material-to-probe

distance 37.

Conclusions

Within the limitations of this study, the following conclusions can be reached:

1. Relative hardness ratios of the two RBC brands demonstrated a weak-to-

moderate correlation between the light probe distance to the material surface

and 20s radiant exposure.

2. The 0.80 RH ratio of two RBC brands was achieved by the majority of LED

LPUs at 1 mm material-to-probe distance.

176

3. The 0.80 RH ratio of two RBC materials was obtained by a minority of LED

LPU's at 4 mm material-to-probe distance.

4. The percentage of LPUs that exhibited inadequate polymerization at 4 mm

material-to-probe distance was observed to be higher than 60%.

5. The average relative hardness reduction over a 3 mm material-to-probe

distance, expressed by the RH profile, was 59% for Tetric EvoCeram and

54.8% for TPH Spectra, respectively.

177

Acknowledgements


University of Toronto. It was supported by grants from the Faculty of Dentistry Research

Institute of the University of Toronto, and an American Academy of Esthetic Dentistry. The

CheckMARC® device was loaned and technically supported by BlueLight Analytics, while

material donations were provided by Ivoclar-Vivadent and Dentsply for which the authors

are very grateful. In addition, the Bluephase Style was graciously loaned by Ivoclar-

Vivadent. The authors are immeasurably thankful to our research assistants Ms. Ana Burilo

and Ms. Ivana Orlovic for their contribution to this project.

178

Figure 5.1 Plastic mold used for the specimens’ production

Figure 5.2. Specimen ready for Knoop microhardness testing

179

Table 5.1 Descriptive statistics for RH ratios recorded for two RBC’s specimens

polymerized at 1mm and 4mm tip-to-material distance. The assumptions of

normality and equal variances among the comparison groups was assumed.

Min Max M SD

Tetric

EvoCeram

1 mm distance .58 .89 .80 .05

4 mm distance .41 .91 .77 .06

TPH Spectra

1 mm distance .37 .90 .81 .06

4 mm distance .31 .88 .77 .08

Figure 5.3 Percentages of LPUs that reached the .80 ratios of RH for each RBC

brand. Significant differences were observed between 1mm/4mm tip-to-target

distances for the two RBC brands, and their RH results (inter-brand and intra-

brand).

69.0

2

8.32

28.3

82.3

37.2

*

*

**

**

180

Figure 5.4 Average Knoop hardness b/t ratios (Mean and SD) produced by multiwave

(n=32) and monowave (n=81) LPUs for the two brands of RBCs (Tetric EvoCeram

and TPH Spectra HV), irradiated for 20s from 1 mm and 4 mm tip-to-target distance.

No statistically significant difference among the groups was found

Figure 5.5 Average RH ratios for Tetric EvoCeram polymerized at 1 mm and 4 mm

tip-to-target distance, using a total of 113 LED LPUs.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

RH 1mm Tetric RH 4mm Tetric RH 1mm Spectra RH 4mm Spectra

Har

ne

ss r

atio

multiwave monowave

181

Figure 5.6 Average RH ratios for TPH Spectra polymerized at 1 mm and 4 mm tip-to-

target distance, using a total of 113 LED LPUs.

Figure 5.7 Q-Q plot showing the average RH ratios of Tetric EvoCeram at 1 mm tip-

to-target distance observed values against normally distributed data (represented by

the line).

182

Figure 5.8 Q-Q plot showing the average RH ratios of Tetric EvoCeram at 4

mm tip-to-target distance observed values against normally distributed data


Figure 5.9 Q-Q plot showing the average RH ratios of TPH Spectra at 1 mm

tip-to-target distance observed values against normally distributed data


183

Figure 5.10 Q-Q plot showing the average RH ratios of TPH Spectra at 4 mm tip-

to-target distance observed values against normally distributed data (represented

by the line).

184

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190

Chapter 6

Manuscript 4

Assessment of Diametral Tensile Strength of Resin Based Composites Polymerized by LED Light-

Polymerization Units in Private Dental Practices in Toronto, Canada

191

Assessment of Diametral Tensile Strength of Resin Based Composites polymerized by

LED Light-Polymerization Units in Private Dental Practices in Toronto, Canada



ON, Canada





Professor, Department of Clinical Dental Sciences, Faculty of Dentistry Dalhousie

University, Halifax, NS, Canada











192

Abstract

Objectives: To evaluate Diametral Tensile Strength (DTS) of two resin-based

composites (RBC), polymerized using LED light-polymerization units (LPUs) in

participating private dental practices. Methods: A 100 dental practices from the Greater

Toronto Area, were recruited. Specimens were made of TPH Spectra (Dentsply),

(camphorquinone (CQ) photoinitiator) and Tetric EvoCeram (Ivoclar-Vivadent),

(CQ/Lucirin-TPO photoinitiators). A plastic mold was used to produce cylindrical specimens

(4mm-thick/6mm-diameter). Each specimen's surface received a 20s radiant exposure.

Specimens (n=5) were stored in dry/dark container (370C/24h). The DTS was conducted in a

universal testing machine (model 8501 Instron Corp), using a cross-head speed at 0.5

mm/min. Fracture load (N) was recorded, and the DTS was calculated. The DTS data of

Tetric EvoCeram and TPH Spectra were analyzed with a paired-samples t-test and a mixed

ANOVA. All statistical tests were completed using a pre-set, α of 0.05, and Pearson

correlations, using a pre-set, α of .001. Results: No significant difference in average DTS

values between the two materials (t (113) =-.72, p=.475) was found. The mean DTS observed

for Tetric EvoCeram was 40MPa (SD=5) and for TPH Spectra 39.9MPa (SD=4). The DTS

values were highly correlated between the two materials (r=.71, p<.001) and the

corresponding LPUs radiant exposure. Conclusions: Although DTS averages two RBCs

were not significantly different, moderately-strong correlations between the 16 J/cm2 of

radiant exposure in 20s and DTS levels of Tetric EvoCeram (r=.54, p<.001) and TPH Spectra

(r=.53, p<.001) were observed.

Keywords: Light Polymerization Unit; Irradiance; Radiant exposure; Diametral

Tensile Strength test; Degree of conversion; Resin-Based Composite; CheckMARC®;

193

Introduction

Adequate RBC photopolymerization is a critical factor in predicting clinical success

and the longevity of direct restorations. Insufficient RBC polymerization is highly correlated

with a substantial reduction in their mechanical properties 1.

The efficient methods to determine the level of RBC polymerization are the degree of

conversion (DC), depth of polymerization (DoP), the degree of crosslinking (DoC),

compressive strength (CS) test, and the diametral tensile strength (DTS) test. Some

mechanical testing procedures have been examined to address the complexity of loading in

clinically relevant situations. Based on literature findings, a positive correlation between CS

and DTS has been observed 2, 3, 4, 5. Furthermore, the fracture resistance (FS) test has

frequently been used for the evaluation of flexural strength and a flexural modulus of RBC

materials by performing a three-point bending test, included in the ISO 4049:2009

standardization testing procedures 6. Although DTS test is not a part of the ISO 4049:2009

standards, it was recognized as technically reliable and a straightforward test for the

assessment of RBC mechanical properties. The DTS test was defined by the ADA/ANSI

Specification No.27 for the RBCs evaluation. The DTS has been designed to test brittle

materials by utilizing a cylinder-type specimen that is subject to compressive load across its

diameter. It has been observed that materials which exhibit plastic deformation would

produce false DTS values. Moreover, they would be anticipated to reveal strain rate

sensitivity 7.

There are some controversies associated with the DTS test, as per experimental

observation that resultant shear forces acting at the apex of the diametral plane have been

suggested to complicate the interpretation of strength, with the possibility that reported data

194

from different studies would not be challenging to interpret 7, 9. However, DTS test can

provide useful information about the effectiveness of RBC polymerization and in conjunction

with other measuring methods can provide meaningful data on the mechanical properties of

given materials 8-16.

The study aimed to assess specific mechanical properties of two RBC brands,

polymerized by LED LPUs in the participating private dental practices, by conducting a DTS

test. The study objectives were to test the following research hypotheses. The first hypothesis

was that the average DTS values of Tetric EvoCeram and TPH Spectra polymerized using

the participating LED LPUs would not show any difference. The second hypothesis was that

LED LPUs with the capacity to produce a 20s radiant exposure of 16 J/cm2, would be

positively correlated with the DTS values of the materials tested. The third hypothesis was

that DTS values of the two RBC brands, polymerized by multiwave LPUs would not be

different from the DTS values polymerized by monowave LPUs.


Specimens for the DTS test were made in the participating practices, by a single

administrator. However, before the specimens’ preparation, the assessment of LPUs in the

participating dental practices was completed by using a commercial, laboratory-grade light

measuring device (CheckMARC®, BlueLight Analytics, Halifax, NS). A 10s radiant

exposure obtained by the CheckMARC® device was multiplied by 2 to represent a 20s

radiant exposure. The study criterion required all LPUs to generate a minimum of 16 J/cm2 of

195

radiant exposure in a 20s cycle. The specimens were fabricated using an opalescent,

cylindrical, and plastic mold (4 mm thick and 6 mm in diameter) (Figure 6.1). Furthermore,

five specimens per BC brand were prepared from the A2 shade of each of two nanohybrid

RBC materials: TPH Spectra (high viscosity) HV (Dentsply, Milford DE) and Tetric

EvoCeram (Ivoclar-Vivadent, Amherst NY). TPH Spectra exhibits a camphorquinone (CQ)

photoinitiator system, while Tetric EvoCeram consists of CQ and Lucirin TPO

photoinitiators 20. The RBC material was inserted into the mold and overlaid with a polyester

strip and a 1 mm microscopic glass-slide on both sides, and then light polymerized by the

LED LPU from the participating dental practice, for the 20s at the top and 20s at the bottom

surface. The polymerized specimens were extracted from the mold and stored in dark

containers at 370C and tested after 24h storage. Specimens were measured with a digital

caliper (Mitutoyo Canada, Mississauga, ON) before testing into a universal testing machine

(Model 8501, Instron Corp, Canton, MA). Each specimen was tested by placing the specimen

on its longitudinal side on the platens of the universal testing machine. The load was applied

vertically on the lateral portion of the cylinder at a crosshead speed of 0.5 mm/min,

producing tensile stress perpendicular to the vertical plane passing through the center of the

specimen (Figure 6.2). The specimens were loaded to failure in compression, and the

fracture load was recorded in Newton (N). Following the compressive test, each specimen

was visually inspected to determine whether the specimen fractured into two halves. Those

specimens fractured into unequal sizes were excluded from the analysis. The DTS was

calculated based on fracture load (F) expressed in Newton (N), diameter, and height of the

specimen, calculated using the formula from the Phillips’ Science of Dental Materials 325:

σt = 2𝐹

𝑑ℎ𝜋 (1)

196

where are: d: diameter (6 mm); h: height (4 mm) of specimens; π: 3.14

To obtain average DTS values of the positive control group (n=5 per each RBC brand), a

reference, multiwave LPU (Bluephase Style, Ivoclar-Vivadent) was used.

• Statistical analyses

Considering that the sample used in this study consisted of 113 light-units, normality

was assumed. Therefore, parametric statistical tests were employed. However, to investigate

the potential impact of the violation of the normality assumption on statistical tests results,

nonparametric tests were also conducted. Whether the results of the parametric and non-

parametric tests deviated, then the results of the non-parametric tests would be reported.

However, it was discovered that the results of non-parametric tests and parametric tests were

identical. Thus, parametric tests were reported.

A paired-samples t-test was used to investigate whether average DTS values were

different between Tetric EvoCeram and TPS Spectra specimens. To further examine whether

LED LPUs with the capacity to deliver at least 16 J/cm2 of radiant exposure had higher levels

of DTS on average, a mixed ANOVA was conducted. The DTS levels of the two RBC

brands were used as a within factor in this analysis, and an indicator of LED LPUs capable of

delivering a minimum radiant exposure level, (yes/no) was used as a between factor. The

analyses were conducted using a pre-set alpha of 0.05.

The 10s radiant exposure data obtained by the CheckMARC® device were multiplied

by 2 to produce the 20s radiant exposure values, which were correlated to the study-required

minimum criterion of 16 J/cm2. It was determined that (n=87) LPUs could satisfy the

197

criterion. Thus, to evaluate whether the association between the DTS values of two RBC

tested and LED LPU radiant exposure values, concerning the study-required energy of 16

J/cm2 exists, the Pearson correlation analysis was used, significant at the 0.001 level. The

SPSS software for Windows, version 22, (SPSS Inc., IBM, Somers, NY) was used for all

statistical analyses.

Results

The results of Shapiro-Wilk tests for normality of the average DTS values were not

significant for Tetric EvoCeram (p=0.081) and TPH Spectra (p=0.097), confirming that data

were normally distributed. Furthermore, the Q-Q plots were also used for normality

assessment, and they are presented in Figures 6.4 and 6.5.

The Levene’s test was used for the assessment of equality of variances across the

comparison groups. The results of Levene’s tests were not significant for all tests (p>0.05),

indicating that assumption of equal variances was satisfied. However, the postulate of equal

variances was violated once, in the independent-samples t-test analysis, where the

appropriate action was taken (by adjusting the degree of freedom). A power analysis using

the G Power program for the paired t-tests revealed that the sample size of 113 was capable

of small effect size with the paired-samples t-tests (Cohen's d=.23). A reference from the

SPSS manual (Green et al., 2010)21 was used to calculate a power analysis for mixed

ANOVA. According to the reference, each cell in the design should have at least 15 cases to

be able to express the minimum power of .80. Thus, the criterion was met in the current

study.

198

A paired-samples t-test showed no significant differences in the average DTS values

(MPa) between two RBC brands (t (112) = -0.72, p=0.475, 95% CI: -0.92; 0.43). The mean

BTS values (Table 6.1) were 39.7 (SD=5) for Tetric EvoCeram and 39.9 (SD=4) for TPH

Spectra. It was observed that DTS levels were highly correlated between the two RBC brands

(r=0.71, p<.001), indicating that LED LPUs which produced higher DTS values on Tetric

EvoCeram specimens, also produced higher DTS values on TPH Spectra specimens.

A mixed ANOVA analysis showed that regardless of RBC brand, the LED LPUs that

were capable of delivering at least 16 J/cm2 of irradiant exposure, also produced higher DTS

levels, F (1, 111) = 18.00, p<.001. A partial ɳ2=0.14 was indicating a medium-to-large effect

size (95% CI: -5.66; -2.06). Specifically, the average DTS values for LED LPUs that did not

meet the criterion was 36.8 MPa (SD=0.8) and for LED LPUs that met the criterion was 40.7

MPa (SD=0.4).

The pool of LED LPUs tested consisted of 32 multiwave and 81 monowave LPUs.

The majority of LED LPUs (n=87, 77.0%) could deliver at least of 16 J/cm2 radiant exposure

in 20s exposure. The average DTS values of the control group were 42.9 MPa for Tetric

EvoCeram, and 43.9 MPa for TPH Spectra.

The Pearson correlation analyses (n=113) showed moderately strong correlations

between the incident irradiance values of LED LPUs and DTS values of Tetric EvoCeram

(r=0.54, p<.001) and TPH Spectra (r=.053, p<.001) RBCs. Furthermore, small-to-moderate,

but significant, correlations were found between LED LPUs capable of delivering at least 16

J/cm2 of radiant exposure (n=87), and average DTS values of Tetric EvoCeram and TPH

Spectra (r=0.37, p<.001, and r=0.31, p<.001), respectively.

199

An independent-samples t-test showed no significant differences in the average DTS

values of Tetric EvoCeram (t (112) = 0.07, p=0.943, 95% CI: -1.70; 1.83), and TPH Spectra

(t (112) = 1.07, p=0.289, 95% CI: -0.68; 2.25), polymerized with multiwave and monowave

LPUs (Table 6.1). The Levene’s tests were significant (p<0.05), therefore, the assumption of

equal variances was violated, however, the tests with adjusted degrees of freedom were used.

Discussion

Adequate photopolymerization is crucial for the expected longevity of RBC

restorations. For a quick and straightforward assessment of polymerization efficiency, the

BTS test is frequently used to quantify the tensile strength of brittle dental restorative

materials 3-5. The appropriateness and reliability of this method to successfully analyze the

capacity of RBC materials to withstand strong masticatory forces in clinically appropriate

situations has been reported 7.

The first research hypothesis, that the average DTS values of Tetric EvoCeram and

TPH Spectra, polymerized LED LPUs in the participating dental practices would not show

any significant difference, was accepted. The results of a paired-samples t-test were not

significant, (t (112 =-0.72, p=0.475), indicating no difference in the average level of BTS

between the two materials. The 95% confidence interval of the difference in the average DTS

level was in the range from -0.92 to 0.43. The mean DTS value observed in the sample for

200

Tetric EvoCeram was 39.7 MPa (SD=5), (95% CI for MD: 38.7; 40.6), and for TPH Spectra

was 39.9 MPa (SD=4), (95% CI for MD: 49.1; 40.7).

Although TPH Spectra and Tetric EvoCeram are Bis-GMA based nanohybrid RBCs,

which exhibit the presence of pre-polymerized filler particles in their compositions, they

differ in the total filler load values, being 60-62 volume percentage for TPH Spectra and 53–

55 volume percentage for Tetric EvoCeram. Furthermore, TPH Spectra exhibits a

camphorquinone (CQ) photoinitiator system, while Tetric EvoCeram has a mix of CQ and

Lucirin TPO photoinitiators 20. Thus, an alternative, Lucirin TPO photoinitiator system

exhibits its maximum peak absorption spectra below 420 nm, in the violet spectral range. It

would be expected that an extended light spectrum of multiwave LPUs yield the higher

average BTS values for Tetric EvoCeram, due to the presence of Lucirin TPO, and its

affinity for the violet range of multiwave LPUs spectral irradiance. However, experimental

data could not support that postulate, as the average DTS values of Tetric EvoCeram were

39.7 MPa, while the average DTS values of TPH Spectra were 39.9 MPa. Although Tetric

EvoCeram exhibits a blend of photoinitiators, which included the Lucerin TPO, and a lower

filler load percentage than of TPH Spectra, the occurrence of the Rayleigh scattering could

be associated with the lower average DTS values of Tetric EvoCeram.

The second hypothesis, that LED LPUs capable of producing at least of 16 J/cm2

radiant exposure in a 20s exposure cycle, would be positively correlated with the average

DTS values of Tetric EvoCeram and TPH Spectra, was accepted. A positive, significant, but

a small-to-moderate correlation between LED LPUs (n=87) capable of producing at least of

16 J/cm2 radiant exposure and the average DTS values of Tetric EvoCeram (r=0.37, p<.001),

and TPH Spectra (r=0.31, p<.001), was observed. Furthermore, the moderately strong

201

correlations between a 40s radiant exposure delivered by LED LPUs (n=113), and the

average DTS values of Tetric EvoCeram (r=0.54, p<.001), and TPH Spectra (r=0.53,

p<.001), were noted. A further analysis unveiled that the average DTS levels were highly

correlated between the two RBC materials (r=0.71, p<.001), indicating that higher or lower

DTS values were observed on both materials depending on the radiant exposure generated by

the participating office’s LPU. Currently, this was the first study that correlated a minimum

level of radiant exposure of LED LPUs in the participating dental practices to the average

DTS level of two RBC materials. Many of published studies have tested a number selected

RBCs with a selected number of LPUs 8-16.

The third research hypothesis, that the average DTS values of two RBC brands,

polymerized by multiwave LPUs would not be different from the DTS values polymerized

by monowave LPUs, was accepted. An independent-samples t-test showed no significant

differences in the average DTS values of Tetric EvoCeram (p=0.943), and TPH Spectra

(p=0.289), polymerized with multiwave and monowave LPUs, as presented in Figure 6.3.

Although it would be expected that multiwave LPUs produce better results with Tetric

EvoCeram, that assumption was not observed. The average DTS values produced by

multiwave LPUs were 39.6 MPa for Tetric EvoCeram, and 39.4 MPa for TPH Spectra, while

the average DTS values produced by monowave LPUs were 39.7 MPa, and 40.1 MPa for

TPH Spectra, respectively.

The current project was the first study that explored DTS test values on a large

sample size of LED LPUs. This study design was dissimilar to all previous studies 8-13. The

decision to use 6mm x 4mm cylindrical specimens was based on a methodology that was

borrowed from previous studies, which, however, was not in full accordance with the

202

ADA/ANSI Specification No.27. Specimens, following the polymerization were stored in

dry containers, instead of the distilled water, as advocated in the ADA specification. The

RBC materials tested complied with the Water Sorption (ISO 4049:2009) standards 6.

According to a pilot project data, no statistically significant difference was found between

two RBC material specimens during the 24h storage in either distilled water or dry

containers, followed by the DTS testing. Furthermore, the compressive load that was applied

in a universal testing machine, with a crosshead speed of 0.5 mm/min. There is no precise

agreement on the employment of different crosshead speed to the specimens for DTS

evaluation. However, the ADA/ANSI Specification No.27 recommends a crosshead speed of

0.75 mm/min. The test selection for 0.5 mm/min of crosshead speed, was based on the

premise that most of reported studies used the same parameter, and the study results showed

that the crosshead speed variations between 0.5 and 1 mm/min were not statistically

significant 19.

When compared with the average DTS values of the control group (42.9 MPa for

Tetric EvoCeram, and 43.9 MPa for TPH Spectra), and the average DTS values produced by

the LPUs tested, it was found that 77.9% LPUs produced lower average DTS values with

Tetric EvoCeram. Similarly, for TPH Spectra, 87.6% LPUs had the average DTS values

below the value of the reference LPU.

The broad spectrum of values recorded in this study could be attributed to a more

substantial number of LED LPUs that were applied for specimens` polymerization.

Furthermore, because travel distance between the participating dental practices was

sometimes reasonably long, the RBC materials could have been exposed to the ambient

temperature during the cold winter season. Thus, the RBCs temperature fluctuation could

203

have some influence on test results. Very shallow concentric grooves on the working faces of

the platens (universal testing machine) were used to prevent specimen’s slipping during the

compressive loading, which eliminated a need for paper slips. Although, paper-points were

not used to prevent contact points due to imperfection of cylindrical shape of the specimens,

however, any specimen fractured into unequal sizes was excluded from the analysis.

Limitations of the current project were associated with the current set-up where the

polymerization light was passing through glass slides, and Mylar strips, which could affect

the recorded DTS values to a certain degree. It would be better if the current set-up supported

the ADA/ANSI Specification No.27 recommendations for specimen's dimension and storage.

However, since the nature of this study was largely comparison of two restorative materials

and a current slight variation in specimen dimensions has little or no effect on the outcome.

Future research should include bulk-fill RBC restoratives evaluation, using similar

approach used in this project. Furthermore, to create a clinically relevant scenario, using the

same methodology, the radiant exposure should follow manufacturers' recommendation

values for the RBC, in respect to the incident irradiance of LPUs tested.

Conclusions

Within the limitations imposed by this study, it may be concluded that:

1. The average DTS values of Tetric EvoCeram and TPH Spectra, polymerized

with LED LPUs in participating dental practices, did not show a statistically

significant difference. The mean DTS values observed in the sample for Tetric

204

EvoCeram was 39.7 MPa (SD=5), and for TPH Spectra, it was 39.9 MPa

(SD=4).

2. A positive, significant, but small-to-moderate correlation between LED LPUs

(n=87) capable of producing at least of 16 J/cm2 radiant exposure in the 20s

and the average DTS values of Tetric EvoCeram (r=0.37, p<.001), and TPH

Spectra (r=0.31, p<.001), was observed.

3. Moderately strong correlations between the 40s radiant exposure delivered by

LED LPUs (n=113), and the average DTS values of Tetric EvoCeram (r=0.54,

p<.001), and TPH Spectra (r=0.53, p<.001) were discovered.

4. The average DTS levels were highly correlated between the two RBC

materials (r=0.71, p<.001), indicating that higher or lower DTS values were

observed with both materials, depending on the radiant exposure applied by

the LPUs in the participating dental practices.

5. No significant differences were observed in the average DTS values of Tetric

EvoCeram (p=0.943) and TPH Spectra (p=0.289) polymerized with

multiwave and monowave LPU.

205

Acknowledgements


University of Toronto. It was supported by grants from the Faculty of Dentistry Research

Institute of the University of Toronto, and an American Academy of Esthetic Dentistry. The

CheckMARC® device was loaned and technically supported by BlueLight Analytics, while

material donations were made by Ivoclar-Vivadent and Dentsply for which the authors are

very grateful. Further, many thanks to Ivoclar-Vivadent for loaning the Bluephase Style as a

study reference LPU. Special thanks to our research assistants Ms. Ana Burilo and Ms. Ivana

Orlovic for their participation in this project.

206

Figure 6.1 A DTS Specimen 4mm thick and 6mm in diameter

Figure 6.2 DTS testing in a universal testing machine.

207

Figure 6.3. The average DTS values (Mean ± SD) of Tetric EvoCeram and TPH

Spectra RBCs polymerized by multiwave (n=32) and monowave (n=81) LPUs,

following the 40s radiant exposure. No significant difference among the groups was

found.

.

Table 6.1 Average BTS values of Tetric EvoCeram and TPH Spectra, polymerized by

monowave (type 0) and multiwave (type 1) LPUs. *Levene’s tests for homogeneity

of group variances were significant for both tests (p<0.05), indicating that assumption

of equal variances was not assumed.

RBC LPU

Type

Mean SD Levene's

test

t p-

value

95% CI for the mean

difference

Lower Upper

Tetric

EvoCer

am

Mono 39.7 5 4.10* 0.07 0.943 -1.70 1.83

Multi 39.6 4

TPH

Spectra

Mono 40.1 5 8.68* 1.07 0.289 -0.68 2.25

Multi 39.4 3

208

Figure 6.4 Q-Q plot showing the average BTS values of TPH Spectra against

normally distributed data (represented by the line).

Figure 6.5 Q-Q plot showing the average BTS values of Tetric EvoCeram against

normally distributed data (represented by the line).

209

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18. Son SA, Roh HM, Hur B, Kwon YH, Park JK. The effect of resin thickness on

polymerization characteristics of silorane-based composite resin. Restor Dent Endod

2014; 39(4):310-318.

19. Sood A, Ramarao S, Carounanidy U. Influence of different crosshead speeds

diametral tensile strength of a methacrylate-based resin composite: An in-vitro study.

J Conserv Dent 2015; 18(3):214-217.

20. Tetric EvoCeram®, scientific documentation 2014. Ivoclar Vivadent AG,

Liechtenstein. Available at: downloads.ivoclarvivadent.com/zoolu-

website/media/document/17021/Tetric+EvoCeram+Bulk+Fill. Accessed December

2017.

21. Green S and Salkind N. Using SPSS for Windows and Macintosh. 6th Edition.

Prentice Hall Press Upper Saddle River, NJ, USA 2010

212

Chapter 7

Findings, Analyses, Discussion, Limitations, and Future

Directions, and Conclusions

213

7.1 Findings

This thesis analyzed the efficacy of 113 LED LPUs used by general dental

practitioners in 100 private practices in the Greater Toronto Area, by conducting a

quantitative assessment of LPUs and polymerization skills of dental professionals, as

well as the characterization of the surface and bulk-properties of two RBCs

polymerized with same LED LPUs. Within the limitations of each of the four studies

that this project comprised of, the following conclusions were made:

1. The study characterized the LED LPUs in 100 participating dental practices

concerning their ability to generate at least 16 J/cm2 of radiant exposure in the 20s

computed exposure time, measured by the Check MARC®.

• The 20s calculated radiant exposure ranged between 3.8 and 59.6 J/cm2 (M=21.7,

SD=8.6).

• The pool of tested LED LPUs consisted of 32 multiwave and 81 monowave LPUs. In

the multiwave group, the Valo (Ultradent, South Jordan, UT) was the most common

(57.6%), while Demi Plus (Kerr, Orange, CA) was the most common (26.3%) in the

monowave group.

• The minimum criterion for radiant exposure of 16 J/cm2 in 20s was met for the

majority of LED LPUs (n=87, 77.0%), where 29 were multiwave LPUs and 58 were

monowave LPUs.

• There were 26 underperforming LPUs (3 multiwave and 23 monowave)

214

• By calculating the ratio of substandard LPUs to a total number of LPUs tested,

(multiwave LPUs (3/32) versus monowave LPUs (23/81)), monowave LPUs

exhibited three times higher possibility to fail the study-required standard of 16 J/cm2

per 20s than multiwave LPUs from the same sample.

• In comparison with a computed 20s, radiant exposure of the reference LED LPU

(Bluephase Style, Ivoclar-Vivadent), which generated 21 J/cm2, 47.8% of LED LPUs

tested, delivered lower radiant exposure values.

• The LED LPUs in the sample were between 1.0 and 5.5 years of age (M=2.6,

SD=1.2) and had damage levels ranging from 0 to 4.0 (M=1.6, SD=1.2).

• The average age of LED LPUs that did not pass the criterion was 3.6 years (SD=1),

while the average age of LED LPUs that passed the criterion was 2.3 years (SD=1).

• The average damage level of LED LPUs that did not pass the criterion was 2.8

(SD=1), compared with 1.3 (SD=1) for LED LPUs that passed the criterion.

• The average age of monowave LPUs was 2.7 years (SD = 1), and the average age of

multiwave LPUs was 2.6 years (SD = 1).

• Similarly, the two types of LPUs were not significantly different on their level of

damage. Specifically, 17.7% of monowave LPUs and 17.6% of multiwave LPUs had

no damage, and 6.3% of monowave and 5.9% of multiwave LPUs had the highest

level of damage (level 4).

• A strong, r=-0.67 (p<.001) negative correlation between the LPU radiant exposure

and its age and a level of damage, was observed.

215

2. The study evaluated the ability of dental practitioners to deliver at least eight J/cm2 of

radiant exposure, in a 10s exposure cycle to the MARC®PS simulated restorations

(before/after instructions), using their LED LPUs.

• Before instructions, 73.6% of practitioners delivered required-energy to anterior

restoration, and 32.2% of practitioners delivered required-energy to posterior

restoration.

• Following instructions, 94.3% of practitioners delivered required-energy to anterior

restoration, and 73.6% of practitioners delivered required-energy to posterior

restoration.

• Dental practitioners, on average, increased delivered energy to the anterior restoration

by 22.6% and the posterior restoration by 30.4% because of instructions.

• Following specific instructions, dental practitioners improved the consistency of

delivered energy to simulated restorations on average by 25.9%.

• The positive control group (graduate students) delivered the following quantities of

energy to the MARC® PS: 9.8 J/cm2 to the anterior restoration, 9.9 J/cm2 to the

posterior restoration before instructions, and 12.6 J/cm2 to the anterior restoration,

and 11.1 J/cm2 to the posterior restoration, post-instructional.

3. The study evaluated relative hardness ratios (RH) of Tetric EvoCeram and TPH

Spectra, polymerized at 1 mm and 4 mm tip-to-target distance with the participating

LPUs.

216

• The relative hardness ratios of the two RBC brands demonstrated a weak-to-moderate

correlation (average r=0.36, p<.001) between the light probe tip-to-target distance,

and a 20s radiant exposure.

• The majority of LED LPUs reached the 0.80 RH level of two RBC brands at 1 mm

tip-to-target distance.

• The majority of LED LPUs did not obtain the 0.80 RH level of two RBC materials at

4 mm tip-to-target distance.

• The percentage of LPUs that exhibited insufficient polymerization at 4 mm tip-to-

target distance was observed to be higher than 60%.

• The average RH ratio reduction over a 3 mm tip-to-target distance, was 59% for

Tetric EvoCeram, and 54.8% for TPH Spectra, respectively.

• In comparison with RH ratio for Tetric EvoCeram obtained by the reference LPU

(0.85 at 1 mm and 0.80 at 4 mm material-to-probe distance), 108 participating LPUs

(95.6%) were below that RH level at 1 mm material-to-probe distance, and 81 LPUs

(71.7%) did not produce that RH level at 4 mm material-to-probe distance

• Similarly, in comparison with RH ratios for Spectra TPH obtained by a reference

LPU (0.84 at 1 mm and 0.83 at 4 mm material-to-probe distance), 97 LPUs (85.8%)

were below that RH level at 1 mm material-to-probe distance, while 101 LPUs

(89.4%) did not yield that RH level at 4 mm material-to-probe distance.

4. The study examined the Diametral Tensile Strength (DTS) of Tetric EvoCeram, and

TPH Spectra RBCs polymerized with the participating LPUs.

217

• The average DTS values of Tetric EvoCeram and TPH Spectra, polymerized with

LED LPUs in the participating dental practices did not show any significant

difference.

• The mean DTS value observed in the sample of Tetric EvoCeram was 39.7 MPa

(SD=5), and for TPH Spectra was 39.9 MPa (SD=4).

• A positive, significant, but small-to-moderate correlation between LED LPUs (n=87)

capable of producing at least of 16 J/cm2 radiant exposure in 20s exposure, and the

average DTS values, produced during a 40s radiant exposure of Tetric EvoCeram

(r=0.37, p<.001), and TPH Spectra (r=0.31, p<.001), were observed.

• A moderately strong correlation between a 40s radiant exposure delivered by LED

LPUs (n=113), and the average DTS values of Tetric EvoCeram (r=0.54, p<.001),

and TPH Spectra (r=0.53, p<.001), were noted.

• The average DTS levels were highly correlated between the two RBC materials

(r=0.71, p<.001), indicating that higher or lower DTS values were observed on both

materials depending on the radiant exposure applied by the participating offices’

LPU.

• No significant differences were observed in the average DTS values of Tetric

EvoCeram (p=0.943), and TPH Spectra (p=0.289), polymerized with multiwave

(n=32) and monowave (n=81) LPUs.

• The average DTS values obtained with a control LED LPU were 42.9 MPa for Tetric

EvoCeram, and 43.9 MPa for TPH Spectra.

• 77.9% LPUs tested produced lower average DTS values with Tetric EvoCeram, in

comparison to the average DTS values of the control LPU.

218

• 87.6% LPUs tested had lower average DTS values with TPH Spectra, in comparison

to average DTS values of the control LPU.

7.2 Analysis

Numerous studies reported that LPUs used in private practices exhibited a substantial

number of clinically unacceptable outputs. The percentage of clinically underperforming

LPUs reported in the literature ranged from 30.3% to 55.5%, depending on the study specific

testing criteria 308-316. Although all of these studies indicated that LPUs employed in dental

practices included a relatively large number of LPUs that produce insufficient irradiance

levels, it was evident that LED LPUs have shown much better results than QTH LPUs.

Although geographic variations, different methodologies, and differences in study

designs presented challenges in making meaningful comparisons, the analysis of a similar

2005 Toronto study would shed some new light on the effectiveness of LPUs in the same

geographic region. Many changes happened in private dental practice, since then the

principal change was the disappearance of QTH LPUs and their replacement with LED

LPUs. The following are the highlights:

• The sample size in the 2005 study involved a total of 214 QTH LPUs. The present

study consisted of only 113 LED LPUs.

219

• There was a tendency of single dental offices becoming aggregated into a group

dental practice. Thus, the current study included 100 dental practices as the 2005

study did.

• The mean reported irradiance in the 2005 study was 526 mW/cm2. The current study

reported 1,082 mW/cm2, which indicated that the irradiance values almost doubled in

the past ten years. The 2005 study reported that LPUs' mean age was 5.6 years, while

the current study reported just being 2.6 years.

• In the 2005 study, an analog radiometer with a range of 0 to 1,000 mW/cm2 (Optilux,

model 100, Kerr, Orange, CA) was used. The current study used a more sophisticated

the CheckMARC®, a laboratory-grade spectroradiometer with a range of 0 to 10,000

mW/cm2, which has a scientific-grade accuracy (an accuracy of +/- 5%).

• In the 2005 study, an incidence irradiance level of 400 mW/cm2 and above was

considered adequate, while the current study promotes that the incident irradiance

level of 800 mW/cm2 or above as appropriate.

• Based on the research criterion from the 2005 study, a total of 30.3% of the LPUs

generated less than the sufficient irradiance values, while the current study exhibited

23% of LPUs that were below the study-required criterion.

• The present study adequately evaluated the optical diameter of the light probe, by

measuring an outer tip diameter using Mitutoyo digital caliper (Mitutoyo Canada,

Mississauga ON) and inputting those values to the CheckMARC® software, which

automatically selected the appropriate active diameter values for the corresponding

incident irradiance calculations.

220

• Evaluation of the level of damage observed on the surface of the light-probe was

completed by using an evaluation scale (0-8), where 0 represents no damage, and 8

represents 100 percent of the damaged light probe.

• The 2005 study used an analog radiometer. However, there are a plethora of reports

associating dental radiometers with erroneous measurements and questionable results

281.

Comparison of raw data presented in both studies, ten years apart, showed that LED LPUs,

compared with QTH LPUs in the same demographics area, demonstrated, on average,

significantly better performance. This improvement could be credited to advances and

significant innovations with LED light sources.

7.3 Discussion

The study identified specific problems associated LPUs among the dental

practitioners in private dental practices in the Greater Toronto Area. The first issue was

associated with the ability of LED LPUs to generate the study-required energy minimum.

The age and level of light probe damage had a significant effect on the required quantities of

energy being produced. A regular test for damage and debris on the light probe should be

part of daily practice routine. Furthermore, a regular incident irradiance test should be

conducted using certified measuring equipment. LED LPUs with a service record of four

years or more should be tested more frequently, as they exhibited the highest potential for

failure.

221

The second issue that this study focused on was the ability of dental practitioners to

deliver adequate quantities of light energy to simulated restorations. To achieve this goal, the

MARC® PS was employed in the participating dental practices. The participants were

assessed for their clinical skills to deliver at least 8 J/cm2 radiant exposure in 10s to simulated

anterior and posterior restorations. The test was conducted in their regular office settings,

which included their choice of eye protection, if used, as a part of their typical daily routine.

Following customized instructions on proper polymerization, participants were retested using

provided blue-light protective glasses. The study revealed that a significant number of dental

practitioners exhibited inadequate polymerization skills, especially when they delivered light

energy to the posterior restoration. It was found that a substantial number of study

participants did not use appropriate eye protection. The post instructional results proved that

the use of protective glasses, along with individualized instructions had substantial effects on

improved photopolymerization technique among dental practitioners tested. It was observed

that a low level of awareness about the importance of photopolymerization among dental

practitioners had a critical effect on their ability to deliver sufficient energy to restorations.

7.4 Limitations and Future Directions

Bias can occur in any phase of a research project, from planning, data collection, and

analysis to the publication phase. The way in which subjects were identified, approached, and

respond to invitations to participate in research can be a significant source of bias, and,

therefore, it is essential for researchers to evaluate selection bias 323. Upon approval by the

University of Toronto Research Ethics Board, the participating private dental practices were

recruited. The 2012 listing of dentists published by the Royal College of Dental Surgeons of

222

Ontario was used to detect prospective dental practices in the Greater Toronto Area. Over

350 targeted practices were initially contacted by a single test administrator via telephone

using the same telephone script approved by the University of Toronto Research Ethics

Board, to determine their eligibility to join the study. However, a total of 250 suitable dental

practices were presented with the research protocol, along with a letter of consent. The

recruitment goal was to secure at least of 100 dental practices for the study. The recruitment

criteria for the study participants included: a general dental practice with proficiency in RBC

placement and physically located within the Greater Toronto Area. The study acceptance was

granted exclusively for participants who have never experienced MARC®PS before. To

proportionally include dental practices in the Greater Toronto Area, the target area of

Toronto was divided into four quadrants with a similar number of dental practices recruited

within each quadrant.

This study may had some inclusion issues to some extent because those dental

practices with possible lower quality of LPUs or questionable proficiency in RBC placements

may have refused to participate in order to avoid embarrassment. The main excuse for those

practices to decline participation was their inability to find time to accommodate the

research. The study exhibited standardized protocols for data collection and data entry. A

single administrator completed the entire study, which substantially reduced any possible

inter-observer variability and performance bias while, at the same time, enabling the test

administrator to evaluate internal validity during the entire research process.

Another potential limitation of this study was a possible conceptual problem with the

10s radiant exposure with the MARC® PS device. If the passing criterion of 16 J/cm2 with a

20s radiant exposure was performed, the results may have been slightly but not significantly

223

different. Furthermore, the 16 J/cm2 radiant exposure per 20s represented the typical value

that was more frequently cited in the literature 16, 94, 188.

The specimens (6 mm diameter and 4 mm height) for DTS test were prepared based

on preceding studies protocols 177, 181, 183, 187, 189, not the ADA/ANSI Specification No. 27 (6

mm diameter and 3 mm height). Although the study results were in line with that of

published findings, by using the ADA/ANSI Specification No. 27, the outcome may have

been more up-to-date, however, the comparative nature of this work places less value on this

aspect.

Future research directives should include the examination of bulk-fill materials

(packable and flowable) using the methodologies outlined in the current study, except for

DTS specimens’ specification, which would include ADA/ANSI Specification No. 27 (6 mm

diameter and 3 mm height). Furthermore, it would be useful to change research population,

thus to see how rural dental practitioners differ from the urban ones and possibly to include

assessment of Dental Public Health professionals in the future projects.

7.5 Conclusions

The findings from the study suggested that improvements in polymerization skills are

required and justified in a large number of participating dental practices. The results showed

that some dental professionals have been delivering insufficient quantities of light energy to

the simulated posterior restorations, which would have a detrimental effect on posterior RBC

polymerization and long-term longevity in clinical settings. Furthermore, clinicians should be

aware that their LPUs may not perform as expected, unless a rigorous maintenance protocol

is implemented, including close inspection of their LPU's light probe and efficient

224

measurement of their LPU's incident irradiance. LED LPUs with a service record of four

years or more, should be tested more frequently as they exhibit the highest potential for

failure.

225

Appendices

226

Appendix I

227

Table 1. Studies on survival of RBC restorations

Principal

author, year

Evaluation

period/study

design

Number of

restorations

/studies

Evaluated

restorations

Survival rate/

Annual Failure

Rate (AFR)

Factors

associated with

RBC failure

Manhart 298,

2004

2-14 years,

18 studies

Class I and II,

amalgams, RBCs,

GICs,

compomers

AFR: amalgams

3.0%

RBCs 2.2%,

GICs 7.2%,

copomers 1.1%

secondary caries,

fracture, marginal

deficiencies, wear

and postoperative

sensitivity

Demarco 300,

2012

5-22 years,

retrospective,

longitudinal

34 studies

Class I and Class II

posterior RBCs

AFRs of

vital teeth

1% to 3%.

AFRs of non-vital

teeth 2% to 12.4%.

tooth type and

location, operator,

socioeconomic,

demographic, and

behavioral

elements

Opdam 301,

2014

6-22 years,

prospective,

retrospective,

longitudinal

25 studies

2,816

restorations

Class I and Class II

RBCs

high/medium caries

risk, with 10-year

AFR of 4.6/4.1%

compared with

1.6% for low-risk

patient

Secondary caries

and tooth fracture

Lempel 302,

2014

10-year

retrospective

study

701

restorations

4 microhybrid RBCs

of Class II

10-year survival

rate of >97.8%.

Number of

surfaces, molars

vs premolars

Pallesen 303,

2015

27-year

prospective

study

99

restorations

60 premolars

39 molar Class II of

3 RBCs

27-year survival

rate

56.5%

Secondary caries

(54.1%) Wear

(21.6%) Fracture

(18.9%)

228

Table 1. Studies on survival of RBC restorations (continued)

Rasines

Alcaraz 304,

2014

5-7 years

retrospective

studies

7 RCT of

3,265 RBCs

and

1935

amalgams

Class II RBC and

amalgam

Restorations

Adults and children

Amalgam vs RBC:

Failure rate

RR 1.86

Secondary caries

RR 2.14

Fracture RR 0.87

Secondary caries,

Fracture and

Restoration loss

Beck 305,

2015

1-17 years,

Prospective

RCTs

88 studies

Class I/II RBCs

premolars/molars

AFR:

1.46% short-term

studies,

1.97% long-term

studies

Fractures,

secondary caries

and marginal gap

Borgia 306,

2017

11,5 years

Retrospective

Longitudinal

study

61 patients,

105 RBC

Restorations

(101 vital

teeth, 4

endo-

treatment)

Class I/II RBC

premolars/molars

103 (98%)

restorations were in

function,

98 (95.1%)

Clinically

successful

2% failed

Statistically higher

mean survival

time of premolars.

Caries-risk factor

shown to be

crucial.

Palotie 307,

2017

13-year

Retrospective

Longitudinal

study

5,542

restorations

RBC (93%)

and

amalgam

(7%)

Class II

premolars/molars

Median survival

time of all

restorations 9.9

years.

No statistical

difference between

RBC/amalgam

restorations

Toot location in

the mouth,

Premolars molars

restoration size

229

Appendix II

230

Table 2. Effectiveness of LPUs in dental practices

Principal

author,

year,

country

Type

of

LPUs

Number

of LPUs

assessed

Methodology Passing

criterion

Conclusion

Dunne 308,

1996,

UK

QTH

49

Depth of

polymerization

(DOP), analog

“LampChecker”

radiometer

5.0 to 7.0 on

analog scale

<50% of LPUs tested

were substandard

Burke 309,

1997,

UK

QTH

98

Assessment of DOP

(Heliotest and Z100)

DOP 2.6 mm

43.5% of LPUs tested

were substandard

Martin 310,

1998,

Australia

QTH

214

Analog radiometer

Demetron

400 mW/cm2 47.7% of LPUs tested

were substandard

Pilo 311,

1999,

Israel

QTH

130

Analog radiometer

Demetron, Knoop

hardness (KH)

300 mW/cm2

55% of LPUs tested

were substandard;

46% of LPUs failed

when KH data were

applied

Solomon 312,

1999,

South

Africa

QTH

35

Analog radiometer

300 mW/cm2


were substandard

El-Mowafy 313, 2005,

Canada

QTH

214

Analog radiometer

Knoop hardness

400 mW/cm2


were substandard

231

Table 2. Effectiveness of LPUs in dental practices (continued)

Santos 314,

2005,

Brazil

QTH

120

Analog radiometer

300 mW/cm2

56% of LPUs tested

were substandard

Hedge 315,

2009,

India

QTH

LED

200

Digital radiometer

400 mW/cm2

88% of LPUs tested

were substandard

AlShaafi 316,

2012,

Saudi

Arabia

QTH

LED

140

Laboratory-grade

spectroradiometer

QTH: 300

mW/cm2,

LED: 600

mW/cm2

59.5% of QTH LPUs

and 31.4% of LED LPUs

were substandard

Maghaireh 317, 2013,

Jordan

QTH

LED

295

Digital radiometer

300 mW/cm2

73.8% of QTH LPUs

and 20.8 %of LED LPUs

were substandard

Sadiku 318,

2010,

Switzerland

QTH

LED

220

Integrating sphere

Not available

40% of LPUs tested

were substandard

Kassim 319,

2013,

Kenya

QTH

LED

PAC

83

Digital radiometer,

DOP, hardness

300 mW/cm2


were substandard

Hao 320,

2015,

China

QTH

LED

196

Digital radiometer

300 mW/cm2

42.4% of QTH LPUs

and 9.4% of LED LPUs

were substandard

Kopperud 17, 2017

Norway

LED

713

Self-reporting

questionnaire

Reported

irradiance

1,000 to 2,000

mW/cm2

78.3% of dentist

unaware of their LPUs

intensity,

32% of dentist without

protective glasses

232

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