Shrinkage and Hardness of Dental Composites2009

VOLUME 40 NUMBER 3 MARCH 2009 203

QUINTESSENCE INTERNATIONAL

Optimized physical properties and minimized

residual shrinkage stresses for light-activated

composites are particularly important in

restorative dentistry. Optimal physical proper-

ties are achieved through adequate polymer-

ization, usually referred to as the degree of

cure, and directly affect the physical properties

and thus clinical performance of composite

restorations.1 Although a high degree of cure

is desirable, it inherently results in more exten-

sive polymerization shrinkage, which may

generate residual shrinkage stresses. Stresses

created by polymerization shrinkage during

composite setting can result in leakage at the

tooth-restoration interface or, where the bond-

ing is adequate, deformation of the tooth/

restoration complex.2 Such effects are clearly

unfavorable because of the possibility of sec-

ondary caries, cuspal fracture, or postopera-

tive sensitivity.

How physical properties develop during

light curing depends in part on the character-

istics of the curing light. High light intensity

(also referred to as power density) provides

faster conversion, but may also produce

higher postgel shrinkage (and thus the poten-

tial for higher shrinkage stresses) during

Shrinkage and hardness of dental compositesacquired with different curing light sourcesStephen S. Clifford, DDS1/Karla Roman-Alicea, DMD2/

Daranee Tantbirojn, DDS, MS, PhD3/Antheunis Versluis, PhD4

Objectives: Curing light sources propel the photopolymerization process. The effect of

3 curing units on polymerization shrinkage and depth of cure was investigated. Method

and Materials: The curing lights were a conventional and a soft-start quartz-tungsten-

halogen (QTH) light source and a light-emitting diode (LED) source. The soft-start QTH

and LED intensity outputs were 9% and 17% less than the conventional QTH source,

respectively. For a 40-second light cure, the light energy was 32% and 14% lower, respec-

tively. The light sources were applied to 4 restorative composites (microfilled, 2 hybrids,

and nanofilled). For each light unitcomposite combination, the development of postgel

shrinkage during polymerization was measured with strain gauges (n = 15), and the

Knoop hardness was tested at 0.5-mm-depth increments to assess degree of cure 15 min-

utes after polymerization (n = 5). The results were statistically analyzed with 2-way ANOVA

at .05 significance level, followed by pairwise comparisons. Results: Both factors, light

source and composite, significantly affected postgel shrinkage and hardness (P < .05).

The conventional QTH unit generally produced the highest shrinkage and hardness (at

composite surface and 2-mm depth). The soft-start QTH unit generated the least shrink-

age but achieved the lowest depth of cure. The resulting values for the LED unit were

mostly in between the results of the other 2 units. Conclusion: Curing lights should

provide sufficient light energy to thoroughly cure composite restorations, which might

be achieved without compromising shrinkage stresses if initial intensity is reduced.

(Quintessence Int 2009;40:203214)

Key words: composite, cure, curing light, hardness, light energy, light intensity, shrinkage,

soft start

1Summer Research Fellow, School of Dentistry, University of

Minnesota, Minneapolis, Minnesota, USA.

2Summer Research Fellow, School of Dentistry, University of

Puerto Rico Medical Sciences, San Juan, Puerto Rico.

3Assistant Professor, Department of Restorative Sciences, School of

Dentistry, University of Minnesota, Minneapolis, Minnesota, USA.

4Research Assistant Professor, Department of Restorative

Sciences, School of Dentistry, University of Minnesota,

Minneapolis, Minnesota, USA.

Correspondence: Dr Antheunis Versluis, Minnesota Dental

Research Center for Biomaterials and Biomechanics, School of

Dentistry, University of Minnesota, 16-212 Moos Tower, 515

Delaware Street SE, Minneapolis, MN 55455. Fax: (612) 626-1484.

Email: [email protected]

Clifford.qxd 4/9/09 11:29 AM Page 203

204 VOLUME 40 NUMBER 3 MARCH 2009


Cl i f ford et a l

polymerization.37 Curing at low light intensi-

ties reduces the rate of polymerization and

residual shrinkage stresses by allowing more

flow, and thus stress relaxation, before the

composite solidifies. Low-intensity curing,

however, may not achieve the desired level of

polymerization and therefore requires addi-

tional light curing at high intensities or light

exposure over a longer period of time.8,9

Various light sources are used in dental

practices.10 Quartz-tungsten-halogen (QTH)

units have been a common source of blue

light for curing restorative composites. The

halogen bulb emits full-spectrum light that is

filtered to a 380- to 520-nm blue wavelength

range, which covers the absorption peak

(468 nm) of camphorquinone, the photoinitia-

tor used in most dental composites.1,11

Because only a small part of the spectral

bulb output is relevant for activating the pho-

toinitiator, the efficiency of a QTH unit is low.

Part of the light energy is released as heat.12,13

More recently, light-emitting diode (LED) cur-

ing units have become commercially avail-

able that feature narrow spectral ranges that

are highly efficient.14,15 The spectral range

emitted by dental LED units is between 440

and 490 nm, specifically targeting cam-

phorquinones maximum absorption.

Given the availability of various curing light

design options with a manifold of restorative

composite compositions,16 the challenge for

clinical practitioners is to maintain optimal

physical properties through thorough poly-

merization while minimizing residual shrinkage

stress if possible. To gain a better under-

standing of the interaction between curing

light design and various composites, we

studied the effect on depth of cure and post-

gel shrinkage of 3 representative types of

light units (conventional QTH, soft-start QTH,

and LED) with comparable light output on 4

light-activated restorative composites. The

hypothesis was that the type of curing light

affects shrinkage stress and degree of cure

differently. Depth of cure was evaluated

using Knoop microhardness, and postgel

shrinkage was measured using a strain-

gauge technique.

METHOD AND MATERIALS

Three Elipar light sources (3M ESPE) with

comparable light output were investigated:

2500 (conventional QTH), TriLight (soft-start

QTH), and Free Light (LED). Details and light

output are listed in Table 1. The intensities

were recorded as a function of time for the

calculation of the applied light energy for a

40-second cure (light intensity multiplied by

time, also referred to as energy density)

using a customized radiometer (Cure Rite,

Model 8000, EFOS), which was connected

to a computer that recorded the light intensi-

ty readings. The 3 curing lights were applied

to 4 commercially available light-activated

restorative composites (A110, Supreme,

Z100, Z250, 3M ESPE) (Table 2).

Shrinkage measurementsThe strain-gauge method17 (Fig 1) was used to

measure the development of postgel shrink-

age for the different curing unit and composite

combinations. Shrinkage strains at the bottom

of the composite samples were measured in

2 perpendicular directions using a biaxial

stacked strain gauge (CEA-06-032WT-120,

Measurements Group). Uncured composite

was placed on the strain gauge. The sample

area attached to the strain-gauge backing

was approximately 9 mm2, while the actual

gauge area was 0.656 mm2. This ensured that

sample boundary artifacts would not affect

the measurement area. The light intensity that

Table 1 Three light-curing units used in this study

Type of Model Light intensity* Light energy

light-curing unit (3M ESPE) (mW/cm2) (mJ/cm2)

Quartz-tungsten-halogen Elipar 2500 634 25,440Light-emitting diode Elipar Free Light 529 21,810Soft-start quartztungsten-halogen Elipar TriLight 579 17,216

*Mean value at output plateau (see Fig 3) (determined with a Cure Rite Model 8000, EFOS radiometer). Mean value calculated from area under the light intensitytime curves (see Fig 3).




Cl i f ford et a l

Fig 1 Experimental design for measuring postgelshrinkage. Shrinkage strain is acquired while a com-posite sample is light cured on the strain gauge; thelight cell records exact light-curing start and duration.

Fig 2 Experimental design for measuring depth of cure. A compositesample in a mold is covered and light cured from 1 direction througha glass slide. After curing, hardness is measured at the surfaces thatwere covered by the glass slide (s is surface hardness measurementlocation at 0-mm depth) and cover plate (d is hardness measurementlocation for depth of cure).

Curing light

Composite

sample

Data

outputStrain gauge

Light cell

Curing light

Composite

sample

Glass slide

Cover

Mold

d s

Table 2 Description of light-activated restorative composites used in this study

Product (3M ESPE) Description of fillers Shade Lot no.

Microfilled Filtek A110 Colloidal silica with an average particle size A2D 3BAAnterior Restorative of 0.04 m (particle size distribution of

0.010.09 m). The filler loading is 40% by volume.

Nanofilled Filtek Supreme Nanosilica filler, particle size 20 nm and A2 Body 3BFUniversal Restorative zirconia/silica nanoclusters with primary

particles sizes 520 nm. The cluster particle size range 0.61.4 m. The filler loading is 59.5% by volume.

Hybrid Z100 Restorative Zirconia/silica filler with a particle size range A2 HE(continuum-filled) 0.013.5 m. The filler loading is 66% by volume. Hybrid Filtek Z250 Zirconia/silica filler. Particle size distribution A2 3XC(continuum-filled) Universal Restorative is 0.013.5 m with an average particle size

of 0.6 m. The filler loading is 60% by volume.

reaches the composite from a light source

diminishes with increasing distance from the

light curing tip.18 In this shrinkage experi-

ment, the distance of the curing light guide

was standardized at 2 mm above the sample.

Samples were light cured for 40 seconds. A

light-sensitive photocell was placed next to

the composite sample. The output of the pho-

tocell was recorded with the strain outputs to

register the exact start and duration of the

light cure. The shrinkage strain was recorded

for 10 minutes after initial light activation. The

relationship between shrinkage strain and

time was obtained by averaging the 2 perpen-

dicular strain components. The sample size

for each light source and composite combi-

nation was 15.

Postgel shrinkage values at 40 seconds

and 10 minutes were used for statistical

analysis. Two-way analysis of variance

(ANOVA) at a significance level of .05 was

performed to determine if there was any dif-

ference in shrinkage as a result of light

sources, composites, or composite*light

source interaction.

Hardness measurementsMicrohardness as a function of depth was

measured (Fig 2) to evaluate the distribution of

degree of cure within the cured composite.19




Cl i f ford et a l

Although not a direct measurement for the

degree of polymerization such as Fourier

Transform Infrared Spectroscopy (FTIR)

Fourier Transform Infrared Spectroscopy20 or

Raman21 techniques, a good correlation has

been shown between the development of

degree of cure and Knoop hardness.20,22,23

This correlation is specific for each resin, and

as such, the microhardness cannot be used

as an absolute number for the degree of cure

across different resins.22 The sample was pre-

pared by packing an uncured composite into

a rectangular slot (2 mm 2 mm 8 mm) of

a plaster mold (green Die-Keen, Heraeus

Kulzer). The top surface was covered with a

brass plate, and the side was covered with a

160-m-thick clear glass slide (cover slip).

The composite was light-cured from the

side through the glass slide for 40 seconds.

The curing tip was placed directly onto the

glass slide, the purpose of which was to create

a flat surface without oxygen inhibition so

that the surface hardness could be meas-

ured. Knoop microhardness tests were per-

formed 15 minutes after curing, using a

Micromet 2004 (Buehler) at 25-g load.

Indentations were placed at 0.5-mm incre-

ments, starting 0.5 mm from the light-cured

edge until the composite was too soft to

measure. In addition, the hardness at 0-mm

depth was measured from the composite

surface cured against the glass slide. The

sample size for each light source and com-

posite was 5. Only the hardness values at the

composite surface (0 mm) and 2-mm depth

were used for statistical analysis.

Two-way ANOVA at a significance level of

.05 was performed to determine if there was

any difference in hardness as a result of light

sources, composites, or composite*light

source interaction.

RESULTS

Each light source had its characteristic irradi-

ation pattern, as shown in Fig 3, where the

light intensity was recorded as a function of

time. The mean intensities of the QTH and

LED units were 634 and 529 mW/cm2,

respectively (n = 3). The light intensity output

700

600

500

400

300

200

100

0

Lig

ht

inte

nsi

ty (

mW

/cm

2)

0 10 20 30 40Time (s)

QTH

LED

Soft-start QTH

Fig 3 Light intensity output during 40-second light cure for the 3 lightsources used in this study: Elipar 2500 (QTH), Elipar Free Light (LED), andElipar TriLight (soft-start QTH). The surface area under the curves repre-sents the light energy (reported in Table 1).




Cl i f ford et a l

tended to be highest in the first few seconds,

after which it leveled off. The output of the

QTH units oscillated ( 10% at approximately

0.3 Hz), while that of the LED unit was stable.

The intensity of the soft-start QTH unit, in the

ramp-curing mode, increased exponentially

in the first 15 seconds, after which it reached

a plateau of 579 mW/cm2 (n = 4). The calcu-

lated light energy (intensity time) for a 40-

second cure of each light source is shown in

Table 1.

Shrinkage resultsShrinkage strain (or postgel shrinkage)

development in each composite, cured with

different light sources, was recorded for 10

minutes. Mean curves were created by calcu-

lating the mean strain-time curves for each

light unitcomposite combination (Fig 4).

During the initial few seconds after the start

of the light cure, the strain values became

positive, indicating thermal expansion, which

is caused by the temperature rise due to the

Fig 4 Development of shrinkage strain (postgel shrinkage) during polymerization for 10 minutes after the start of light cure.Curves are the mean of each light sourcecomposite combination (n = 15), where positive values indicate expansion and neg-ative values contraction.

500

0

500

1,000

1,500

2,000

2,500

3,000

3,500

Po

stg

el

shri

nk

ag

e (

mic

rost

rain

)

500

0

500

1,000

1,500

2,000

2,500

3,000

3,500

Po

stg

el

shri

nk

ag

e (

mic

rost

rain

)

500

0

500

1,000

1,500

2,000

2,500

3,000

3,500

Po

stg

el

shri

nk

ag

e (

mic

rost

rain

)500

0

500

1,000

1,500

2,000

2,500

3,000

3,500

Po

stg

el

shri

nk

ag

e (

mic

rost

rain

)

100 200 300 400 500 600 100 200 300 400 500 600

100 200 300 400 500 600 100 200 300 400 500 600

QTH

LED

Soft-start QTH

QTH

LED

Soft-start QTH

QTH

LED

Soft-start QTH

QTH

LED

Soft-start QTH

Time (s) Time (s)

Time (s) Time (s)

A110 Supreme

Z100 Z250




Cl i f ford et a l

exothermic reaction and the heat induced by

the light source. After the initial expansion,

the strain value became negative, indicating

that the polymerization shrinkage contribu-

tion had overtaken the thermal expansion

effects. The soft-start curing unit generated

the slowest development in shrinkage strain

during the first 40 seconds. When the curing

light was turned off, the thermal strain contri-

bution from the curing light was taken away.

As a result, the strain curve shows a drop at

the 40-second time interval. Contraction

strain continued to develop at a decreasing

rate after the curing light was turned off,

practically leveling off at 10 minutes.

Mean strain values and standard devia-

tions at 40 seconds and 10 minutes were

compiled (Table 3). Two-way ANOVA indicat-

ed that curing lights, types of composite, and

the composite*light interaction significantly

affected the postgel shrinkage (P < .05).

Vertical lines in Table 3 connect mean values

within each composite that were not signifi-

cantly different (pairwise comparisons,

P > .05 / 3 = .0167). Figure 5 shows the same

data in graphical form. All composites cured

with the soft-start QTH unit had significantly

less postgel shrinkage at 40 seconds. This

trend was maintained after 10 minutes,

except for Supreme, for which the difference

between the soft-start QTH and LED units

was not significant. The conventional QTH

light source created the highest strain values

in 3 of 4 composites evaluated.

Table 3 and Fig 5 also show the differ-

ences between composites cured with the

same light source. Z100 had the highest post-

gel shrinkage, followed by A110, Supreme,

and Z250. At 10 minutes, these values were

significantly different when the composites

were cured with the conventional QTH and

LED light sources. The differences among

Z100, A110, and Supreme were less when the

soft-start unit was used. Z250 consistently

showed the lowest postgel shrinkage values.

Table 3 Postgel shrinkage (mean SD microstrains; n = 15) at 40 seconds and 10 minutes after initial curing

Light-curing unitA110 Supreme Z100 Z250

Postgel shrinkage at 40 secondsQTH 1,784 71a 1,496 117b 2,242 163c 1,208 121d

LED 1,620 166e 1,328 56f 2,042 77g 1,290 86f

Soft-start QTH 1,153 131h 1,005 74i 1,384 327j 900 69i

Postgel shrinkage at 10 minutesQTH 2,754 81k 2,489 151l 3,045 168m 1,938 137n

LED 2,485 220o 2,263 78p 2,744 85q 2,010 108r

Soft-start QTH 2,014 175s 2,130 176s,t 2,196 347t 1,662 106u

Vertical lines connect results within each composite that are not significantly different. Same letter denotes mean values withineach light unit that are not significantly different. (Two-way ANOVA, pairwise comparisons; P > .0167).

Fig 5 Postgel shrinkage (mean and SD microstrains; n = 15) at 40 secondsand 10 minutes after the initial curing. Lowercase letters group values forcomposites that were not significantly different for the same curing light,while capital letters group values for curing lights that were not significant-ly different for the same composite (2-way ANOVA, pairwise comparisons;P = .0167).

A110 Supreme Z100 Z250

40 s

10 min

3,500

3,000

2,500

2,000

1,500

1,000

500

0

Sh

rin

ka

ge

str

ain

(1

0

6)

QTH LED Soft-startQTH







Cl i f ford et a l

Hardness resultsKnoop hardness profiles were determined at

various depths, as an indication of the

achieved degree of polymerization in the

cured composite (Fig 6). For all light sources

and composites, hardness values decreased

with increasing depth. In general, the hybrid

composite Z100 had the highest hardness

values, while the lowest values were found

for the anterior microfilled composite A110.

The differences between curing lights were

the largest for the nanofilled composite

Supreme and hybrid composite Z250.

The hardness values at the surface

(0 mm) and 2-mm depth were used for the

statistical analysis (Table 4). Vertical lines

connect mean values within each compos-

ite that are not significantly different (pair-

wise comparisons, P > .0167). Figure 7

shows the same data in graphical form. The

surface hardness values of 2 composites,

A110 with the lowest hardness values and

Z100 with the highest hardness values, were

not significantly affected by the different light

sources. The conventional QTH unit general-

ly produced the highest hardness values,

Fig 6 Knoop microhardness (mean and SD) of 4 composites cured by various light sources,measured 15 minutes after light curing (n = 5).

Kn

oo

p h

ard

ne

ss

Kn

oo

p h

ard

ne

ss

Kn

oo

p h

ard

ne

ss

Kn

oo

p h

ard

ne

ss

Depth (mm) Depth (mm)

Depth (mm) Depth (mm)

A110 Supreme

Z100 Z250

80

70

60

50

40

30

20

10

0

80

70

60

50

40

30

20

10

0

80

70

60

50

40

30

20

10

0

80

70

60

50

40

30

20

10

0

QTH

LED

Soft-start QTH

QTH

LED

Soft-start QTH

QTH

LED

Soft-start QTH

QTH

LED

Soft-start QTH

0 0.5 1 1.5 2 2.5 3 3.5 0 0.5 1 1.5 2 2.5 3 3.5

0 0.5 1 1.5 2 2.5 3 3.5 0 0.5 1 1.5 2 2.5 3 3.5




Cl i f ford et a l

especially in Supreme, where the difference

was significant. The soft-start QTH unit pro-

duced a significantly lower hardness value

in Z250. At 2-mm depth, the hardness was

generally less than half of the surface value.

Two-way ANOVA showed that curing lights,

types of composite, and composite*light

interaction significantly affected the hard-

ness (P < .05).

DISCUSSION

Clinicians, researchers, and dental industries

likewise perceive polymerization shrinkage,

which threatens the adhesive bond and

restoration longevity, as one of the most chal-

lenging properties of restorative com-

posites.1 Apart from improvements in the

resin matrix chemistry, curing light philoso-

phy and clinical techniques have brought

about some reduction in polymerization

Table 4 Microhardness (mean SD) at composite surface (0 mm) and at 2-mmdepth measured after 15 minutes postcuring

Light-curing unitA110 Supreme Z100 Z250

Microhardness of composite surface (0 mm)QTH 29.9 3.3 55.0 2.7 61.9 4.7 52.7 1.4LED 28.7 6.3 36.5 2.3 58.9 3.4 50.2 2.5Soft-start QTH 33.8 4.6 44.1 7.1 64.3 8.9 36.3 5.8Microhardness at 2-mm depthQTH 14.8 6.8 28.4 3.0 23.0 6.6 38.4 4.0LED 10.0 6.1 18.2 3.4 28.1 7.3 27.2 3.8Soft-start QTH 9.4 3.6 17.8 2.3 18.2 3.6 12.5 4.7

Vertical lines and bracket connect results within each composite that are not significantly different (2-way ANOVA, pairwise com-parisons; P > .0167).

Fig 7 Knoop microhardness (mean and SD) at composite surface (0 mm)and at 2-mm depth measured after 15-minute postcuring. Capital lettersgroup values for different curing lights that were not significantly differentfor the same composite (2-way ANOVA, pairwise comparisons; P > .0167).

A110 Supreme Z100 Z250

0 mm

2 mm

75

60

45

30

15

0

Kn

oo

p h

ard

ne

ss








Cl i f ford et a l

shrinkage stress development.4 Reduction in

shrinkage, however, should not compromise

other properties, especially the degree of

cure. Although poorly cured composite has

lower shrinkage, it will not attain its optimal

mechanical and biocompatibility properties.

Therefore, this study, in its assessment of

different curing lights, not only measured

polymerization shrinkage but also evaluated

hardness as a function of depth to assess the

degree of cure. Curing light source technology

develops rapidly. The units used in this study

have already been superseded by improved

models at publication time.15 However, the

objective of this study was to explore general

principles of curing light characteristics that

remain relevant irrespective of particular

curing light models. This was accomplished

by choosing different light source types with

similar outputs (see Table 1).

How do the curing lights differ in output?Light intensity is an important factor for the

performance of a curing light, which can be

easily determined using one of the many

available radiometers. Although the exact

interpretation of radiometer readings may be

less than straightforward, they are generally

considered acceptable for measuring curing

light output.24,25 In the current study, 2 evalu-

ated light-curing units emitted radiation from

a QTH lamp, which was filtered to a blue light

spectrum with a wavelength between 380

and 520 nm. The LED curing source used

junctions of doped semiconductors to gener-

ate blue light mainly in the wavelength range

of 440 to 490 nm. According to the manufac-

turer, the optimal match for the cam-

phorquinone photoinitiator (468 nm) ensures

that polymerization performance is similar to

that of a QTH unit, even though an LED unit

may record a lower light intensity on the

CureRite radiometer (EFOS).

It is important to note that besides the dif-

ferences in light spectra between curing

sources, each radiometer may also have its

own filter. The radiometer used in this study

contained a selective filter between 400 and

500 nm, according to its manufacturer.

Therefore, a higher curing light intensity

measured by a particular light meter does

not necessarily indicate a better light curing

source, because the filter may measure a

wider spectrum. Furthermore, different cur-

ing lights may have different thermal outputs

due to infrared radiation. It is well-known that

thermal effects can affect the rate of polymer-

ization reactions. In this study the tested com-

mercial curing lights and radiometer readings

were taken on face value.

Do the curing lights decreaseshrinkage stress?The clinical concern about polymerization

shrinkage is not so much the physical con-

traction but rather the development of resid-

ual stresses. In other words, shrinkage is

not the same as shrinkage stress. How

much shrinkage stress is generated

depends on many factors,26 such as

mechanical properties of the composite and

its substrates, cavity and substrate geometry,

bonding conditions, and of course polymer-

ization shrinkage. However, not all polymer-

ization shrinkage is relevant for the resulting

shrinkage stress.27 Only the so-called postgel

shrinkage (ie, the shrinkage after a composite

has become too rigid to relax stresses

through flow) is relevant for residual shrink-

age stresses. This postgel shrinkage can be

measured using a strain-gauge technique,

which excludes shrinkage that is not able to

generate stresses.7

Because polymerization shrinkage is the

result of the dimensional changes that take

place within the resin when its components

react and cross-link to form a polymer

network,1 it is not surprising that different

shrinkage values would be measured for the

different composites.16 Two-way ANOVA con-

firmed that difference in shrinkage strain of

each composite, averaged across the curing

lights, was highly significant (P < .0001).

Z100 and Z250, both hybrid composites,

had the highest and lowest shrinkage values

in the present study. The relatively high poly-

merization shrinkage of Z100 results from

the amount of a low molecular weight com-

ponent, triethylene glycol dimethacrylate

(TEGDMA). The shrinkage properties were

improved for Z250 by replacing TEGDMA

with higher molecular weight resins. The

anterior composite, A110, also resulted in




Cl i f ford et a l

relatively high polymerization shrinkage

because of the low molecular weight compo-

nent and low filler loading. The filler loading

of A110 is 40% by volume, compared to 60%

by volume filler loading for Z250. According

to the manufacturer, the nanofilled compos-

ite Supreme has the same resin system but a

slightly lower filler loading than Z250. The

reaction of nanofillers with curing light may

also have an effect on the resulting shrinkage

strain.

It was to be expected that different com-

posites would result in different shrinkage

values. The results of this study show that the

amount of postgel shrinkage, and thus

potential shrinkage stress, also vary for differ-

ent curing lights. Statistical analysis indicated

that the differences between composites

depended on the light source, and vice

versa. The conventional QTH unit, which

recorded the highest light intensity and total

energy, created the highest strain values in 3

out of 4 composites (see Table 3 and Fig 5).

Except in the hybrid composite Z250, the

LED unit was associated with lower shrink-

age strain than the conventional QTH light.

The soft-start QTH unit generated the least

amount of shrinkage strain. This outcome

seems to support the concept of slow-start

or ramped curing, which allows more time

for flow and stress relaxation before compos-

ite becomes solid.2830 The slower strain

development of the soft-start unit can

be seen in the initial segment of postgel

shrinkage curves in Fig 4. The disparity in

shrinkage between curing units was more

profound at 40 seconds than after 10 min-

utes (see Table 3 and Fig 5). The clinical sig-

nificance of this observation is that shrinkage

stress is most critical in the initial phase of

curing when the bonding between compos-

ite and cavity wall is not yet well-developed.

Do the curing lights cure the composites?A lower shrinkage strain value can be the

result of a better light-cure technique, where

postgel shrinkage is delayed, or it can be the

result of incomplete polymerization.31

Clinically, a high level of polymerization is

essential to attain the required physical prop-

erties and biocompatibility.1 Hardness is

often used to assess the achieved degree of

cure,32 which is justified based on its proven

correlation with degree of cure.20,22,23 It

should be reemphasized, however, that this

correlation between hardness and degree of

cure only applies within the same composite

group. Using hardness values to compare

the degree of cure between different com-

posites is invalid.

The results show that the highest hard-

ness values were at the composite surface,

while they decreased with increasing depth

from the exposed surface (Fig 6). The hard-

ness values at the composite surface (0 mm)

were not significantly different for A110 and

Z100 between the 3 light sources (see Table

4 and Fig 7), despite the differences in light-

intensity outputs. The hardness values may

have reached a saturated level at the surface,

indicating a complete cure. At 2-mm depth,

however, the achieved degree of cure was

consistently lower (28% to 73% of the sur-

face value), and the differences between the

3 curing lights became significant. With the

exception of Z100, composites cured with

the conventional QTH unit achieved the high-

est hardness (and thus degree of cure) and

the soft-start QTH unit the lowest. This sug-

gests that adequate surface hardness may

not ensure sufficient subsurface polymerization

of a restoration.

What is the relation betweenshrinkage stress and degree of cure?It is persuasive to speculate that there is a

correlation between shrinkage and hardness.

As discussed before, hardness correlates

with degree of cure. Because a well-cured

composite must have a higher density (and

thus total shrinkage) than an under-cured

composite, it seems intuitive that hardness

and shrinkage stress should also correlate.

However, shrinkage stress can vary even

when the composite has attained the same

degree of cure.7 Shrinkage stress is appar-

ently not directly related to the degree of

cure. Consequently, evaluating curing-light

units based on only degree of cure indicators

(such as hardness, density, and total shrink-

age) do not adequately assess all polymer-

ization effects that are important for clinical

assessments. Curing lights should thus also




Cl i f ford et a l

be tested for the development of residual

shrinkage stress effects.

Shrinkage stress has been shown to be

highly affected by the intensity of the initial

light exposure.4,33 High intensity values dur-

ing the first seconds of polymerization resulted

in higher postgel shrinkage strains, and thus

potentially higher shrinkage stresses.7 The

soft-start curing technique was proposed to

lower the initial intensity to reduce the devel-

opment of shrinkage stresses.2830 Because

it has also been shown that degree of cure

depends on the applied total light energy

(light intensity multiplied by the exposure

time),34 soft-start modes should not have to

compromise degree of cure if sufficient energy

is ensured, either by an increased final light

intensity or increased exposure time.8,9

The literature reports mixed results for the

effectiveness of soft-start curing. Besides refer-

ences that suggest positive effects for soft-

start light curing, others have reported that it

had no effect or even worsened marginal

adaptation or microleakage.3537 Although

these interfacial qualities are often associated

with shrinkage stresses, there are other factors

that more directly determine the quality of an

adhesive bond. The present study found that

the lowest postgel shrinkage was achieved

with the soft-start QTH unit. However, hard-

ness measurements indicated that the com-

posites did not attain the same degree of cure

with the soft-start QTH unit as with the other

curing sources. The lower hardness likely

resulted because the total light energy was

reduced by about 25% due to the exponential

soft-start profile (see Fig 3). Increasing the final

light intensity level or extending the total expo-

sure time beyond 40 seconds may eliminate

the differences between the acquired degree

of cure of the soft-start QTH light and the other

2 curing units.

CONCLUSION

This studys objective was to evaluate the

effect of different light characteristics found

in 3 commercially available curing lights on

the curing results of restorative composites.

Because degree of cure and shrinkage

stress are both crucial for clinical perform-

ance of composites, and since degree of

cure and shrinkage stress do not have a

direct correlation, both variables were

assessed through the combination of hard-

ness and postgel shrinkage measurements.

It was found that both the curing unit and the

type of composite significantly affected the

postgel shrinkage and hardness. The soft-

start QTH curing unit reduced postgel

shrinkage (and thus potential shrinkage

stress) in most of the composites tested but

produced optimal hardness only at the sur-

face of 2 composites. The conventional QTH

unit (highest light intensity output) usually

provided favorable hardness, but this was

associated with high postgel shrinkage. The

tested LED unit, with its intermediate light

intensity output, achieved intermediate val-

ues for both shrinkage strain and hardness.

The ideal conditions for a high degree of

cure and a low postgel shrinkage were not

easy to obtain together. This likely requires a

soft-start light cure followed by a higher inten-

sity or extended exposure time.

ACKNOWLEDGMENTS

This research was supported by the Minnesota Dental

Research Center for Biomaterials and Biomechanics and

by NIH grant 5T35DE07098.The authors thank Dr James

S. Hodges for his statistical advice.

REFERENCES

1. Ferracane JL. Current trends in dental composites.

Crit Rev Oral Biol Med 1995;6:302318.

2. Tantbirojn D, Versluis A, Pintado MR, DeLong R,

Douglas WH. Tooth deformation patterns in molars

after composite restoration. Dent Mater 2004;20:

535542.

3. Bouschlicher MR, Vargas MA, Boyer DB. Effect of

composite type, light intensity, configuration factor

and laser polymerization on polymerization con-

traction forces. Am J Dent 1997;10:8896.

4. Watts DC, al Hindi A. Intrinsic soft-start polymerisa-

tion shrinkage-kinetics in an acrylate-based resin-

composite. Dent Mater 1999;15:3945.

5. Emami N, Sderholm KJM, Berglund LA. Effect of

light power density variations on bulk curing prop-

erties of dental composites. J Dent 2003;31:189196.




Cl i f ford et a l

6. Soh MS, Yap AUJ, Siow KS. Post-gel shrinkage with

different modes of LED and halogen light curing

units. Oper Dent 2004;29:317324.

7. Versluis A,Tantbirojn D, Douglas WH. Distribution of

transient properties during polymerization of a

light-initiated restorative composite. Dent Mater

2004;20:543553.

8. Yap AUJ, Seneviratne C. Influence of light energy

density on effectiveness of composite cure. Oper

Dent 2001;26:460466.

9. Fan PL, Schumacher RM, Azzolin K, Geary R,

Eichmiller FC. Curing-light intensity and depth of

cure of resin-based composites tested according to

international standards. J Am Dent Assoc 2002;

133:429434.

10. Rueggeberg F. Contemporary issues in photocur-

ing. Compend Contin Educ Dent 1999;20(suppl

25):S4S15.

11. Stansbury JW. Curing dental resins and composites

by photopolymerization. J Esthet Dent 2000;

12:300308.

12. Kleverlaan CJ, de Gee AJ. Curing efficiency and heat

generation of various resin composites cured with

high-intensity halogen lights. Eur J Oral Sci 2004;

112:8488.

13. Asmussen E, Peutzfeldt A.Temperature rise induced

by some light emitting diode and quartz-tungsten-

halogen curing units. Eur J Oral Sci 2005;113:9698.

14. Mills RW, Jandt KD, Ashworth SH. Dental composite

depth of cure with halogen and blue light emitting

diode technology. Br Dent J 1999;186:388391.

15. Wiggins KM, Hartung M, Althoff O, Wastian C, Mitra

SB. Curing performance of a new-generation light-

emitting diode dental curing unit. J Am Dent Assoc

2004;135:14711479.

16. Price RBT, Felix CA, Andreou P. Effects of resin com-

posite composition and irradiation distance on the

performance of curing lights. Biomaterials 2004;

25:44654477.

17. Sakaguchi RL, Versluis A, Douglas WH. Analysis of

strain gauge method for measurement of post-gel

shrinkage in resin composites. Dent Mater 1997;13:

233239.

18. Felix CA, Price RBT. The effect of distance from light

source on light intensity from curing lights. J Adhes

Dent 2003;5:283291.

19. Watts DC, Amer O, Combe EC. Characteristics of vis-

ible-light-activated composite systems. Br Dent J

1984;156:209215.

20. Rueggeberg FA, Craig RG. Correlation of parameters

used to estimate monomer conversion in a light-

cured composite. J Dent Res 1988;67:932937.

21. Shin WS, Li XF, Schwartz B, Wunder SL, Baran GR.

Determination of the degree of cure of dental

resins using Raman and FT-Raman spectroscopy.

Dent Mater 1993;9:317324.

22. Ferracane JL. Correlation between hardness and

degree of conversion during the setting reaction of

unfilled dental restorative resins. Dent Mater 1985;

1:1114.

23. Santos GB, Medeiros IS, Fellows CE, Muench A, Braga

RR. Composite depth of cure obtained with QTH

and LED units assessed by microhardness and

micro-Raman spectroscopy. Oper Dent 2007;31:

7983.

24. Hansen EK, Asmussen E. Reliability of three dental

radiometers. Scand J Dent Res 1993;101:115119.

25. Dunne SM, Davies BR, Millar BJ. A survey of the

effectiveness of dental light-curing units and a

comparison of light testing devices. Br Dent J 1996;

180:411416.

26. Versluis A, Tantbirojn D, Pintado MR, DeLong R,

Douglas WH. Residual shrinkage stress distributions

in molars after composite restoration. Dent Mater

2004;20:554564.

27. Bowen RL. Properties of a silica-reinforced polymer

for dental restorations. J Am Dent Assoc 1963;66:

5764.

28. Goracci G, Mori G, Casa de Martinis L. Curing light

intensity and marginal leakage of resin composite

restorations. Quintessence Int 1996;27:355362.

29. Mehl A, Hickel R, Kunzelmann KH. Physical proper-

ties and gap formation of light-cured composites

with and without softstart-polymerization. J Dent

1997;25:321330.

30. Leinfelder KF. Ask the expert. What intensity is best

in light curing? J Am Dent Assoc 1999;130:534.

31. Yap AUJ, Soh MS, Siow KS. Effectiveness of compos-

ite cure with pulse activation and soft-start poly-

merization. Oper Dent 2002;27:4449.

32. de Jong LCG, Opdam NJM, Bronckhorst EM, Roeters

JJM, Wolke JGC, Geitenbeek B. The effectiveness of

different polymerization protocols for class II com-

posite resin restorations. J Dent 2007;35:513520.

33. Barros GKP, Aguiar FHB, Santos AJS, Lovadino JR.

Effect of different intensity light curing modes on

microleakage of two resin composite restorations.

Oper Dent 2003;28:642646.

34. Suh BI.Controlling and understanding the polymer-

ization shrinkageinduced stresses in light-cured

composites. Compend Contin Educ Dent 1999;

20(suppl 25):S34S41.

35. Sahafi A, Peutzfeldt A, Asmussen E. Soft-start poly-

merization and marginal gap formation in vitro. Am

J Dent 2001;14:145147.

36. Amaral CM, Peris AR, Ambrosano GM, Pimenta LA.

Microleakage and gap formation of resin compos-

ite restorations polymerized with different tech-

niques. Am J Dent 2004;17:156160.

37. Lindberg A, van Dijken JWV, Hrstedt P. In vivo inter-

facial adaptation of class II resin composite restora-

tions with and without a flowable resin composite

liner. Clin Oral Invest 2005;9:7783.


Text1: COPYRIGHT 2008 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART OF THIS ARTICLE MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER

Shrinkage and Hardness of Dental Composites2009

Documents

Transcript of Shrinkage and Hardness of Dental Composites2009