The Importance of Choosing the Right Laser Irradiation Parameters for Effective Laser Ablation of...

5/26/2018 The Importance of Choosing the Right Laser Irradiation Parameters for Effective Laser Ablation of Dental Hard Tissue

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University Of Salahaddin

College Of Science

Department Of Physics

The Importance of Choosing the Right Laser

Irradiation Parameters for Effective Laser

Ablation of Dental Hard Tissue

Prepared By

Ali Mahmood Ali

Supervised By

Eman E. Said

May

2014 A.D.


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Abstract

Various kinds of laser-dental hard tissue interaction mechanisms may occur by

altering the laser irradiation parameters(laser wavelength, pulse duration, fluence,

and intensity). The goals of this collection are to determine the most suitableinteraction mechanism for preparing dental hard tissues, and to determine the most

suitable laser type to do that interaction mechanism with less potential for

collateral damage tothe tissues surrounding the target tissue.


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TA B L E O F C O N T E N T S

C H A P T E R 1

The Energy That Causes the

Target Ablation: The Laser

1-1 Characteristics of Laser Systems 1

1-2 Parameters of Laser Beam 4

1-3 Laser delivery systems and modes of

operation 5

C H A P T E R 2

The Target Tissue That is Will

Be Exposed to Laser Beam: theTooth

2-1 Tooth Structure and Its Chemical

Composition 8

2-2 The Properties of Dental Hard

Tissues and the Thermal Relaxation

Time 10

2-2-1 Optical Properties 10

2-2-2 Thermal Properties 14

2-2-3 Thermal Relaxation Time 15

C H A P T E R 3

The Mechanisms of Laser-Tissue

Interaction and the Ablation of

Dental Hard Tissues

18

3-1 Photochemical Interaction 19

3-2 Thermal Interaction 20

3-3 Photoablation 24

3-4 Plasma-Induced Ablations 25

3-5 Photodisruption 26

Conclusion 28

References 30


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C H A P T E R 1

The Energy That Causes the Target Ablation: The Laser

1-1 Characteristics of Laser Systems

Transitions between two energy levels in an atom can occur by stimulated

absorption, spontaneous emission, and stimulated emission. Einstein, in 1917, was

the first to point out a third possibility, stimulated emission, in which an incident

photon of energy hvcauses a transition from upper state E1to lower state E01,Figure 1. The basis of laser system is based on this theory.

Figure 1: Schematic drawing of stimulated emission 3.

Laser energy is unique in that laser light is coherent this means that laser light has

four distinct properties that distinguish it from regular light. Ideal laser light is

monochromatic (composed of a single wavelength of light), collimated (the light

waves run parallel to each other instead of diverging), and uniphasic (the peaks

and valleys of the waves are synchronous [Figs. 2-1 and 2-2]). It is also extremely

intense 2.


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Figure 2-1: Regular light, showing the different wavelengths

present and the random spread of the beam. Laser

demonstrating uniform, coherent light. 2

Figure 2-2: Laser light, showing monochromatic wavelength,

collimation, and uniformity of phase, which constitute

coherent light. 2

An important result of these four properties is that laser light can be targeted with

great precision and is extremely powerful. Because the laser beam does not

diverge significantly over distance, the source can be positioned at great length

from the target tissue or can be very efficiently focused down to a small spot with

a convex focusing lens 2. Fundamental components of a laser energy producing

devices are illustrated in figure 3 andas follows:

a. Active lasing medium


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b. Energy source

c. Optical (Resonating) cavity

d. Cooling system

e. Delivery system. 3

Figure 3: Laser system illustration 3

The lasing medium can be a solid (e.g. Ruby laser), a liquid (e.g. Dye laser) or a

gas (e.g. Argon laser). The different types of laser are always named according to

the lasing medium. For example, the Nd:YAG laser has a solid lasing medium

which is a crystal of Yttrium Aluminum Garnet (YAG), doped with Neodymium

(Nd) . When the doping material is changed from Neodymium to Erbium (Er) for

the YAG laser, this laser is named the Er:YAG laser .The CO2laser has a carbon

dioxide gas as the lasing medium. The atoms or molecules of the lasing medium

are required to be excited in order to emit photons of laser light 4. The pump

delivers energy to the active medium so that there can be a population inversion.

Population inversion is an ensemble of atoms having a greater population density

in the upper state than the lower state 5. Usually pumping process is performed in

one of the following two ways:


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(i) Optically, i.e. by the continuous wave or pulsed light emitted by a powerful

lamp or by a laser beam. The light Pumping sources include flash-lamps for

producing (pulse current), arc lamps (continuous current) and other lasers (laser

pump). Flash lamps generate much higher intensities.

(ii) Electrically, i.e. by a continuous wave, radio-frequency, or pulsed current

flowing in a conductive medium such as an ionized gas or a semiconductor 6.

Of the main components of laser system is resonator cavity that surrounds active

lasingmedium and energy source for pumping. Photons generated in the cavity

travel inside the cavity and reflected by two parallel mirrors one on either pole of

lasing cavity, so that they can be reflected in between until a parallel beam profileis yielded. During excitation procedure, especially with high power laser systems,

also excessive amount of heat is generated that may rise up to levels that may

damage the system. Even low power laser systems may necessitate a cooling

system around the lasing medium and energy source 3.

1-2 Parameters of Laser Beam

The clinician can adjust many parameters of the laser instrument emission, except

the wavelength, which has its unique photon energy .these photon produce a tissue

effect, known in basic physics as work. The ability to perform work is termed

energy and is expressed as joules or milli joules. The measurement of the work

completed over time is called power, and is measured in watts. One watt equals 1

joule delivered for 1 second, and the power can be selected by the operator on

each device. As discussed in a subsequent section, unless set in a continuous

mode, lasers can produce multiple pulses of energy in one second. The length of

each pulse, called pulse width or pulse duration, can be a short as a few ten-

thousandths of a second on certain instruments. The word hertz describes pulses

per second. Each pulse of laser light can have a much higher peak power, which is

numerically expressed as the energy per pulse divided by the pulse duration. For

those lasers with millisecond pulse durations, individual pulses of hundreds orthousands of watts could be produced 7. Depending on the length of on/off periods


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(pulse duration/intervals between pulses) transferred energy varies, as well as

amplitude (Figure 4).

Figure 4: Transferred energy amount is related with pulse duration and intervals between twopulses. Given energy is related with magnitude of power (W) and pulse length in one pulse

cycle, and repetition rate (Hz) in unit time. 3

Though same amount of energy output is yielded; different pulse durations or

repetitions rates affect the target in different ways 3. Once the beam is focused, the

total energy it delivers is a function of the intensity of the beam, the time ofexposure, and the area affected. These are used to calculate the exposure dose

"Fluence 8. Fluence, expressed in joules per square centimeter, is also called

energy density; the term power density describes the watts per square centimeter 7.

1-3 Laser delivery systems and modes of operation

Scientific and commercial lasers produce highly collimated beams, but such a

beam is potentially dangerous in clinical situations 8. Therefore, the collimated

beam is directed to the target site by various delivery systems 9. And several

different delivery methods exist, depending upon wavelength, operating power,

desired spot size and accessibility of the target 10. The shorter wavelength

instruments (visible and near infrared radiation), like argon, diode, and Nd:YAG,have small, flexible glass fiber-optic delivery systems, with bare fibers that are


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usually used in contact with the target tissue 11.In addition,these fibers are small

in size (the diameter of fiber cores ranges from 5 to 500 m), which enables

minimally invasive surgical techniques (MIS) 12, 13.Unfortunately, not all

wavelengths (e.g., CO2, Er: YAG) (10600nm, 2940 nm) can be transmittedthrough the currently used quartz fiber-optic fibers 2. Therefore, some

manufacturers have chosen to use semiflexible hollow wareguides or rigid

sectional articulated arms to deliver the laser energy to the surgical site. Some of

these systems use additional small quartz or sapphire tips that attach to the

operating handpiece; other systems simply are used out of contact with the tissue.

In addition, the erbium family of dental lasers uses a water spray for hard-tissue

procedures11

. Although articulated arm delivery is functional for superficialtissues, it is less than glass fibers for deeper tissues or areas of difficult access,

such as the oral cavity, the hollow waveguidesystem has dramatically improved

the dentist's ability to provide convenient, precise delivery within the oral cavity 2.

There are two basic modes of laser emissions:

1. Continuous wave (CW) mode: in this basic mode the laser emits continuous and

constant laser energy as long as the device is activated 4. Continuous wave output

transfers energy to the target without interruption that creates high thermal effect

at collision site. Its mostly used to destroy the aimed target by loading excessive

energy and rise the temperature until its burned out (Figure 5a) 3. These lasers,

which emitted in CW mode,are sometimes equipped with a mechanical shutter

with a time circuit or a digital mechanism to produce gated or super-pulsed

energy. Pulse durations can range from tenths of a second to several hundred

microseconds 14.

2. Free-running pulse mode: this mode is produced by a flash lamp 7or by pulsed

current. With free-running pulse, very short bursts of laser energy, in the scale of a

few ten-thousandths of a second, emanate from the instrument (Figure 5b) 11.


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Figure 5: Working modes of laser. Line graphic show (a.) continuous wave and (b.) pulsed

beam profiles. 3

Longer pulses act like CW and tend to create more thermal damage on the target

and heat it easily; while short pulses create mechanical beat effect with less or

minor thermal damage like hammering. With this purpose shorter pulse rates

(ultrafast lasers) were introduced at microsecond (10-6 second), nanosecond (10-

9 second), picosecond (10-12 second) and femtosecond (10-15 second) ranges.

Depending on the type of laser system optically methods like mode locking or

pumping power modulation (gain switching) or Q switching techniques are used to

generate pulse mode operations 3.


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C H A P T E R 2

The Target Tissue That is Will Be Exposed to Laser Beam: the

Tooth

2-1 Tooth Structure and Its Chemical Composition

A clear understanding of laser and target tissue interactions enables the clinician to

choose the appropriate wavelength for specific procedures 9.The tooth tissues can

best be explained by describing some of their chemical, optical, and thermalproperties. The teeth are composed of enamel, pulp-dentin complex,and

cementum.Dentin forms the largest portion of the tooth structure, extending

almost the full length of the tooth. Externally, dentin is covered by enamel on the

anatomic crown and cementum on the anatomic root. Internally, dentin forms the

walls of the pulp cavity (pulp chamber and pulp canal[s]) (Figure 6) 8.

Figure 6: Human molar. 16


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The pulp contains nerves, blood vessels, fibroblasts, and lymphocytes, while the

mineralized organs (hard tissues) of the tooth include enamel, dentin, and

cementum. Enamel makes up the uppermost 12 mm of the tooth crown and

contains a high mineral content, giving it a high modulus but also making itsusceptible to cracking 17. Chemically, Enamel is the hardest biological substance

of human body and comprises 92% hidroxyapatite, 6% water and 2% organic

matter (insoluble protein fibers), while dentin composition is 47% Hidroxyapatite,

30% organic matter (strong meshwork of collagen fibers)and 23% water by

volume 18.The junction between enamel and dentine is called the amelodentinal

junction (ADJ). The degree of mineralization increases from the ADJ to the

surface of the tooth, and that deciduous teeth have a lower mineral concentrationand higher porosity than permanent teeth. Hidroxyapatite (HA) has the chemical

formula Ca10 (PO4)6 (HO)2, but substituents such as Cl-, F-,Na+, K+, Mg+2and

CO3-2exist in the crystal lattice. The last substituent, carbonate, is the most

important of all, representing 3% to 5% by weight. This mineral is organized in

hexagonal crystallites that have an average diameter of 40 nm and length of 40 nm

to 1 m (see Fig. 7), which in turn organize into larger structures, called the

enamel rods or prisms. The enamel rods have an average diameter ofapproximately 5 m, and extend from the enamel-dentine junction to the free

surface of the tooth, being approximately perpendicular to that surface 15.

(a) A schematic representation of a single crystallite.

(b) Electron micrograph of enamel crystallites. Organic material and water exist in the spaces

between the crystallites.

Figure 7: Enamel crystallites 15.


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Water and organic material are mainly located in micropores at the interface

between crystallites; the region of highest porosity is the boundary between

enamel rods, the rod sheath (figure 8) 15.

Figure 8: Illustration of an enamel cross-section 19.

2-2 The Properties of Dental Hard Tissues and the Thermal Relaxation

Time

2-2-1 Optical Properties

The optical properties of hard tissue play a major role in the ablation process 20.

Laser beams may reflect off, transmit through, scatter (break up) within, or be

absorbed by organic target tissue 2. When the laser light reflects off the surface

without penetration or interaction of the light energy with the tissue defined as

"Reflection" 4. The second effect is transmission, in this way, the beam enters the

medium, but there is no interaction between the incident beam and the medium.

The beam will emerge distally, unchanged or partially refracted 21. This effect is

also highly dependent on the wavelength of laser light. Water, for example, is

relatively transparent to the diode (810nm-980nm) and Nd:YAG (1064 nm)

wavelengths ( figure 10) 14. If some laser energy is absorbed into a component of


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the tissue, this is known as "Absorption". Lastly, when the laser light is scattered

within the tissue without producing a noticeable effect on the tissue, this is known

as "Scattering 4. Scattering of the laser beam could cause heat transfer to the

tissue adjacent to the surgical site, and unwanted damage could occur. However abeam deflected in different directions would be useful in facilitating the curing of

composite resin 14.

Figure 9: incident laser light interaction with tissue event possibilities. 21

Selecting the appropriate laser for a given procedure usually is a simple matter of

determining which laser wavelength is best absorbed by the target tissue while

producing the least reflection, scatter, and transmission 2. The total attenuation of

light intensity is then given by the extinction coefficient t= a+s22. For the

efficient and gentle laser ablation with minimal thermal side effects, a small

light penetration depth is needed 20. Therefore, for cutting the dental hard tissue

(enamel dentin and cementum), the suitable wavelength of the laser beam should

be maximally absorbed by hydroxyapatite (HA) and water 4. Regarding

wavelength absorption graphic in (figure 10), erbium (Er:YAG and Er:Cr:YSGG)

family and CO2lasers had a great affinity to water and hydroxyl-apatite crystal

and their photons leave their energy on HA and water. Therefore, health care

professionals dealing with tooth or bone in their operational practice or researchers

should prefer wavelengths between 2780 to 10600nm to obtain highest efficacy 3.


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Figure 10: Outline absorption coefficients (hydroxyapatite and water) relative to laserwavelength 21.

The CO2laser is strongly absorbed by the mineral of dental hard tissues near =9

m, due to the phosphate group of hydroxyapatite. The absorption coefficient of

dental enamel has been determined to be approximately 8000 cm at =9.6 m

and 5250 cm at =9.3 m, which is approximately 10 times higher than for the

conventional =10.6 m CO2laser wavelength used in medicine today and is

markedly higher than for any other laser wavelengths throughout the visible and

IR spectra (see table 3.1) 23. But, a very high (>> 1000 cm-1) absorption coefficient

like CO2laser near =9 m will imply that most of the energy will be absorbed in

a layer of material less than 10 m thick. Therefore, the depth of material ablated

per pulse will be somewhat small, which indicates that ablation will proceed

slowly unless high repetition rates are used. However, repetition rates may not be

raised indefinitely, since at some point heat accumulation will occur from one

pulse to the next and thermal damage will be more likely to occur. On the other

hand, if the material has an absorption coefficient lower than 100 cm-1, most of the

radiation will be absorbed in a layer over 0.1 mm thick, which clearly does not

enable us to obtain the necessary precision. This implies that the most adequate

wavelengths will have an absorption coefficient in enamel and dentine between


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100 and 1000 cm-1, perhaps slightly 15. Examples of this include the Er:YAG laser

at (2.94 m) and the CO2laser at (10.6 m) (see table 1).The Er:YAG (2.94

m)and the CO2(10.6 m)lasers are absorbed equally well by enamel (absorption

coefficient

800 cm-1

for both wavelengths ) and have similarly low reflectance(5 13%) in enamel.The reflectance of enamel varies widely in the mid-IR

spectral region, between a minimum of 5% near 3m to a maximum of 50% at 9.6

m. Scattering is known to be very small at CO2wavelengths. Other wavelengths

in this region were not investigated, although in general it is expected to be

small 15.

Table 1: Optical properties of human dental enamel obtained experimentally. 15

Wavelength Absorption

coefficient (cm)

Scattering

coefficient (cm)

Reflectance (%)

HeNe (633 nm) 66 66

Er:YSGG (2.79 m) 47747 52%

Er:YAG (2.94 m) 79585 51%

CO2(9.3 m) 5500 small 37.50.5

CO2(9.6 m) 8000 small 49.41

CO2(10.6 m) 81962

82525

small 13.20.2

The light attenuation in material with negligible scattering is described by the

Lambert-Beer law: 20

I (z) = I0e-z (1)

I0 , is the incident irradiance , is the linear optical absorption coefficient and I(z)

is the irradiance at the depth z in the absorber. The depth at which the irradiance

drops to the 1/e (~ 37%) level is called optical penetration depth d and is given

by: 20

d = 1 / a (2)


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2-2-2 Thermal Properties

Considering the laser irradiation in dental hard tissues, it is necessary to know andto understand the thermal behavior of these tissues when submitted to heating 24.

During the laser-tissue interaction, the main cause of the resulting thermal damage

is the heat transfer, which includes heat conduction, heat convection and heat

radiation. Usually, due to the moderate temperature achieved in most laser-tissue

interactions and the low perfusivity of most tissues, heat radiation and convection

can be neglected. Heat conduction is the primary mechanism by which heat is

transferred to unexposed tissue structures25

. Thermal parameters include thermaldiffusivity and thermal conductivity, which refer to the rate and amount of heat

diffusion through a medium, respectively. Thermal diffusivity is an important

thermo-physical parameter given by: 26

= / C (3)

Where is the thermal diffusivity, is the thermal conductivity, is the density

and C is the heat capacity26

. Several studies about thermal parametersmeasurement in hard dental tissues have been published. Results of these studies

are summarized in table 2.

Table 2: Thermal parameters of dental hard tissues (enamel and dentin) and water.24

Thermal parameter Enamel Dentin Water

Specific Heat (J/g C) 0.71 1.59 4.18

Thermal conductivity (W/cm C). 9.3410- 5.6910- 6.110-

Thermal diffusivity (cm/s) 4.69 10-3 1.8610-3 1.310-3

Although these thermal values are well-established in literature and can be used

for supporting clinical applications, it is important to consider that all parameters

were measured at room temperatures. In the moment of laser irradiation of dentalhard tissues, the temperature increase can lead several chemical and ultra-


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structural changes on enamel and dentin; as a consequence, the tissue thermal

characteristics of tissue may change during laser irradiation. Dentin and cementum

have higher water and organic compound percentage when compared to the

enamel and, due to this composition, they are more susceptible to heat storage thanthe enamel. Dentin has low thermal conductivity values and offers more risk when

lasers irradiate in deeper regions, considering that dentinal tubules area and

density increase at deepest regions, and subsequently, can easily propagate the

generated heat. As an example, considering the use of CO2lasers in dentistry

(wavelength of 9.6 m or 10.6 m), the absorption coefficient for dentin tissue is

lower than enamel due to its low inorganic content; also, the thermal diffusivity is

approximately three times smaller, which can lead a less heat dissipation amountand, as a consequence, can induce higher pulp heating 24. A rise in pulpal

temperature results in a hyperaemic reaction of the pulpal blood flow. This

intensified blood circulation is reversible if the intrapulpal temperature rise ranges

between 6-12C. If more than 12 C rise, an irreversible pulp necrosis occurs 27.

2-2-3 Thermal Relaxation Time

There are some other thermal parameters related to the heat propagation. The

thermalpenetration length (z thermal) is a parameter that describes the propagation

extension per time, and it isgiven by: 24

z thermal(t) =(4t)1/2 (4)

Whereis the tissue thermal diffusivity and tis the time. Other importantparameter is the thermal relaxation time ( thermal), which is obtained

mathematicallycorrelating the optical penetration length (from eq. 2) with thermal

penetration length (from eq.4) (if the laser spot is large compared to the depth of

penetration of radiation):24, 15

d = z thermal (5)

Then, t is called thermal relaxation time thermal


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1 / a= (4 thermal) (6)

thermal= 1 / 4.a2. (7)

The thermal relaxation time describes the necessary time to the heat propagatesfrom the surface of irradiation until the optical penetration length and is

particularly important when the intention is to cause a localized thermal damage,

with minimal effect in adjacent structures. This parameter can be interpreted as

follows: if the time of the laser pulse (p) is smaller than the relaxation time, the

heat would not propagate until a distance given by the optical penetration length d

(fig.11-a, fig. 11-b). So the thermal damage will happen only in the first layer

where the heat is generated. On the other hand, if the time of the laser pulse (p) ishigher than the relaxation time, the heat would propagate for multiple of the

optical penetration length d, resulting in a thermal damage in a bigger volume to

the adjacent structures (fig.11-c) 24. Therefore, the optimal pulse durations are

ultimately a function of the optical properties of the material. For high absorption

wavelengths, with absorption coefficients on the order of 100 to 10000 cm1, the

thermal relaxation times are on the order of milliseconds to tenths of

microseconds, as can be seen in Table 315

.

Table 3: Thermal relaxation time for various optical absorption coefficients. Calculated using

the material parameters for enamel: thermal diffusivity = 0.47 mm/s, density = 3100 kg/m,

specific heat = 880 J/kg/C. The thermal relaxation times for dentine are similar.15

Absorption coefficient

(cm)

Optical penetration

depth (m)

Thermal relax. time (s)

1 10000 50

10 1000 0.5

100 100 0.005

1000 10 5 10

10000 1 5 10


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Pulse durations longer than the values references in Table 3 will inevitably cause

more extensive thermal damage and will thus be less preferable than shorter

pulses, unless some sort of cooling mechanism such as water spraying is used 15.

Figure 11: Heat propagation in biological tissue. Optical penetration depth d is the depth at

which the irradiance drops to the 1/e (~ 37%) level (i.e., I = 0.37 I0).


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C H A P T E R 3

The Mechanisms of Laser-Tissue Interaction and the Ablation of

Dental Hard Tissues

Various kinds of interaction mechanisms may occur when laser light is applied to

biological tissue 28. For laser tissue interaction, there are mainly five categories of

interaction types:

1. Photochemical interactions

2. Thermal interactions

3. Photoablation

4. Plasma-induced ablation

5. Photodisruption 25

However, most interactions do not fall clearly into one of these categories, andoften multiple processes are competing with each other 25. All these seemingly

different interaction types share a single common datum: the characteristic energy

density ranges from approximately 1 J/cm to 1000 J/cm. It is surprising, that the

power density varies over 15 orders of magnitude. A double logarithmic map with

the five basic interaction types is shown in (fig.12) as found in several

experiments. The ordinate expresses the applied power density or irradiance in

W/cm2the abscissa represents the exposure time in seconds 28.


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Figure 12: Double logarithmic plot of the power density as a function of exposure time. Thecircles show the laser parameters required from a given type of interaction with biological

tissue 29.

3-1 Photochemical Interaction

The group of photochemical interactions originates from empirical observations

that light can induce chemical effects and reaction within macromolecules ortissues. Photochemical interaction occurs at very low power densities (typically 1

W/cm) and long exposure times in the range of seconds to CW 28. There are

photochemical effects that the laser can stimulate chemical reactions, such as the

curing of composite resin 14. Careful selection of laser parameters yields a

radiation distribution inside the tissue that is determined by scattering. In most

cases wavelengths in the visible range are used because of their efficiency and

their high optical penetration depths 10.


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3-2 Thermal Interaction

The thermal interaction is of primary importance for surgical applications10

.Energy of the photons is absorbed by the tissue and transformed into heat, and,

depending on heat propagation and deposition in tissues, the photothermal effects

originate 3. The basic parameters that govern thermal effects are summarized in the

following figure 13. Heat generation is determined by laser parameters and optical

tissue properties (irradiance, exposure time and the absorption coefficient, which

is a function of the laser wavelength). Heat transport is characterized by thermal

tissue properties such as heat conductivity and heat capacity. Finally, Heat effects;depend on the type of tissue and the temperature achieved inside the tissue 30.

Figure 13: Flow chart with important parameters for modeling thermal interaction. 30

Thermal interaction represents a large group of interaction types, where the

increase in local temperature is the significant parameters change. Thermal effects

can be induced by either CW or pulsed laser radiation 28. Thermal interactions

usually happen for pulse duration of s or higher 25. With photothermal effects,

there is no specific pathway, and the photons may be absorbed by any biomolecule

and still lead to a thermal effect 31. However, depending on the duration and peak

value of the tissue temperature achieved different effects like vaporization,carbonization, and melting may be distinguished 28. Vaporization at temperatures


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above 100 C leads to destruction of the cellular water. The increase in

temperature leads to an increase in pressure as water within the hydroxyapatite of

a dental hard tissue tries to expand in volume. This leads to localized

microexplosions and is thus sometimes referred to as a thermomechanical effect10

.Through this mechanism, whole tissue fragments are ejected and a hole is cut in

the tooth, with little or no alteration to the mineral itself (Fig.14) 32.The resulting

ablation is called thermal decomposition (thermomechanical ablation) and must be

distinguished from photoablation which is discussed in a following section. This

vaporization is sometimes advantageous, since the vapor generated carries away

excess heat and helps to prevent any further increase in the temperature of

adjacent tissue10

. Carbonization happens at temperature above about 150C andwill lead to a blackening in color 25. Finally beyond 300 C, melting can occur

depending on the target material 10. For example, Enamel surface melting requires

heating up to 12000C 33.

Figure14. Theoretical zones of tissue change associated with hard dental tissue exposure tolaser light 21.

For the ablation process to be effective and sparing for the surrounding tissue, a

fast energy deposition is obviously required. It is necessary to guarantee that the


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internal pressure build-up is faster than the heat diffusion. Only in this case will

the main part of the deposited energy be used for the ablation itself and the heat

will leave the tissue together with the ejected tissue particles (debris) and vapor. It

is obvious that the ablation efficiency directly influences the thermal sideeffects 20. In addition, to minimize the thermal damage of the surrounding dental

tissue during laser treatment, should be selected a wavelength that is preferentially

absorbed by the target tissue with a laser exposure duration that should be shorter

than the thermal relaxation time of the tissue 27. Therefore, CW or long-pulse

lasers are not appropriate to drill tunnels through enamel or dentine in a clinically

safe manner and with the necessary precision 15. The Continuous-wave lasers and

pulsed lasers with pulse durations in the microsecond range but higher than therelaxation time generate considerable heat in the region of the pulp chamber

during the irradiation process 27. The adjustment of repetition rate is important to

assure that the inter-pulse period is longer than the thermal relaxation time of

tissues; in this way, it is possible that the temperature of the irradiated tissues

decrease between laser pulses 34. Due to the dependence of the optical penetration

depth on the absorption coefficient athe thermal relaxation time becomes

proportional to thermal~a -2and is dependent on the wavelength. As a result, thethermal relaxation time is the shortest at the wavelength where absorption is the

strongest. Stronger absorption leads to steeper temperature gradients and, with

that, to faster heat diffusion 20. The Er:YAG (2.94 m)and the CO2 (10.6 m)lasers

which are absorbed equally well by enamel (absorption coefficient ~ 800 cmfor

both wavelengths)15, for enamel with thermal diffusivity = 4.710-3cm/s 24, the

thermal relaxation time for these lasers is (~8.3 s).Therefore, to achieve efficient

thermomechanical ablation with minimum damage to the surrounding dental

enamel tissue, the pulse duration of the Er:YAG and CO 2lasers must be

significantly less than 8.31 s. (This value is only estimation due to the variation

in the material constants of tooth tissue. In (Ivanenko et al.,2005) the values

estimated for the two thermal relaxation times from the literature material

constants are 11.3 s and 5.6 s for 2.94, and 10.6 m, respectively.) .

Theoretically, (lateral) thermal damage of tissues is limited when the laser

intensity is high and the interaction time is short: e.g., the Q-switched Er:YAG


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laser operating with a pulse length below the thermal relaxation time of the

irradiated tissue. However, (Dayem et al) found that nanosecond Er:YAG laser

pulses used at a high repetition rate and high power resulted in crack formation at

the edges or the bottom of the crater, because of an abrupt rise in temperatureand/or the stress transients induced In contrast, the free-running Er:YAG laser was

considered to be very effective, although the reduced energy deposition time of

short pulses results in smaller volume heating, and the threshold energy for

ablation decreases (see Table 4) 35. The cracks and fissures are certainly not

desirable since they may serve as an origin for the development of new decay.

Table 4: acoustical relaxation time for Each Er:YAG (2.94 m) and CO2(10.6 m) lasers.

[When the speed of sound (vS) in the enamel = 6500 m/s]36.

Wavelength Absorption coefficient

(cm)

Acoustic relax. time

ac(ns)

Er:YAG (2.94 m) 79585 15

2170 20

1.930.19

0.71

CO2(10.6 m) 81962

3080 20

1.880.14

0.5

A laser pulse whose duration is shorter than a= dABS/vSbuilds up mechanical

energy within the optically affected zone in the form of acoustic waves. This

condition is called the stress confinement condition 37. Under stress confinement

conditions, the stress transients are normally much higher than the quasi-static

thermal stress generated in the material and, consequently, mechanical damagesuch as cracks is much more likely to occur 15. One can conclude that, using

microsecond-long pulses, the thermal interactions dominate and the transient

stresses induced in the material are likely not playing a determinant role in

ablation 37, 15. With sub-microsecond laser pulses, photomechanical interactions

result from the conversion of laser energy into mechanical energy by the rapid

temperature increase 37, 15. Additionally a sufficient quantity of driving material

is necessary for the micro-explosions, i.e. enough tissue water 20. Because


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externally applied water, serves multiple purposes during ablation. The first

purpose, and easiest to understand, is that of a coolant: applied water is

indispensable to cool the ablation site for IR lasers. Secondly, water plays a role

during ablation, and influences the ablation rates15

.

3-3 Photoablation

In the case of UV laser radiation, the ablation mechanism is usually described as

photoablation or direct photoablation. This means the laser photons have enough

energy to break molecular bonds i.e. the photon energy is high enough to create

repulsive states in which the molecule breaks apart, thus causing tissue ablation 20.

At pulse durations in the range of nanoseconds, the typical threshold values of

power densities for this type of interaction are 107108W / cm230.Lasers emitting

in this region are ArF at 193 nm (6.4 eV), KrF at 248 nm (5.0 eV), XeCl at 308

nm (4.0 eV) and XeF at 351 nm (3.5 eV), among others. The absorbers in this

region will most likely be the covalent bonds existing in the organic material

making up enamel and dentine. Under these conditions the material decomposes

because the energy of the photons is sufficient to break existing covalent bonds, as

can be seen in Table 5. The excess energy not used in the dissociation of the

molecule remains in the molecular fragments as kinetic energy and these

subsequently leave the material 15.

Table 5: Dissociation energies of selected chemical bonds 39.


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The fact that the main absorber of UV radiation should be the organic material

existing in dental hard tissue, conjugated with its inhomogeneous distribution in

enamel and dentine, suggests that the mesostructure of the material will play a role

during ablation. It also indicates that the preferably ablated regions at theirradiated surfaces should be the dentinal tubule content, in dentine, and the rod

boundaries in enamel. This may have an influence on the surface morphology of

the irradiated site and, consequently, on the ability of the surface to bond to filling

materials subsequently to ablation. In general, ablation with UV wavelengths does

not lead to significant melting or carbonization, thus making these lasers

potentially good choices to drill long tunnels through enamel and dentine 15.

Although they provide very precise cuts there is a concern of potentiallydangerous mutagenic cell effects caused by the energetic UV photons 38.

Additionally, the ablation rates in the UV are extremely small, measuring typically

in the tens of nm/pulse regime 20. Furthermore, the UV pulse duration is rather

close to the acoustic relaxation time of materials, which means that it is possible

that these lasers induce mechanical damage (cracks) in the material, because of the

stress transients created 15.

3-4 Plasma-Induced Ablations

The laser creates numerous ionized molecules and free electrons. These ionized

molecules and electrons are ejected from the surface and form a localized cloud

called plasma (which reaches a state called optical breakdown) 37, 15. Picosecond

(p


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picosecond pulses are 1011W / cm2, whereas the corresponding electric field

amounts to approximately 107V / cm, which is comparable to the intramolecular

Coulomb electric fields and provides the necessary conditions for plasma

ionization30

. One should note that the depth of the layer in which most of theenergy is deposited is approximately 1 m. Kruger et al. investigated the outcome

of a = 615 nm femtosecond laser (pulse duration = 300 fs, repetition rate = 3 Hz,

fluence per pulse = 0.5 3 J/cm2, 100 pulses per spot) on enamel and dentine,

without water cooling, and found that both materials could be ablated very

precisely, with little or no evidence of melting, charring or cracking, and using

much lower fluencies (1 J/cm2) than those necessary with longer laser pulses 15.

3-5 Photodisruption

For pulse durations between 10 ps and 100 ns and high radiation intensity (1011

1016W/cm2) 15, optical breakdown is associated with shock wave formation 30.

When the laser pulse duration exceeds the onset of plasma formation, the plasma

further absorbs and scatters the incident laser energy, which consequently gives

rise to plasma shielding. As a result of this, the plasma expands rapidly and

eventually collapses. The rapid expansion of plasma can induce shock waves that

produce photomechanical fragmentation 37. Additionally, if the optical breakdown

takes place inside soft tissues or fluids, cavitation which occurs when focusing the

laser beam not on the surface of a tissue but into the tissue, and jet formation may

be observed. Since the effect appears mechanical impact, the most appropriate

term to use is disruption (Figure 15) 30.


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Figure 15: Schematic illustrations of shock wave induced ablation. (a) Theinitiation of optical breakdown (ionization process). (b) Plasma formation and its

shielding of the incident light in an early stage. (c) Plasma expansion

accompanied with generation of shock waves. (d) Photoacoustical ablation with

mechanical fractures inside the crater37

.

The shock waves in photodisruption are able to travel outside the optical

breakdown area, thus causing damage outside it also. This damage is, non-

intuitively, more intense with longer (nanosecond) pulses, where it can affect areas

on the order of millimeters, than with shorter pulses. The mechanism in question

at these pulse durations and irradiation intensities strongly suggests that these will

be less than adequate to ablate brittle materials such as dental hard tissues, because

of the risk of developing deep cracks 15.


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Conclusion

Dental hard tissue experiences ablation by doing thermaly, opticaly, or chemicaly

interaction with laser beam. A laser so as to be used instead of conventional tissue

removal methods, it it should be able to do somethings that they can not do. But,

neither of the types of laser-tissue interaction (photothermal, photochemical,

photo-optical, photomechanical) is entirely useful, and completely safe. Because

all of them can expose undesirable effect to the tissues surrounding the target

tissue. Therefore, before a laser being comparised with the conventional tissue

removaltechnique, must be determined that which one of the laser-hard tissueinteraction mechanisms have more good sides. The most importance one between

the desired effects which required from a laser in the processing the hard tissue is

decreasing the risk of damage to the surrounding tissues. For example, occuring

such event as (crack generation on the irradiated surfaces, carbonisation, pulp

necrosis, or DNA mutation in dental hard tissues) is never wanted. For this reason,

must be avoided from occring the UV specific photo ablation which causes

mutation in DNA and the photo-disruption which generates the shock waves,

which in turn are able to form cracks. The plasma mediated ablation mechanismand the photo-thermal interaction will remain. Although the plasma mediated

ablation technique has got very effective efficacies (it does not display thermal or

mechanical damage) , its ablation rate is very low which determines that the

operation duration will have increased for removing a great volme of target tissue,

which in turn is an undesired condition. n addition, the ablated surfaces appear to

be very smooth with this technique and thereby decreases the adhesion of

composite material to the tooth structure. Conversely, in photo-thermal effect,laser with an appropriate wavelength, pulse duration, and fluence produces micro-

explosions during hard tissue ablation that result in microscopic and macroscopic

irregularities increase the adhesion of the composite resin. The laser device, which

is operated in the plasma induced ablation technique, is very expensive 15. n many

directions, it seems that utilization of photo-thermal effect will be more suitable.in

order to benefit from photo-thermal effect, the law of thermal relaxation time must

be understood. This law function as association linkage between the opticalpenetration depth and the thermal penetration depth.therefore, this is exceedingly


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important in controlling thermal damage to the surrounding tissue. The Er:YAG

(at = 2.9 m) and the CO2laser (at = 10.6 m), which operate in free-running

mode, are deemed suitable lasers, which achieve photo-thermal interaction with

the dental hard tissue. Although they irradiate the dental hard tissue with pulseduration longer than the thermal relaxation time, an effect that can cause

undesirable thermal damage, this problem is solved by using laser supported with

water spray. The ablation rates of enamel by Er:YAG lasers are higher when a

water layer is previously applied to the ablation site. On the other hand, ablation

rates by the CO2laser at 9.6 m when water is used are lower than when water is

not applied, and it is likely that the same behavior is observed for = 10.6 m,

since these wavelength is highly absorbed by the mineral, the added water in effectdecreases the efficiency of the energy coupling to the material thus making

ablation less efficient 15.According to this collection, among the many laser-hard

tissue interaction mechanism that have been expressed, one of the most suitable is

the photothermal interaction. In addition, the free-running Er:YAG laser is the

most suitable one to achieve this interaction.


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Of Caries Prevention. Laser Physics, 19 (7), 14631469.

34- Matos, A., de Azevedo, C., da Ana, P., Botta, S., & Zezell, D. (2012). LaserTechnology for Caries Removal, Contemporary Approach to Dental Caries, Dr. Ming-Yu

Li (Ed.), ISBN: 978-953-51-0305-9, InTech, Available

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http://ebookbrowsee.net/lecture7-sph-618-pdf-d217392676http://ebookbrowsee.net/lecture7-sph-618-pdf-d217392676http://www.google.iq/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0CDYQFjAC&url=http%3A%2F%2Fwww.ucl.ac.uk%2Fmedphys%2Fstaff%2Fpeople%2Fbcox&ei=McFcU6joAoO74AS9noGIAw&usg=AFQjCNED1-dYZ4RmqeyZ4X8wg84-focfsw&bvm=bv.65397613,d.bGQhttp://www.intechopen.com/books/contemporary-approach-to-dental-caries/laser-technology-for-cariesremovalhttp://www.intechopen.com/books/contemporary-approach-to-dental-caries/laser-technology-for-cariesremovalhttp://www.intechopen.com/books/contemporary-approach-to-dental-caries/laser-technology-for-cariesremovalhttp://www.intechopen.com/books/contemporary-approach-to-dental-caries/laser-technology-for-cariesremovalhttp://www.google.iq/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0CDYQFjAC&url=http%3A%2F%2Fwww.ucl.ac.uk%2Fmedphys%2Fstaff%2Fpeople%2Fbcox&ei=McFcU6joAoO74AS9noGIAw&usg=AFQjCNED1-dYZ4RmqeyZ4X8wg84-focfsw&bvm=bv.65397613,d.bGQhttp://ebookbrowsee.net/lecture7-sph-618-pdf-d217392676http://ebookbrowsee.net/lecture7-sph-618-pdf-d217392676

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