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Like normal light, laser light can interact with tissue in four basic ways1 as follows:
(1) Reflection: some light reflects back off the surface, its energy neither penetrating nor interacting with tissue.
(2) Transmission: some (light) may be transmitted through tissue, albeit unchanged as if transparent to the laser beam and without interaction between the incident beam and the tissue.
(3) Scatter: some light may penetrate the tissue and be scattered without causing a noticeable effect on the tissue2 .Scattering causes some lessening of light energy with distance, together with distortion in the beam, whereby rays proceed in an uncontrolled direction through the medium. Moreover, back-scatter can occur as the laser beam hits the tissue, most commonly in short wavelengths, e.g. diode, Nd:YAG (≥50% back-scatter).
(4) Absorption: some light may be absorbed into a component of the tissue, whereby there will be transference of energy to the tissue, i.e. the incident energy of the beam is attenuated by the medium and transferred into another form. In clinical dentistry, depending on the value of the energy, there is conversion into heat or, in the case of very low values, photobiostimulation of receptor tissue sites (e.g. sun-bathing - the stimulation of ‘tanning' melanocytes by low-grade UV sunlight versus the damaging sun-burn with higher exposure values)
Laser wavelength absorption and tissue composition
Laser tissue interactions, as described above, are not exclusive and occur in varying proportions within tissues depending on the chemical and or molecular variation found within such complex biological systems. The degree of interaction is usually proportional to the level of absorption of a particular wavelength by tissue. Tissue elements that absorb a particular wavelength or spectrum of light energy to a high degree are called chromophores. All (organic) matter has the property of ‘absorption specificity' which determines how it reacts to incident radiation. Indeed, the preferential absorption of specific wavelengths of radiant energy by chromophores within tissues accounts for the unique interactions that occur between the monochromatic light energy of lasers and various tissue elements. Laser wavelengths thus affect certain, inter-related components of the target tissue, that is: its water content; colour; and chemical composition. In dentistry, oral tissue comprises one or more chromophores - haemoglobin, melanin and allied pigmented proteins, (carbonated) hydroxyapatite, and water. Generally speaking, any predominantly pigmented tissue absorbs shorter laser wavelengths (i.e. visible and near infra-red), whereas non-pigmented tissue absorbs longer wavelengths. Consequently, absorption peaks of water and (carbonated) hydroxyapatite, coincident with Er:YAG, Er:YSGG and CO2 wavelengths, would support the potentially advantageous use of these lasers in hard tissue management. Moreover, oral soft tissues mainly comprise water, which predominantly controls the tissue effects of laser emissions within the infrared spectrum, such as CO2. Therefore, CO2 laser energy is absorbed very efficiently by tissue fluids with minimal penetration beyond the surface2. Conversely, water is comparatively transparent to the emission of the Nd:YAG laser, which accounts for its tendency to penetrate deeper into tissue. In this way, whereas CO2 wavelength might penetrate oral epithelia to a depth of 0.1-0.2 mm, Nd: YAG and diode wavelengths can result in an equivalent-power penetration of 4-6 mm.3
Light Absorption in Tissue
Absorption characteristics for various wavelengths in four absorption media (oxyhaemoglobin, melanin, hydroxyapatite and water). The absorption coefficient is plotted as a function of the wavelength, and the absorption coefficient for a given material is plotted on this graph. A high absorption coefficient means the given laser wavelength is well absorbed in the selected medium. A low absorption corresponds with a greater degree of transparency allowing the light to penetrate deeper into the medium. Note that the vertical scale is logarithmic; that is, each grid line is equivalent to a change of the absorption coefficient by 1 order of magnitude (factor 10).
The overriding beneficial effect of laser energy is absorption of the light by the target tissue and the transfer of laser energy, thus causing a tissue interaction (Photobiological Effect). There are four basic interactions that can occur following absorption of laser energy:
(1) Photochemical (Photochemolysis): certain wavelengths of laser light are absorbed by naturally occurring chromophores or wavelength- specific light absorbing substances that are able to induce certain biochemical reactions at cellular level. Derivatives of naturally occurring chromophores or dyes have been used as photosensitizers to induce biological reactions within tissues for both diagnostic and therapeutic applications. Photochemical interactions include photobiostimulation, photodynamic therapy, and tissue fluorescence. Certain biological pigments, upon absorbing laser light, can fluoresce, which can be used for detecting teeth caries. Lasers can also be used in a non- surgical mode for biostimulation or more rapid wound healing, pain relief, increased collagen growth and a general anti- inflammatory effect. Photodynamic interaction is demonstrated by PAD (Photo-Activated Disinfection) in which a 635nm laser used to activate a dye solution of tolonium chloride placed in a carious cavity or root canal. Activation of the tolonium chloride releases oxygen species which disrupt the membranes of micro-organisms found in caries, periodontal pockets and root canals.
(2) Photothermal (Photothermolysis): light energy absorbed by the tissues is transformed into heat energy which then produces tissue effects as follows:
- Coagulation and haemostasis: from 60oC to 70oC, this is the secondary effects through conduction of the heat generated.
- Photopyrolysis: from 65oC to 90oC, target tissue proteins undergo permanent morphological change (protein denaturation) as result of dissociation of covalent bonds.
- Photovaporolysis: at 100oC +, inter- and intra-cellular water in soft tissue and interstitial water in hard tissue is vaporised. This destructive phase transfer results in expansive volume change, which can aid the ablative effect of the laser by dissociating large tissue elements. This will be carried onto a further phase: transfer to hydrocarbon gases and production of residual carbon (carbonization).4
The amount of laser energy absorbed by the tissue largely determines the thermal interaction produced and is in turn dependant on the wavelength of the laser light to a great degree, but also on other parameters such as spot size, power density, pulse duration and frequency, and the optical properties and composition of the tissue irradiated. The CO2 (10600nm) is highly absorbed by the water content of oral soft tissues, whereby 90% of the energy is absorbed within the first 100 microns of penetrating the tissue surface5. Hence, even at relatively low power densities using a focused beam, there is rapid tissue vaporization of the water with charring and burning of the organic content of the tissue.
Photothermal interaction causes the irradiated target tissue to absorb the laser energy and converts it into heat, thereby producing a direct temperature rise in the irradiated tissue volume. When this energy is applied for long enough, heat conduction will cause a temperature rise in surrounding tissues as well. Hence, thermal effects, such as coagulation necrosis, are produced indirectly in collateral areas and are one of the mechanisms responsible for haemostasis when cutting or vaporizing with a laser.
(3) Thermal relaxation
Heat dissipation or diffusion from the irradiated tissue site will determine the extent of collateral damage seen and is largely dependant on the thermal conductivity of the tissue. The time required for diffusion of the heat or ‘thermal relaxation time' is defined as the time required for the accumulated heat energy within the tissue mass to cool to 37% of its original value6. The degree of heat conduction and rate of tissue cooling both determine the extent of collateral tissue damage for a given wavelength of laser light and tissue type. The composition of the tissue in terms of its structure, water content and vascularity will greatly determine heat conduction/tissue cooling and therefore collateral damage. Moreover, factors such as the volume and surface area of tissue irradiated will also influence the rate of heat dissipation.
With continuous laser emission there is no thermal relaxation time, but with pulsed emissions there are brief periods of time allowing for heat dissipation or cooling between pulses7. Tissues should be allowed a period of cooling approximately three times their thermal relaxation time to avoid accumulation of heat energy in surrounding tissue and therefore collateral damage. This can be managed effectively using a combination of appropriate power density and pulse duration for the desired procedure8, 9.
Factors that influence thermal relaxation are summarized as follows:
- Laser absorption characteristics of the target tissue
- Laser emission mode : continuous wave or pulsed emission
- Laser incident power
- Laser power density
- Beam movement: relative to tissue site; rapid laser beam movement will reduce heat build-up and aid thermal relaxation.
- Endogenous coolant: water content and vascularity of the tissue.
- Exogenous coolant: water, air, pre-cooling of tissue.10, 11
(4) Photomechanical and photoelectrical:
These are non- thermal interactions produced by high energy, short pulsed laser light, including: photodisruption, photodisassociation, photoplasmolysis and photoacoustic interaction. Absorption of laser energy pulses results in rapid expansion or generation of shock waves that are capable of rupturing intermolecular and atomic bonds (photo-disruption or photodisassociation ). Thus, the laser beam's energy is transformed into vibration or kinetic energy. A pulse of laser energy on hard dentinal tissues can produce a shock wave, which might explode or pulverize the tissue, creating an abraded crater. This is an example of the photoacoustic effect of laserlight.12
Photoplasmolysis is a process of tissue removal through the formation of electrically charged ions and particles that exist in‘plasma' state, a semi-gaseous, high -energy state which is neither solid, liquid, or gas.13 This process is observed in ultra-short pulsed lasers, e.g. Nd: YAG, Er:YAG, with pulse widths of <100 μs. Ionization of atoms occurs at very high-energy densities followed by plasma formation. The plasma state is maintained by the absorption of energy from the incident laser beam and, through electron vibrations, causes the rapid expansion and contraction that produces the disruptive shock waves that break apart target materials in photoplasmolysis. Photoplasmolysis is achieved photonically in soft tissue and thermionically in hard tissue and is characterised by flashes and popping sounds during laser use. Plasma formation can be beneficial, in that extremely high ablative energies can be produced, but also disruptive as it can ‘shield' the target from further incident light, through the phenomenon of a plasma acting as a ‘superabsorber' of electromagnetic radiation. However, photoplasmolysis is considered to be a rare occurrence at the therapeutic levels of laser power as used in dental procedures.
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