Laser Tissue Interaction

optical maser Tissue InteractionLaser-Tissue interactionLike normal shadowy, ocular maser set about grass interact with interweave in four basic ways1 as follows(1) Reflection some settle reflects back off the surface, its elan vital neither penetrating nor interacting with weave.(2) Transmission some ( argus-eyed) whitethorn be transmitted through meander paper, albeit unchanged as if transparent to the laser commit and without interaction between the hazard strike and the tissue.(3) Scatter some light may sound the tissue and be scattered without causing a noticeable printing 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 drop dead as the laser beam hits the tissue, most commonly in short wavelengths, e.g. diode, NdYAG (50% back-scatter).(4) Absorption some light may be absorbed into a component o f 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 foster of the energy, there is conversion into commove or, in the case of very low values, photobio excitant of receptor tissue come ins (e.g. sun-bathing the stimulation of tanning melanocytes by low-grade UV sunlight versus the damaging sun-burn with gamyer exposure values)Laser wavelength submergence and tissue themeLaser tissue interactions, as described above, are not exclusive and give in varying proportions indoors 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 immersion 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 ducking specificity which cracks 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 and so affect certain(p), inter-related components of the fanny tissue, that is its water content comment 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, coinciding with ErYAG, ErYSGG and CO2 wavelengths, would support the potentially advantage ous use of these lasers in hard tissue management. Moreover, oral light tissues mainly comprise water, which predominantly controls the tissue effects of laser emissions within the infrared light 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 NdYAG 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.3Light Absorption in TissueAbsorption 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 habituated material is plotted on this graph. A high absorption c oefficient means the given laser wavelength is hearty 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).Photobiological EffectThe 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 riveting substances that are able to induce certain biochemical reactions at cellular level. Derivatives of naturally occurring chromophores or dyes have been used as photosensi tizers 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, put out 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, periodontic pockets and root canals.(2) Photo caloric (Photothermolysis) light energy absorbed by the tissues is transformed into heat energy which then affirms tissue e ffects as follows Coagulation and hemostasia 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 mischievous 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 mathematical product of residual carbon (carbonization).4The amount of laser energy absorbed by the tissue largely determines the thermic interaction produced and is in turn dependant on the wavelength of the laser light to a great degree, scarce also on other parameters such as spot size, power dumbness, shiver duration and frequenc y, 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 drying up 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 relaxationHeat dissipation or diffusion fr om 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 delineate as the time required for the accumulated heat energy within the tissue mass to serene 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 pulse s7. 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 followsLaser absorption characteristics of the target tissueLaser emission mode continuous wave or pulsed emissionLaser incident powerLaser power density Beam movement relative to tissue site rapid laser beam movement will reduce heat build-up and aid thermal relaxation. endogenetic coolant water content and vascularity of the tissue.Exogenous coolant water, air, pre-cooling of tissue.10, 11(4) Photomechanical and photoelectricalThese are non- thermal interactions produced by high energy, short pulsed laser light, including photodisruption, photodisassociation, photoplasmolysis and photoacoustic inter action. Absorption of laser energy pulses results in rapid expansion or coevals of shock waves that are capable of rupturing intermolecular and atomic bonds (photo-disruption or photodisassociation ). Thus, the laser beams 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 member of tissue removal through the formation of electrically charged ions and particles that exist inplasma 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, ErYAG, with pulse widths of