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RO0111

08 I I special _ laser The diode laser (from 810nm to 1,064nm) and the Nd:YAG (1,064nm) belong to the near infrared region of the electromagnetic spectrum of light. They interact primarily with soft tissue by diffusion (scattering). The Nd:YAG laser has a greater depth of penetration in soft tissues (up to 5mm), while the diode laser is more superficial (up to 3mm). Their beam is selectively absorbed by haemoglobin, oxy- haemoglobin and melanin, and has photo-thermal effects on tissue. Therefore, their use in dentistry is limitedtothevaporisationandincisionofsofttissue. They are also used for dental whitening with a laser beam, by thermal activation of the reagent. In endo- dontics, they currently represent the best system for decontamination, owing to their ability to penetrate the dentinal walls (up to 750µ with the 810nm diode laser;upto1mmwiththeNd:YAG)8 andfortheaffin- ity of these wavelengths with bacteria, destroying them through photo-thermal effects.16 TheErbiumlasers(2,780nmand2,940nm)belong to the medium infrared region and their beam is pri- marily absorbed superficially by soft tissue between 100and300µandupto400µbythedentinalwalls.8,17 The chromophore target is water, which is why theiruseindentistryextendsfromsofttohardtissue. Owing to the water content of the mucosa, gingiva, dentineandcarioustissue,Erbiumlasersvaporiseand affect these tissues thermally. The explosion of the watermoleculesgeneratesaphotomechanicaleffect that contributes to the ablative and cleaning process (Fig. 3).18–20 Parameters that influence the emission of laser energy Laser energy is emitted in different ways with various instruments. In diode lasers, the energy is emittedinacontinuouswave(CWmode).Amechan- ical interruption of the energy emission is possible (properly called ‘gated’ or ‘chopped’ and improperly called‘pulsed’),allowingforbettercontrolofthermal emission. The pulse duration and intervals are in milliseconds or microseconds (time on/off). TheNd:YAGlaserandtheErbiumfamilyemitlaser energy in a pulsed mode (also called free-running pulse),sothateachpulse(orimpulse)hasabeginning time, increase and an end time, referred to as a Gaussian progression. Between pulses, the tissue has time to cool (thermal relaxation time), allowing for better control of thermal effects (Fig. 4). The Erbium lasers also work with an integrated water spray, which has the double function of both cleaning and cooling. In the pulse mode, a string of Fig. 3_Coefficients of tissue absorption. roots1_2011 Table I_Laser light emission parameters. P power (in W) E energy (in J) R pulse repetition rate (in Hz) Pd power density or density of power (in W/cm2) F fluence or density of energy (in J/cm2) P(W) average power = E x R PP(W) peak power = E; length of single pulse (in seconds) Table I Fig. 3