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I 09 special _ laser I roots1_2011 pulsesisemittedwithadifferentpulserepetitionrate (improperlycalled‘frequency’)referredtoastheHertz rate (generally from 2 to 50 pulses) per second. The higheremissionrepetitionrateactsinasimilarwayto the CW mode, while the lower repetition rate allows foralongertimeforthermalrelaxation.Theemission frequency (pulse repetition rate) influences the aver- age power emitted, according to the formula shown in Table I. Another important parameter to consider is the ‘shape’ of the pulse, which describes the efficiency and the dispersion of the ablative energy in the form of thermal energy. The length of the pulse, from microseconds to milliseconds, is responsible for the principal thermal effects. Shorter pulses, from a few microseconds(<100)tonanoseconds,areresponsible for photomechanical effects. The length of the pulse affects the peak power of each single pulse, accord- ingtotheformulainTableI.Dentallasersavailableon the market today are free-running pulsed lasers, the Nd:YAG with pulses of 100 to 200µs and the Erbium lasers with pulses of 50 to 1,000µs. Furthermore, diode lasers emit energy in CW that can be mechani- cally interrupted to allow the emission of energy with pulse duration of milliseconds or microseconds depending on the laser model. Effectsoflaserlightonbacteriaanddentinalwalls In endodontics, lasers use the photo-thermal and photomechanical effects resulting from the interaction of different wavelengths and different parameters on the target tissues. These are dentine, the smear layer, debris, residual pulp and bacteria in all their various aggregate forms. Using different outputs, all the wavelengths destroy the cell wall due to their photo-thermal effect.Becauseofthestructuralcharacteristicsofthe different cell walls, gram-negative bacteria are more easily destroyed with less energy and radiation than gram-positive bacteria.16 The near infrared lasers are not absorbed by hard dentinal tissues and have no ablative effect on dentinal surfaces. The thermal effect of the radiation penetrates up to 1mm into the dentinal walls, allowing for a decontaminating effectondeeperdentinelayers.8 Themediuminfrared lasers are well absorbed by the water content of the dentinal walls and consequently have a superficial ablative and decontaminating effect on the root- canal surface.8,16 Thethermaleffectofthelasers,utilisedforitsbac- tericidal effect, must be controlled to avoid damage to the dentinal walls. Laser irradiation at the correct parametersvaporisesthesmearlayerandtheorganic dentinal structure (collagen fibres) with characteris- tics of superficial fusion. Only the Erbium lasers have a superficial ablative effect on the dentine, which appears more prevalent in the intertubular areas richerinwaterthaninthemorecalcifiedperi-tubular areas.Whenincorrectparametersormodesofuseare employed, thermal damage is evident with extensive areas of melting, recrystallisation of the mineral matrix (bubble), and superficial microfractures con- comitantwithinternalandexternalradicularcarbon- isation. With a very short pulse length (less than 150µs), the Erbium laser reaches peak power using very low energy (less than 50mJ). The use of minimally ablative energy minimises the undesirable ablative and thermal effects on dentinal walls while the peak power offers the advantage of the phenomena of water molecule excitation (target chromophore) and the successive creation of the photomechanical and photoacoustic effects (shock waves) of the irrigant solutionsintroducedintherootcanalonthedentinal walls.Theseeffectsareextremelyefficientincleaning the smear layer from the dentinal walls, in removing the bacterial biofilm and in the canal decontamina- tion, and will be discussed in Part II.10–13_ Editorial note: A complete list of references is available fromthepublisher. Fig. 4_Methods of laser light emission. Prof Giovanni Olivi University of Genoa DI.S.TI.B.MO Department of Restorative Dentistry Genoa,Italy Private Practice Piazza F.Cucchi,3 00152 Rome Italy olivi.g@tiscali.it _contact roots continuous wave mode gated mode pulsed mode Fig. 4