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I 07 special _ laser I roots1_2011 efficiency of lasers in combination with commonly used irrigants, such as 17% EDTA, 10% citric acid and 5.25% sodium hypochlorite.9 The action of the chelating substances facilitates the penetration of laserlight,whichcanpenetrateintothedentinalwalls up to 1mm in depth and have a stronger decontami- nating effect than chemical agents.8,9 Other studies have investigated the ability of certain wavelengths to activate the irrigating solutions within the canal. This technique, which is termed laser-activated irrigation, has been shown to be statistically more effective in removing debris and the smear layer in rootcanalscomparedwithtraditionaltechniquesand ultrasound.10–12 ArecentstudybyDiVitoetal.demon- stratedthattheuseoftheErbiumlaseratsubablative energy density using a radial and stripped tip in combination with EDTA irrigation results in effective debris and smear layer removal without any thermal damage to the organic dentinal structure.13 _Electromagnetic spectrum of light and laser classification Lasers are classified according to their location on the electromagnetic spectrum of light. They can bevisibleandinvisible,near,mediumandfarinfrared laser. Owing to optical physics, the function of the various lasers in clinical use differs (Fig. 1). In the visible spectrum of light, the green light laser (KTP, aneodymiumduplicateof532nm)wasintroducedin dentistryinrecentyears.Therehavebeenfewstudies concerning this wavelength. Its delivery through a flexible optical fibre of 200µ allows its use in endo- dontics for canal decontamination and has shown positive results.14,15 Near infrared lasers (from 803nm to 1,340nm) were the first to be used for root decontamination. In particular, the Nd:YAG (1,064nm), introduced at the beginningofthe1990s,deliverslaserenergythrough an optical fibre.5 The medium infrared lasers, the Erbium (2,780nm and 2,940nm) laser family, also produced at the beginning of the 1990s, have been equipped with flexible, fine tips only since the begin- ning of this century and have been used and studied in endodontic applications. The far infrared laser CO2 (10,600nm) was the first to be used in endodontics for decontamination and apical dentine melting in retrograde surgery. It is no longer used in this field with the exception of vital pulp therapy (pulpotomy and pulp coagulation). The lasers considered here for endodontic applications are the near infrared laser— diode (810, 940, 980 and 1,064nm) and Nd:YAG (1,064nm)—andthemediuminfraredlasers—Erbium, Chromium:YSGG(Er,Cr:YSGG;2,780nm)andErbium: YAG (2,940nm). A brief introduction to the basic physics of laser–tissue interaction is essential for understanding the use of lasers in endodontics. _Scientific basis for the use of lasers in endodontics Laser–tissueinteraction Theinteractionoflightonatargetfollowstherules of optical physics. Light can be reflected, absorbed, diffused or transmitted. _Reflection is the phenomenon of a beam of laser light hitting a target and being reflected for lack of affinity. It is therefore obligatory to wear protective eyewear to avoid accidental damage to the eyes. _Absorption is the phenomenon of the energy inci- dent on tissue with affinity being absorbed and thereby exerting its biological effects. _Diffusion is the phenomenon of the incident light penetrating to a depth in a non-uniform manner with respect to the point of interaction, creating biological effects at a distance from the surface. _Transmission is the phenomenon of the laser beam being able to pass through tissue without affinity and having no effect. The interaction of laser light and tissue occurs when there is optical affinity between them. This interaction is specific and selective based on absorp- tion and diffusion. The less affinity, the more light will be reflected or transmitted (Fig. 2). Effectsoflaserlightontissue The interaction of the laser beam on target tissue, via absorption or diffusion, creates biological effects responsible for therapeutic aspects that can be summarised as: _photo-thermal effects; _photomechanical effects (this includes photo- acoustic effects); and _photochemical effects. Fig. 2_Laser–tissue interaction. Fig. 2