The erbium lasers are solid-state lasers produced with different types of matrix crystals. Some of them, such as Er:YAG (X = 2.94 |im), Er,Cr:YSGG (X = 2.78 |im), Er:YLF (X = 2.81 ^m), Er:YAG (X = 2.73 ^m) and CTE:YAG (X = 2.69 ^m), were already studied for ablation of dental hard tissues (Altshuler et al., 1994). From all of them, the most popular and with commercially available equipments for dentistry are Er:YAG and Er,Cr:YSGG.
Comparing the absorption of Er:YAG with Er,Cr:YSGG lasers by dental hard tissues, it is possible to observe that Er:YAG have a strong interaction with OH- from water molecules contained in the teeth, while Er,Cr:YSGG is better absorbed by water and OH- contents of hydroxyapatite (Figure 2)(Ana et al., 2006). Due to this fact, Er:YAG promotes surface temperatures up to 300o C at the ablation threshold, and Er,Cr:YSGG reaches 800o C during ablation of enamel (Fried et al., 1996).
Although it have been tested some erbium lasers operating in the Q-switched mode (with pulse duration in the range of ns) (Fried, 2000), the commercially available erbium lasers operate in free running mode, with pulse duration of 150-400 ^s. This pulse duration is shorter than the thermal relaxation time of dental hard tissues (< 1ms) (Niemz, 2004), which provide heat dissipation during ablation and avoid excessive heat transmission to the pulp, for instance. However, at higher energy per pulses, erbium lasers can induce thermal injuries to dental hard tissue, such as the presence of microcracks, melting or even carbonization. In this way, during the clinical application, it is essential to use the correct set up of laser parameters and the use of an adequate air-water spray to provide proper refrigeration and avoid these side effects. However, although the presence of a thin layer of water can increase the ablation process (Fried at al., 2002), an excessive water layer can decrease erbium interaction with dental hard tissues (Niemz, 2004); in this way, the use of saliva suction is recommended.
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Fig. 2. Absorbance of water and hydroxyapatite and their relation with Er:YAG and Er,Cr:YSGG lasers (Ana et al., 2006).
Er:YAG lasers became popular for using in dental hard tissues at the end of 1980s, when researchers tested the Er:YAG for ablation of enamel, dentin and caries lesions on extracted teeth (Hibst & Keller, 1989). Since then, several studies have been performed to determine parameters and conditions for a safe and efficient application in daily practice for soft and hard tissue applications (Altshuler et al., 1994; De Moor & Delme, 2010; Eberhard et al., 2005; Fried et al., 1996, 2002; Hibst & Keller, 1989; Moldes et al., 2009; Neves et al., 2010; Stock et al., 1997; White et al., 1994). On the other hand, the popularity of Er,Cr:YSGG laser started later, since the first studies tested the possibility of ablation of dental hard tissues on early 90's. In vitro (Altshuler et al., 1994; Ana et al., 2007; Bachmann et al., 2009; Botta et al., 2011; Dundar & Guzel, 2011; Fried et al., 1996; Moldes et al., 2009; Obeidi et al., 2009; Stock et al., 1997; Tachibana et al., 2008) and in vivo (Yazici et al., 2010; Yilmaz et al., 2011) studies confirmed the feasibility of this wavelength for several applications on dental hard tissues, such as cavity preparation and caries removal.
During cavity preparation, the ablation of sound enamel by Er:YAG laser promotes cavities with rough enamel margins, with irregular and rugged walls, with depth that depends on the energy density and pulse width (Navarro et al., 2010). As well, it is reported the absence of smear layer, cracks, carbonization or melting if the adequate parameters and refrigeration were used (Botta et al., 2009). As Er:YAG, the enamel cavities produced by Er,Cr:YSGG laser irradiation present their floor with fissures and conical craters with sharp enamel projections and, in some areas, with the exposition of the enamel rods. The roughness of cavities is also dependent on the energy densities used (Ana et al., 2007; Olivi et al., 2010; Tachibana et al., 2008).
In sound dentin, due to the differences in composition and morphology, erbium lasers promote a higher removal of peritubular than the intertubular dentin. In this way, both Er:YAG and Er,Cr:YSGG promote the formation of rough surfaces with opened dentinal tubules, absence of smear layer, cracks or melting, with protrusion of peritubular dentin due to its less amount of water when compared to the intertubular dentin (Botta et al., 2009, 2011). The irregularities promoted by laser irradiation vary according to the energy density applied. Considering these facts, the differences in temperature rises at the ablation threshold promoted by Er:YAG and Er,Cr:YSGG seem to be unable to induce significant distinct morphological effects during cavity preparation.
In contrast, the use of high-speed drills for cavity preparation promotes enamel and dentin cavities flattened, with smooth internal walls and geometrically well-defined shapes, with closed dentinal tubules and presence of smear layer (Botta et al., 2009; Navarro et al., 2010). These characteristics, as well as the changes in chemical and crystalline structure in remaining tissue promoted by laser irradiation, must be taken into consideration in order to choose an appropriate adhesive system for composite restoration, since the adhesive systems interact in a different way with laser or bur treated tissues (Moretto et al., 2011).
Erbium lasers are also effective on removal of dental caries. In vitro studies revealed that Er:YAG and Er,Cr:YSGG can selectively remove dental caries due to the higher amount of water and organic content when compared to sound tissues (Eberhard et al., 2008; Tachibana et al., 2008); in this way, it is possible to obtain a conservative therapy, with no removal of sound tissue and lack of thermal damages. However, the adjustment of laser energy density in commercial equipments is sometimes difficult to promote the selective ablation of infected dentin and in order to preserve the affected dentin, and the clinical results still depend on the experience and knowledge of the professional, added to the use of manual instruments for correct diagnosis of remaining tissue. In fact, clinical trials report the well acceptance of patients (Dommisch et al., 2008; Krause et al., 2008), the maintenance of pulp vitality and marginal seal, the good quality of restorations and the absence of secondary caries even after two years (Yazici et al., 2010). Also, it is reported that these lasers can fulfill the requirements of Minimal Invasive Dentistry, due to the possibility of conservation of the sound tissue structure during caries removal and to the possibility of surface decontamination of affected dentin (Kornblit et al., 2008).
To determine an end point for caries removal, there are some equipments that associate Er:YAG laser irradiation to the diagnosis by laser fluorescence (Dommisch et al., 2008; Eberhard et al., 2008; Jepsen et al., 2008; Krause et al., 2008;). The essential principle of this application is that the fluorescence of sound tissue differs from the fluorescence of carious tissue due to the variation in chemical composition, such as the presence of proteins, bacteria and other contents. In this equipment, the fluorescence is induced by red laser that emits at the wavelength of 655 nm, and the Er:YAG laser is turned off when significant changes on fluorescence are detected during caries removal, established by a cut-off value that indicates that all decayed tissue was removed. Some in vitro (Eberhard et al., 2008; Jepsen et al., 2008) and in vivo (Dommisch et al., 2008; Krause et al., 2008) studies showed the feasibility of this equipment; however, there is no consensus about the correct values of cutoff in different clinical conditions. Also, it must be emphasized that there are limitations of this technique mainly in dentin (Eberhard et al., 2008; Krause et al., 2008), when false-positive can be reported due to the presence of pigments in affected or tertiary dentin, for instance, which should not be removed. In this way, the association of manual instruments and is still necessary to assure a safe and correct clinical removal of dental caries.
Clinical trials have demonstrated that Er:YAG and Er,Cr:YSGG lasers can be considered a safe and efficient treatment for caries removal, since it is reported pulpal response and histological effects similar to those obtained by the use of conventional bur. Also, due to the lack of noise, pressure, discomfort and sometimes the necessity of local anesthesia, it is reported a good compliance of patients, mainly the pediatric ones. However, it must be emphasized that the time necessary to remove caries by laser irradiation is almost two or three times longer than the bur treatment, depending on the repetition rate and energy density (Navarro et al., 2010; Yamada et al., 2001). The increase of energy density and repetition rate can lead to discomfort and pain to patients, besides increasing the surface temperatures. For this reason the strategies used for improving the laser ablation speed are limited (Navarro et al., 2010).
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