Even with the higher repetition rates and the application of air-water coolant during the cutting process, commercially high intensity infrared lasers still cannot cut dental hard tissues with the same speed or the same precision than those promoted by drills (White et al., 1994). In this way, studies were performed to verify the possibility of using ultra short pulse lasers (USPLs) for cutting dental hard tissues, considering the success of using the USPL for precise cutting in industry and in medicine (ophthalmology) (Niemz, 2004).
The USPLs were first developed to allow spectroscopic and electrical conductivity measurements (Strickland & Mourou, 1985) and, according to Strassl et al. (2008), studies concerning the use of USPLs for medical applications started more than 15 years ago. In fact, one of the first studies that report the use of lasers with pulse widths of ps and fs was performed by Stern et al. (1989), relating applications for corneal ablation. Since then, efforts were made to understand the effects of these lasers on biological tissues and to develop of practically applicable systems. Although the majority of the studies report the use of laboratorial equipments for biological purposes, nowadays it is possible to find commercially available equipments for ophthalmology and for laboratorial use; this fact indicates that, in a near future, commercial equipments can be available for dentistry applications too.
The USPLs are lasers with pulse duration ranging from 100 fs to 500 ps, with power densities above 1011 W/cm2 in solids (Niemz, 2004). The main characteristics of these complex systems (Freitas et al., 2010) are the very low pulse duration and the high precision that can be acquired due to the extremely small focalization area, in which a peak power up to 1.5 TW (Freitas et al., 2010) can be obtained. Also, these lasers can operate at repetition rates higher than 15 kHz and energy per pulse of hundreds of ^J (Wieger et al., 2006). In this way, these lasers offer the advantage of promoting precise smooth ablation without a heat-affected zone, effects that cannot be controlled when using lasers with pulse duration of ^s or ns. Some researchers report that the main advantage of using the USPLs in dentistry is to achieve a controlled material removal and, as a consequence, reducing the pain caused by the vibration and friction heat (Kruger et al., 1999). According to Neev et al. (1996), the main advantages of USPLs are: the decreased energy density to ablate the material; minimal mechanical and thermal damages due to the extremely short laser pulses; minimal dependence of the tissue composition for ablation; precision in the ablation depth; low noise level in comparison with high-speed bur; ability to texture surface and precise spatial control.
The USPLs are solid-state lasers, such as Nd:YLF, Ti:Al2O3 , Cr:LiSAF (Alexandrite), Cr:BeAl204, Cr:LiSGaF, Cn:LiCAF, Cr:YAG, Ti:Al2O3/Nd:glass, Er:glass. These lasers interact with the tissues by a mechanism called plasma-induced ablation or plasma mediated ablation, in which the phenomenon of optical breakdown occurs. In a few words, the ablation is caused by plasma ionization, in which laser irradiation produces an extremely high electric field that forces the ionization of the molecules and atoms, promoting a breakdown and, then, the ablation or ejection of target tissue (Niemz, 2004). During the cutting, it is possible to observe the formation of a bright plasma spark, and a typical low noise, characteristic of plasma formation.
Considering the strictly short pulse durations and the low energy per pulse in USPLs systems, it is possible to infer that the ablation process is practically not dependent on the wavelength or the composition and absorption characteristics of the tissue (Perry et al., 1999). Also, the removal of ablated material is faster than the heat propagation on the tissue, i.e., the pulse length is lower than the heat conduction time of target tissue (Perry et al., 1999); in this way, there is no transmission of heat to pulp or surrounding tissues, for example, as well, no thermal damages to the irradiated tissues. Other advantage of using USPLs in dentistry is that these systems can remove any kind of restorative material, including amalgam (Freitas et al., 2010), which is not possible using other systems due to the reflection of light or overheating of the material.
Although they are characterized by extremely high peak powers, the USPLs uses lower energy densities when compared to laser with pulse width of ^s. The reduction of the energy density is because the femtosecond laser energy densities necessary for micromachining are an order of magnitude lower than those in the nanosecond-laser case for equal wavelength and repetition rate (Kruger et al., 1999).
The ablation of dental hard tissues with USPLs were investigated by Niemz et al. (1995), using a system with pulse length of 30 ps. These authors reported enamel cavities with good precision and absence of thermal damages when compared with cavities performed by lasers operating with pulse length of ^s and ns. Further researches confirmed that the application of USPLs with pulse length of few femtoseconds almost completely avoids thermal damages and the formation of microcracks on irradiated tissues and on surrounding ones (Kruger et al., 1999; Freitas et al., 2010). It must be pointed out that lasers that operate with pulse length of ^s can generate the formation of microcracks on irradiated tissue depending on the energy density, and these thermal damages can be responsible for the development of secondary caries (Apel et al., 2005).
Other studies were performed to verify the feasibility of removing restorative materials with USPLs, since these lasers can ablate any kind of material. Also, the literature reports the selectivity on removing different materials due to the different nature of interaction of USPLs with dielectric or metal materials, for instance (Freitas et al., 2010). In this way, it is easier to adjust a laser fluence that can be bellow or above the ablation threshold of a specific material. Literature studies determined that the threshold fluence for ablating enamel with USPL is higher than the fluence for ablating dentin and, in the same way that using erbium lasers, it is easier and faster to ablate dentin than enamel, which suggests selectivity to the tissue removal (Lizarelli et al., 2008; Niemz et al., 2004; Strassl et al., 2008; Wieger et al., 2006;). In 2006, Wieger et al. used a picosecond Nd:YVO4 laser for ablation of sound dentin, and it was observed the production of a microretentive pattern with opened tubules and the absence of microcracks or melting. These authors also compared the ablation rate (i.e. the ablation volume per laser pulse) of seven types of composite resins, and showed that the ablation rates of restorative materials are much higher than that measured on dentin, demonstrating that the removal of restorative materials is faster than dental hard tissue. Another study performed by Freitas et al. (2010) determined the ablation threshold fluence for removal of amalgam and composite resin restorations by a femtosecond chirped Ti:sapphire laser. In this work it was also demonstrated the selectivity of USPLs in the material removal process suggesting a selective preparation, preserving health tooth structure.
Concerning the removal of dental caries, literature studies reported that the threshold fluence for carious dentin is lower than that for sound dentin, also suggesting a selective removal of caries (Niemz, 2004). A recent study (Schelle et al., 2011) that used a Nd:YAG laser with 8 ps pulse duration confirm that the ablation threshold for carious dentin is lower than that for sound dentin and it was obtained good precision even when removing caries. These findings suggest that the USPLs are promising tools for selective removal of dental caries; however, the literature is scarce considering the applications of USPLs for selective removal of dental caries in order to establish suitable equipments and parameters. Also, there are no studies that relate the possibility of selective removal of infected dentin and preserving the affected dentin, for instance.
Although it is reported the possibility of precise removal of tissue with USPLs, it should be pointed out that the time required for a cavity preparation with USPLs is higher than the time required when using a laser with pulse duration of ^s or a drill, even with the higher repetition rate of the available systems (Kruger et al., 1999). Although there is some commercially available equipment for ophthalmology, the application of USPLs in dentistry for cavity preparation and caries removal is not yet a routine technique, and the cost and complexity of systems still represent a problem to be solved.
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