**Laser Technology for Caries Removal**

Adriana Bona Matos, Cynthia Soares de Azevedo, Patrícia Aparecida da Ana, Sergio Brossi Botta and Denise Maria Zezell *University of São Paulo, School of Dentistry and Nuclear and Energetic Research Institute, Brazil*

#### **1. Introduction**

Laser technology has been in the scope of dentistry community since Stern & Sognnaes (1964) studied laser application on dental hard tissues. Lasers have become an attractive instrument for many dental procedures including soft tissues surgery (Sperandio et al., 2011), decontamination (Benedicenti et al., 2008; Koba et al., 1998) and for assuring antiinflammatory effects (Lang-Bicuto et al., 2008). In restorative dentistry, laser has been used successfully for cavity preparation (De Moor et al., 2010; Obeidi et al., 2009), caries prevention (Namoour et al., 2011; Rechmann et al., 2011; Zezell et al., 2009), caries decontamination (Namour et al., 2011) and caries removal (Neves et al., 2011; White et al., 1993). For that, high intensity lasers are indicated, which are able to promote controlled temperature rise in a small and specific area of dental hard tissue (Ana et al., 2007). Depending on the temperature rise and the interaction of laser irradiation with dental tissues, it is possible to produce specific micro structural and/or mechanical changes related to a correct clinical application.

The use of lasers for cavity preparation and caries removal is based on the ablation mechanism, in which dental hard tissue can be removed by thermal and/or mechanical effect during laser irradiation (Seka et al., 1996). This mechanism relies on the type of tissue to be irradiated, as well as the characteristics of laser equipments. The knowledge of laser wavelength, laser emission, pulse duration, pulse energy, repetition rate, beam spot size, delivery method, laser beam characteristics (Ana et al., 2006), and optical properties of the tissue, such as the refractive index, the scattering coefficient (μs), the absorption coefficient (μa), and the scattering anisotropy (Featherstone, 2000a) are necessary to assure better clinical results without thermal or mechanical damages to the dental hard tissue.

For irradiation in dental hard tissues, the most frequent laser systems used are Nd:YAG λ = 1.064 µm), Argon (λ = 0.488 µm), Ho:YLF (λ = 2.065 µm), Ho:YAG (λ = 2.100 µm), Er:YAG (λ = 2.940 µm), Er,Cr:YSGG (λ = 2.780 µm), Diode (λ = 0.810 µm) and CO2 (λ = 9.300 µm or 9.600 µm or 10.600 µm). With the exception of the argon laser, these lasers emit in infrared range of electromagnetic spectrum, and a good number of equipment operates at the free running mode, with pulse durations of microseconds (µs). Considering that laser wavelength must be absorbed by enamel and dentin to assure the efficient caries removal and cavity preparation (Seka et al., 1996), the most successful laser systems for this purpose

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due to a *thermomechanical* effect (Fried, 2000; Niemz, 1995; Seka et al., 1996). For shorter pulses, such as femtosecond laser pulses, the ablation occurs due to non-linear interactions

The thermal ablation process that occurs in dental hard tissues is also known as explosive (water-mediated) tissue removal (Fried, 2000; Niemz, 1995; Seka et al., 1996). In a few words, this process can be explained as a result of the fast heating of the subsurface water confined by the hard tissue matrix, due to the higher interaction with infrared laser irradiation. The heating of these water molecules leads to an increase on molecular vibration and, consequently, an increase on subsurface pressures that can exceed the strength of the above tissue. Finally, it can be noted an "explosion" of tissue due to the material failure, resulting in the material removal. This process happens in temperatures below the melting point of dental hard tissues (around 1200oC) and varies according to the laser wavelength (e.g., Er:YAG reaches 300o C at the ablation threshold, while Er,Cr:YSGG reaches 800o C and CO2 9.6 µm reaches 1000o C) ((Seka et al., 1996; Fried et al., 1996). This process has been studied for the past 30 years, with the intention of choosing the best laser wavelength and parameter to effectively promote tissue removal or selective caries removal with minimal thermal consequences (Stern & Sognnaes, 1964; White et al., 1993; Neves et al., 2010; Ana et al., 2007; Seka et al., 1996; Tachibana et al., 2008; Moldes et al., 2009; Botta et al., 2009; Dundar &

For understanding how laser irradiation can provide a more conservative treatment of caries lesions, the chemical composition of target tissue must be known by the professional. Human enamel is composed by 95% hydroxyapatite (Ca10(PO4)6(OH2)), 4% water and 1% collagen fibers (Gwinnett, 1992); as well as human dentine contains 70% hydroxyapatite, 20% collagen fibers and 10% water (Ziip & Bosch, 1993). Considering the differences in composition and the higher resonance of Er:YAG (λ = 2.94 µm) by water (λ = 3 µm), we can infer that Er:YAG laser can ablate dentin faster than enamel. The same rule is valid when comparing carious tissue with sound ones, taking into account that decayed tissues have a significant higher amount of water. In this way, in a clinical application, professionals can observe easier caries removal when compared to the removal of sound surrounding tissues, and this fact can influence the laser irradiation parameters that should be used for different

Dental enamel and dentin have a weak absorption in the visible (400–700 nm) and nearinfrared (1064 nm) wavelength ranges; however, absorption bands of water and carbonated hydroxyapatite is found from 2.7 to 11 µm (Figure 1) (Ana et al., 2006; Fried 2000). The optical penetration of Nd:YAG on enamel is significantly high, indicating that the dentin irradiation with Nd:YAG laser can affect the pulp tissue in case of high energy densities, long exposure or in the absence of a photoabsorber (Boari et al., 2009). However, Nd:YAG laser can be indicated for removal of stained caries tissue, promoting a selective removal of caries lesion without pulpal damages due to the higher interaction of Nd:YAG by pigments (Seka et al., 1996), as it was demonstrated by a clinical trial performed by White *et al*. (1993). Considering the use of Er,Cr:YSGG laser, literature evidences (Stock et al., 1997) that the 2.78 µm is strongly absorbed by the dental hard tissue since the optical absorption coefficient of enamel is about 7000 cm−1. In this way, the optical penetration is a few micrometers smaller

with the tissue resulting in a plasma-mediated ablation.

Gunzel, 2011).

application.

than the obtained by Er:YAG laser.

are erbium and CO2 (λ = 9.6 µm) lasers. However, the CO2 (λ = 9.6 µm) systems are not commercially available for applications in dentistry.

Considering the advances in technology for the development of ultra short pulse lasers (USPLs) (Niemz, 1995; Strickland & Mourou, 1985), efforts have been implemented to understand their interaction with dental hard tissues and to determine safe and proper parameters to provide a future clinical application in dentistry (Altshuler et al., 1994; Freitas et al., 2010; Kruger et al., 1999; Lizarelli et al., 2008; Strassl et al., 2008). Due to the extremely short pulse length, these systems promote precise cutting and have a strong potential for obtaining well-defined cavities and controlled caries removal (Niemz, 2004; Serbin et al., 2002). Also, due to the use of low energies per pulse, it is possible to adjust parameters bellow the ablation threshold for sound tissue which, at the same time, can ablate and remove the carious tissue (Niemz, 2004; Strassl et al., 2008). In this way, the selective removal of carious tissue could be seen as a minimal intervention that does not depend on the professional experience, but essentially relies on the tissue chemistry (Serbin et al., 2002).

The operation of laser systems and interactions with dental hard tissues, the clinical diagnosis and the knowledge of the characteristics of the tissue to be irradiated are extremely important to assure a well-succeeded therapy. Professionals must evaluate the mineralization degree and chemical composition of the tissue to be removed, the extension and localization of caries, the activity degree of lesions and the interference of the irradiation on the restorative procedure.

In this chapter, focus will be given on the last developments concerning the use of highintensity lasers in restorative dentistry, describing the different laser wavelengths, the mechanisms of interaction with dental hard tissue and the influence of pulse width on removing these tissues. Also, the effects of laser irradiation on carious tissues will be described, and the possibility of removing dental caries with laser irradiation will be discussed to help dentists to choose a suitable equipment and technique for improving their clinical practice.
