Influence of pulse width on tissue removal

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Although the pulse duration of most commercial lasers (range of ^s) is shorter than the thermal relaxation time of dental hard tissues, laser ablation promotes irregular cavities (depending on composition of target tissue), desiccation of the surface (due to the removal of underlying water) and the presence of few microcracks (related to the energy density), the amount of water coolant and the repetition rate must be adjusted during the clinical procedure.

The adjustment of repetition rate is important to assure that the inter-pulse period is longer than the thermal relaxation time of tissues; in this way, it is possible that the temperature of the irradiated tissues decrease between laser pulses (McDonald et al., 2001). Another strategy for cooling the tissue during laser irradiation is reducing the pulse duration (Seka et al., 1995). Depending on the pulse duration (<1 ps), the process of ablation is changed and the non-linear processes (or non-thermal ones) take place (Ana et al., 2006; Freitas et al., 2010; Kruger et al., 2008; McDonald et al., 2001; Niemz, 2004; Strassl et al., 2008).

According to Niemz (1995), lasers with pulse durations in the range of ms (10-3 s), ^s (10-6 s) or ns (10-9 s) generate considerable heat during ablation of dental hard tissues, in a mechanism mediated by thermal interaction. On the other hand, lasers with pulse durations of ps (10-12 s) and fs (10-15 s) ablate the tissues by forming an ionizing plasma. These lasers, commonly called as USPL (ultra short pulse lasers), operates at very high repetition rate (larger than 15 kHz) and energy per pulse typically of hundreds of ^J (Wieger et al., 2006).

Although USPLs have extremely higher repetition rate (> KHz) and peak power (up to TW), previous studies relate that a single ultra short laser pulse removes significantly less volume of dental tissue when compared to conventional Er:YAG laser removal (Strassl et al., 2008). This fact occurs due to the differences in focal size and penetration depth of USPLs (which are severely lower when compared to Er:YAG lasers that operate at pulse width of ^s); in this way, the pulse repetition rate had to be increased in USPLs to obtain a similar ablation volume than those obtained by Er:YAG (Wieger et al., 2006).

Some literature studies compared the morphological aspects, as well the depth of craters during ablation of dental hard tissues with lasers operating with distinct pulse widths. Niemz (1995) relates that the Nd:YLF laser (X = 1053 nm) operating with pulse duration of 30 ps provide cavity preparation on sound and decayed enamel without severe thermal or mechanical damages, with negligible shock-wave effects. Also, in the same paper, they showed that the ablation of carious enamel was 10 times more efficient than the ablation of sound enamel. A study performed by McDonald et al. (2001) showed that the total deposited energy on tissue as well the laser pulse duration change the crater depth generated on dentin, and the Nd:YAG with pulse width of 35 ps is unable to promote carbonization of dentin in comparison with a Nd:YAG laser with pulse width of ms.

The heating of dental hard tissues can induce composition and crystallographic changes on these tissues which are dependent on temperature rises. In this way, both morphological aspects and chemical analysis are indicative of thermal effects of lasers on enamel and dentin. A study performed by Kamata et al. (2004) showed that the chemical properties of hydroxyapatite (HAp) are unchanged after ablation with lasers operating with pulse widths of 50 fs, 500 fs and 2 ps. These results suggest that USPLs do not significantly increase the temperature of HAp. On the other hand, the use of Nd:YAG operating with pulse duration of 6 ns and 200 ns on enamel promote melting and recrystallization of this tissue (Antunes et al., 2005), indicating temperature rises up to 1200° C. Also, with the pulse duration of 6 ns, Nd:YAG promoted changes on organic content of enamel and dentin (Antunes et al., 2006).

Thermal measurements were performed using a laser with pulse width of fs on enamel using thermocouples, and it was detected temperature rises about 2o C on enamel surfaces after a 8 ms train of 70 fs pulses (Pike et al., 2007). This fact indicates that the USPLs do not induce significant thermal rises on surfaces and on surrounding tissues and can be used with safety even without refrigeration.

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