The effect of Nd:YAG radiation at nanosecond pulse duration on dentine crater depth

Femtosecond laser ablation of enamel

The surface topographical, compositional, and structural modifications induced in human enamel by femtosecond laser ablation is studied. The laser treatments were performed using a Yb:KYW chirped-pulse-regenerative amplification laser system (560 fs and 1030 nm) and fluences up to 14  J/cm2. The ablation surfaces were studied by scanning electron microscopy, grazing incidence x-ray diffraction, and micro-Raman spectroscopy. Regardless of the fluence, the ablation surfaces were covered by a layer of resolidified material, indicating that ablation is accompanied by melting of hydroxyapatite. This layer presented pores and exploded gas bubbles, created by the release of gaseous decomposition products of hydroxyapatite (CO2 and H2O) within the liquid phase. In the specimen treated with 1-kHz repetition frequency and 14  J/cm2, thickness of the resolidified material is in the range of 300 to 900 nm. The micro-Raman analysis revealed that the resolidified material contains amorphous calcium phosphate, while grazing incidence x-ray diffraction analysis allowed detecting traces of a calcium phosphate other than hydroxyapatite, probably β-tricalcium phosphate Ca3(PO4)2, at the surface of this specimen. The present results show that the ablation of enamel involves melting of enamel’s hydroxyapatite, but the thickness of the altered layer is very small and thermal damage of the remaining material is negligible.

Ablation of carious dental tissue using an ultrashort pulsed laser (USPL) system

The aim of the study was to investigate the efficiency of caries removal employing an ultrashort pulsed laser (USPL) and to compare the results regarding to the ablation rate of sound enamel and dentin including surface texture. The study was performed with 59 freshly extracted carious human teeth. Two cavities with an edge length of 1 × 1 mm per tooth were created: one in the dental decay and one in sound hard tissue. For this purpose a 9-W Nd:YVO4 laser with a center wavelength of 1,064 nm and a pulse duration of 8 ps at a repetition rate of 500 kHz was used. A scanner system moved the laser beam across the surface with a scan speed of 2,000 mm/s. Ablated volume and roughness R z of the cavity ground were measured using an optical profilometer. Subsequently, the specimens were cut to undecalcified sections for histological investigations. The removal of dental decay (dentin, 14.9 mm(3)/min; enamel, 12.8 mm(3)/min) was significantly higher (p < 0.05) compared to the removal of sound tissues (dentin, 4.2 mm(3)/min; enamel, 3.8 mm(3)/min). The arithmetic means of the surface roughness R z were 8.5 μm in carious enamel, 15.43 μm in carious dentin, 4.83 μm in sound enamel and 5.52 μm in sound dentin. Light microscopic investigations did not indicate any side effects in the surrounding tissues. Regarding the ablation rate of dental decay using the USPL system, caries removal seems to be much more efficient for cavity preparation.