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

OBJECTIVES: The effect of alteration of laser parameters on laser-dentine interaction, in particular the effect of pulse duration, has not been well documented. The aim of this in vitro study was to determine the effect on dentine crater depth of Nd:YAG laser pulse duration, and total delivered energy, dentine site and the presence or absence of dye. METHODS: Ninety-six sound third molars were extracted and sectioned transversely to provide 192 upper and lower cut surfaces. The upper surfaces were painted with a layer of dye (IR5) suitable for absorption at 1064 nm. The specimens were divided into 16 sub-groups and exposed to two Nd:YAG lasers; one of pulse duration 7 ms and the second of pulse duration 35 ps. Both lasers operated in a non-contact mode (spot diameter 165 microm) with repetition rates of 10.5 and 10 Hz, respectively. Four total energies (2.28, 2.64, 3.6, 4.2 J) were delivered to eight dyed and eight undyed sub-groups. Eight outer and five inner sites were irradiated on each specimen. Dentine crater depth was measured five times using a Reflex Microscope and a three-dimensional centre of gravity derived. An upper and lower specimen were taken from each sub-group and viewed under a SEM. RESULTS: ANOVA and multiple regression analysis were applied and the following factors were found to have a statistically significant effect on crater depth (p<0.0001): total delivered energy, pulse duration and inner/outer location. CONCLUSIONS: Increasing energy and pulse duration produced deeper craters. Similarly inner dentine sites produced deeper crater depths. Only craters produced at the ms pulse duration were carbonised. It would appear that laser-dentine interaction has a non-thermal component at picosecond pulse duration.