Thermal and morphological effects of the pulsed Nd:YAG laser on root canal surfaces

To investigate the sequential thermal and morphological effects of the pulsed Nd:YAG laser on root canal surfaces.

Effectiveness and safety are two critical aspects of laser application for root canal instrumentation. To date, few studies have synchronously focused on the efficacy of root canal cleanliness and thermal changes in root surfaces irradiated by the Nd:YAG laser.

Single-root human premolars (n = 144) were sectioned at the cementoenamel junction. The root canals were instrumented and divided into three groups: group 1 (15 Hz), group 2 (20 Hz), and group 3 (15 Hz with black ink). Each group was divided into eight subgroups according to the laser power settings used. The temperature elevations of the apical and coronal root surfaces were measured with a thermocouple measurement system during laser irradiation. Then the roots were bisected longitudinally and examined with the scanning electron microscope.

There was a positive correlation between laser energy level and temperature elevation of the root surfaces of all three groups. The temperature elevation in group 3 was the highest, while that in group 1 was the lowest at all power settings. More strikingly, with increasing laser power and frequency, there were corresponding morphological changes seen in the root canal wall, such as removal of the smear layer and melting and recrystallization of the dentin. At 2.0 W, the temperature elevations seen in all three groups were within the biologically tolerable thermal limit, but efficient removal of the smear layer was achieved only in groups 2 and 3. Additionally, the thermal and morphological changes seen at the apical third of the root were greater than those seen at the coronal third.

This study provides useful information on the choice of appropriate energy parameters to use during application of pulsed Nd:YAG energy for root canal therapy.