Phase, Compositional, and Morphological Changes of Human Dentin after Nd:YAG Laser Treatment Chun-Pin Lin, DDS, MS, PhD, Bor-Shiunn Lee, DDS, MS, Feng-Huei Lin, PhD, Sang-Heng Kok, DDS, and Wan-Hong Lan, DDS, DDsc Although techniques for repairing root fracture have been proposed, the prognosis is generally poor. If the fusion of a root fracture by laser is possible, it will offer an alternative to extraction. Our group has attempted to use lasers to fuse a low melting-point bioactive glass to fractured den- tin. This report is focused on the phase, composi- tional, and morphological changes observed by means of X-ray diffractometer, Fourier transform- ing infrared spectroscopy, and scanning electron microscopy-energy dispersive X-ray spectroscopy in human dentin after exposure to Nd:YAG laser. The irradiation energies were from 150 mJ/ pulse-10 pps-4 s to 150 mJ/pulse-30 pps-4 s. After exposure to Nd:YAG laser, dentin showed four peaks on the X-ray diffractometer that corre- sponding to -tricalcium phosphate (TCP) and -TCP at 2  30.78 degrees/34.21 degrees and 32.47 degrees/33.05 degrees, respectively. The peaks of -TCP and -TCP gradually increased in intensity with the elevation of irradiation energy. In Fourier transforming infrared analysis, two absorp- tion bands at 2200 cm 1 and 2015 cm 1 could be traced on dentin treated by Nd:YAG laser with the irradiation energies beyond 150 mJ/pulse-10 pps-4 s. The energy dispersive X-ray results showed that the calcium/phosphorus ratios of the irradiated area proportionally increased with the elevation of irradiation energy. The laser energies of 150 mJ/ pulse-30 pps-4 s and 150 mJ/pulse-20 pps-4 s could result in the -TCP formation and collagen breakdown. However, the formation of glass-like melted substances without -TCP at the irradiated site was induced by the energy output of 150 mJ/ pulse-10 pps-4 s. Scanning electron micrographs also revealed that the laser energy of 150 mJ/ pulse-10 pps-4 s was sufficient to prompt melting and recrystallization of dentin crystals without cracking. Therefore, we suggest that the irradia- tion energy of Nd:YAG laser used to fuse a low melting-point bioactive glass to dentin is 150 mJ/ pulse-10 pps-4 s. It has been 30 yr since lasers were first used in dentistry. Despite substantial development in this field in the United States, only Er:YAG laser has been granted marketing permission for use on dental hard tissues. The responses of soft tissues to lasers of different wavelengths are fairly well known, but the reactions of hard tissues are just being understood. Nd:YAG laser has long been investigated for its application in dental treatment. Previous studies suggested that Nd:YAG laser may function as an alterna- tive or adjunctive therapy in the treatment of periodontally dis- eased root surfaces (1), endodontically infected teeth (2), incipient dental caries (3), and dentin hypersensitivity (4 – 6). Arakawa et al. (7) induced root fractures, irradiated them by Nd:YAG laser with air/water surface cooling, and then filled the fractures with a paste of tricalcium phosphate (TCP). Levy and Koubi (8) tested the permeability of a cracked root after crack lines had been filled with a TCP melted by a Nd:YAG laser. However, the decomposition temperature for pure apatite has been reported to be up to 1500°C (9), at which damage including multiple cracks of dentin may occur. The effect of lasers on dentin is caused mainly by the changes in temperature that can be extremely high at the irradiated spot even for a short action time. Consequently, the dentin melts, vaporizes, and a crater is formed at the irradiation site. Laser energy causes a quick local temperature rise and prompts melting, recrystallization, and decomposition of the apatite crystals. Heat conduction should lead to a thermal equalization between the pronouncedly heated components and the surrounding tissues when a laser pulse of the order of a few microseconds is used. The phase transformation and structural changes of the enamel at different temperature intervals have been investigated (10). However, similar changes in the dentin are less well known. According to the study of Kantola (11), recrystallization of the dentin occurred during CO 2 laser irradiation. Simultaneously, growth in the crystal size was observed, and dentin of a low order of crystallinity changed structurally in such a way that it came to closely resemble the crystalline structure of the hydroxyapatite of normal enamel. Similar results were obtained with Nd:YAG laser that produced significant recrystallization and grain growth of the JOURNAL OF ENDODONTICS Printed in U.S.A. Copyright © 2001 by The American Association of Endodontists VOL. 27, NO. 6, JUNE 2001 389