Peripheral thermal and mechanical damage to dentin with microsecond and sub-microsecond 9.6 μm, 2.79 μm, and 0.355 μm laser pulses

Background and Objectives: It is desirable to minimize peripheral thermal damage during laser irradiation, since thermal damage to collagen and mineral compromises the bond strength to restorative materials in dentin and inhibits healing and osteointegration in bone. There were two primary objectives of this study. The first objective was to measure the degree of thermal damage peripheral to incisions in dentin produced with lasers resonant to the specific absorption bands of water, collagen, and hydroxyapatite with varying pulse duration using polarized-light microscopy (PLM). The second objective was to use synchrotron radiation infrared spectromicroscopy (SR-FTIR) to identify the specific chemical nature of the optical changes observed under PLM in the respective zones of thermal damage peripheral to the laser incisions. Study Design/Materials and Methods: Precise incisions were produced in 3 × 3 mm² blocks of human dentin using CO₂ (9.6 μm), Er:YSGG (2.79 μm), and Nd:YAG (355 nm) lasers with and without a computer controlled water-spray. Optical coherence tomography (OCT) was used to obtain optical cross-sections of each incision to determine the rate of ablation. The peripheral thermal damage zone around each incision was analyzed using PLM and SR-FTIR. Results: Thermally induced chemical changes to both mineral and the collagen matrix were observed with SR-FTIR with a 10 μm spatial resolution and those changes were correlated with optical changes observed with PLM. Minimal (<10 μm) thermal damage was observed for pulse durations less than the thermal relaxation time (T_r) of the deposited laser energy, with and without applied water at 9.6 μm and with only applied water at 2.79 μm. For pulse durations greater than T_r, greater peripheral thermal damage was observed for both IR laser wavelengths with and without the water-spray. There was minimal thermal damage for 355 nm laser pulses less than T_r with and without applied water; however, extensive mechanical damage (cracks) was observed. Conclusions: High resolution SR-FTIR is well suited for characterization of the chemical changes that occur due to thermal damage peripheral to laser incisions in proteinaceous hard tissues. Sub-microsecond pulsed IR lasers resonant with water and mineral absorption bands ablate dentin efficiently with minimal thermal damage. Similar laser parameters are expected to apply to the ablation of alveolar bone.