The adverse effects of radiotherapy on the structure of dental hard tissues and longevity of dental restoration

Purpose: The main goal of this study was to evaluate the impact of different ionizing radiation doses on the mineral (carbonate/phosphate ratio, crystallinity index [CI]) and organic (amide III/phosphate, amide I sub-band ratios) structures, as well as the microhardness, of enamel and dentin, along with their influence on the bonding strength stability of the etch-and-rinse (ER) and self-etch (SE) dental adhesive strategies. Materials and methods: Enamel and dentin human tissue specimens were irradiated (with 0, 20, 40, and 70 Gy radiation doses, respectively) and sectioned to perform an attenuated total reflection-Fourier transform IR spectroscopy assay (ATR-FTIR) and the Vickers microhardness (VHN) test to conduct a biochemical and biomechanical evaluation of the tissues. Regarding the adhesive properties, restored enamel and dentin specimens exposed to the same radiation doses were submitted to microshear bond strength (μSBS) tests for enamel in immediate time (IM) and to microtensile bond strength (μTBS) tests after for IM and 12-month (12 M) period of time, Mann–Whitney U tests were implemented, using the ATR-FTIR data for significant differences (α < 0.05), and three- and two-way analyses of variance, along with post-testing, were performed on the μTBS and μSBS data (MPa), respectively (Tukey post hoc test at α = 0.05). Results: The ATR-FTIR results showed a significant decrease (p <.05) in the amide III/phosphate ratio after 20 Gy for the enamel and after 40 Gy for the dentin. The CI was significantly reduced for both tissues after a dose of 70 Gy (p <.05). All radiation doses significantly decreased microhardness values, relative to the respective enamel and dentin controls (p <.05). In both tissues and adhesive strategies, the decrease in bond strength was influenced by ionizing radiation starting from 40 Gy. The ER strategy showed high percentages of enamel cohesive failure. In general, ER in both tissues showed greater and more stable bond strength than SE against increased radiation doses and long term. Conclusions: It is possible to conclude that structural alterations of enamel and dentin are generated by all radiation doses, decreasing the microhardness of dental hard tissues and influencing bond strength over time, starting at 40 Gy radiation dose. The etch-and-rinse strategy demonstrates better adhesive performance but generates cohesive fractures in the enamel.