Effect of Radiotherapy on Nano-Mechanical Properties and Chemical Composition of Tooth Enamel and Dentin

Abstract

The purpose of this study was to understand radiotherapy-induced dental lesions characterized by enamel delamination near the dentin-enamel junction (DEJ) by evaluating enamel and dentin nano-mechanical properties and chemical composition before and after in vitro and in vivo radiotherapy. For the in vitro group, sections from non-carious third molars were exposed to 2 Gy fractions, 5 days/week for 7 weeks for a total of 70 Gy. For the in vivo group, five teeth extracted from three patients who had previously undergone radiotherapy for head or neck cancer were collected, all teeth had doses greater than 60 Gy. Nanoindentation was used to evaluate modulus and hardness, while Raman microspectroscopy was used to measure protein/mineral ratios, carbonate/phosphate ratios, and phosphate peak width. All measures were completed prior to and following in vitro simulated radiation and after in vivo radiation at the same four buccal and lingual sites 500 and 30 microns from the DEJ in enamel and dentin (E-500, E-30, D-30 and D-500). With significance set at P≤0.05 across testing, the modulus and hardness of enamel and dentin significantly increased following in vitro and in vivo radiation at all locations. Following in vivo radiation, there was a significant decrease in the protein/mineral ratio, the carbonate/phosphate ratio, and phosphate peak width in enamel and dentin. However, following in vitro radiation while some similar significant chemical compositional changes occurred, they did not occur across all sites as with in vivo radiation. Radiotherapy produced an increase in the stiffness of enamel and dentin near the DEJ. Increased stiffness is speculated to be the result of a radiation-induced decrease in the protein content in the in vitro and in vivo specimens. Another possible contribution to increased stiffness could be related to a potential increase in the crystallinity of hydroxyapatite in enamel noted with in vivo specimens. Collectively, such changes in mechanical properties and chemical composition could be linked to DEJ biomechanical failure leading to enamel delamination that occurs post-radiotherapy. However, other analyses are required for a better understanding of radiotherapy-induced effects on tooth structure to improve preventive and restorative treatments for oral cancer patients.