Radiotherapy effects on enamel and dentin nano-mechanical properties

Our previous clinical study indicates tooth-level radiation dose significantly impacts post-radiation dentition breakdown. Three tiers of dose-response were identified: minimal damage (below 30Gy), 2-3x increase (between 30 and 60Gy-salivary gland threshold), and 10x increase in tooth damage at ≥60Gy suggesting a direct effect on tooth structure.  Objectives: To understand the mechanism of radiotherapy-induced dental lesions characterized by shear fracture of enamel near the dentin-enamel junction (DEJ) that suggests decreased interface stability.  Methods: An in vitro model was used to examine the effects of radiotherapy, elapsed time following radiation, and occlusal function on the nano-mechanical properties of enamel and dentin near the DEJ.  Initial studies were completed on 16 extracted, unerupted third molars with one-half of the teeth exposed to 70 Gy or no radiation followed by 1 or 3 mo PBS storage at 37°C. Half the teeth from each radiation dose/elapsed time group were exposed to cyclic loading simulating 3 years of function.  Following these conditions, the crown of each tooth was sectioned buccolingually to generate a 2-mm-thick slice centered on the mesiobuccal/mesiolingual cusps. After sequential polishing, linear nanoindentation mappings were done on the buccal and lingual surface of each section starting and ending approximately 500 microns from the DEJ in enamel and dentin, respectively.  Results: Based on the nanoindentation data, Young's modulus and hardness were significantly (p<.05) lower across the mappings of enamel as a function of radiation.  Significant main and interaction effects were observed for radiation, radiation*time, and radiation*load for dentin modulus and hardness (p<.05). Conclusions: This preliminary evidence suggests that radiation, in conjunction with elapsed time and occlusal load, has a significant effect on the nano-mechanical properties of enamel and dentin near the DEJ. Besides extending the elapsed time, next steps are to discern the underlying mechanism for the property changes.  Supported by NIH/NIDCR R01DE21462