Influence of environment on the mechanical behaviour of mature human enamel

Objectives To investigate the role of different environments in regulating the mechanical behaviour of mature human enamel. Methods Healthy enamel samples were subjected to different environmental treatments such as ethanol dehydration, water re-hydration, desiccation at room temperature, and after heating (burnt) to 300 °C. Nanoindentation tests were done on all samples for determination of elastic modulus, hardness and indentation creep behaviour. Scanning electron microscopy (SEM) was used to observe surfaces and indentation impressions of different treated samples. Results Statistically significant differences of the mechanical properties were found following the various treatments. Burnt sample had the highest elastic modulus and hardness of 115 and 6 GPa, respectively, while the re-hydrated sample showed the lowest values of 95 and 4 GPa. The creep deformation showed the inverse response to the environment-induced elastic modulus results with negligible creep found for the burnt specimens. SEM showed that, although no significant structural changes were found for burnt samples after heating, there was much more cracking about the residual indentation impression. Conclusions Because of the chemical and thermal stability of hydroxyapatite under the experimental conditions investigated, differences of mechanical behaviour of enamel are rationalized in terms of changes to the matrix proteins and loss of water within enamel. These results indicate that matrix proteins play an important role in regulating the mechanical behaviour of enamel as a biocomposite.