Enamel crystallite strength and wear: nanoscale responses of teeth to chewing loads

The nanoscale responses of teeth to chewing loads are poorly understood. This has contributed to debate concerning the aetiology of enamel wear and resistance to fracture. Here we develop a new model for reactions of individual hydroxyapatite nanofibres to varying loads and directions of force. Hydroxyapatite nanofibres, or crystallites, composed of chains of bonded nanospheres, are the fundamental building blocks of enamel. This study indicates that these nanofibres respond to contact pressure in three distinct ways depending on force magnitude and direction: (i) plucking (nanosphere loss when the strength of the bonding protein 'glue' is exceeded), (ii) plastic deformation (compression to gradually bend nanofibres and squeeze the protein layer), and (iii) fragmentation (nanofibres fracture when the strength of H-bonds that bind smaller nanoparticles into nanospheres is exceeded). Critical contact pressure to initiate plucking is the lowest, followed by plastic deformation, and then fragmentation. Further, lower contact pressures are required for a response with shear forces applied perpendicular to the long axes of crystallites than with crushing forces parallel to them alone. These nanoscale responses are explained as a function of the interfacial nanochemical bonding between and within individual crystallites. In other words, nanochemistry plays a critical role in the responses of enamel to varying chewing loads.