Misorientation and enhanced hardness in tooth enamel

Teeth are subjected to extreme, repetitive forces on a daily basis. Human enamel exerts forces up to 770 Newtons, hundreds of times per day, and must remain functional for decades¹. By comparison, great white shark enameloid exerts 7400 Newtons of force when biting, but they shed their teeth regularly². The mechanical stress the teeth undergo suggests there are structural features in enamel that prevent catastrophic failure. We used PIC (polarization-dependent imaging contrast)^3,4 mapping at the calcium L-edge⁵ to reveal the crystal orientations within rods in mammalian enamel and in bundles in fish enameloid. Analysis of PIC maps from the enamel(oid)⁶ of diverse animals indicates that c-axis orientations of adjacent crystals are slightly misoriented by small angles. The observed misorientation is positively correlated with the hardness and elastic modulus, suggesting that crystal misorientation is related to, and possibly causes, these enhanced properties.
1 S Varga 2010, 10.1093/ejo/cjq097
2 S Wroe 2008, 10.1111/j.1469-7998.2008.00494.x
3 P Gilbert 2011, 10.1073/pnas.1107917108
4 C Killian 2011, 10.1002/adfm.201001546
5 C Stifler 2018, 10.1021/jacs.8b05547
6 M Marcus 2017, 10.1021/acsnano.7b05044