Microstructure, elemental composition and mechanical properties of enamel and dentine in the polar bear Ursus maritimus

Objective: To investigate the microstructure, elemental composition and mechanical properties of polar bear teeth.Design: Incisors, canines and fourth premolar teeth of two subadult male museum specimens were analysed. Teeth were measured, photographed, embedded in Epoxy resin, sectioned, polished and etched for scanning electron microscope (SEM) imaging, elemental composition and nanomechanical testing analyses.Results: The thickness of enamel ranged from 350-430 mu m in canines, 220-330 mu m in incisors and 320-510 mu m in premolars. SEM images showed distinct transversely-oriented undulating Hunter Schreger bands from the enamel-dentine junction (EDJ) to the outer enamel surface. Enamel prisms had a hexagonal shape, with open prism sheaths. Prisms measured 6-8 mu m in diameter. The EDJ was straight with no evidence of scalloping. Larger tubules adjacent to the EDJ were observed in the mantle dentine zone. Enamel Hardness and Elastic modulus values were higher in premolars (6.9 GPa and 269 GPa), followed by canines (6.5 GPa and 230 GPa) and incisors (4.9 GPa and 187 GPa). Dentine Hardness and Elastic modulus values were higher in canines. CaO and P2O5 were the components with higher oxide weight percentage in both enamel and dentine.Conclusions: Understanding the microstructure, elemental composition and mechanical properties of polar bear teeth can help elucidate the biology and functional morphology of this globally threatened species and could be used as a proxy for studies with fossil ursids.