Screwy microstructure of hydroxyapatite sheets of shank bone

Scanning electron microscope (SEM) observation shows that a shank bone is a kind of bioceramic composite consisting of inorganic hydroxyapatite and organic collagen protein. The hydroxyapatite possesses laminated microstructure and the each hydroxyapatite layer is composed of numerous long and thin hydroxyapatite sheets. The hydroxyapatite sheets in an arbitrary hydroxyapatite layer make an almost fixed angle with the hydroxyapatite sheets in their adjacent hydroxyapatite layers which compose a kind of particular screwy microstructure. The maximum pullout energy of the screwy microstructure, which is closely related to the fracture toughness of the bone, is theoretically investigated and compared with that of the parallel microstructure of the sheets. It shows that the maximum pullout energy of the screwy microstructure is markedly larger than that of the parallel microstructure, which is experimentally verified.