Unique alignment and texture of biological apatite crystallites in typical calcified tissues analyzed by microbeam x-ray diffractometer system

Preferential orientation of biological apatite (Ap) crystallites in typical calcified tissues of rabbit ulna, rabbit skull, and monkey dentulous mandible was investigated using a microbeam X-ray diffractometer, with a beam spot of 100 μm in diameter, to clarify relationship between the Ap orientation and mechanical function. Preferential alignment of the c-axis of the biological Ap was evaluated by the relative intensity between (002) and (310) diffraction peaks. Preferential alignment of biological Ap in each calcified tissue varied depending on the shape and stress condition in vivo; that is, the c-axes of biological Ap in the rabbit ulna and the rabbit skull bone were preferentially observed as a one-dimensional orientation along the longitudinal axis and a two-dimensional orientation along the surface, respectively. Precise analysis of the preferential alignment along the skull surface showed an elliptical distribution of the c-axis of biological Ap elongating along the suture inside the skull surface of both lamina exterior and interior. The c-axis of biological Ap in a monkey dentulous mandible basically aligned along the mesiodistal direction in the flat bone, but this alignment changed along the normal direction to the flat bone surface parallel to the biting direction near the tooth, due to the force of mastication. It was concluded that the microscale measurement of biological Ap texture is one of the useful new methods for evaluating mechanical function and stress distribution in vivo in calcified tissues.