Relationship between Preferential Alignment of Biological Apatite and Young's Modulus at First Molar in Human Mandible Cortical Bone

Biological apatite (BAp) crystallite c-axis alignment is known to be a factor in the mechanical function of bone. However, reports are lacking on mechanical properties in the alveolar area and base of the mandible area of the human mandible, which has complex occlusal force. In addition, there are many unclear mechanical properties concerning the relationship with BAp crystallite alignment. We therefore aimed to elucidate the relationship between BAp crystallite alignment and Young's modulus in the alveolar area and base of the mandible area in human mandible cortical bone. The effect of occlusal force on bone quality at first molar in the mandible was also discussed. Mandibular specimens including teeth were obtained from 6 Japanese adult cadavers (mean age: 63.0 +/- 12.1 years) housed at the Department of Anatomy, Tokyo Dental College. A microbeam X-ray diffraction system was used to determine BAp crystallite alignment along the mesiodistal direction (longitudinal axis) in each specimen. Young's modulus was also measured at the same sites along the mesiodistal direction by nanoindentation and its relationship with BAp crystallite alignment examined. BAp crystallite alignment at first molar in human mandible cortical bone was low in the alveolar part but high in the base of mandible. The same tendency was observed for Young's modulus at the same sites. Furthermore, a positive correlation was recognized between BAp crystallite alignment and Young's modulus (p < 0.01). The present results showed a close relationship between BAp crystallite alignment and Young's modulus, suggesting the importance of determining BAp crystallite alignment when evaluating bone strength. Determination of BAp crystallite alignment at first molar in human mandible may be useful in determining the effects of occlusal force.