Analysis of Biological Apatite Crystal Orientation in Anterior Cortical Bone of Human Mandible Using Microbeam X-ray Diffractometry

The human jaw is a unique bone that facilitates mastication. The structural properties of the jaw are determined by mechanical stresses transmitted via the teeth. However, it is very difficult to evaluate the impact of these mechanical stresses on bone. In recent years, it has become clear that the orientation of biological apatite (BAp) crystals is closely related to local stress, and is thought to respond more acutely to local stress than bone mineral density (BMD). Few studies have been conducted on BAp crystal alignment in response to mechanical stress in the human jaw, which has a complex masticatory function. The purpose of this study was to quantitatively evaluate BMD and BAp crystal orientation using micro-computed tomography (micro-CT) and microbeam X-ray diffractometry in the anterior cortical bone of human mandible. The intensity and direction of mechanical stresses in both the alveolar area and mandibular base were compared. The mandibular central incisor region in Japanese bone samples was designated as the region of interest and BMD and BAp crystal orientation in the alveolar area and mandibular base measured. Bone samples were imaged by micro-CT and the data obtained converted into BMD values. BAp crystal orientation was determined by both reflection- and transmission-based microbeam X-ray diffractometry. The diffraction intensity ratio was calculated using X-ray diffraction peaks of (002) and (310). The results showed no difference in BMD values among regions. BAp crystals were oriented predominantly in the mesiodistal direction in the mandibular base and along the direction of masticatory force in the alveolar area. These findings suggest that the mandibular base exhibits long bone-like characteristics, with the mandibular condyle acting as the head of the bone, while in the alveolar area alignment takes place in the direction of masticatory force resulting from mechanical stress exerted via the teeth. Qualitative evaluation revealed clear differences between the mandibular base and alveolar area, suggesting that BAp crystal orientation offers a more precise indicator of bone quality than BMD. [doi:10.2320/matertrans.M2011329]