Alignment of Biological Apatite Crystallites in Premolar and Molar Region in Cortical Bone of Human Dentate Mandible

The mandible exists in a unique mechanical environment subjected to occlusal force and other functional pressures. In dentulous humans, differences in occlusal force and position are believed to be closely linked to the mandible's structural characteristics. The mechanical environment that this bone occupies is difficult to evaluate qualitatively, and it is therefore assessed indirectly by quantitative determination of bone mineral density (BMD). However, in order to understand the loading environment of the human mandible, we must also evaluate local bone quality. In the present study, we therefore sought to identify the structural properties of each tooth in the dentulous human mandible by qualitatively evaluating the crystalline orientation of biological apatite (BAp) as a marker of bone quality. After dividing the area from the alveolar ridge to the apical foramen surrounding the first and second premolar and molar regions of the Japanese dentulous mandible, we measured BAp crystalline orientation and BMD at a total of 4 sites including 2 on the buccal side and 2 on the lingual side. As a result, the orientation of BAp crystallites showed a significant difference between premolar and molar regions, namely preferred orientation in a direction vertical to the occlusal plane (Y-axis) was high for the premolar region and low for the molar region (p < 0.01). These findings indicated that quantitative evaluation of BAp crystallite orientations in the mandibular cortical bone has been revealed to be an effective parameter regarding the local loads that are applied to the human mandible. It was suggested that the distribution map of BAp crystallite orientations in the mandibular cortical bone based on this study would be valuable for planning the prosthodontic treatment in molar and/or premolar regions.