Correlation between Hydroxyapatite Crystallite Orientation and Ultrasonic Wave Velocities in Bovine Cortical Bone
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The mineral component of bone is mainly composed of calcium phosphate, constituting 70% of total bone mass almost entirely in the form of hydroxyapatite (HAp) crystals. HAp crystals have a hexagonal system and uniaxial elastic anisotropy. The objective of this study was to investigate the effect of HAp crystallite preference on macroscopic elasticity. Ultrasonic longitudinal wave velocity and the orientation of HAp crystallites in bovine cortical bone are discussed, considering microstructure, density, and bone mineral density (BMD). Eighty cube samples of cortical bone were made from two bovine femurs. The orientation of HAp crystallites was evaluated by integrated intensity ratio of (0002) peak using an X-ray diffractometer. Ultrasonic longitudinal wave velocity was investigated with a conventional pulse system. The intensity ratio of HAp crystallites and velocity were measured in three orthogonal directions; most HAp crystallites aligned in the axial direction of the femurs. Our results demonstrate a linear correlation between velocity and intensity ratio in the axial direction. Significant correlation between velocity and BMD values was observed; however, the correlation disappeared if we focused on the identical type of microstructure. In conclusion, differences in microstructure type have an impact on density and BMD, which clearly affects the velocity. In addition, at the nanoscopic level, HAp crystallites aligned in the axial direction also affected the velocity and anisotropy.
KeywordsBone density technology Ultrasound Crystal structure/crystallinity Bone histology/histomorphometry Bone architecture/structure
This study was partly supported by the Academic Frontier Research Project of the “New Frontier of Biomedical Engineering Research” by Doshisha University and the Ministry of Education, Culture, Sports, Science, and Technology (Japan) and a bilateral joint project between the The Centre National De La Recherche Scientifique (CNRS) and the Japan Society for Promotion of Science supported by the Japan Society for Promotion of Science. We also thank Gregory O’Dowd of the English Department of Hamamatsu University School of Medicine for his assistance in proofreading the manuscript.
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