Design of biomaterials for bone replacement based on parameters determining bone quality

In addition to the bone mineral density (BMD), new parameters determining the bone quality have been investigated recently because bone is a well-organized hierarchical composite at various scale levels. BMD is nothing but the density of biological apatite (BAp); however, the crystallographic orientation of BAp crystallites corresponds to the rotation of BAp crystallites, and these two parameters are independent. In other words, the degree of BAp orientation might be a parameter determining bone quality. Thus, our group has studied the preferential degree of the BAp c-axis orientation in normal, pathological, and regenerated bones using micro beam X-ray diffraction. The preferential degree of the BAp c-axis orientation strongly depends on factors such as the bone position, in vivo stress distribution, bone growth, degree of pathology and bone regeneration, turnover rate, activity of bone cells, and gene defects. Correlations are clearly observed among the BAp orientation, in vivo stress distribution, and mechanical function in normal, pathological, and regenerated bones, including mandibles. Because of the anisotropy of bone microstructure based on the BAp orientation distribution, implants for bone replacement should be developed by taking into account the structural and/or material anisotropy. For example, an implant with unidirectional-elongated-through pores was implanted such that the elongated pore direction was parallel or perpendicular to the mesiodistal axis in the mandible with one-dimensional orientation of the BAp c-axis. The degree of calcification and the subsequent orientation of BAp during bone regeneration are greater in the elongated pore parallel to the mesiodistal axis than in the pore perpendicular to the axis, indicating that BMD and BAp orientation of the newly formed bone can be controlled by the elongated pore direction; this is closely related to the anisotropic bone micro-structure and in vivo principal stress direction. The electron beam melting (EBM) method, which is a rapid prototyping technique for developing anisotropic-shaped implants, is a promising candidate for artificially fabricating arbitrarily shaped nonporous and porous materials and even customized medical devices. This technique can be used to fabricate implants having arbitrary morphology from the raw powder metals without the use of a casting mould. Because the BAp orientation distribution strongly influences the mechanical, chemical, and biological functions of bones, methods for controlling the BAp orientation should be developed.