Recently, there have been remarkable advances in medical techniques for regenerating bone defects. To determine the degree of bone regeneration, it is essential to develop a new method that can analyze microstructure and related mechanical function. Here, quantitative analysis of the orientation distribution of biological apatite (BAp) crystallites by a microbeam X-ray diffractometer system is proposed as a new index of bone quality for the evaluation of regenerated bone microstructure. Preferential alignment of the BAp c-axis in the rabbit ulna and skull bone, regenerated by controlled release of basic fibroblast growth factor (bFGF) was investigated. The BAp c-axis orientation was evaluated by the relative intensity between the (002) and (310) diffraction peaks, or the three-dimensional texture for the (002) peak. It was found that new bone in the defects was initially produced without preferential alignment of the BAp c-axis, and subsequently reproduced to recover towards the original alignment. In other words, the BAp density recovered prior to the BAp orientation. Perfect recovery of BAp alignment was not achieved in the ulna and skull defects after 4 weeks and 12 weeks, respectively. Apparent recovery of the macroscopic shape and bio-mineralization of BAp was almost complete in the ulna defect after 4 weeks. However, an additional 2 weeks was required for complete repair of BAp orientation. It is finally concluded that orientation distribution of BAp crystallites offers an effective means of evaluating the degree of microstructural regeneration, and also the related mechanical function, in regenerated hard tissues.

HAp and the related calcium phosphates have been of great interest as biological materials for regenerating hard tissues, but their solubility should be improved to fit the remodeling cycle of hard tissues in vivo before their practical use for bone grafts. Control of the grain size of hydroxyapatite (HAp) matrix and change in constituent phase of the surface layer were examined to improve the dissolution rate in HAp ceramics. Surface and boundary layers composed of calcium phosphate phases with high solubility were formed by annealing at 1350 C in a vacuum, and the apparent mass transfer coefficient in the initial stage of the solubility experiment remarkably increased in an acetate buffer solution at pH of 4.0. The soluble surface and boundary layers are composed of tetracalcium diphosphate monoxide (TTCP), alpha-tricalcium phosphates (alpha-TCP) and amorphous calcium oxide (CaO) with high solubility, and their formation mechanism was discussed. Refinement of the grain size of HAp matrix also contributed to a slight increase in the apparent solubility.