Jianxi Lu

This study was undertaken to understand the biodegradation mechanisms of calcium phosphate (Ca-P) biomaterials with different crystallization. Two types of sintered Ca-P porous ceramic (HA and beta-TCP) and a Ca-P bone cement (CPC) were implanted into cavities drilled in rabbit femoral and tibiae condyles. The results have shown that a material biodegradation was rapid in the beta-TCP and the CPC, but very weak in the HA. This biodegradation presented a decrease of material volume from the periphery to the center as well as a particle formation causing phagocytosis by numerous macrophages and multinucleated giant cells in the CPC. In the beta-TCP, there was a peripheral and central decrease of material volume as well as an absence of particle formation or visible phagocytosis. The process of biodegradation is considered to be directly influenced by the type of material crystallization. The sintered bioceramics processed at a high temperature exhibit good crystallization and are primarily degraded by a process dependent on interstitial liquids. However, the bone cement is formed by physicochemical crystallization and is degraded through a dissolution process associated with a cellular process.

In order to understand the influence of the implantation site on bone biomaterial evaluation, we implanted cylinders of HA and beta-TCP ceramics in the femoral diaphysis and condyle of rabbits. After 3, 8, 12, and 24 weeks of implantation, histological investigation and histomorphometry were performed on undecalcified samples. Our results show that spontaneous bone healing in the empty cavities is significantly different (p < 0.05) between cortical (SBH > 80%) and cancellous bone sites (SBH < 31%) and that no new bone is formed in marrow tissue. For both porous ceramics, the highest osteogenesis was obtained in the cortical site. Osteogenesis was intermediate in the cancellous site and weak in the medullar site. The material biodegradation was the strongest in the medullar site and higher in the cancellous site than in the cortical site. Both activities were better in the beta-TCP than in the HA (p > 0.05). The marrow tissue presents a foreign-body reaction more reliable, sensitive, and durable than other bone tissues. Therefore, the cancellous bone site is a good site for evaluation of the biofunctionality of biomaterials because of the equilibrium of the osteogenesis and the biodegradation activities, but marrow tissue seems to be better for testing material biocompatibility in vivo.

It is noticeable that porous architectural characteristics of the biomaterials play an important role in revascularization of the scaffold. However, there has been no consensus regarding the optimal conditions for vascularization, including macropore size, shape, interconnection, and the arrangement of macropores, due to the failure to accurately control porous structure of biomaterials. To investigate the effect of the porous structure parameters on vascularization of the biomaterials, an accurate control of these parameters is required. In this study, porous β-tricalcium phosphate (β-TCP) with accurately controlled pore parameters is fabricated by using assembled organic microspheres as templates combined with casting technique. Using this technique, we produced a series of disk-type β-TCP with variable pore sizes and variable interconnections to evaluate the influence of macropore size and interconnection on the vascularization of bioceramic material in vivo. The vascularization of β-TCP implanted in the rabbit model is evaluated by histomorphology and single photon emission computed tomography. The results showed that the pore parameters affect not only the size of the blood vessels growing into the porous structure but also the number of blood vessels formed in the pores of the bioceramic. The increase in pore size only resulted in an increase in size of the blood vessels growing into the macroporous of the bioceramic scaffolds. However, with the increase in size of interconnection, both the size and number of the blood vessels formed in the macroporous increased. Therefore, we conclude that the size of the interconnections is more important for vascularization in the scaffold compared with the pore size. On the other hand, there was no significant difference in vascularization in the scaffolds with pores size above 400 μm, and there was no marked increase in extent of vascularization with further increase in pore size above 400 μm, indicating that the upper limit of pore size for vascularization is 400 μm.