Role of interconnections in porous bioceramics on bone recolonization in vitro and in vivo

The interconnections in a porous biomaterial are the pathways between the p ores. They conduct cells and vessels between pores. Thus they favour bone i ngrowth inside ceramics. The aim of our study was to determine the effect o n bone ingrowth of interconnections in two ceramics: hydroxyapatite (HA) an d beta-tricalcium phosphate (beta-TCP) with the same porosity of about 50% and a mean pores size of 100-300 mu m and a mean interconnection size of 30 -100 mu m In vitro, four discs for osteoblast culture were studied after 14 and 28 days of incubation. The results show that human osteoblasts can pen etrate interconnections over 20 mu m in size, and colonize and proliferate inside macropores, but the most favourable size is over 40 mu m. In vivo, e ight cylinders were implanted in the middle shaft of both rabbit femurs for 12 or 24 weeks. The histomorphometric results show that interconnections i n porous ceramics favour bone ingrowth inside the macropores. In the HA gro up the rate of calcification and bone ingrowth do not differ, and chondroid tissue is observed inside pores. But in beta-TCP, the calcification rate a nd the bone ingrowth increased significantly. At week 12 significant correl ation between new bone ingrowth and the size of the interconnections is obs erved between new bone ingrowth and the density of pores. In conclusion we notice that in vivo a 20 mu m interconnection size only allows cell penetra tion and chondroid tissue formation; however the size of the interconnectio ns must be over 50 mu m to favour new bone ingrowth inside the pores. We pr opose the concept of "interconnection density" which expresses the quantity of links between pores of porous materials. It assures cell proliferation and differentiation with blood circulation and extracellular liquid exchang e. In resorbable materials, pore density and interconnection density are mo re important than their size, contrary to unresorbable materials in which t he sizes and the densities are equally important. (C) 1999 Kluwer Academic Publishers.