PORE-SIZE OF POROUS HYDROXYAPATITE AS THE CELL-SUBSTRATUM CONTROLS BMP-INDUCED OSTEOGENESIS

To elucidate the biochemical mechanism of osteogenesis, the effect of matrix geometry upon the osteogenesis induced by bone morphogenetic pr otein (BMP) was studied. A series of five porous hydroxyapatites with different pore sizes, 106-212, 212-300, 300-400, 400-500, and 500-600 mu m, was prepared. A block (approximately 5 x 5 x 1 mm, 40.0 mg) of e ach hydroxyapatite ceramics was combined with 4 mu g of recombinant hu man BMP-2 and implanted subcutaneously into the back skin of rat. Oste oinductive ability of each implant was estimated by quantifying osteoc alcin content and alkaline phosphatase activity in the implant up to 4 wk after implantation. In the ceramics of 106-212 mu m, the highest a lkaline phosphatase activity was found 2 wk after implantation, and th e highest osteocalcin content 4 wk after implantation, consistent with the results observed with particulate porous hydroxyapatite [Kuboki, Y. et al. (1995) Connect. Tissue Res. 32: 219-226]. Comparison of the alkaline phosphatase activities at 2 wk and the osteocalcin contents a t 4 wk after implantation revealed that the highest amount of bone was produced in the ceramics implants with pore size of 300-400 mu m. In the ceramics with smaller or larger pore sizes, the amount of bone for mation decreased as the pore size deviated from 300-400 mu m. The resu lts indicated that the optimal pore size for attachment, differentiati on and growth of osteoblasts and vascularization is approximately 300- 400 mu m. This study using chemically identical but geometrically diff erent cell substrata is the first demonstration that a matrix with a c ertain geometrical size is most favorable for cell differentiation.