Bone differentiation in porous hydroxyapatite in baboons is regulated by the geometry of the substratum: implications for reconstructive craniofacial surgery.

Previous studies have shown the morphogenesis of bone in a porous hydroxyapatite substratum when implanted intramuscularly in baboons. This hydroxyapatite-induced osteogenesis model was used to study the effect of geometry and pore size of the substratum on bone differentiation. Two distinct geometric configurations of porous hydroxyapatite of different pore size were obtained after hydrothermal conversion of the calcium carbonate exoskeleton of corals (genera Porites and Goniopora). Substrata were blocks of hydroxyapatite in rod configuration of 200- and 500-microns pore size and granular hydroxyapatite (400 to 620 microns in diameter) of 200- and 500-microns pore size. A total of 64 granular hydroxyapatite implants and 32 hydroxyapatite rods were implanted intramuscularly in 8 subadult male baboons (Papio ursinus). Specimens were harvested on days 60 and 90 and subjected to histologic and histomorphometric analysis. With the exception of an island of bone that formed in two implants of granular hydroxyapatite of 500-microns pore size, bone differentiation occurred only in blocks of hydroxyapatite in rod configuration of either pore size at both observation periods. The lack of bone formation in the granular hydroxyapatite indicates the critical role of geometry of the substratum in bone differentiation. These results in primates may have implications for the appropriate selection of porous bone substitutes for reconstructive craniofacial surgery in humans.