BONE DIFFERENTIATION IN POROUS HYDROXYAPATITE IN BABOONS IS REGULATEDBY THE GEOMETRY OF THE SUBSTRATUM - IMPLICATIONS FOR RECONSTRUCTIVE CRANIOFACIAL SURGERY

Previous studies have shown the morphogenesis of bone in a porous hydr oxyapatite 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 diff erent pore size were obtained after hydrothermal conversion of the cal cium carbonate exoskeleton of corals (genera Porites and Goniopora). S ubstrata were blocks of hydroxyapatite in rod configuration of 200- an d 500-mum pore size and granular hydroxyapatite (400 to 620 mum in dia meter) of 200- and 500-mum pore size. A total of 64 granular hydroxyap atite implants and 32 hydroxyapatite rods were implanted intramuscular ly in 8 subadult male baboons (Papio ursinus). Specimens were harveste d on days 60 and 90 and subjected to histologic and histomorphometric analysis. With the exception of an island of bone that formed in two i mplants of granular hydroxyapatite of 500-mum pore size, bone differen tiation occurred only in blocks of hydroxyapatite in rod configuration of either pore size at both observation periods. The lack of bone for mation in the granular hydroxyapatite indicates the critical role of g eometry of the substratum in bone differentiation. These results in pr imates may have implications for the appropriate selection of porous b one substitutes for reconstructive craniofacial surgery in humans.