GEOMETRY OF POROUS HYDROXYAPATITE IMPLANTS INFLUENCES OSTEOGENESIS INBABOONS (PAPIO-URSINUS)

Coral-derived porous hydroxyapatite disks of two geometric configurati ons were implanted in 16 calvarial defects, 25 mm in diameter, prepare d in eight subadult male baboons (Papio ursinus). To determine whether the orientation of the exoskeletal microstructures (corallites) influ ences bone ingrowth and osteogenesis, hydroxyapatite disks were prepar ed by cutting the coral either longitudinally or transversally, to obt ain two distinct implant geometries defined by the orientation of the corallites. In the same animals, 16 defects were left untreated to ass ess the regeneration potential of the subadult baboon calvaria. Disks of both geometric configurations were also implanted in heterotopic in tramuscular sites to investigate their osteoinductive potential. Histo morphometric analysis on undecalcified and decalcified sections prepar ed from specimens harvested on day 90 after surgery showed that greate r amounts of bone formed in porous hydroxyapatites cut in the longitud inal plane when compared with hydroxyapatites cut in the transversal p lane (p < 0.01). Bone formed in hydroxyapatite specimens harvested fro m the intramuscular sites, but no difference in bone formation was fou nd between the two geometric configurations. Untreated defects showed limited osteogenesis, comparable with a previous series of untreated d efects identically prepared in adult baboon calvariae. This finding su ggests that skeletally mature, adult primates may not be a requirement for evaluation of craniofacial. bone substitutes. These results in a primate indicate that hydroxyapatite disks are able to induce bone dif ferentiation when implanted in intramuscular sites, and that the geome tric configuration of the porous hydroxyapatite influences bone ingrow th and osteogenesis in orthotopic calvarial sites. This should be take n into consideration when designing porous bone substitutes for cranio facial applications.