Behavior of dense and porous hydroxyapatite implants and tissue response in rat femoral defects

Porous and dense hydroxyapatite cylinders (PHA and DHA) were implanted into cavities produced in rat femora and the sites of implantation were examined at different times over a period of 24 weeks by microradiologic and histological techniques. Microradiographs showed the presence of a layer of trabecular bone around the implants, which became more radiopaque and thinner along the experimental time. The microradiologic methodology used was suitable for the evaluation of the interface between hydroxyapatite and newly formed bone in nondecalcified materials. Microscopic observations showed that young bone grew over the surface of both types of implants after 1 and 2 weeks of surgery and that bone also grew inside PHA implants. Progressive bone absorption was observed in both types of implants after the fourth week. A layer of fibrous tissue was formed in the interface between new bone and DHA. Mature bone with haversian systems surrounded DHA implants and filled the pores of PHA implants throughout the experimental period. The pores of PHA implants were smaller than those commonly reported, which should have been a disadvantage, although it was observed that the extra cellular fluid induced disintegration of the ceramic granules, allowing the gradual growth of bone tissue into the spaces among them, without the interposition of fibrous tissue.