Resorption of synthetic porous hydroxyapatite and replacement by newly formed bone.

Synthetic porous hydroxyapatite (HA) is commonly used as a bone substitute for bone defects which, previously, would have been treated by autogenous bone grafting. HA has been thought to be a nonbiodegradable material that remains as it is implanted. However, after long-term follow-up, some authors report that the margin of implanted HA blocks or granules is absorbed, suggesting that HA is biodegradable. We experienced a patient in whom synthetic HA blocks implanted in a bone defect of the ilium after the harvesting of full-thickness bone for grafting were extensively absorbed and replaced by newly formed bone 6 years and 7 months after the implantation. Therefore, we conclude that HA is biodegradable. Sintering temperature, porosity, and pore diameter seem to influence the biodegradability of HA.