Dissolution of dense carbonate apatite subcutaneously implanted in Wistar rats

Hydroxyapatite (HA) is used as a biomaterial in orthopedic applications because it is similar in composition to bone mineral; however, carbonate apatite (CHA) is closer in chemical composition to bone mineral because bone mineral contains significant amounts of carbonate, yet there have been few reports comparing biological responses to HA and CHA. It is generally agreed that bone forms a bond of some kind to HA, and there is conflicting evidence as to whether HA is resorbed in vivo or not. However, comparative reports generally agree that beta-tricalcium phosphate (beta-TCP) is removed faster than HA from an implant site by an undetermined combination of resorption and/or dissolution. The work reported here attempts to provide the first directly comparable subcutaneous dissolution data for dense sintered HA, beta-TCP, and CHA in rats. The weight losses of HA, beta-TCP, and 3.2 wt % CHA were approximately 60, 520, and 320 mg m(-2) day(-1), respectively. Histological sections did not show any evidence of giant cells, and all of the samples were encapsulated with fibrous tissue. beta-TCP and (to a lesser degree) CHA were found to be resistant to dissolution around the edges of the sample. An X-ray analysis did not indicate that any phase transformation had occurred in the dissolution resistant region.