Histological and histochemical analyses of acid and alkaline phosphatases around hydroxyapatite-coated implants: a time course study in rabbit

One of the concerns pertaining to hydroxyapatite (HA)-coated dental implants is their possible dissolution over time, with ultimate loss of the implant. HA-coated dental implants were inserted medially into rabbit femoral articular knee joints. The implants were retrieved after 1 week, 1 month, 3 months and 6 months. At 1 week, many alkaline phosphatase (ALP)-positive osteoblasts were observed near the implant surface. No signs of degradation of the coating were present. At 1 month, only few ALP-positive osteoblasts were present in the marrow spaces at the interface with the coating. Many acid phosphatase (ACP)-positive cells were observed near the coating. In many marrow spaces the HA coating was reduced in thickness. At 3 months, a further reduction of the thickness of the coating, in the areas in contact with marrow spaces, was seen. Many ACP-positive cells were present in tight contact with the coating. Few ALP-positive osteoblasts were present. At 6 months, in many areas in contact with marrow spaces, the HA coating was completely resorbed. The histological and histochemical results seem to point to the existence of two types of ACP-positive cells: multinuclear, often in the process of phagocytosing HA particles, and mononuclear, with a morphology similar to osteoclasts.