Pellets of well-characterized microporous hydroxyapatite (HA) ceramic were implanted in hamsters in two nonosseous sites: (1) in the fatty tissue of the gingival crease, far from bony tissue and (2) in intraperitoneal sites. The implants in site 1 were placed directly in contact with tissues, cells, and extracellular fluids while the implants in site 2 were placed in special chambers made of plexiglass cylinders covered in both ends with millipore filters, preventing contact with tissues and cells, but not with extracellular fluids. The hamsters were sacrificed and the implants recovered after 8, 16, 30, 150, and 365 days. The pellets were characterized using x-ray diffraction, infrared absorption, thermogravimetry, scanning and transmission electron microscopy, and calcium and phosphate analyses before and after implantation. Physicochemical analyses of HA ceramic implants before and after implantation demonstrated the formation of new material which was significantly different from the HA ceramic in terms of the following: (a) morphology (size of shape) of crystals; (b) intimate association of the inorganic phase of the new material with an organic phase similar to inorganic/organic association in bone; (c) the inorganic phase of the new material is a CO3-apatite, similar to that of bone, while the HA in ceramic is CO3-free; (d) electron diffraction of apatite of new material is similar to that of bone apatite. This study also demonstrated that the new material associated with the HA ceramics implanted in two different nonosseous sites were identical in spite of the differences in their microenvironment (cellular and acellular).

Calcium phosphate coatings on dental implants enhance integration of the material. Resorption of the ceramic coatings has raised some concern about the behavior of the bone-implant interfaces after the coating disappearance. Substitution of the OH- ions by fluoride in the hydroxylapatite (HA) lattice makes the calcium phosphate more stable. We investigated the degradation rate of dental implants with 50- and 100-microm coatings of HA, fluorapatite (FA), or fluorhydroxylapatite (FHA). The implants were inserted in dog jaws and retrieved for histological analysis after 3, 6, and 12 months. The thickness of the calcium phosphate coatings was evaluated using an image analysis device. A relative resorption index and its standard deviation were studied. HA and FA coatings (even at 100-microm thickness) were almost totally degraded within the implantation period. In contrast, the FHA coatings did not show significant degradation during the same period. The standard deviation showed that the resorption process for FHA with thicknesses of 50 or 100 microm was the same. Such a difference was not observed between the 50- and 100-microm thick coatings of FA and HA. In conclusion, the FHA coatings showed good integration in the bone tissue and lasted much longer than classic calcium phosphate coatings.

A TEM study was made of the response to synthetic hydroxyapatite prepared in powder form and implanted for 6 and 12 months respectively in infrabony lesions in 2 adult patients with chronic periodontitis and tooth mobility. Round or oval‐shaped crystal aggregates, ranging in diameter from 1 to 20 μm, were surrounded by connective tissue free of inflammatory cells. The aggregates were made up of loosely‐packed individual synthetic hydroxyapatite crystals, with a mean diameter of 128.12±14.57 nm, separated by an amorphous matrix containing few collagen fibrils. In the 6‐month implants small apatite crystals, of a size similar to those found in adjacent alveolar bone and giving similar diffraction patterns, appeared in the center of the aggregates between the relatively large crystals of synthetic hydroxyapatite. These new apatite crystals filled the amorphous matrix progressively from the center to the periphery of the aggregate. The latter was surrounded either by fibroblasts or by osteoblasts and osteoid tissue. In the 12‐month samples a calcified collagenous bone matrix enveloped the crystal aggregates. Typical osteoclasts, lacking a brush border, were evident around certain aggregates. Some osteoclasts contained large vacuoles filled with synthetic hydroxyapatite crystals.