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Apatite ceramics composed of synthetic hydroxyapatite, Ca10(PO4)6(OH)2 were implanted in mandibles of adults dogs. The histological observations indicated that the apatite ceramic was closely contacted with newly formed bone tissue without any rejection phenomenon up to 2 weeks. An electron micrograph of non-decalcified ultra-thin section at 8 weeks showed that the apatite ceramic was directly bonded to newly formed bone and the mineralized bone tissue was grown into the micro pores of the ceramics independent on the pore size. It seemed that the outer surface of the apatite was exchanged by bone tissue. From these results it was considered that the apatite ceramic was a successful implant material in dental and surgical fields.

Hydroxyapatite (HA)-coated implants were developed to promote osseointegration of titanium implants and to overcome the mechanical drawbacks of solid HA implants. Although many clinical reports on the prognosis of HA-coated implants have reported high success rates, the risks of dissolution and weakening of the coating have been noted. We hypothesized that the chemical and mechanical stability of HA coating are affected by its microstructural characteristics. The present study investigates differences in the microstructures of available HA-coated implants, before and after implantation into the coxal bones of dogs for periods ranging from 3 weeks to 10 months and under the coxal periosteum of dogs for 10 months. The results of transmission electron microscopy and energy-dispersive x-ray analysis revealed that crystallization of super-fine HA crystals occurred in the amorphous phase of the HA coating and progressed over time. This crystallization weakens HA-coated implants by making the amorphous phase brittle, causing stress accumulation within the coating, and causing a decrease in the binding strength between the coating and the substrate. Furthermore, the HA coating dissolved in soft tissue. Dissolution started with the super-fine HA crystals in the crystallized portion that was originally part of the amorphous phase.

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