Hydroxyapatites (HAPs), which are synthetic ceramics (complex calcium phosphates), are very similar to bone and tooth minerals in their complex and have a biocompatibility. Recently, HAPs are used as a bone implant material in variety of situations, eg, surgical reconstruction, augmentation of the facial skeleton, also used to fill acquired and congenital bone defects.
Autogenous particulate cancellous bone and marrow (PCBM) had been known to have an active osteogenesis. These implant materials produce desirable effects on regeneration in osseous defects.
The aim of this investigation was to ascertain experimentally the biocompatibility, osteoinductive and/or conductive ability of a combined implant material with HAP and autogenous bone in an ectopic situation. Mixed type of materials, porous HAP granules and PCBM (volume ratio, HAP: PCBM=2: 1), were implanted into adult dogs using an vessel attached type incubating chamber which enabled to make the circumstances being free from the influences of recipient site. This chamber was attached to bilateral carotid artery, HAP-PCBM mixed group was implanted on one side and HAP alone group on the other side. Periodical (7, 14, 28, 60, 90 days) histopathological examinations were carried out by light and electron microscopies after implantation.
The results were as follows:
New bone formation was not observed in HAP alone group throughout the experimental period (90 days). Implanted HAP was encapsulated with fibrous tissue and these tissues were infiltrated with a few inflammatory cells. Macrophages and multinuclear giant cells (MGC) were observed on the HAP-tissue interface (MGC, I supposed to be a fusion cell type of macrophage). HAP resorption occurred by the activities of macrophage and MGC.
After 28 days, a newly formed calcified bone matrix surrounded by transplanted trabecular bone chips was observed in HAP·PCBM group. These new bones, initially grew toward the HAP granules, were formed along with the HAP's surface. The osteoconductive ability of HAP in an ectopic site was also confirmed with this results. Under the same experimental conditions, a lamellar bone formation, which was a matured type of bone formation, was confirmed after 90 days.
MGC have a different ultramicroscopical features depending on the contact materials. Namely, MGC surrounding a newly formed bone matrix had a ruffled border and clear zone, which were characteristic in osteoclast, but N1GC around HAP or transplanted bone chips had not such organelles.