MIGRATION OF HYDROXYAPATITE ONLAYS INTO THE MANDIBLE AND NASAL BONE AND LOCAL BONE TURNOVER IN GROWING RABBITS

To determine the effects of local bone turnover on the migration of ma croporous hydroxyapatite onlays in the nasal bone and mandibular ramus , we performed histomorphometric analyses of the underlying bone area in 41 New Zealand White rabbits from the age of 4 weeks. The hydroxyap atite implants were placed under the periosteum of the right nasal bon e (a depository bone onto its periosteal surface and endosteal resorpt ive) and the mandibular ramus (resorptive onto its outer surface). The corresponding left sides were sham operated. Following fluorescence b one labeling, composite specimens of the hydroxyapatite block includin g both sides of the nasal bone and mandible were removed at 0 (n = 1), 3, 6, 9, 12, and 16 weeks postoperatively (n = 8, respectively) and p rocessed to yield undecalcified sections. Bone-bone marrow interfaces in the entire area within 200 mu m beneath the base of the hydroxyapat ite and in the counter-area on the sham-operated side were measured un der a light microscope. In all grafted specimens, the hydroxyapatite m atrix was directly united with the underlying tissue by bone ingrowth. However, the sinking of the hy droxyapatite graft in the nasal bone w as significant at 3 weeks postoperatively and gradually increased ther eafter. In the mandible, the sinking became significant at 6 weeks. In the nasal bone, the bone area density beneath the graft showed a time -dependent decrease during the experimental period, but in the mandibu lar bone, the value was initially decreased at 3 weeks and then recove red to baseline level. In both bones, parameters of bone resorption, s uch as osteoclast number and osteoclast surface, were significantly in creased from 3 weeks. While the parameters of bone formation, such as osteoblast surface and mineralizing surface, were significantly decrea sed from 3 weeks in the nasal bone, they were significantly increased in the mandible. Mineral apposition rate showed a significant decrease in both bones. Our data indicate that while the bone area density ben eath the hydroxyapatite seemed to depend on bone formation, increased bone resorption would be more critical for the remodeling of underlyin g bony architecture in the migration of the hydroxyapatite graft.