Bone formation in the presence of phagocytosable hydroxyapatite particles

Small particles of biomaterials used in orthopaedic surgery have been shown to induce the resorption of bone. The purpose of this study was to determine whether phagocytosable particles of hydroxyapatite had an adverse effect on bone ingrowth. Bone harvest chambers were implanted bilaterally in the proximal tibial metaphyses of 13 mature rabbits. The bone harvest chamber has a transverse 1-mm wide pore, providing a continuous canal through the chamber for tissue ingrowth. After an initial 6-week period for osseointegration of the bone harvest chambers, the contents of the canal were harvested at 3-or 6-week intervals. Hydroxyapatite particles (diameter, 5 mu) were mixed with a carrier solution, 1% sodium hyaluronate, and implanted in the canal of one chamber in each animal. The contralateral chamber was implanted with the carrier only and served as a control. Histological sections from the tissue harvested from the chambers were evaluated by light microscopy and histomorphometry, and the area of bone ingrowth was measured as a percentage of total area in each section. At 3 weeks there was more bone in the hydroxyapatite sections than in controls; at 6 weeks there was no difference. Hydroxyapatite particles were incorporated within the matrix of new ingrown bone at both time periods. There was no evidence of granuloma formation or inflammation. Previous studies have shown that particles of high density polyethylene and bone cement adversely affect bone ingrowth in this model. The present results suggest that hydroxyapatite particles, small enough to be phagocytosed by macrophages, did not have such effects.