THE EFFECTS OF ALLOPLASTIC IMPLANT ONLAYS ON BONE IN THE RABBIT MANDIBLE

The relative importance of the factors that contribute to the bone ero sion seen underlying alloplastic implants remains unknown. This study was designed to examine the effects of different alloplastic onlays on the mandible in a rabbit animal model. Three alloplastic materials (s oft solid silicone, methyl methacrylate, and porous polyethylene) were examined to determine the importance of implant composition in bone r esorption. Thin (1.5-mm) and thick (6-mm) onlays were used to determin e the relative importance of implant thickness. Fluorescent markers we re administered at different time intervals to evaluate the patterns o f new bone formation. A total of 96 implants placed on the mandibles o f 16 animals were examined grossly, histologically, and under fluoresc ent microscopy after 12 and 24 weeks. The frequency and degree of bone erosion were related both to the type of alloplast and to the thickne ss of the implant. Silicone implants were associated with the greatest amount of bone resorption (p < 0.01). Thicker silicone implants were associated with a greater degree of thinning (p < 0.05). Bone resorpti on beneath the implants was documented by the presence of osteoclasts actively eroding bone and by changes in the normal patterns of the flu orescent markers. Both the thin methyl methacrylate and the thin porou s polyethylene implants had a low incidence of bone thinning. The thic k methyl methacrylate implants caused more erosion than the thin impla nts of the same material, although this was less than the silicone gro ups. Increasing the thickness of porous polyethylene implants did not adversely affect the underlying bone. In conclusion, implant compositi on was an important variable in this model. Silicone was associated wi th the greatest frequency and degree of bone erosion.