Bone induction in a composite allogeneic bone/alloplastic implant.
This study evaluated the morphogenetic properties of the bone matrix as a biological delivery system in a composite allogeneic bone/alloplastic implant placed in extraskeletal recipient sites in rats. Seventy-two allogeneic diaphyseal bone cylinders were chemosterilized to obtain autolyzed antigen-extracted allogeneic (AAA) bone and implanted in twenty 140- to 160-day-old rats. In 30 implants, porous hydroxyapatite biomatrix (HAB) was inserted into the AAA bone cylinders creating the composite implants. Serial sections from the specimens harvested at 23, 30, 45, 53, and 63 days showed that 84% of the AAA bone cylinders induced the differentiation of variable amounts of endochondral bone and that in 61% bone formed in the interior of the cylinders; 74% of the composite implants had no bone-inductive effect, recruiting instead macrophages and multinucleated giant cells at the HAB/mesenchymal interface. In all instances bone failed to form within the porous HAB or in proximity to the AAA bone matrix facing the interior of the cylinders. When present, bone formed only in localized areas within the matrix. Histologic analysis showed that high deposits of induced bone are correlated to high levels of matrix vascular invasion and mesenchymal cell aggregation. Within the limits of this study on rodents, the results suggest that the HAB inhibits and delays the local differentiation of induced bone in the composite implants.