Osteoinduction of hydroxyapatite/β-tricalcium phosphate bioceramics in mice with a fractured fibula

Many studies have shown that calcium phosphate ceramics can induce bone formation in non-osseous sites without the application of any osteoinductive biomolecules, but the mechanisms of this phenomenon (intrinsic osteoinduction of bioceramics) remain unclear. In this study, we compared the intrinsic osteoinduction of porous hydroxyapatite/beta-tricalcium phosphate (HA/beta-TCP) implanted in mice at different sites. In 30 mice the left fibula was fractured and the right fibula was kept intact. A porous HA/beta-TCP cylinder was implanted into both the left (group 1) and right (group 2) leg muscles of each animal. In addition, two HA/beta-TCP cylinders were bilaterally implanted into leg subcutaneous pockets (group 3) in each of the remaining 15 mice. New bone formation was studied in the three groups by histology, histomorphometry and immunostaining. In group 1 new bone was observed at week 6 and bone marrow appeared at week 12. In group 2 new bone was observed at week 8 and bone marrow appeared at week 12. The new bone area percentage in group 1 was significantly higher than in group 2 at both weeks 8 and 12. In contrast, group 3 did not show any new bone within the period studied. These differences were explained based on the location of the implants and thus their proximity to the osteogenic environment of fracture healing. The results support the hypothesis that intrinsic osteoinduction by calcium phosphate ceramics is the result of adsorption of osteoinductive substances on the surface. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.