Osteogenesis in bone defects in rats: the effects of hydroxyapatite and demineralized bone matrix.

Hydroxyapatite (HA) and Demineralized Bone Matrix (DBM) are being investigated as potential osteogenic agents with hopes that these substances can be used to induce bone formation in non-union fractures. This study was done to determine the relative effects of HA and DBM implanted as moldable phospholipid composites in bone defects that result in non-unions. We studied 22 ten-month-old Long-Evans male rats with 5.0 mm unilateral radial defects implanted with HA, DBM, and a combination of both substances. Control defects were left unfilled. Eight weeks after implantation, the histological sections demonstrated a decrease in bone formation with HA relative to controls. The HA crystals were encapsulated by connective tissue stroma made up of collagenous elements, fibroblasts, and blood vessels. There were no indications of bone formation within the fibrous stroma. 45Calcium, alkaline phosphatase, and bone gla protein (BGP) assays demonstrated a 16% increase in bone formation in rats implanted with DBM, an 80% decrease in groups implanted with HA (p = 0.01) and an 80% decrease with DBM plus HA (p = 0.01). Radiologic analysis corresponds well with histological and biochemical results. We conclude that osteogenesis in non-union defects is enhanced by the implantation of DBM, while HA interferes with bone formation in the rat model. In the presence of both substances, HA appears to impede new bone growth, negating any positive effects seen with DBM.