Preclinical evaluation of injectable bone substitute materials.
BACKGROUND In recent clinical trials demonstrating the efficacy of recombinant human bone morphogenetic protein-2 (rhBMP-2) for the acceleration of bone-healing, investigators used carriers requiring open surgery for administration. In this study, we used a nonhuman primate fibular osteotomy model to evaluate injectable rhBMP-2/carrier formulations that can be administered in closed fractures. METHODS The fibular osteotomy model was first characterized by evaluating surgically harvested fibular segments containing untreated osteotomy sites (controls) from seventy adult male Cynomolgus monkeys at eight weeks (twenty-four monkeys), ten weeks (thirty-four), twelve weeks (six), and fourteen weeks (six). Fibular segments, from twenty-four animals, in which an osteotomy had not been performed served as normal controls (intact). The contralateral limb of twenty-four of the animals was then used to evaluate the effect of rhBMP-2 administered, three hours after the osteotomy, in eight carrier formulations (buffer, calcium phosphate paste, and hyaluronan gel, hyaluronan paste, and gelatin foam formulated with and without tricalcium phosphate granules). Each carrier was used in three monkeys. At ten weeks, the fibulae with the treated osteotomy sites were harvested and were compared with the contralateral, untreated osteotomized fibulae (paired control). The most promising carrier, calcium phosphate paste (alpha bone substitute material, or alpha-BSM), was then evaluated in eleven additional animals. The outcomes included the findings on radiographs made weekly until the time of fibular harvest, the callus area, the biomechanical properties, and the histologic findings. RESULTS Radiographic and histologic studies confirmed complete bridging of the control osteotomy sites in most animals by fourteen weeks. The mean torsional stiffness and maximum torque of the control osteotomy sites were 42.7% and 53.7%, 55.2% and 60.4%, 66.7% and 66.4% of the mean torsional stiffness and maximum torque of the intact fibulae at eight, ten, and twelve weeks, respectively, but they were not substantially different from the mean torsional stiffness and maximum torque of the intact fibulae at fourteen weeks (82.3% and 79.8%). In the carrier screening study, outcome measures of healing were more consistently enhanced in the rhBMP-2/alpha-BSM-treated osteotomy sites. In the confirmatory study, the mean callus area, torsional stiffness, and maximum torque were 86%, 72%, and 68% greater in the rhBMP-2/alpha-BSM-treated osteotomy sites than in the paired-control osteotomy sites at ten weeks (p < 0.001). The torsional stiffness and maximum torque in the rhBMP-2/alpha-BSM-treated osteotomy sites were equal to those in the intact fibulae, whereas those parameters in the paired-control osteotomy sites were only 55% and 58%, respectively, of the torsional stiffness and maximum torque of the intact fibulae. Histologic analysis confirmed complete osseous bridging of the rhBMP-2/alpha-BSM-treated osteotomy sites but incomplete bridging of the paired-control osteotomy sites at ten weeks. CONCLUSIONS A single percutaneous injection of rhBMP-2/alpha-BSM accelerates the healing of fibular osteotomy sites in nonhuman primates by approximately 40% compared with the healing of untreated osteotomy sites.