The effects of BMP-7 in a rat posterolateral intertransverse process fusion model

The use of autologous bone grafting is an essential component in spine fusion because it is the key factor in achieving long-term stable arthrodesis between spinal motion segments. However, harvesting autologous iliac crest bone graft can be associated with significant morbidity and its supply is limited. Although no current substitute for autologous graft is available, multiple studies have already established the success of bone morphogenetic proteins (BMPs) in augmenting spinal fusion in models using larger animals. The purpose of our study was to evaluate the ability of BMP to augment a posterolateral intertransverse process single-level fusion in a rat. To our knowledge, this model has not been used to evaluate the effects of recombinant BMPs. A posterolateral intertransverse process fusion was attempted in white male Sprague-Dawley rats. The following are the four study groups: insoluble collagen bone matrix (ICBM) alone, 3 microg BMP-7 + 25 mg ICBM, 10 microg BMP-7 + 25 mg ICBM, and a sham group with no implanted material. The animals were killed on postoperative day 21 and were evaluated for signs of clinical and/or radiographic fusion. All of the rats in the 10 microg BMP-7 + 25 mg ICBM group demonstrated clinical fusion and had solid bilateral fusion masses on radiographs. None of the rats in the sham group, ICBM group, or 3 microg BMP-7 + 25 mg ICBM group fused clinically; however, the rats in the 3 microg BMP-7 + 25 mg ICBM group did show evidence of new bone formation. Our study demonstrates that a rat posterolateral intertransverse process fusion model is inexpensive and efficient and produces consistent results. It also shows that BMP can augment fusion in a rat and that dosing plays a role in successful fusion. This is consistent with results that have been studied in larger animal models.