Evaluation of carriers of bone morphogenetic protein for spinal fusion

Study Design. Posterolateral lumbar transverse process fusion in a rabbit m odel was performed using two different carriers for recombinant human morph ogenetic protein-2, one having a porous structure acid the other being a Ty pe 1 collagen sheet. Objectives. To compare the effectiveness of two different carriers for reco mbinant human morphogenetic protein-2 in achieving lumbar intertransverse p rocess arthrodesis. Summary of Background Data. The application of osteoinductive growth factor s at various anatomic sites, such as in long bones and spinal segments, has been performed experimentally by many researchers. Although many carriers of osteoinductive factors have been reported, the most effective carrier ha s not been established. We have reported the efficacy of sintered bovine bo ne, True Bone Ceramics, which is coated with Type I collagen as a carrier o f recombinant human bone morphogenetic protein-2 in achieving lumbar intert ransverse process arthrodesis. True Bone Ceramics is a crystallized form of bone minerals made from sintering bovine bone at high temperatures and pos sesses natural trabecular structure, The crystalline character of True Bo n e Cera mics is similar to that of artificial hydroxyapatite. In this study we focused on the structure of two different carriers to facilitate osteosy nthesis in lumbar arthrodesis. Methods, Fifty-four adult rabbits underwent bilateral lumbar intertransvers e process arthrodesis at L4-L5. The animals were divided into five groups a nd had implants placed as follows: Group 1, autograft group, harvested auto logous corticocancellous bone from the posterior iliac crest; Group 2. TBC group, True Bone Ceramics alone; Group 3, TBC-TBMP group, True Bone Ceramic s coated with Type I collagen infiltrated with 100 mug of recombinant human bone morphogenetic protein-2; Group 4, collagen group, Type I collagen she et; and Group 6, collagen-BMPgroup, implanted collagen sheet containing 100 mug of recombinant human bone morphogenetic protein-2. Spinal fusion was e valuated by radiographic analysis, manual palpation, biomechanical testing, and histologic examination at both 3 and 6 weeks after surgery. Results. Radiographs in the TBC-TBMP group showed a continuous trabecular p attern within the intertransverse area at 3 weeks after surgery. The fusion mass in the intertransverse area was more prominent than in the other grou ps. At 3 weeks after surgery the TBC-TBMP group had higher fusion rates bas ed on manual palpation, and the fusions showed significantly higher tensile strength and stiffness. The histologic findings in the TBC-TBMP group at 3 weeks after surgery showed a cortical bone rim around the edge of the fusi on mass, and contiguous new bone appearing between the recipient bone and t he matrix of TBC without evidence of foreign body formation. In the collage n-BMP group, less mature bone formation was present within the grafted area and the new bone was not contiguous, even at 6 weeks after surgery. Conclusions. As a carrier for recombinant human bone morphogenetic protein- 2, True Bone Ceramics, possessing a bony or porous structure, was more effe ctive than a Type 1 collagen sheet in achieving a faster and stronger lumba r spinal fusion in a rabbit model.