Experimental spinal fusion with use of recombinant human bone morphogenetic protein 2

Study Design. Posterolateral lumbar spinal fusion with use of recombinant h uman bone morphogenetic protein 2 (rhBMP-P) was tested in rabbits by implan ting composites of rhBMP-2 and collagen carrier. Objectives. To examine the bone-formation-inducing activity of rhBMP-2 and find the optimal amount of rhBMP to add to a collagen carrier to constitute bone-formation-inducing implants to be substituted for bone graft in poste rolateral spinal fusion in rabbits. Summary of Background Data. In animal models, rhBMP-2-impregnated collagen has been successfully used for posterolateral spinal fusion, indicating tha t it is a potential substitute for the autogenous corticocancellous bone gr aft currently used most routinely in posterolateral lumbar spinal fusion. Methods. Nine rabbits were divided into three equal groups. The bilateral L 4-L5 transverse processes were exposed, and collagen strips impregnated wit h rhBMP-2 (10, 50, or 200 mu g) were placed on the left transverse processe s, and collagen strips alone were inserted on the right. All rabbits were k illed 24 weeks after surgery. The implanted sites were assessed for new bon e formation and bony fusion by radiography and histologic examination. Results. New bone formation was noted in intertransverse spaces on the left side of all rabbits except one (10 mu g rhBMP-2). Twelve weeks after impla ntation, no new bone formation was seen on the right side of all animals. T he newly formed bone masses were significantly larger in the 50-mu g and 20 0-mu g rhBMP-2 groups than in the 10-mu g rhBMP-2 group (P < 0.01), but the re was no significant difference between bone formation in the 50-mu g and 200-mu g groups (P = 0.647). Conclusions. The rhBMP-2/collagen composite implant was an effective bone g raft substitute for achieving posterolateral spinal fusion. When combined w ith a collagen carrier, the optimal rhBMP-2 dose for achieving posterolater al spinal fusion seemed to be approximately 50 mu g per segment in rabbits.