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

Posterolateral lumbar spinal fusion with use of recombinant human bone morphogenetic protein 2 (rhBMP-2) was tested in rabbits by implanting composites of rhBMP-2 and collagen carrier.

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 posterolateral spinal fusion in rabbits.

In animal models, rhBMP-2--impregnated collagen has been successfully used for posterolateral spinal fusion, indicating that it is a potential substitute for the autogenous corticocancellous bone graft currently used most routinely in posterolateral lumbar spinal fusion.

Nine rabbits were divided into three equal groups. The bilateral L4-L5 transverse processes were exposed, and collagen strips impregnated with rhBMP-2 (10, 50, or 200 micrograms) were placed on the left transverse processes, and collagen strips alone were inserted on the right. All rabbits were killed 24 weeks after surgery. The implanted sites were assessed for new bone formation and bony fusion by radiography and histologic examination.

New bone formation was noted in intertransverse spaces on the left side of all rabbits except one (10 micrograms rhBMP-2). Twelve weeks after implantation, no new bone formation was seen on the right side of all animals. The newly formed bone masses were significantly larger in the 50-microgram and 200-microgram rhBMP-2 groups than in the 10-microgram rhBMP-2 group (P < 0.01), but there was no significant difference between bone formation in the 50-microgram and 200-microgram groups (P = 0.647).

The rhBMP-2/collagen composite implant was an effective bone graft substitute for achieving posterolateral spinal fusion. When combined with a collagen carrier, the optimal rhBMP-2 dose for achieving posterolateral spinal fusion seemed to be approximately 50 micrograms per segment in rabbits.