Using rhBMP-2 along with adequate stabilization yields better fusion rates in the anterior cervical spine than those likely without the protein, a new study shows. But without added stabilization, surgeons run the risk that “aggressive graft remodeling” will lead to instability and ultimately nonunion in these procedures, said one of the authors.
Lead investigator Ben B. Pradhan, MD, MSE, of the Spine Institute at Saint John’s Heath Center, Santa Monica, Calif., attributed the superior fusion rates he found associated with using recombinant human bone morphogenetic protein-2 (rhBMP-2) in part to the added stability provided by a plate.
Pradhan and colleagues chronicled the fusion process in 41 consecutive patients (20 women and 21 men) who underwent anterior cervical discectomy and fusion (ACDF) with fibular allograft, rhBMP-2 (InFuse, Medtronic Sofamor Danek) and plating. The population included 10 one-level, 11 two-level, 18 three-level, and two four-level fusions, with an average age of 55 years. Researchers took radiographs at three, six, 12, 18, and 24 months, and CT scans from three to 10 months.
They compared those results to three previously published historical controls of the senior author (Rick B. Delamarter, MD), which included ACDF surgeries with allograft but no BMP.
“Fusion with BMP was uniformly 100% across one-, two-, three- or even four-level ACDF surgeries and compared favorably to historical controls without BMP,” Pradhan said.
Using radiographs, Pradhan showed how the outline of the allograft is still visible after complete fusion at 12 months following an ACDF without rhBMP compared to ACDF with rhBMP-2. “With the use of BMP-2, note the early graft and endplate resorption and significant trabecular continuity as early as three months,” he said. “At six months, the outline of the allograft is barely visible,” said Pradhan, at the North American Spine Society 20th Annual Meeting. The investigators also used CT scans to document rapid graft and endplate resorption.
Aggressive remodeling
Without additional stabilization, there exists a risk of aggressive graft resorption, subsequent instability and nonunion.
“The problem is there is such intense remodeling occurring early with rhBMP-2 that we really cannot tell on plain X-rays how much structural strength the construct has, which could affect our management of these patients,” Pradhan said. “The clinical relevance for all this is that while BMP-2 certainly increases osseogenic activity, it may not obviate adjunct stabilization when used with structured allografts because of the aggressive remodeling.”
Pradhan said he believes their study radiographically verified that bone graft incorporation occurs in two distinct but overlapping stages: an initial resorptive phase followed by a bone forming phase.
“Based on the study we can divide the fusion process with BMP into four CT radiographic stages: Stage 1 is inflammatory; stage 2 can be called resorption dominated; stage 3 is bone formation, and stage 4 is final incorporation.”
Editor’s note: The FDA has not approved rhBMP-2 for anterior cervical spine fusion.
For more information:
- Pradhan BB, Bae HW, Kropf MA, Patel V, Delamarter RB. Graft resorption with rhBMP-2 in anterior cervical discectomy and fusion: a radiographic characterization of the effect of rhBMP-2 on structural allografts. Presented at the North American Spine Society 20th Annual Meeting. Sept. 28-Oct. 1, 2005. Philadelphia.