Data lags while surgical biologic options for fusion used for spinal deformity grow

BMPs may surpass other biologic agents, but more follow-up is needed, investigator says.

Issue: April 2007

Orthopedic surgeons have a long list of options to choose from when it comes to biologic facilitators for fusion in adult spinal deformity. However, they would benefit from more clinical evidence to support their choices, according to Keith H. Bridwell, MD.

Keith H. Bridwell, MD
Keith H. Bridwell

He discussed the existing evidence for the various biologic facilitators and called for more studies on the products at the Scoliosis Research Society 41st Annual Meeting.

“I think certain biologic products clearly [increase the fusion rate], and if they can ultimately reduce the pseudarthrosis rate and reduce the harvesting of the autogenous graft, it will decrease morbidity,” Bridwell said. “But … it’s hard to weigh the immediate costs and price of a technologic advance vs. the long-term financial savings.”

Platelet gel, DBM, sponges and allograft

Bridwell eliminated platelet gel as an optimal facilitator, based on a 2005 study by Carreon and colleagues who found a substantial nonunion rate with platelet gel and autologous iliac crest bone graft.

Another facilitator is collagen matrix sponge with autologous bone marrow; but Bridwell said no human data exists for this facilitator. In terms of animal studies, “One study shows that the concept works nicely. Another study suggests that it didn’t work at all. So we have two studies with conflicting data,” Bridwell said.

Of demineralized bone matrix (DBM), Bridwell said quality investigations are still necessary.

He referred to a 2004 study by Cammisa and colleagues, which found roughly equivalent fusion rates when comparing DBM and autograft composite to iliac crest autograft alone. However the fusion rates were low, at 52% and 54%,

This patient underwent primary surgical treatment for a spinal deformity (left) from T10 to S1. She is shown on the right at 3.5 years postoperatively. Although she appears to be fused well at this point, Bridwell reminds surgeons that 5 to 10 years of follow-up is necessary to determine a solid fusion.

Additional views of the patient preoperatively (left) and postoperatively (right) show where surgeons used 108 mg of BMP-2 posteriorly with no iliac bone graft, but aggressive local harvesting. Anteriorly, they used 48 mg of BMP-2 with sponges, but no additional products in the anterior disc spaces.

Images: Bridwell KH

“So maybe there is some benefit to DBMs, but only as a graft extender,” Bridwell said.

As for fresh frozen allografts, structural allografts achieve fusion when protected with posterior segmental spinal instrumentation, but surgeons do not have the evidence to determine whether morsellized allograft achieves the same fusion, Bridwell said.

BMP-2 vs. BMP-7

BMP-2 and BMP-7 have shown an increase in fusion rates while eliminating iliac crest harvesting and reducing the need for anterior surgery, Bridwell said. However, surgeons are limited in their ability to assess a solid fusion with these products.

“Probably the best we can do is analyze oblique X-rays and CTs,” he said. “To some extent, if you analyze all the studies, many use purely the product and many are combining [BMPs] with various quantities of autogenous bone graft and that makes it hard to analyze.”

It is especially important to have substantial follow-up with adult spinal deformity.

“You have to have an absolute minimum of a 5-year follow-up and perhaps even a 10-year follow-up until you know whether or not a long adult spinal deformity fusion is solid or not,” Bridwell said.

Some studies suggest that BMP-2 is more effective than BMP-7, but “that’s probably just a difference in the dosage, concentration and carrier,” Bridwell said. But while several studies suggest that BMP-7 is effective, some of these studies also demonstrate lower fusion rates than with iliac crest bone graft. Studies on BMP-2, specifically by Singh and colleagues, demonstrated that a high dosage posteriorly is more effective than using iliac crest bone graft. Other studies have shown high fusion rates when combining lower doses of BMP-2 with appropriate bulking agents and large quantities of iliac or local bone graft, Bridwell said.

“In the anterior spine, with dosages in the range of 8 mg to 12 mg (rhBMP-2) per level, protected with either structural allografts or with titanium cages, clearly the product does work,” Bridwell said. “In my practice, it eliminates the morbidity associated with graft harvesting for the anterior fusions.”

For more information:

Bridwell KH. Biologics. From Adult symposium: Do modern techniques change outcome in adult scoliosis surgery? Presented at the Scoliosis Research Society 41st Annual Meeting and Pre-Meeting Course. Sept. 13-16, 2006. Monterey, Calif.

Keith H. Bridwell, MD, Washington University School of Medicine, Campus Box 8233, 660 S. Euclid Avenue, St. Louis, MO 63110; 314-747-2533; Bridwellk@wustl.edu. He is a consultant for DePuy and Medtronic Sofamor Danek. Washington University receives research money from Medtronic Sofamor Danek to study generic outcomes on patients with spinal disorders.