Patients with previous lumbar surgery achieve lower fusion rates with rhBMP-2

Lower posterolateral fusion rates may be due to commercially available dose, concentration of rhBMP-2.

CHICAGO — A recent study suggests that patients with prior lumbar surgery may benefit from a higher dose and concentration of rhBMP-2 than what is now commercially available.

Researchers retrospectively reviewed 25 patients who underwent posterolateral fusion with rhBMP-2 (recombinant human bone morphogenetic protein-2) and resorbable ceramic granules. They found that 86% of patients without prior lumbar surgery achieved solid bilateral fusion.

However, “Our patients with prior surgery had significantly lower BMP fusion rates with only 28% achieving solid bilateral fusion and another 61% achieving solid unilateral fusion,” Edward C. Sun, MD, of SpineCare Medical Group in Daly City, Calif., said at the American Academy of Orthopaedic Surgeons 73rd Annual Meeting.

He noted that this study was the first to investigate the use of rhBMP-2 in posterolateral fusion for patients with prior surgery.

Most patients with prior lumbar surgery

Study patients had an average age of 53 years (range, 35 years to 83 years) and 18 patients (72%) underwent previous spinal surgeries. Most patients had failed back surgery syndrome: eight patients with pseudoarthrosis, another eight patients with adjacent level degeneration and two patients with prior laminectomies, Sun said.

All patients underwent posterolateral fusion using the commercially available 1.5 mg/cc of rhBMP-2 and resorbable ceramic granules (15% HA/85% beta-tricalcium phosphate). Surgeons concurrently used iliac crest bone graft in six patients (24%).

At one year postop researchers assessed the patients’ fusion rates using CT scan with 2 mm cuts, Sun said. They graded the fusion rates using a modified Bridwell-Lenke classification system. Fusion status results were as follows:

11 patients (44%) were rated as Grade I — solid bilateral fusion with trabeculated transverse processes
12 patients (48%) were rated as Grade II — thick unilateral fusion mass
Two patients (8%) were rated as Grade III — clear pseudoarthrosis with defects in fusion mass bilaterally

Researchers did not rate any patients as Grade IV or definite graft resorption. Again, looking at the effect of prior surgery, 86% of those without prior surgery achieved solid bilateral fusion, and 28% with prior surgery achieved solid bilateral fusion — a statistically significant difference (P<.05), according to Sun.

“The use of spinal instrumentation and also the concurrent use of iliac crest bone graft also show a higher trend towards fusion, although the difference is not statistically significant (P>.05),” Sun said.

Further study necessary

The researchers concluded that rhBMP-2 and ceramic granules can be effective for posterolateral fusion — even in patients with failed prior surgery — since 92% of patients achieved either solid bilateral or unilateral fusion. But their 44% Grade I fusion rate is lower than reported rates in previous studies for two reasons, Sun said.

“The majority of our patients had failed back surgery with either pseudoarthrosis or adjacent level degeneration; and therefore, there was a much more difficult biological environment for fusion,” he said.

The rhBMP-2 concentration and dose also played a significant role. Previous reports used concentrations of 2 mg/cc per level, while the commercially available rhBMP-2 has a concentration of 1.5 mg/cc per level, Sun said. Previous clinical studies also used doses of 40 mg/level, while this study used 12 mg/level, equivalent to a large Infuse pack.

“More studies are needed to see whether a higher dose and concentration of BMP should be used for patients with revision surgery or to obtain a higher fusion rate,” Sun said.

For more information:
Sun E, Slosar P, Reynolds J. Assessment of posterolateral fusion rates with rhBMP-2 and resorbable ceramic granules. #116. Presented at the American Academy of Orthopaedic Surgeons 73rd Annual Meeting. March 22-26, 2006. Chicago.