Much progress has been made in spinal fusions since 2002 when the FDA approved the use of bone graft substitutes. Bone graft substitutes replace bone harvested from the patient or from a donor.
At first, bone morphogenetic protein referred to as rhBMP-2 was just used for single level spinal fusions. The surgery is done from the front of the body. The procedure is called an anterior lumbar interbody fusion (ALIF).
Over time, results with rhBMP-2 have been so good, its uses have expanded. Surgeons started using less total BMP dose, using it in posterior spinal fusions, and for two-level lumbar spinal fusions. Surgery time was less, blood loss was reduced, and hospital stay was shorter.
Now, in this study, the first set of results using rhBMP-2 with multiple levels is reported. In the past, solid fusion of long spinal segments was difficult to achieve. This was true even with autograft harvest (bone taken from the patient’s pelvis). The procedure is called an iliac crest bone graft (ICBG).
Too many risk factors interfered with the results. Osteoporosis, not enough bone for the graft, previous failed fusions were just a few of the problems encountered with spinal fusions. The current use of RhBMP-2 placed on an absorbable collage sponge (ACS) and put in the disc space has been very successful with one and two segment levels.
Three groups of patients were included in this study. All patients had multilevel spinal deformities requiring multilevel spinal fusion. The fusion went down to the sacrum.
The first group had an anterior(through the abdomen) spinal fusion with rhBMP-2. The material was placed on a collagen sponge. The sponge was placed inside a titanium mesh cage. The cage was inserted into the interbody spaces (between the vertebrae where the discs used to be). Metal supports (rods and screws) were used posteriorly to help hold the bones in place until healing occurred. This part of the surgery is called posterior instrumented fusion.
The second group also had rhBMP-2 but it was applied to the posterolateral (side and back) spine. Use of rhBMP-2 in the posterolateral location has not been approved specifically by the FDA. So this use is called off-label. In this group, bone removed was ground up and used as a bone extender. This is done by combining harvested bone with the rhBMP. The mixture is placed on the sponge and placed in the interbody space.
The third group had a posterolateral spinal fusion with BMP but without the bone extender. This group had previous failed spinal fusions. They made up a special category called compassionate use surgeries. High-dose BMP-2 was used but without adding any of the patient’s bone to extend it. Instead a special graft extender or bulking agent in granule form called TCP-HA was added to the BMP.
Number of spinal levels fused averaged from two to five levels among the three groups. Average dose amount of rhBMP-2 used ranged from 10 mg/level to 40 mg/level.
Everyone was followed on a regular basis for at least two years. X-rays were used to assess the fusion. CT scans were only used when there were questions about the fusion as seen on X-rays. CT scans are more expensive than plain X-rays and expose patients to more radiation.
The fusion site was scored with a number (one through five) and given a grade (A through E). The scoring and grading was based on the description of the fusion. For example, Grade A (fusion score of one) was given to solid fusions present on both sides (bilateral). Grade D (score of four) indicated the graft was probably not fused. Grade E (score of five) was given when the fusion could not be assessed. Intermediate grades (B and C) are on a continuum from definite fusion to no fusion with special qualifiers for each.
The results showed that rhBMP-2 had excellent fusion rates (95 per cent) in all three groups. Solid fusion was achieved with low-doses, high-doses, and when combined with local bone or an extender. Pseudoarthrosis (formation of a false joint with movement at the fusion site) was very low using BMP. No known causes or risk factors could be determined to help predict in advance who might be affected by pseudoarthrosis.
Outcomes were equally good when performed using an anterior or posterior approach. At the end of two years, no one had any complications (loosening, shifting, pullout, or breakage) of the instrumentation used (rods, screws). And results were good even when patients smoked, had other health issues, or had previous (failed) spinal fusion.
BMP may actually be a superior choice over iliac crest bone graft (ICBG). A 95 per cent fusion rate with BMP is higher than reported in other studies using ICBG. The factors that can interfere with successful ICBG don’t seem to influence results with BMP.
In summary, the authors did, indeed, show that BMP fusion rates for multiple level lumbar spinal fusions are equal (if not better) than iliac crest bone grafts (ICBG). And lower amounts of BMP were used with the same good outcomes. Likewise, BMP used alone had the same results as BMP used with graft extenders.
The authors make note of the fact that sometimes fusion failures (pseudoarthrosis) occur as much as four years after the surgery. For that reason, the patients in this study will continue to be followed up to see what the long-term results are with rhBMP-2 as a substitute for bone graft.