Warning: Biologic Agents Must Be Used With Caution in Spinal FusionSurgeons continue to look for ways to fuse the lumbar spine with the best results and fewest problems. One of the biggest problems with spinal fusion is the need for bone graft material. Bone chips taken from the patient's hip (iliac crest) work but often leave undesirable pain, infection, and swelling at the donor site. New biologic agents might be able to speed up the fusion process and do it without the use of bone graft material. But there have been some reports of serious side effects. This is a report of four cases of ectopic bone formation following the use of rhBMP-2 or recombinant human bone morphogenetic protein. rhBMP-2 is a growth factor that can be used to stimulate bone growth.
Ectopic bone formation means that the product used worked! It stimulated the growth of bone (bone formation). But unfortunately, not in the right place (ectopic). Instead of creating new bone around the fusion site (between the vertebral bones), bone formed inside the spinal canal and around the spinal nerve roots. In one case, the bone had completely encased the L4-L5 spinal nerve roots. The pressure from the bone on the nerve tissue caused serious neurologic problems.
In the four patient cases presented, the authors make it clear that the use of rhBMP to enhance bone growth was off-label. That means it wasn't used as it was intended and for what it has been tested. Right now, rhBMP-2 has been approved by the FDA for use in spinal fusions. In its liquid form, it is placed on a sponge (called the carrier). The sponge is put inside a tiny cage that is then implanted between the two vertebrae. The disc that normally sits between the two vertebral bones has been removed first in a procedure called a discectomy to make room for the intervertebral cage.
In its off-label use, the procedure just described with the sponge carrier inside the cage is still used. But in addition, the surgeon has put some rhBMP-2 in the space left by the discectomy before inserting the cage (that's the off-label use). The rhBMP is pushed up against what's left of the disc lining (the annulus fibrosis) then the cage is set behind it. This pushes the cage back a bit from where it would normally rest without the additional rhBMP-2. The cage ends up being flush or even with the back wall of the vertebral bone.
Why did ectopic bone form in these four cases? There were a total of 37 patients who had a minimally invasive spinal fusion. The approach to the fusion procedure was from the side at an angle called transforaminal lumbar interbody (TLIF) fusion. Everything went well with the operation and early post-operative response. Patients got pain relief and improved function. X-rays showed that the fusion was successful. But 29 to 51 months later, new symptoms developed that led to the diagnosis of ectopic bone formation.
What went wrong? Well, maybe nothing in terms of the ability to use the rhBMP-2 off-label as presented. It does help stimulate bone growth for bone fusion. But the position of the rhBMP-2 and the cage back so far might be a problem. Without any protective barriers between the rhBMP and the cage or between the cage and the spinal nerve root, the rhBMP could leak out and stimulate bone growth outside the intervertebral space. It's possible that using so much rhBMP-2 was also a factor -- a lower dosage might work better.
The surgeons who tried this method have modified it now. They have built in protective layers using crushed bone and a layer of fibrin glue. A layer of bone is placed between the rhBMP-2 and the annular covering at the front of the empty disc space. Then the rhBMP-2 goes in, followed by another protective layer of bone. The cage filled with the rhBMP-2 carrier is next. And the final layer (fibrin glue) is placed behind the cage. They also pay close attention to where the cage is placed. Putting it as far forward as possible (and as far away from the nerve tissue) may help reduce the problem of neural compression.
The authors conclude that off-label use of rhBMP-2 to augment spinal fusion is still a good idea and a way to avoid the problems associated with iliac crest autografts. And given the fact that it's often difficult to get enough bone graft material from the patient, an alternative approach to fusion is a welcome idea. However, patients should be selected carefully for the procedure and given the facts about the potential side effects.
Knowing that it is a delayed response, surgeons may want to follow-up with patients longer than they would otherwise. Watching for any signs of ectopic bone formation and intervening earlier than later might make a difference. If ectopic bone formation does occur, additional treatment (including a possible second surgery) may be needed. This problem is rare, so a standard treatment protocol has not been developed yet. More research is needed in both the use of off-label rhBMP-2 for lumbar spine fusions and treatment response if and when it happens.
Nan-Fu Chen, MD, et al. Symptomatic Ectopic Bone Formation After Off-Label Use of Recombinant Human Bone Morphogenetic Protein-2 in Transforaminal Lumbar Interbody Fusion. In Journal of Neurosurgery:Spine. January 2010. Vol. 12. No. 1. Pp. 40-46.
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