Review of Various Types of Bone Graft Substitute for Lumbar Fusion
It's time. Time to take a closer look at materials used as a substitute for bone graft in lumbar fusions. How well do they work? Do they all give equally good results? How do they stack up against the patient's own bone donated for the procedure? This systematic review examines each one of these questions searching for some answers.
There are different reasons to fuse the spine and different ways to do the fusion procedure. Surgeons are studying the results of fusion techniques to get an idea of which one works best for each problem. Fusing two or more vertebral bones together prevents motion at those segments and stabilizes the spine. Instrumentation such as screws or metal plates and screws may be used to perform the fusion. Bone grafting using the patient's own harvested bone or bone from a bone bank (either without instrumentation) is another approach.
More recently, bone graft substitutes have come onto the scene. These materials made from bone are called recombinant human bone morphogenetic proteins (rhBMP). There are two types of recombinant BMPs available: rhBMP-2 and rhBMP-7. BMP bone graft materials rely on growth factors within the bone to do two things: make more bone cells (osteoinductive) and build a scaffold for the new bone cells to live (osteoconductive).
Bone graft materials are useful because they eliminate the risk of rejection with donor bone and the risk of infection with bone harvested from the patient. There are other potential problems with using patient-harvested bone for the graft.
For example, the donor site can be very painful or sore for a long time. It takes time in the operating room to gain access and then remove the donor bone, which can increase the risk of complications. And then after all that, the bone graft may fail and the patient ends up with a nonunion fusion. In other words, the vertebral bones still move when they shouldn't so the spine still isn't stable.
Bone cells from a bone bank are really only osteoconductive -- they provide a framework for the body to fill in with its own live bone cells. The donated bone is no longer alive, so it is not osteoinductive (able to make new bone cells). That's where bone substitutes like bone morphogenetic proteins (BMPs) come in handy. BMPs contain both the bone mineral matrix and growth factors, which makes them osteoconductive (provides a scaffold) AND osteoinductive (helps fill the scaffold in with new bone). Fresh bone graft taken at the time of surgery from the patient does contain live bone cells that are thought to survive and create new bone.
So, all that brings us to the question: how well do BMPs really work? The authors of this review didn't actually do any studies of their own. They reviewed the literature already published looking for good quality studies on bone substitutes for lumbar fusion. They limited their search to articles in English. They included studies that used either an anterior (from the front of the spine) surgical approach or posterior (from the back of the spine) surgical approach.
Data collected from the studies included operating time, blood loss, length of hospital stay, surgical approach, and type of fusion material. Results were measured and compared using these factors as well as X-rays evaluating the fusion site. Patient pain, function, and disability were measured using a popular and well-known tool called the Oswestry Disability Index (ODI).
A total of 17 studies made the cut based on the standards set for good quality research methods, the use of English, and fusion for lumbar degenerative osteoarthritis. Some of the studies investigated the results of other bone graft substitutes such as demineralized bone matrix DBM), platelet gels, and activated or autologous growth factor (AGF).
Each of these bone graft materials are prepared and used in a slightly different way to achieve the desired results. BMP-2 is mixed in water and placed on a collagen sponge then placed in a fusion cage between two vertebral bodies (where the disc used to be located). BMP-7 is mixed in a saline (salt) solution and combined with collagen then painted around the sides of the bones to be fused together. Collagen is the basic building material of all soft tissues and bone.
When everything was studied and analyzed, they found that the rhBMP-2 had the best results. Patients receiving the rhBMP-2 bone graft substitute had more stable unions/fusions when compared with rhBMP-7, demineralized bone matrix (DBM), platelet gels, and growth factors. There was less blood loss and shorter operating times with the rhBMP-2. rhBMP-2 outperformed rhBMP-7 in all areas except change in function as measured by the Oswestry Index.
The conclusion of this systematic review of bone graft substitutes for lumbar fusion is that rhBMP is a useful alternative to donor bone or bone harvested from the patient. It is safe and cost-effective. The cost savings come from fewer complications during and after surgery with less blood loss, shorter operating times, and fewer infections. More studies are really needed before any final conclusions can be made.
Rajender Agarwal, MD, MPH, et al. Osteoinductive Bone Graft Substitutes for Lumbar Fusion: A Systematic Review. In Journal of Neurosurgery: Spine. December 2009. Vol. 11. No. 6. Pp. 729-740.
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