BMP: The What and the Who
The title of this article asks two completely different but equally important questions. First, the "what." RhBMP-2 (recombinant bone morphogenetic protein, BMP) is a genetically engineered protein which both recruits bone forming cells to the surgical area and "turns on" local cells to the bone-making process. BMP is used for conditions requiring spinal fusion and stimulation of bone growth.
In the initial experiments done by Marshall Urist almost 50 years ago, he was able to identify a mixture of proteins isolated from the bone marrow, which were activated when the bone was damaged. It was not until the late 1980's that the individual protein components could be separated and identified. This is important. The tests used to determine the relative potency of the individual protein components involved placing a small amount of the material beneath the skin of test animals. Some, specifically rhBMP-2, were able to stimulate immature local mesenchymal (soft-tissue) cells to become bone-forming cells.
The finding was both an amazing scientific discovery and a warning. Consider the possibilities of a material that could alter the way a body forms bone and heals itself. If BMP placed under the skin could turn tissues to bone, what about the nearby blood vessels and nerves? What if they also turned to bone? Think of the possibilities for injury. The key to controlling this powerful material involves understanding the mechanisms that are involved in the stimulation and regulation of bone formation. The difficulty for surgeons is knowing where and how to place the material so that bone forms in the correct places, but does not form in areas which could injure the patient.
Is BMP the right choice for you?
Now the "who." Recently, rhBMP-2 has been approved by the U.S. Food and Drug Administration for use in certain spinal fusion procedures. Therefore, any patient needing spinal fusion may soon be a candidate for BMP. However, since BMP can only be used in certain parts of the spine at this time, it is essential that you talk to your surgeon to see if BMP is an option for you. It is important to remember that BMP is an adjunct (or aid) to fusion, and is not a cure-all. The patient must still meet the criteria for spinal fusion to be eligible for BMP treatment.
The basics of spinal fusion involve a surgical procedure in which adjacent bones or segments of the spine are locked together to eliminate motion (and hopefully pain) caused by degeneration of the bones and joints. Such motion causes pain but may also cause progressive nerve damage as well. Typically, the surgeon uses some type of fixation hardware (such as screws and rods) to hold the spinal bones together while placing graft material (usually bone marrow taken from the patient's hip) around the spine to act as slowly hardening "concrete." The hardware keeps the spine from moving while the bony concrete sets. Once fusion occurs (typically 3-9 months later), the bones are permanently locked together. The hardware is only necessary for that first year of healing, but is generally left in place to avoid another procedure to remove it.
There are dozens of spinal conditions that require spinal fusion surgery. All deal with abnormal movement or alignment of the spine. Infections, tumors, traumatic injuries, and degenerative arthritis seem to account for the majority of the spinal fusion operations performed in the United States.
At this time, BMP has been approved for the treatment of L5-S1 degenerative disease and used along with titanium spacers (called cages) placed anteriorly (from the front of the spine). The anterior approach was chosen because it is the easiest way to shelter the BMP within the spine. By using a collagen sponge soaked with BMP and then placing it within the cage, there is little possibility that the material could seep into unwanted places and form bone where it is not needed. Degenerative disease was chosen as the first condition to be treated with BMP because of concerns from experts in the field that BMP could possibly stimulate the growth of tumor cells or spread infection if used improperly or in the wrong patient. Although the animal studies to date indicate that BMP is safe even when used in the presence of tumor cells or infection, most of us agree upon a cautious course when it comes to trying this material in large groups of human patients. Similarly, BMP has not been approved for use in children, whose developing bodies may react differently to the substance.
Clearly, the medical judgments and technical skills necessary to properly choose eligible patients and implant the material require someone who has received adequate training. Check with your surgeon regarding the specific training he or she has received to implant this material to help ensure your own safety.
Is BMP better than bone?
Patients ask this question all the time. There are basically two reasons to use BMP instead of the patient's own hip bone. First, the animal studies and the preliminary human studies show a higher and more successful fusion rate from BMP when compared to hip bone. That means that the likelihood of a solid fusion occurring and the bones healing together properly is better when the surgeon uses BMP. The second reason is that by using BMP the surgeon does not have to take any hip bone. When the marrow or cancellous bone is taken from the patient's hip there is always some additional pain and blood loss. It may be minimal, but it still requires additional surgery. The site where the bone is taken from (donor site) has a risk of long-lasting pain, infection, and nerve damage as well. It simply makes the most sense to use BMP since it provides a higher fusion rate with a lower risk of complications and less surgery.
The Today and Tomorrow of BMP
Currently, BMP has only been recommended and approved for use in certain types of spinal surgery. Given the ability of BMP to form new bone, scientists have had some difficulty finding a way to put BMP in other parts of the spine while keeping it in the right place. However, studies are ongoing to identify a carrier material that will grab BMP, hold it in just the right position for fusion to occur, and keep it from seeping out where it is unable to stimulate the fusion process.
rhBMP-2 has recently received clearance from the Food and Drug Administration (FDA) for specific uses. Consult your surgeon to learn if you are a candidate.