Bone Grafts: New Developments
Many spinal conditions that are treated with surgery require a procedure called spinal fusion. Spinal fusion is the joining or fusing of one or more vertebrae to reduce pain and stabilize the spine. During a spinal fusion, a wide variety of implants, screws and cages may be used to enhance the fusion. However, the fusion is only considered successful when the bones grow together biologically and form a solid mass. To help achieve this biological fusion, bone grafts or other biological products that promote bone growth must be used. Currently there are many different types of biologics that surgeons can use to help attain solid spinal fusion. It is very important for patients and surgeons to understand all the options and how each of these biologics work. Since the success of each varies, careful consideration needs to be used when choosing a graft material.
Autogenous Bone Graft
The gold standard of bone grafting is taking the patient's own bone. This is called autogenous bone graft. This means that at the time of surgery, the surgeon makes a separate incision and takes a small piece of bone from an area of the body where it is not needed. Typically, autogenous bone grafts are taken from the pelvis or iliac crest. Autogenous bone grafting has excellent fusion rates and has become the standard by which all other biologics are measured. Many surgeons prefer autogenous bone grafts because there is no risk of the body rejecting the graft since it came from the patient's own body.
There are disadvantages of autogenous bone grafting including the need for an additional incision, pain and soreness which often last well after the surgery is healed, as well as possible complications such as increased blood loss and prolonged time in the operating room. Complications such as these occur in about 10%-35% of patients and vary in their severity. Even when using a patient's own bone, 100% fusion rates are not always achieved, which is why other fusion techniques have been developed.
Allograft Bone Graft
In an effort to minimize the problems associated with taking the patient's own bone, a number of other fusion techniques have been developed that use biological products as bone graft extenders or as bone graft replacements. One common source of bone graft replacement or extender is the use of allograft bone. An allograft bone graft is bone harvested from cadavers or deceased individuals who have donated their bone for use in the treatment of living patients. This is commonly used in many forms for spinal fusions ranging from cervical interbody fusions to lumbar interbody fusions and can provide excellent structural support.
The disadvantage of allograft structural bone is that it does not promote bone growth very well and therefore is very weak at stimulating a spinal fusion. Although it is used successfully for short-level fusions in the cervical spine, it is not a powerful enough biological stimulant to allow us to successfully use this to achieve a spinal fusion in the thoracic or lumbar spine. Studies have shown that when using allograft bone as the only graft material, the fusion rates in the thoracic and lumbar spine are extremely poor and the failure rate is very high.
Demineralized Bone Matrix
Sometimes allograft bone can be demineralized, a process by which some of the proteins that stimulate bone formation are extracted from the bone. These proteins are processed and available in various forms. This type of product is called demineralized bone matrix and can be readily used in place of or as an extender to the patient's own bone. Although it has successfully fused spines in animal studies, there is no proof that this is a powerful enough stimulus to successfully fuse a human spine and is therefore not recommended for use without the addition of the patient's own bone. It is only recommended as a bone graft extender and not a replacement.
Other Graft Extenders
There are several substances such as ceramics, calcium phosphates, and other synthetic materials which have similar biomechanical properties and structure to that of cadaver bone; however, they are not biologically active and do not stimulate a spinal fusion by themselves. These products are currently only recommended for use as bone graft extenders. There has been recent interest in supercharging these materials by adding bone marrow aspirate. This is a procedure in which bone marrow cells are taken up in a syringe and soaked onto the structural carrier such as the ceramics or cadaver bone. Since by themselves these products are not biologically active, the addition of the patient's bone marrow cells can give them more biological activity. This is currently being tested; however, there are no good long-term studies in humans showing this to be efficacious in stimulating spinal fusion. To date, the results have shown this technique to be inferior to using the patient's own autogenous bone graft. There are several synthetic carriers being developed which also will need stimulation with bone marrow cells to have some type of biological activity. Unfortunately studies have shown that these carriers also fall far short of using the patient's own bone.
Another area of biotechnology interest is the use of blood products such as platelet gels that are taken from the patient's own blood. This gel-like material is created by isolating a concentration of platelets, which are important clotting factors; from the patient's own blood. The platelet gel contains many growth factors that can help in bone formation and can play a key role in the formation and maturation of bony spinal fusion. The advantage of using platelet gels is that they are easily removed from the patient's blood with very few complications. The major disadvantage is that they do not contain osteoinductive proteins, which means they are not powerful enough stimulants to induce bone formation. They can be used as graft extenders but not graft replacements. All of the products discussed above have not shown to be as effective as using the patient's own autogenous bone for spinal fusion. They are all considered graft extenders and can supplement the patient's own bone graft but should not be used in place of the patient's own bone.
The next category of products is termed growth and differentiation factors. These are very powerful stimulants for bone formation and can be used as graft replacements. These proteins play a key role in the body's own natural bone-forming process and are found naturally at sites of spinal fusion. These proteins can be produced, concentrated and placed in the body in areas where bone formation is needed and are powerful enough to stimulate bone formation without the need for taking the patient's own bone. These proteins were discovered by Dr. Marshall Urist, an Orthopaedic Surgeon at the UCLA Department of Orthopaedic Surgery in Los Angeles, CA. Through his groundbreaking work, he discovered these proteins and named them bone-morphogenetic proteins, or BMPs.
There are several different BMPs naturally found in the body and many play a critical role in bone formation. The most promising proteins are BMP-2 and BMP-7. These two proteins have been extensively studied in animals and humans with very promising results. Both proteins have shown to successfully stimulate spinal fusion equally or even superiorly to autogenous bone graft. They accomplish this by stimulating "regular" cells to turn into bone forming cells. This in turn results in a solid spinal fusion. BMP-2, the most extensively studied growth factor, has shown to achieve spinal fusions faster and with higher success rates when used alone as compared to using the patient's autogenous bone graft. The use of BMP-2 in spinal fusions will eliminate the need for taking the patient's own bone as well as the side effects and potential complications of the grafting procedure. BMP-2 has received preliminary FDA approval and will most likely be the first growth factor available for general use in human patients for spinal fusion.
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.
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