Not All Bone Grafts Are Created Equal
Most patients considering spinal fusion surgery don't think to ask what type of bone graft will be used. But not all bone grafts are created equal. This is important to remember because most of the responsibility for the success of the fusion rests squarely on the shoulders of the bone graft.
So what makes a good bone graft? All bone grafts are judged on three elements: osteoconductive scaffolding, osteoinductive growth factors, and the ability to incorporate with osteogenetic progenitor cells found in the human body. Though the terms may be difficult, the concept is relatively simple. These elements have been successfully achieved by the use of a particular graft taken from a patient's own body. In fact, up until recently, the "gold standard" of all bone grafts, was actually a graft surgically removed from the iliac crest (hip area) of the patient about to have spinal fusion surgery. This bone graft is called an "autograft."
The problem with an autograft, is the procedure needed to access the bone and remove it can cause complications of its own. Studies show: increased operative time, increased patient recovery time, and chronic pain at the location where the bone graft was taken as common complications of this procedure. Also, not everyone has bone suitable for use during surgery. A patient suffering from osteoporosis or who smokes is typically not a good candidate for autograft fusions. Because of these problems with autograft many physicians and researchers have spent long hours trying to find or manufacture options that hold up to the high standards of autograft, without the complications.
This is where those three elements become extremely important. All bone grafts are compared to autograft and must be tested by these elements in humans. Much like in a court case, there is a burden of proof; bone grafts must prove that they measure up to this "gold standard."
Osteoconductive Scaffolds
Osteoconductive scaffolds are materials that create an environment where bone can grow. This material is not what causes bone to grow, but simply the framework the bone can cling to as it grows. Think of a fusion as an ice cube. The three elements need to create ice are: water, freezing temperature, and some type of structural mold. The osteoconductive scaffold is the mold. The mold does not make the water, nor does it cause the water to freeze, it simply creates an environment that allows the water to create a cube as it freezes. In this same way an osteoconductive scaffold is a special material conducive to allowing bone graft to grow into a solid piece of bone, creating a fusion.
Osteoinductive Growth Factors and Osteogenetic Progenitor Cells
Osteoinductive growth factors are capable of inducing new bone formation. In order to be considered osteoinductive, the substance must contain one or more growth factors such as bone morphogenetic proteins (BMPs). Osteoinductive growth factors utilize osteogenetic progenitor cells, bone-forming tissue, existing in the human body. This creates a two-part equation: the material that bone is made up of and the factors that will help that bone material to grow. If you go back to our ice analogy, the osteogenetic progenitor cells are represented by the water, while the osteoinductive growth factors are the freezing temperature. Yes, ice is made up of water. But you can have all of the water in the world, and never achieve ice without the freezing temperature.
The osteoinductive portion of the equation is where the major differences are found between bone grafts. Though some bone grafts contain actual bone, often bone marrow aspirate, the potential for a fusion to occur is heavily dependent on having osteoinductive growth factors with the optimal amount of BMP, as evidence has suggested that only BMPs are capable of initiating the process of bone formation. When an osteoinductive material is used, the growth factors in this material signals the body to start forming bone in the area where the material is placed. Just because real bone is used, does not mean it has the most effective level of BMP.
A perfect example of a product using real bone that has not proven itself as equal or better than the "gold standard" in humans, is a product that combines bone marrow aspirate (BMA) with an osteoconductive scaffold. The bone marrow aspirate is applied to a manufactured osteoconductive scaffold, becoming a bone graft. But like autograft, bone marrow aspirate is harvested from a patient through a procedure where a long, 11 to 15-gauge needle, is inserted into the patient's iliac crest (hip area). This procedure is repeated over and over until 10-20mL (cc) of bone marrow is removed, often requiring the needle to be inserted up to 12 times. This procedure is most commonly used as a test for cancer, though during that procedure only 2mL of bone marrow is removed, and it usually only requires 1-2 needle insertions.
The problems and complications that occur when using bone marrow aspirate are similar to those that occur with autograft taken from the iliac crest. According to the National Institutes of Health, most patients complain of pain during this procedure. Besides pain, bleeding is the most common complication, and bruising at the donor site can also occur. According the May 2002 supplement of Orthopedics, no human studies have currently been performed to test the osteoinductive properties of bone marrow aspirate. So at this time it is impossible to state whether or not using this type of graft will result in a solid fusion in humans.
Currently there is only one bone graft available that has been proven to meet all three elements, meeting the burden of proof. rhBMP2, or INFUSE® Bone Graft, not only utilizes an osteoconductive scaffold, contains an effective level of BMP to be osteoinductive and initiate the process of bone formation, but also does not require ANY second procedures. INFUSE® Bone Graft does not need autograft or bone marrow aspirate. It is completely stand-alone, and has been proven in human studies to meet the level of fusion set by the "gold-standard," inside the LT-CAGE® Lumbar Fusion Device. Learn more about INFUSE® Bone Graft at www.infusebonegraft.com.
Other combinations of BMPs used in other products, have not proven so effective. According to the American Academy of Orthopaedic Surgeons there has been at least one example of an osteoinductive product (meaning it contained some combination of BMPs) that has been marketed and implanted in humans that has failed to demonstrate bone induction in rats.
When considering bone graft options, the bar has been raised from that of the "gold standard." Not only must the bone graft prove safety and effectiveness by matching the gold standard's criteria, but also prove to be easier on the patient and perform in even this most challenging of environment. For this reason, INFUSE® Bone Graft (rhBMP2) is quickly replacing autograft as the "gold standard." INFUSE® Bone Graft meets this new challenge and has in turn increased the potential for a successful fusion in cage surgery.
Boden, Scott, M.D. "Osteoinductive bone graft substitutes: Burden of proof," American Academy of Orthopaedic Surgeons, Bulletin 51, February 2003. http://www.aaos.org/wordhtml/bulletin/feb03/ acdnws6.htm (14 April 2003).
Vaccaro, Alexander, M.D. "The Role of the Osteoconductive Scaffold in Synthetic Bone Graft," Orthopedics, May 2002. http://www. orthobluejournal.com/supp/0502/vaccaro/ (14 April 2003).
Grant, Warren. "Preparing for a Needle Aspiration Biopsy," Patient Information Publications, National Institutes of Health, August 1999. http://www.cc.nih.gov/ccc/patient_education/ pepubs/needle.html (23 April 2003).
It is important that you discuss the potential risks, complications, and benefits of the INFUSE® Bone Graft/LT-CAGE® Lumbar Tapered Fusion Device with your doctor prior to receiving treatment, and that you rely on your physician's judgment. Only your doctor can determine whether you are a suitable candidate for this treatment.
