Bone graft is used in spinal fusion procedures to help stimulate new bone growth that will fuse (join) two or more vertebrae together. Spinal fusion is not spontaneous, but a process that occurs as the spine heals. Bone graft is essential to spinal stabilization and is used with instrumentation (eg, screws) to limit motion and promote fusion of the vertebrae.
There are three basic types of bone graft, each with particular advantages and disadvantages; autograft, allograft, and man-made substitutes.
Bone graft is classified as either osteoinductive or osteoconductive.
Osteoinductive means the graft material induces or stimulates bone to grow. This type of graft includes Bone Morphogenetic Protein (BMP). BMP is a naturally occurring human protein that has been isolated and reproduced for surgical use.
Osteoconductive means the graft material does not stimulate bone growth, but provides a scaffold for new bone cells to attach to, grow along, and fill in during the fusion process.
Autograft is bone taken from the patient's body. The primary source for autograft is bony shavings or pieces of spinal bone collected during the decompression and preparation of the vertebra. If more autograft is needed, some may be harvested (taken) from the patient's iliac crest (hip bone). A wedge is taken from the middle portion of this bone. The bony wedge can be shaped and used as an interbody implant. It can also be ground into small particles to fill a cavity (void) or packed into and around an implant. Other sources for autograft bone include the patient's rib or fibula. The fibula is a non-weight-bearing bone in the lower leg.
Advantages: Autograft bone is living tissue that is an exact match to the patient. There is no risk for disease transmission and the bone is osteoinductive.
Disadvantages: With the exception of bony shavings or pieces taken from the spine, autograft bone requires a separate surgical incision. Patients frequently report hip pain after bone is removed from the iliac crest.
Allograft bone comes from a bone bank, i.e., organ donor. The donated bone is sterilized to reduce the risk of disease transmission. Allograft bone can be demineralized to remove calcium and other minerals, leaving collagen and proteins (including BMPs) that may be osteoinductive, if present in sufficient quantity.
Demineralized bone is available in different forms including paste, putty, and gel. Allograft bone can be mixed with autograft bone to increase the probability for solid fusion.
Advantages: Allograft use eliminates pain and recovery time associated with harvesting autograft bone. It is osteoconductive and, depending on its form, may be strong enough to support load (withstand weight). The supply of allograft is plentiful and it is available in different forms to suit specific surgical needs.
Disadvantages: There is risk for disease transmission. Although allograft is carefully screened and processed, a small risk of contamination remains.
Bone graft substitutes are a growing number of man-made materials such as ceramic, plastic, glass, calcium compounds, and bioresorbable substances. The term bioresorbable means the body absorbs it over time.
Advantages: The supply of man-made substitutes is virtually limitless. They do not pose a risk for disease transmission. The materials are osteoconductive. Bone graft substitutes can be used to fill a bony void or close gaps between bone.
Disadvantages: The materials do not contain bone morphogenetic proteins so they are not osteoinductive. To increase the probability for solid fusion, bone graft substitutes (depending on the type) must be mixed or filled with autograft bone to improve the probability for solid fusion.
BMP is formally termed recombinant human bone morphogenetic protein-2 or rhBMP-2. It is commercially available in a pure, freeze-dried powder. The protein does not come from a human donor, but is generated in a laboratory. BMP-2 stimulates bone growth where it is implanted (placed).
During surgery, the BMP product is mixed and soaked into a collagen sponge. Collagen is a protein found in human skin and other tissue. The sponge is inserted into an interbody device (cage) and implanted into the spine. As the BMPs induce bone growth, the sponge is absorbed by the body as new bone takes its place.
Advantages: BMP is the only synthetic (man-made) bone graft material that is osteoinductive. It eliminates the need to harvest autograft bone from the patient. The risk of disease transmission from donor bone is eliminated.
Disadvantages: BMP use is limited by the ways the collagen sponge can be implanted. Sometimes the expense and insurance coverage limitations can contribute to its exclusion as a treatment option.
Deciding which type of bone graft to use is based on your surgeon's recommendation. Considerations include the spinal level(s) to be surgically treated, implant use (if any), and weight-bearing demands of the level(s). While graft packed into a void or implant does not generally support load (bear weight), it must provide an adequate scaffold for bone to completely grow through the implant to provide a secure and solid fusion.
Spine surgery is an important decision. While the final decision is yours to make, the benefits and potentials risks and complications require careful consideration. We look forward to answering your questions and helping you to resolve your concerns about any treatment offered by Piedmont Spine Center.