The Role of Bone Graft in Spinal Fusion Surgery

Bone graft refers to fragments of bone used in spinal fusion surgery. The purpose of a spinal fusion (arthrodesis) procedure is to link or weld bones together. Many different spinal conditions cause instability and/ or pain (e.g., degenerative disc disease, scoliosis, trauma) and require treatment with a spinal fusion. Spine surgeons can use bone graft to stop motion between 2 or more vertebral bodies, stabilize a corrected spinal deformity, or repair spinal fractures.

Bone graft doesn't heal or fuse the spine immediately - instead, bone graft provides a foundation or scaffold for the patient's body to grow new bone. Bone graft can stimulate new bone production. When new bone grows and solidifies, fusion occurs. Although instrumentation (e.g., screws, rods) is often used for initial stabilization (postoperative), it is the healing of bone that welds vertebrae together to create long-term stability.

There are two general types of bone grafts: real bone and bone graft substitutes.

Autograft: The Patient's Bone
An autograft is transplanted bone taken (called harvesting) from the patient's body. Autograft is considered the gold standard of bone grafts because it contains the patient's bone cells, proteins, and calcified matrix; all of which help to stimulate healing of the fusion. The advantages of autograft include a greater chance for fusion success and negligible risk of disease transmission. A drawback to using autograft is postoperative pain associated with the procedure to harvest the patient's bone.

Bone may be harvested from one of the patient's iliac crests (pelvic bones) or another source, such as a rib or the spine. A bone graft harvesting procedure presents a separate set of surgical risks including: postoperative pain, nerve injury, or surgical wound problems. Sometimes, because of these disadvantages and the possibility the patient has poor quality bone, the surgeon decides to use another type of bone graft.

Allograft: Donor Bone
Allograft is bone from another individual, usually obtained from a tissue bank. Tissue banks are agencies that harvest bone and other tissues from cadavers for medical purposes. In spine surgery, allograft is generally prepared for use by freezing or freeze-drying, which helps to limit the chances of graft rejection. Allograft bone does not contain living bone cells and therefore is not as effective at stimulating fusion compared to autograft bone. But, often, it is sufficient.

Tissue banks screen donors, oversee bone recovery, test and sterilize donations, and store it for use. Many tissue banks are accredited by the American Association of Tissue Banks. In 2005, the U.S. Food and Drug Administration set up new regulations about human cell and tissue processing which includes rules about donor eligibility. These practice guidelines help to reduce the risk of tissue contamination and the spread of disease.

What is a bone graft substitute?
Bone graft substitutes are man-made, or a manipulated version of a natural product. As an alternative to autograft and allograft, they are generally safe and can provide a foundation for the patient's body to produce its own bone. Bone graft substitutes share similar properties with human bone including a porous structure and/ or proteins to stimulate healing.

Demineralized Bone Matrix (DBM)
Demineralized bone matrix is allograft bone that has undergone a process whereby the mineral content has been removed. Demineralization helps to expose bone-forming proteins (collagen, growth factors) hidden within the bone structure that can stimulate healing. DBM is usually considered a bone graft extender rather than a replacement because its ability to fuse the human spine alone is not proven. DBM can be mixed with regular bone to obtain more graft volume and is available in chip, granule, gel, powder, or putty forms.

Ceramic-Based Bone Graft Extenders
These include calcium phosphate, calcium sulfate, and bioglass. These ceramic-based products are also best used in combination with other sources of bone because they contain a calcium matrix for fusion, but no cells or proteins to stimulate the process. Ceramic-based products do not present a risk for disease transfer, but may occasionally cause inflammation. These products are available in porous and mesh forms.

Recombinant Human Bone Morphogenetic Protein (rhBMP-2)
Researchers found that a group of related bone proteins can stimulate bone formation, even in areas outside the skeleton. These proteins are often called bone morphogenetic proteins and can be found in trace amounts in human bone, or can be produced in larger amounts by genetic engineering.

The U.S. Food and Drug Administration recently approved rhBMP-2 for use in certain spinal surgeries. To use this product, a dissolvable sponge soaked with rhBMP-2 is implanted into the spine. It appears that bone formation is strongly stimulated, as it causes the body's own cells to become active. BMP begins the process of fusion, which continues even after the protein and sponge dissolve, leaving living bone behind.