Bone graft options for spine fusion surgery

Obtaining a solid spine fusion

To achieve a solid spine fusion, three processes are necessary:

• Osteoconduction—this refers to the scaffolding that is needed for new bone to grow on.

• Osteogenicity—this refers to the transmittal of live bone cells or osteoblasts

• Osteoinduction—this is the process whereby specific proteins such as bone morphogenetic proteins (BMPs) induce the bone to grow.

Autograft bone (patient’s own bone)

Bone that is harvested from the patient (autologous bone graft, or autograft bone) has two of these properties because it has both the calcium scaffolding (osteoconduction) and it is estimated that some 15% of the bone cells survive the transplantation (osteogenicity). However, the third property—osteoinduction—may not be sufficiently available in the patient’s own bone. Although small amounts of osteoinductive proteins are present in all bone matrix, since autograft is mineralized bone, these osteoinductive proteins are not exposed and may have very limited activity.

Allograft bone (cadaver bone)

Donor bone, which is bone from a cadaver and is referred to as allograft bone, has only the osteoconductive property. It does not contain bone cells or exposed osteoinductive proteins, and has only a calcium scaffolding. Although donor bone seems to work well elsewhere in the spine (e.g. neck) it is generally not sufficient for a spine fusion in the lumbar spine (lower back).

Allograft bone has been shown to not work well in a posterior lateral fusion, which is a common type of spine fusion, when compared with autologous bone graft (patient’s own bone). Sometimes allograft bone is used anteriorly (in the front of the spine) as an interbody device (bone dowel), but autologous bone harvested from the patient’s pelvis is almost always used along with it. The interbody device provides the structural support and the harvested bone graft from the patient’s pelvis is what eventually fuses.

Similar to the patient’s own bone, structural allograft bone comes fully mineralized so the osteoinductive proteins are not exposed and readily active. Recent developments have seen the advent of surfaced demineralized allograft that can combine the structural integrity of bulk allograft with the osteoinductivity of demineralized bone matrix (see explanation of demineralized bone matrix in the Bone graft substitutes section).

Bone graft substitutes

A third option, which is a newer area of development in spine fusion surgery, is the field of bone graft substitutes. There are several types of bone graft substitutes currently available for use as an adjunct to, or as a substitute for, the above two bone graft options. The types of bone graft substitutes and their development status/availability are discussed in the next page.