Bone graft for spinal fusion surgeries may either be harvested from the patient (autologous bone) or from a cadaver (allograft bone).
Autologous bone is harvested from the patient's pelvic bone (iliac crest) and provides the spinal fusion with a calcium scaffolding for the new bone to grow on (conduction). In addition, autologous bone also contains bone-growing cells (osteoblasts) and bone-growing proteins (bone morphogenic proteins).
Allograft bone simply provides a calcium scaffolding and does not have any bone-growing cells or bone-growing proteins. In the lumbar spine, allograft bone is restricted for use in ALIF or PLIF procedures in which bone graft is placed in compression (the compression aids the healing process for the bone). In a posterolateral gutter spine fusion, in which the bone is placed in tension, allograft bone by itself will not heal well (although allograft chips combined with autograft may be used to extend the harvested bone graft).
Autologous bone, in which the bone is harvested from the patient’s body during spine surgery, has the obvious disadvantage of higher post-operative pain. Most of the pain associated with bone graft harvesting is either from too much muscle stripping or from cutting the small sensory nerves (cluneals) that lie in the fat layer over the pelvis (iliac crest). With careful surgical technique, both of these pitfalls may be avoided.
In posterolateral gutter spine fusion and PLIF procedures, a single incision can be used for the spinal fusion surgery and to harvest the bone from the pelvis (iliac crest). The pelvis can be approached through a plane that has no nerves or blood vessels, and only the top portion of the crest needs to be stripped of its muscles (gluteal muscles). Use of this surgical technique minimizes the blood loss and post-operative pain associated with bone graft harvesting.
Bone harvested for ALIF procedures is done through a separate incision (one inch to two inches long) over the iliac crest. Again, only the very top portion of the iliac crest needs to be removed and the soft cancellous (spongy) bone from in between the cortical (hard) layers of bone is scooped out.
Scooping the bone out of the pelvic bone does not result in a lot of pain because there are no nerve fibers inside the bone. However, care must be taken to avoid the sensory nerve in this region (lateral femoral cutaneous nerve) as damage to this nerve can produce pain and numbness in the front of the thigh (meralgia parasthetica). In general, this approach should be associated with minimal post-operative pain or discomfort because limited soft tissue stripping is needed.
There are currently several products on the market and in development that act either as a bone graft extender or substitute. Demineralized bone matrix (bone that has had the calcium removed) has been available for the past several years. It carries some of the bone morphogenic proteins that the body uses to induce bone formation. There are also calcium hydroxyappetite products or coral, both of which have structures similar to bone and act as scaffolding for new bone.
There has been a lot of excitement among spine surgeons awaiting the new bone morphogenic protein products that are expected to be strong inducers of bone growth (osteoinductive). These new products will be relatively expensive, but will probably be able to grow bone even better than the patient’s own bone and bone graft harvesting may no longer be necessary.
