Spine surgeons use a combination of stability (usually with spinal instrumentation) and some sort of bone grafting to get a fusion. Basically, they are trying to fool the body into thinking that it is injured and needs to heal and try to tilt the balance in favor of healing with bone, thus getting a fusion. The alternative is to heal with fibrous tissue, which basically is a failure of the bone to mend together, also known as a pseudoarthrosis or failed back surgery syndrome.
To favor the balance of bone production, the bone graft needs three elements:
- Osteogenesis. Cells that produce bone need to be brought in with the fusion material
- Osteoconduction. The cells need a scaffolding to grow on
- Osteoinduction. The cells need to be told through autocrine/paracrine factors to upregulate the function of bone producing cells (osteoblasts), turn off bone eating cells (osteoclasts) , and bring in blood vessels to carry nutrients to the fusion mass (angiogenesis)
Learn more about Elements of a Spine Fusion
Patient's own bone
The gold standard for now and the foreseeable future has been the patients' own bone graft (autograft). It can be harvested from the spine itself during the spine surgery, but the best bone is obtained from in between the hard outer crests of the pelvis (iliac crest bone graft). This soft, spongy cancellous bone basically carries all three of the above factors, but harvesting the bone graft carries with it a not insignificant amount of morbidity, especially postoperative pain. For this reason, there has been a considerable amount of interest in finding a suitable bone graft substitute that would not need to be harvested from the patient.
Read more about Bone Graft Site Pain and Morbidity After Spinal Fusion
The original alternative source of bone graft was from cadavers (allograft bone). It can come in various forms from ground up chips, to demineralized bone, to stem cells. There is a lot of debate over the efficacy of each type of graft. They each carry different potential for osteoconduction or osteoinduction, but only the stem cells carry the potential for osteogenesis. By and large, allograft bone is readily obtained (at least in the USA), but some forms are quite expensive (especially stem cells).
The primary disadvantage of allograft bone is the potential to transmit infectious material from one individual to another. Outright wound infections after implantation is quite rare and really not a major deterrent to their use. However, they have the potential of carrying small proteins segments known as prions that can transmit infections. The classic disease is something known as Jacob-Kreutzfeld disease, which is transmitted by a prion similar to the one that transmits Mad Cow disease in cows. As a matter of fact, because of European outbreaks of Mad Cow disease, human allograft tissue is not used in most European countries, and if it is, almost all the material comes from the USA. Theoretically, other diseases that we do not now know the cause of (including Alzheimer's, Parkinson's, or any other slowly degenerative neurological disease) may be prion disseminated. This is only a theoretical concern now, but a factor to consider.
Bone graft substitutes
Bone graft substitutes have been developed for the very reason that there is a potential of transmitting diseases with allogenic material. Again, there are a lot of different choices, but in general, bone graft substitutes are made of some sort of porous, bone friendly material that is slowly resorbed as the body lays down its own bone during the fusion process. The materials are for the most part only osteoconductive, although the osteoinductive and osteogenic components can be added by including a bone marrow aspirate harvested from the iliac crest and combined with the bone graft substitute.
The main controversy surrounding the absorbable bone graft substitutes is how fast they resorb.
- If they resorb before the fusion has been completed, then fibrous tissue will replace the bone tissue (pseudarthosis).
- If it does not resorb, then the patients' bone cannot replace the substance which can also lead to a pseudarthrosis.
Bone morphogenic protein (BMP)
A bone graft substitute that is in a class all by its own is bone morphogenic protein. The only one approved for use in the spine as a bone graft substitute is BMP2 (Infuse), made by Medtronic. It has been widely used by spine surgeons since about 2004. It is in a class by its own because it is a powerful growth hormone that acts as a very strong osteoinductive agent. BMP2 is a naturally occurring growth hormone we all have in our bodies, in picogram amounts. In the commercial form, pharmacological doses of BMP2 (Infuse) are introduced into the fusion area in milligram amounts (millions of times the naturally occurring amounts). It has been shown to reliably grow bone, but there is a considerable amount of potential morbidity from using massive amounts of a growth hormone. In 2011, Dr. Eugene Carragee documented many of these potential dangers in an article in The Spine Journal.
My personal preference is to use a bone graft substitute where possible to avoid any and all chance of transmitting anything that may be a problem later on. This being said, I will still use some iliac crest graft in certain posterior lumbar fusion surgery, and will rarely use BMP2 in very difficult fusion surgeries (i.e. a multilevel revision for a pseudarthrosis).
The bottom line is that there is no one perfect bone graft to use for spine fusion surgery. All surgeons have their preferences, but patients should educate themselves well enough on the myriad of bone graft choices to be involved in the process of choosing a bone graft substitute.