Spinal fusion is a very common back and neck surgery. In simplest terms, the goal of a spinal fusion is to fuse 2 spinal bones (vertebrae) together, creating 1 solid bone and restoring spinal stability or restoring alignment. To stimulate the bone fusion, a spine surgeon will use additional bone called bone graft. This graft material is necessary to “trick” the body into causing living bone to cross the gap and join to the other side. This article will review the different types of bone graft available today.
Your spine surgeon may use 1 type of bone graft or a combination in your surgery. Your doctor will consider your age, specific spine condition, and overall health when deciding what type(s) of bone graft to use.
Bone graft falls into 1 of 3 categories:
Real bone refers to bone graft coming from your own body (known as autograft or autogenous) or harvested from a cadaver donor (called allograft).
Bone graft substitutes are materials that attempt to mimic the benefits of transplanted bone without the side effects that rarely but potentially can occur.
Today, human bone autografts and allografts are the most common bone graft options for spinal fusion patients. However, bone graft substitutes are widely used and account for more than 2 million surgeries annually worldwide, according to a 2015 review of bone graft substitutes published in World Journal of Orthopedics.1
Below you can learn about the different types of bone grafts.
Autograft bone graft is taken from your own body, most likely from your hip (really the pelvis), ribs, or leg. The surgeon may also take bone from your spine, known as local autograft. Local autograft comes from the vertebra itself and may be from removed bone spurs, lamina, or parts of the spinous process removed during fusion surgeries that remove bone to decompress pinched nerves. Since this bone has to be removed to decompress the nerves, it can be recycled to be used as graft.
Autograft bone is the most popular type of bone graft because it has no risk of disease transmission, and your body accepts the transplant well because the bone is coming from your own body. Because your body is already familiar with the bone, it creates the optimal environment for a healthy bone fusion.
While the benefits of autograft are numerous and hard for bone graft substitutes to replicate, there are some drawbacks. Taking bone from your own body means your surgical time can be longer (if it is not taken from the vertebra) because it adds an additional surgical site to your procedure. If taken from the pelvis, this also ups the cost of your surgery and can result in more post-operative pain. However, the upside is that this bone is much better as it contains your own cells that immediately go to work to start the fusion process.
Allograft is bone that comes from a donor—this type of bone graft is harvested from a cadaver. Allografts do not form new bone the way autograft does as this bone does not have any bone forming cells that are “alive." Rather, it provides a framework for the new bone to grow through like a scaffold to allow the bone cells to grow. Eventually, the new bone cells will remodel and replace the donor bone. To prevent disease transmission, allograft bone is thoroughly disinfected.
While allograft has the benefit of not requiring the additional surgical site associated with autograft, the downsides include a small risk of disease transmission and reduced bone strength due to the elimination of bone growth cells and proteins when the bone is disinfected. The time to fusion is also elongated as there are no host bone cells in this graft.
Demineralized bone matrix (DBM)
Demineralized bone matrix (DBM) is allograft bone that has had its mineral content (calcium) removed. What’s left in the bone are protein-based growth-stimulating substances, such as collagen, proteins, and growth factors. DBM comes in a variety of forms—powder, granules, putty, gel, chip—and its processed nature makes it low risk from a disease transmission perspective. However, DBM isn’t strong enough to promote bone fusion by itself, so it’s typically combined with other grafts.
Ceramic-based substitutes and synthetic bone graft extenders
Ceramics, calcium sulfate, calcium phosphate, and other synthetic bone graft extenders are common bone graft substitutes. Their composition is similar to allograft. And, like allograft, synthetic substitutes encourage new bone growth on their surface before dissolving once the new bone is in place.
Ceramics and synthetic bone graft extenders boast a host of benefits: They have zero risk for disease transmission, are nontoxic, easily sterilized, and may be crafted into different sizes and shapes. The main drawback is their limited strength. These grafts show promise operating as a scaffold (providing a framework for bone growth), but they can’t stimulate bone growth on their own.
Bone morphogenetic proteins (BMPs)
You naturally produce bone morphogenetic proteins (BMPs), and these substances will encourage bone healing after fusion as these substances stimulate your own stem cells to turn into bone cells. They’ve also been shown to reduce adverse reactions to allograft and bone graft substitutes. While a large amount of bone is necessary to extract just a small amount of BMP, scientists are genetically engineering the proteins to make them more accessible. There are potential risks of using BMP, so the surgeon needs to have experience with this substance. Uses of this material can be “off label” or used outside the official use criteria. BMP needs to be used with a structural agent, such as bone graft or substitutes.
A bone graft composite is the combination of substances to achieve the greatest bone fusion success. Common combinations include collagen and ceramic to mimic the structure of a patient’s own bone, DBM and bone marrow cells to stimulate new bone growth, and BMP and ceramics.
Authors in the World Journal of Orthopedics review pointed to some emerging areas that hint at what’s to come for bone graft substitutes. Researchers are seeing promise in the bone healing properties of bone marrow-derived mesenchymal stem cells, gene therapy, and tissue engineering. At this stage, these technologies have only shown promise in animal studies, but they may offer a peek into what’s on the horizon for spinal fusion bone grafting.
Autograft and allograft are currently the most frequently used bone graft types for spinal fusion surgery, but new biomedical materials are advancing the case for bone graft substitutes. With many successful bone graft options available today, your surgeon will select the right type(s) for you to give your spinal fusion the best chance of healing properly.