Spinal fusion is usually performed after a discectomy. The goal of spinal fusion is to stop the motion caused by segmental instability. This reduces the mechanical pain caused from excess motion in the spinal segment.
In this procedure, the surgeon fills the space left after the disc is removed with a block of bone called a bone graft. This graft may be taken from your pelvis, or it may be obtained from a bone bank. Bone graft from a bone bank is given by organ donors and stored under sterile conditions until it is needed for operations such as spinal fusion. The bone goes through a rigorous testing procedure, similar to a blood transfusion. This is in order to reduce the risk of passing on diseases, such as AIDS or hepatitis, to the recipient.
Placing a bone graft between two or more vertebrae causes the vertebrae to grow together, or fuse. If your back or neck pain is caused by segmental instability, a spinal fusion may also be recommended even if you do not have radiculopathy.
When doing a spinal fusion, the bone graft may simply be wedged in tight between the vertebrae, which holds the graft in place. In recent years, there has been an increase in the use of metal plates, pedicle (ped-i-cul) screws, and rods to try to improve the success of the fusion. Many different types of metal implants are used, and all of them try to hold the vertebrae in position while the fusion heals. Bone heals best when it is held still, without motion between the pieces trying to heal together. The healing of a fusion is no different than healing a fractured bone, such as a broken arm. In particular, the cervical spine is a difficult part of the body to hold still.
There are two basic types of spinal fusion:
Anterior Interbody Fusion
In the anterior (which refers to the front side of your body) interbody fusion, a bone graft is placed between two vertebrae after the disc is removed. During the healing process, the vertebrae grow together, creating a solid piece of bone out of the two vertebrae. When surgery is needed, the anterior interbody cervical fusion is used to treat most problems in the neck caused by degenerative disc disease. These include unrelieved neck pain and pressure on the nerve roots caused by bone spurs or a herniated disc.
Bone Graft to Provide Support
Bone graft is also used for structure. Rather than crush the bone into fine pieces, larger pieces of bone are used to fill a gap between two bones. For example, if the surgeon removes a vertebra or disc, he or she may place a chunk of bone graft into the space. Because bone is rigid, it will hold the bones apart while the body grows to the ends of the graft. Over time the entire piece of bone that was grafted will be “remodeled” and replaced by the body with new bone. The time it takes to fuse depends on the size of the piece of bone that was used. It is sometimes a slow process that may take several years.
Types of Bone Used in Bone Graft
Two types of bone are used in bone graft procedures: autograft (auto-graft), which is bone taken from your own body; and allograft (al-oh-graft), which is bone that comes from a bone bank. The most common approach is to use your own bone whenever possible. Sometimes your surgeon may mix allograft with autograft.
Allograft can come from many types of bones and in many different forms. Allograft is usually removed from organ donors and placed in bone banks. Bone banks test it for diseases such as hepatitis and AIDS — just like testing done in a blood bank. Allograft does not contain any living cells, so it has fewer chemicals to stimulate growth of new bone. The disadvantage of an allograft is that it may not always grow as well or as quickly as an autograft, but a bone-growing protein can be added to the area to make up for what the bone graft lacks. The advantage to using allograft is that you do not have to donate the bone graft. This makes your surgery shorter, and there may be less pain afterward. The allograft carries a very small risk of transferring infectious diseases even though it is carefully tested.
Allograft is very useful when your operation requires more bone graft than your own body can supply. Some major spine fusions need a lot of bone graft. Some surgeries need large pieces of structural bone graft, which may cause problems in the area of your body where the bone was removed.
Advancements in Bone Graft Technology
Much research is being done to design bone graft substitutes, chemicals, and devices that stimulate the bones to fuse. It is well known that electrical current stimulates bone to grow. Many surgeons use electrical stimulation devices during the first weeks after surgery to speed up fusion. Some artificial bone graft materials have been developed. For instance, sea coral, harvested from oceans, is sometimes used as a structural bone replacement. Other developments include:
- Demineralized bone matrix (DBM) — a type of allograft developed from bones in a bone bank. The bone has the calcium removed and can be turned into a putty, sheet, or gel. The material can then be added to a graft site to improve the fusion.
- Autologous growth factor (AGF) — a solution used to stimulate bone growth. It is created in a laboratory from blood platelets (the clotting part of blood). The mixture is usually used in combination with some form of structural support, such as autograft or fusion cages.
- Bone morphogenic protein (BMP) — a chemical added to bone graft to improve bone growth when it is added to a fusion site.