During a spinal fusion, a bone graft is used to join two or more vertebrae. The vertebrae grow together during the healing process, creating a solid piece of bone. The bone graft helps the vertebrae heal together, or fuse. The bone graft is usually taken from the pelvis at the time of surgery. However, some surgeons prefer to use bone graft from a bone bank (called allograft).
Your surgeon can use an anterior (from the front) approach, a posterior (from the back) approach, or a combined approach to lumbar fusion surgery.
In some cases, it may be best to consider placing the bone graft between the vertebral bodies. The anterior interbody approach allows the surgeon to remove the intervertebral disc from the front and place the bone graft between the vertebrae.
This operation is usually done by making an incision in the abdomen, just above the pelvic bone. The organs in the abdomen, such as the intestines, kidneys, and blood vessels, are moved to the side to allow the surgeon to see the front of the spine. The surgeon then locates the problem disc and removes it. Bone graft is placed into the area between the vertebrae where the disc has been removed.
The posterior approach is done from the back of the patient. This approach can be just a fusion of the vertebral bones or it can include removal of the problem disc. If the disc is removed, it is replaced with a bone graft. The surgeon moves the spinal nerves to one side and inserts the bone graft between the vertebral bodies. This is called a posterior lumbar interbody fusion.
With a posterior approach, an incision is made in the middle of the lower back over the area of the spine that is going to be fused. The muscles are moved to the side so that the surgeon can see the back surface of the vertebrae. Once the spine is visible, the lamina of the vertebra is removed to take pressure off the dura and nerve roots. This allows the surgeon to see areas of pressure on the nerve roots caused by bone spurs, a bulging disc, or thickening of the ligaments. The surgeon can remove or trim these structures to relieve the pressure on the nerves. Once the surgeon is satisfied that all pressure has been removed from the nerves, a fusion is performed. When operating from the backside of the spine, the most common method of performing a spinal fusion is to place strips of bone graft over the back surface of the vertebrae.
Working between the vertebrae from the back of the patient has limitations. The surgeon is limited by the fact that the spinal nerves are constantly in the way. These nerves can only be moved a slight amount to either side. This limits the ability to see the area. There is also limited room to use instruments and place implants. For these reasons, many surgeons prefer to make a separate incision in the abdomen and actually perform two operations-one from the front of the spine and one from the back. The two operations are usually performed at the same time, but they may be done several days apart.
In the past, spinal fusions of the lumbar spine were performed without any internal fixation. The surgeon simply roughed up the bone, placed bone graft material around the vertebrae, and hoped the bones would fuse. Sometimes, patients were placed in a body cast to try to hold the vertebrae still while healing. The patient was lucky to have a 70 percent chance of successful fusion.
Surgery of the spine continues to be a challenging and difficult area. The vertebrae are small, so there is not much room to place small instruments. Also, many nerves can get in the way of putting screws into the vertebral body. And a large amount of stress is put on the lower back, so finding a metal device that is able to hold the bones together can be difficult.
Over the past two decades, some very inventive devices have been designed that have changed the way surgeons perform spinal fusions.
Pedicle Screws and Rods
The surgeon may use some type of metal screws, plates, and rods to hold the vertebrae in place while the spine fusion heals. Designed to stabilize and hold the bones together while the fusion heals, these devices have greatly improved the success rate of fusion in the lower back.
A new type of device, called an intervertebral fusion cage, can be used to perform a spinal fusion between two or more vertebrae. These implants are designed to spread the two vertebrae apart while the fusion heals.