Posterior Thoracic Fusion
Fusion is the process of joining bones with bone grafts, adding bone graft or bone graft substitute to an area of the spine to set up a biological response that causes the bone to grow between the two vertebral elements and thereby stop the motion at that segment. The fusion process essentially “tricks” the body into thinking it has a fracture.
Posterior cervical fusion is performed on the back of the mid-back region for reasons such as instability of the spine, fracture, degenerative disc disease, or stabilization for tumors. The goal with fusion is to stabilize the spine so that pain or deformity is reduced.
THE GENERAL PROCEDURE:
1. Surgical approach
2. Preparation of the fusion bed
3. Bone graft and fusion
In a posterior thoracic fusion, the surgical approach to the spine is from the back through a midline incision. An extension of the vertebral body in this area (transverse process) is a bone that serves as a muscle attachment site, and the site of attachment of the ribs. The large back muscles that attach to the transverse processes are elevated up to create a bed to lay the bone graft on. The soft tissue and cartilage of the facet joints are then removed, preparing the surfaces for bone graft.
Bone graft is often obtained from the pelvis (the iliac crest). Alternatively, bone graft substitutes and extenders can be used. Next, the bone graft material is laid out in the posterolateral portion of the spine, and packed into the facet joints. The back muscles are then released over the bone graft, creating tension to hold the bone graft in place.
The body responds by building bone between the moving segments to stop them from moving. Standard posterior fusion is achieved between the facets and the transverse process of adjacent vertebrae. Instrumentation is usually added to hold the vertebrae together to help increase the chance of fusion.
However, a solid fusion is not always achieved. There are a few factors that patients can control that are important in determining whether or not a fusion grows in solidly, including:
Smoking cessation. It is generally advisable to quit smoking prior to a spinal fusion procedure, as nicotine is a direct toxin to bone graft and will prevent the bone from forming.
Limited motion. Bone forms better if motion is limited, so patients are often advised to avoid bending, lifting, and twisting for three months after spinal fusion surgery.
Since fusion will take at least three months to set up, some type of immobilization is recommended. Often, bracing will be recommended. You may be asked to wear a thoracolumbar brace for a weeks to months after surgery. The bracing is often used after surgery for comfort and to decrease the motion of the neck to allow fusion.
The activity level is gradually increased. Patients are encouraged to walk as much as possible but to avoid lifting or binding early on. Strengthening and physical therapy can be started at three months post-operative if the fusion appears to be progressing well.
It should be noted that the time to fusion can vary, and usually use of the patient’s own bone or use of instrumentation can result in a quicker fusion. It usually takes approximately three months, but can take up to 6 to 9 months, for the bone graft to fuse the facets together. Heavy lifting, bending and twisting are usually limited until the fusion is noted to be solid.
Signs of infection like swelling, redness or draining at the incision site, and fever should be checked out by the surgeon immediately. Keep in mind, everybody is different, and therefore the amount of time it takes to return to normal activities is different for every patient. Discomfort should decrease a little each day. Most patients will benefit from a postoperative exercise program or supervised physical therapy after surgery.
Radiographs will be taken at regular intervals to assess fusion. Further studies such as CT scan, MRI, or CT myelogram may be necessary if the pain continues or if the fusion is in question.
The principal risk from a fusion is that it does not heal. In general, allograft bone does not heal quite as well as autograft bone, but both yield good results when used in the anterior cervical spine.
If a graft is used without instrumentation, there is a small chance (1% to 2%) of a graft dislodgment or extrusion. If this happens, another operation may be necessary to reinsert the bone graft, and instrumentation (plates) can then be used to hold it in place.
The principal disadvantage with using autograft bone is that another incision needs to be made over the hip to harvest the bone graft. Possible complications associated with taking out bone graft include:
Damage to the lateral femoral cutaneous nerve (a sensory nerve that supplies sensation to the front of the thigh)
The chances of a complication increase with the size of the bone graft. The bone graft is an important part of the procedure, and many patients find the bone graft harvest site to be more painful than the cervical surgery itself.
If allograft is used, there is a theoretical risk of transmission of an infection from a donor. The risk of contracting HIV from a graft has been estimated to be between 1 in 200,000 to 1 in 1 million.
In skilled hands, this is a very safe procedure. Possible reported risks and complications of anterior cervical fusion may include: