Posterior Lumbar 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 a procedure that is performed on the low 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:
- The surgical approach to the spine is from the back through a midline incision.
- The muscles are dissected off of the lamina, and the facet joints and lateral mass are identified.
2. Preparation of the fusion bed:
- The soft tissue and cartilage of the facet joints are then removed.
- The surfaces are meticulously prepared for bone graft by burring or with a gouge. This allows surface area for the ingrowth of bone.
3. Bone graft and fusion:
- 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.
In a posterolateral gutter fusion, the surgical approach to the spine is from the back through a midline incision that is approximately three inches to six inches long. The muscles are dissected off of the lamina, and the facet joints are identified. The soft tissue and cartilage of the facet joints are then removed, preparing the surfaces for bone graft. A small extension of the vertebral body in this area (transverse process) is a bone that serves as a muscle attachment site. 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). Most surgeons work through the same incision to obtain the bone graft and to perform the spinal fusion. 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 may be added to hold the vertebrae together to help increase the chance of fusion.
The success rate for posterior fusion in the treatment of refractory discogenic back pain is only 60-70%. The success rate is higher when there is frank instability due to fracture or to spondylolisthesis.
The selection of the appropriate patient for this surgery has been blamed for the relatively poor results when fusion is used for treatment of discogenic back pain. Other possible causes of poor results are that the actual pain-causer, the disc, is not addressed. See discussion of PLIF - Posterior Lumbar Interbody 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 up to three months after spinal fusion surgery depending on the amount of instability.
The principal risk of this type of surgery is that a solid fusion will not be obtained (nonunion) and further surgery to re-fuse the spine may be necessary. Most fusion will set up within three months, and will continue to get stronger for one to two years. Once a solid fusion is achieved it is very unlikely that it will ever break.
Recurrent pain after a successful spinal fusion procedure is more likely due to a “transfer” lesion at the motion segment above or below the fusion, because stress is transferred to the next level and may cause that vertebral segment to degenerate and breakdown.
Nonunion rates of between 10% and 40% have been quoted in the medical literature. Nonunion rates are higher for patients who have had prior surgery, patients who smoke or are obese, patients who have multiple level fusion surgery, and for patients who have been treated with radiation for cancer.
In skilled hands, this is a very safe procedure. Possible reported risks and complications of posterior cervical fusion may include but are not limited to:
- Damage to the spinal cord or nerve roots
- Continued pain
- Adjacent disc disease
- Injury to the vertebral artery
- Blood clots
- Bowel or bladder incontinence
Pain medications are provided either with PCA (patient controlled anesthesia), through the intervenous catheter, or by mouth.
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 neck brace for a few days to weeks after surgery. Lumbar 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.