Anterior 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 (see spinal disorders).
Anterior lumbar fusion is performed from the front of the low back regions for reasons such as instability of the spine, degenerative disc disease, or fracture. The goal with fusion is to stabilize the spine so that pain or deformity is reduced.
There are multiple different methods for obtaining a spinal fusion. One method is the Anterior Lumbar Interbody Fusion (ALIF). This type of spinal fusion, which involves placing bone graft with or without allograft strut graft or instrumentation in the disc space, has a long history. The ALIF approach has the advantage that, unlike the PLIF and posterolateral gutter approaches, both the back muscles and nerves remain undisturbed. Another advantage is that placing the bone graft in the front of the spine places it in compression, and bone in compression tends to fuse better.
THE GENERAL PROCEDURE:
1. Surgical approach:
- A three-inch to five-inch incision is made on the left side of the abdomen or alternatively in the midline. The abdominal muscles are retracted to the side.
- Since the anterior abdominal muscle in the midline (rectus abdominis) runs vertically, it does not need to be cut and easily retracts to the side. The abdominal contents lay inside a large sack (peritoneum) that can also be retracted, thus allowing the surgeon access to the front of the spine.
- Alternatively, the peritoneum can be cut and the abdominal contents retracted to approach the spine. This is performed most commonly at L5-S1.
- The large blood vessels that continue to the legs (aorta and vena cava) are gently retracted off of the anterior spine.This part of the procedure is often performed by a general or vascular surgeon.
2. Disc removal:
- A needle is then inserted into the disc space and an x-ray is done to confirm that the surgeon is at the correct level of the spine
- After the correct disc space has been identified on x-ray, the disc is then removed by first cutting the outer annulus fibrosis (fibrous ring around the disc) and removing the nucleus pulposus (the soft inner core of the disc)
- Dissection is carried out from the front to back of the disc.
3. Preparation of the fusion bed:
- After the disc is removed, a space remains between the vertebral bodies.
- Disc space shavers and spacers may be used to template the height, width, and depth of bone graft that is needed.
- The surfaces of the vertebral bodies are meticulously prepared for bone graft by burring any irregularities. This allows surface area for the ingrowth of bone.
4. Bone graft and fusion:
Cancellous bone graft is often obtained from the pelvis (the iliac crest). Alternatively or additionally, bone graft substitutes and extenders can be used.
Since the lumbar disc spaces are tall, often a structural, weight-bearing component is necessary to maintain the disc height and lordosis. An allograft strut structural bone or instrumentation is often employed.
The combination of the structural component (either allograft strut or instrumentation) and non-structural component (cancellous autograft or allograft and/or bone graft substitutes or extenders) are inserted into the disc space.
Some ALIF procedures will be done using a minilaparotomy (one small incision) or with an endoscope (a scope that allows the surgery to be done through several one-inch incisions). The results with either procedure are equivalent and the type of approach used should depend mostly on which procedure the surgeon is most comfortable using.
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.
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.
The procedure is performed in close proximity to the large blood vessels that go to the legs (I have figures to show this). Damage to these large blood vessels may result in excessive blood loss. Quoted rates in the medical literature put this risk at 1% to 15%. There is also a small risk of injury to the bowel or ureter, since they are in the proximity of the lower lumbar discs.
For males, another risk unique to this approach is that approaching the L5-S1 (lumbar segment 5 and sacral segment 1) disc space from the front has a risk of creating a condition known as retrograde ejaculation.
In males, there is a small risk of retrograde ejaculation with anterior approaches at L5-S1. The nerves that control ejaculation lay over the front of the L5-S1 disc. They are very sensitive, and therefore even with retraction the normal coordination of ejaculation can be disrupted resulting in the ejaculation occurring backward into the bladder. It should be noted that erection and sex drive are not affected. Fortunately, retrograde ejaculation happens in less than 1% of cases (but in some studies is higher) and tends to resolve over time (a few months to a year).
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:
- Graft site chronic pain (which happens 10% to 25% of the time)
- Hip fracture
- Damage to the lateral femoral cutaneous nerve (a sensory nerve that supplies sensation to the front of the thigh)
The chances of a graft-related 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 lumbar 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 lumbar fusion may include:
- Retrograde ejaculation (in men)
- Injury to the ureter
- Injury to the bowel
- Incisional hernia
- Injury to the diaphragm or kidney
- Damage to the spinal cord or nerve roots
- Continued pain
- Adjacent disc disease
- Graft extrusion
Pain medications are provided either with PCA (patient controlled anesthesia), through the intervenous catheter, or by mouth.
The bowels can be slow to wake up post-operatively after an anterior approach. Often the diet is advanced slowly to not result in abdominal discomfort or distension.
Since fusion will take at least three months to set up, some type of immobilization is recommended. Often, bracing will be recommended. Hospital stay usually ranges from 2-5 days. 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.
A stand-alone one-level spinal fusion is usually performed with an anterior cage or a piece of allograft to maintain the disc height. Often, anterior lumbar fusion is combined with a posterior instrumentation. The fusion rates for anterior and posterior surgery are very high, and in fact in some studies are 100%.
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 to the vertebral body bone. Heavy lifting and overhead work 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.