Overview

Anterior lumbar interbody fusion (ALIF) surgery

Doctor Cloward first performed an anterior lumbar interbody fusion (ALIF) surgery in the 1950's for treatment of low back pain for degenerative spine conditions. The procedure did not gain relative favor because of fairly high nonunion rates (30-40%). In the 1990's, however, there was a resurgence of popularity for anterior (from the front) lumbar interbody fusion surgery because of the advent of new threaded titanium cages that held the disc space better and allowed for a higher fusion rate.

While the ALIF is still a widely available spine fusion technique, this type of procedure is often combined with a posterior approach (anterior/posterior fusions) because of the need to provide more rigid fixation than an anterior approach alone provides.

In cases where there is not a lot of instability, an ALIF alone can be sufficient. Generally, this is true in cases of one level degenerative disc disease where there is a lot of disc space collapse. For patients who have a "tall" disc, or for those with instability (e.g. isthmic spondylolisthesis), an anterior approach to spine fusion may not provide adequate stability. In these clinical situations the anterior lumbar interbody fusion may be supplemented with a posterior (from the back) instrumentation and fusion to provide additional support to the fused level of the spine.

Anterior lumbar interbody fusion description
The anterior lumbar interbody fusion (ALIF) is similar to the posterior lumbar interbody fusion (PLIF), except that in the ALIF the disc space is fused by approaching the spine through the abdomen instead of through the lower back.

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 spine surgeon access to the front of the spine.

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 minilaparotomy allows better visualization and can be done with a minimal amount of postoperative pain. Most spine surgeons use the open, minilaparotomy approach.

• The endoscopic approach has more limited visualization, and it usually leads to larger surgical times and carries with it a much higher technical learning curve for the surgeon.

The results with either procedure are equivalent and the type of approach used should depend mostly on which procedure the spine surgeon is most comfortable using. The endoscopic approach has largely fallen out of favor because of the technical difficulties associated with it, and it has not been proven to generally lessen postoperative pain or hasten the healing process.

The large blood vessels that continue to the legs (aorta and vena cava) lay on top of the spine, so many spine surgeons will perform this surgery in conjunction with a vascular surgeon who mobilizes the large blood vessels. After the blood vessels have been moved aside, the disc material is removed and bone graft, or bone graft and anterior interbody cages, is inserted.

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.

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%.

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. There are very small nerves directly over the disc interspace that control a valve that causes the ejaculate to be expelled outward during intercourse. By dissecting over the disc space the nerves can stop working, and without this coordinating innervation to the valve, the ejaculate takes the path of least resistance, which is up into the bladder. The sensation of ejaculating is largely the same, but it makes conception very difficult (special harvesting techniques can be utilized). Fortunately, retrograde ejaculation happens in less than 1% of cases and tends to resolve over time (a few months to a year). This complication does not result in impotence as these nerves do not control erection.

In general, the principal risk of this type of spine surgery is that a solid fusion will not be obtained (nonunion) and further surgery to re-fuse the spine may be necessary. Fusion rates for an ALIF should be as high as 90-95%.

Nonunion rates are higher for patients who have had prior lower back 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.   Not all patients who have a nonunion will need to have another fusion procedure. As long as the joint is stable, and the patient's symptoms are better, more back surgery is not necessary.

Other than nonunion, the risks of a spinal fusion surgery include infection or bleeding. These complications are fairly uncommon (approximately 1% to 3% occurrence). In addition, there is a risk of achieving a successful fusion, but the patient's pain does not subside.

ALIF spinal implants and bone grafts
Although initially the only spinal implants that were available were threaded titanium cages, there is now an assortment of cages in different shapes, sizes and materials.

One limitation with a threaded titanium cage is that the larger the size of the disc space the more bone that is removed from the subchondral endplates (the strong bone at the bottom and top of the vertebral body). This weakens the bone and leads to an increase in the subsidence rate (where the cages subside into the vertebral body).

One potential way to avoid subsidence is to use a more rectangular cage that sits in between the vertebral endplates, and allows the endplates to support device. It is not yet known whether or not these newer cages with more surfaces area will lessen subsidence rates. Fixation can be achieved by using porous metal that binds to the bony endplates by friction.

Because an anterior interbody spine fusion surgery relies on the strength of the vertebral body to keep from subsiding, and absolute contraindication to doing an anterior interbody fusion (without posterior supporting instrumentation) is osteoporosis. The cages do not fail by breaking. They fail because the bone in the vertebral endplates may not be strong enough to support the cages. This leads to a failure of the endplates, with the cage subsiding into the vertebral bodies.

In general, anterior cages are not strictly fixation devices for spine fusion. Pedicle screws used with posterior instrumentation systems provide excellent spinal fixation. Anterior intervertebral devices should be thought of as an interference type of fixation. They are implanted in between the vertebral bodies and do not strictly fixate the two vertebral bodies to each other. Until the bone knits them together, gravity and the ligamentous tension of the residual disc space are all that holds it together in the spine.

Because of this difference in mechanics, stand alone anterior fixation is best limited to collapsed disc spaces. It works better at L5-S1 where there is little motion. At L4-L5 there is more flexion/extension motion, and this allows more motion through the cages. Lastly, they work better in one-level spine fusions than two-level fusions, and most spine surgeons feel they should not be used as a stand alone device for three level fusions.

A problem with titanium cages has been that it is difficult to assess spine fusions postoperatively because the metal impedes evaluation by x-ray. One solution has been to use radiolucent cages (made of either carbon fiber or PEEK). Postoperatively the cages allow much better visualization of the healing bone. Unfortunately, they do not adhere to the bony endplates well and are rarely used by themselves. They are usually used in conjunction with either an anterior/posterior fusion or a PLIF and supplemented with pedicle screws.

There are many innovations and technical improvements being developed, and although no one cage is the best, there are certain cages that work well for certain indications. As with any other spine fusion procedure, the implant used is largely dictated by what the treating spine surgeon prefers and has had the most success with in the past.

Whether a spine surgeon approaches the disc space from an anterior approach or from one of the posterior approaches (PLIF, TLIF) is largely dependent on how comfortable the surgeon is with the anterior approach and operating around the aorta and vena cava. Most spine surgeons have not had a great deal of experience doing the procedure by themselves, and not all spine surgeons have access to a skilled vascular surgeon to help them with the approach. Therefore, for many surgeons a posterior approach for lumbar spine fusion surgery may be more practical.

An ALIF spine surgery can also be done with an allograft bone implant. Allograft bone (cadaveric bone) can be milled to a shape like a titanium implant (cylindrical), or more commonly, it is a femoral ring that can be shaped by the physician to fit the disc space. Generally, allograft bone is not as strong as other implants. In cases where the lower back surgery is being done as an anterior/posterior approach, it is strong enough, but most spine surgeons are leery about using it as a stand-alone device (e.g. no posterior instrumentation to help support it). Allograft bone tends to cause resorption of the patient's own bone (osteolysis) at the graft/vertebral endplate interface early in the postoperative course, and can lead to further instability.

In the past, the patient's own bone had been used (autologous bone). This required a large bone graft to be taken from the patient's iliac crest, and had a fairly high complication rate (such as postoperative chronic pain, infection, pelvic fracture). Also, this bone is not all that strong and required supplementation with posterior instrumentation. It is not as strong or supportive as allograft bone.

By: Peter F. Ullrich, Jr., MD
September 8, 1999 (Updated January 20, 2004)