Mini-Open Anterior Lumbar Interbody Fusion (ALIF)

Introduction

The anterior lumbar interbody fusion (ALIF) is a powerful and versatile tool for obtaining interbody fusion for a variety of conditions. Anterior fusion for disk disease was first described by Burns, and Harmon first described a retroperitoneal approach in 1950. The current mini-open perirectus approach was first described by Crock. Since that time many studies have been released using this approach for a variety of indications. Although used predominantly to obtain fusion, it is also excellent for restoring lumbar lordosis and can be used for indirect decompression, especially in the setting of foraminal stenosis. The technique is used predominantly at L4-5 and L5-S1, but can be used at L3-4 and, in rare cases in the author’s experience, at L2-3. It can be used for one-, two-, and, in slender patients, three-level procedures.

This surgery is most commonly done as a team approach with a vascular or general surgeon, but can be done safely by a properly trained spinal surgeon without a vascular surgeon.

Indications

The mini-open ALIF can be used in almost any indication for lumbar fusion from L3 to the sacrum. It was initially described for fusion for back pain in disk degeneration. It has been shown to provide good results in isthmic spondylolisthesis when combined with posterior instrumentation. The author has also used the ALIF extensively in the setting of degenerative spondylolisthesis without central stenosis and iatrogenic spondylolisthesis after previous laminectomy. With the high failure rates seen in long fusions to the sacrum for adult deformity, it has become common to use the ALIF at the L5-S1 level to improve fusion rate. The ability to release the anterior longitudinal ligament and place a trapezoidal cage or graft in the disk space makes it a powerful tool for restoration of lordosis. The procedure can have very high fusion rates especially with the use of recombinant human bone morphogenetic protein (rhBMP2) and, as a result, is often used in the treatment of symptomatic pseudarthrosis. This is the author’s preferred method of treating pseudarthrosis after posterolateral or posterior-based interbody fusions (PLIF and TLIF). It has also been used in the treatment of septic discitis. This is a difficult indication because of the presence of significant lymphatic and inflammatory tissues around the vascular bifurcation (my experience). In patients with symptomatic foraminal stenosis who have failed direct decompression owing to the lack of vertical height in the foramen, an ALIF can significantly increase foraminal volume and be a good treatment option.

Limitations

The limitations to the mini ALIF approach are generally either anatomic, approach, or diagnosis related. The anatomic plane used for this approach is prerenal. As a result, when trying to reach the upper lumbar levels the renal artery and kidney prevent access to the disk. As a result, the upper limit is generally the L3-4 level. The approach corridor should be collinear to the disk to allow appropriate disk preparation and implant placement. In patients with a high sacral slope, a collinear approach is blocked by the symphysis pubis and is anatomically not possible. The vascular anatomy of the lumbosacral junction is quite variable. In some cases the location of the bifurcation can make access to the disk difficult or impossible.

Obesity is a relative contraindication to the ALIF approach. The width of the access corridor is limited by the width of the rectus sheath. In obese patients, particularly those with higher retroperitoneal or intraperitoneal fat deposits, the distance to the spine and the narrow corridor make the approach very difficult. In an obese patient with mostly prefascial fat, a longer skin incision can be used to reach the rectus sheath, and the approach may be possible. The author tends to use a body mass index of more than 35 as a cut-off for the ALIF approach.

Although it has been shown that increasing disk height with an ALIF can increase central canal diameter, use of the ALIF approach for the treatment of severe central stenosis is probably not warranted. Similarly, the treatment of central stenosis in a patient with no listhesis and normal disk height would not be effective.

As mentioned previously, this approach can be used for the treatment of discitis and osteomyelitis. The limiting factor here is how much thickening of the perivascular tissues around the bifurcation exists. This thickened tissue makes identifying and mobilizing the great vessels difficult. Careful study of the preoperative imaging is necessary to see if the approach would be safe in this setting.

The ability to develop the retroperitoneal plane and mobilize the vessels is the key to safely reaching the disk space. In patients who have had previous retroperitoneal surgery or radiation, the planes no longer exist and the ALIF is not an option. Previous abdominal surgery is generally not a limitation. The exception to this is in patients with pelvic floor reconstructions where a sling has been tacked up to the sacrum. It may be necessary to obtain previous operative notes to see if this was done. The necessity of mobilizing the left common iliac vein and location of the inferior vena cave makes a left-sided retroperitoneal approach mandatory for L3-4 and L4-5. In the author’s experience, in some patients with previous left-sided ALIF where the left common iliac is relatively lateral, a right-sided approach can be used to access L5-S1 and is sometimes also used in an isolated L5-S1 fusion to preserve the left-sided retroperitoneum for potential future surgery.

Technique

Preoperative Assessment

When assessing patients for possible anterior interbody fusion several factors need to be considered. The first consideration is whether the disk can be accessed successfully. In morbidly obese individuals the mini-open ALIF approach is sometimes impossible owing to the distance from skin to spine. In the author’s practice those with a body mass index of greater than 35 are usually not candidates. Previous retroperitoneal surgery or radiation are also contraindications. One exception to this in the author’s practice is a previous left-sided ALIF surgery if attempting to access L5-S1. In many cases it possible to do these through a right-sided approach.

Careful review of the preoperative imaging will also identify patients unsuited to this approach. A standing lateral x-ray study, including the symphysis pubis, should always be performed. If a projected line from the superior endplate of S1 does not pass above the symphysis, the disk space will be inaccessible and an alternate fusion technique should be used ( Fig. 6.1 ). In addition, the x-ray study will show the relationship of the spinal levels to superficial bony landmarks to guide incision placement ( Fig. 6.2 ).

Careful review of a computed tomography scan or magnetic resonance image will usually give a good view of the vascular anatomy and will, in some cases, reveal anatomy unfavorable to the procedure. Computed tomography and magnetic resonance imaging are also very useful in patients where a surgeon is considering an ALIF for infection. The dense concentration of lymphatic tissue related to the periaortic lymphatics can make this approach very difficult in these patients.

Operative Technique

Anesthesia and Positioning

Mini ALIF is performed under general anesthesia with neuromuscular blockade. Not having skeletal relaxation makes the approach much more difficult. A Foley catheter should be placed because a full bladder can interfere with the approach. The patient is positioned supine on the surgeon’s table of choice. A flexible table with a kidney rest placed under the level of interest can be used to increase lumbar lordosis. The author’s preference is a flat radiolucent table with an inflatable arterial line bag placed under the level of interest to be inflated or deflated as necessary during the procedure. In three-level procedures, increasing the lumbar lordosis will often make access to L3-4 very difficult so the author do not inflate the bag until this level is done. In addition, if the incision has been made too low to access the upper of two disks, reducing any applied lordosis can ease exposure.

Incision

Some authors advocate using the umbilicus as a guide to placing the incision. In the author’s experience this is a highly mobile and unreliable landmark. In many patients’ once anesthetized, the sacral promontory can be directly palpated and used as a landmark. The preoperative x-ray can be used to compare the location of the anterior superior iliac spine to the level of interest ( Fig. 6.3 ). The top of the iliac crest can also be used, but it is less reliable in obese patients. When in doubt, fluoroscopy may be utilized to identify the level. Incision should be colinear with the inferior endplate for single level procedures; midpoint of the intervening vertebral body for two levels and colinear with the middle disk in three levels. A transverse skin incision can be used in single or multilevel procedures. The rectus is divided transversely in single level procedures and obliquely inferomedial to superolateral for a multilevel procedure.