Axial Lumbar Interbody Fusion

Overview

Axial lumbar interbody fusion (AXIALIF) is a relatively new, minimally invasive approach to fusion of L5–S1 and L4–L5. This approach allows access to the disk space axially through the body of S1. By making use of this approach, the surgeon can avoid the morbidity usually associated with anterior or posterior approaches, including damage to paraspinal muscles and retraction of the thecal sac and nerve root; it also avoids risk to great vessels and risks of mobilization of the peritoneal contents. The tradeoff is that this approach exposes the patient to risk of rectal injury and presacral hematoma. AXIALIF also limits the surgeon’s ability to completely visualize the disk space and may limit the ability to correct the deformity. Thus patients should be selected very carefully for this approach.

Anatomy Review

The relevant bony anatomy for axial interbody fusion includes the sacrum, coccyx, ilia, and the lowest two lumbar vertebrae. The anteriorly concave shape of the sacrum allows axial instrumentation to be placed across the L5–S1 and L4–L5 disks through the anterior cortex of the sacrum.

The ligamentous anatomy of the sacrum and ilia define the access point through the parietal pelvic fascia. The sacrospinous and sacrotuberous ligaments, along with the coccyx, define an arch caudal to the tip of the sacrum. Deep to this arch, the pubococcygeus and coccygeus muscles form a hiatus, which is exploited to gain entry into the pelvis in the presacral space. In this space, the presacral fat layer provides a plane of dissection between the rectum and the sacrum. The midline sacrum is relatively devoid of neurologic structures, however, if dissection is carried laterally, the sacral plexus and sacral nerve roots are encountered. The median sacral artery arises from the aorta above the bifurcation and can be found near the midline of the sacrum along with its associated venous plexus.

The approach to the S1–S2 junction for the AXIALIF procedure exploits the potential space of the presacral space. This space is bordered posteriorly by the parietal pelvic fascia covering the presacral vessels and sympathetic trunk and is bordered anteriorly by the fascia propria of the rectum. The space is contiguous with the retroperitoneum superiorly and is closed by the levator ani inferiorly. A recent anatomic study by Xiang and colleagues confirmed the presence of a rectosacral fascia in 94% of specimens. This fascia arises from the parietal presacral fascia between S2 and S4 and runs anteroinferiorly to join with the fascia propria of the rectum; the rectosacral fascia divides the presacral space into superior and inferior presacral spaces. The authors of this study suggest that the rectosacral fascia be penetrated semisharply, because blunt dissection may rupture the rectum or presacral vessels. This study also divided the presacral space into five layers. From posterior to anterior they are the 1) sacral periosteum, 2) parietal presacral fascia, 3) rectosacral fascia, 4) autonomic nerve fascia, and 5) fascia propria of the rectum ( Fig. 52-1 ). The plane of dissection should be between the parietal presacral fascia and rectosacral fascia inferiorly and the rectosacral fascia and autonomic nerve fascia superiorly.

Pelvic splanchnic nerves arise from the anterior roots of S2–S4, pass anterior to the parietal fascia at the lateral foramina, and run inferolaterally down the pelvic wall. These nerves join with the inferior hypogastric nerve to form the inferior hypogastric plexus, which is found approximately 1 cm from the midline of the sacrum. Branches from this plexus then run medially to enter the mesorectum. Anterior mobilization of the mesorectum is limited by the length of these branches to 19 to 25 mm. These nerves are the limits of safe surgical dissection and define a safe corridor of approximately 2 cm mediolateral and 4 cm anteroposterior.

The great vessels lie far outside the above mentioned corridor. The aorta consistently bifurcates above L5, and the iliac vessels are consistently lateral to the sacral foramina. The vascular structures of concern are the sacral vessels, middle and lateral, and the vessels of the hypogastric plexus. Straying posteriorly through the parietal fascia or laterally into the plexus can put these vessels at risk.

Indications and Contraindications

This technique is indicated for arthrodesis of the L5–S1 disk or L5–S1 and L4–L5 disks. The general indications for arthrodesis at these levels include pseudarthrosis, degenerative disk disease, and grade 1 or 2 spondylolisthesis. Additionally, AXIALIF is an interbody technique that must be supplemented with additional fixation, especially for rotational stability. There are no absolute indications for this procedure, although the implant is well suited to resist shear forces associated with spondylolisthesis after reduction. Relative indications include factors that make other approaches to fusion at this level difficult or impossible. These include:

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Previous anterior surgery, especially with significant scarring or adhesions, provided the presacral space has not been violated
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Severe abdominal obesity
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Abnormal vascular anatomy precluding an anterior approach, as long as the anterior sacrum is free of such vessels

Contraindications to AXIALIF include:

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Previous presacral surgery with significant scarring or rectal adhesion to the sacrum ( Fig. 52-2 )

Figure 52-2

Presacral scarring.
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History of pelvic radiation
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Inflammatory bowel disease
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Close approximation of rectum to sacrum on preoperative imaging ( Fig. 52-3, B )

Figure 52-3

A, Large presacral vessel on magnetic resonance imaging. B, Bowel adherent to sacrum, large vessel.
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Significant deformity of L5–S1 disk level
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Steep L5–S1 inclination or other lumbosacral anatomy that precludes safe screw placement
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Large middle sacral vessels (see Fig. 52-3 )

Operative Technique

Equipment

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Jackson table with leg sling, which allows patient positioning with hips flexed and abdomen free of pressure
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C-arm fluoroscopy

Patient Preparation and Positioning

Because this procedure is always combined with posterior instrumentation, the authors prefer to complete any decompression and/or reduction maneuvers before beginning AXIALIF. This optimizes positioning for screw placement and minimizes time spent in the prone position after violating the presacral space.

Before surgery, a complete bowel prep is mandatory to reduce risk of infection following a rectal injury. Preoperative imaging is critical for planning the trajectory and excluding contraindications to this procedure ( Fig. 52-4 ). Pelvic magnetic resonance imaging (MRI) is useful to visualize the location of the vessels and bowel. Additionally, antibiotic prophylaxis should be given following the guidelines for elective intraperitoneal colon or rectal surgery. The patient should be positioned prone on a Jackson table with the hips flexed to facilitate access to the presacral space. The leg sling on the Jackson table, set to the loosest setting, is suggested ( Fig. 52-5 ). Placing the patient in Trendelenburg position can aid access. The anus should be isolated from the sterile field, and the field should allow visualization of the posterior inferior iliac spines and palpation of the ischial tuberosities and coccyx. Treat this procedure as completely separate from any subsequent fixation. We also prefer to scrub the surgical site with chlorhexidine gluconate or Betadine sponges before standard surgical preparation.