Anterior Lumbar Interbody Fusion

Illustrative Case

A 58-year-old woman with L5-S1 spondylolisthesis and bilateral neuroforaminal narrowing with associated radiculopathy (JGH) (Figure 24-1).

Figure 24-1 ▪ There is a grade 1 spondylolisthesis at L5-S1 on standing x-ray, which completely reduces in the supine position for the magnetic resonance image.

Figure 24-1 ▪ (continued) There is associated bilateral foraminal stenosis at L5-S1 but no canal stenosis, consistent with an isthmic spondylolisthesis.

Radiologic Assessment

Carefully review the preoperative MRI to evaluate vessel anatomy at the desired disk level; scrutinize the location of bifurcation of the aorta and vena cava, and location of the common iliac arteries and veins as it relates to the target disk space. Note the degree of calcification of the vessels. Look for fat plane between the iliac vein and the spine (Figures 24-2 and 24-3).
Review the plain x-rays for orientation of the target disk and the trajectory required to gain access to it.

Figure 24-2 ▪ Note on the axial MRI cut through the L5-S1 disk the positions of the iliac veins and arteries. As is often but not always the case at L5-S1, there is a working zone in between the vessels that can be utilized to access the disk with minimal vascular mobilization needed. Because the axial cut was not made exactly collinear with the disk space, the S1 pedicles are seen. However, on cross-localization with the sagittal MRI, this axial cut is the one that corresponds to the anterior disk space and thus represents the vascular anatomy that will be encountered when exposing the disk anteriorly.

Figure 24-3 ▪ Note at L4-5, the iliac vessels lie directly anterior to the disk space and thus would need to be mobilized if anterior lumbar interbody fusion were performed at this level.

Positioning

Check pedal pulses before and after surgery. If these are not palpable, a vascular Doppler machine should be used to document the status of pulses preoperatively.
Patient is positioned supine on a regular reversed OR table with a bump under the sacrum.
For L4-5 and L5-S1 access, the table is then placed in Trendelenburg to allow for a more upright access to these (typically lordotic) disk space(s).

Anesthesia and Neuromonitoring Concerns

General endotracheal anesthesia
Typically, no need for neuromonitoring
We prefer complete muscle relaxation for ease of approach.

Localization of Incision

We prefer a left-sided pararectus retroperitoneal approach.
- The rationale for this approach is that the aorta is more resilient than the vena cava, and the trans-oblique muscle approach is frequently complicated by incisional hernia.
- May consider transperitoneal or right-sided approach if prior surgery on left side.
Various skin incisions may be used.
- The Pfannenstiel incision can be used for L5-S1 exposures but is less extensile if further proximal exposure becomes necessary.
- The direct midline and paramedian incisions are useful for multilevel exposures. This is our preference (Figure 24-4).
  
  Figure 24-4 ▪ Approximate locations of incision for anterior lumbar interbody fusion at various levels.
For L5-S1, center the incision over sacral promontory, which may be palpable in thinner patients.
- If needed, incision is localized with a lateral C-arm image, keeping in mind that the trajectory needed to access the disk may require that the incision be placed where the path of this trajectory meets the skin, rather than directly over the disk space itself (Figure 24-5).

Figure 24-5 ▪ In a different patient, note the optimal “line of sight” (red dotted lines) to access L5-S1 versus L4-5. Because of lordosis, the ideal location of the incision at the level of the skin is typically more distal at L4-S1 than the actual locations of the anterior disk spaces themselves.

Approach

The layered configuration of the anterior abdominal wall changes depending on whether the approach is above or below the arcuate line.
- Above the arcuate line, the layers are skin, subcutaneous fat, anterior rectus sheath (aponeurosis of the external and internal oblique muscles), rectus muscle, posterior rectus sheath (aponeurosis of the internal oblique and transversus abdominis muscles), transversalis fascia, and peritoneum.
- Below the arcuate line, the posterior rectus sheath is not present, and thus the rectus muscle lies directly on the transversalis fascia.
The retroperitoneal approach goes through the abdominal wall to the layer of the transversalis fascia and then progresses laterally until this fascia ends, exposing the retroperitoneal fat.

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