Posterior Lumbar Fusion and Pedicle Screw Instrumentation

Illustrative Case

A 65-year-old woman with spinal stenosis and neurogenic claudication, and anterolisthesis at L3-4 and L4-5 (Figure 20-1).

Figure 20-1 ▪ Anteroposterior (A) and lateral (B) x-rays demonstrating spondylolisthesis at L3-4 and L4-5.

Figure 20-1 ▪ (continued) MRI scans (C-F) demonstrate right sided foraminal stenosis at L4-5 from a foraminal disc herniation (D), as well as moderate lateral recess stenosis at L4-5 (F).

Radiologic Assessment

On radiographs
- Scrutinize the preoperative x-rays for bone quality, presence of instability, or spinal deformity. Look at pedicle morphology, size, and orientation, and whether the pedicles appear sclerotic.
- Look for any segmentation anomalies such as a lumbarized sacral vertebra, and ensure that the naming of the spinal levels is consistent between the preoperative x-rays and the magnetic resonance imaging (MRI) or computed tomography (CT) images. Mark the levels on the preoperative x-rays.
On axial and sagittal MRI images
- Look for location and extent of stenosis centrally as well as in the lateral recess and foramina. Look for any area of potential scarring that might create a challenge with decompression and risk a dural tear.
On CT, if obtained
- Look at bony landmarks for the pedicle start site, measure pedicle screw length and width.

Positioning

The patient is placed prone on a radiolucent Jackson frame.
The head and face are placed in a well-padded face holder, with eyes free of pressure. Place tape over the eyelids and make a mark along the eyelid with a black marker over the tape to ensure easy visualization of the eyes using the mirror when positioned prone.
Ensure that the neck is in neutral position and is not hyperextended. If the neck is hyperextended, consider adding an extra layer to the chest pad to elevate the chest and reduce neck hyperextension.
Position the arms at less than 90° of abduction to avoid rotator cuff impingement. Allow the arms to hang down slightly, in 10° of forward flexion. Ensure the axilla is clear of any padding to avoid a brachial plexus palsy.
Pad the medial elbows to protect the ulnar nerve.
The chest pad is placed just proximal to the xiphoid process and distal to the axilla. In women, ensure the breasts are tucked and nipples are pressure-free. For women with breast implants, use foam donuts to further pad the breasts.
The iliac pads are placed two fingerbreadths distal to the anterior superior iliac spine. This will allow enough room for the abdomen to hang free and reduce epidural bleeding.
If increased lumbar lordosis is desired, the chest pad may be moved slightly cranial, and iliac pads slightly caudal, to take advantage of the force of gravity in restoring lumbar lordosis. Additionally, the hips can be extended further by placing sheets under the thighs.
Place foam donuts under the knees to avoid undue pressure on the skin overlying the patellae.
Using blankets or pillows, allow about 30° of knee flexion to reduce stretch on the sciatic nerve (Figures 20-2A, B and 20-3).

Figure 20-2 ▪ A, B, Jackson table used for posterolateral lumbar fusion. Note position of chest and hip pads, donut cutouts for the knees, and blankets to allow for knee flexion.

Figure 20-3 ▪ Patient positioned prone for posterolateral lumbar fusion. If greater lordosis is desired, the chest pads are placed more cranially and the hip pads more distally.

Anesthesia and Neuromonitoring Concerns

We do not routinely use neuromonitoring for posterior spinal decompression and fusion below the level of the spinal cord, with several exceptions:
- If the patient has significant documented cervical spondylotic myelopathy, somatosensory evoked potential (SSEP) neuromonitoring may be used.
- If any deformity correction is undertaken, then SSEP and TcMEP neuromonitoring is used under total intravenous anesthesia.

Localization of Incision

We use long spinal needles to localize the incision.¹
With the patient sterilely prepped (but not draped), place two sterile spinal needles at the expected cranial and caudal ends of the incision three fingerbreadths lateral to the midline. Ensure that these needles are exactly perpendicular to the floor (and not the patient’s skin).
Mark the location of the needles on the skin using a sterile skin marker.
Obtain a lateral plain radiograph with the needles in place.
Adjust the cranial and caudal ends of the incision based on the location of the spinal needles.
We prefer to print this x-ray, mark the levels with a marker, and place on the viewbox in the operating room for later reference.
Intraoperatively, once two consecutive facet joints are exposed, we place two sterile spinal needles superficially into the facet joints and obtain a second lateral plain x-ray to confirm intraoperative localization. The capsules of the facet joints are marked with a sterile marker prior to removing the needles so as to serve as a constant landmark with respect to levels. This second x-ray is also printed, marked, and placed on the viewbox in the operating room for repeated reference ( Figures 20-4, 20-5, 20-6).

Figure 20-4 ▪ Localization of skin incision by taking a lateral x-ray with two needles positioned at the anticipated cranial and caudal extents of the incision.

Figure 20-5 ▪ Localizing x-ray. Based on the position of the needles, the location of the skin incision is adjusted as needed for the extent of exposure.

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