Microdiskectomy

Radiologic Assessment—Key Factors to Consider on X-ray, MRI, CT

The location of the disk herniation is key to planning the operative approach.
- On axial MRI, determine whether the herniation is central, paracentral, foraminal, or extraforaminal (“far lateral”).
- On sagittal MRI, determine whether the herniated fragment is at the level of the disk space or has migrated cephalad or caudal.
- Central and paracentral disk herniations that are contained within the annulus can usually be accessed through a hemilaminotomy. Extruded disk fragments should be noted and may require extension of the decompression above or below (Figure 18-1).
- Sometimes, there is a disk herniation in the setting of significant spinal stenosis. In such cases, a laminectomy with diskectomy may be the most prudent choice for thorough canal decompression and safe, complete removal of the herniation.
  
  Figure 18-1 ▪ Midsagittal and axial T2 MRI cuts of an L3-4 posterolateral, paracentral disk herniation amenable to treatment with a midline approach.
Intraforaminal herniations may be retrieved through a hemilaminotomy with partial medial facetectomy approach, a paraspinal Wiltse approach (Figure 18-2), or may occasionally require facetectomy and fusion if not fully accessible without removing the facet.
Far lateral disk herniations will require a paraspinal, Wiltse approach.

Figure 18-2 ▪ Right parasagittal and axial T2 MRI cuts from a patient with a right-sided disk herniation in the intraforaminal zone, which was treated with the Wiltse approach. The arrows point to the location of the intraforaminal herniation.

Positioning

Prone over a Wilson frame (Figure 18-4).

Figure 18-4 ▪ Wilson frame positioning. Note that after the skin prep was applied two spinal needles were placed in preparation for the localizing radiograph.

Anesthesia/Neuromonitoring Concerns

We prefer to perform this under general anesthesia.

Localization of Incision

The surface anatomy is palpated to help the surgeon in identifying the appropriate level. The curvature of the sacrum at its junction with the L5 spinous process is often palpable, and subsequent spinous processes can be counted up from there. In addition, the L4-5 disk space is roughly at the level of the iliac crests, and this can serve as an additional landmark.
After prepping the skin, a 20-gauge spinal needle is advanced through the skin roughly 2 cm off of the midline, toward the facet joint. It is imperative that the needle not be inserted at the midline, as this would risk penetrating the canal. Alternatively, an anteroposterior C-arm image with two metallic markers on the skin can be used to guide initial skin incision.
After needle placement, a single lateral lumbar radiograph is obtained with portable digital radiography. The position of the needle is noted and then marked on the skin (Figure 18-5).
The subsequent incision is adjusted up or down as necessary relative to this mark.

Figure 18-5 ▪ Lateral lumbar radiograph showing preoperative needle localization for a planned L5-S1 diskectomy. The subsequent incision is adjusted up or down as necessary relative to the needles. In this case, the incision needs to be centered slightly distal to the upper needle.

Approach

Midline Approach

A skin incision is made directly centered over the spinous processes for a distance of roughly 15 to 20 mm.
The dissection is continued down to the spinous process and then the fascia and muscle are subperiosteally elevated off the spinous process and lamina on the side of the pathology. The uninvolved side is not dissected.
The dissection is carried to the medial border of the facet joint, with the muscle bluntly elevated off the facet using a Cobb. Very little muscle needs to be detached from the facet (Figure 18-6).
As soon as the lamina-facet junction is well visualized, a spinal needle should be placed at the lamina-facet junction and definitive localization on C-arm or digital radiograph should be performed (Figure 18-7). The needle position should be well marked with ink for future reference.

Figure 18-6 ▪ After unilateral exposure on the right side to the medial facet joint. The facet capsule, as well as the lamina above and below, is identified.

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