C1-2 Instrumentation and Fusion

Christopher T. Martin

John M. Rhee

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

Unrecognized vertebral artery anomalies are a potentially catastrophic source of complications.
A computed tomography (CT) angiogram is recommended in many cases involving C1-2 instrumentation, and certainly in those demonstrating congenital or erosive bony abnormalities, which may be associated with anomalies of the vertebral artery as well.

CT scan provides assessment of feasibility of C1 lateral mass and C2 fixation, and is mandatory to obtain preoperatively.

A standard C1 lateral mass screw is placed through the midpoint of the lateral mass.

A parasagittal cut through this plane should demonstrate a bony thickness of at least 3.5 mm for the placement of a screw (Figure 7-1).

The axial cut through the C1 lateral mass demonstrates the relative position of the vertebral foramen and therefore the medial-lateral angle for screw insertion (Figure 7-2).

Figure 7-1 ▪ Left parasagittal, midline, and right parasagittal computed tomography myelogram images of a 74-year-old F patient with severe C1-2 arthrosis. The C1-2 facet joint is collapsed, with near complete loss of the C1 lateral mass height. The C2 pars is sclerotic. In this patient, the severe bony destruction will make instrumentation placement much more challenging than usual.

Figure 7-2 ▪ Axial CT cuts of the patient in Figure 7-1 showing the position of the vertebral foramen relative to the lateral mass at C1. The red arrows mark the trajectory for the C1 lateral mass screws.

A C2 pars screw runs in the isthmus of bone connecting the inferior and superior articular processes of C2. The trajectory of this screw is relatively vertically oriented. To determine feasibility, there must be a parasagittal cut demonstrating adequate bony thickness without encroachment of the vertebral artery within the foramen transversarium. Typically, this cut is the one obtained just lateral to the lateral portion of the spinal canal (Figure 7-3).

Figure 7-3 ▪ Parasagittal computed tomography cuts of the patient in Figure 7-2 showing the maximal width of the pars and the planned trajectory for the C2 pars screws. In this case, there are some small bony erosions present, but these should not interfere with the screw, and the path is small but accessible. The red arrows demonstrate the ideal trajectory for screw placement.

A C2 pedicle screw, by contrast, runs more horizontally and connects the posterior elements to the vertebral body of C2 (like any pedicle screw in the thoracolumbar spine). The start site is at the upper border of the superior laminar arch, just lateral to the lateral aspect of the canal, and then runs anteriorly into the vertebral body, medial to the vertebral artery (Figure 7-4).
- Axial CT scans can help identify feasibility of C2 pedicle screws.¹
  
  Figure 7-4 ▪ Computed tomography scans showing the difference in trajectory and alignment between a C2 pars (left column) and a C2 pedicle screw (right column).
High-riding vertebral arteries can narrow the C2 pars and pedicle, limiting the bony area available for screw fixation. In such cases, alternative forms of C2 fixation, such as translaminar screws, may be necessary.

The appearance and position of the vertebral artery should be carefully reviewed for anomalies. A particularly dangerous variant if unrecognized is the persistent first intersegmental artery (Figure 7-5), in which a branch of the vertebral artery runs through the C1-2 neural foramen along with the C2 nerve. Depending on whether the artery is fenestrated and also runs in its usual position on the cephalad aspect of the C1 laminar arch, the presence of this variant may preclude placement of C1 lateral mass screws. A C1 screw may be placed through the arch if there is no artery running in the usual position along the superior laminar arch.

Figure 7-5 ▪ Persistent first intersegmental artery. Note that the right vertebral artery runs dorsal to the lateral mass of C1, rather than along the superior edge of the laminar arch (normal left vertebral artery). In a fenestrated artery, there is a branch that runs both above and below the laminar arch. These arterial anomalies are often associated with bony anomalies, such as the incomplete laminar arch with a spina bifida depicted in this diagram.

Degenerative and rheumatologic conditions can result in significant destruction of the C1 lateral mass, resulting in collapse of the C1-2 joint, which can limit the area for placement of C1 lateral mass screws (Figure 7-1). If the inferior aspect of the laminar arch is sufficiently thick, the screw may be placed into the arch or the undersurface “notched” and placed partly into the lateral mass and partly in the arch. Careful scrutiny of the parasagittal CT through the lateral mass is needed to determine the optimal start site for C1 screw placement.

Positioning

The patient is positioned prone in Mayfield tongs tongs, similar to the description in the Laminoplasty chapter.
The head should be in neutral rotation and neutral flexion. We use fluoro to determine alignment before locking in the Mayfield. “Normal” C1-2 Cobb angle is approximately 30°, but the actual angle in a given patient may vary. We generally try to mimic the neutral lateral x-ray, unless there is a segmental deformity through C1-2 requiring correction.
The arms are tucked at the sides, the knees are bent, and the table is in reverse Trendelenburg so that the neck is positioned roughly parallel to the floor (Figure 7-6).
The prep should include the bilateral iliac crests to allow for structural autograft harvest.

Figure 7-6 ▪ Preoperative positioning.

Anesthesia/Neuromonitoring Concerns

For myelopathic cases, the mean arterial pressure should be maintained above 80 to 85 mm Hg.
In general, we use monitoring in myelopathic cases as detailed in the chapter on neuromonitoring.

Localization of Incision