Atlantoaxial instability resulting from

Nonunions

Neurologic and musculoskeletal examination.

Preoperative imaging should include plain radiographs (Figure 11-3, A), computed tomography (CT) (Figure 11-3, B), CT angiography, and magnetic resonance imaging (MRI) (Figure 11-3, C) of the cervical spine.

Noninvasive magnetic resonance angiography (MRA) can be utilized to evaluate vertebral artery injury, patency, and/or dominance (in lieu of CT angiography that requires administration of dye contrast).

The posterior arch of C1 and the C1-2 facet joint are key anatomic landmarks for the placement of C1 lateral mass screws. The dorsal root ganglion of C2 lies just posterior to the starting point of the C1 screw and must be gently retracted caudally for adequate exposure (Figure 11-4, A). The starting point for the C1 screw is at the midpoint of the inferior portion of the C1 lateral mass at its junction with the posterior arch. The more superior and medial trajectory of the screws, when compared with transarticular screws, decreases the risk of vertebral artery injury (Figure 11-4, B and C)

The ponticulus posticus or congenital arcuate foramen is a common bony anomaly of the atlas (Young et al, 2005) (Figure 11-4, D). It is a bony arch on the cephalad aspect of the C1 lamina that contains the vertebral artery. If present, it can easily be confused with the lamina of C1 and must be identified during the posterior dissection and placement of C1 lateral mass screws to prevent vertebral artery injury.

After an awake fiberoptic nasotracheal intubation is performed, a nasogastric tube is inserted for intraoperative gastric drainage.

If the patient is immobilized in a halo vest preoperatively, either the halo can be left in place and attached directly to the Mayfield headholder using an adapter or it can be removed. If the halo ring is removed, the patient is placed in Mayfield tongs and a hard cervical collar before being turned into the prone position. In coordination with anesthesia, the surgeon stands at the head of the hospital bed and stabilizes the patient’s neck. The patient is cautiously turned in the prone position on the operating table with the torso on bolsters or a four-poster frame. The Mayfield tongs or the halo ring is fixed to the operating table using a Mayfield headholder with the neck in a neutral position (Figure 11-5, A and B).

All bony prominences are well padded, and the patient’s arms are secured by their side using a folded sheet that is tucked beneath them.

Using fluoroscopic C-arm, proper alignment of the atlantoaxial bony structures is confirmed with the radiograph centered at C1-2. The lateral fluoroscopic image must not be oblique at C1-2; otherwise, malpositioning of the drill can result in erroneous screw placement (Figure 11-6).

If necessary, adjustments can be made while the patient is in the Mayfield headholder, to obtain reduction. Reduction should be confirmed on fluoroscopic radiograph. If possible, extreme positions of the neck should be avoided.

Somatosensory evoked potential (SSEP) and transcranial motor evoked potential (MEP) monitoring are neurophysiologic spinal cord monitoring methods that can be utilized intraoperatively. Baseline readings can be obtained before and after placing the patient in the prone position.

An electric razor is used to remove all hair from the patient’s occipital, suboccipital, and neck regions. If a definitive fusion is being performed, the posterior iliac crest is also shaved for bone graft harvesting.

The skin surfaces of the neck and posterior iliac crest are prepared and draped in a sterile fashion.

Using the inion of the occiput cranially, and the protuberance of the vertebral prominens caudally, the midline is identified and marked from the occiput to C3-4 with a sterile marker.

The subcutaneous skin of the planned skin incision can be infiltrated with 0.5% lidocaine containing epinephrine diluted 1:100,000.

A 10-blade scalpel is used to sharply incise the skin in the midline from the occiput to C3-4.

Bovie electrocautery is used for the subcutaneous dissection down to and through the underlying ligamentum nuchae. Midline dissection of the nuchal ligament provides a relatively avascular dissection and decreases the risk of injury to the greater and third occipital nerves. Self-retaining retractors are inserted for adequate visualization.

At the cephalad end of the incision, a 1.5-cm fascial cuff of trapezius, along the nuchal ridge, can be elevated to facilitate lateral exposure of C1-2, but this is not usually necessary. Subperiosteal dissection of the paraspinous muscular insertions from the suboccipital bone is completed.

The midline tubercle of the arch of C1 and the larger spinous process of C2 are used as palpable landmarks during dissection. Starting at the midline, the periosteum of C1 and the tip of the spinous processes of C2 and C3 are incised sharply.

Careful subperiosteal dissection is continued from C3 to C1, starting in the midline and proceeding laterally. Periosteal elevators can facilitate the subperiosteal dissection of the paraspinous muscles as they are swept laterally. The lateral masses and pedicles of C3 and C2 are exposed with care not to disturb the C2-3 facet capsules.

The C1-2 joint can be exposed with dissection over the superior surface of the C2 pars. Significant venous bleeding can be encountered with dissection around the venous plexus of the C2 nerve. This can effectively be controlled with bipolar electrocautery, thrombin-soaked Gelfoam, cotton pledgets, and various commercial gelatin thrombin preparations.

To decrease the risk of injuring the vertebral artery on the cephalic surface of the C1 lamina, identify the lamina and follow the caudal edge of the posterior arch during exposure of C1. If present, the ponticulus posticus or congenital arcuate foramen must be identified during the posterior dissection, because it can easily be confused with the lamina of C1 (Young et al, 2005).

The dissection is complete with exposure of the suboccipital rim of the foramen magnum.