There are a variety of halo vest devices available on the market, and the spine surgeon should become familiar with several of these. When used as cervical traction, the halo ring is placed with the anticipation that it will be eventually secured to a jacket/vest to allow early mobilization. The patient may be positioned either supine or, if not contraindicated, upright. Halo rings are available in various sizes, and a halo to skull clearance of 1.0 to 1.5 cm should be obtained. The hair is clipped at the planned pin sites and the skin is prepared with Betadine and infiltrated with local anesthetic. Four proposed pin sites are chosen. The ventral pin sites are located approximately 1 cm above the lateral third of the eyebrow, avoiding injury to the supraorbital nerve. The dorsal pins are placed in a channel diagonally opposite the ventral pins and situated approximately 1.5 cm above the ears. Before the halo ring is placed, it is held in place by an assistant. Current systems have suction-tipped pins to hold the halo in position until the skull pins are placed, thus permitting one person to place the halo safely. Hexagonal lock nuts are loosely placed on each pin outside the ring prior to advancing the pin. The pins should be placed so that they are perpendicular to the skull. The skull pins are then gently advanced until the outer layer of dermis is penetrated. The ventral and diagonally opposed dorsal pins are simultaneously tightened by hand, then are tightened to a maximum of 6 kg using the torque screwdriver. Alternate tightening of opposed skulls pins prevents asymmetric displacement of the halo ring. The pins should penetrate only the outer table of the skull. Once the pins are properly placed, the hexagonal nut is tightened against the halo ring, thus securing the position of the pins and preventing their backing out. Twenty-four hours later, the hexagonal nuts should be loosened and the skull pins tightened to 6 kg of pressure with the torque screwdriver. The hexagonal nuts are then retightened, and no further tightening of the pins is necessary as this risks inadvertent penetration of the inner table of the skull. A cervical spine radiograph should be obtained to check spinal alignment. The halo ring may be used to provide cervical traction if needed and may be converted at an appropriate time to a halo vest device.

The patient is carefully intubated and general anesthesia administered with the patient supine on a stretcher adjacent to the operating table. Meticulous care is required during intubation to prevent flexion or extension of the head and neck. Fiberoptic intubation may be used— allowing for better visualization during intubation. In patients with atlantoaxial instability, awake fiberoptic intubation is recommended to allow continuous monitoring of patients’ neurologic status. Electrophysiologic monitoring with somatosensory evoked potentials and motor evoked potentials should be employed. Baseline potential recordings may be obtained prior to intubation, or alternatively, the patient may be intubated fiberoptically and a wake-up test performed immediately after intubation and once again after positioning.

Once adequate general anesthesia is obtained, a threepoint Mayfield head fixation device is applied to provide rigid head and neck stabilization intraoperatively. The patient is then carefully log rolled onto the operative table in the prone position. The headholder is then affixed to the operating table with the neck in neutral position and slight flexion of the head. Confirmation of proper vertebral alignment using fluoroscopy or static lateral cervical spine x-ray is essential. The skin is then prepped and draped from the occiput to the mid- or lower cervical spine in the standard fashion. Prior to skin incision, the skin may be infiltrated with 1% lidocaine with 1:100,000 epinephrine to facilitate hemostasis. A midline incision is then made from the subocciput to the fourth or fifth cervical vertebrae. Using monopolar cautery and sharp dissection, the midline incision is carried deeper, staying within the avascular intermuscular septum or ligamentum nuchae. The spinous process and lamina of C2 are exposed in a subperiosteal fashion, taking care not to expose the articular processes below C2. The paraspinal muscles are retracted laterally. The dorsal arch of the atlas is then exposed by subperiosteal dissection. Blunt subperiosteal dissection with a Cobb or periosteal elevator should be used along the lateral portions of the C1 arch to avoid injury to the vertebral artery and venous plexus. Similarly, bipolar cautery should be used, and monopolar cautery avoided, when exposing the lateral portions of the C1 arch. The posterior caudal aspect of C1 arch and the dorsal rostral surface of the axis are curetted to remove any soft tissues. The occiput should not be extensively exposed or decorticated, thereby avoiding inadvertent fusion to the skull base (Fig. 138.1).

The C1 dorsal arch and C2 lamina are exposed circumferentially with curettes to facilitate wire passage. A 20-or 24-gauge stainless steel wire or braided titanium cable (either single wire or a “looped” double wire) is passed under the C1 ring in the caudal-rostral direction. This is accomplished using a right-angled clamp or, alternatively, by tying the leading loop of wire or cable to a suture and then passing the blunt end of the suture needle in a sublaminar fashion. Safe passage of the loop may be performed in the midline directly under the spinous process, as there is substantial space in this region. An adequate removal of ligamentum flavum should be performed prior to passage of the sublaminar cable. Similarly, the bone surfaces should be decorticated prior to cable passage in order to allow for bone fusion without risking damage to the cables with the air drill. Depending on the technique used, the loop may then be cut into two single strands, which are then placed laterally. Careful dissection of the atlantoaxial membrane facilitates moving the cables to each side, and great caution must be exercised during this maneuver to avoid injury to the dura and spinal cord.

Autologous bone graft is harvested from the dorsal iliac crest superficial to the sacroiliac joint. The posterior superior iliac spine (PSIS) is palpated and prepared as a separate surgical region for any posterior cervical procedure. A diagonal or vertical incision approximately 2 to 3 cm in length is made centered over the PSIS well medial to the
superior cluneal nerves. Horizontal incision risks injury to the superior cluneal nerves. Cortical bone overlying the iliac wing is exposed subperiosteally using monopolar cautery and an elevator. The gluteal musculature is then carefully dissected in a subperiosteal fashion from the dorsolateral surface of the iliac bone. Great care should be taken to preserve the gluteal musculature, as minimizing gluteal dissection decreases postoperative graft site pain. An osteotome or oscillating saw is used to harvest a tricortical graft, and the rostral cortical margin is then removed to create a bicortical graft. The two cortices will interface the inferior rim of the C1 dorsal arch and the rostral margin of C2 lamina so that the bone graft and C1-C2 cancellous regions are closely approximated. The surgeon needs to ensure that the height of the graft is adequate for optimal bone graft-spine contact. Thus, many surgeons choose to oversize the initial bone harvest and then tailor the graft to optimal size. Bone graft length should also be carefully considered. For the Sonntag cable technique, the graft should be at least 4 to 5 cm in length so that it may be incorporated into the construct. For interlaminar clamp technique, the graft must be shorter because the clamps attach lateral to the graft. Regardless of the dorsal fixation technique used, additional bone graft may be morselized and placed between C1 and C2 lamina into the lateral aspects of the construct.

Described in 1939, the Gallie technique involves the placement of a wire under C1 and attached to the spinous process of C2 below with an intervening H-shaped bone graft (8). The graft is typically a corticocancellous iliac crest bone graft approximately 1.5 cm by 3 cm and notched caudally in the midline to contour the C2 spinous process during compression. Two variations of the technique exist, both of which eliminate the need for placement of sublaminar wires at C2. A 20-gauge wire is passed under the C1 dorsal arch, the two free wire ends are placed through the leading wire loop, then placed over the bone graft, and either through or under the C2 spinous process. Tightening the construct secures the onlay bone graft to the dorsal arch of C1 and the rostral laminar surface of C2. Alternatively, a doubleloop technique may be used, consisting of a sublaminar wire placed caudal to rostral around the arch of C1 with the cranial loop brought caudally to over the bone graft and under the C2 spinous process. The remaining free ends of wire are then placed transversely over the bone graft and tightened to secure the construct. The primary drawback of this technique is that it represents a single, midline point of fixation. Biomechanically, the Gallie technique is capable of preventing ventral translation and flexion but does not optimally restrict extension and rotational forces. There is also concern when more than one spinal segment is spanned using sublaminar wires, as there is increased risk of encroachment on neural elements within the spinal canal during the passage of wires, as well as the risk of chronic encroachment of the wires over time. This is particularly significant in cases in which ventral translation of the atlas results in spinal canal stenosis. Therefore, although easy to perform, this method is not very stable and requires a rigid orthosis, preferably a halo, for postoperative immobilization. Biomechanically, the Gallie technique is capable of preventing ventral translation and flexion but does not optimally restrict extension and rotational forces (Fig. 138.1).

In 1978, Brooks and Jenkins (4) refined the sublaminar wiring technique by using dual sublaminar wires at C1 and C2 with intervening bilateral interlaminar bone grafts. By incorporating bilateral, interlaminar bone grafts, this strategy overcame the rotational deficiencies seen with the Gallie fusion. For this technique, the dual wire is passed caudorostrally under the C2 lamina and C1 dorsal arch. Two separate rectangular corticocancellous grafts are harvested and placed between the caudal margin of the C1 dorsal arch and rostral margin of the C2 lamina. The bilateral dual wires are then tightened to compress and secure the bone grafts. Biomechanical studies have shown that this technique is superior to the Gallie technique in resisting extension and axial rotational forces. However, the Brooks technique carries the additional risk of wire passage at two levels instead of a single level (Fig. 138.1).

In the Dickman et al. modification of the Gallie technique, the use of sublaminar wires under C2 is eliminated and, instead, the spinous process of C2 serves as the fixation point. Intraoperatively, the C1-C2 interlaminar space is widened. The caudal surface of the C2 spinous process is notched, and a loop of wire is then passed sublaminarly in the caudorostral direction under the C1 lamina. A single corticocancellous autograft or allograft strut is used, similar to the Gallie procedure. The wire is then passed over the graft, which can be notched to accept and secure the wire, and is then secured to the undersurface of the C2 spinous process. This secures the bone graft in the interlaminar space. The free ends of the wire are then brought over the C2 laminar surface and secured either by twisting or with a crimp device after adequate tightness has been achieved. The use of a braided multistrand wire facilitates safe wire passage because of its decreased rigidity, resulting in a lower rate of morbidity and allowing for greater tightening and improved strength and fixation (Fig. 138.1).