Posterior Atlantoaxial Screw Fixation
Traditionally, atlantoaxial instability has been treated surgically using posterior C1-C2 wiring and bone grafts. However, bone grafts and wires do not provide rigid internal fixation of C1-C2, and they have high nonunion rates unless supplemented with a halo brace.1–4 There are several techniques to fixate C1 and C2 with screws. Magerl5 initially developed posterior atlantoaxial facet screw fixation as a way to achieve rigid internal fixation of C1-C2 by eliminating motion, promoting arthrodesis, and treating instability. The technique is useful but has risks as well as advantages. Alternatively, the technique of C1-C2 fixation utilizing the lateral mass of C1 and pars interarticularis, lamina or pedicle of C2 with screw-rod fixation is an excellent method of achieving solid fixation, as described by Harms6 and modified by Goel.7,8 This chapter describes the operative techniques for posterior atlantoaxial facet screw fixation, C1 lateral mass screw fixation, C2 pars and C2 lamina screw fixation, and strategies to minimize or avoid complications.
Indications
Atlantoaxial instability
Contraindications
Aberrant course of vertebral artery (as seen on computed tomography [CT] or magnetic resonance angiography)
Destruction of C2 pars interarticularis and C1 lateral mass
Severe osteoporosis
Preoperative Evaluation
Flexion and extension radiographs (evaluate reducibility and instability)
CT with sagittal reconstructions through the screw path ( Fig. 40.1 )
Patient Positioning and Preparation
Prone
Flexed position of the neck ( Fig. 40.2 )
Fluoroscopic monitoring during neck positioning (lateral C-arm)
Sagittal reformatted computed tomography images through the C2 pars interarticularis and the C1 lateral mass. (A) Normal bone structures permit insertion of a transarticular screw. (B) In this patient, an aberrant foramen transversarium precluded placing a screw on one side. 
Neck flexion helps to obtain the appropriate trajectory for drilling and screw insertion. (A) If the neck cannot be flexed, percutaneous drilling is performed. (B) When the neck can be flexed, drilling often can be performed directly through the incision used to expose C1 and C2. The patient is positioned while the cervical alignment is monitored with real-time fluoroscopy to avoid subluxation and neurological injury. (From Marcotte P, Dickman C, Sonntag VK, Karahalios DG, Drabier J. Posterior atlanto-axial facet screw fixation. J Neurosurg 1993;79:234–237. Reprinted with permission.)
Head fixation with Mayfield skull clamp and halo ring
Intraoperative somatosensory evoked potential monitoring
Operative Technique of Transarticular Screw Fixation
Neck flexion helps obtain the proper trajectory for the drills and screws, which should be almost parallel to the posterior surface of the spine ( Fig. 40.2 ). Lateral fluoroscopic monitoring is used to assess spinal alignment during positioning, drilling, and screw insertion.
A posterior cervical incision is made to access the atlas and axis. The incision extends from the inion to the spinous process of C7 or to the spinous process of C3, if percutaneous drilling technique is planned. The skin over the upper thoracic level is prepared if percutaneous access is needed for drilling ( Fig. 40.2A ).
C1 and C2 are exposed using a subperiosteal dissection. The C2 pars interarticularis and the C1-C2 articular surfaces are exposed and visualized directly. The ligamentum flavum is removed adjacent to the C2 laminae and pars interarticularis. A thin Kirschner wire (K-wire) or a no. 4 Penfield dissector is placed directly along the upper surface of the C2 pars interarticularis into the atlantoaxial facet joint and retracted upward to displace the C2 nerve root and venous plexus superiorly. Removal of the ligament and retraction of the C2 nerve root enable direct visualization of the C2 pars interarticularis and C1-C2 facet joint during drilling ( Fig. 40.3 ). Bleeding around the C2 root from the venous plexus is controlled by gentle packing with Surgicel (Ethicon, Somerville, NJ) and bipolar cauterization. Various commercially available products utilizing preparations of powdered Gelfoam (DuPuy, Raynham, MA) and thrombin (e.g., FloSeal [Baxter, Deerfield, IL], Surgifoam [Ferrosan, Soeborg, Denmark]) are extremely effective for this as well.
If needed, the atlas and axis are realigned by manual reduction. Anterior atlantoaxial subluxations are reduced by gently displacing C2 anteriorly and by pulling C1 posteriorly. Opposite forces are applied for posterior C1-C2 subluxations. A wire or braided cable is placed around the ring of C1 for traction and for later fixation of an interspinous bone graft. Direct traction of C2 is applied with an Allis clamp attached to the C2 spinous process. The C2 spinous process is retracted gently toward the base of the occiput to obtain an ideal trajectory for drilling.
The drill enters the caudal aspect of the C2 inferior facet, 2 to 3 mm lateral to the medial edge of the C2-C3 facet ( Fig. 40.3 ). Occasionally, the entry point is adjusted 1 to 2 mm to compensate for the altered anatomical relationships of the vertebrae. First, the posterior cortical bone of C2 is penetrated with a bone awl or high-speed drill. This step precisely directs the drill insertion for the pilot hole to prevent the bit from migrating as drilling begins.
Lateral fluoroscopic monitoring is used to adjust the drill trajectory toward the dorsal cortex of the anterior arch of C1 ( Fig. 40.4 ). In the anteroposterior direction, the drill is placed through the central axis of the C2 pars interarticularis. A sagittal trajectory between 0 and 10 degrees medially is required ( Fig. 40.5 ). Typically, the screw trajectory is straight in a sagittal orientation. The sagittal trajectory is oriented along the central axis of the C2 pars interarticularis, which is visualized directly by retracting the C2 root during the drilling.
Cannulated or noncannulated screws can be used for C1-C2 fixation. The tract for the screw is created with a drill while the path is monitored with lateral plane fluoroscopy and direct visualization. A K-wire is used for drilling when cannulated screws are used. For noncannulated screws, a wider cylindrical drill bit (2.5 mm diameter) is used to drill the pilot hole.
After the length of the screw has been measured, the screw is inserted into the bone with a screwdriver. On lateral fluoroscopic imaging, the tip of the screw should be positioned behind the dorsal cortical rim of the anterior arch of C1 ( Fig. 40.6 ). Screws ventral to this cortical rim can penetrate through C1 anteriorly. Screws should not extend cephalad or they could cross the occipitoatlantal joint.
As the screws cross the joint space into Cl, the atlas and axis become rigidly coupled. The surgeon can feel the vertebrae lock together with characteristic stiffness as the screws are inserted. The screw heads are positioned flush against the bone surfaces or recessed slightly into the bone to prevent the screws from levering against the C2-C3 joint space. The screws should not be overtightened because the screw will shear through the cortex of the C2 pars interarticularis and facet, destroying its purchase in the bone.
After the screws have been inserted bilaterally, a bicortical interspinous strut graft is placed using autologous iliac crest bone.l The graft is fitted precisely and wired into position to compress it between C1 and C2 ( Fig. 40.7 ). The graft provides the substrate for fusion. The wire provides three-point fixation (two screws plus the wire) and compresses the graft between C1 and C2 to promote fusion.
Percutaneous Drilling and Screw Insertion
After C1 and C2 have been exposed subperiosteally through a neck incision, the surgeon must decide whether the proper screw trajectory can be achieved directly through the incision or whether a percutaneous method is needed. If the patient′s neck can be flexed adequately, pilot holes are drilled directly through the neck incision used to expose C1 and C2 ( Fig. 40.2B ). Often, however, the patient′s neck cannot be flexed sufficiently without dislocating C1-C2; therefore, percutaneous drilling is used to achieve the proper drill trajectory ( Fig. 40.2A ). Long tools, long drill bits, and a tissue sheath are required for percutaneous screw insertion ( Fig. 40.6 ). Percutaneous drilling should not be used exclusively without the subperiosteal exposure of C1 and C2 to visualize the proper medial-lateral screw trajectory and to prepare the entry point for the drill on the C2 inferior facet. Also, the bone surfaces must be exposed to prepare them for fusion. The percutaneous drilling is performed only after C1 and C2 have been exposed directly through the neck incision. Many surgeons prefer the percutaneous technique for routine use, arguing that it results in less injury to the musculoligamentous structures between C3 and C7, leaves less dead space, and promotes more rapid recovery due to the incision being less than half as long and avoiding subperiosteal muscle dissection of the mid to lower cervical spine.
The position of the percutaneous tunnel is judged by holding a long instrument adjacent to the patient′s neck and thorax and imaging the trajectory with fluoroscopy. Stab incisions are made in the skin lateral to the midline in the upper thoracic region. A tunneler inserted into a tissue sheath is inserted through the stab incisions to the soft tissues of the neck. The tip of the tissue sheath is positioned adjacent to the C2 inferior facet. A handle on the tissue sheath is used to manipulate the sheath to alter the trajectory of the drills and screws.
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