As shown in Figure 15.1, the patient is positioned prone in a Mayfield skull clamp for control of the head in multiple planes. The cervical spine must be in neutral position without rotational, coronal, or sagittal imbalance. This is accomplished with a “military tuck” that involves translating the head dorsally while flexing the chin slightly. Positioning the patient this way is essential because adjustment will not be possible after instrumentation, as can be done in screw-rod constructs.

Arms are secured to the side, and all bony prominences are padded. The shoulders may be taped to the foot of the bed to optimize fluoroscopic visualization. In addition, a Jackson spine frame should be considered to facilitate fluoroscopy in the anteroposterior (AP) plane, as well as radiolucent head-holder in certain situations where adequate AP fluoroscopy may not be feasible otherwise.

Proper screw placement is achieved by drilling through the superior facet into the body of the inferior facet of selected joints. To this end, a midline skin incision is planned cranial to the targeted levels such that a downward trajectory may be achieved. AP fluoroscopy is useful to help identify the midline, particularly in obese individuals. Local anesthetic is injected transdermally before the incision is made. After incision, a self-containing retractor is placed in the wound.

Using AP fluoroscopy, a small fascial incision is made over the superior surface of the desired lateral mass with the aid of monopolar cautery. The lateral mass selected is above the joint space chosen for fixation. Safe screw insertion necessitates a starting point in the middle of the superior facet. Through the fascial incision a cannulated drill guide is docked on the superior surface of the lateral mass. Under biplanar fluoroscopy (Figs. 15.2 and 15.3), an air-powered drill is used to create a 14 to 16 mm pilot hole with the aid of a small gauge short Kirschner wire (K-wire). Drilling is directed caudally and laterally.

Air-powered systems minimize the downward force needed to breach the cortical bone surfaces, thus reducing the risk of migration of the drill bit or rotation of the vertebral segments. Various cannulated small fragment screw systems can be used (Fig. 15.4). These are generally used for extremity trauma fixation and are thus quite prevalent and accessible.

Aiming laterally and caudally avoids the vertebral artery and cervical nerve root. The risk of injury to these structures is low in the superior facet where instrumentation is dorsal, but higher in the inferior facet where the screw tip will be positioned more ventrally after crossing two cortical surfaces of the joint. While securing a secure bony purchase, however, it is important to pick a steep trajectory in order to avoid too ventral a position for the screw tip. Proximal levels, C2-4 are more challenging considering that the occiput has the propensity to impede an accurate and steep trajectory.

A K-wire is left in place after appropriate drill depth has been obtained in the inferior lateral mass. The K-wire is utilized for tap and screw placement, during which one must be mindful of the small amount of distal purchase the wire has. Meticulous control is employed to obviate potential extraction or advancement. A screw is then placed and carefully advanced to firmly engage both lateral masses. Other sides and levels are subsequently instrumented in the same fashion.