Object: Stabilization of the cervical spine can be challenging when instrumentation involves the axis. Fixation with C1-2 transarticular screws combined with posterior wiring and bone graft placement has yielded excellent fusion rates, but the technique is technically demanding and places the vertebral arteries (VAs) at risk. Placement of screws in the pars interarticularis of C-2 as described by Harms and Melcher has allowed rigid fixation with greater ease and theoretically decreases the risk to the VA. However, fluoroscopy is suggested to avoid penetration laterally, medially, and superiorly to avoid damage to the VA, spinal cord, and C1-2 joint, respectively. The authors describe how, after meticulous dissection of the C-2 pars interarticularis, such screws can be placed accurately and safely without the use of fluoroscopy.
Methods: Prospective follow-up was performed in 55 consecutive patients who underwent instrumented fusion of C-2 by a single surgeon. The causes of spinal instability and type and extent of instrumentation were documented. All patients underwent preoperative CT or MR imaging scans to determine the suitability of C-2 screw placement. Intraoperatively, screws were placed following dissection of the posterior pars interarticularis. Postoperative CT scans were performed to determine the extent of cortical breach. Patients underwent clinical follow-up, and complications were recorded as vascular or neurological. A CT-based grading system was created to characterize such breaches objectively by location and magnitude via percentage of screw diameter beyond the cortical edge (0 = none; I = < 25% of screw diameter; II = 26-50%; III = 51-75%; IV = 76-100%).
Results: One-hundred consecutive screws were placed in the pedicle of the axis by a single surgeon using external landmarks only. In 10 cases, only 1 screw was placed because of a preexisting VA anatomy or bone abnormality noted preoperatively. In no case was screw placement aborted because of complications noted during drilling. Early complications occurred in 2 patients and were limited to 1 wound infection and 1 transient C-2 radiculopathy. There were 15 total breaches (15%), 2 of which occurred in the same patient. Twelve breaches were lateral (80%), and 3 were superior (20%). There were no medial breaches. The magnitude of the breach was classified as I in 10 cases (66.7% of breaches), II in 3 cases (20% of breaches), III in 1 case (6.7%), and IV in 1 case (6.7%).
Conclusions: Free-hand placement of screws in the C-2 pedicle can be done safely and effectively without the use of intraoperative fluoroscopy or navigation when the pars interarticularis/pedicle is assessed preoperatively with CT or MR imaging and found to be suitable for screw placement. When breaches do occur, they are overwhelmingly lateral in location, breach < 50% of the screw diameter, and in the authors' experience, are not clinically significant.