There is no absolute contraindication for posterior cervical navigation. The indications for posterior cervical instrumentation include instability from traumatic, neoplastic, infectious, iatrogenic, or congenital etiologies, as well as multilevel cervical spinal stenosis with degenerative instability, deformity, or kyphosis. The use of CT image-guided navigation in the cervical spine is typically indicated in regions where visualization of the deep bony anatomy is extremely beneficial. At the occipitocervical junction, identifying the location of the occipital protuberance, spinal canal, and vertebral artery are necessary in order to safely introduce spinal instrumentation. In the subaxial and cervicothoracic junctions, the overlying soft tissue and bony structures of the shoulder girdles often obstruct conventional fluoroscopy, making navigation a superior alternative. Aside from improving accuracy and safety of cervical instrumentation, image-guided neuronavigation has the added benefit of smaller incisions and less soft tissue mobilization.

Carefully reviewing all available preoperative advanced imaging including CT, MRI, and radiographs of the cervical spine, may reveal anomalous anatomy such as presence of a ponticulus posticus. With a prevalence of 16.7% (range, 4.3%–52.9%), a ponticulus posticus can easily be mistaken for a thickened posterior arch, leading to a potentially devastating vertebral artery injury during C1 screw fixation. C2 and subaxial dorsal anatomy can also be highly variable. Atretic or congenitally abnormal bony C2 pedicles, abnormal subaxial cervical landmarks and variable anatomy of the intervertebral foramen can be identified with preoperative imaging review. The potential hazards associated with these anomalies can be carefully and safely avoided intraoperatively with neuronavigation systems.

Room layout: We use the Medtronic mobile CT scanner O-arm® Surgical Imaging with StealthStation® Navigation System (Medtronic, Inc., Minneapolis, MN). The StealthStation® Navigation System and optical tracking camera are typically placed at the head or foot of the bed for optimal visualization of the surgical field. We use either a Skytron 3100 with gel pads or the Mizuho OSI® Jackson table with the arms tucked at the sides. The patients are positioned prone and the cranium is secured with a radiolucent Mayfield® head-holder. Care is taken not to use any table extenders that drop below the table as this may block the entry, closure, and imaging of the mobile CT scanner.

Reference frame: The selection of an ideal optical dynamic reference frame (DRF) is dependent on the type of cervical surgery and exposure that is warranted. The two main options for reference frames are a spinous process clamp affixed to any of the C2–T1 spinous processes and a Mayfield extension arm that allows positioning of the DRF near the occiput ( Figs. 2.1 and 2.2 ). The DRF is typically affixed between the StealthStation optical tracking camera and the working surgical level to provide the best uninterrupted line of sight to both the DRF and navigated spinal instruments. The DRF should be placed out of the surgical working area with the camera positioned at either the patient’s head or the foot of the bed. The motor-evoked and somatosensory potentials are routinely monitored, and electromyographic monitoring is performed in all patients.

Optical Dynamic Reference Frame attached to the Mayfield extension arm.

Optical Dynamic Reference Frame attached with a spinous process clamp.

A straight midline incision is made to expose the levels of interest. After complete exposure, retractors are placed and left in during the O-arm image acquisition so as not to be adjusted after the O-arm spin and instrument registration. The O-arm is then brought into the protected field for image acquisition ( Fig. 2.3 ). The anesthesiologist is asked to hold ventilation throughout the entire scan and resume respiration after image acquisition is completed. If possible, the tidal volumes are reduced to at least 30% of the baseline to reduce chest wall excursion during the navigated instrumentation portion of the surgery. All neuronavigation instruments are then registered and utilized for intraoperative planning of screw placement including choice of screw diameter, length, and trajectory based upon the relevant anatomy ( Fig. 2.4 ).

Medtronic mobile CT scanner O-arm® Surgical Imaging.

StealthStation® Navigation System (Medtronic, Inc.) display.

The insertion of posterior cervical screws with navigation can be achieved according to several different methods. In general, in our practice, a high-speed drill is used initially to identify the ideal starting points on the bony surface anatomy. This starting point is confirmed with navigation and the deep bony anatomy analyzed to determine the ideal screw trajectory. In all cases, navigation is used as a confirmatory tool and as an adjunct to the direct visual and tactile information available from the surgical field.

With minor adjustments made with navigation, the anatomic entry points and trajectories for the various screws are as follows: C1 lateral mass screws are initially determined by palpation of the medial and lateral edges of the C1–C2 facet joint. The starting point is selected directly beneath the edge of the posterior arch in the center of the C1 lateral mass, with a sagittal trajectory parallel to the posterior arch of C1, angled approximately 10 degrees medially ( Fig. 2.5 ). Care is taken to identify and protect the C2 nerve running just below the C1 start point ( Fig. 2.6 ). A pilot hole is drilled with a 2.4-mm or 2.9-mm drill bit, and the trajectory is confirmed with navigation. The software on the StealthStation workstation allows users to “save” the drilled trajectory as a projection and provides the ability to measure screw length and width. Finally, the screw tract is palpated with a ball probe to confirm the absence of a bony wall breach, and a final screw is inserted into its ideal trajectory, as determined by CT-guided navigation ( Fig. 2.7 ).

Placement of C1 lateral mass screws using StealthStation® Navigation System.

Posterior atlantoaxial anatomy showing C1 lateral mass screw start point just above the C2 nerve.

(From Pittman JL, Bransford RJ. Cervical Spine Surgery: Standard and Advanced Techniques. 2019:201–206.)

Placement of C1 lateral mass screws using StealthStation® Navigation System.

The C2 pedicle screw starting point is at the midpoint of the C2 pars with the drill bit guide directed into the central cancellous channel of the C2 pedicle by navigation. Generally, C2 pedicle screw placement begins 3 to 4 mm above the C2–C3 facet, angled 20 to 30 degrees medial and 20 to 30 degrees cephalad. The exposure of the medial and superior aspect of the C2 pedicle within the spinal canal allows the direct visualization of any medial bony wall breach with instrumentation. Due to gross variations in pedicle orientation, size, and vertebral artery anatomy, the C2 pedicle screw length and trajectory can be highly variable ( Fig. 2.8 ). Care is always taken to review the preoperative advanced imaging at the occipitocervical junction in order to anticipate any anatomic variations.

Placement of C2 pedicle screws using StealthStation® Navigation System.

Other non-pedicle trajectories for the instrumentation of the C2 can be easily achieved with navigation guidance. The C2 pars screws begin 3 mm above the C2–C3 facet and 3 mm lateral to the lamina-lateral mass junction, angled 10 to 15 degrees medial and 35 degrees cephalad ( Fig. 2.9 ). The C2 transarticular screws start 2 to 3 mm superior to the facet and 3 mm lateral to the lamina-lateral mass junction, aimed 10 to 15 degrees medial to the anterior tubercle of C1. The C2 translaminar screws have an offset starting point with a trajectory slightly less than the slope of the lamina in relation to the vertebral body. With the ability to visualize the deep bony anatomy, all of the non-pedicle C2 screw trajectories can be determined confidently, and screw head alignment for rod insertion can be planned better.

Starting point and trajectory for C2 pars screws.

The subaxial lateral mass screws from C3 to C7 follow the modified Anderson technique, with a starting point approximately 1 mm medial to the center of the lateral mass, a trajectory parallel to the facet joint, and approximately 30 degrees of lateral angulation in the parasagittal plane ( Fig. 2.10 ). Although free-hand and fluoroscopic techniques are well described, navigation is particularly helpful in preventing adjacent segment degeneration by avoiding screw violation of the proximal and distal facet joints. Additionally, navigation allows the easy alignment of all the lateral mass screw starting points to help facilitate eventual rod placement.

Placement of subaxial lateral mass screws using StealthStation® Navigation System.

The traditional subaxial C3–C7 pedicle screws without navigation have not been considered as a first-line instrumentation choice. With image-guided navigation, subaxial C3–C7 pedicle screws have been gaining popularity, especially in the revision setting in patients with permissible anatomy. In addition, C7 pedicle screws have been increasingly utilized for instrumentation of the cervicothoracic junction. The trajectory of the pedicle screws can be variable, from 25 to 45 degrees medial as one progresses from C3 to C7. Ultimately, the ideal axial and sagittal trajectories are best determined with navigation.

CT image-guided navigation is also utilized with occipital plate placement. There is great variability of the posterior skull and bony anatomy. The location of the external occipital protuberance can be identified visually with direct dissection. However, the identification of the internal bony anatomy of the occipital cranium is the critical portion for instrumentation purposes. With navigation, the visualization and localization of the internal occipital protuberance, midline inferior keel, and inferior nuchal line are extremely straightforward. Understanding the depth of screws and selecting the thickest portion of the occiput allows for maximal bony purchase of the occipital screws and plate. In addition, knowing the thinnest regions of the posterior occipital bone (particularly in the occipital squamous bone laterally) helps with avoiding injury to internal structures such as the transverse sinus, cerebellar vasculature, or cerebellum itself.

In general, for all screw instrumentation in the occipitocervical spine, all instruments including the probe, high-speed drill, awl, pedicle finder probes, screw tap, and screw are calibrated for navigation purposes. In our hands, specifically, a navigation probe is initially placed on the appropriate starting entry point based on the standard anatomic surface landmarks. Reformatted CT images centered on the corresponding point in the image data set will be displayed, allowing the selection of the appropriate entry point and screw trajectory in the axial, sagittal, and coronal planes. The navigated probe can be placed in different positions and angles, to check the ideal trajectory in real time. The cortical entry site is prepared with a navigated high-speed drill, while the appropriate trajectory in both the sagittal and axial planes is checked. The entry hole is then pre-drilled with a navigated drill/awl and then probed for sidewall breach and to determine depth. A navigated tap is used, followed by final screws that are inserted with the navigated screwdriver into their ideal trajectories.

In order to reduce the radiation burden on the patient, we do not routinely perform confirmatory intraoperative CT scans for navigated placed screws. However, upright plain radiographs are obtained postoperatively in the hospital and again at 3 and 12 months after surgery. In the case of intraoperative breach, navigation inaccuracy, surgeon request, or intraoperative neuromonitoring changes there is a low threshold to repeat a CT scan for confirmation of instrumentation placement.

The patients are evaluated immediately postoperatively for any signs of neurologic deficit or clinical symptoms associated with the remote possibility of a malpositioned screw during the inpatient hospitalization and outpatient settings. Except for patients with osteoporosis, or those in whom poor bone quality is identified intraoperatively, those with stable instrumentation do not require postoperative immobilization with a cervical collar. Drains are routinely placed during surgery and ultimately removed after the output reaches a threshold minimum. All sutures are buried under the subcutaneous skin, dressings are applied, and the first follow-up is scheduled within the first 2 weeks.