24.1 Introduction

The atlantoaxial articulation is a unique and highly stressed joint complex that supports and protects the craniocervical junction while permitting tremendous range of motion. While uncommon relative to other spinal disorders, acquired degeneration or instability of the C1–C2 articulation may result in severe disability due to pain or neurologic impairment. As such, fusion of this joint complex may be indicated for the relief of pain or prevention of neurologic deterioration in the setting of traumatic, degenerative, inflammatory, or other acquired atlantoaxial instability.

Historical methods for atlantoaxial fusion involved open decortication and bone grafting techniques but relied primarily upon external stabilization via collar, traction, or halo vest immobilization to achieve a biomechanical environment permissive of osseous fusion. ^{1 ,​ 2} More sophisticated wiring techniques subsequently achieved additional construct stability when used in combination with structural autografts, but still necessitated prolonged external immobilization as an adjunct. ^{3 ,​ 4} Additional posterior hook-and-clamp-based fixation systems have also been described. ⁵ The widespread adoption of screw fixation techniques in the subaxial cervical spine prompted investigation of additional screw fixation tracts within the atlantoaxial articulation. Magerl first described clinical experience with the C1–C2 transarticular screw trajectory for stabilization and fusion. ^{6 ,​ 7} This elegant technique achieves excellent biomechanical stability but is technically demanding and dependent on the variable neurovascular anatomy of the atlantoaxial region. Indeed, it has been estimated that up to 23% of patients may have anatomy that is not permissive of safe bilateral transarticular screw passage. ⁸ Harms ⁹ and Goel ¹⁰ each reported on their clinical experience with segmental C1–C2 fixation, leading to increased adoption of this technique for a variety of indications, including fractures, degenerative and inflammatory arthritis, and congenital malformations and/or instability. In particular, polyaxial locking screws have facilitated greater flexibility in screw trajectories to achieve fixation even in the setting of traumatically altered or congenitally aberrant anatomy. When combined with direct decortication of the C1–C2 joints and/or onlay interlaminar bone grafts, satisfactory clinical outcomes and high rates of osseous fusion have been observed following both transarticular and segmental fixation techniques. ^{11 ,​ 12 ,​ 13 ,​ 14}

24.2 Background and Rationale

Frameless stereotactic navigation techniques have evolved in parallel for a variety of indications, notably intracranial and spinal procedures. The ability to recognize the spatial position and orientation of an instrument in real time when co-registered with a patient’s specific three-dimensional cross-sectional imaging is of intuitive benefit to the surgeon. However, this data must ultimately be incorporated along with direct visual and tactile feedback to guide intraoperative decision-making. The anatomy of the C1–C2 articulation is particularly unforgiving with respect to dissection and instrumentation. The upper cervical spinal cord is in close proximity and is at risk of injury with medial deviation into the canal. The C2 nerves overlay the C1 pars screw starting point and C1–C2 joint, and are enveloped by a robust epidural venous plexus. The vertebral arteries course through the transverse foramina of C2 lateral and inferior to the C2 pedicle, and cranially over the arch of C1 in a highly variable fashion. ^{15 ,​ 16 ,​ 17} In addition, the occiput-C1 articulation is at risk with cranial deviation of the C1 pars screw trajectory. The relative lack of bone stock within the typical transarticular, C2 pedicle, and C1 pars screw trajectories leaves little room for error or re-drilling of cannulation attempts. These regions are also difficult to visualize with standard C-arm fluoroscopy due to the multiple superimposed bony contours from overlapping structures.

Regardless of the means of fixation employed, the posterior cervical approach involves significant morbidity in terms of muscular dissection, potential blood loss, and risk of infection. Rapid prototyping techniques have emerged to facilitate fabrication of custom patient-specific models or drill guides based upon preoperative imaging studies to potentially improve the consistency of instrumentation placement and decrease intraoperative radiation exposure. ^{18 ,​ 19 ,​ 20 ,​ 21 ,​ 22 ,​ 23 ,​ 24} However, use of such drill guides still necessitates an open surgical exposure with attendant soft tissue disruption and blood loss. Other surgeons have reported on less-invasive exposures and/or percutaneous techniques for decreasing the approach-related morbidity in C1–C2 fusion. ^{25 ,​ 26 ,​ 27 ,​ 28 ,​ 29 ,​ 30} These techniques have relied heavily on the use of intraoperative fluoroscopic imaging. While the use of intraoperative navigation and robotic assistance has been investigated primarily as a means of increasing the safety and consistency of instrumentation placement in the C1–C2 region, it offers the potential to simultaneously facilitate less-invasive approaches and reduce surgical morbidity. These potential benefits must be weighed carefully against other factors, such as operative time, cost, and radiation exposure to the patient and surgical team when deciding whether or not to employ intraoperative navigation.

24.3 Surgical Technique

The senior author’s specific technique and navigation system has been previously reported. ³¹ In general, the patient is positioned prone on a radiolucent table with the head fixed in a Mayfield clamp. Cervical alignment is assessed with cross-table lateral imaging and closed reduction maneuvers may be performed as indicated. If required by the navigation system, a fiducial array may be attached to the Mayfield adapter at the head of the bed. The room is set up with the navigation detector at the head of the bed and the display screen in the surgeon’s line of sight (Fig. 24‑1). The patient is prepped and draped in a standard fashion, and a sterile fiducial array is attached to the adapter through the drape (Fig. 24‑2). A midline subperiosteal exposure of the C1–C2 region may be performed, or less-invasive approaches for percutaneous instrumentation may be considered. An intraoperative scan is then obtained using either fiducial registration or mapping of multiple points directly on the exposed osseous structures. The desired surgical instruments are then co-registered with the navigation scan. These may include a probe, drill guide, screwdriver, and/or burr at the surgeon’s discretion. The navigated probe may be used to visualize the planned screw trajectories to aid determining the extent of exposure. The navigated burr may be used to notch the C1 lamina if desired to facilitate placement of instruments for C1 screw placement (Fig. 24‑3 a). The navigated drill is used to verify the position and trajectory while drilling a starting point and recess for the navigated drill guide (Fig. 24‑3 b, c). The navigation tract is locked as surgical plan (Fig. 24‑4). Using the surgical plan the navigated drill guide then maintains a consistent trajectory during drilling of the screw tract and a new surgical plan is fixed (Fig. 24‑5 , Fig. 24‑6). A screw of appropriate length is selected to permit clearance of the C1 lamina (either fully threaded or with a smooth shaft to prevent C2 nerve irritation) and is then placed using the navigated screwdriver and the previous surgical plan. The process may be repeated for the desired C2 screw trajectory (pedicle, pars, or translaminar). Alternatively, the technique may also be used for C1–C2 transarticular screw placement. The construct is completed with rods, set screws, and/or cross links as indicated, and second intraoperative scan may be performed to confirm satisfactory appearance of the construct. Direct decortication of the C1–C2 joints may be performed prior to insertion of instrumentation and/or placement, and fixation of onlay grafts may be subsequently performed per the surgeon’s preferred fusion technique.

24.4 Caveats and Potential Pitfalls

Navigation can be a useful adjunct for performing complex spinal surgery. It is especially useful in the setting of altered or aberrant anatomy, and especially at the craniocervical junction where there is little room for error. This includes cases of trauma, rheumatoid arthritis or other subluxating conditions, revisions, and cases with aberrant vertebral artery or bony anatomy. Additionally, for most systems, it allows confirmation of implant placement while still in the OR.

There are however some general and specific issues regarding utilizing navigation at the craniocervical junction that should be considered. First, we highly recommend that surgeons become working “experts” in all aspects of the navigation system, as nothing can be more frustrating than having scrub assistants and/or radiology technicians who are not familiar with these systems. In addition, having the image acquisition system in the room and ready for operation in advance tremendously helps with workflow and avoids needless delays. We also feel that learning these techniques without the aid of navigation or using navigation as a confirmation is critical. This knowledge of appropriate starting points and trajectories helps the surgeon realize when the navigation may be “off.” Additionally, if the system is not functioning properly during a case, the surgeon must rely on experience with anatomic and basic fluoroscopic techniques. Navigation is a virtual system, so it is important to periodically check the accuracy of the system by touching a specific anatomical structure such as the notch of the C2 spinous process. If there is any concern for accuracy, the practitioner should have a low threshold for reacquiring source images. Additionally, even under normal circumstances, there is more potential relative motion at the craniocervical junction when applying pressure with drill guides, drills, and screws, and especially in the setting of trauma. Because of this, there is some practitioner variability regarding the placement of the fiducial array. Some advocate attaching it to the Mayfield cranial clamp as described above. The advantage is that the array is out of the operative field. The disadvantage is the potential for relative motion between the clamp and the spine. The other option is to attach it to a structure in the operative field such as the C2 or C3 spinous process. The disadvantage is that it can be cumbersome to work around especially since the trajectory of the screws requires the detector for the system to be placed at the head of the bed. Additionally, because of the potential for relative motion with placement of retractors, generally we recommend leaving the retractors in place for the image acquisition, where usually we would remove the retractors to avoid artifact with the spine. This increased mobility at C1–C2 is most likely a pitfall for robotic systems also.

Another way to utilize navigation is in a hybrid capacity in which navigation is used to determine starting points and plan trajectories, and then to use live fluoroscopy for drilling and screw placement. This can be done with standard fluoroscopy or by draping in image acquisition equipment and then using it in its fluoroscopic capacity. This is the technique we use for navigation assistant odontoid screw placement. ³²