Percutaneous, Navigated Minimally Invasive Posterior Cervical Pedicle Screw Fixation

Technique

For the subaxial cervical spine, 2 lateral incisions were made paracentrally on the posterior neck over the spinal segments to be stabilized using intraoperative navigation (Stealth Station 8, Medtronic, Minneapolis, MN). Separate fascial incisions were used for each level to be instrumented. For atlantoaxial fixation, bilateral 2-cm stab incisions were made approximately 5 cm from the midline, over the C1 lateral masses, and separate bilateral 2-cm stab incisions were made approximately 4 cm from the midline at the C6-C7 level parallel to the extended angle of the C2 pars. A combination of sharp and blunt dissection was carried down through the fascia and muscle to the spine without the use of a retractor system. The Proficient Minimally Invasive System (PROMIS; Spine Wave, Shelton, CT) with reduction towers designed for use in the cervical spine was used for screw placement. The rods were then passed via the percutaneous technique. Postplacement intraoperative CT was performed to confirm accurate screw placement.

Preoperative Evaluation

All patients were evaluated with a preoperative cervical spine CT to measure the diameter of the pedicles under consideration of fixation. In patients with traumatic injury, a CT angiogram of the neck was also performed to evaluate for any preexisting vertebral artery injury or aberrant tortuosity that might affect operative planning. A cervical spine MRI was obtained as well to determine whether the pathology required anterior or posterior decompression in conjunction with percutaneous pedicle screw placement.

Positioning

The patient was placed in a Mayfield frame and positioned prone on Jackson table with a Mayfield attachment or cervical management system. The alignment of the cervicothoracic junction was maintained in a neutral position if fixating across the cervicothoracic junction. In addition, the head was maintained in a neutral position. When performing C1-C2 fixation, a slightly flexed position provided better access to the C1 lateral masses, but the patient was repositioned appropriately once screws were placed prior to rod placement. Taping the shoulders down allowed for access to the lateral neck. Sterile prep was performed in traditional fashion. It was of critical importance to prep extremely wide on the neck due to the unusually lateral-to-medial angle required for percutaneous cervical pedicle screw placement (Figure 1A).

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Figure 1

(A) Preparation, draping, and placement of spinous process clamp and navigation array for the subaxial percutaneous pedicle screw technique. (B) Drilling of a pilot hole along the trajectory of the pedicle. (C) Skin incision down to the layer of the investing fascia.

Navigation

Cone-beam CT intraoperative imaging was obtained using the O-Arm 2.0 (Medtronic, Minneapolis, MN) for use with an intraoperative navigation system. For subaxial fixation, a fiducial was placed on a spinous process caudal to the lowest instrumented vertebra. This midline incision required for spinous process exposure was approximately 2.5 cm and involved very superficial release of the cervical fascia and paraspinal musculature to allow for clamp placement. The fiducial was unlikely to be obstructive even if unintentionally placed on the spinous process of a planned instrumented vertebrae, given the unusually wide angle of screw placement. The spinous process clamp provided more consistent accuracy, given small micromovements that may have occurred throughout the procedure, which can be worsened in cases of cervical instability. For atlantoaxial fixation, a fiducial attachment was placed directly on the Mayfield retractor.

Screw Fixation

The instrumentation system is designed to allow for true percutaneous pedicle screw placement in the cervical spine. Screws are available with various diameters (3.8, 4.2, 4.5, and 5.5 mm) and with lengths ranging from 10 to 90 mm. Screws were attached to a reduction tower system specifically designed for use in the cervical spine. Cobalt chrome or titanium rods with diameters of 3.5 or 4.0 mm were then placed via percutaneous technique.

Technique: Subaxial Pedicle Screws

Once the fiducial array had been placed on the spinous process of the lowest vertebral body to be instrumented and an intraoperative CT had been performed, the next step was planning the percutaneous incision. The navigated wand with an extended projection was used to establish the necessary entry point to provide the desired trajectory. Once each entry point was identified, a linear incision through the dermis was made down to the level of investing fascia along the course of the entry points. This incision provided a more cosmetic closure as opposed to several small stab incisions, which could have been used as an alternative. A standard small, self-retaining Weitlaner retractor was used (Figure 1C). The navigated wand was again used to determine the percutaneous entry point through the fascia. Monopolar electrocautery was used to go down through the fascia, and the muscle fibers were split bluntly cranial caudally. Any deep fascia layers were released with the Bovie to allow for the placement of the navigated MH80 matchstick drill onto the lateral mass (alternatively, a navigated 2-mm burr could have been used) to drill a pilot hole. The navigated pilot hole allowed for efficient drilling, tapping, and screw placement with minimal skiving. The trajectory was confirmed and the navigated drill was advanced into the lateral mass along the axis of the pedicle (Figure 1B). If the pedicle was large enough in diameter, the drill was advanced at least midway through the pedicle. A 3.0-mm navigated tap was used for 3.5- and 4.0-mm screws, whereas a 4.0-mm navigated tap was used for 4.5- and 5.0-mm screws. Navigation software was then used to select appropriate screw length and size. A navigated screw and reduction tower were then placed using navigation (Figure 2B).

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Figure 2

(A) Percutaneous placement of rod. (B) Pedicle screw placement. (C) Skin closure. (D) Final construct.

After placement of pedicle screws on both sides, a second intraoperative CT was performed to confirm appropriate placement. A rod was then passed using the percutaneous technique and tested to confirm it was through all reduction towers (Figure 2A). Capture of the rod through each reduction tower was confirmed with a testing device that indicated the presence of the rod. The locking caps were placed and final tightened. Rod length was often confirmed visually or with direct palpation. If there was any concern or if additional alignment adjustments were desired, final intraoperative imaging was obtained with anterior-posterior and lateral fluoroscopy with the O-arm or C-arm (Figure 2D). The rod holder was then removed. Superficial cervical fascia were then closed with interrupted 2-0 resorbable sutures. Subcutaneous inverted 2-0 resorbable sutures were then placed for wound closure and topical skin adhesive was placed on the skin (Figure 2C).

Technique: C1-C2 fixation

Either a C1-C2 transarticular screw or C1 lateral mass and C2 pars screw/rod fixation was achieved with this navigated, percutaneous posterior cervical technique. The fiducial array was placed on the Mayfield frame and intraoperative CT imaging was obtained. For a C1-C2 transarticular screw, the navigated wand was used to place a skin and fascial stab incision parallel to the extended angle of the C2 pars and in line with the cervical lateral masses. Bilateral stab incisions were made at approximately the C6-C7 level, and the projected image from the navigated wand was used to approximate the screw trajectory. If the patient's anatomy and body habitus allowed for C1-C2 transarticular screw fixation, a navigated pilot hole was made just above the C2-C3 lateral mass in the midline of the lateral mass. Pilot hole placement was followed by navigated drilling, tapping, and screw placement through the C2 pars, across the C1-2 joint, and into the C1 lateral mass. The C1-C2 transarticular did not require a tulip. If a C1-C2 transarticular screw trajectory was not feasible or desirable, C1 lateral mass and C2 pars screw/rod fixation was accomplished by modifying this technique. Shorter C2 pars screws with tulips were placed in the fashion described above. Separate bilateral stab incisions were made directly above C1 lateral masses and pilot hole placement was followed by navigated drilling, tapping, and screw placement directly into the C1 lateral masses. Extended tab screws were not required. Rods were then placed directly into the C1-C2 screw tulips and locking caps are engaged.

Technical Note

The soft tissue and fascia surrounding the lateral masses can cause significant difficulty with cannulating the initial pilot hole. To prevent this from occurring, it is important to ensure there is an appropriate corridor through the lateral neck musculature and fascia. If there is any resistance encountered with cannulating the entry point or obtaining screw purchase, we recommend reorienting with the navigated wand and widening any muscle and fascia near the lateral mass entry point.

To reduce the catastrophic risk of bilateral vertebral artery injury, screws should be placed on 1 side at a time. If there is any concern for significant breach or inaccurate navigation, an intraoperative CT should be performed prior to proceeding with the contralateral side to confirm there has not been significant foraminal breach and potential vertebral artery compromise.

In cases of traumatic instability, especially with C1-C2 bony and ligamentous injury, special care must be taken to avoid exerting undue pressure onto the spine during the drilling and tapping portions of the procedure to avoid inaccurate screw placement. The C2 pars screws should be placed prior to C1 lateral mass screw placement. Exacting pilot hole placement and soft tissue release facilitates accurate screw placement.

Rods should not be passed until all screws have been placed. Due to the significant ligamentous laxity in the cervical spine and the small margin of error, placement of the rod can reduce the accuracy of the navigation on the contralateral side.

Potential Indications

The MIS, navigated percutaneous cervical technique described in the present article can be used for stand-alone posterior cervical fixation or in conjunction with MIS or open decompression. It can be used in conjunction with anterior cervical decompression as part of a single surgery or a staged anterior-posterior procedure. It can also be adapted for use with intraoperative robot-assisted navigation. From our early experience, we have identified patients who would potentially benefit most from implementation of this posterior MIS technique. The use of percutaneous cervical pedicle screws from a supplemental perspective following anterior surgery helps reduce the approach-related morbidity of a combined anterior-posterior operation. Patients with a high risk of wound healing issues, such as multi-trauma patients, may benefit substantially from this technique. Avoiding a large midline incision, with significant potential for the dead space of traditional approaches, is especially advantageous in patient populations with spinal cord injury, osteomyelitis, metastatic tumor, or end-stage renal disease.^7,25,26

Patients undergoing anterior cervical corpectomy with significant risk of construct failure may benefit from this posterior MIS technique, especially patients with osteoporosis, metastatic disease, infection, or end-stage renal disease who otherwise may not receive posterior stabilization due to approach-related morbidity. Patients who have undergone multi-level corpectomy are also potential candidates for percutaneous stabilization.

Cervical fracture dislocation is another indication for this technique. After reduction of the fracture with traction with or without addition of anterior decompression and fusion, percutaneous pedicle screw fixation can be used as a primary or secondary means of fixation. In addition, unstable cervical spine fractures with preserved alignment and no spinal cord compression may also benefit from this technique. We have found this technique particularly useful in the subset of patients with cervical spine diffuse idiopathic skeletal hyperostosis fractures. Two patients with diffuse idiopathic skeletal hyperostosis with unstable cervical fractures in our series were able to be reduced and decompressed anteriorly with a 1-level anterior cervical discectomy and fusion and then received long-segment fixation posteriorly via this technique. Acute or chronic type II odontoid fractures, especially in the elderly, represent another patient population amenable to this MIS approach.

This study reports the initial experience with this technique for all primary surgeons and therefore reflects the expected learning curve as it relates to the radiographic and clinical outcomes. In addition, this technique should be reserved for spinal surgeons who have significant open and MIS navigation experience in the thoracic and lumbar spine. Intraoperative navigation alone requires a separate learning curve, and it is not recommended that comfort with it be built with this technique. The primary surgeons in this study each had at least 5 years of experience with intraoperative CT-guided navigation prior to implementing this technique.

The limitations of this report on our initial experience with a novel MIS percutaneous technique include retrospective nature of the study as well as the relatively small number of patients and the lack of clinical correlation with validated patient-reported outcome measures. This report is intended to be a feasibility and safety analysis with radiographic outcome. Future, prospective study should focus on a comparative analysis to open posterior cervical procedures including clinical outcomes and economic measures.