A Morphometric Study of the C2 Pedicle and Pars - Part 1

Objective

To provide anatomic and clinical information about the pertinent surgical anatomy for C2 pedicular vs. pars screws.

Summary of Background Data

The C2 pars has been repeatedly referred to as the C2 pedicle.

Methods The CT scans of 14 patients with a variety of cervical disorders were assessed on an image–guided surgery workstation. Bilateral “virtual” 3.5 mm screws were placed in the C2 pedicles and pars. An anatomic analysis along the path of each screw was conducted to assess ideal screw trajectory, isthmic morphology, proximity to neurovascular anatomy, and dorsal entry point.

Results

The C2 pedicle was greater in height and narrower in width than the pars. The mean pedicle screw axial and sagittal trajectories were 45° medial (24º–56°) and 41º cephalad (23º–58º), respectively. The mean values for the pars screws were 7.3º medial (10º lateral–21º medial) and 58.2º cephalad (49º–68º), respectively. The mean proximity of the pedicle screws to the spinal canal was 1.60 mm (0.00–2.73 mm) and 2.04 mm (0.00–3.75 mm) for the pars screws. The mean proximity of the pedicle screws to the C2 transverse foramen was 1.40 mm (0.00–5.14 mm) and 1.24 mm (0.00–3.58 mm) for the pars screws.

Conclusion

The C2 pedicle and pars are anatomically and surgically distinct structures. There was significant variability in the course of the vertebral artery from patient–to–patient, as well as side–to–side differences in the same patient. As such, the vertebral artery was the most vulnerable structure to potential injury. This study emphasizes the need for detailed preoperative imaging and planning for dorsal screw placement into the C2 pedicle or pars.

Introduction

Posterior cervical screw fixation is a commonly used method of instrumentation among spine surgeons. The lateral mass is the most common location for screw placement; however, screw pullout and loosening remain a clinical problem. This has led to increasing interest in the cervical pedicle as an alternative site of fixation. The area defined as the pars in this study has been a source of debate, for it has also been called the C2 pedicle by numerous authors (Figure 1). This discrepancy arises from the opinion that the C2 superior articular process, because of its anterior location, is part of the C2 body and not a true lateral mass. However, from a developmental stand point, this area is part of the neural arch, not the C2 body. Thus, a screw placed across the C2 pars into the superior articular process is anatomically a pars screw, not a pedicle screw. As our experience and technical abilities continue to grow, transpedicular cervical fixation may become more common. C2 fixation is pivotal in the management of many cervical disorders. Therefore, we feel a clarification or standardization of terms for the C2 pedicle vs. pars is necessary. The purpose of this study was to assess the pertinent osseous surgical anatomy of the C2 pedicle vs. the C2 pars.

morphometric study c2 pedicle pars
Figure 1. For the purpose of this study, the C2 pars was defined as the area between the inferior and superior articular process and the C2 pedicle as the area bridging the dorsal elements to the true C2 body.

Materials & Methods

The CT scans of fourteen patients with a variety of upper cervical disorders were loaded into an image–guided surgery workstation. The ideal pathways for C2 pedicle and pars screws were then determined bilaterally, yielding 28 virtual pedicle and pars screw trajectories (Figure 2). Each optimal screw position was individualized based on the patient’s osseous and neurovascular anatomy. Using a method previously described by Foley et al3, assessment included isthmic morphology, screw proximity to neurovascular structures, and the ideal screw trajectory, length, and dorsal entry point. All linear measurements were made directly utilizing the planning station’s measurement function (accurate to within 0.01 mm). Angular measurements were made using a standard goniometer accurate to one degree.

morphometric study c2 pedicle pars

Image–Guided Workstation (StealthStationTM)
morphometric study c2 pedicle pars
Figure 2. The virtual pathway for a C2 pedicle screw (top) and C2 pars screw (bottom) are demonstrated. The different views illustrate the relevant anatomy along the planned screw pathway: the screw's sagittal (trajectory view 1) and axial plane (trajectory view 2) as well as the anatomy as seen perpendicular to the tip of the screw (probe's eye view).

Results

The average age of the study group was 50.1 years. There were seven females and seven males. The primary diagnosis was trauma in 8 patients (odontoid non–union(3), os odontoideum(3), transverse ligament rupture(1), and failed odontoid screw(1)), cervical spondylosis in 2 patients, rheumatoid arthritis in 2 patients, ossification of the posterior longitudinal ligament (OPLL) in 1 patient and a benign osseous tumor (aneurysmal bone cyst) in 1 patient. The plans for 28 pedicle and pars screws were constructed. Figures 3 to 6 demonstrate the main anatomic parameters identified in this study. The mean screw length was 26 mm for pedicular screws (range 20.6–32.7 mm) vs. 22 mm for pars screws (range 19.4–24.7 mm). The isthmus of the pedicle averaged 9.26 mm from the screw entry point (range 6.10 – 13.6 mm). This figure was 16.0 mm for the isthmus of the pars (range 11.8 – 23.1 mm). Violation of the spinal canal was virtually unavoidable for both the pedicle and the pars screws in one patient (Figure 7). Violation of the foramen occurred in three patients for both the pedicle and the pars screws. One of these three patients was also the patient with screw violation of the canal, giving a virtual unsafe perforation rate of 21% in this group.

 Isthmic Morphology ~ Pedicle
morphometric study c2 pedicle pars
  Isthmic Morphology ~ Pars
morphometric study c2 pedicle pars
Figure 3. These images demonstrate the typical morphology of the C2 pedicle and pars as seen in cross–section perpendicular to the screw trajectory (probe's eye view) at the isthmus. The C2 pedicle was significantly greater in height and narrower than the C2 pars.

Screw Trajectory ~ Pedicle

morphometric study c2 pedicle pars

morphometric study c2 pedicle pars

Figure 4. The axial trajectory was measured from the mid–sagittal anatomic plane and the sagittal trajectory relative to the C2 endplate. Note the wide variability, particularly for the pars screw, where in three patients a lateral rather than medial trajectory was required in order to avoid injury to the vertebral artery.

 Screw Trajectory ~ Pars
morphometric study c2 pedicle pars
 
Updated on: 02/01/10
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A Morphometric Study of the C2 Pedicle and Pars - Part 2
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