Abstract Study Design
A Morphometric Study of the C2 Pedicle and Pars (Page II)
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| Figure 5. The dorsal entry point was measured as the distance lateral to the junction of the lamina and inferior articular process and the distance superior to the inferior aspect of the articular process. The C2 pedicle screw has a superior and lateral starting point that is grossly different from the C2 pars entry point. | |
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| Figure 6. The proximity of the virtual screws (3.5 mm) to the spinal canal and the foramen transversarium (vertebral artery) was determined at the isthmus of the C2 pedicle and pars using the probe's eye view. There was no statistically significant difference in neurovascular proximity between the pedicle and pars screws. | |
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| Figure 7. Illustration of the right C2 pedicle (A) and pars (B) isthmus in a patient with rheumatoid arthritis. Regardless of the trajectory chosen, placement of a 3.5 mm screw across the isthmus of the pedicle or pars was not possible without cortical violation. |
Discussion
The morphology and dimensions of the C2 pedicle found in this study are consistent with published cadaveric data4,5. Our data on the C2 pars is consistent with that described by Ebraheim2 and Xu7; however, this structure was defined as the C2 pedicle in both of these papers. Numerous other authors have also denoted this area as the C2 pedicle. The results of our study clearly show that the C2 pedicle and pars are distinct structures. More importantly, this study illustrates and reaffirms the innate variability of the vertebral artery at the C2 level. This variability increases the risk of vertebral artery injury when placing any type of dorsal screw into C2. This risk is also compounded by the thicker medial wall of the pars and pedicle, which gives a drill or screw a path of least resistance directly into the foramen. As well, the patient’s underlying disease process may also significantly affect the osseous anatomy of this area. Not only do anatomic variations occur from patient to patient but also from side to side within the same patient (Figure 8).
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| Figure 8. Illustrated are the right and left isthmic cross sectional (probe's eye view) images of the C2 pedicle (above) and pars (below) in a patient with cervical spondylosis. Note the side–to–side variability in the course of the vertebral artery. This inherent variability of the vascular and osseous anatomy of C2 necessitates detailed preoperative planning and individualization prior to dorsal fixation at this level. |
Anatomically, the vertebral artery was most vulnerable to injury at the isthmus of the pars or pedicle (where anatomic constraints were “tight”). Placement of a C2 pedicle or pars screw would have been unsafe in 21% of our patients. Due to the significant anatomical variability and risk in this area, the placement of a C2 pars screw should be individualized for each patient and side based on detailed pre–operative planning rather than prescribing set guidelines1,6 for the placement of C2 pars screws.
This study also points out a major advantage of the image–guided surgery planning station. Even if it is not used for intraoperative navigation, the system allows the surgeon to conveniently view a patient’s unique anatomy along any chosen pathway. This process facilitates surgical individualization in these technically demanding cases. This study also illustrates the utility of an image–guidance planning station as an anatomic research tool, obviating the need for cadaveric dissection. Based on our findings, the C2 pars screw has a much more accessible entry point that conforms with current instrumentation systems and techniques as compared to the C2 pedicle screw. The C2 pedicle has a superior and lateral entry point that is very close to the vertebral artery and requires additional exposure as well as percutaneous drill and screw placement to achieve the medial trajectory required. This point of entry does not conform with most occipital or subaxial fixation systems.
Conclusions
- The C2 pedicle and pars are anatomically and surgically distinct structures.
- Screw placement into either places the vertebral artery at risk.
- The C2 pars screw is technically a much easier alternative.
- This study also demonstrates the importance of detailed pre–operative planning for any type of C2 dorsal screw fixation.
References
1. An H, Gordin R, Renner K. Anatomic considerations for plate–screw fixation of the cervical spine. Spine 1991; 16(10 Suppl):S548–551
2. Ebraheim N, Rollins J, Xu R, Jackson W. Anatomic consideration of C2 pedicle screw placement. Spine 1996;21(6):691– 695.
3. Foley K, Silveri C, Shah S, Garfin S, Vaccaro A. Atlantoaxial transarticular screw fixation: risk assessment and bone morphology using an image guidance system. Spine 1998. In press.
4. Karaikovic E, Daubs M, Madsen R, Gaines R. Morphologic characteristics of human cervical pedicles. Spine 1997;22(5):493–500.
5. Panjabi M, Duranceau J, Goel V, Oxland T, Takata K. Cervical human vertebrae: quantitative three–dimensional anatomy of the middle and lower regions. Spine 1991;16(8 Suppl):S861–869.
6. Roy–Camille RR, Sailant G, Mazel C. Internal fixation of the unstable cervical spine by posterior osteosynthesis with plate and screws. In Cervical Spine Research Society (ed). The Cervical Spine Ed 2. Philadelphia, JB Lippincott, 1989, pp.390–404.
7. Xu R, Nadaud M, Ebraheim N, Yeasting R. Morphology of the second cervical vertebra and the posterior projection of the C2 pedicle axis. Spine 1995;20(3):259–263.