Study Design. This anatomic study investigated the surgical and radiographic anatomy of the C2 pedicle as it relates to transpedicular screw placement in occipitocervical stabilization surgery.
Objectives. To establish anatomic guidelines for the placement of C2 pedicle screws.
Summary of Background Data. Several studies have put forth structured, algorithmic guidelines for the placement of C2 pedicle screws, but there have been no reports describing the use of anatomic landmarks to guide placement of these screws.
Methods. The C2 pedicles in ten cadaveric spines were evaluated using computer assisted tomography (CT) measurements. The specimens were imaged and the medial-lateral and rostral-caudal angulations of each pedicle were measured and recorded in 1º increments using a digital goniometer. The pedicle height, width and length were also measured from the CT images. At this point, pedicle screws were placed based on the above measurements in conjunction with direct visualization of the C2 pedicle through the C1-C2 interlaminar space. The distances of the screw entry point to the midline, C2-C3 joint line and the medial aspect of the vertebral artery were also measured. Repeat scans were then obtained to assess screw placement.
Results. The average pedicle height, width, and length measured 9.1 mm, 7.9 mm, and 16.6 mm respectively, and the medial inclination and rostrocaudal angulation averaged 35.2º and 38.8º respectively. The cortex of the pedicle was not violated in any of the twenty pedicles.
Conclusions. Adequate preoperative studies in conjunction with direct visualization of the C2 pedicle make transpedicular fixation safe and effective.
The surgical management of occipitocervical instability has evolved in the past two decades secondary to advances in instrumentation materials and our understanding of spinal biomechanics. Prior to the use of rigid internal fixation, all attempts to stabilize this segment of the spine required supplemental external fixation.(2,6-8 ) Many of these methods utilized sublaminar wires, which adds significant risk to the surgery. Although Leconte(11) was the first to describe the use of C2 pedicle screws in the management of traumatic spondylolisthesis in 1964, it was not until twenty years later that Borne(4) published the first series of patients treated with this technique. Borne's work demonstrated that using the C2 pedicle as an anchor for the internal stabilization of occipitocervical instability could be performed safely and provided excellent results. The rigid fixation obviated the need for postoperative external immobilization, but the technique did not gain widespread attention until Roy-Camille(13,14 )reported its use in the treatment of C2-related instability in 1989 and again in 1991. These reports and the increasing popularity of lateral mass plating has led to a push to incorporate the C2 pedicle into these constructs for both traumatic and nontraumatic occipitocervical instability. Several anatomical studies(1,9,15 ) of the second cervical vertebra for the purpose of setting forth guidelines for the placement of pedicle screws have been published, but adherence to such algorithmic approaches fails to take into account individual variations and too often results in cortical breach of the pedicle (Figure 1). As no two patients are identical, a surgeon must have a full appreciation of each patient's anatomy. Such a working knowledge allows the surgeon to use the C2 pedicle as an anchor in occipitocervical stabilization with minimal risk of damaging neurovascular structures. In this study, we describe the anatomy of the C2 pedicle and its landmarks as they relate to the surgeon.
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Figure 1A. Previous anatomical studies put forth these angles as guidelines for C2 pedicle screw placement. 1B. Previous guidelines for screw entry point: 5 mm inferior to the superior border of the C2 lamina and 7 mm lateral to the lateral border of the spinal canal.
MATERIALS AND METHODS
In this anatomical study we used ten cadaveric disarticulated cervical spine/head specimens with an equal male to female ratio. Each specimen was placed in a three-point fixation device in the prone position. A posterior midline incision was then made from the occiput to approximately the level of C4. The soft tissue was removed from the posterior elements out to the lateral border of the inferior articulating process of C2. Attention was focused on the C1-C2 interlaminar space, the C2-C3 facet joints, and the medial aspect of the vertebral artery. Once these were exposed, the ligamentum flavum was incised and removed exposing the C2 nerve root and the superior and medial aspects of the C2 pedicle. At this point, each of the pedicles was drilled using the superior-medial aspect as a guide, and 18-gauge Kirschner wires were placed in each pedicle and compared to a midline wire representing the approximate axial and sagittal planes. The medial-lateral and rostral-caudal angulations were then made and recorded using a digital caliper and goniometer. The specimens were then imaged using computer assisted tomography (CT), and the same measurements were made using a digital goniometer and the sagittal plane and the inferior endplate of the C2 body as references. The CT measurements closely approximated those made manually. Because the CT measurements accurately compared to those made manually on the first four specimens, the manual method was abandoned for the remaining specimens. Using the CT images, the following measurements were then recorded: pedicle height, width, and length. 3.5 mm pedicle screws were then placed unilaterally using the superior-medial border of the pedicle as a guide, and the specimens were reimaged in both the axial and sagittal planes in an effort to evaluate the possibility of cortical breach (Figures 2 &3). The screws were then removed, and new screws were placed in the contralateral side followed by imaging. We chose to image one side at a time in order to decrease the chance of metallic artifact interfering with the view of the pedicle and any possible breach of the cortex.
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Figure 2. Two pedicle screws have been placed in this cadaveric specimen.
Figure 3A. Axial image after placement of the C2 pedicle screw. There has been no breach of the pedicle. 3B. Sagittal image of the same pedicle screw. Again, there has been no violation of the pedicle.
In the 20 pedicles examined in this study, the average pedicle height, width, and length measured 9.1mm (7.4-11mm), 7.9mm (6.4-9mm), and 16.6mm (15.9-17mm) respectively, and the average medial inclination and rostrocaudal angulation measured 35.2º (29-41º) and 38.8º (22-52º) respectively (Table 1;Figure 4).
Table 1 Pedicle Dimensions and Angles (Left/Right)
|Cadaver||Height (mm)||Width (mm)||Length (mm)||M-L Angle*||R-C AngleÝ|
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Figures 4A. The average medial-lateral measured 35.2º. 4B. The average rostral-caudal angle measured 38.8º.
The images obtained after screw placement revealed that the cortex of the pedicle was not violated in any of the twenty pedicles (see CT scans above). During the dissection, measurements were made from surrounding structures to the screw entry point in an effort to establish useful landmarks to guide the surgeon during surgery. These structures included the midline represented by the center of the C2 spinous process, the C2-C3 joint line, and the medial aspect of the vertebral artery. The average distance from the C2 spinous process, C2-3 joint line and the medial aspect of the vertebral artery measured 26 mm, 9.1 mm, and 1.38 mm respectively. These values are shown in Figure 5.
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Figure 5. Useful landmarks in surgery. A. The distance from the midline to entry point - 26/25.9 mm (L/R). B. The distance from the C2-C3 facet - 9.1/9.1 mm. C. The distance form the medial border of the vertebral artery 1.37/1.38 mm.
Benzel(3) points out that the pedicle of second cervical vertebra is actually the segment that joins the body with the posterior elements and not the portion located between the superior and inferior articulating processes. This part of the vertebra in the remainder of the spine is termed the pars interarticularis. However, the vast majority of the literature devoted to this topic labels the pars interarticularis as the pedicle. In an effort to minimize any confusion, we have chosen the more common, although anatomically incorrect, definition for the C2 pedicle.
The average dimensions of the pedicles measured in this study do not differ significantly from those published in previous studies(1,9,10,12,15) with the exception of pedicle length. Previous anatomical (1,10,12,15 )reports defined the length as the distance from the posterior surface of the C2 inferior articulating process to the anterior surface of the body. We chose to define the length as the distance from the posterior surface of the inferior articulating process to the junction of the pedicle with the body. This accounts for the shorter length in our study.
Xu et al(15) in their anatomical study of the C2 pedicle reported that the average medial angulation of the pedicle was 30º and the rostral-caudal inclination was 20º cephalad to the transverse plane. They also reported that the proper screw entry point based on these angles was located at a point 5mm inferior to the superior border of the C2 lamina and 7mm lateral to the lateral border of the spinal canal. In an effort to test these values, the same group in a later study5 compared screws placed using the above values with those placed using the superior-medial border of the pedicle as a guide. They found that there was a higher incidence of cortical breach using the structured approach and recommended that C2 pedicle screws be placed using visualization of the pedicle. In the present study, we placed the screws under direct visualization and then measured the angles as well as the distance of the entry point from structures routinely encountered at time of surgery. The average medial-lateral angulation in the present study measured 35.2º, and the average rostral-caudal angulation measured 38.8º. The medial-lateral angulation differs slightly from the previous guideline, but the rostal-caudal inclination is almost twice that of the previous study. Such differences in angulation likely account for the increased incidence of cortical breach in previous study.
We chose to use the superior-medial border of the pedicle as a guide for screw placement as opposed to a structured algorithmic approach that fails to take into account individual anatomic variations. The structured method described by Xu et al(15) revolves around the intersection of imaginary planes, which often contributes to poor screw placement and increased risk of neurovascular injury. Using the superior-medial surface of the pedicle as a guide, one has an increased likelihood of locating the optimal entry point, but at times, the anatomy may be distorted. In such cases, one needs reliable aids in locating the entry site. By measuring distances from structures routinely encountered in surgery to our entry points, we were able to establish a fairly standard set of guidelines. The average distance from the midline to the entry point in the current study was 26mm, and the average distance from the C2-C3 joint line to the entry point was 9mm. If the surgical dissection extends laterally to the medial aspect of the vertebral artery, then the surgeon can use that landmark as a guide as well. We found the average distance from that point to our entry site to be 14mm. These values, like the ones set forth by Xu et al,(15) have the drawback of not taking into account individual variations, and for that reason should not be used blindly. They should be used as an adjunct when either direct visualization is not possible or the regional anatomy is distorted.
The C2 pedicle provides a good anchor for occipitocervical stabilization constructs and can be used safely is the surgical anatomy of this region is fully appreciated. Such appreciation cannot be obtained through algorithmic approaches. Adequate preoperative studies in conjunction with direct visualization of the C2 pedicle enables the surgeon to tailor the screw placement with minimal risk to the patient and makes transpedicular screw fixation safe and effective.