Comparison of perpendicular to the coronal plane versus medial inclination for atlas pedicle screw insertion: an anatomic and radiological study in human cadavers

The technique of C1 pedicle screw placement was initially developed by Resnick and Benzel [17]: the screw was inserted through the posterior arch or “pedicle” into the lateral mass. Since then, multiple studies have investigated the anatomy and feasibility of C1 pedicle screw placement [10‐12, 18‐22]. Most investigations insisted that the mediolateral width at the isthmus of the C1 pedicle was large enough to insert a 3.5-mm diameter screw [11, 12]. In fact, these authors measured the external pedicle wall width, including the cortex thickness and medullary cavity width of C1 posterior arch. Gebauer et al. [12] reported that the width of the posterior arch at the vertebral artery (VA) groove was 9.8 ± 1.7 mm for males and 8.9 ± 0.7 mm for females. Meanwhile, Ma et al. [11] found that it was on average 8.46 ± 0.57 mm. In our research, external pedicle wall width along trajectory P was 9.3 ± 1.5 mm for males and 7.0 ± 1.6 mm for females, also greater than 3.5-mm or 4.0-mm screw diameter, in agreement with the studies published by Gebauer et al. and Ma et al. The external pedicle wall width along trajectory I was even greater than that along trajectory P, due to larger available mediolateral width.

However, it has been demonstrated that the stability of cervical pedicle screw largely depends on the internal characteristics of the pedicle, and not so much on its external dimensions [23]. Unfortunately, reports on the C1 pedicle’s medullary cavity width are scarce. In recent years, multiplanar CT renders possibility of the precise measurement of the inner as well as outer pedicle diameters [24]. Measurement of C1 pedicle’s medullary cavity width on axial CT image is becoming more and more important for choosing an optimal trajectory, and evaluating the safety of C1 pedicle screw trajectory.

Different from sub-axial cervical vertebra, the vertebral canal of C1 consists of two parts, which contains the odontoid process and the spinal cord. So we designed two trajectories named trajectory P and trajectory I for C1 pedicle screw insertion. As demonstrated above, the rate of lateral wall perforation was higher along trajectory P (29.4 %) than trajectory I (8.8 %). Two reasons could explain these differences. First, the medullary cavity width along trajectory P is lesser than that along trajectory I by 1.4 mm; in addition, medullary cavity width <3.5 mm was found in 16.1 % of pedicles along trajectory P, and only 2.9 % along trajectory I. Another reason is that the lateral wall of C1 pedicle is thinner than the medial wall, similar to sub-axial cervical vertebra [23‐25]. As a result, the lateral wall perforation is more frequent than the medial one. If trajectory P is not wide enough to contain a pedicle screw, trajectory I would be a better choice. In fact, any trajectory with entry points from trajectory P to trajectory I and introversion direction from parallel to C1 midline to parallel to the lateral border tangent to the odontoid foramen and vertebral canal of C1 could insert pedicle screw. If a trajectory with small transverse angle could not contain a 3.5-mm diameter pedicle screw, more adequate introversion of the transverse angle with more lateral entry points could make a wider medullary cavity width in order to minimize the risk of lateral wall perforation. The individual maximum medial inclination angle might be transverse angle of trajectory I measured pre-operatively on CT images.

Numerous studies have indicated that the limited superior-inferior height of C1 posterior arch under the vertebral artery groove restricts C1 pedicle screw application [18‐20]. If the height is less than 3.5 or 4.0 mm screw diameter, the superior or inferior wall would be penetrated during screw placement. Our research demonstrated that the superior-inferior height of the posterior arch under the vertebral artery groove was 4.3 ± 0.9 mm, which was similar to data reported by Ma et al. [18] and Qian et al.[19] The superior-inferior height of the posterior arch <3.5 mm screw diameter was found 22.0 % (15/68) of pedicles. However, the main handicap to C1 pedicle screw insertion was injury of the VA running along the vertebral artery groove, or venous plexus and C2 nerve root. Deviant direction of pedicle screw insertion on sagittal plane would result in penetration of the posterior arch even with superior-inferior height of the posterior arch larger than screw diameter. We took a measurement to prevent injury of the neurovascular structure. Superior and inferior wall of C1 posterior arch could be subperiosteally dissected. Then, the direction of C1 pedicle on sagittal plane was evident. Drilling holes along the C1 pedicle trajectory and screw insertion could be performed following this direction.

Many researchers agreed that the C1 posterior arches could be divided into four different categories: superficial groove, deep groove, semi-foramen–shaped, and foramen-shaped [18, 21]. In the latter three categories, the superior wall of the posterior arch is difficult to be subperiosteally dissected. However, we found that the superior wall of the C1 posterior arch was flat along trajectory I and groove-shaped along trajectory P, likely because the C1 lamina became gradually thin laterally (Fig. 3). This was in agreement with Kim et al. [22], who showed that orientation of C1 lamina was changed from vertical to horizontal direction in the posterior view. Therefore, exposure of the superior wall of the C1 posterior arch by subperiosteum dissection was easier along trajectory I than trajectory P. Inferior wall of C1 posterior arch could also be subperiosteally dissected. They were useful to determine sagittal direction of pedicle screw insertion and avoid inferior or superior wall perforation.

Several limitations of the current study should also be mentioned. Limited to the number of cadavers, the data of the study are primarily used only as a reference. More measurements could be taken to minimize the deviation of C1 pedicle screw insertion from preoperatively determined direction on CT images. In addition, clinical researches need to be performed on the safety and possibility of pedicle screw insertion along different trajectories.