Abstract
Background There are currently no studies that directly compare the previously established 2-dimensional (2D) sagittal technique with 3-dimensional (3D) templating for C2 pedicle screw.
Objective To verify the accuracy of sagittal radiological analysis for safe placement of a C2 pedicle screw by performing a direct comparison between 2D planning with 3D templating methods.
Methods In this retrospective analysis, forty-six sets of computed tomography scans that contained 2-mm bony cuts and 2D reconstructions in the axial, sagittal, and coronal planes of skeletally mature patients were analyzed. StealthStation S7 (Medtronic Surgical Navigation, Minneapolis, Minnesota, United States) trajectory planning was used to plan the ideal placement, maximum diameter pedicle screw into the C2 pedicle. Based on the parameters of ≤3 mm screw diameter as high risk, >3 mm and <5 mm as moderate risk, and ≥5 mm as low risk, frequency and percentage values were calculated for the left, right, and bilateral pedicle screws.
Results Out of the 46 patients analyzed in this study, only 1 patient (2.2%) was classified as low risk (≥5 mm) bilaterally, 5 were classified as high risk (≤3 mm) bilaterally (10.8%), and 25 patients (54.3%) showed variability in pedicle width between the left and right sides. With analysis of both left and right pedicle, 7 out of 92 pedicles (7.6%) analyzed were classified as low risk (≥5 mm), 67 out of 92 (72.8%) were at moderate risk (>3 mm and <5 mm), and 18 out of 92 (19.6%) were at high risk (≤3 mm).
Conclusion Both the previously described 2D sagittal planning method and the current 3D templating method allow for accurate preoperative planning for the placement of ≤4 mm C2 pedicle screws, which is important given the limited availability and amount of resources utilized for the 3D templating model. However, the 3D templating method more precisely identifies C2 pedicles where 3.0 to 4.5 mm screws can feasibly be placed.
Level of Evidence 3.
Introduction
Numerous methods of posterior surgical fixation of the C2 vertebra have been described, primarily for the purpose of occipitocervical and C1 to C2 fusion.1,2 These methods include sublaminar wiring, hooks, clamps, transarticular screws, lateral mass screws, and pedicle screw fixation. Pedicle screw fixation has been shown to have increased construct stiffness and pullout strength in comparison to wire fixation and lateral mass fixation.3–5 The ability to perform segmental fixation offers a distinct advantage in the preservation of motion and fusion levels. The unique anatomy and variability of C2 make instrumentation challenging, as there is a significant risk of vertebral artery and neurological injury.6,7 There have been reports of up to 18% of individuals with a high-riding foramen transversarium, placing the vertebral artery at significant risk.8
There are numerous methods of clinically and radiologically evaluating the C2 pedicle to determine its suitability for pedicle screw placement, most of which involve digital templating software to preoperatively plan screw trajectory.9–14 This complex, expensive software may not be readily available to all surgeons for preoperative planning.
We previously described a 2-dimensional (2D) technique for evaluating the C2 pedicle for placement of a screw using commonly available sagittal reconstruction computed tomography (CT) images.15,16 The study revealed high intraobserver reliability scores in determining the risk of placing a C2 pedicle screw, and the clinical data confirmed the safety of using this technique. A potential limitation of this approach is the dependence on the surgeon’s clinical judgment of pedicle height, width, and angulation, especially in relation to the vertebral artery foramen.16 Resnick et al and Yoshida et al introduced 3-dimensional (3D) templating as a viable technique to preoperatively plan for C2 pedicle screw placement.9,10 This process involves the use of a computer-assisted navigation system to reconstruct (3D) images to visualize the anatomy of the pedicle and outline screw trajectories in multiple planes, which may enhance the surgeon’s ability to safely place a C2 pedicle screw.
There are no previous reports of a direct comparison of 2D planning to 3D templating. Thus, directly comparing the 2 techniques may identify the superiority of 1 technique over the other in determining the feasibility of safe placement of a C2 pedicle screw. The purpose of this study was to verify the accuracy of sagittal radiological analysis for safe placement of a C2 pedicle screw by performing a direct comparison between 2D planning with 3D templating methods.
Methods
Forty-six sets of standard screening cervical CT scans of skeletally mature patients were used. These CT scans contained 2-mm bony cuts and 2D reconstructions in the axial, sagittal, and coronal planes. These CT scans were previously used to determine the risk of placing a C2 pedicle screw using the 2D radiological analysis of the sagittal CT reconstruction views using the technique described by Marco et al.16 In this technique, the lateral-most aspect of the spinal canal on the left and right sides is identified separately. The medial wall cut of the pedicle is then identified by the finding of complete bony continuity between the vertebral body and the lamina.
The total number of sequential 2-mm slices from the medial wall of the pedicle up to, but not including, the vertebral artery foramen cut is then recorded. A sagittal image is considered to contain the foramen when a full semicircle of the vertebral artery foramen is found. A full semicircle is considered present when its height is greater than or equal to its radius. Passage of a pedicle screw is considered at low risk of vertebral artery injury when 3 slices (ie, 6 mm of pedicle width) lateral to the canal without the presence of a semicircle of the vertebral artery foramen. Pedicles with 0 to 1 slice showing bony continuity before visualizing the semicircle of the vertebral artery foramen are considered at high risk of vertebral artery injury, and pedicles with 2 slices showing bony continuity before visualizing the semicircle slice are considered at moderate risk of injury. The figure below is an example of the evaluation of the C2 pedicle of a patient with at least 4 slices from the medial wall slice before the semicircle slice of the foramen is visualized, thus indicating a low risk of vertebral artery injury.
Our technique (Figure 1) set parameters for pedicle screw placement as follows:
Series of sagittal computed tomography cuts from the spinal canal through the pedicle. This patient has 4 slices of bony continuity that do not show the vertebral artery foramen. Thus, slices 1 through 4 represent the safe zone where a long pedicle screw can be placed. Reproduced/adapted with permission from Marco et al. Radiological analysis of C2 to predict safe placement of pedicle screws. Int J Spine Surg. 2018;12(1):30–36. https://doi.org/10.14444/5006.
CT slice (2 mm): High risk
CT slices (4 mm): Moderate risk
CT slices (6 mm): Low risk
StealthStation S7 (Medtronic Surgical Navigation) trajectory planning was utilized in order to plan the ideal placement of the maximum-diameter pedicle screw into the C2 pedicle for each of the CT scans. Left and right screw diameter measurements were recorded, and parameters were set defining screw placement as high risk/unacceptable, moderate risk, and low risk/safe. These are defined as:
Possible screw diameter ≤3 mm = High risk/unacceptable
Possible screw diameter >3 mm and <5 mm = Moderate risk/“risky”
Possible screw diameter ≥5 mm = Low risk/safe
Frequency and percentage values were calculated based on left and right pedicles separately (92 total pedicles) and bilateral pedicle screws (46 patients). Fisher’s exact test was then utilized to provide P values, with P < 0.05 considered significant, in order to provide a direct comparison of the results from the current study to the results from the 2D planning, which also defined parameters of screw placement as high risk, moderate risk, and low risk based on sagittal reconstruction CT scan cuts (Figure 1).
(A) Three-dimensional (3D) StealthStation S7 (Medtronic Surgical Navigation) trajectory planning shown with left and right screw placement. (B) Sagittal (left) and coronal (right) views of the same screws placed in different planes.
Results
We found that only 2.2% (1/46) of the patients had sufficient pedicle width bilaterally to permit the safe (low risk) placement of a pedicle screw ≥5 mm in diameter (Table 1). A total of 10.8% (5/46) were found to be at high risk bilaterally, with a possible screw diameter of ≤3 mm. A total of 54.3% (25/46) showed variability in pedicle width between the left and right sides. There were no significant differences in identifying low- and high-risk pedicles bilaterally between the 2 planning methods.
Comparison of C2 pedicle risk profiles between the 2D and 3D techniques in bilateral pedicle screw placement.
Of the total pedicles analyzed, a pedicle diameter ≥5 mm (low risk/safe) occurred in 7.6% (7/92), whereas 72.8% (67/92) were at moderate risk with a pedicle diameter >3 mm, but <5 mm (Table 2). Lastly, 19.6% (18/92) were at high risk/unacceptable, measuring ≤3 mm. These results were all significantly different compared with the prior 2D planning method.
Comparison of C2 pedicle risk profiles between the 2D and 3D techniques in pedicle screw placement.
Discussion
The goal of this study was to verify the accuracy of sagittal radiological analysis for safe placement of a C2 pedicle screw. Our previously described technique of sagittal radiological analysis involved evaluation of standard 2 mm sagittal CT scan images taken through the C2 vertebra.16
This previous technique leaves some room for error by defining ≥4 mm as moderate risk, which allows for placement of 4.0 mm or smaller screws. However, it is limited by the 2mm- CT cuts and the inability to precisely define a >3 mm and <5 mm pedicle diameter that can accommodate 3.0 to 4.5 mm screws. Considering the routinely available smallest and largest multiaxial cervical pedicle screw size ranges from 3.0 to 4.5 mm, we modified our definition of moderate risk to >3 to <5 mm in diameter to be more objective and clinically applicable. Additionally, we chose 3 to 5 mm to accommodate a 4mm- screw safely because for most patients, choosing 3.5 is somewhat awkward.
Using these new parameters in this study, by utilizing the 3D trajectory planning/templating software, we were able to obtain a more exact determination of possible pedicle screw size for each of the 92 pedicles analyzed.
In our comparison, we found that a lower percentage of pedicles were “high-risk.” A total of 19.6% (18/92) were found to be high risk for pedicle screw placement, measuring ≤3 mm, compared with 34% (31/92) from the previous study. Additionally, 10.8% (5/46) of patients were found to be at high risk bilaterally, as compared with 15% (7/46) in our previous study. This can potentially be explained by the more detailed and multidimensional abilities of the Stealth templating software. It allows for more overall pedicle screws to be placed. With a higher number of pedicles in the original study being deemed as high risk, the sagittal radiological analysis technique potentially provides a safer (more cautious) prediction of screw placement, as 1 would not likely attempt to place a pedicle screw if it was determined to be “high risk” (unable to place a screw of at least 3.5 mm diameter) preoperatively. Alternatively, a shorter pars screw can be placed in these patients.
In contrast to our findings, Resnick et al9 found that 9% of pedicles were at high risk for vertebral artery injury during placement of a C2 pedicle screw, and Yoshida et al10 found that 10% of the pedicles that they analyzed with the Stealth system were at high risk for vertebral artery injury. Resnick et al9 and Yoshida et al10 reported an overall feasibility rate of 91% and 90%, respectively, for the placement of pedicle screws. This is an increase from the previous Paramore et al17 study, which found that 18% to 23% of patients had a high-riding transverse foramen, which made screw placement not possible. Resnick et al9 attributed this difference to their ability to manipulate these 2D images and their proposed safe zone for screw placement of 4 mm. Our current study utilized a 5-mm pedicle width as a parameter for “low risk/safe” screw placement. Another point to consider is that these studies only considered whether there would be any neurological or vascular damage, rather than whether the trajectory was actually a viable plan.9
The current study revealed that 72.8% (67/92) of pedicles were at moderate risk of pedicle screw placement, meaning nearly 3 quarters of the pedicle screws were measured between 3 and 5 mm. This is significantly different than the 38% (35/92) moderate-risk pedicles from our initial study. This finding also relates to the more precise nature of the Stealth templating. It is likely that the majority of C2 pedicle screws, whether 3.5, 4.0, or 4.5 mm, are placed into pedicles that would be deemed as moderate risk in the current study as well as our original study. The 3D templating method offers no distinct advantage over the sagittal radiological analysis method in the placement of moderate-risk pedicle screws. With either method, the realization that the pedicle is at “moderate risk” of vertebral artery injury and the subsequent intraoperative techniques is the most critical portions of screw placement. Particular attention to the entry point of the screw and screw trajectory (cephalad and medial screw orientation following the patient’s medial and superior pedicle border) is of utmost importance during drilling and screw placement to avoid violation of the vertebral artery foramen. There are multiple descriptions and methods of determining the actual entry point and drill trajectory.2,18
Ultimately, the choice of technique should be based on patient anatomy and surgeon preference as opposed to any single/dogmatic approach. Both sagittal radiological analysis and 3D templating allow satisfactory and accurate identification of those pedicles at risk for vertebral artery and neurological injury during pedicle screw placement. As before, the sagittal radiological analysis has been shown to be readily available and reproducible. It can also be considered comparable to the more complex templating software in preoperatively determining the pedicles that are at significant risk for injury.
There are a few different limitations to this study. The first is the small sample size of patients, 46 CT scans, used for the comparison. Even though the number analyzed is less than the other studies (Resnick et al and Yoshida et al) that utilized 60 and 62 patients, respectively, our study still provides us with enough information to make the comparison between the 2D and 3D methods. Another limitation to the study is that clinical outcome measurements were not analyzed, and the study was entirely radiologically based.
Despite these limitations, to our knowledge, this is the first study to verify the accuracy of sagittal radiological analysis for safe placement of a C2 pedicle screw. The verification of similar results between the 2D and 3D techniques is important given the limited availability and amount of resources utilized for the 3D templating model. By establishing the accuracy of our previous 2D model, this technique may continue to be considered for the planning of C2 pedicle screw placement.
Conclusion
Both the previously described 2D sagittal planning method and the current 3D templating method allow for accurate preoperative planning for the placement of ≤4 mm C2 pedicle screws. However, the 3D templating method more precisely identifies C2 pedicles where 3.0 to 4.5 mm screws can feasibly be placed.
Footnotes
Funding The authors received no financial support for the research, authorship, and/or publication of this article.
Disclosures and Conficts of Interest Rex Marco has the following disclosures: DePuy, A Johnson and Johnson Company: Paid presenter or speaker; Globus Medical: IP royalties.
- This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2025 ISASS. To see more or order reprints or permissions, see http://ijssurgery.com.
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