Radiographic feasibility of novel cervical pedicle inlet screw placement during posterior cervical fusion

Introduction

Posterior cervical instrumentation is widely implemented in modern spine surgery for various indications including degenerative, traumatic, infectious, or neoplastic causes. Lateral mass screw fixation is the most commonly utilized method for posterior cervical fusion. Advantages of lateral mass screws include avoidance of neurovascular structures and ability to use free-hand techniques with acceptable accuracy (1). The Magerl and Roy-Camille techniques have been consistently validated as safe methods of lateral mass screw placement without the need for navigation (2). Despite the popular use of lateral mass fixation, screw loosening or pullout, particularly at the upper or lower end of the construct, is the most common complication observed with lateral mass screws (3). Factors related to lateral mass screw failure include limitations in screw size, bone quality, and facet joint violation.

Traditional cervical pedicle screw placement provides superior biomechanical advantages in comparison to lateral mass screws (4,5). Adoption of cervical pedicle screw fixation has been limited by the elevated risk of neurovascular injury due to the narrow transpedicular corridor (6). In addition, traditional cervical pedicle screws may not be feasible given the small pedicle size in the cervical spine or an aberrant course of the vertebral artery. The “key slot” technique, described by Lee et al., describes a free-hand technique of traditional pedicle screw placement (7). While navigation has improved the accuracy of cervical pedicle screw placement, the incidence of malposition in the literature still remains up to 20.5% (8). While some still argue for the use of traditional cervical pedicle screw placement, particularly in cervical deformity, lateral mass fixation is the mainstay trajectory for posterior cervical instrumentation.

In 2021, Gelinne et al. published a small case series of three patients describing a novel screw trajectory in the subaxial cervical spine (9). The cervical pedicle inlet screw has a similar trajectory of a traditional pedicle screw but stops short of the pedicle isthmus, thus limiting the risk to the neurovascular structures. In 2025, Martin and colleagues investigated the biomechanical advantages of this technique in cadaveric models. Their findings demonstrated a 51% greater pullout strength in comparison to lateral mass screws with 100% accuracy when using navigation for placement (10). The superior pullout strength may be related to the bicortical purchase available at the pedicle isthmus.

The cervical pedicle inlet screw is a novel, promising trajectory that may provide superior biomechanical advantage in comparison to lateral mass screws while limiting the risk of neurovascular injury. However, the feasibility of its use has not been investigated in clinical practice. This study aims to radiographically investigate the feasibility of cervical pedicle inlet screws in the subaxial cervical spine. We present this article in accordance with the STROBE reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-2025-aw-218/rc).

Methods

This is a retrospective cohort analyzing patients consecutively undergoing posterior cervical fusion for cervical myelopathy. Surgical intervention was performed by multiple surgeons at University of Kansas Medical Center. Patients were collected from 2022 to 2024. Patient data including demographic, operative, and radiographic films were collected. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This retrospective analysis was approved by the University of Kansas Medical Center Institutional Review Board (No. IRB#00150790). Informed consent was not necessary due to the retrospective nature of this study.

Using a 3D spine navigation workstation (Brainlab, Munich, Germany) cervical pedicle inlet and traditional pedicle screw trajectories were manually planned on post-operative computed tomography (CT) from C3 to C6. Post-operative imaging was utilized to make direct comparisons with actual lateral mass screws placed during surgery. C7 was excluded due to the widely accepted use of traditional pedicle screw instrumentation given the large pedicle size afforded at this level. An example of the trajectories is provided in Figure 1. The trajectory for the traditional cervical pedicle screw placement was undertaken using previously reported techniques in the literature (5). The starting position for screw placement was made at the supero-lateral portion of the lateral mass with an inferior-medial trajectory towards the anterior cortex of the vertebral body. The cervical pedicle inlet screw trajectory follows the same course but terminated at the pedicle isthmus before breaching the pedicle. Trajectory feasibility was determined if the proposed screw was entirely contained within the cortical boundaries on 3D reconstruction. Traditional cervical pedicle screws were deemed not feasible if the pedicle size was less than 3.5 mm in greatest diameter or within 1 mm of the vertebral artery. Trajectories were placed using multi-planar views including axial, sagittal, and in-line pedicle views.

Figure 1 Example trajectories of a traditional cervical pedicle screw (right) and cervical pedicle inlet screw (left) in the axial plane.

Data including screw length and diameter, medialization angle, and distance from screw placement to the midline was collected. Optimal screw selection was determined using intervals of 0.5 mm for diameter and intervals of 2 mm for screw length. The proposed cervical pedicle inlet screws were compared to the actual lateral mass screws placed at the index surgery. The authors of this study hypothesize that pedicle inlet screw trajectories will be universally feasible and allow for larger screw placement in comparison to lateral mass screws placed at the index surgery. No patient-level outcomes were reported in this study due to the nature of this study exploring radiographic feasibility.

Statistical analysis

Continuous data is presented as the mean with standard deviation. Categorical data is presented as the proportional percentage. The Pearson’s Chi-squared test was used to assess the difference between proportions in categorical variables. T-test was used to assess differences between continuous variables. To assess significant differences of continuous data between multiple groups, analysis of variance (ANOVA) was utilized. Simple linear regression was utilized to determine the relationship between cervical level and pedicle inlet screw size. Non-parametric variables were compared using the Mann-Whitney U test. P value <0.05 was considered statistically significant. The data was analyzed using the International Business Machines (IBM)^® Statistical Package for the Social Sciences (SPSS)^® software (IBM, New York, U.S.).

Results

Patient data is listed in Table 1. Thirty patients (60% male, mean age: 65.5±7.7 years) were included in this study. Nineteen (63.3%) patients were Caucasian. Average body mass index was 31.2±4.8 kg/m². All patients underwent posterior cervical decompression and fusion for cervical myelopathy. The average number of instrumented levels was 4.6±1.2. Lateral mass screws were most frequently 3.5 mm × 14 mm (60%), 3.5 mm × 12 mm (30%), and 4 mm × 14 mm (9%). Two hundred and thirty-four total trajectories (117 bilaterally) were planned from C3 to C6. Trajectories at the C6 level were excluded in three patients due to image quality.

Traditional pedicle screw placement

Traditional pedicle screws ranged from 3.5 mm × 18 mm to 3.5 mm × 32 mm. The most frequent screw placement was 3.5 mm × 26 mm (40.1%), 3.5 mm × 24 mm (27.8%), and 3.5 mm × 28 mm (20.1%). Traditional pedicle screw placement was not feasible in 69 (29.5%) trajectories. Seventeen trajectories were due to aberrant vertebral artery anatomy. Fifty-two trajectories were not possible due to a pedicle diameter of less than 3.5 mm.

Pedicle inlet screw placement

Pedicle inlet screws ranged from 4.0 mm × 14mm to 4.5 mm × 24 mm. The most frequent pedicle inlet screw placed was 4.5 mm × 16 mm (34%), 4.5 mm × 14 mm (33%), and 4.5 mm × 18 mm (17%). Pedicle inlet trajectories were feasible in all 234 planned screws (117 levels bilaterally). The average distance from midline to the starting point of the trajectory for both traditional pedicle and pedicle inlet screws was 26.9±2.7 mm. The average medialization angle was 42.6°±5.2°. Average pedicle inlet screw length by cervical level is demonstrated in Figure 2. Average pedicle inlet screw significantly differed by cervical level (C3: 14.7±1.8 mm, C4: 15.7±2.2 mm, C5: 16.1±2.2 mm, C6: 16.9±2.3 mm; P<0.01) via ANOVA analysis.

Figure 2 Pedicle inlet screw length by cervical level. Data is presented as the mean with standard deviation. A trendline from the linear regression analysis is displayed.

Simple linear regression was used to determine the relationship between the cervical level and pedicle inlet screw length. The fitted regression model was: screw length =0.6731 (level) + 14.16. The overall regression was statistically significant (R²=0.11, F(1, 232) =28.83, P<0.001). Our analysis demonstrated that the more caudal level was a modest but significant predictor of pedicle inlet screw length (β=0.33, SE =0.13, P<0.001).

Comparative analysis

All pedicle inlet screws exceeded lateral mass screws in diameter. Sixty-six percent of pedicle inlet screws were longer in screw length which was statistically significant (P=0.011). Screw length was longer in all traditional pedicle screw trajectories in comparison to pedicle inlet screws. Fifty-four percent of pedicle inlet trajectories were larger in screw diameter in comparison to traditional pedicle screw placement which was statistically significant (P=0.02).

Discussion

This is the first radiographic analysis to assess the clinical feasibility of the novel cervical pedicle inlet screw trajectory. Pedicle inlet screws were universally feasible in all patients. In contrast, 29.5% of traditional pedicle screw trajectories were not possible due to pedicle size constraints or aberrant vertebral artery anatomy. The pedicle inlet trajectory consistently allowed for larger screw dimensions in comparison to lateral mass fixation. While traditional pedicle screws were longer, 54% of cervical pedicle inlet screw trajectories were larger in diameter. This finding is likely due to avoidance of the tapering width of the cervical pedicle by stopping at the pedicle isthmus. In addition, a trend was observed for longer pedicle inlet screws at more caudal levels. While statistically significant, this finding does not imply predictive utility in surgical planning of pedicle inlet screws. This finding differs from lateral mass fixation where it is typical to use the same screw dimension at all cervical levels.

While demonstrating suboptimal rates of screw loosening or pullout, lateral mass screws still continue to be the most widely utilized method of posterior cervical fixation. This is largely due to the technical feasibility of free-hand techniques and lower risk of neurovascular injury. Complications due to this include additional hardware failure, pseudoarthrosis, and iatrogenic deformity. These downstream sequalae inevitably lead to revision surgery and worse patient outcomes.

Traditional cervical pedicle screw placement has been shown extensively in the literature to have superior biomechanical advantage regarding pullout strength in comparison to lateral mass screws (11). While some have adopted this technique, its routine use has been limited by an elevated risk of neurovascular injury, which may be up to 9% (12). While rare, this complication can result in devastating consequences. Even with the use of navigated techniques, this trajectory may still not be possible due to patient anatomy highlighted by our study.

Our findings demonstrate that pedicle inlet screws are similar in radiographic feasibility to lateral mass fixation. With prior evidence in the literature demonstrating a biomechanical advantage (10), we believe this is a potential solution to mitigate the risk of screw failure with lateral mass fixation while limiting risk of neurovascular injury. This trajectory may also serve as a reliable bailout strategy for failed lateral mass fixation. An additional advantage of cervical pedicle inlet screw is demonstrated with the starting position on the lateral mass. The starting point, in comparison to lateral mass fixation, mitigates alignment concerns in long constructs when instrumenting into the craniocervical junction or proximal thoracic spine.

While navigation-assisted screw placement is recommended for pedicle inlet trajectories currently, future studies to describe a potential free-hand technique are necessary. The authors posit that a free-hand technique is possible due to less risk of medial or lateral pedicle breach. In addition, the superior biomechanical advantage of pedicle inlet screws may limit additional levels of instrumentation necessary with lateral mass fixation, although further studies are necessary to investigate this theory. Lastly, further clinical data with long-term follow-up remains absent in the literature.

Limitations

There are several inherent limitations in the present study. The retrospective, single-cohort nature of our study design prevents some generalizability of our findings. In addition, our sample size is appropriately powered but may benefit from a larger patient cohort. While 3D planning navigation software has been shown to be highly accurate, this study fails to take into consideration intraoperative factors including surgeon skill, surgeon-specific preferences, radiation exposure, and operative time. In addition, post-surgical findings including laminectomy and instrumentation may influence the pedicle inlet trajectories. These factors may affect the feasibility of pedicle inlet screw placement in real clinical practice which is an inherent limitation of the study. However, the authors believe that demonstrating pedicle inlet screw feasibility will provide helpful information to promote the adoption of this novel trajectory and future research efforts.

Conclusions

This is the first radiographic analysis to assess clinical feasibility of the novel pedicle inlet screw trajectory. Pedicle inlet screws were universally feasible and consistently allowed for larger screw dimensions in comparison to lateral mass fixation. Combined with prior evidence of biomechanical superiority, these findings support pedicle inlet screws as a feasible and potentially advantageous alternative for subaxial cervical fixation.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-2025-aw-218/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-2025-aw-218/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-2025-aw-218/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-2025-aw-218/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This retrospective analysis was approved by the University of Kansas Medical Center Institutional Review Board (No. IRB#00150790). Informed consent was not necessary due to the retrospective nature of this study.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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