Radiological studies for the best position and trajectory of the anterior cervical pedicle screw in the lower cervical spine in normal populations

Authors: Panlop Tirawanish, Werasak Sutipornpalangkul, Monchai Ruangchainikom

Abstract

Background

Anterior cervical transpedicular screw (ACTPS) surgery has gained popularity due to its strong biomechanical fixation via a single anterior approach, especially in cases like osteoporosis requiring three-column stabilization. However, precise entry point and trajectory are essential, as malpositioning can cause serious complications such as spinal cord or vertebral artery injury. This study aims to evaluate a novel preoperative planning program designed to improve accuracy in locating the optimal entry point and trajectory for ACTPS.

Methods

This study included normal patients who underwent cervical spine computed tomography (CT) scans. All pedicles of the lower cervical spine were measured in length, diameter, angle of trajectory in the sagittal, coronal, and axial planes after finding the best position for the placement of the pedicle screw. The entry point was identified and classified into grid zones (zones 1 to 4) on the anterior surface of the vertebral body.

Results

In the axial plane, the safe zones for the inserted pedicle screw were in zone 2 and zone 3. The entry points screws started with the mean lengths of the distance from the midline axis being 1.7±0.5 mm and the mean angulations of the entry points of the screws were 46.2°±2.6°. The screw lengths were longer in men (32.9±1.8 mm) than in women (30.7±1.6 mm) all cervical levels and the screw lengths at C6–C7 levels were gradually longer than the levels of C3–C5. In the sagittal plane, the best entry points of all ACTPS trajectories started from the upper half of the vertebral body. The trajectory of the screws was nearly parallel (sagittal angulation mean 1.4°±0.6°) to the anterior boarder of the upper vertebral endplate, incline from the caudal (C3) to the cephalad (C7). In the coronal plane, the average diameters of the pedicle gradually increased from C3 to C7. The larger pedicle diameter was found in men (3.3±0.3 mm) than in women (3.1±0.2 mm) at all cervical levels with statistical significance.

Conclusions

The optimal entry points for ACTPS in the lower cervical spine are located on the midline axis and near the superior vertebral border of the vertebral body, with trajectory angulations in the axial plane of 46.2°±2.6° and nearly parallel to the superior vertebral border in the sagittal plane, at 1.4°±0.6°. The pedicle screw lengths range from around 31.8±1.7 mm. ACTPS is a feasible option for lower cervical spine fixation. Our CT-based measurements, which include essential anatomical parameters of the cervical spine, will assist surgeons in performing this procedure.

Introduction

In multilevel cervical spine reconstructive surgery, pseudarthrosis is the most common major complication, especially in patients with osteoporosis or three-column injuries. From the review of the literature, the rates of pseudarthrosis in multilevel corpectomy were in the range of 30–100% due to their weak biomechanics reconstructive fixation (1,2). Therefore, these patients need posterior cervical fixation or postoperative external fixation such as immobilization of the halo vest to prevent early reconstruction failure. Posterior cervical fixation has many disadvantages and surgical risks, such as increased postoperative pain from the second surgical approach, higher incidence of surgical infections, more blood loss, and increased operative time. In addition, immobilization of the halo vest has many complications such as pin tract infections, dysphagia, neck stiffness, and other morbid effects on patients.

Nowadays, many spine surgeons are interested in transpedicular cervical techniques from the anterior approach of the cervical spine. After Aramomi* et al. (3) presented their fixation techniques, anterior cervical transpedicular screw (ACTPS) fixation has been used as the feasible technique in multiple level cervical spine injuries. There is a great deal of literature on cadaveric anatomy that supports the advantages of excellent biomechanics of the ACTS system (4-7). Koller et al. *series (6,8) studies in anatomical, biomechanical properties and clinical applications of ACTPS fixation revealed that ACTPS has the ultimate pull-out strength more than the vertebral body screw (VBS) system significantly. Furthermore, the ACTPS system is safe and has clinical feasibility via the anterior cervical approach. In addition to these, there was a literature in cadaveric studies showing that the pull-out strength of ACTPS was approximately 2.5 times that of VBS and the immediate stability in 2 level corpectomy cervical reconstruction was superior to the anterior plate with VBS fixation and the same as the posterior fixation of the transpedicular screw (TPS) (9). This technique is worth studying, especially for patients who have multilevel cervical disease with three column injuries or osteoporosis. Despite its advantages, there were serious intraoperative complications of pedicle screw fixation, such as spinal cord or vertebral artery injuries (10,11). Therefore, the precision of the pedicle screw trajectory is very important. In the past, the accuracy rate for pedicle screws fixation was only 78.3% in the conventional manual transpedicular technique and only 86.9% in the fluoroscopic assisted pedicle axis view technique (12). Subsequently, there was a computer-assisted navigation (CAN) technique to improve the results of the screw position. There were many publications that supported computed tomography (CT) navigation to have higher precision compared to fluoroscopy or virtual fluoroscopy navigation assistance (13,14). The study that compared the accuracy of fluoroscopy and CAN for the placement of ACTPS in the cadaveric experience showed that the CAN-guided technique had an accuracy rate superior to the fluoroscopic-assisted technique (66.7% and 42.6%) (15).

Currently, O-arm navigation plays an important role in spinal operations to improve the accuracy of screw fixation and decrease the operative time. Chachan* et al. *reported the accuracy of cervical screws fixation in 92–95% without grade III screw breaches in the Gertzbein classification (16).

Several publications studied the entry points and the trajectory of the screws in cervical spines. Yukawa* et al. (12) showed anatomical parameters with angulations between ACTPS and the anterior boarder of vertebrae in the range of 35°–50°, tilted angulations in sagittal view were 45° at C3–C6 and 40° at C7 vertebrae, and the entry points of ACTS were typically about 2 mm in surface area below the upper end and central to the transverse plane. In this literature, there were no studies on the length of pedicle screws, the entry point zone of the screws, or the largest diameter of pedicle screws along the coronal plane. In 2014, Zhao et al. (17) studied cervical parameters from three-dimensional (3D) reconstruction CT, the results consisted of the angulations between ACTPS to the anterior boarder of the vertebrae in a range of 38°–45° with gradually increasing and decreasing C5–C7, the angulations inclined between ACTPS to the anterior boarder of the vertebrae in the sagittal view were −8.9° (cranial tilted) to 8.5° (caudal tilted), the pedicle screws in the axial and sagittal view were 32.4–35.2 mm without statistical significance of the gender, the zone of the entry points of screws usually started from zones 2 or 3 in the axial view and the zone of the entry points of screws usually started from zones 1 or 2 in the sagittal view. There were no studies of distance from the midline axis and the largest diameter of pedicle screws along the coronal plane in this literature. In 2018, Li et al. *(18) presented cervical anatomical parameters from ACTPS CT navigation in a cervical 3D printing model, the results showed the axial angulation approximately 40°, the sagittal angulation approximately 40°, the length above 30 mm, and the transpedicular screws pedicles were greater than 3.5 mm. In each screw trajectory, there was an adjustable range of approximately 5° on the sagittal surfaces and 4° on the axial surfaces. There were no studies of distance from the midline axis and zone of entry points of screws in this literature.

Due to a small anatomical landmark on the anterior surface of the cervical vertebrae, it is difficult to find the best entry points for screws with the longest and largest diameter to pass through the pedicle to achieve the strongest biomechanical fixation, but to minimize the risks of major structural injuries and decrease the operative time, it is important to find one. Therefore, the objective of our study is to find the best entry points and ACTPS path through IplanWorkstaionIGS 3.0 software^®^ (Brainlab Company, Munich, Germany) with the Brainlab navigation program to apply in the O arm navigation in lower cervical spine surgery in real clinical patients. Additionally, our study was also designed so that the study consisted of factors related to general populations.

This study aims to evaluate a novel preoperative planning program designed to improve accuracy in locating the optimal entry point and trajectory for ACTPS. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-24-1971/rc).

Methods

Ethics approval

This observation study was conducted at the Siriraj Hospital Department of Orthopedic Surgery, Mahidol University, Bangkok, Thailand. Data were obtained from the hospital’s electronic medical records. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Ethics Committee board of the Siriraj Hospital Faculty of Medicine, Mahidol University, Bangkok, Thailand (COA No. Si 694/2019) and individual consent for this retrospective analysis was waived. The ethics review board guidelines and regulations were followed for all materials and methods.

Subjects

After approval by the Siriraj Ethics Committee, 71 normal patients who underwent CT scans of the brain with the cervical spine due to evaluations related to head injuries from January 2018 to September 2019 at Siriraj Hospital were enrolled. All cases were selected and confirmed by three spine surgeons (P.T., W.S. and M.R.) at Faculty of Medicine, Siriraj Hospital, that there were no obvious lesions or defects in the lower cervical spine, without any appearance of spondylosis, and that they had no history of cervical spine injury or had performed cervical surgery before. Databases that included patient demographics and CT scan results were recorded using Faculty of Medicine, Siriraj Hospital electronic medical record program.

Radiographic measurement

After receiving informed consent, all patients underwent 16-channel digital CT scan. Each plane cut thickness was 1 mm. and the size of the plane pixel was 0.5 mm * 0.5 mm. 3D reconstructions were performed with IplanWorkstaionIGS 3.0 software^®^ (Brainlab Company) with Brainlab elements image fusion technology from Brainlab Company. The 3D reconstruction images were created from the original images. These 3D images could be rotated 360° and expanded to see all the details for the clearest construction of the vertebral edges (Figure 1A-1D).

Due to the high resolution of the 3D reconstruction image, the measurement of all parameters could be correlated and shown in all planes with precise results. The C3–C7 vertebral bodies were divided into 3 planes (axial, coronal and sagittal plane). An anterior vertebral edge of each axial and sagittal plane was further divided into four equal zones (zones 1 to 4) as demonstrated in Figures 2,3. In the axial plane (Figure 2), the midpoint of the intersection of the vertebrae and pedicle was used as a reference for planning the ACTPS trajectory. In the sagittal plane (Figure 3), the reference and trajectory of the screws were analyzed. In the coronal plane (Figure 4), we used for recording diameters of the transpedicular screws.

All measurements were performed by two spine surgeons (P.T. and M.R.). There were interobserver agreements between two surgeons before recording the actual outcomes. We used a Pearson coefficient of correlation to determine the interobserver and intraobserver reliability. The correlation value (r) of 0 to 0.25 indicated poor reliability, 0.25 to 0.50 indicated fair reliability, 0.5 to 0.75 indicated good reliability and more than 0.75 indicated excellent reliability.

Statistical analysis

The demographic and clinical characteristics of the patients were analyzed and descriptively reported. The distribution of continuous numeric data was verified using the Shapiro-Wilk test. The results are presented as mean ± standard deviation and frequency (percentage). Multivariate logistic regression analysis was performed on each raw data. Student’s t-tests and Chi-square test were used to assess differences between men and women. All analyzes were performed using the SPSS program version 18.0 (SPSS Inc., Chicago, IL, USA). We consider P<0.05 as significant.

Results

Demographic results

There were 28 women and 43 men, ranging in age from 18 to 75 years old (mean 36.3±11.3 years old). We analyzed the data at each vertebral level to correlate with sides and sex.

Radiographic results

In the axial plane

The average lengths of the pedicle screws (A–B) were 31.8±1.7 mm. The mean angulations (D angle) of the entry points of the C3–C7 screws were 46.2°±2.6°. The mean distance from the screw entry point (C–G) was 1.7±0.5 mm. The lengths were significantly different between the sexes, in male longer than in female on all cervical spines. The lengths of the screws at the lower cervical levels of C6–C7 gradually increased compared to those at the upper cervical levels of C3–C5. There was no significant difference in the lengths of the screw trajectory between the sides of the vertebrae. There was no significantly different angulation of the screw trajectory (D angle) and distance from a midline axis of the screw entry points (C–G) at the same level of vertebrae among the left and right sides, genders (P>0.05) (Table 1). The entry points of the C3–C7 screws started mainly from the anterior edge of the vertebrae in zone 2 for the insertion of the right-side pedicle screw and zone 3 for the insertion of the left-side pedicle screw (Table 2). The entry point zone does not have significant differences on both sides and gender (P>0.05). There was no starting point for screws in zones 1 or 4 of the anterior vertebral edge (Table 2).

In the sagittal plane

The mean of the angulations was almost parallel (sagittal angulation mean 1.4°±0.6°) to the superior endplate. At levels C3–C4, the trajectory of the screws was caudally inclined and the trajectory of the screws at levels C6–C7 was in the cephalad direction (from lordosis curve in the cervical spine). There were no significant differences in the angulations of the trajectory of the screws on the sides of the vertebrae or by sex (Table 3). The entry points in the sagittal plane of the C3–C7 screws started from the anterior edge of the vertebrae in zone 1 or zone 2 but there was no starting point for the screws in zones 3 or 4. There were no significant differences on the left and right sides or by sex (Table 4).

In the coronal plane

We measured the diameters of the pedicle screws by increasing by 0.5 mm each measurement time, according to the screw model in the preoperative program of the IplanWorkstaionIGS 3.0 software^®^. The mean of the largest diameters of pedicle screws in the coronal plane that are safe for pedicle breakage were 3.2±0.3 mm. They gradually increased in diameter of the pedicle screws of C3–C7 and the average largest diameters of the pedicle screws were in the C7 vertebrae. The average diameters of the pedicle screws were larger in men (3.3±0.3 mm) than in women (3.1±0.2 mm) and significantly different (Table 5), but there was no significant difference on the sides of the vertebrae.

Discussion

Our study did not demonstrate significant differences in screw trajectory angulation, screw entry points zones or distance from the midline axis of anterior cervical pedicle screws in the lower cervical spine between different sides and sex in a normal population. The screw entry points were located mainly in zones 2 or 3 in the axial plane, with a mean distance from the midline axis of 1.7±0.5 mm. The mean angulation of the entry points of the C3–C7 screw was 46.2°±2.6°, and the lengths of the pedicle screws ranged from 31.8±1.7 mm. In the sagittal plane, the screw entry points were mainly in zones 1 or 2, with a mean angulation nearly parallel to the superior endplate (1.4°±0.6°), resulting in a caudal trajectory at levels C3–C4 and a cephalad trajectory at levels C6–C7 due to the lordotic curve of the cervical spine. We found a significant difference in the length and diameter of anterior cervical pedicle screws between men and women, with longer and larger screws observed in men (32.9±1.8 mm in length and 3.3±0.3 mm in diameter) compared to women (30.7±1.6 mm in length and 3.1±0.2 mm in diameter).

In the axial plane, we had similar results as in the previous literature (17,18). There was no significantly different angulation of the screw trajectory, the zone of the screw entry points, the distance from the midline axis of the screw entry points at the same level of vertebrae between the left and right sides and the sexes. The mean distance from the midline axis of the entry points of the screws was 1.7±0.5 mm and the mean angulations of the entry points of the C3–C7 screws were 46.2°±2.6°, same as the report by Yukawa* et al. (12). The lengths of the pedicle screws were in the range of 31.8±1.7 mm, similar to the reports of the reports of the reports by Li et al. (18) and Zhao et al. (17), but the screws lengths of our study were statistically significant differences between sexes, in men (32.9±1.8 mm) longer than in women (30.7±1.6 mm) (P value <0.05) in all cervical levels and the lengths of screws at the levels of C6–C7 were gradually longer than the levels of C3–C5 [difference results of Zhao et al. (17)]. However, our findings are consistent with the results of Hasanain et al. *(19), which showed clear differences in cervical screw lengths between males and females, with the longest screws at the C7 level. Aside from this, the measurements on both sides of the spine were comparable.

In the sagittal plane, there were no significantly different angulations of the screw trajectory, the zone of entry points on the sides of the vertebrae and the genders. The entry points in the sagittal plane of the C3–C7 screws started from the anterior edge of the vertebrae in zone 1 or zone 2 similar to the report of Zhao* et al. *(17). With our reference technique in the tilted angle using the superior vertebral boarder (superior endplate) of the lower cervical spine for an easier and more notable point of reference in the surgical field, the results of the tilted angle were totally different from those of the previous literature (17,18). We found that the mean angulations were nearly parallel, in the range of 1.4°±0.6° to superior endplate.

In the coronal plane, the anatomy of the cervical vertebra study by Panjabi* et al. (20) published that the diameter of the transverse pedicle at the level of C3 in the transverse plane was 5.1 mm and in the sagittal plane was 6.7 mm and the diameter would increase to 6.6 mm at the level of C7 in the transverse plane and 7.6 mm in the sagittal plane. However, there were limitations in cervical anatomy of the pedicle morphology for insertion of transpedicular screws (21). In the report by Li et al. (18), the largest diameters of pedicle screws in the coronal plane that were safe for pedicle breakage were in the range of 3–4 mm. In our study, we found that the largest diameters of the pedicle screws were similar to the report by Li et al. *(18), but there was a statistically significant difference (P value <0.05) of average diameters of the pedicle screws between men and women (male > female). As in clinical surgery, we have to increase the diameter size of the transpedicular screws 0.5 mm each, so the data on the diameters of the pedicle screws between men (3.3±0.3 mm) and women (3.1±0.2 mm) was not different in real surgery. There were no significantly different diameters of pedicle screws on the sides of the vertebrae and in the age groups. According to the diameters of the C3–C7 pedicle screws that gradually increased, the diameter of the lower C pedicle screws (C6–C7) appeared to be larger than the average, where male pedicle screws were larger than female.

In general, sex was significantly associated with screw length and diameter, but the side of the vertebrae did not appear to be related to cervical spinal alignment. There was only one optimal trajectory for the fixation of ACTPS at each vertebral level from C3 to C7. Attempting an alternative trajectory on the opposite side was not feasible due to the proximity of the screw entry points to the midline of the anterior vertebral border, which could result in screw collisions.

Strengths and limitations

Our study had many strengths. First, our study showed all 3D parameters that affected the best position of the entry points and the trajectory of the anterior cervical pedicle screw in the lower cervical spine. Second, the 3D reconstructions image programs were created as the original images, which could be rotated in 360° and expanded to see every detail for the clearest construction of vertebral edges. And third, we used two independent evaluators blinded in the evaluation to prevent biased measurement of radiograph. The main limitation of our study is that the participants were Thais and that we had a relatively small sample size. The cervical parameters observed in our population may not be applicable to other populations.

Conclusions

The optimal ACTPS entry points in the lower cervical spine are located on the midline axis and near the superior vertebral border of the vertebral body, with trajectory angulations in the axial plane of 46.2°±2.6° and nearly parallel to the superior vertebral border in the sagittal plane, at 1.4°±0.6°. The pedicle screw lengths range from around 31.8±1.7 mm. ACTPS is a feasible option for lower cervical spine fixation. Our CT-based measurements, which include essential anatomical parameters of the cervical spine, will assist surgeons in performing this procedure.

Supplementary