Endoscopic Transnasal Approach to Atlantoaxial Decompression and C1–2 Fixation in Basilar Invagination of Adults: A Feasibility Study

Article information

Neurospine. 2025;22(2):543-555

Publication date (electronic) : 2025 June 30

doi : https://doi.org/10.14245/ns.2449320.660

Jiahai Ding ¹^,²

, Shao Xie ¹

, Yuliang Chen ¹^,²

, Runchuan Zhou ¹^,², Syed Matiullah Azizi ¹, Xiaoya Huang ²

, Yang Xiong ¹^,²

, Yuancheng Yao ¹^,²

, Yushun Zhang ¹^,²

, Yong Liu ¹

, Lei Wang^,¹

¹Department of Neurosurgery, the Affiliated Hospital of Xuzhou Medical University, Xuzhou, China

²The First Clinical College of Xuzhou Medical University, Xuzhou, China

Corresponding Author Lei Wang Department of Neurosurgery, the Affiliated Hospital of Xuzhou Medical University, 99 Huaihai West Road, Xuzhou, 221002 Jiangsu Province, China Email: wl@xzhmu.edu.cn

Received 2024 December 5; Revised 2025 January 24; Accepted 2025 February 4.

Abstract

Objective

To explore a surgical technique for completing ventral bone decompression and C1–2 plate-screw fixation in the craniocervical junction (CVJ) through nasal approach by stage I at the imaging and physical anatomy levels, and to evaluate its feasibility.

Methods

Radiographic parameters of 80 patients with basilar invagination (BI) and 56 with normal CVJ anatomy were retrospectively analyzed. Three-dimensional (3D) reconstructions were performed in 31 patients with BI. Key anatomical landmarks, screw entry points, and fixation trajectories were evaluated. Customized plate-screw constructs were designed. Finally, surgical feasibility was tested on a 3D-printed anatomical model and a cadaveric.

Results

In 80 BI patients, the average distances between 4 screw insertion points were 16.04 mm, 21.10 mm, 6.83 mm, and 7.10 mm. C2 lateral mass oblique lengths were 16.81 mm (right) and 17.12 mm (left); C1 lengths were 18.71 mm (right) and 19.07 mm (left), with significant differences between C1 and C2 (p<0.001). A 28.5×14.1-mm titanium plate with 16 mm screws was successfully implanted via the nasal route in the polyether ether ketone 3D-printed BI model and the cadaveric. Radiology indicated that the screws were all in the lateral mass and the plates fit tightly.

Conclusion

In BI, transnasal odontoidectomy and plate-screw fixation of C1–2 are feasible theoretically. This may enable a new alternative approach for nasal minimally invasive decompression and immobilization, following the completion of biomechanics and clinical trials.

Keywords: Basilar invagination; Craniovertebral junction; Transnasal; C1–2 fixation; Odontoidectomy

INTRODUCTION

Craniovertebral junction (CVJ) pathologies are highly complex, and the treatment options available are diverse [1,2]. Basilar invagination (BI) is a developmental malformation of the bone tissue in the foramen magnum area. The odontoid process protrudes upward or backward, compressing the spinal cord and causing a series of clinical symptoms [3]. It is often combined with Chiari malformation, atlantoaxial dislocation (AAD), atlanto-occipital fusion (AOF), syringomyelia, etc [4,5]. The purpose of manual treatment is to decompress and stabilize the CVJ [6]. Although posterior decompression, combined with joint distraction and occipitocervical or C1–2 fixation, is considered a safe and effective approach, it does not adequately address the abnormal proliferative tissue around the ventral odontoid process [7,8]. As a result, secondary anterior decompression or odontoid resection has been employed to alleviate ventral compression [9]. However, both secondary surgery and anterior decompression combined with posterior fixation inevitably face challenges, including extensive muscle dissection in the posterior approach, postoperative axial pain, blood loss, vertebral artery injury risks, and the inherent risks of secondary procedures [10].

Traditionally, the transoral approach for decompression and fixation has been an alternative. However, it is limited by the patient’s oral anatomy, high infection rates, and slow postoperative recovery [11,12]. While lateral cervical approaches have also been explored and applied in a limited number of cases [13], the advent of endoscopic endonasal approaches (EEAs) for CVJ treatment represents a significant breakthrough. The non-invasive nature of EEA, combined with advancements in CVJ surgical techniques, makes the transnasal approach a promising option for BI [14,15].

Endonasal resection of the odontoid process has already been established as a safe and mature technique [9,16,17]. However, the use of the endonasal route for CVJ fixation remains varied and primarily confined to cadaveric studies. Feasibility and reliability of C1–2 joint screw fixation through the endonasal approach have been demonstrated in cadaveric studies. Other cadaveric research has confirmed the feasibility of placing screws into the occipital condyles (OC) and the C1 lateral mass via the endonasal route [18,19]. Additionally, recent studies utilizing 3-dimensional (3D) printing technology have designed a plate-screw system for occipitocervical fixation that can be implanted through the nasal passage [20]. Although these studies validate the feasibility of C1–2 joint screw insertion, C0–1 plate-screw fixation, and occipital-C2 plate-screw fixation (including OC screws and C1–2 joint screws), they are based on cadaveric specimens without BI, and fail to address the CVJ fixation in cases where the anterior arch of C1 and the odontoid process are resected [20].

Furthermore, there are no studies on the fixation of plate-screw fixation of C1–2 after odontoidectomy by nasal approach. In this study, we analyzed the imaging features of 80 patients with BI and developed a novel plate-screw fixation system for C1–2. Utilizing 3D reconstruction and printing technologies, we simulated and reconstructed the anatomical structures of BI patients. Through radiological and anatomical simulation, we explored the feasibility of a single-stage endonasal procedure for CVJ decompression, odontoid resection, and C1–2 fixation, assessing its technical viability and clinical potential.

MATERIALS AND METHODS

This study primarily encompasses the following objectives:

(1) Analyze the radiological characteristics of CVJ deformities and assess the theoretical feasibility of transnasal access.
(2) Create 3D-printed anatomical models using computed tomography (CT)-based reconstructions of bony structures.
(3) Identify key anatomical landmarks, assess optimal screw entry points, and evaluate the viability of transnasal internal fixation.
(4) Determine ideal screw trajectories and plate contours through anatomical and radiological evaluation of the CVJ.

1. Patient and Data collection

A retrospective analysis was conducted on 80 patients diagnosed with BI between September 1, 2019, and September 1, 2023 (Fig. 1). The study was approved by the Ethical Committee for Medical Research at the Affiliated Hospital of Xuzhou Medical University (XYFY2024-KL255), and informed consent was obtained. Inclusion criteria: (1) Radiological diagnosis of BI (Table 1); (2) Availability of complete CT or magnetic resonance imaging (MRI) data. Exclusion criteria: (1) age <18 years; (2) basilar skull fractures; (3) postoperative CT imaging only. A control group of 56 adults with normal CVJ anatomy was included. Patients without complete transnasal pathway CT data were excluded, resulting in 31 BI and 34 control patients in the BI 3D and non-BI 3D groups. Age and gender distributions were recorded.

Fig. 1.

Flowchart of patient selection and experimental design. CT, computed tomography; MRI, magnetic resonance imaging; BI, Basilar invagination; 3D, 3-dimensional.

Table 1.

Imaging diagnostic indicators of basilar invagination (n=80)

2. Imaging Measurements

Using 3D CT data, we performed measurements of the anterior atlanto-dental interval, clivus canal angle (CCA), and the maximum diameter of the superior articular facets (SAFs) on both the sagittal and coronal views at the C2 level (Fig. 2A and B). We also documented the incidence of AOF and C2–3 vertebral segmental malformation in both groups.

Fig. 2.

(A and B) Red lines represent the maximum diameter of the superior articular facets on both the coronal and sagittal views. The yellow line is the vertical tangent to the front edge of the C2 lateral mass. (C) The coronal section of the yellow line in panel B. And distance from the center of W-shape lateral mass points (W-L-1R, W-L-1L, W-L-2R, W-L-2L) to the superior or inferior margins and medial edges of the lateral mass were 2 mm. L1 (distance between W-L-2R and W-L-2L) and L2 (distance between W-L-1R and W-L-1L) were marked with black lines, L3 (distance between W-L-1R and W-L-2R), and L4 (distance between W-L-1L and W-L-2L) were marked with white lines. (D) Schematic diagram of plate parameters. (E) The left and right of lateral mass oblique length in C1 and C2 were marked with a white line (Screw-C1-R; Screw-C1-L; Screw-C2-R; Screw-C2-L). (F) The blue, green, and yellow lines represent NFL, SFL and NAL (the distance from the posterior margin of the hard palate to the lowest point of the C2 vertebral body’s posterior aspect) respectively. In the window is a 3-dimensional (3D) view of Mimics, and the cylinder represents a transnasal instrument. (G) NCL: the vertical distance between the nasal bone superior edge and the endoscope was marked with a pink line. (H) W-Angle L/R-A: the angle between the cylinder and the midline. (I) W-Angle L/R-S: the angle between this plane and the C1 lower articular facet. (J) The red line indicates the full length of the nasal septum, and the pink line is the length of the posterior nasal septum covered by the device. The ratio of the latter to the former is recorded as NS%. (K) 3D reconstruction of the patient 33’s head and upper cervical spine and 3D reconstruction of the atlas-axis vertebrae and partial fusion of C3 and the occipital bone; L, 1:1 scale 3D printed skull model made of PEEK. NFL, nasal-front palate line; SFL, soft nasal-front palate line; NAL, nasal-atlantoaxial line; NCL, nasal bone-cylinder line.

For obtaining parameters of plate-screw system, theoretical screw insertion points were set on the medial sides of C1 and C2 lateral masses, defined as W-shape lateral mass points (W-L-1R, W-L-1L, W-L-2R, W-L-2L) (Fig. 2C). To unify the measurement standards, in coronal, distance from the center of warranty land points to the superior or inferior margins and medial edges of the lateral mass were 2 mm (Fig. 2C). Distances between adjacent insertion points (L1–4) were measured and plate width were calculated using: YL4/3=L4/32-L2-L1222+2+1.75*2 (Fig. 2C and D). And the lateral mass oblique length of C1 and C2 were also measured (Fig. 2E).

Data measurements for transnasal access to verify the feasibility of this surgery included: nasal-front palate line (NFL) and soft nasal-front palate line (SFL) (Fig. 2F); nasal-atlantoaxial line (NAL) (Fig. 2F); nasal bone-cylinder line (NCL) (Fig. 2G); screw angles (W-angle L/R-A/S) (Fig. 2H and I); cylinder distance from the nasal septum and nasal septum coverage percentage (NS%) (Fig. 2J).

3. 3D Construction and Print

CT DICOM datasets (each patient’s cranial base CT scans consisted of approximately 200 slices, with an inter-slice spacing ranging from 0.35 mm to 0.7 mm) from 31 BI patients were imported into Mimics Medical 21.0 for 3D reconstruction using grayscale values of 226–3,045 Hounsfield unit. In order to verify the feasibility of the surgery at the bony level, we selected a representative case to perform reconstruction and 3D printing of the skull base and upper cervical spine: Patient 33, male, 52 years old, from the 3D-BI group was selected to undergo 1:1 3D printing of the reconstructed atlantoaxial joint (C1–2) and CVJ using transparent resin and polyether ether ketone (PEEK) material (Fig. 2K and L).

4. Procedure

1) Bony models

Screws and plates, custom-designed based on study data, were placed in PVC molds and transparent resin models with C1 anterior arch and odontoid resection for Screw trajectory simulation. Meanwhile, transnasal approach simulation was performed on the PEEK model. Bilateral nostrils were used, with nasal turbinate/septum resection if needed. Guide holes were drilled for screw insertion, ensuring alignment with the plate. Postop-erative imaging was obtained by Philips mobile C-arm x-ray machine.

2) Cadaveric dissection

Cadaveric dissection was performed on a formalin-fixed male specimen aged 50 years, which included an intact head and upper cervical spine. Prior to the dissection, the head was securely fixed in a Mayfield 3-pin headrest to ensure stability during the procedure. The surgery utilized a standard rigid endoscope (4-mm diameter, 18-cm length, with 0° and 30° lenses) connected to a TL400 LED light source and an IMAGE1 S 4K NIR camera system. The procedure was conducted by 2 neurosurgeons employing a bimanual technique. Access to the surgical field was established through the right nasal cavity, with partial resection of the posterior nasal septum and the upper margin of the hard palate. The nasopharyngeal mucosa overlying the upper nasopharynx and the floor of the sphenoid sinus was disrupted to expose the underlying structures. A combination of rongeurs and a high-speed drill was used to clear the muscles and ligaments, providing access to the lower clivus, anterior arch of C1, and the lateral masses of C1 and C2 for screw placement. The anterior arch of the atlas was drilled out using a high-speed drill, and guide holes were created with a custom-angled drill bit for precise screw placement. Screws and plates were then inserted, and postoperative high-resolution CT scans (100 slices, 1-mm slice spacing) were performed.

5. Data Analysis Methods

Data were analyzed using IBM SPSS Statistics ver. 27.0 (IBM Co., Armonk, NY, USA) and GraphPad Prism 9.3.0 (GraphPad Software Inc., La Jolla, CA, USA). Continuous variables were presented as mean±standard deviation or median (interquartile range) based on distribution, and categorical variables as frequencies (%). Group comparisons were performed using chi-square tests for categorical data and t-tests or Mann-Whitney U-tests for continuous data. A p-value <0.05 was considered statistically significant.

RESULTS

1. Demographic and Radiological Characteristics

As detailed in Table 2, 80 patients diagnosed with BI were included in this study, with a mean age of 52.88 years. The BI group had a male representation of 29, like the non-BI group. The CCA in the BI group (129.82°) was significantly reduced compared to the non-BI group (153.36°) (p<0.001). This difference persisted in the BI 3D and none-BI 3D subgroups (p<0.001). Furthermore, the maximum diameters of the SAF at C2 were significantly smaller in the BI group across sagittal and coronal planes (p<0.05). The incidences of AAD, AOF, and C2–3 vertebral segment malformations in the BI group were 48.75%, 71.25%, and 22.5%, respectively.

Table 2.

Characteristics of clinical and images

Average lengths of L1–4 were 16.04 mm, 21.10 mm, 6.826 mm, and 7.095 mm, respectively (Fig. 3A). Lateral mass oblique lengths in C2 averaged 16.81 mm (right) and 17.12 mm (left), while C1 measured 18.71 mm (right) and 19.07 mm (left) (Fig. 3B).

Fig. 3.

(A) Column bar graph plot (mean with standard deviation [SD]) of L1–4. (B) Column mean, error bars plot (mean with SD) of screw-C1/2-L/R. The normality of the distributions for L1, L2, L3, L4, and screw were assessed using the Kolmogorov- Smirnov (KS) test. The KS distance and corresponding p-values for each variable are as follows: L1: 0.09670, p=0.0618; L2: 0.09369, p=0.0794; L3: 0.05182, p>0.1000; L4: 0.06075, p>0.1000; screw-C2-L: 0.09081, p=0.0999; screw-C2-R: 0.09313, p=0.0831; screw-C1-L: 0.07024, p>0.1000; screw-C1-R: 0.09344, p=0.0810; All variables were found to follow a normal distribution (p>0.05). (C) composite pie chart of nasal septum deviation. (D) Box plot (min to max) of SFL, NFL, YL4, and YL3. (E) Column bar graph plot (mean with SD) of NS, NS%, NCL, and NAL of 3-dimensional (3D) groups. (F) X-Y chart of NAL of BI- 3D group and NCL. (G) Floating bars (min to max) plot (line at median) of W-angle. Normal data is presented as mean±SD and nonnormal data as median (interquartile range). Comparisons between groups were conducted using the chi-square test (χ²) for categorical variables and either an unpaired t-test or a Mann-Whitney U-test for continuous variables, depending on the distribution of the data. A p-value of <0.05 was considered statistically significant. NS, nasal septum; NCL, nasal bone-cylinder line; NAL, nasal-atlantoaxial line; BI, Basilar invagination; L A, left axial; R A, right axial; L S, left sagittal; R S, right sagittal.

In the 3D group, 31 BI and 34 non-BI patients were analyzed. 19% of BI had right septum deviation (Fig. 3C). The mean plate widths were 13.74 mm on the left (range, 10.76–16.61 mm) and 14.04 mm on the right (range, 10.58–17.69 mm), with no statistical difference (p=0.1281) (Fig. 3D). Nasal measurements yielded an average SFL of 18.04 mm and NAL of 28.40 mm, with NAL showing a significant difference between groups (2.16 vs. -2.38, p<0.001) (Fig. 3E). Linear regression between NAL and NCL indicated an R² of 0.4419 (p<0.001) (Fig. 3F). No significant differences were observed between W-angle left axial and W-angle right axial (11.78° vs. 11.94°, p=0.884) or between W-angle left sagittal and W-angle right sagittal (15.53° vs. 11.92°, p=0.254) (Fig. 3G).

2. The Display of C1–2 Plate-Screw and Actual Screw Implant Area

The C1–2 plate with a convex curvature was designed with a size of 28, 28.5, and 29×14.1 mm and the self-tapping screws had cap diameter of 3.5 mm and lengths of 14, 16, and 18 mm, all made from titanium alloy (Fig. 4A and B).

Fig. 4.

(A) Schematic diagram of plate-screw. (B) Physical image of 28.5×14.1-mm plate and 16-mm screws. (C) Schematic diagram of the actual screw implant area (blue mark) on the polyvinyl chloride (PVC) educational model. (D) Plate-screw implantation diagram on the PVC model. (E and F) Plate-screw implantation diagram on the transparent model, and the screw trajectory on the lateral mass; key steps of the endoscopic procedure via the transnasal approach. (G) Identification of the C1 anterior arch and drilling the C1 anterior arch. (H) Drilling the odontoid process into an "eggshell" shape. (I) anatomical identification of the entry point. (J) implantation of the plate-screw. (K) fixation in position. (L) superior view indicating the odontoid process has been removed, the space in the foramen magnum increased. (M and N) radiographs confirm the screws are fully within the lateral masses.

Actual screw implant area: the C1 lateral mass was divided into 3 equal parts horizontally and vertically, with the inner and lower zones designated for screw placement. Similarly, C2’s lateral mass was partitioned, with the upper one-third selected for screw insertion. C1 screws were oriented outward and upward, while C2 screws were directed outward and downward toward the pedicle, with angles determined preoperatively from CT scans (Fig. 4C).

3. Surgical Exposure and Radiological Evaluation

Screws were implanted successfully in both the PVC and transparent resin models, with the plate fitting well against the surface (Fig. 4D). In the transparent model, the screw trajectories were parallel to the joint surface, with angles of approximately 10° and 22° relative to the midline on the C2 lateral masses and 25° and 15° on C1, respectively (Fig. 4E and F). Under endoscopic guidance, the anterior arch of C1 was resected to a width of approximately 1 cm, allowing clear visualization of the odontoid process and its apex. The odontoid process was gradually resected from its center in a shell-like fashion. The plate was successfully introduced through the transnasal route. The screws were placed parallel to the joint surface at an angle of 15°–30° relative to the midline along the screw implant area, with subsequent reduction of C2 (Fig. 4G–K). Postoperative radiographs confirmed successful screw placement, with the plate fitting well. The foramen magnum and spinal cord space were notably enlarged compared to preoperative views (Fig. 4I–N).

In the cadaveric, partial resection of the posterior edge of the hard palate and posterior nasal septum was performed to address anatomical limitations of the surgical approach. Approximately 1 cm of the anterior arch of C1 was drilled, exposing the odontoid process, which remained intact. Custom-designed drill bits were employed to create guide holes in the designated screw placement areas, facilitating the successful insertion of screws and plates (Fig. 5A–G). Postoperative 3D CT imaging demonstrated the following outcomes: two 16-mm screws were accurately placed in the right/left lateral masses of C1 at angles of 10° and 15° relative to the midline; two 14-mm screw was inserted into the right/left lateral mass of C2 at an angle of 10° and 8° relative to the midline; The plate aligned seamlessly with the anterior structures of the atlantoaxial complex (Fig. 5H–K).

Fig. 5.

(A) Establishing the nasal passage. (B) the nasopharyngeal mucosa was destroyed to the exposure of craniocervical junction. (C) The titanium plate was inserted into the nasal cavity with a grasping forceps. (D) The screw insertion point was determined. (E) The guide hole was drilled with a customized grinding drill bit. (F) The anterior arch of C1 was removed and the 4 screw tracks were exposed. (G) The plate-screw was implanted using a customized screwdriver. Postoperative 3-dimensional (3D) computed tomography (CT) showed that the plate-screw structure was in place. (H) 3D reconstruction. (I–K) the coronal, axial, and sagittal positions of CT.

DISCUSSION

1. Approaches for CVJ

Persistent ventral compression following posterior fixation may necessitate anterior approaches [2,3,6,21,22], but surgical risk significantly increased in combined surgery [23-25]. Transoral decompression and fixation are well-established for managing odontoid fractures and AADs. However, this approach involves significant soft tissue disruption, higher infection risks, and postoperative complications such as dysphagia [26-28]. Transcervical approaches to the CVJ have also been explored in a limited number of studies, but they are less suitable for concurrent odontoidectomy [13].

The EEA might be a promising minimally invasive alternative for decompression and stabilization of the CVJ [14,29], providing direct access to CVJ without requiring extensive incisions, thereby minimizing soft tissue damage and reducing the risk of postoperative infections [30,31]. What’s more, EEA offers a more direct surgical pathway and superior exposure from the clivus to the C2 vertebra, facilitating precise identification of critical anatomical landmarks such as the anterior arch of C1, the odontoid process, and the clivus [31]. This approach is particularly suitable for integration with intraoperative navigation and customized implants, potentially improving the accuracy of screw placement and plate fixation [32]. The feasibility of few C1–2 fusion methods have also been demonstrated in cadaveric studies [18-20,33]. These factors make it a compelling option for BI patients requiring ventral decompression and fixation. Notably, preserving portions of the C1 anterior arch and its associated ligaments has been reported to maintain CVJ stability without the need for fixation [17]. However, a high level of expertise in endoscopic techniques and a thorough understanding of nasal anatomy are required to perform this approach [32]. The narrow operative corridor may restrict the use of instruments, especially in cases involving severe anatomical deformities or extensive decompression. Furthermore, postoperative complications, including cerebrospinal fluid leakage, nasal infections, and nasal crusting, remain potential risks [34]. Overall, EEA is well-suited for precise interventions targeting the anterior CVJ structures, but its application should be carefully weighed against patient-specific conditions and surgical requirements. Combining this approach with posterior stabilization techniques can further optimize clinical outcomes. Based on these considerations, we propose a novel minimally invasive technique involving the EEA for partial removal of the C1 anterior arch and odontoid process, followed by C1–2 plate-screw fixation.

2. Imaging Features of BI and Parameters of the Endonasal Approach

While the EEA has been widely applied to treat various CVJ pathologies, our study focuses specifically on the anatomical and imaging characteristics of BI. Compared to non-BI group, BI patients exhibited a reduced CCA and smaller SAF dimensions, consistent with biomechanical alterations often associated with AAD and AOF. The literature suggests that the feasibility of the endonasal approach depends not only on technical proficiency and equipment but also on the anatomical constraints imposed by the narrow nasal corridor [18,19]. Established reference lines, such as the midline, nasopalatal line, retro-palatal line, and nasoaxial line, have been used to delineate surgical reach and localization of BI lesions [35-37]. While the endonasal approach provides excellent access to the clivus and upper CVJ, its downward reach is limited. Our data showed that when surgical instruments are directed downward along the posterior hard palate to the inferior articular surface of C2 (W-L point), the distance between the instruments and the superior margin of the nasal bone averages approximately 1.3 cm. This additional space may enhance maneuverability and reduce external nasal structure constraints. To further simplify clinical application, we defined a novel parameter, the NAL, which differed significantly between the BI and non-BI groups. While the linear correlation between NCL and NAL was weak in the BI group, NAL may serve as a useful CT-based indicator for assessing the reachability of the W-L2 point via the endonasal route. This warrants larger sample sizes and improved correlation analysis methods for validation. Meanwhile, dual-nostril access appeared more favorable for achieving adequate exposure and maneuverability without more extensive bony resection, as confirmed by observations on printed 3D models. However, in cadaveric specimens, partial resection of the posterior nasal septum and posterior edge of the hard palate was required to obtain sufficient operative space [37]. This need for bony removal may be attributed to the loss of soft tissue elasticity in cadavers and the variability inherent in individual specimens. Thus, further studies are required to refine these findings and optimize surgical techniques.

3. Plate-Screw and Implantation for CVJ Stabilization

O–C1 and C1–2 transarticular screw fixation via lateral cervical techniques may provide reliable biomechanical stability comparable to traditional posterior methods [33,38-40]. Customized 3D-printed plate-screw constructs have been explored for C0–1 fixation, but limited screw depth into C2 raises concerns about their long-term stability, particularly for the C0–C1–C2 complex [20].

Given the anatomical constraints of the nasal corridor, traditional transoral techniques, such as the transoral atlantoaxial reduction plate system, cannot accommodate plate placement. To address this, we developed a standardized plate model and 16-mm screws, based on measurements of L1–4 and optimized screw angles. The curvature of the plates is designed to conform to the C2 vertebral body, and our data indicate that YL3 and YL4 dimensions are statistically smaller than those of the superior and nasal floor lines (SFL and NFL), making the plate design compatible with most nasal corridors. Our findings showed that the inferior articular surface of C1 forms an average axial and sagittal angle of 10°–15° relative to the surgical trajectory in the endonasal approach. Transparent material models demonstrated that screws need to be angled off the midline to achieve sufficient depth and holding strength. Based on the radiological differences between the BI and non-BI group, we customized angled drills and screwdrivers to facilitate screw placement in our cadaveric specimens. A certain deviation was observed between the actual insertion angles and the theoretical angles, which might be attributed to bone osteoporosis. In future studies, we plan to incorporate technologies such as neuronavigation to further enhance the accuracy and safety of screw placement, while also considering factors such as vertebral artery variation that affect screw placement [25,32,41]. The use of 1:1 3Dprinted BI models provided valuable insights into the anatomical challenges of BI, as cadaveric specimens with significant CVJ abnormalities are scarce. Furthermore, integrating universal screws and locking mechanisms into the plate design enhanced both flexibility and stability during implantation. Model implantation results aligned with the screw implantation area, but further studies are needed to optimize screw holding strength and validate these findings in clinical settings.

4. Limitations

This study has limitations, including a small sample size, variable imaging quality, and potential software synthesis errors. Larger cohorts are necessary to validate our findings. Additionally, due to incomplete MRI data, spinal cord compression quantification was limited. While theoretical analysis, bony models and cadaveric dissection demonstrated feasibility, further mechanical testing and clinical trials are needed. Lastly, our study focused solely on BI, and its applicability to other CVJ pathologies remains unexplored. Future integration of customized instruments and refined techniques may expand the utility of this approach for broader CVJ disease management.

CONCLUSION

In patients with BI, the EEA for odontoid process resection followed by C1–2 fixation is theoretically feasible based on 3Dprinted anatomical models and cadaveric specimen. This method offers a potential minimally invasive solution for ventral decompression and stabilization. However, further biomechanical testing and clinical trials would confirm the reliability and safety of this technique, paving the way for its broader application in CVJ pathologies.

Notes

Conflict of Interest

The authors have nothing to disclose.

Funding/Support

This research was funded by Jiangsu Provincial Key Medical Discipline, grant number ZDXK202228 and Xuzhou Science and Technology Plan Project, grant number KC20139.

Author Contribution

Conceptualization: LW; Visualization: SX, YC, LW; Writing – original draft: JD, YC, XH; Writing – review & editing: JD, RZ, SMA, YX, YY, YZ, YL, LW.

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Variable	Median (IQR)
Chamberlain line (mm)	9.37 (7.30,12.54)
McGregor line (mm)	10.80 (8.95,13.75)
McRae line (mm)	4.60 (3.21,6.25)

Variable	BI group (n = 80)	Non-BI group (n = 57)	χ²/Z/t-value	p-value	BI 3D group (n = 31)	Non-BI 3D group (n = 34)	χ²/Z/t-value	p-value
Male sex	29 (36.25)	30 (52.63)	3.64	0.056	12 (38.71)	14 (41.18)	0.04	0.839
Age (yr)	52.88 ± 10.41	64.00 (51.00–73.50)	-3.66	< 0.001*	54.39 ± 11.09	60.50 (49.00–69.25)	-1.393	0.164
CCA (°)	129.82 ± 13.07	153.36 ± 8.99	-12.49	< 0.001*	130.0 ± 12.67	153.64 ± 10.10	-8.354	< 0.001*
Maximum diameter of SAF in C2 (mm)
Left coronal	14.07 (12.86–15.91)	15.10 (14.30–15.90)	-2.64	0.008	-	-	-	-
Left sagittal	14.78 ± 1.80	15.44 ± 1.13	-2.65	0.009	-	-	-	-
Right coronal	13.93 (12.70–15.72)	15.00 (14.00–16.20)	-3.49	< 0.001*	-	-	-	-
Right sagittal	14.62 ± 1.94	15.50 ± 1.32	-3.15	0.002*	-	-	-	-
ADI (mm)	3.70 ± 2.84	-	-	-	4.00 ± 3.17	-	-	-
AAD	39 (48.75)	0	-	-	16 (51.61)	0	-	-
AOF	57 (71.25)	0	-	-	22 (70.97)	0	-	-
C2–3 Vs	18 (22.50)	0	-	-	7 (22.58)	0	-	-