Personalized and precise prediction of cage width and implantation angle in transforaminal lumbar interbody fusion: an image analysis of combined application of CT and Surgimap

Introduction

Degenerative lumbar spine diseases constitute a ubiquitous health issue globally, particularly prominent among aging populations. The pathogenesis of these conditions involves complex biological processes, including the degeneration of intervertebral discs, facet joints, and other spinal structures. This degeneration can lead to pain, reduced mobility, and, in severe cases, neurological deficits, thereby affecting the quality of life of millions of individuals and frequently necessitating surgical intervention (1-4). Transforaminal lumbar interbody fusion (TLIF) is a classical and widely performed surgical procedure in lumbar spine surgery (5,6), aiming to alleviate pain and restore functional stability in patients with lumbar degenerative diseases, such as lumbar spondylolisthesis (7), degenerative disc disease, and spinal stenosis. TLIF involves the precise placement of an interbody cage within the vertebral interspace (8), which necessitates accurate preoperative planning to ensure optimal fusion, minimize surgical trauma, and prevent complications such as cage migration or nerve root injury. The dimensions of the cage, particularly its width and the implantation angle, are crucial factors influencing the success and outcome of the surgery.

A study comparing the biomechanical properties of different cage designs reported that the width of the cage significantly influences the range of motion (ROM) and stress distribution across the spinal segments (9). Specifically, narrower cages may lead to increased motion at the surgical level, which may compromise the stability of the fusion and potentially result in complications such as cage subsidence (10). Additionally, biomechanical studies have reported that the anterior positioning of the cage can lead to a greater gain in segmental lordosis and overall spinal alignment compared to posterior placements (11-13).

TLIF surgery involves the oblique insertion of a single cage into the intervertebral space. Ideally, a wide cage is placed horizontally at the anterior aspect of the intervertebral space (14). Notably, the cage is more easily positioned in cases with a greater initial implantation angle. Nonetheless, comprehensive studies regarding the initial implantation angle and the maximum width of the cage in TLIF are lacking.

This study aimed to address this gap by performing image analysis of standard preoperative and postoperative computed tomography (CT) scans and three-dimensional (3D) reconstructed images, which were imported into Surgimap software (Nemaris Inc., New York, NY, USA) for calibrating, mapping, measurement, and analysis. The objective is to develop a reference model for precise and personalized prediction of the width and implantation angle of the cage in TLIF surgery. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-216/rc).

Methods

Study design and consent

This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Medical Ethics Committee of Shanghai Ninth People’s Hospital (approval No. SH9H-2024-T443-1). The requirement for individual consent for this retrospective analysis was waived.

Study participants

From January 2018 to May 2023, a total of 93 patients (52 men and 41 women) with different lumbar spine diseases were enrolled in the study. The baseline characteristics of the participants are shown in Table 1. Eligible patients were diagnosed with a single-level lumbar degenerative disease at L4/5, verified by CT and magnetic resonance imaging (MRI), and consistent symptoms. The patients underwent TLIF surgery because conservative treatment had been ineffective. Patients with spinal malignancies, infections, or a history of prior lumbar spine surgery requiring further revision surgery were excluded from the study.

The main steps of TLIF include decompression, fixation, and fusion (15). All patients underwent routine CT scans before and after the surgery.

Imaging measurement

Actual cage insert angle, and possible cage width

In the cross-sectional midline corresponding to the lumbar 4/5 intervertebral space level on the postoperative CT sagittal midline view (Figure 1A), the distance FO between the anterior and posterior margins was marked. The dural sac and traveling nerve roots were carefully pulled to the midline using a nerve root retractor. During the implantation of the interbody fusion cage, one side was positioned tangentially to the rod, and the other side is placed tangentially to point O. As shown in the Figure 1B, the angle between the long side of the fusion cage and the central axis, indicated by the green angle line, represents the angle of cage implantation. The white line segment, w, also represents the radius, r, of the dashed circle, indicating the width of the cage (Figure 1B).

Figure 1 CT imaging of a 54-year-old female patient and the schematic diagram of measurement methods. (A) Mid-sagittal CT image. (B) CT cross-sectional image corresponding to the L4/5 intervertebral space. On the middle line, the anterior margin of the intervertebral space is labeled as F and the posterior margin as O; the white solid circle represents the rod; the orange dashed-line rectangle indicates the cage; the width of the cage, w, is equal to the radius, r, of the dashed-line circle, which is indicated by a white line segment; the light blue solid circle denotes the dural sac, which is pulled and protected to the midline by the blue nerve root retractor; the green angular line represents the ACIA. (C) On the corresponding preoperative CT cross-sectional abdominal window, at the midline, the distance between the anterior and posterior margins of the intervertebral space measures 4.12 cm. (D) On the postoperative CT cross-sectional bone window, a line segment measuring 4.12 cm is drawn on the midline. (E) The image is calibrated in Surgimap. (F) A white circle with a radius of 10 mm and a green angular line are drawn. (G) A green angular line with an angle of 20° and a white circle that is tangent to this line is drawn. ACIA, actual cage insert angle; CT, computed tomography.

On the midline of the abdominal window cross-section corresponding to the intended fusion segment intervertebral space before surgery, the distance from the anterior margin to the posterior margin was measured (Figure 1C: 4.12 cm). Post-surgery, a line segment of 4.12 cm in length was drawn on the midline of the bone window at the same level (Figure 1D). The images were imported into Surgimap software (Version 2.3.1.5), and a blue line segment was used for calibration (Figure 1E). With “O” as the center, a circle with a radius of 10 mm was drawn (taking a 10 mm wide fusion cage as an example), and a green angle line was also drawn, with one side tangent to both the rod and the circle, and the other side overlapping with the midline. This angle line (34.6°) represents the actual cage insert angle (ACIA, Figure 1F). Similarly, the angle line can be drawn first, taking the hypothetical cage insert angle (HCIA) of 20° as an example, and then drawing a white circle tangent to it. The radius of the circle indicates the maximum possible cage width (PCW) of 17 mm that can be selected when inserting at a 20° angle, as shown in Figure 1G. The ACIA of two types of cages (10, 12 mm) and the PCW ranging from 15° to 50° were measured and compared (cases with PCW <1 mm were excluded from the analysis).

Distance between nerve root and middle-line, and possible cage insert angle

On the cross-sectional plane corresponding to the inferior endplate of L4 on the preoperative sagittal midline CT scan (Figure 2A), the greater the distance from the projection of the exiting nerve root to the midline (DN-M, Figure 2B), the larger the safe space for oblique insertion of the fusion cage. During cage implantation, one side of the cage was placed tangentially to the exiting nerve root, and the other side was positioned tangentially to point O. The maximum possible cage insert angle (PCIA) is shown in Figure 2C, representing the angle between the long side of the cage and the central axis, indicated by the green angle line.

Figure 2 CT imaging of a 54-year-old female patient and the schematic diagram of measurement methods without rods. (A) Mid-sagittal CT image. (B) CT cross-sectional image corresponding to the inferior endplate of L4. The yellow dashed oval represents the exiting nerve root projection; the light blue solid circle represents the dural sac; D-NM stands for the distance from the exiting nerve root to the midline. (C) The light blue rectangle represents the interbody fusion cage; the width of the fusion cage, denoted as w, which is also the radius r of the red dashed circle, is represented by a red line segment; the green angular line represents the PCIA. (D) At the midline, the distance between the anterior and posterior margins of the intervertebral space measures 4.09 cm. (E) The image is calibrated in Surgimap. (F) A red circle with a radius of 12 mm is drawn. (G) A green angular line is drawn. CT, computed tomography; PCIA, possible cage insert angle.

First, the measurements and calibrations were performed according to the method described above (Figure 2D,2E). Then, with “O” as the center, a circle with a radius of 12 mm was drawn (taking a 12 mm-wide fusion cage as an example) (Figure 2F). Next, a green angle line was drawn, with one side tangent to both the exiting nerve root and the circle, and the other side overlapping with the midline. This angle line (57.8°) represents the PCIA (Figure 2G). D-NM on both sides and PCIA of cages with different widths on both sides were measured and compared.

In addition, our method was based on CT scans and Surgimap software, which allowed us to obtain accurate measurement data and reduce errors. The accuracy of the linear data was 0.1 mm, and the accuracy of the angle was 0.1°.

Statistical analysis

Intraobserver and interobserver reliability were assessed using intraclass correlation coefficients (ICCs) after all parameters were measured twice by the same observer on two separate occasions and once by a different observer. If the ICC fell between 0.82 and 0.98, then the intraobserver and interobserver measurements were consistently reliable. Therefore, measurements obtained by one observer were used in the analysis.

Continuous variables were expressed as mean ± standard deviation. The matched or unmatched t-test was used to evaluate the difference between groups. In this study, P<0.05 was considered statistically significant. All statistical analyses were performed using the software SPSS 22.0 (IBM Corp., Armonk, NY, USA).

Results

ACIA

In the 50 patients with a 10 mm cage, the implantation angle was 28.56°±4.65°, and for the 43 patients with a 12 mm cage, the implantation angle was 27.26°±5.54°. No statistically significant difference in ACIA was observed between the two groups, as shown in Table 1 and Figure 3A.

Figure 3 ACIA and PCW. (A) Scatter plot comparing the ACIA between 10 mm and 12 mm cage and (B) changes in the PCW at different HCIA ranging from 15° to 50°. Data are presented as the means ± SD. ns, non-significant; *, P<0.05; ****, P<0.0001. ACIA, actual cage insert angle; CW, cage width; D-NM, the distance from the exiting nerve root to the midline; HCIA, hypothetical cage insert angle; PCW, possible cage width; SD, standard deviation.

PCW

A greater initial implantation angle implies a more vertical placement, allowing a larger cage width. When implanted at 15°, a cage width of 17.56±2.53 mm can be selected, as displayed in Table 2 and Figure 3B.

D-NM

At the L4/5 level, the D-NM on the left side (19.92±2.62 mm) was significantly smaller than that on the right side (20.85±2.99 mm, P<0.001), as shown in Table 3 and Figure 4A.

Figure 4 D-NM and PCIA. Scatter plot comparing the (A) D-NM, (B) PCIA with CW =10 mm, and (C) PCIA with CW =12 mm between left and right. Data are presented as the means ± SD. ns, non-significant; *, P<0.05; ***, P<0.001. CW, cage width; D-NM, the distance from the exiting nerve root to the midline; PCIA, possible cage insert angle; SD, standard deviation.

PCIA

In the absence of a rod, the maximum initial implantation angle of the 10 mm-wide cage was 60.77°±9.20° on the left side and 61.85°±7.45° on the right side, demonstrating no significant difference between the two sides. The maximum initial implantation angle of the 12 mm-wide cage was 54.07°±10.07° on the left side, which was significantly less than the 55.58°±8.37° on the right side (P<0.05, as shown in Table 3 and Figure 4B,4C).

Discussion

This study analyzed the CT images of patients who underwent single-level TLIF surgery using Surgimap software to measure the position of the exiting nerve root, the maximum width of the cage, and the initial implantation angle of the cage. The findings indicate that the distance from the exiting nerve root to the midline was approximately 20 mm at the L4/5 intervertebral space. When pedicle screws and rods were pre-placed, the initial implantation angle of the cage was 27–28°; theoretically, a cage with a maximum width of 17.6 mm can be used. Without the restriction of pedicle screws and rods, the theoretical maximum initial implantation angle of the cage was approximately 60°.

The implantation step of the TLIF procedure involves the oblique placement of a single cage between the traveling nerve root (dural sac) and the exiting nerve root. More expansive cages cover a larger surface area, facilitating a more uniform load distribution on the endplates, thereby diminishing cage subsidence (16-20). Placement of the cage horizontally in front of the intervertebral space can restore lumbar lordosis and reduce its migration and retropulsion (21,22). In summary, a wide cage placed horizontally in the anterior intervertebral space can achieve better stability and fusion results, while also reducing the occurrence of complications (23,24).

However, previous studies have largely overlooked the width of the cage in open TLIF surgery and its horizontal placement. In recent years, endoscopic intervertebral fusion surgery has emerged as a popular technique, but the minimally invasive approach has also limited the size and implantation angle of the cage (25,26).

The cage needs to be tapped into the intervertebral space; the larger the initial implantation angle (relative to the vertical plane), the easier it is to tap the device into a horizontal position on the contralateral side within the intervertebral space. Previous studies have reported an initial angle of 20–30° to 45° or oblique implantation (14,27,28). Nonetheless, the specific angle has not been clearly defined and in the case of pre-placed pedicle screws and rods, a 45° fusion cage implantation angle is almost impossible.

This study clarifies the exact maximum initial angles for the commonly used cage widths (10, 12 mm) in TLIF, thereby providing a data foundation for subsequent research aimed at increasing the insertion angle of fusion cages (Figure S1), and also proposes the maximum cage widths that can be selected at different initial implantation angles, providing a theoretical basis for the clinical use of larger cages.

Additionally, this study suggests that the presence of nerve root variations or intradural migration can be determined by observing the course of the nerve roots and measuring D-NM in the CT abdominal window (Figure S2), thereby facilitating preoperative planning, enhancing operational safety, and reducing the occurrence of complications. Furthermore, this study also proposes the theoretical maximum initial implantation angle in the absence of screw-rod constraints. This implies that if the initial angle of the cage exceeds this limit, it may cause damage to the exiting nerve roots.

Nerve root retractors during TLIF surgery are essential for the protection and retraction of the dural sac and the traveling nerve roots. We believe that retracting to the midline is safe. With this in mind, the data were measured and analyzed. Instead of memorizing these average values, the method should be mastered and individualized preoperative measurements should be carried out for each patient to develop a personalized surgical plan.

Nevertheless, the limitations of the present study should be acknowledged. Firstly, the size of the sample was relatively small, and it was restricted to Chinese adults. Data measurements may vary among other races and in children, although the same measurement methodology could be applied. Secondly, when studying the implantation angle of the fusion device, our focus was on assessing the position of the rod and nerve roots, but other influencing factors such as abnormally proliferating bone spurs and soft tissues such as skin and muscle were overlooked. Lastly, the traction of the dural sac to the midline was estimated for the convenience of measurement; the specific degree of traction during surgery could not be accurately measured.

Conclusions

The size and implantation angle of cages in TLIF surgery is determined by the spatial position of pedicle screws and rods and the location of the exiting nerve root. The cage width and implantation angle could be efficiently evaluated by measurement and analysis by combining CT images and Surgimap software. This approach facilitates the individualization of surgical procedures.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-216/rc

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-216/dss

Funding: This study was supported by Shanghai Orthopedic Digital Medical Research Center (grant No. 2023ZZ02026); Shanghai Science and Technology Innovation Action Plan for 2023 (grant No. 23ZR1447400); and the National Natural Science Foundation of China (grant No. 32170770).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-216/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. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Medical Ethics Committee of Shanghai Ninth People’s Hospital (approval No. SH9H-2024-T443-1). The requirement for individual consent for this retrospective analysis was waived.

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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