Divergent morphological changes of Kambin’s triangle in degenerative spondylolisthesis: a 3D MRI/CT fusion study

Divergent morphological changes of Kambin’s triangle in degenerative spondylolisthesis: a 3D MRI/CT fusion study | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Divergent morphological changes of Kambin’s triangle in degenerative spondylolisthesis: a 3D MRI/CT fusion study Katsuhisa Yamada, Ken Nagahama, Yoshinori Hyugaji, Tomohiro Miyashita, and 11 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8579454/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 4 You are reading this latest preprint version Abstract Purpose The transforaminal (TF) approach through Kambin’s triangle is the cornerstone of minimally invasive spinal surgery. Although degenerative lumbar spondylolisthesis (DS) is a common indication, its quantitative effect on the three-dimensional (3D) anatomy of the safety zone remains unclear. We aimed to evaluate the 3D morphological alterations of the TF safety zone in DS using magnetic resonance imaging (MRI)/computed tomography (CT) fusion imaging. Methods We retrospectively analysed 60 patients (120 sides) categorised into three groups (n = 20 each): no-slip, Meyerding gade 1, and gade 2 spondylolisthesis. Using a validated 3D MRI/CT fusion model, we simulated a 45° transforaminal surgical trajectory at the L4/5 level. The width (horizontal distance from the superior articular process [SAP] to the exiting nerve root), height (inter-endplate distance), and total safety zone area were quantified. Results A divergent response was observed as vertebral slippage progressed. Width significantly increased with slip degree (no-slip: 6.2 mm; grade 2:7.5 mm; r = 0.26, p < 0.05), reflecting relative SAP posterior displacement. Conversely, height demonstrated a negative correlation (no-slip: 4.8 mm; grade 2:3.5 mm; r=-0.67, p < 0.01) due to disc height loss. The total area demonstrated a significant negative correlation with slip percentage (r = -0.40, p < 0.01). Conclusions The TF safety zone 3D morphology was significantly altered by DS progression and characterised by a severe height decrease and a paradoxical width increase. This structural remodelling flattened the safety zone profile. Our quantitative analysis provided a new anatomical baseline for understanding the unique spatial constraints of the TF approach in vertebral slippage. transforaminal approach Kambin’s triangle degenerative spondylolisthesis MRI/CT fusion imaging full-endoscopic lumbar surgery Figures Figure 1 Figure 2 Figure 3 Introduction The transforaminal (TF) approach, which accesses the intervertebral disc and spinal canal through Kambin’s triangle, has become a cornerstone of minimally invasive spine surgery [ 1 – 4 ]. It is used for procedures ranging from full-endoscopic discectomy (FED) to TF lumbar interbody fusion, including percutaneous endoscopic transforaminal lumbar interbody fusion (PETLIF) [ 5 – 12 ]. By limiting trauma to the posterior elements, including the facet joints and paraspinal muscles, the TF approach facilitates rapid postoperative recovery [ 11 , 12 ]. The safety and efficacy of the TF approach depend on the anatomical dimensions of the safety zone within Kambin’s triangle, which is bounded by the exiting nerve root (ENR), superior articular process (SAP), and vertebral endplates [ 1 , 3 , 4 ]. Because surgical manoeuvres within this corridor inherently risk neural injury, accurate morphological assessment and a detailed understanding of this zone are critical[ 13 ]. Earlier studies used cadaveric or two-dimensional imaging methods, but these approaches were limited by altered tissue tension or an inability to simultaneously visualize complex bone–nerve relationships in three dimensions [ 14 – 16 ]. Recent three-dimensional (3D) imaging advancements, particularly artificial intelligence (AI)-based 3D magnetic resonance imaging/computed tomography (MRI/CT) fusion imaging, have enabled high-fidelity morphological evaluations [ 8 , 13 , 17 ]. Previous studies using this technology have demonstrated that the morphology of this zone varies across lumbar levels and is influenced by the surgical approach angle [ 8 , 13 ]. Nevertheless, these anatomical evaluations have primarily focused on relatively normal spinal alignments or general degenerative changes, leaving a clinical knowledge gap regarding specific pathological conditions. Degenerative lumbar spondylolisthesis (DS) is among the most common pathologies encountered in clinical practice, characterized by vertebral slippage and concomitant narrowing of the disc space [ 11 , 12 ]. These pathological changes may significantly alter the 3D spatial relationship between the ENR and surrounding bony structures. For spine surgeons utilising the TF approach, such alterations may narrow the safe working corridor, potentially increasing ENR injury risk, which is among the most critical full-endoscopic surgery complications. Nevertheless, the quantitative effect of vertebral slippage on the 3D morphology of the TF safety zone remains unclear. Here, we hypothesised that the TF safety zone undergoes progressive structural remodelling with increasing vertebral slippage. This study aimed to quantitatively evaluate the 3D morphological characteristics of the safety zone in patients with varying grades of DS. By simulating the actual endoscopic surgical trajectory using a validated 3D MRI/CT fusion model, we aimed to provide a comprehensive anatomical basis for the TF approach in patients with lumbar DS. Materials and methods Patients This study was approved by the relevant institutional review board, and written informed consent was obtained from all participants. We retrospectively analysed 60 patients who underwent 3D MRI/CT fusion imaging of the lumbar spine between April 2020 and January 2024. To enable balanced statistical comparisons across vertebral slippage severity and to reduce selection bias, we randomly sampled 20 subjects from each of three cohorts: no-slip, grade 1 spondylolisthesis, and grade 2 spondylolisthesis (the detailed criteria for these grades are described below) [ 18 ]. Inclusion criteria were age 55 years or older to evaluate the morphological characteristics of degenerative spondylolisthesis. The anterior L4 vertebral slippage (%slip) on mid-sagittal CT images was measured as a percentage of the anteroposterior diameter of the L5 vertebral body. According to the Meyerding classification, patients were categorized as no-slip, grade 1 (%slip < 25%), or grade 2 (25% or greater). Exclusion criteria were lumbar spondylolysis, lumbar degenerative scoliosis of 10 degrees or more, prior lumbar surgery, and congenital anomalies such as lumbarization or sacralisation. 3D MRI/CT fusion imaging of lumbar nerve/spine The 3D MRI/CT fusion images of the lumbar nerve and spine were generated as previously described [ 8 , 13 ]. Lumbar MRI was performed using a 1.5-T system (Signa Creator; GE Healthcare, Chicago, IL, USA) with a 3D multiple-echo recombined gradient-echo sequence. CT scans were acquired using a 64-channel multislice CT scanner (Revolution Maxima; GE Healthcare, Chicago, IL, USA) with a 1.25-mm slice thickness. To standardise imaging position between MRI and CT, a custom-made trunk support pillow was used to maintain the same posture [ 13 ]. AI-based software automatically extracted lumbar nerve roots from MRI volume data using a dedicated workstation (SYNAPSE VINCENT; Fujifilm Co., Ltd., Tokyo, Japan), markedly reducing reconstruction time [ 8 , 13 ]. The resulting 3D nerve images were fused with CT-derived bony structures on a workstation to create high-fidelity 3D models simultaneously depicting the ENR, SAP, and vertebral endplate morphology [ 13 ]. These MRI/CT fusion images enable evaluation of the spatial relationship between neural tissue and bone [ 13 ]. Simulation of the transforaminal surgical trajectory for L4/5 To reproduce the surgical trajectory of the full-endoscopic TF approach, the fused 3D MRI/CT model was first aligned with the axial plane of the upper L5 endplate. Subsequently, the model was rotated 45 ° towards the intended approach side, an angle validated to represent the TF trajectory used in clinical practice, such as FED-TF or PETLIF [ 7 , 9 , 13 ]. Morphological assessment of the safety zone At the L4/5 level, the safety zone for the TF approach was defined on the 45-degree rotated plane as the anatomical corridor bounded anteriorly by the ENR, posteriorly by the SAP, superiorly by the caudal endplate of L4, and inferiorly by the cephalic endplate of L5 (Fig. 1 ). Three morphological parameters were quantified (Fig. 1 ): Width: The horizontal distance between the SAP and ENR was measured at the centre of the L4/5 intervertebral disc. Height: Vertical distance between the caudal endplate of L4 and cephalic endplate of L5 at the midpoint of the width. Area: The total area of the safety zone as defined above. Overall, 120 sides were analysed bilaterally across the 60 subjects. Statistical Analysis Sample size was determined based on the feasibility and technical requirements of AI-based 3D MRI/CT fusion imaging. To ensure balanced statistical comparisons across the three Meyerding grades and minimize selection bias, 20 patients were randomly sampled from each group. Analysis of 120 independent sides provided sufficient statistical power to detect significant morphological alterations within the safety zone. Post-hoc power analysis using the observed effect sizes confirmed that the sample size was sufficient to detect significant differences in the primary parameters, with power (1 − β) exceeding 0.80. The normality of data distribution was assessed using the Shapiro-Wilk test. Continuous variables with a normal distribution are expressed as mean ± standard deviation. Conversely, side-to-side differences were calculated as absolute values and are presented descriptively as median (interquartile range [IQR]: 25th–75th percentile) Differences among the no-slip, grade 1, and grade 2 groups were evaluated using one-way analysis of variance. Categorical variables were evaluated using the chi-squared or Fisher’s exact test.The relationships between the L4 vertebral slip percentage (%slip) and morphological parameters were assessed using Pearson’s correlation coefficients. Statistical significance was set at p < 0.05. Results Demographic data for the study subjects Sixty subjects were included, with 20 randomly selected cases per group: no-slip, grade 1, and grade 2 slips. Age and sex were similar across groups (Table 1 ). On midline sagittal CT evaluation, the mean L4 vertebral slippage rate was 18.7% in grade 1 and 26.7% in grade 2 (p < 0.01). In the midsagittal CT evaluation, mean disc height was significantly lower in the vertebral slippage groups (7.1 mm for both grades 1 and 2) than in the no-slip group (10.3 mm; p < 0.01) (Table 1 ). Table 1 Demographic data of patients No-slip Grade 1 Grade 2 p-value Sex, n Female 16 16 17 0.89 Male 4 4 3 Age, years 73.1 ± 6.8 72.6 ± 6.9 71.4 ± 6.7 0.73 < 59 years, n 0 0 2 0.56 60–69 years, n 7 6 4 70–79 years, n 10 11 11 80–89 years, n 3 3 3 Slip percentage, % 0 18.7 ± 3.5 26.7 ± 1.4 < 0.01 Disc height, mm 10.3 ± 1.8 7.1 ± 1.6 7.1 ± 1.9 < 0.01 Data are presented as number or mean ± standard deviation Comparison of morphometric parameters for the safety zone among Meyerding grades The morphometric parameters of the safety zone in Kambin’s triangle were compared among the three Meyerding grades (Table 2 , Fig. 2 ). The mean of the safety zone width increased with advancing grade, from 6.2 mm in the no-slip group to 7.1 mm in grade 1 and 7.5 mm in grade 2, with a significant difference between the no-slip and grade 2 groups (p = 0.02). Conversely, safety zone height decreased significantly and progressively as slip grade increased, from 4.8 mm in the no-slip group to 4.0 mm in grade 1 and 3.5 mm in grade 2. Consistent with these changes, total safety zone area was reduced in grade 2 (26.8 mm 2 ) compared with the no-slip (31.4 mm 2 ) and grade 1 (32.4 mm 2 ) groups. Table 2 Analysis of safety zone of transforaminal approach for L4-5 level (n = 60) Group (n) Mean (mm) Right (mm) Left (mm) Difference between right and left (absolute value), median [IQR] Width (mm) (distance between SAP and ENR) No-slip (20) 6.2 ± 2.2 5.4 ± 2.2 7.0 ± 1.9 2.1 [1.1, 2.8] Grade 1 (20) 7.1 ± 2.3 7.2 ± 2.7 6.9 ± 1.7 1.7 [1.2, 2.8] Grade 2 (20) 7.5 ± 2.8 7.0 ± 2.6 7.9 ± 3.0 0.9 [0.5, 3.0] Total (60) 6.9 ± 2.1 6.5 ± 2.6 7.3 ± 2.2 1.7 [0.8, 2.9] Height (mm) (disc height in safety zone) No-slip (20) 4.8 ± 1.6 4.7 ± 1.6 4.8 ± 1.5 0.6 [0.4, 1.2] Grade 1 (20) 4.0 ± 0.9 3.8 ± 0.9 4.3 ± 0.9 0.7 [0.3, 1.0] Grade 2 (20) 3.5 ± 1.8 3.5 ± 1.8 3.6 ± 1.8 0.4 [0.2, 0.9] Total (60) 4.1 ± 1.6 4.0 ± 1.6 4.2 ± 1.5 0.6 [0.3, 1.0] Area (mm2) No-slip (20) 31.4 ± 13.7 27.9 ± 13.9 34.8 ± 12.6 6.9 [5.5, 12.9] Grade 1 (20) 32.4 ± 12.2 29.5 ± 13.4 35.4 ± 1.0 10.0 [5.9, 14.9] Grade 2 (20) 26.8 ± 17.0 25.9 ± 16.7 27.7 ± 17.2 6.2 [1.6, 12.9] Total (60) 30.2 ± 14.6 27.8 ± 14.8 32.7 ± 14.0 7.8 [4.7, 13.7] Data are presented as mean ± standard deviation. SAP, superior articular process; ENR, exiting nerve root; IQR, interquartile range. Furthermore, the absolute values of the differences between the right and left sides of each parameter were evaluated (Table 2 ). The median width difference ranged from 0.9 mm to 2.1 mm across the grades, while the median side-to-side difference for height was 0.6 mm (IQR: 0.3–1.0). The absolute difference in area was 7.8 mm 2 (IQR: 4.7–13.7), with the right side tending to be larger in all grades. Correlation between %slip and morphometric parameters Correlation analysis further elucidated the impact of vertebral slippage on morphometric parameters (Fig. 3 ). A divergent response was observed as vertebral slippage progressed: width significantly increased (r = 0.26, p < 0.05), whereas height exhibited a strong negative correlation with %slip (r = -0.67, p < 0.01). The total area demonstrated a significant negative correlation with %slip (r = -0.40, p < 0.01). Discussion Here, we quantitatively evaluated 3D TF safety zone morphology in patients with lumbar DS using an MRI/CT fusion model. The analysis demonstrated that vertebral slippage progression significantly altered safety zone anatomy within Kambin’s triangle. Specifically, safety zone height exhibited a marked, progressive decrease with advancing slippage, whereas width paradoxically increased. Accordingly, total safety zone area showed an overall downward trend as slip increased. Collectively, these findings indicate that DS progression fundamentally reshapes the surgical corridor, primarily through compromised vertical clearance. The most notable finding was the distinctive safety zone reshaping with vertebral slippage, a phenomenon not fully captured by conventional anatomical models or 2D image analyses [ 14 , 15 , 19 – 21 ]. The 3D MRI/CT fusion analysis demonstrated a divergent morphological response: width increased (r = 0.26), whereas height decreased substantially (r = -0.67). The paradoxical width increase was likely driven by relative posterior SAP displacement as the L4 vertebra translated anteriorly; as the SAP (posterior safety zone boundary) shifts away from the ENR, the horizontal corridor expands. Nonetheless, this horizontal gain is offset by pronounced height reduction. Hasegawa et al. [ 19 ] reported a significant positive correlation between posterior disc height and foraminal height and identified disc space narrowing as a key driver of vertical foraminal reduction, consistent with the present findings. Similarly, Cinotti et al. [ 21 ] demonstrated that disc narrowing significantly reduces foraminal vertical diameter (height) but does not significantly affect sagittal dimensions (width), likely because their analysis did not include pathological conditions such as spondylolisthesis [ 21 ]. This fundamental distinction supports the present study's contribution. Thus, in DS, the safety zone becomes wider horizontally but shallower vertically, a structural remodelling that complicates the surgical approach despite relatively preserved total area. These results provide clinically relevant insights for TF procedures, including full-endoscopic TF-lumbar interbody fusion (TF-LIF) [ 12 ]. Procedures such as PETLIF, one of the TF-LIF techniques, aim for indirect decompression and are well-suited for degenerative lumbar spondylolisthesis [ 9 , 11 , 12 ]. The marked decrease in safety zone height and area in severe DS suggests that standard interbody cage insertion without corrective manoeuvres carries a high ENR injury risk. Clinically, it is also important to recognize that intraoperative vertebral slippage reduction may paradoxically narrow safety zone width by shifting the SAP closer to the ENR. Therefore, to maintain a safe working space for cage insertion, surgeons should prioritize height restoration via adequate disc space distraction and perform aggressive foraminoplasty (SAP resection) to expand the safety zone width [ 9 , 10 , 12 ]. Emphasis on securing vertical clearance is essential to compensate for potential horizontal narrowing during the reduction process. Regarding side-to-side differences, our study revealed that each individual exhibits a degree of asymmetry. In TF-LIF procedures, surgeons can strategically select the wider side as the approach side based on 3D measurements to maximise the safety margin [ 9 ]. Because these subtle variations are challenging to appreciate in conventional 2D imaging, AI-based 3D MRI/CT fusion imaging represents an indispensable tool for case-specific surgical simulations and tailored planning [ 12 ]. This study had some limitations. First, all measurements were obtained in the supine position, whereas morphology may differ in the prone surgical position. Prior studies indicate that spinal alignment and foraminal dimensions are position-dependent, particularly in structurally unstable conditions such as DS. Because the safety zone reportedly widens in the prone position, the dimensions measured here may underestimate the true intraoperative working zone. Second, DS grading and evaluation relied on static CT images. Clinically, vertebral slippage severity is typically graded using standing lateral radiographs; because CT is performed supine without weight-bearing, vertebral translation may be reduced compared with the weight-bearing state. Consequently, the true association between slippage severity and morphological change may be even stronger than observed. Nevertheless, CT was necessary to enable high-precision 3D bony architecture analysis. Finally, the simulation assumed a fixed 45° approach angle, but patient-specific anatomy (e.g., high iliac crests or narrow disc spaces) may necessitate trajectory modification in practice. Future studies incorporating dynamic imaging or intraoperative measurements are warranted to further validate these morphological changes. Conclusion The 3D morphology of the TF safety zone was significantly altered by DS progression, which was characterised by a severe decrease in height and a paradoxical increase in width. This structural remodelling resulted in a flattened safety zone profile, suggesting that surgeons may need to focus on securing the vertical clearance, particularly if intraoperative slip reduction is planned, to compensate for potential changes in the horizontal corridor. Our quantitative analysis provided a new anatomical baseline for understanding the unique spatial constraints of the transforaminal approach in the presence of vertebral slippage. Declarations Funding: This work was supported in part by NSK Nakanishi Foundation. Author Contribution K.Y. and K.N. contributed to the conception and design of the study. Data acquisition was performed by K.Y., K.N., Y.H., and T.M. Analysis and interpretation of data were carried out by K.Y., Y.H. and T.M. Drafting of the manuscript was done by K.Y. Critical revision for important intellectual content was performed by K.N. Statistical analysis was conducted by K.Y. Funding acquisition was handled by K.Y. Administrative, technical, or material support was provided by K.N., Y.H. T.M. and T.K. Supervision was managed by Y.A., H.S., and N.I. All authors reviewed and approved the manuscript. Data Availability All data generated or analyzed during this study are included in this published article. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8579454","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":619905982,"identity":"ee887da7-cd07-431a-94e9-4e54c2a32058","order_by":0,"name":"Katsuhisa 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Hospital","correspondingAuthor":false,"prefix":"","firstName":"Terufumi","middleName":"","lastName":"Kokabu","suffix":""},{"id":619905987,"identity":"5d58378b-540b-492f-83e3-ef60a685c50d","order_by":5,"name":"Daisuke Ukeba","email":"","orcid":"","institution":"Hokkaido University Hospital","correspondingAuthor":false,"prefix":"","firstName":"Daisuke","middleName":"","lastName":"Ukeba","suffix":""},{"id":619905988,"identity":"0d161bf0-ec9a-4a56-a029-05f736aaa719","order_by":6,"name":"Tomoya Sato","email":"","orcid":"","institution":"Hokkaido University Hospital","correspondingAuthor":false,"prefix":"","firstName":"Tomoya","middleName":"","lastName":"Sato","suffix":""},{"id":619905989,"identity":"20ed618f-fe6a-4f4c-a644-e2f8c20acdf2","order_by":7,"name":"Tsutomu Endo","email":"","orcid":"","institution":"Hokkaido University Hospital","correspondingAuthor":false,"prefix":"","firstName":"Tsutomu","middleName":"","lastName":"Endo","suffix":""},{"id":619905990,"identity":"3feda7da-b238-49bc-9250-5cbdc892a557","order_by":8,"name":"Takashi Ohnishi","email":"","orcid":"","institution":"Hokkaido University Hospital","correspondingAuthor":false,"prefix":"","firstName":"Takashi","middleName":"","lastName":"Ohnishi","suffix":""},{"id":619905991,"identity":"b130a519-6c3d-41f1-bb24-39373df123bb","order_by":9,"name":"Ryo Fujita","email":"","orcid":"","institution":"Hokkaido University Hospital","correspondingAuthor":false,"prefix":"","firstName":"Ryo","middleName":"","lastName":"Fujita","suffix":""},{"id":619905992,"identity":"c3ef9908-ea1e-4869-a6e4-89e601afca38","order_by":10,"name":"Yoshinao Koike","email":"","orcid":"","institution":"Hokkaido University Hospital","correspondingAuthor":false,"prefix":"","firstName":"Yoshinao","middleName":"","lastName":"Koike","suffix":""},{"id":619905993,"identity":"9ef577ea-e19f-4393-a92e-e341efeac4dc","order_by":11,"name":"Yohei Sodeyama","email":"","orcid":"","institution":"Hokkaido University Hospital","correspondingAuthor":false,"prefix":"","firstName":"Yohei","middleName":"","lastName":"Sodeyama","suffix":""},{"id":619905994,"identity":"fa4cd7ef-d01f-45fc-89cd-75355cfb5ffa","order_by":12,"name":"Yuichiro Abe","email":"","orcid":"","institution":"Sapporo Medical Research","correspondingAuthor":false,"prefix":"","firstName":"Yuichiro","middleName":"","lastName":"Abe","suffix":""},{"id":619905995,"identity":"cb395281-b096-4fb1-acd6-8f0f3538d9e6","order_by":13,"name":"Hideki Sudo","email":"","orcid":"","institution":"Hokkaido University Hospital","correspondingAuthor":false,"prefix":"","firstName":"Hideki","middleName":"","lastName":"Sudo","suffix":""},{"id":619905996,"identity":"d8aec541-2dc9-43d2-bd9a-b21bfe10feb3","order_by":14,"name":"Norimasa Iwasaki","email":"","orcid":"","institution":"Hokkaido University Hospital","correspondingAuthor":false,"prefix":"","firstName":"Norimasa","middleName":"","lastName":"Iwasaki","suffix":""}],"badges":[],"createdAt":"2026-01-12 08:54:01","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8579454/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8579454/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":107246937,"identity":"9c1188a1-298b-4242-a696-874df0feea47","added_by":"auto","created_at":"2026-04-19 08:10:56","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1942054,"visible":true,"origin":"","legend":"\u003cp\u003e3D morphological assessment of the safety zone within Kambin’s triangle for transforaminal approach using MRI/CT fusion imaging. Representative 3D MRI/CT fusion images simulating the transforaminal surgical trajectory at the L4/5 level. The model is rotated 45 degrees to reproduce the surgical trajectory for the full-endoscopic transforaminal approach. The red trapezoid delineates the safety zone, bounded by the exiting nerve root (ENR), the superior articular process (SAP), and the vertebral endplates. Quantitative parameters include: (A) Width (the horizontal distance between the SAP and ENR, measured at the centre of the L4/5 intervertebral disc), (B) Height (the vertical distance between the caudal and cephalic endplates), and (C) Area (the total surface area within the defined boundaries).\u003c/p\u003e","description":"","filename":"Fig.1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8579454/v1/ec4f2d1a1a6de230b2344f0f.jpg"},{"id":107246855,"identity":"9d744a60-c468-422d-a19e-4d48592c85be","added_by":"auto","created_at":"2026-04-19 08:10:37","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1242020,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of morphometric parameters across Meyerding grades. Box-and-whisker plots showing the distribution of width, height, and area of the safety zone in the no-slip, grade 1, and grade 2 spondylolisthesis. Asterisks (*) indicate statistical significance at p \u0026lt; 0.05.\u003c/p\u003e","description":"","filename":"Fig.2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8579454/v1/7ec0023cf0f4d62ca9726173.jpg"},{"id":107246905,"identity":"846bcdb8-93ee-43dd-90c3-5b87d50bd765","added_by":"auto","created_at":"2026-04-19 08:10:49","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1122557,"visible":true,"origin":"","legend":"\u003cp\u003eCorrelation between vertebral slippage percentage and morphometric parameters of the safety zone. Scatter plots illustrating the relationships between the percentage of L4 vertebral slippage (%slip) and the safety zone dimensions. (A) Width shows a significant positive correlation with %slip (r = 0.26, p \u0026lt; 0.05). (B) Height shows a significant negative correlation with %slip (r = -0.67, p \u0026lt; 0.01). (C) Area demonstrates a significant negative correlation with %slip (r = -0.40, p \u0026lt; 0.01).\u003c/p\u003e","description":"","filename":"Fig.3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8579454/v1/860055cff1a4fd2299d28524.jpg"},{"id":107246973,"identity":"39a1ee19-cee1-4da7-b1f6-f3e2455d59cc","added_by":"auto","created_at":"2026-04-19 08:11:06","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4610618,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8579454/v1/75fbcf37-a869-4b33-b211-ddb651d05a3c.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Divergent morphological changes of Kambin’s triangle in degenerative spondylolisthesis: a 3D MRI/CT fusion study","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe transforaminal (TF) approach, which accesses the intervertebral disc and spinal canal through Kambin\u0026rsquo;s triangle, has become a cornerstone of minimally invasive spine surgery [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. It is used for procedures ranging from full-endoscopic discectomy (FED) to TF lumbar interbody fusion, including percutaneous endoscopic transforaminal lumbar interbody fusion (PETLIF) [\u003cspan additionalcitationids=\"CR6 CR7 CR8 CR9 CR10 CR11\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. By limiting trauma to the posterior elements, including the facet joints and paraspinal muscles, the TF approach facilitates rapid postoperative recovery [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe safety and efficacy of the TF approach depend on the anatomical dimensions of the safety zone within Kambin\u0026rsquo;s triangle, which is bounded by the exiting nerve root (ENR), superior articular process (SAP), and vertebral endplates [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Because surgical manoeuvres within this corridor inherently risk neural injury, accurate morphological assessment and a detailed understanding of this zone are critical[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Earlier studies used cadaveric or two-dimensional imaging methods, but these approaches were limited by altered tissue tension or an inability to simultaneously visualize complex bone\u0026ndash;nerve relationships in three dimensions [\u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eRecent three-dimensional (3D) imaging advancements, particularly artificial intelligence (AI)-based 3D magnetic resonance imaging/computed tomography (MRI/CT) fusion imaging, have enabled high-fidelity morphological evaluations [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Previous studies using this technology have demonstrated that the morphology of this zone varies across lumbar levels and is influenced by the surgical approach angle [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Nevertheless, these anatomical evaluations have primarily focused on relatively normal spinal alignments or general degenerative changes, leaving a clinical knowledge gap regarding specific pathological conditions.\u003c/p\u003e \u003cp\u003eDegenerative lumbar spondylolisthesis (DS) is among the most common pathologies encountered in clinical practice, characterized by vertebral slippage and concomitant narrowing of the disc space [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. These pathological changes may significantly alter the 3D spatial relationship between the ENR and surrounding bony structures. For spine surgeons utilising the TF approach, such alterations may narrow the safe working corridor, potentially increasing ENR injury risk, which is among the most critical full-endoscopic surgery complications. Nevertheless, the quantitative effect of vertebral slippage on the 3D morphology of the TF safety zone remains unclear.\u003c/p\u003e \u003cp\u003eHere, we hypothesised that the TF safety zone undergoes progressive structural remodelling with increasing vertebral slippage. This study aimed to quantitatively evaluate the 3D morphological characteristics of the safety zone in patients with varying grades of DS. By simulating the actual endoscopic surgical trajectory using a validated 3D MRI/CT fusion model, we aimed to provide a comprehensive anatomical basis for the TF approach in patients with lumbar DS.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003ePatients\u003c/h2\u003e \u003cp\u003eThis study was approved by the relevant institutional review board, and written informed consent was obtained from all participants. We retrospectively analysed 60 patients who underwent 3D MRI/CT fusion imaging of the lumbar spine between April 2020 and January 2024. To enable balanced statistical comparisons across vertebral slippage severity and to reduce selection bias, we randomly sampled 20 subjects from each of three cohorts: no-slip, grade 1 spondylolisthesis, and grade 2 spondylolisthesis (the detailed criteria for these grades are described below) [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Inclusion criteria were age 55 years or older to evaluate the morphological characteristics of degenerative spondylolisthesis.\u003c/p\u003e \u003cp\u003eThe anterior L4 vertebral slippage (%slip) on mid-sagittal CT images was measured as a percentage of the anteroposterior diameter of the L5 vertebral body. According to the Meyerding classification, patients were categorized as no-slip, grade 1 (%slip\u0026thinsp;\u0026lt;\u0026thinsp;25%), or grade 2 (25% or greater). Exclusion criteria were lumbar spondylolysis, lumbar degenerative scoliosis of 10 degrees or more, prior lumbar surgery, and congenital anomalies such as lumbarization or sacralisation.\u003c/p\u003e \u003cp\u003e \u003cb\u003e3D MRI/CT fusion imaging of lumbar nerve/spine\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe 3D MRI/CT fusion images of the lumbar nerve and spine were generated as previously described [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Lumbar MRI was performed using a 1.5-T system (Signa Creator; GE Healthcare, Chicago, IL, USA) with a 3D multiple-echo recombined gradient-echo sequence. CT scans were acquired using a 64-channel multislice CT scanner (Revolution Maxima; GE Healthcare, Chicago, IL, USA) with a 1.25-mm slice thickness. To standardise imaging position between MRI and CT, a custom-made trunk support pillow was used to maintain the same posture [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. AI-based software automatically extracted lumbar nerve roots from MRI volume data using a dedicated workstation (SYNAPSE VINCENT; Fujifilm Co., Ltd., Tokyo, Japan), markedly reducing reconstruction time [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The resulting 3D nerve images were fused with CT-derived bony structures on a workstation to create high-fidelity 3D models simultaneously depicting the ENR, SAP, and vertebral endplate morphology [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. These MRI/CT fusion images enable evaluation of the spatial relationship between neural tissue and bone [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSimulation of the transforaminal surgical trajectory for L4/5\u003c/h3\u003e\n\u003cp\u003eTo reproduce the surgical trajectory of the full-endoscopic TF approach, the fused 3D MRI/CT model was first aligned with the axial plane of the upper L5 endplate. Subsequently, the model was rotated 45 \u0026deg; towards the intended approach side, an angle validated to represent the TF trajectory used in clinical practice, such as FED-TF or PETLIF [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e\n\u003ch3\u003eMorphological assessment of the safety zone\u003c/h3\u003e\n\u003cp\u003eAt the L4/5 level, the safety zone for the TF approach was defined on the 45-degree rotated plane as the anatomical corridor bounded anteriorly by the ENR, posteriorly by the SAP, superiorly by the caudal endplate of L4, and inferiorly by the cephalic endplate of L5 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThree morphological parameters were quantified (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e):\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eWidth: The horizontal distance between the SAP and ENR was measured at the centre of the L4/5 intervertebral disc.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eHeight: Vertical distance between the caudal endplate of L4 and cephalic endplate of L5 at the midpoint of the width.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eArea: The total area of the safety zone as defined above.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eOverall, 120 sides were analysed bilaterally across the 60 subjects.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eStatistical Analysis\u003c/h2\u003e \u003cp\u003eSample size was determined based on the feasibility and technical requirements of AI-based 3D MRI/CT fusion imaging. To ensure balanced statistical comparisons across the three Meyerding grades and minimize selection bias, 20 patients were randomly sampled from each group. Analysis of 120 independent sides provided sufficient statistical power to detect significant morphological alterations within the safety zone. Post-hoc power analysis using the observed effect sizes confirmed that the sample size was sufficient to detect significant differences in the primary parameters, with power (1\u0026thinsp;\u0026minus;\u0026thinsp;β) exceeding 0.80.\u003c/p\u003e \u003cp\u003eThe normality of data distribution was assessed using the Shapiro-Wilk test. Continuous variables with a normal distribution are expressed as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation. Conversely, side-to-side differences were calculated as absolute values and are presented descriptively as median (interquartile range [IQR]: 25th\u0026ndash;75th percentile) Differences among the no-slip, grade 1, and grade 2 groups were evaluated using one-way analysis of variance. Categorical variables were evaluated using the chi-squared or Fisher\u0026rsquo;s exact test.The relationships between the L4 vertebral slip percentage (%slip) and morphological parameters were assessed using Pearson\u0026rsquo;s correlation coefficients. Statistical significance was set at p\u0026thinsp;\u0026lt;\u0026thinsp;0.05.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eDemographic data for the study subjects\u003c/h2\u003e \u003cp\u003eSixty subjects were included, with 20 randomly selected cases per group: no-slip, grade 1, and grade 2 slips. Age and sex were similar across groups (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). On midline sagittal CT evaluation, the mean L4 vertebral slippage rate was 18.7% in grade 1 and 26.7% in grade 2 (p\u0026thinsp;\u0026lt;\u0026thinsp;0.01). In the midsagittal CT evaluation, mean disc height was significantly lower in the vertebral slippage groups (7.1 mm for both grades 1 and 2) than in the no-slip group (10.3 mm; p\u0026thinsp;\u0026lt;\u0026thinsp;0.01) (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDemographic data of patients\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNo-slip\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eGrade 1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eGrade 2\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003ep-value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSex, n\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFemale\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.89\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMale\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAge, years\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e73.1\u0026thinsp;\u0026plusmn;\u0026thinsp;6.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e72.6\u0026thinsp;\u0026plusmn;\u0026thinsp;6.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e71.4\u0026thinsp;\u0026plusmn;\u0026thinsp;6.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.73\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;59 years, n\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.56\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003e60\u0026ndash;69 years, n\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003e70\u0026ndash;79 years, n\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003e80\u0026ndash;89 years, n\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eSlip percentage, %\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e18.7\u0026thinsp;\u0026plusmn;\u0026thinsp;3.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e26.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eDisc height, mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.1\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e7.1\u0026thinsp;\u0026plusmn;\u0026thinsp;1.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003eData are presented as number or mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eComparison of morphometric parameters for the safety zone among Meyerding grades\u003c/h3\u003e\n\u003cp\u003eThe morphometric parameters of the safety zone in Kambin\u0026rsquo;s triangle were compared among the three Meyerding grades (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The mean of the safety zone width increased with advancing grade, from 6.2 mm in the no-slip group to 7.1 mm in grade 1 and 7.5 mm in grade 2, with a significant difference between the no-slip and grade 2 groups (p\u0026thinsp;=\u0026thinsp;0.02). Conversely, safety zone height decreased significantly and progressively as slip grade increased, from 4.8 mm in the no-slip group to 4.0 mm in grade 1 and 3.5 mm in grade 2. Consistent with these changes, total safety zone area was reduced in grade 2 (26.8 mm\u003csup\u003e2\u003c/sup\u003e) compared with the no-slip (31.4 mm\u003csup\u003e2\u003c/sup\u003e) and grade 1 (32.4 mm\u003csup\u003e2\u003c/sup\u003e) groups.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAnalysis of safety zone of transforaminal approach for L4-5 level (n\u0026thinsp;=\u0026thinsp;60)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGroup (n)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMean (mm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eRight (mm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eLeft (mm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eDifference between right and left (absolute value), median [IQR]\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"5\" nameend=\"c5\" namest=\"c1\"\u003e \u003cp\u003eWidth (mm) (distance between SAP and ENR)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNo-slip (20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.4\u0026thinsp;\u0026plusmn;\u0026thinsp;2.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.0\u0026thinsp;\u0026plusmn;\u0026thinsp;1.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.1 [1.1, 2.8]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGrade 1 (20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7.1\u0026thinsp;\u0026plusmn;\u0026thinsp;2.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e7.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.9\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.7 [1.2, 2.8]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGrade 2 (20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7.5\u0026thinsp;\u0026plusmn;\u0026thinsp;2.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e7.0\u0026thinsp;\u0026plusmn;\u0026thinsp;2.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.9\u0026thinsp;\u0026plusmn;\u0026thinsp;3.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.9 [0.5, 3.0]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTotal (60)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6.9\u0026thinsp;\u0026plusmn;\u0026thinsp;2.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6.5\u0026thinsp;\u0026plusmn;\u0026thinsp;2.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.3\u0026thinsp;\u0026plusmn;\u0026thinsp;2.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.7 [0.8, 2.9]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"5\" nameend=\"c5\" namest=\"c1\"\u003e \u003cp\u003eHeight (mm) (disc height in safety zone)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNo-slip (20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.6 [0.4, 1.2]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGrade 1 (20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.7 [0.3, 1.0]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGrade 2 (20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3.5\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.5\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.4 [0.2, 0.9]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTotal (60)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.1\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.0\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.2\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.6 [0.3, 1.0]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"5\" nameend=\"c5\" namest=\"c1\"\u003e \u003cp\u003eArea (mm2)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNo-slip (20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e31.4\u0026thinsp;\u0026plusmn;\u0026thinsp;13.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e27.9\u0026thinsp;\u0026plusmn;\u0026thinsp;13.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e34.8\u0026thinsp;\u0026plusmn;\u0026thinsp;12.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e6.9 [5.5, 12.9]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGrade 1 (20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e32.4\u0026thinsp;\u0026plusmn;\u0026thinsp;12.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e29.5\u0026thinsp;\u0026plusmn;\u0026thinsp;13.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e35.4\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e10.0 [5.9, 14.9]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGrade 2 (20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e26.8\u0026thinsp;\u0026plusmn;\u0026thinsp;17.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e25.9\u0026thinsp;\u0026plusmn;\u0026thinsp;16.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e27.7\u0026thinsp;\u0026plusmn;\u0026thinsp;17.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e6.2 [1.6, 12.9]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTotal (60)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e30.2\u0026thinsp;\u0026plusmn;\u0026thinsp;14.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e27.8\u0026thinsp;\u0026plusmn;\u0026thinsp;14.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e32.7\u0026thinsp;\u0026plusmn;\u0026thinsp;14.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e7.8 [4.7, 13.7]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003eData are presented as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation. SAP, superior articular process; ENR, exiting nerve root; IQR, interquartile range.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFurthermore, the absolute values of the differences between the right and left sides of each parameter were evaluated (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The median width difference ranged from 0.9 mm to 2.1 mm across the grades, while the median side-to-side difference for height was 0.6 mm (IQR: 0.3\u0026ndash;1.0). The absolute difference in area was 7.8 mm\u003csup\u003e2\u003c/sup\u003e (IQR: 4.7\u0026ndash;13.7), with the right side tending to be larger in all grades.\u003c/p\u003e\n\u003ch3\u003eCorrelation between %slip and morphometric parameters\u003c/h3\u003e\n\u003cp\u003eCorrelation analysis further elucidated the impact of vertebral slippage on morphometric parameters (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). A divergent response was observed as vertebral slippage progressed: width significantly increased (r\u0026thinsp;=\u0026thinsp;0.26, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), whereas height exhibited a strong negative correlation with %slip (r = -0.67, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01). The total area demonstrated a significant negative correlation with %slip (r = -0.40, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eHere, we quantitatively evaluated 3D TF safety zone morphology in patients with lumbar DS using an MRI/CT fusion model. The analysis demonstrated that vertebral slippage progression significantly altered safety zone anatomy within Kambin\u0026rsquo;s triangle. Specifically, safety zone height exhibited a marked, progressive decrease with advancing slippage, whereas width paradoxically increased. Accordingly, total safety zone area showed an overall downward trend as slip increased. Collectively, these findings indicate that DS progression fundamentally reshapes the surgical corridor, primarily through compromised vertical clearance.\u003c/p\u003e \u003cp\u003eThe most notable finding was the distinctive safety zone reshaping with vertebral slippage, a phenomenon not fully captured by conventional anatomical models or 2D image analyses [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan additionalcitationids=\"CR20\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The 3D MRI/CT fusion analysis demonstrated a divergent morphological response: width increased (r\u0026thinsp;=\u0026thinsp;0.26), whereas height decreased substantially (r = -0.67). The paradoxical width increase was likely driven by relative posterior SAP displacement as the L4 vertebra translated anteriorly; as the SAP (posterior safety zone boundary) shifts away from the ENR, the horizontal corridor expands. Nonetheless, this horizontal gain is offset by pronounced height reduction. Hasegawa et al. [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] reported a significant positive correlation between posterior disc height and foraminal height and identified disc space narrowing as a key driver of vertical foraminal reduction, consistent with the present findings. Similarly, Cinotti et al. [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] demonstrated that disc narrowing significantly reduces foraminal vertical diameter (height) but does not significantly affect sagittal dimensions (width), likely because their analysis did not include pathological conditions such as spondylolisthesis [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. This fundamental distinction supports the present study's contribution. Thus, in DS, the safety zone becomes wider horizontally but shallower vertically, a structural remodelling that complicates the surgical approach despite relatively preserved total area.\u003c/p\u003e \u003cp\u003eThese results provide clinically relevant insights for TF procedures, including full-endoscopic TF-lumbar interbody fusion (TF-LIF) [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Procedures such as PETLIF, one of the TF-LIF techniques, aim for indirect decompression and are well-suited for degenerative lumbar spondylolisthesis [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. The marked decrease in safety zone height and area in severe DS suggests that standard interbody cage insertion without corrective manoeuvres carries a high ENR injury risk. Clinically, it is also important to recognize that intraoperative vertebral slippage reduction may paradoxically narrow safety zone width by shifting the SAP closer to the ENR. Therefore, to maintain a safe working space for cage insertion, surgeons should prioritize height restoration via adequate disc space distraction and perform aggressive foraminoplasty (SAP resection) to expand the safety zone width [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Emphasis on securing vertical clearance is essential to compensate for potential horizontal narrowing during the reduction process.\u003c/p\u003e \u003cp\u003eRegarding side-to-side differences, our study revealed that each individual exhibits a degree of asymmetry. In TF-LIF procedures, surgeons can strategically select the wider side as the approach side based on 3D measurements to maximise the safety margin [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Because these subtle variations are challenging to appreciate in conventional 2D imaging, AI-based 3D MRI/CT fusion imaging represents an indispensable tool for case-specific surgical simulations and tailored planning [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThis study had some limitations. First, all measurements were obtained in the supine position, whereas morphology may differ in the prone surgical position. Prior studies indicate that spinal alignment and foraminal dimensions are position-dependent, particularly in structurally unstable conditions such as DS. Because the safety zone reportedly widens in the prone position, the dimensions measured here may underestimate the true intraoperative working zone. Second, DS grading and evaluation relied on static CT images. Clinically, vertebral slippage severity is typically graded using standing lateral radiographs; because CT is performed supine without weight-bearing, vertebral translation may be reduced compared with the weight-bearing state. Consequently, the true association between slippage severity and morphological change may be even stronger than observed. Nevertheless, CT was necessary to enable high-precision 3D bony architecture analysis. Finally, the simulation assumed a fixed 45\u0026deg; approach angle, but patient-specific anatomy (e.g., high iliac crests or narrow disc spaces) may necessitate trajectory modification in practice. Future studies incorporating dynamic imaging or intraoperative measurements are warranted to further validate these morphological changes.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe 3D morphology of the TF safety zone was significantly altered by DS progression, which was characterised by a severe decrease in height and a paradoxical increase in width. This structural remodelling resulted in a flattened safety zone profile, suggesting that surgeons may need to focus on securing the vertical clearance, particularly if intraoperative slip reduction is planned, to compensate for potential changes in the horizontal corridor. Our quantitative analysis provided a new anatomical baseline for understanding the unique spatial constraints of the transforaminal approach in the presence of vertebral slippage.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding:\u003c/h2\u003e \u003cp\u003eThis work was supported in part by NSK Nakanishi Foundation.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eK.Y. and K.N. contributed to the conception and design of the study. Data acquisition was performed by K.Y., K.N., Y.H., and T.M. Analysis and interpretation of data were carried out by K.Y., Y.H. and T.M. Drafting of the manuscript was done by K.Y. Critical revision for important intellectual content was performed by K.N. Statistical analysis was conducted by K.Y. Funding acquisition was handled by K.Y. Administrative, technical, or material support was provided by K.N., Y.H. T.M. and T.K. Supervision was managed by Y.A., H.S., and N.I. All authors reviewed and approved the manuscript.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eAll data generated or analyzed during this study are included in this published article.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eFanous AA, Tumial\u0026aacute;n LM, Wang MY (2019) Kambin\u0026rsquo;s triangle: Definition and new classification schema. J Neurosurg Spine 32:390\u0026ndash;398. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3171/2019.8.SPINE181475\u003c/span\u003e\u003cspan address=\"10.3171/2019.8.SPINE181475\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKambin P, Schaffer JL (1989) Percutaneous lumbar discectomy. Review of 100 patients and current practice. 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Spine 27:223\u0026ndash;229. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1097/00007632-200202010-00002\u003c/span\u003e\u003cspan address=\"10.1097/00007632-200202010-00002\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"european-spine-journal","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"esjo","sideBox":"Learn more about [European Spine Journal](http://link.springer.com/journal/586)","snPcode":"586","submissionUrl":"https://submission.springernature.com/new-submission/586/3","title":"European Spine Journal","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"transforaminal approach, Kambin’s triangle, degenerative spondylolisthesis, MRI/CT fusion imaging, full-endoscopic lumbar surgery","lastPublishedDoi":"10.21203/rs.3.rs-8579454/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8579454/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003ePurpose\u003c/h2\u003e \u003cp\u003eThe transforaminal (TF) approach through Kambin\u0026rsquo;s triangle is the cornerstone of minimally invasive spinal surgery. Although degenerative lumbar spondylolisthesis (DS) is a common indication, its quantitative effect on the three-dimensional (3D) anatomy of the safety zone remains unclear. We aimed to evaluate the 3D morphological alterations of the TF safety zone in DS using magnetic resonance imaging (MRI)/computed tomography (CT) fusion imaging.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eWe retrospectively analysed 60 patients (120 sides) categorised into three groups (n\u0026thinsp;=\u0026thinsp;20 each): no-slip, Meyerding gade 1, and gade 2 spondylolisthesis. Using a validated 3D MRI/CT fusion model, we simulated a 45\u0026deg; transforaminal surgical trajectory at the L4/5 level. The width (horizontal distance from the superior articular process [SAP] to the exiting nerve root), height (inter-endplate distance), and total safety zone area were quantified.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eA divergent response was observed as vertebral slippage progressed. Width significantly increased with slip degree (no-slip: 6.2 mm; grade 2:7.5 mm; r\u0026thinsp;=\u0026thinsp;0.26, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), reflecting relative SAP posterior displacement. Conversely, height demonstrated a negative correlation (no-slip: 4.8 mm; grade 2:3.5 mm; r=-0.67, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01) due to disc height loss. The total area demonstrated a significant negative correlation with slip percentage (r = -0.40, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01).\u003c/p\u003e\u003ch2\u003eConclusions\u003c/h2\u003e \u003cp\u003eThe TF safety zone 3D morphology was significantly altered by DS progression and characterised by a severe height decrease and a paradoxical width increase. This structural remodelling flattened the safety zone profile. Our quantitative analysis provided a new anatomical baseline for understanding the unique spatial constraints of the TF approach in vertebral slippage.\u003c/p\u003e","manuscriptTitle":"Divergent morphological changes of Kambin’s triangle in degenerative spondylolisthesis: a 3D MRI/CT fusion study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-19 08:09:52","doi":"10.21203/rs.3.rs-8579454/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewersInvited","content":"","date":"2026-04-08T22:05:39+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-16T07:29:53+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-16T07:27:13+00:00","index":"","fulltext":""},{"type":"submitted","content":"European Spine Journal","date":"2026-01-12T08:39:14+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"european-spine-journal","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"esjo","sideBox":"Learn more about [European Spine Journal](http://link.springer.com/journal/586)","snPcode":"586","submissionUrl":"https://submission.springernature.com/new-submission/586/3","title":"European Spine Journal","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"9b8aa229-3c09-4dd9-bd1e-732b3c5fa407","owner":[],"postedDate":"April 19th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-19T08:09:53+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-19 08:09:52","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8579454","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8579454","identity":"rs-8579454","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}