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This study is a retrospective analysis approved by the Institutional Ethics Committee of Nantong First People's Hospital. The trial has been registered on ClinicalTrials.gov (Registration Number: NCT06833099). As all participants' health information was anonymized, informed consent was not required. The consumables and equipment used are listed in the Table of Materials.
1. Study population
The study included clinical data and preoperative imaging records of 436 patients who underwent percutaneous endoscopic interlaminar discectomy (PEID) for low back pain and leg pain due to L5-S1 disc herniation at Nantong First People's Hospital between January 1, 2020, and June 30, 2024. Based on exclusion criteria, 309 patients were ultimately included, among whom recurrent lumbar disc herniation (rLDH) was confirmed through postoperative Visual Analog Scale (VAS) scores and follow-up imaging. Of the 309 patients, 33 experienced rLDH post-surgery, while the remaining 276 had significant relief of low back and leg pain, with VAS scores reduced by over 60%. Imaging variables for all participants were derived from preoperative X-ray, CT, and MRI examinations, combined with detailed clinical information, including gender, age, height, weight, BMI, VAS scores, and other relevant variables (Figure 1).
2. Inclusion and exclusion criteria
Inclusion criteria for rLDH: (1) Patients with L5-S1 lumbar disc herniation who underwent single-segment PEID. (2) Comprehensive imaging examinations completed within one month preoperatively. (3) Postoperative VAS score reduction ≥60%, followed by an increase in score and confirmation by imaging. (4) No other abnormalities detected on imaging. (5) Minimum follow-up period of 6 months.
Inclusion criteria for Non-rLDH: (1) Patients with L5-S1 lumbar disc herniation who underwent single-segment PEID. (2) Comprehensive imaging examinations completed within one month preoperatively. (3) Postoperative VAS score reduction ≥60%, with no recurrence. (4) No other abnormalities detected on imaging. (5) Minimum follow-up period of 6 months.
Exclusion criteria: (1) Presence of other pathological conditions causing low back pain, such as disc infection, spinal tumors, metabolic bone diseases, or osteoporosis. (2) History of lumbar disc or other spinal surgeries. (3) Poor imaging quality or incomplete examination data. (4) Loss to follow-up.
3. Categorical and continuous variables
Preoperative clinical characteristics and imaging data of patients were statistically analyzed and measured (Table 1 and Table 2). Categorical variables were used to distinguish basic disease characteristics, lifestyle factors, and other variables, while continuous variables represented specific measurements describing patients' physiological status and preoperative imaging changes. To reduce bias, strict quality control measures were implemented. Two radiologists and spine surgeons with over 10 years of clinical experience were responsible for the statistical analysis and measurement of imaging data. For complex cases, the two doctors resolved issues through joint consultation to ensure accuracy and consistency in data processing.
Categorical variables included: gender, diabetes, hypertension, cardiovascular and cerebrovascular diseases (CCD), scoliosis, spinal stenosis, triggering factors (e.g., strenuous activity, sprain, cold exposure, impact, or no clear trigger), disease duration (over 6 months or less than 6 months), numbness or weakness, protrusion type (contained or non-contained), disc degeneration (Grades I, II, III and IV, V), adjacent disc degeneration (Grades I, II, III and IV, V), Modic changes, and disc calcification.
Continuous variables included: age, surgical duration, height, weight, body mass index (BMI), maximum disc herniation diameter (MDHD), posterior disc height (PDH), distance between two vertebrae centers (DBTVC), facet joint orientation angle (FJOA), disc angle (DA), sacral slope angle (SSA), lumbar lordosis angle (LLA), and posterior disc height index (PDHI, PDHI = PDH / DBTVC).
4. Data cleaning and variable selection
First, the data file "zjkj3" was read into R (version 4.3.1, https://www.r-project.org/, Platform: x86_64-w64-mingw32/x64 (64-bit), Copyright (C) 2023 The R Foundation for Statistical Computing) using the read_excel function and stored in a data variable. The character encoding selected was UTF-8 (system default), which is a widely used encoding method capable of supporting the written languages of most countries around the world. The select function was then used to separate the target variable from the feature variables. To ensure reproducibility of the data split, a fixed random seed of 3 was set, and the dataset was divided into a training set (80%) and a test set (20%). Finally, a LASSO regression function was defined, and a portion of the training data was randomly sampled using the glmnet function to fit the LASSO regression. The optimal regularization parameter λ was determined using L1 regularization and cross-validation (default 10-fold), while ensuring no duplicate patient data was mixed between the training and test sets. The cross-validation error for each λ value was calculated to determine the optimal λ value.
5. Model development and evaluation
SVM (Support Vector Machine): This model maps training data to a hyperplane, maximizing the margin between two classes to predict the target. The specific hyperparameter settings for the SVM model in this study are: kernel = "linear", cost = 1. Supplemental Figure 1 depicts the confusion matrices for SVM.
DT (Decision Tree): This model recursively splits data to create a tree structure for predicting the target variable. The specific hyperparameter setting for the DT model in this study is: max_depth = 3. Supplemental Figure 2 depicts the confusion matrices for DT.
ADA (AdaBoost): This model combines multiple weak learners (typically decision trees) to create a strong learner, improving classification performance. The specific hyperparameter settings for the ADA model in this study are: n_estimators = 150, learning_rate = 0.1, seed = 80. Supplemental Figure 3 depicts the confusion matrices for ADA.
LGBM (Light Gradient Boosting Machine): This model uses a histogram algorithm to split continuous features, speeding up the training process and reducing memory load, making it an efficient and fast gradient boosting framework, particularly suitable for large-scale datasets. The specific hyperparameter settings for the LGBM model in this study are: num_leaves = 5, learning_rate = 0.05, n_estimators = 50. Supplemental Figure 4 depicts the confusion matrices for LGBM.
RF (Random Forest): This model constructs multiple decision trees and combines their predictions to improve accuracy and control overfitting. The specific hyperparameter settings for the RF model in this study are: ntree = 310, mtry = 1, maxnodes = 10, max_depth = 1, seed = 80. Supplemental Figure 5 depicts the confusion matrices for RF.
XGB (Extreme Gradient Boosting): This model is an enhanced algorithm based on decision trees, utilizing a new generalized gradient boosting decision tree algorithm to accelerate model construction, with strong applicability for classification and regression tasks. The specific hyperparameter settings for the XGB model in this study are: nrounds = 100, max_depth = 2, eta = 0.39, gamma = 0, colsample_bytree = 0.88, seed = 80. Supplemental Figure 6 depicts the confusion matrices for XGB.
The performance of the six models was evaluated using the following metrics: test set ROC AUC, accuracy, sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and F1 score. The calculation formula is as follows22:

TP: True Positive; TN: True Negative; FP: False Positive; FN: False Negative.
6. Grid search and hyperparameter tuning
To optimize model performance, grid search was utilized to explore various combinations of hyperparameters. Through iterative tuning and evaluation, the optimal hyperparameter combination was identified to maximize the ROC AUC values for both the train and test sets, thereby improving the overall model performance. The ranges of adjusted hyperparameters are presented in Table 3. The grid search is provided in Supplemental Figure 7, Supplemental Figure 8, and Supplemental Table 1, Supplemental Table 2, Supplemental Table 3, and Supplemental Table 4.
7. Variable importance ranking
The top-performing models-Random Forest (RF) and Extreme Gradient Boosting (XGB)-were selected for variable importance ranking. These rankings aid in identifying the most critical predictors of L5-S1 rLDH following PEID.
8. Statistical analysis
All statistical analyses were performed using R software. Categorical variables were reported as percentages, while continuous variables were expressed as mean ± standard deviation. Clinical characteristics and imaging parameters were compared between the non-recurrent group (non-rLDH, n=276) and the recurrent group (rLDH, n=33).