Preoperative MRI Features and Outcomes for Lumbar Disc Herniation Treated with Percutaneous Endoscopic Lumbar Discectomy: A Retrospective Study

Summary

This retrospective study of 101 PELD patients showed that preoperative MRI features, particularly Modic II (unfavorable) and DHI > 25% (protective), were closely associated with 1-year prognosis. These findings can guide surgical decisions and are not used as static predictive factors. Complete neural decompression remains the ultimate determining factor for success.

Abstract

Percutaneous endoscopic lumbar discectomy (PELD) is widely used for lumbar disc herniation (LDH), yet outcomes vary. Preoperative magnetic resonance imaging (MRI) features may predict prognosis, but systematic research on their correlation with surgical efficacy is lacking. The purpose of this study is to retrospectively analyze the relationship between preoperative MRI morphological characteristics and clinical outcomes in patients receiving PELD treatment for lumbar disc herniation, with the aim of providing a reference for preoperative evaluation, outcome prediction, and surgical strategy optimization. In this retrospective study, 101 single-level LDH patients undergoing PELD (2022.1–2024.9) with ≥1-year follow-up were included. Preoperative MRI parameters [Pfirrmann grade, Modic changes, herniation type/location/migration, canal occupation ratio, disc height index (DHI)] and clinical outcomes [Visual Analog Scale (VAS), Oswestry Disability Index (ODI), modified MacNab] were analyzed. Patients were grouped by 1-year MacNab scores. MRI features were compared between groups, with Pearson correlation and multivariate logistic regression used to identify predictors of a favorable prognosis. Of 101 patients, 76 (75.2%) had favorable outcomes and 25 (24.8%) unfavorable outcomes at 1-year follow-up. The unfavorable group showed significantly higher rates of disc extrusion/sequestration, larger herniation area, and Modic type II changes, but lower intervertebral height and foraminal area (P < 0.05). Canal occupation ratio and herniation area positively correlated with pain VAS and ODI, while DHI and foraminal area showed negative correlations. Modic type II changes independently predicted unfavorable prognosis, whereas DHI > 25% was a protective factor. Preoperative MRI features are closely associated with PELD outcomes, with Modic type II and reduced DHI identifying high-risk patients. This multidimensional MRI evaluation enables preoperative risk stratification and guides individualized surgical decision-making, including consideration of foraminoplasty or alternative approaches when adverse imaging features are present. Complete neural decompression, however, remains the ultimate determinant of success.

Introduction

Lumbar disc herniation (LDH) is primarily characterized by disc degeneration, annulus fibrosus rupture, and nucleus pulposus herniation compressing nerve roots or the cauda equina. Clinically, the condition is characterized by manifestations including sciatica, lower limb numbness and weakness, and intermittent claudication. These symptoms severely compromise patients’ daily life and working ability, thus placing a substantial economic burden on individuals, families, and society^1,2. Although traditional open surgery for LDH allows direct removal of the herniated nucleus pulposus, it is associated with drawbacks, including substantial trauma, significant blood loss, delayed postoperative recovery, and disruption of spinal stability³. Percutaneous endoscopic lumbar discectomy (PELD) has evolved over more than two decades since Yeung and Tsou⁴ first reported the transforaminal endoscopic discectomy (YESS technique) in 1997, and has now become a mainstream minimally invasive approach for treating LDH. PELD utilizes a working channel approximately 6–7 mm in diameter to remove herniated disc tissue under direct endoscopic visualization, offering notable advantages such as minimal trauma, preserved spinal stability^5,6. Numerous clinical studies have confirmed that PELD achieves excellent or good outcomes in 84%–94% of LDH cases, comparable to those of traditional open surgery⁷. However, clinical practice reveals that even with strict adherence to surgical indications and standardized procedures performed by the same surgical team, postoperative outcomes following PELD exhibit considerable inter-individual variability. While some patients experience significant pain relief and functional recovery, approximately 20%–30% report residual low back or leg pain, sensory abnormalities, suboptimal functional recovery, or even recurrence^8,9. This phenomenon has emerged as a significant clinical challenge for spine surgeons. Recently, the concept of defining objective and subjective “end-points of decompression” has been proposed as a critical strategy to minimize surgical failures and optimize outcomes in transforaminal endoscopic lumbar surgery¹⁰.

Preoperative magnetic resonance imaging (MRI) is the preferred imaging modality for evaluating LDH, providing clear visualization of disc morphology, herniation extent, nerve compression, and the structure of the spinal canal and neural foramina. MRI morphological parameters not only aid in confirming diagnosis and surgical indications but may also offer predictive value for postoperative outcomes. The Pfirrmann grading system, based on T2-weighted MRI signal intensity, disc structure, disc height, and distinction between the nucleus pulposus and annulus fibrosus, classifies disc degeneration into grades 1–5 and is widely used in clinical assessment¹¹. Modic changes refer to signal alterations in the vertebral endplates and adjacent bone marrow on MRI, categorized into type I (edematous), type II (fatty), and type III (sclerotic), closely associated with disc degeneration, low back pain, and postoperative recurrence¹². An increasing number of studies have investigated the association between MRI morphological features and clinical outcomes following PELD. However, current research predominantly focuses on single or a few MRI parameters, lacking a systematic evaluation of multiple morphological indicators. Moreover, studies on the relationship between quantitative parameters—such as spinal canal occupation ratio, foraminal area, and disc height index (DHI)—and PELD outcomes remain insufficient¹³. Furthermore, recent advancements in minimally invasive spinal surgery have evolved from standalone endoscopic or channel techniques toward intelligent minimally invasive approaches integrating robotics, artificial intelligence, and imaging guidance. The integration of robotic-assisted systems has further addressed instrumentation challenges in confined surgical spaces, representing a critical advancement in minimally invasive spinal surgery technology¹⁴.

Based on the aforementioned research background and existing knowledge gaps, this single-center retrospective observational study aims to systematically collect clinical data and preoperative MRI morphological features from patients undergoing PELD for single-level LDH. A multidimensional MRI morphological evaluation system will be constructed, and in conjunction with one-year postoperative clinical outcomes, the associations between various MRI morphological parameters and PELD prognosis will be thoroughly analyzed to identify independent risk and protective factors influencing unfavorable postoperative outcomes. The primary objectives of this study include: determining the overall clinical efficacy of PELD for single-level LDH and the distribution of one-year prognostic outcomes; comparing differences in preoperative MRI morphological parameters between patients with favorable and unfavorable prognoses; identifying independent predictors of favorable postoperative outcomes through multivariate logistic regression analysis; and establishing a multidimensional preoperative MRI evaluation system that integrates both qualitative (Modic subtypes, herniation grade) and quantitative parameters (canal occupation ratio, DHI, foraminal area, herniation area) to enable more precise postoperative risk stratification and guide individualized surgical decision-making.

Protocol

This study was conducted in strict adherence to the principles of the Declaration of Helsinki¹⁵ and was approved by the Institutional Ethics Committee of North China Medical Health Group XingTai General Hospital (Approval No. ZCKT-2026-0018). All patients were informed of the study purpose, treatment protocols, and potential risks, and provided written informed consent. All study data were anonymized to ensure patient privacy.

Research subjects
This single-center retrospective observational study included patients with single-level LDH who underwent PELD in the orthopedics department of our hospital from January 2022 to September 2024. Among 123 initially screened patients, 101 were ultimately included after applying strict inclusion and exclusion criteria, all of whom completed at least one year of follow-up. Based on the modified MacNab criteria at one-year post-surgery, patients were classified into the favorable group (n = 76) and the unfavorable group (n = 25). The detailed process is shown in Figure 1.

Figure 1: Research flowchart. Flowchart of patient selection and grouping. Of 123 assessed patients, 101 with single-level LDH undergoing PELD were included and divided into Favorable (n = 76) and Unfavorable (n = 25) groups based on 1-year modified MacNab criteria. The analytical framework covered general clinical data, MRI parameters, and clinical outcomes (VAS, ODI, MacNab). Please click here to view a larger version of this figure.

Inclusion criteria
Age between 18 and 75 years; diagnosed with single-level LDH at L3/4, L4/5, or L5/S1 based on clinical symptoms, physical examination, and MRI/CT findings; presence of definite symptoms and signs of nerve root compression, with no response to at least 6 weeks of systematic conservative treatment (including bed rest, medication, physical therapy, etc.); underwent PELD surgery; postoperative follow-up duration of at least 1 year with complete clinical data.

Exclusion criteria
Multilevel LDH or a history of previous lumbar surgery; concomitant lumbar spondylolisthesis, lumbar spinal stenosis, spinal tumors, spinal infections, or spinal fractures; presence of cauda equina syndrome requiring emergency surgery; poor preoperative MRI image quality precluding accurate measurement; severe comorbid medical conditions (e.g., cardiac, pulmonary, hepatic, or renal insufficiency) rendering patients unfit for surgery; loss to follow-up or incomplete clinical data.

Surgical procedure
All surgeries were performed by the same experienced team of spine surgeons. The surgical approach (transforaminal, TF, or interlaminar, IL) was determined based on a comprehensive evaluation of the herniation morphology and anatomical constraints. Specifically, the TF approach was preferentially adopted for foraminal, extraforaminal, or central/paracentral herniations located below the pedicle level. Conversely, the IL approach was selected for highly migrated or sequestered fragments, particularly those migrating cephalad to the L5 pedicle or in patients with a high iliac crest, as previously described by Choi et al.¹⁶. This differentiated selection strategy ensured that the surgical trajectory aligned optimally with the target lesion.

Procedures were conducted under local anesthesia with or without intravenous sedation, with patients positioned prone. The target disc space was localized using C-arm fluoroscopy. A puncture needle was advanced into the spinal canal through Kambin's triangle or the interlaminar space, followed by guidewire insertion and sequential dilation to establish a working channel. A working endoscope (diameter 6.9 mm, 30° viewing angle) was introduced, and herniated nucleus pulposus tissue was removed under direct visualization to achieve nerve root decompression. Annuloplasty and hemostasis were performed using a radiofrequency probe. The working channel was then withdrawn, and the skin incision was sutured.

In cases presenting with preoperative DHI ≤25% or severe foraminal narrowing on MRI, additional foraminoplasty using a reamer or high-speed drill was performed at the discretion of the attending surgeon to enlarge Kambin's working zone and facilitate channel placement. However, given the retrospective design, the execution of this foraminoplasty was neither strictly standardized nor systematically documented in the operative notes, which precludes a quantitative analysis of its specific impact on clinical outcomes in this cohort.

Observation indicators
In this study, T0, T1, T2, and T3 were defined as preoperative, 1 week postoperative, 3 months postoperative, and 1 year postoperative, respectively.

General clinical data
Patient data, including age, sex, body mass index (BMI), disease duration, smoking history, history of diabetes mellitus, history of hypertension, surgical level, and surgical approach, were collected.

Measurement of MRI morphological parameters
All patients underwent preoperative lumbar MRI, including sagittal T1WI, T2WI, and axial T2WI sequences. Two experienced radiologists independently measured the parameters in a double-blind manner, with the average values used for analysis. Pfirrmann grade: Assessed on sagittal T2WI based on disc signal intensity, structure, height, and distinction between nucleus pulposus and annulus fibrosus, graded 1–5¹¹. Modic changes: Evaluated as signal changes in vertebral endplates and adjacent bone marrow, classified as none, type I (hypointense on T1WI, hyperintense on T2WI), type II (hyperintense on T1WI, iso- or slightly hyperintense on T2WI), or type III (hypointense on both T1WI and T2WI)¹². Herniation type: Classified on axial T2WI as bulging, protrusion, extrusion, or sequestration¹⁷. Herniation location: Categorized as central, paracentral, foraminal, or far lateral⁴. Extent of migration: Measured on sagittal T2WI as the vertical distance from the upper or lower margin of the herniated fragment to the disc level, classified according to Lee et al.¹⁸ as no migration (grade 0), low-grade migration (<50% of disc height, grades 1–2), or high-grade migration (≥50% of disc height, grades 3–4). Spinal canal occupation ratio: Calculated on axial T2WI at the level of maximal herniation as (anteroposterior diameter of the herniation / anteroposterior diameter of the spinal canal) × 100%^19,20. DHI: Measured on sagittal T2WI as [(anterior disc height + posterior disc height) / 2] / [(anterior-posterior diameter of upper vertebral body + anterior-posterior diameter of lower vertebral body) / 2] × 100%²¹. Foraminal area: Manually delineated and measured on axial T2WI at the maximal foraminal level using ImageJ software^22,23. Maximum herniation area: Measured as the cross-sectional area of the herniated fragment on axial T2WI at the level of maximal herniation²⁴.

Clinical outcome evaluation
Pain Assessment: Pain intensity was evaluated using the Visual Analog Scale (VAS) for low back pain (VAS-Back) and leg pain (VAS-Leg), with scores ranging from 0 (no pain) to 10 (worst imaginable pain)^25,26. Functional Disability: Lumbar function was assessed using the Oswestry Disability Index (ODI)^27,28. Modified MacNab Criteria: Evaluated at the 1-year follow-up, outcomes were classified as excellent (complete symptom resolution, return to normal activities), good (occasional pain, no interference with work or daily life), fair (improved symptoms but activity limitations), or poor (no improvement or worsened symptoms)^29,30. “Excellent” and “good” outcomes were defined as a favorable prognosis, while “fair” and “poor” outcomes were defined as an unfavorable prognosis. The modified MacNab criteria were selected as the primary outcome measure because they provide a global, patient-centered assessment of surgical success that integrates pain relief, functional recovery, and return to daily activities—domains that are collectively more relevant to clinical decision-making than any single continuous scale. Additionally, the MacNab classification has been widely utilized and validated in PELD literature^29,30, facilitating direct comparison with prior studies.

Sample size estimation
Sample size estimation was performed using G*Power software³¹. As a retrospective observational study, the sample size was determined based on the total number of patients meeting the inclusion and exclusion criteria during the study period, and its adequacy was verified through post-hoc power analysis. Based on previous relevant studies⁸, assuming an incidence of unfavorable prognosis following PELD of approximately 25% and using disc extrusion/sequestration as the primary exposure factor, a minimum of 89 participants was required with a two-sided α = 0.05 and a power (1-β) of 80%. A total of 101 patients were ultimately included in this study, meeting the statistical requirements.

Statistical analysis
Statistical analyses were performed using SPSS. Normality of continuous variables was assessed by the Shapiro-Wilk test. Normally distributed data were presented as mean ± SD and compared using independent-samples and paired t-tests. Non-normally distributed data were shown as median (Q1, Q3) and analyzed via the Mann-Whitney U test. Categorical variables were compared using the chi-square test. Pearson correlation and multivariate logistic regression were applied. All tests were two-sided, and P < 0.05 was deemed statistically significant.

For multivariate logistic regression analysis, the dependent variable was encoded as favorable prognosis = 1 and unfavorable prognosis = 0, resulting in 76 positive events. The number of covariates included in the final model must comply with the EPV principle³². Used variance inflation factor (VIF) to evaluate multicollinearity among MRI parameters; a VIF value <5 is considered acceptable.

Results

General information about patients
A total of 101 patients were included in this study. As shown in Table 1, patients in the unfavorable group were significantly older (P < 0.05) and had a longer disease duration (P < 0.05). No statistically significant differences were observed between the two groups regarding sex, BMI, smoking history, comorbidities, surgical level, or surgical approach (P > 0.05).

Discussion

This retrospective study analyzed 101 patients with single-level LDH who underwent PELD, aiming to investigate the correlation between preoperative MRI morphological features, clinical factors, and 1-year postoperative clinical outcomes. The findings demonstrated that PELD achieved favorable overall clinical efficacy for LDH, with an excellent-to-good rate (favorable group) of 75.2% at the 1-year follow-up. However, 24.8% of patients experienced unfavorable outcomes (unfavorable group). This result is highly consistent w...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
C-arm Fluoroscope	Philips	718133	Intraoperative localization of target disc space
G*Power	Heinrich-Heine-Universität Düsseldorf	Version 3.1.9.7	Performed post-hoc power analysis for sample size.
ImageJ Software	National Institutes of Health (NIH)	Version 1.54r	Measure foraminal area & herniation area
MRI Scanner (3.0T Siemens Skyra)	Siemens Healthineers, Erlangen, Germany	syngo MR E11	Preoperative lumbar MRI: sagittal T1WI, T2WI, and axial T2WI sequences using a dedicated spine coil.
Radiofrequency System	Elliquence	IEC6-SU170	Intraoperative annuloplasty and hemostasis
Statistical Package for the Social Sciences (SPSS)	IBM	Version 25.0	Used for all statistical analyses.