Minimally invasive spine surgery has undergone remarkable evolution over the past 2 decades, with full-endoscopic lumbar decompression becoming one of its most promising techniques [1]. The interlaminar endoscopic route offers a familiar posterior trajectory that enables direct access to central, paracentral, and migrated disc fragments while preserving musculoligamentous integrity. The approach is particularly advantageous at the L5–S1 level, where a high iliac crest limits the feasibility of the transforaminal corridor.

Despite these benefits, the technical complexity of interlaminar endoscopic decompression varies considerably among patients. Factors such as interlaminar space anatomy, facet joint orientation, ligamentum flavum (LF) thickness, and canal dimensions significantly affect the maneuverability of instruments and visualization. These variations influence operative time, blood loss, fluoroscopy duration, and the likelihood of conversion to microscopic surgery. However, objective literature defining clinical and radiological predictors of surgical ease in interlaminar endoscopy remains scarce.

Anatomically, interlaminar window height and width are primary determinants of surgical exposure [2]. A narrow window or high iliac crest restricts endoscope angulation, increasing operative time and fatigue. Similarly, facet hypertrophy and steep orientation reduce the working space, especially at L4–5 [3]. Previous morphometric studies using magnetic resonance imaging (MRI) and computed tomography have defined normative dimensions and their level-wise variations. Reduced interlaminar window dimensions and increased LF thickness correlate with central canal stenosis and surgical difficulty in microscopic decompressions. Yet, these associations remain underexplored in full-endoscopic procedures, where visualization is more constrained [4].

Beyond anatomy, patient-specific clinical factors—such as high body mass index (BMI), diabetes, and prior lumbar surgery—also impact operative ease by altering tissue planes, promoting fibrosis, or increasing bleeding. The present prospective observational study was designed to identify preoperative predictors of surgical ease in patients undergoing interlaminar endoscopic lumbar decompression [5]. We hypothesized that a narrow interlaminar window, facet hypertrophy, and LF thickness >4 mm would independently predict higher technical difficulty. A secondary aim was to develop and internally validate an Endoscopic Ease Index (EEI), a composite score correlating intraoperative metrics—such as operative time, bleeding, field clarity, and surgeon-rated difficulty—with preoperative parameters, enabling surgeons to anticipate challenges and optimize efficiency.

This prospective observational study was conducted in the Department of Neurosurgery at our institute between January 2021 and May 2025. Consecutive adults undergoing interlaminar endoscopic lumbar decompression for disc herniation or degenerative canal stenosis were enrolled after taking prior informed consent.

Patients aged ≥18 years with unilateral or bilateral radiculopathy due to MRI-confirmed disc herniation or central/lateral recess stenosis were included. Exclusion criteria were multilevel decompression, combined transforaminal–interlaminar procedures, spinal infection, trauma, neoplasm, fixed spondylolisthesis ≥ grade II, prior fusion at the index level, or severe medical comorbidities.

All surgeries were performed by a single senior endoscopic spine surgeon (≥100 prior cases). Under general or spinal anesthesia, patients were positioned prone on a radiolucent frame with flexed hips and knees. Following fluoroscopic localization, a 1-cm incision was made over the interlaminar space. Sequential dilators and a working cannula were docked on the LF, and an 8-mm endoscope (Richard Wolf GmbH, Germany) was introduced under continuous saline irrigation. Limited laminotomy and partial flavectomy were performed as required, followed by decompression using endoscopic forceps and bipolar cautery [6]. The wound was closed without drainage.

The EEI was defined a priori as a composite measure of intraoperative technical difficulty. It was designed to capture multiple dimensions of procedural complexity, including temporal factors, technical exposure, anatomical resistance, adverse intraoperative events, and surgeon perception (Table 1).

This prospective observational study was conducted using a predefined protocol in which EEI components, radiological parameters, and outcome measures were established prior to patient enrolment.

EEI incorporated 10 predefined intraoperative parameters. Operating time, docking time, and fluoroscopy usage were treated as continuous variables and standardized as z-scores. Bleeding was graded on a 4-point scale (0=minimal bleeding without field obscuration; 1=mild bleeding, easily controlled; 2=moderate bleeding causing intermittent field obscuration; 3=severe bleeding with persistent obscuration).

Visual field clarity was graded on a 5-point scale (1=poor visualization throughout; 2=frequently obscured field; 3=moderate clarity with intermittent obscuration; 4=mostly clear field with minimal interruptions; 5=consistently clear visualization). Surgeon-rated difficulty was scored on a 5-point Likert scale (1=very easy; 2=easy; 3=moderately difficult; 4=difficult; 5=extremely difficult).

Penalty-based variables were incorporated to reflect clinically meaningful intraoperative challenges. An unplanned laminotomy extension or additional facet undercutting was assigned +1 point. Dural exposure or unintended durotomy was assigned +2 points, while conversion to microscopic surgery was assigned +3 points, reflecting escalating technical consequence.

The final EEI score was calculated as the sum of standardized, ordinal, and penalty components. Because EEI includes standardized continuous variables, it does not have a fixed upper limit; however, penalty-based components allow a maximum additive burden of +8 points. To categorize surgical difficulty, the upper tertile of EEI values was selected a priori to define technically difficult cases.

Preoperative data included demographics, BMI, symptom duration, comorbidities, and prior surgery. MRI morphometric parameters assessed by a blinded neurosurgeon and radiologist included disc morphology, migration (Lee classification) [7], calcification, interlaminar window size, facet hypertrophy and orientation, LF thickness, canal and foraminal stenosis (Schizas and Lee grades) [8], sacral slope, iliac crest height, disc height, and epidural fat thickness. Facet hypertrophy was assessed on axial T2-weighted MRI at the index level and graded according to medial encroachment into the spinal canal: grade 0 (none), grade 1 (mild without canal compromise), grade 2 (moderate with partial canal encroachment), and grade 3 (severe with significant canal or lateral recess compromise). Grades ≥2 were considered clinically significant. Interlaminar window height and LF thickness were measured on sagittal and axial T2-weighted MRI images, respectively, as illustrated in Figure 1. Interobserver reliability was measured using intraclass correlation coefficient (ICC) and weighted kappa (excellent if ICC >0.8, κ >0.7).

Visual analogue scale (VAS; leg pain) was recorded preoperatively, on postoperative day 1, and at 6 weeks; Oswestry Disability Index (ODI) was measured preoperatively and at 3 months. Early complications were documented.

Data were analyzed using IBM SPSS Statistics ver. 26.0 (IBM Co., USA) and R v4.3.2 (R Foundation for Statistical Computing, Austria). Group comparisons employed t-test, Mann-Whitney U, or chi-square tests. Variables demonstrating p<0.20 on univariate analysis, along with clinically relevant covariates identified from prior literature, were entered into the multivariate logistic regression model. Model performance was evaluated using receiver operating characteristic curve (AUC), Hosmer-Lemeshow test, and bootstrap validation (1,000 resamples). A nomogram was generated to estimate preoperative difficulty probability.

This study was approved by the Institutional Ethics Committee of Sri Venkateswara Institute of Medical Sciences, Tirupati (approval number: 1577).

Using an objective EEI, nearly one-third of cases in the present study were categorized as technically difficult. The most significant independent predictors of increased surgical difficulty were LF thickness >4 mm, interlaminar window height <11 mm, and facet hypertrophy grade ≥2. These findings underscore the importance of detailed preoperative morphometric evaluation when planning interlaminar endoscopic lumbar decompression.

Comparative endoscopic studies have demonstrated that anatomical constraints significantly influence the choice and feasibility of interlaminar versus transforaminal approaches. Previous morphometric studies have demonstrated that the dimensions of the endoscopic working corridor and the relationship between the nerve root and the superior articular process significantly influence surgical accessibility and technical complexity during endoscopic lumbar discectomy [9]. This observation is consistent with our finding that facet hypertrophy independently predicts technical difficulty in interlaminar endoscopy.

Prospective randomized evidence has further demonstrated that, despite greater technical demands, full-endoscopic lumbar discectomy achieves clinical outcomes comparable to conventional microscopic techniques. Ruetten et al. reported equivalent pain relief and functional outcomes between endoscopic and microsurgical approaches, despite differences in operative exposure and technical execution [1]. These data support the premise that technical difficulty does not inherently compromise surgical efficacy when procedures are performed by experienced surgeons.

Long-term outcome studies reinforce this concept. Parker et al. [10] demonstrated sustained pain relief following lumbar discectomy, suggesting that operative complexity and technical nuances do not necessarily translate into inferior long-term clinical outcomes. In the present study, postoperative VAS and ODI improvements were comparable between difficult and nondifficult cases, further supporting this observation.

From a safety perspective, complication profiles differ between open and minimally invasive lumbar spine procedures. Shih et al. [11] reported that minimally invasive techniques are associated with complication rates comparable to or lower than open surgery when appropriate patient selection and surgical expertise are ensured. Anticipating anatomical difficulty, rather than avoiding endoscopic approaches, therefore appears to be the more appropriate strategy.

Advances in endoscopic techniques have expanded indications beyond central and paracentral pathology. Choi [12] demonstrated favorable outcomes using percutaneous endoscopic techniques for extraforaminal lumbar disc herniations, highlighting the growing technical versatility of endoscopic surgery. However, such expanded indications further emphasize the need for meticulous anatomical assessment and careful preoperative planning.

Endoscopic decompression for lumbar spinal stenosis remains technically demanding, particularly in the presence of hypertrophic LF and facet joints. Ahn [13] highlighted that endoscopic stenosis surgery is highly sensitive to anatomical constraints and requires a steep learning curve, with surgical difficulty driven predominantly by osseoligamentous morphology rather than patient-related factors. This aligns with our observation that clinical variables such as obesity and prior surgery were not independent predictors after adjustment for anatomical parameters.

Finally, a detailed understanding of interlaminar surgical anatomy remains fundamental to safe and efficient endoscopic practice. Komp and Ruetten [14] described the surgical anatomy of the lumbar interlaminar window in detail, emphasizing the importance of corridor geometry, laminar inclination, and facet orientation in determining endoscopic accessibility. Building on this anatomical foundation, our study quantitatively links these morphometric features to an objective measure of surgical difficulty through the EEI.

The EEI integrates time-based, visual, and surgeon-assessed parameters into a structured framework for estimating intraoperative technical complexity. By correlating EEI with measurable anatomical predictors, this study provides a reproducible tool for preoperative risk stratification. Future multicentric validation and incorporation of 3-dimensional morphometric and artificial intelligence-based modeling may further refine predictive accuracy and enhance endoscopic training paradigms.

Anatomical factors such as LF thickness >4 mm, interlaminar window height <11 mm, and facet hypertrophy ≥ grade 2 are independently associated with increased technical difficulty in interlaminar endoscopic lumbar decompression. The EEI provides an objective assessment of technical complexity. Anticipating these morphometric predictors enables better preoperative planning, safer execution, and optimized training, ensuring that increased technical difficulty does not compromise surgical outcomes in experienced hands.