The sciatic nerve (SN) is the principal continuation of the sacral plexus (L4–S3) and the largest peripheral nerve in the human body, traversing the deep gluteal region before descending into the posterior thigh. It lies between the greater trochanter and the ischial tuberosity, deep to the gluteus maximus, and typically bifurcates at the popliteal fossa into tibial and common peroneal branches ¹.

However, extensive cadaveric and imaging evidence demonstrates that the SN displays considerable variability in its level of division and its relationship to the piriformis muscle ²,³. Beaton and Anson’s (1937) classification remains foundational, describing six patterns ranging from the classical undivided nerve below the piriformis (Type I) to multiple high-division and piercing variants (Types II–VI) ⁴. Such deviations, though often asymptomatic, carry direct clinical relevance. During posterior hip arthroplasty or acetabular fixation, an unrecognized variant may increase the risk of iatrogenic nerve injury ⁵. Similarly, in regional anesthesia, early bifurcation of the SN may cause incomplete sciatic nerve blocks ⁶.

The depth of the nerve from the skin and its distance to bony landmarks are critical parameters guiding safe needle trajectories and surgical dissections ⁷. Population-specific morphometric data are especially necessary in South Asian cohorts, where pelvic dimensions and gluteal muscle bulk differ from Western norms. Despite abundant cadaveric literature, in-vivo studies integrating HRUS, MRI, and intraoperative correlation remain limited ⁸,⁹.

Therefore, the present prospective anatomical study aimed to quantify the prevalence and morphometric characteristics of sciatic nerve variations in the gluteal region, classify them using standard criteria, and establish normative data through imaging validation. By linking anatomical observation with radiological precision, the study bridges traditional morphology and modern clinical application, providing surgeons, anesthesiologists, and anatomists with a population-relevant anatomical reference.

A prospective observational study was conducted over 18 months at a tertiary-care teaching hospital in collaboration between the Departments of Anatomy, Radiology, and Orthopaedics/Anaesthesiology.

Participants:
Three hundred adults (≥ 18 years) undergoing posterior hip procedures, gluteal pain evaluation, or sciatic nerve blocks were recruited after informed consent. Exclusion criteria included prior gluteal surgery or trauma, neuromuscular disorders, and MRI contraindications.

Imaging protocol:

• HRUS: Curvilinear 2–5 MHz probe (linear 5–12 MHz for lean subjects). Measurements were taken along a longitudinal line between the GT and IT.
• MRI: 1.5/3 T scanners using axial and oblique T1/T2 fat-suppressed sequences.
  Both modalities assessed the course, bifurcation level, and morphometric parameters: skin-to-nerve depth, SN–GT distance, SN–IT distance, and nerve diameter.
  Variations were classified per Beaton–Anson Types I–VI. Intraoperative visualization during posterior hip surgeries (n = 62) served as gold standard for validation.

Sample size:

Calculated for an expected 20 % prevalence, α = 0.05, precision ± 5 %, yielding n = 273; rounded to 300.

Data analysis:

Continuous variables expressed as mean ± SD; categorical as percentages. Chi-square and t-tests assessed group differences; logistic regression determined correlates of variant presence. Inter-observer reliability used intraclass correlation coefficient (ICC) and Cohen’s κ. Significance p < 0.05.

Ethics:
Institutional Ethics Committee approval obtained. Study posed minimal risk; confidentiality maintained.

Table 1. Demographic profile of study participants (n = 300)

*Significant at p < 0.05; NS = not significant.

Table 2. Distribution of sciatic-nerve variations according to Beaton & Anson classification (n = 288)

Table 3. Side-wise distribution and bilaterality of sciatic-nerve variations

Variants occurred slightly more often on the right side; ~85 % of subjects displayed bilateral symmetry.

Table 4. Morphometric parameters of the sciatic nerve in the gluteal region (HRUS-based)

*Significant at p < 0.05. GT = greater trochanter; IT = ischial tuberosity.

Table 5. Comparative diagnostic performance of imaging modalities (n = 62 intra-operative confirmations)

MRI demonstrated slightly higher diagnostic accuracy, though HRUS remained highly reliable.

Table 6. Association between demographic variables and presence of sciatic-nerve variant (binary logistic regression)

*Significant at p < 0.05.

Table 7. Inter-observer reliability for imaging measurements (n = 60)

The present study demonstrates that nearly one-quarter of the population exhibits high-division or variant courses of the sciatic nerve. This prevalence (28.1 %) agrees with previous Indian cadaveric reports (20–30 %) ¹⁰ and Mediterranean data ¹¹, but exceeds the 10–20 % rates in East Asian and Western cohorts ¹². Such variability likely reflects ethnic and morphometric influences on pelvic development.

The measured mean skin-to-nerve depth (≈ 5 cm) corroborates Rajasekaran et al. (2016) and has direct procedural relevance. Increased depth in males aligns with greater gluteal muscle mass, whereas higher BMI correlated with variant presence—potentially due to altered pelvic tilt and muscle orientation. These findings reinforce the need for individualized needle length and insertion angle during posterior approaches.

The study also validates imaging as a reliable anatomical tool. MRI achieved near-perfect sensitivity and specificity, while HRUS provided real-time guidance suitable for anesthesia practice. High inter-observer ICC (> 0.85) confirms reproducibility, strengthening its pedagogical and clinical utility. Importantly, the integration of imaging and intraoperative data bridges the gap between cadaveric anatomy and living-body variation.

Anatomically, high-division variants alter the topographic relationships of the nerve with the piriformis, gemelli, and obturator internus, predisposing to entrapment syndromes and procedural complications. Surgeons must anticipate such variants during posterior hip exposure, while anesthesiologists should employ ultrasound mapping to ensure complete nerve blockade.

Approximately one in four individuals demonstrates non-classical sciatic nerve anatomy in the gluteal region. MRI and HRUS reliably delineate these variations, enabling safer surgical corridors and more effective nerve blocks. The morphometric reference values established herein provide a region-specific anatomical framework for clinicians and educators alike.

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