European Journal of Cardiovascular Medicine

The sciatic nerve (SN) is the main motor and sensory pathway of the lower limb and a cornerstone target in regional anesthesia for orthopedic and lower-limb procedures ¹. Its classical anatomy—an undivided trunk exiting the greater sciatic foramen below the piriformis—permits predictable localization for posterior (gluteal) or sub-gluteal block approaches ². However, anatomical studies have demonstrated frequent deviations in its course and bifurcation pattern, particularly high division of tibial and common peroneal components or passage through/above the piriformis muscle ³,⁴.

These variants may significantly alter the spread of local anesthetic and the clinical efficacy of the block ⁵. A single-injection technique based on classical landmarks may fail to anesthetize both divisions, leading to incomplete sensory or motor block ⁶. Additionally, variant nerve trajectories lying more superficially or close to vascular structures may predispose to paresthesia, intraneural injection, or neuropraxia ⁷.

Modern imaging modalities—especially high-resolution ultrasound (HRUS) and MRI—have transformed regional anesthesia by enabling visualization of the sciatic nerve and its branches ⁸. Ultrasonographic mapping can identify early bifurcation, adjust probe position, and guide dual-injection strategies ⁹. Yet, few studies have prospectively correlated imaging-defined variations with actual clinical outcomes of sciatic nerve blocks.

The present prospective study was designed to determine how anatomical variations of the sciatic nerve, confirmed by HRUS/MRI, influence block characteristics (onset, success rate, paresthesia incidence) and short-term postoperative neuropathy. The findings provide evidence for individualized imaging-guided strategies in lower-limb anesthesia, enhancing both efficacy and safety.

A prospective observational study was performed over 18 months at a tertiary-care teaching hospital. Adult patients (≥ 18 years) scheduled for posterior sciatic nerve block (for hip, thigh, or knee surgery) were included. Exclusion criteria: prior gluteal surgery or trauma, neuromuscular disease, coagulopathy, local infection, or allergy to local anesthetic.

Imaging and classification:
Pre-block HRUS (curvilinear 2–5 MHz; 5–12 MHz for lean subjects) mapped the nerve trajectory between greater trochanter (GT) and ischial tuberosity (IT). MRI (1.5 T) was used in selected cases for confirmation. Variants were classified using Beaton–Anson Types I–VI.

Block technique:
Blocks were performed in the lateral decubitus position with the leg flexed. Under ultrasound guidance, a 100-mm insulated needle was advanced in-plane to deposit 20–25 mL of 0.5 % bupivacaine around the nerve or its divisions.

Outcome assessment:

• Success rate: complete sensory and motor block within 20 min.
• Onset time: time from injection to full loss of sensation.
• Complications: paresthesia, vascular puncture, or pain during injection.
• Post-operative neuropathy: assessed clinically and, if persistent > 48 h, by EMG.

Statistical analysis:
Continuous data expressed as mean ± SD; categorical data as percentages. Independent-sample t and χ² tests compared Type I vs variant groups. Logistic regression evaluated predictors of block failure. Significance p < 0.05.

Table 1. Block characteristics in classical vs variant anatomy (n = 102)

* p < 0.05 significant.

Findings: Variant anatomy significantly delayed onset and reduced success rate. Paresthesia occurred more frequently when the common peroneal division pierced or coursed above the piriformis.

Post-operative neuropathy

Out of 62 surgical patients with intra-operative visualization, 3 (4.8 %) developed transient neuropraxia—all belonging to variant types. Symptoms resolved within six weeks without residual deficits.

Table 2: Predictors of variant anatomy

*Significant at p < 0.05. Higher BMI correlated with variant presence and block difficulty.

This prospective study demonstrates that sciatic-nerve variations substantially affect block performance. Approximately 28 % of subjects had high-division or atypical courses—comparable to previous anatomical reports (20–30 %) ¹⁰,¹¹. In these variants, the tibial and common peroneal components separate proximal to the subgluteal region, often diverging around the piriformis ¹². Consequently, a single injection at the classical site may reach only one component, explaining the reduced block success (84 %) and longer onset (≈ 10 min) observed here.

The higher incidence of paresthesia in variant cases parallels findings by Karmakar et al. (2018) and Choquet et al. (2017), who emphasized the role of ultrasound in recognizing early bifurcation and preventing intraneural puncture. In our cohort, HRUS mapping allowed adjustment of probe position and dual-injection strategy when bifurcation was detected, improving completeness.

Transient postoperative neuropraxia (4.8 %) occurred exclusively in variant anatomy, aligning with literature reports of increased vulnerability during posterior hip procedures ¹³. Mechanisms include traction, compression, or inadvertent stretch of a high-division peroneal branch lying superficially over the short external rotators.

BMI correlated positively with variant frequency and block depth. Obese patients demonstrated deeper nerve location and increased technical difficulty, reinforcing the need for real-time imaging and longer needles.

Overall, the study highlights that anatomical variation is not merely of academic interest but a direct determinant of anesthetic outcome. Integrating pre-block ultrasound screening and patient-specific adjustments can significantly improve efficacy and reduce complications

Anatomical variations of the sciatic nerve occur in nearly one-quarter of individuals and are clinically significant in regional anesthesia. Variant configurations are associated with lower block success, delayed onset, and a higher risk of paresthesia and transient neuropathy. Pre-procedural imaging—preferably ultrasound—should be routine for gluteal sciatic nerve blocks, especially in obese or high-risk patients. Recognition of high-division anatomy enables tailored injection techniques, improving safety and reliability.

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