Classical popliteal sciatic block versus Crosswise Approach to Popliteal Sciatic (CAPS) block for below-knee surgery: a randomised controlled trial

Ultrasound-guided popliteal sciatic nerve block is frequently used for below-knee surgery. The classical approach often requires lateral decubitus or prone positioning, which may be uncomfortable for patients with lower limb injuries. The Crosswise Approach to Popliteal Sciatic (CAPS) block allows the procedure to be performed with the patient supine. This randomised trial compared the classical approach with CAPS in terms of patient comfort, block performance, and clinical outcomes.

Eighty American Society of Anesthesiologists Physical Status I-III patients undergoing unilateral below-knee surgery were randomised to receive either a classical popliteal sciatic block in the lateral position (Group A) or a CAPS block in the supine position (Group B). All procedures were ultrasound-guided with 20 mL of 0.5% ropivacaine after lignocaine skin infiltration. All patients also received a femoral nerve block. Pain scores (verbal numerical rating scale [VNRS]) during positioning and block performance, fentanyl requirements, block performance time, sensory onset time, block success, adverse effects, and patient satisfaction were assessed.

All blocks were successful. Median highest pain scores were low in both groups (1.5 [classical] vs. 3.0 [CAPS] VNRS; p = 0.050). The proportion of patients reporting no pain (VNRS = 0) did not differ significantly between groups (30.0% [classical] vs. 15.0% [CAPS]; p = 0.180). Fentanyl supplementation (any dose) was required in 45% of Group A and 60% of Group B (p = 0.365). Block performance time was shorter with CAPS (10.5 min) than the classical approach (12.0 min; p = 0.002). Sensory onset time was similar (~ 20 min; p = 0.919). No major complications occurred. Transient sensory symptoms lasting more than 24 h occurred in two patients (5.0%) in Group A and one (2.5%) in Group B. Patient satisfaction remained high in both groups, with 75.0% of Group A and 67.5% of Group B rating satisfaction as 4 or 5 on a 5-point Likert scale (p = 0.120).

Both techniques provide effective and safe anaesthesia for below-knee surgery with comparable clinical outcomes. CAPS provides a logistical advantage by allowing the procedure to be performed in the supine position, which may be of value in patients unable to tolerate lateral decubitus positioning, as well as a modest reduction in procedural time (approximately 1.5 min). Either technique may be selected based on patient factors and operator preference.

ClinicalTrials.gov (NCT07134608), registered 21 March 2025. Retrospectively registered.