A new lateral approach to the parasacral sciatic nerve block: an anatomical study

Abstract

Background: Sciatic nerve block is a commonly used technique for providing anesthesia and analgesia to the lower extremity. At the parasacral level, the nerve block is classically performed via a posterior approach in lateral decubitus position causing patient's discomfort. Therefore, we aimed to conduct an anatomical study describing a new lateral approach to the parasacral sciatic nerve in supine position.

Methods: The skin entry point was located on the vertical line through the greater trochanter (GT) at the midpoint between the anterior superior iliac spine (ASIS) level and the GT. The angle to the skin was 10° dorsally oriented. According to these palpable anatomical landmarks, the parasacral lateral approach was simulated bilaterally in four cadavers in supine position. Anatomical dissection allowed assessment of the needle tip position with regard to the sciatic nerve. Then, to refine the anatomical description of this new lateral approach, 40 pelvic computer tomography (CT) examinations were retrospectively selected and post-processed to bilaterally simulate the needle route to the sciatic nerve. The skin-nerve distance, the optimal angle to the skin, and the sciatic nerve anteroposterior diameter at parasacral and ischial tuberosity levels, respectively were recorded by two independent readers.

Results: Cadaver dissection showed that the needle tip was placed in the vicinity of the sciatic nerve in 8/8 cases. Then, CT-simulated lateral approach demonstrated a mean skin-nerve distance of 128 mm (81-173), and a 12° dorsally oriented (5-22) optimal angle to the skin. The sciatic nerve anteroposterior diameter was 10 mm (7-15) at the parasacral level, and 7 mm (5-10) more caudally at the ischial tuberosity level. No significant intra- or inter-observer variability was observed.

Conclusion: This study describes a new lateral approach to the parasacral sciatic nerve block in supine position. These anatomical results should be confirmed by further clinical studies.