Sonographic Evaluation of Sciatic Nerve Anatomy and Subcutaneous Depth at Its Termination in Posterior Thigh: Importance in Popliteal Blocks

Introduction: Sciatic nerve (SN) is about 2cm wide, forms in the pelvis from ventral rami of L4-S3 spinal nerves and leaves the pelvis by passing out via greater sciatic foramen inferior to piriformis.It travels in the posterior compartment of thigh where it is crossed by long head of biceps femoris, and terminates by dividing into tibial and common peroneal nerves proximal to knee near the apex of popliteal fossa.Cross sectional area (CSA) of SN at mid-thigh and the level of termination may vary.This is important in respect to clinical as well as treatment purpose for the performance of popliteal block.Popliteal nerve block is the block of SN in the popliteal fossa, it is ideal for surgeries of lower leg, particularly below the knee, foot and ankle.It anesthetizes the same dermatomes as both the anterior and lateral approaches to the SN.Variability in level of termination and subcutaneous depth may account for the frequent failures associated with popliteal block.Ultrasound guided sciatic nerve blockade when performed in a systematic manner, is associated with a high success rate.Aims & Objectives: Present study was done to evaluate sciatic nerve morphometry and its depth from skin with the help of high resolution ultrasonography (HRUS) and highlight importance of relevant anatomy in relation to popliteal nerve block. Material & Methods:Study was conducted in the Department of Anatomy, King George's Medical University, Lucknow, Uttar Pradesh, India in 50 volunteer students of 1 st year MBBS 2018 batch (25 males & 25 females).Sonography was done with the help of Esaote Europe My Lab 40 ultrasound machine (installed in the Department of Anatomy, KGMU) to observe Cross sectional area, perimeter, level of termination of nerve and its depth from skin at a particular site.Results: CSA ranged from 0.22-0.35±0.028cm 2 and perimeter ranged from 15.23 -30.33±2.92mm The mean CSA of SN was equal on both sides ie.0.27± 0.028 cm 2 on right and 0.27±-0.025cm 2 on left.The perimeter of SN on right side was 21.27±2.92mm and left side 20.29±2.05mm.The depth of SN from skin on right side was 19.16±1.70 mm while on left side 19.16±1.70mm.The level of termination was 77.65±4.31mm on right side while 77.26±4.43mm on left side proximal to popliteal crease.