Adductor Canal Blocks: An Observational Ultrasound Study in Volunteers to Identify the Relationship of the True Adductor Canal to Commonly Described Block Approaches and a Review of the Literature

Abstract

Background There is controversy over the site at which the ultrasound-guided adductor canal blocks (ACB) should be performed, and the anatomic relationship of these sites to the true adductor canal (AC). Most studies describe performing the ACB at the anatomical mid-point of the thigh (mid-thigh ACB, mtACB), or 2-3 cm above the inferior border of AC (distal ACB, dACB). The aim of the study was to determine the relationship of these approaches to the true anatomical AC in volunteers. Methods Using ultrasonography and surface landmarks, we characterized the AC anatomy of both lower limbs in 60 adult volunteers (30 males, 30 females). The primary outcome variable was the distance from the mid-thigh approach to the superior border of AC. Calculated secondary measurements were the distance between the 2 approaches and the length of AC. Results The (median [IQR]) needle entry point for mtACB was above the superior border of the AC in both males (5.5 cm [4.6-7.0]) and females (6.6 cm [5.8-7.3]) (P = 0.045 [95% CI of the difference in medians, -1.63 to 0.00 cm]). The median distance between the needle entry points of mtACB and dACB in males vs females were not different (median difference: 0.63 cm; 95% CI, -0.25 to 1.50). The length of the adductor canal was 1.5 cm longer in males compared to females (95% CI, 1.00 to 2.25 cm) Conclusions AC blocks performed at mid-thigh or more proximal are outside the anatomical adductor canal. A review of recent literature shows 3 different sites where AC blocks are performed; the majority of the blocks are performed in the mid-thigh region and hence outside of the true adductor canal.