David F. Johnston, Rakesh V. Sondekoppam, Robert Giffin, Robert Litchfield, Sugantha Ganapathy

Adductor canal block (ACB) is popular for knee analgesia because of its favorable effect on quadriceps strength. The aim of this study was to find the minimum volume of local anesthetic, which can be injected into the ACB that would result in quadriceps weakness. This nonrandomized study used an up-and-down sequential allocation design. Twenty-six patients scheduled to undergo arthroscopic knee surgery received an ultrasound-guided ACB preoperatively. The initial volume of ropivacaine 0.5% injected was 30 mL, which was subsequently increased or decreased by 2 mL, depending on whether the previous subject had a 30% reduction in quadriceps function. The minimum effective volume in 50% of patients was determined using Dixon-Massey up-and-down method. The effective volume in 95% of patients was then calculated using probit transformation. The ED50 (minimum effective anesthetic volume in 50% of the subjects) needed for a 30% decrease in quadriceps power was 46.5 mL (95% confidence interval, 45.01-50.43 mL), and estimated ED95 (minimum effective anesthetic volume in 95% of the subjects) was 50.32 mL (95% confidence interval, 48.66-67.26 mL). The local anesthetic volume injected correlated with degree of quadriceps weakness at 20 minutes postblock (P < 0.001) and in the postanesthesia recovery unit (P = 0.032). Significant quadriceps weakness is unlikely when clinically representative volumes of 0.5% ropivacaine is used for ACB performed using sonographic landmarks. This study was registered at ClinicalTrials.gov, identifier NCT02541552.