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Background: Adductor canal blocks (ACBs) are commonly employed in multimodal pain control for total knee arthroplasty (TKA) and minimize motor blockade compared with femoral nerve blocks. Quadriceps weakness may be associated with ACBs. The purpose of this study was to quantify the prevalence of clinically relevant quadriceps weakness after a single-shot ACB and to identify the factors that are associated with its diagnosis. Methods: The study group consisted of 1,083 retrospectively reviewed consecutive TKAs that were performed with ACBs at an academic hip and knee center. Quadriceps weakness was quantified with a standardized rating system during the initial physical therapy evaluation, and 23 potential covariates were analyzed. Results: The prevalence of quadriceps weakness was 9%. Increasing the dose of the ACB anesthetic per unit of body mass index (BMI) increased the probability of quadriceps weakness by 5.0 times (95% confidence interval [CI], 1.9 to 13.3; p = 0.001). The highest probability of quadriceps weakness (52.9%) was associated with women who received the highest anesthetic dose per unit of BMI and an epinephrine extender but no corticosteroid extender. Conclusions: The optimal volume of local anesthetic in ACBs to maintain pain control while minimizing quadriceps weakness has not yet been defined. Our observation that quadriceps weakness was associated with increasing doses of ACB anesthetic per unit of BMI suggests that more than traditional structural canal-fill parameters (i.e., filling the distal aspect of the adductor canal without spreading to the femoral triangle) should be considered when choosing injectates and injectate volumes for ACBs. Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

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