LAUSR

Background Our previous study reported that when a respective 20 mL of ropivacaine was used for adductor canal block (ACB) combined with infiltration between the popliteal artery and capsule of the posterior knee (IPACK) block for patients undergoing total knee arthroplasty (TKA), the minimum concentration of 0.275% could provide successful block in almost all patients. Based on the results, the primary objective of this study was to investigate the minimum effective volume (MEV90, defined as the volume which can provide successful block in 90% of patients) of ACB + IPACK block. Methods This double-blind, randomized dose-finding trial was based on a biased coin up-and-down sequential design, where the volume of ropivacaine administered to a given patient depended on the previous patient’s response. The first patient received 15 mL of 0.275% ropivacaine for ACB and again for IPACK. If the block failed, the next subject received a 1 mL higher volume for ACB and IPACK, respectively; otherwise, the next subject received either a 1 mL lower volume (probability of 0.11) or the same volume (probability of 0.89). The primary outcome was whether the block was successful. Block success was defined as the patient did not suffer significant pain and did not receive rescue analgesia within 6 h after surgery. Then the MEV90 was estimated by isotonic regression. Results Based on analysis of 53 patients, the MEV90 was 17.99 mL (95% CI 17.47–18.61 mL), MEV95 was 18.48 mL (95% CI 17.45–18.98 mL) and MEV99 was 18.90 mL (95% CI 17.38–19.07 mL). Patients whose blocks succeeded had significantly lower NRS pain scores, lower morphine consumption, and shorter hospitalization. Conclusion 0.275% ropivacaine in 17.99 mL respectively can provide successful ACB + IPACK block in 90% of patients undergoing TKA. The minimum effective volume (MEV90) of ACB + IPACK block was 17.99 mL.