Background and Objectives: To evaluate if psoas compartment block requires a larger concentration of mepivacaine to block the femoral nerve than does an anterior 3-in-1 femoral nerve block.

Methods: Forty eight patients undergoing anterior cruciate ligament repair were randomly allocated to receive an anterior 3-in-1 femoral block (femoral group, n = 24) or a posterior psoas compartment block (psoas group, n = 24) with 30 mL of mepivacaine. The concentration of the injected solution was varied for consecutive patients using an up-and-down staircase method (initial concentration: 1%; up-and-down steps: 0.1%).

Results: The minimum effective anesthetic concentration of mepivacaine blocking the femoral nerve in 50% of cases (ED₅₀) was 1.06% ± 0.31% (95% confidence interval [CI], 0.45%-1.68%) in the femoral group and 1.03% ± 0.21% (95% CI, 0.6%-1.45%) in the psoas group (P = .83). The lateral femoral cutaneous and obturator nerves were blocked in 4 (16%) and 5 (20%) femoral group patients as compared with 20 (83%) and 19 (80%) psoas group patients (P = .005 and P = .0005, respectively). Intraoperative analgesic supplementation was required by 15 (60%) and 5 (20%) patients in the femoral and psoas groups, respectively (P = .01).

Conclusions: Using a posterior psoas compartment approach to the lumbar plexus does not increase the minimum effective anesthetic concentration of mepivacaine required to block the femoral nerve as compared with the anterior 3-in-1 approach, and provides better quality of intraoperative anesthesia due to the more reliable block of the lateral femoral cutaneous and obturator nerves.