Obturator versus femoral nerve block for analgesia after total knee arthroplasty

BACKGROUND: Both femoral and obturator nerve blocks have been suggested to be useful in relieving pain after total knee arthroplasty (TKA). We sought to compare their efficacy. METHODS: Sixty patients undergoing elective unilateral TKA under spinal anesthesia received in a randomized, double-blind manner a femoral, obturator, or sham nerve block at the end of surgery. Blocks were performed using nerve stimulation and 20 mL, bupivacaine 0.5% containing epinephrine 5 mu g/mL. Patient-controlled, IV analgesia with fentanyl, celecoxib 100 mg PO bid, and acetaminophen 650 mg PO every 6 h were started on arrival in the recovery room. Pain (0-10 numeric rating scale, NRS) at rest and with movement, analgesic use, and side effects were recorded for 48 h. Maximum knee flexion and total days in hospital were recorded as functional outcomes. RESULTS: There were no significant differences in the obturator block group and the control group in any outcome variable. With baseline pain scores subtracted, femoral block resulted in less pain at rest compared with control (NRS difference from baseline 2.1 +/- 0.4 SEM vs 3.4 +/- 0.4, respectively; P = 0.02) and less pain with movement (NRS difference 2.6 +/- 0.6, 4.3 +/- 0.6, P = 0.05) at recovery room discharge. Neither block had a significant effect on opioid use, functional outcome, or side effects. Only one (5%) patient with femoral block developed obturator motor block. CONCLUSION: Femoral nerve blocks rarely block the obturator nerve. Single-injection femoral nerve block improved multimodal analgesia after spinal anesthesia for TKA, but this effect did not persist beyond the day of surgery. Obturator nerve block alone was of no benefit.