['Postoperative pain is severe after total knee arthroplasty (TKA). Therefore, femoral nerve block (FNB) is commonly used as an adjuvant to spinal anesthesia for TKA. Some anesthesia providers perform this preoperatively, while others perform it postoperatively. To our knowledge, no study has compared the relative benefits of the timing of performing the procedure. In this study, we investigated whether preoperative FNB would provide better analgesic effects than postoperative FNB in patients undergoing unilateral TKA.', 'In this double-blind, randomized, controlled trial, we divided 82 patients (ASA physical status I-III) undergoing unilateral TKA into four groups: (1) a pre-treatment group, in which FNB was performed with 0.4 mL/kg 0.375% bupivacaine plus 1:200,000 epinephrine after spinal anesthesia but before the operation; (2) a post-treatment group, in which FNB was performed with the same drugs at similar dosages immediately after the operation; (3) a pre-control group, in which FNB was performed with normal saline in the same volume as the tested drugs before the operation; and (4) a post-control group, in which FNB was performed with normal saline in the same volume as the tested drug after the operation. At 2, 4, 6, 24, 48 and 72 postoperative hours, we recorded cumulative morphine consumption, visual analog pain scales (VAS), the time of first request for morphine and its side effects. We also measured knee maximum flexion range of motion once a day for 3 days. Our primary aim was to obtain cumulative morphine consumption in 24 hours.', 'Within the postoperative 24 hours, we found significant differences in cumulative morphine consumption between patients who received true FNB and those who did not (at 24 hours, treatment groups = 45.6 ± 31.7 and 33.5 ± 20.6 mg vs. controls = 70.8 ± 31.2 and 78.8 ± 37.7 mg, p < 0.001). We also found significant differences in VAS (at 24 hours, p < 0.001) and time to first request of morphine (p = 0.005) between the treatment group and the sham group. However, there were no significant differences in these values between the pre-surgical treatment group and the post-surgical treatment group. Beyond 24 hours, there were no significant differences in morphine consumption or maximum flexion range on day 2 and day 3 among the four groups.'] Does single-injection femoral nerve block lack preemptive effect on postoperative pain and morphine consumption in total knee arthroplasty?