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Neurostimulation is commonly used to perform femoral nerve block. Ultrasound can be used to identify nerve structures and guide needle placement. The aim of this study was to compare postoperative analgesic efficiency when femoral nerve catheters were sited using ultrasound (in-plane approach) combined with neurostimulation or neurostimulation alone, for total knee arthroplasty.Ninety-three patients were enrolled in this prospective, randomized, blind study to receive a continuous femoral nerve block performed using conventional neurostimulation or ultrasound guidance combined with neurostimulation. The primary endpoint was the postoperative 48 h total local anaesthetic consumption. Other outcomes included visual analogue scale scores at rest, after 12, 24 and 48 h; total oral opioid dose; onset time of femoral block; procedural time; knee flexion and complications. Results are expressed with medians (25–75th percentiles), mean ± SD and n (%), as appropriate.The onset time of sensory and motor block was faster in the ultrasound group [11 (6–17) min] than in the conventional group [16 (11–23) min, P = 0.009]. Total local anaesthetic dose was reduced [299 ± 45 vs. 333 ± 48 ml, (difference = 34.0 ml, 95% confidence interval 15.6–52.5 ml), P = 0.0003] and the time to the first analgesic request was lengthened [11 (7–13) vs. 7 (4–12) h, P = 0.034] in the ultrasound group. Visual analogue scale scores at 12, 24 and 48 h were lower in the ultrasound group as well as visual analogue scale scores during knee flexion at 48 h [14.5 (11.0–23.0) vs. 28.5 (21.0–43.5) mm, P < 0.0001]. Total oral morphine doses were 20 (0–40) vs. 40 (20–60) mg (P = 0.0065). Durations of hospital stay were comparable in both groups.Continuous perineural femoral catheter placement using ultrasound combined with neurostimulation and an in-plane approach reduces total doses of local anaesthetic, morphine consumption and improves postoperative pain management by comparison with neurostimulation alone.