BACKGROUND: The lateral femoral cutaneous nerve (LFCN) block may be used for post-operative pain management in patients undergoing total hip arthroplasty. The aim of this trial was to investigate the sensory coverage of the posterior and the lateral incision lines and the involvement of the femoral nerve after an LFCN block.

METHODS: The study was a randomised, blinded trial in 20 healthy volunteers. All subjects received a bilateral LFCN block randomised to 8 ml ropivacaine on the right side and 8 ml isotonic saline on the left side, or vice versa. An orthopaedic surgeon depicted the incision lines (invisible to the investigators) prior to block performance. The distribution of the blocked area and the coverage of the incision lines were assessed with temperature discrimination and pinprick test before unblinding the incision lines. Pain during tonic heat stimulation and involvement of the femoral nerve by measuring quadriceps strength were assessed.

RESULTS: The mean difference in block coverage of the posterior (primary outcome) and the lateral incision lines tested with temperature discrimination were 5.8% (95% CI: -2.2 to 14.0%, P = 0.146) and 18.9% (95% CI: 6.5-31.4%, P = 0.005), respectively, comparing the active with the placebo side. A varying anatomic distribution area was observed. No clinically significant differences for experimental pain and quadriceps muscle strength were found. The block failure rate was 15%.

CONCLUSION: An LFCN block consisting of 8 ml 0.75% ropivacaine had limited coverage of the posterior and lateral incision lines.