Lateral femoral cutaneous nerve block with different volumes of Ropivacaine: a randomized trial in healthy volunteers

Background Nerve block of the lateral femoral cutaneous nerve (LFCN) is a predominantly sensory block. It reduces pain following total hip arthroplasty (THA), but the non-responder rate is high. We hypothesized, that an increased volume of ropivacaine, would result in greater coverage of incisions used for THA. Methods We conducted a randomized, blinded trial in 20 healthy volunteers. Participants were randomized to receive bilateral LFCN-blocks with 8 mL ropivacaine 0.75% on the left side and 16 mL ropivacaine 0.75% on the right side, or vice versa. Allocation was blinded to both participants and outcome assessors. Before nerve block performance, incision lines for posterior and lateral THA approaches were depicted with invisible ultraviolet-paint, thereby securing sufficient blinding during outcome assessment. The blocked area was mapped using temperature and mechanical discrimination tests. Quadriceps muscle strength was monitored. Primary outcome was coverage of the posterior incision line assessed by temperature discrimination test. Results We found no difference in coverage of the posterior or lateral incision lines when comparing LFCN-blocks with 8 mL versus 16 mL of ropivacaine. The blocked area was significantly larger in the 16 mL group, assessed by both temperature discrimination test ( p = 0.012) and mechanical discrimination test ( p = 0.034). We observed no difference between groups regarding quadriceps muscle strength ( p = 1.0). Conclusions A LFCN-block with increased volume of ropivacaine from 8 mL to 16 mL did not result in a greater coverage of posterior or lateral incision lines used for THA, but in a larger blocked sensory area. Trial registration Clinicaltrials.gov: NCT03138668 . Registered 3rd of May 2017.