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

Abstract

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.

Keywords: Anatomy; Clinical trial; Lateral femoral cutaneous nerve block; Regional anesthetics; Ropivacaine; Total hip arthroplasty.

Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Fig. 1

Ultrasound image of the target nerve. The image is not from an actual trial participant. SAR = sartorius muscle. LFCN = lateral femoral cutaneous nerve. TFL = tensor facia latae muscle. * marks the anterior superior iliac spine

Fig. 2

Mapping of blocked areas. Sketch showing how to draw guiding lines for the mapping of the blocked area, 0A marks the point of the greater trochanter. Red lines border the blocked area which was mapped using temperature discrimination test. Green lines border the blocked area which was mapped using pinprick. The purple line corresponds to the posterior incision line. The blue line corresponds to the lateral incision line. The drawn sketch has previously been published in “Sensory distribution of the lateral femoral cutaneous nerve block – a randomised, blinded trial.” Nersesjan M, Acta Anaesthesiologica Scandinavica volume 62 issue 6, Copyright© 2018, The Acta Anaesthesiologica Scandinavica Foundation. Published by John Wiley & Sons Ltd.

Fig. 3

Consort flowchart

Fig. 4

Area dot-plots. Blocked areas assessed with temperature discrimination for each participant, identification number given. The green dot marks 0A (the point of the greater trochanter). Blue lines represent the side given 8 mL. Orange lines represent the side given 16 mL