Impact of leg movement on skin-adductor canal distance: a potential cause for catheter tip displacement?

Purpose Catheter-based adductor canal blocks are an effective pain management strategy for total knee arthroplasty. Nevertheless, catheter-based techniques may fail if the tip migrates because of leg movement. This observational study used ultrasound to measure the distance from the skin to the adductor canal (SAC). We tested the hypothesis that leg movements mimicking those occurring during postoperative physiotherapy change the SAC distance. Methods Following total knee arthroplasty under spinal anesthesia, the SAC distance was measured using ultrasound in 40 patients. The leg was passively moved into five standardized positions: neutral, neutral position with manual lateral displacement of the skin, 30° external rotation, straight leg raised to 30°, and knee and hip flexed to 90°. Results The mean (standard deviation) SAC distance change from the neutral position was +1.0 (0.6) cm with manual displacement, +0.3 (0.4) cm with external rotation, +0.4 (0.4) cm with straight leg raise, and +0.6 (0.5) cm with leg flexion ( P < 0.001 for all positions). SAC distance changes did not correlate with height, body mass index, or leg circumference. Conclusion Passive leg movements in five standardized positions increase the SAC distance. We speculate that the altered SAC distance associated with passive leg movement may contribute to catheter tip dislodgement and adductor canal block failure. Trial registration www.clinicaltrials.gov , NCT03562559; registered 19 June, 2018.