Effect of local anesthetic volume for popliteal plexus block on motor nerve conduction and muscle function in the leg: a randomized clinical trial in healthy volunteers.

Abstract

Background and objectives: Popliteal plexus block (PPB) has shown moderate improvements in multimodal analgesia following total knee arthroplasty (TKA) while preserving motor function. However, the optimal analgesic volume for PPB remains unknown, and concerns exist regarding potential volume-dependent motor nerve involvement. This study investigates whether increasing the volume of local anesthetic for PPB affects muscle function and motor nerve involvement.

Methods: In this randomized, controlled, blinded trial, 40 healthy volunteers received one active nerve block and one sham block in each leg. Participants were randomized into three PPB groups receiving 10 mL, 20 mL, or 30 mL of 1% lidocaine, with 20 observations per group. Additionally, 10 femoral nerve blocks and 10 sciatic nerve blocks were included as reference groups to confirm motor nerve involvement. The primary outcome was relative changes in maximum voluntary isometric contraction (MVIC) of ankle plantar- and dorsiflexion. Secondary outcomes included relative changes in MVIC of knee extension, relative changes in compound muscle action potential of the gastrocnemius, anterior tibial, vastus medialis, and vastus lateralis muscles and frequency of saphenous nerve involvement.

Results: There were no significant differences in motor function between the three PPB groups for any MVIC measures (p≥0.1). Compound muscle action potential changes did not indicate clinically relevant motor nerve involvement across PPB groups. Saphenous nerve involvement was inconsistent, affecting 40% (10 mL), 40% (20 mL), and 60% (30 mL) of cases (p=0.3), suggesting ineffective sensory block of the femoral nerve's medial knee contributions.

Conclusion: Increasing the volume of local anesthetic for PPB does not impair muscle function or significantly involve motor nerves, supporting its motor-sparing efficacy. PPB inconsistently affects the femoral nerve's sensory contributions to the knee, emphasizing its role as an adjunct to femoral triangle or adductor canal blocks in clinical practice. Concerns of muscle impairment should not hinder future clinical trials from exploration of the optimal analgesic volume for PPB.

Trial registration number: NCT05464862.