Adductor Canal Block Motor Sparing — FRCA Final MCQ

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ModerateRegional AnaesthesiaAdductor Canal Block Motor SparingFRCA Final

A 28-year-old man is undergoing anterior cruciate ligament reconstruction under general anaesthesia. He receives an ultrasound-guided single-shot adductor canal block with 20 mL ropivacaine 0.5%. Compared with an otherwise equivalent femoral nerve block, which movement is expected to show the greatest relative preservation of motor strength in the immediate postoperative period?

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Correct answer: D — Knee extension

The correct answer is D: knee extension. A femoral nerve block anaesthetises the proximal motor branches supplying the quadriceps and therefore commonly weakens knee extension. An adductor canal block targets more distal, predominantly sensory femoral branches and consequently preserves quadriceps strength better, although motor sparing is relative rather than absolute because the nerve to vastus medialis or proximal spread may be involved. Ankle dorsiflexion and foot eversion are mediated by deep and superficial fibular nerves respectively. Knee flexion is predominantly mediated by sciatic-innervated hamstrings, while hip extension is mediated mainly by the inferior gluteal nerve and sciatic contributions. These movements are therefore not the characteristic motor-sparing distinction between adductor canal and femoral nerve blocks.

Reference: Abdallah FW et al. Adductor Canal Block Provides Noninferior Analgesia and Superior Quadriceps Strength Compared with Femoral Nerve Block in Anterior Cruciate Ligament Reconstruction. Anesthesiology. 2016;124:1053–1064. https://pubmed.ncbi.nlm.nih.gov/26938989/