A 30-year-old man has foot drop after a knee dislocation. Ankle dorsiflexion and eversion are weak, but inversion of the ankle is strong. Where is the lesion?

• A L4-L5 disc prolapse compressing the L5 root
• B Common peroneal nerve at the fibular neck ✓
• C Deep peroneal nerve only
• D Sciatic nerve in the thigh

Explanation

Preserved inversion localises the lesion distal to the origin of the tibial division contribution to inversion. Tibialis posterior, the main invertor, is supplied by the tibial nerve, so a pure common peroneal lesion at the fibular neck spares inversion while abolishing dorsiflexion and eversion. An L5 radiculopathy would weaken both dorsiflexion and inversion, since L5 contributes to tibialis posterior. A deep peroneal-only lesion would spare eversion, which is supplied by the superficial branch.

Reference: Bailey and Love's Short Practice of Surgery, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.