A 32-year-old male labourer from Delhi presents with acute onset weakness of foot dorsiflexion and toe extension following a knee injury sustained while climbing stairs. On examination, he has foot drop and inability to evert the foot. Sensation over the dorsum of the foot and lateral leg is intact. Which nerve is most likely injured?

Explanation

## Clinical Presentation Analysis **Key Point:** The combination of foot drop, weakness of dorsiflexion, and toe extension with preserved sensation indicates a **common peroneal (fibular) nerve injury**. ### Anatomical Basis The common peroneal nerve is the **lateral terminal branch of the sciatic nerve**, arising at the popliteal fossa. It is highly vulnerable to injury at the **fibular neck** due to its superficial course around the fibular head. ### Motor Deficits in Common Peroneal Nerve Injury | Function | Nerve Branch | Status in This Case | |----------|--------------|---------------------| | Dorsiflexion of foot | Deep peroneal | **Weak/Absent** | | Toe extension | Deep peroneal | **Weak/Absent** | | Foot eversion | Superficial peroneal | **Weak/Absent** | | Plantarflexion | Tibial nerve | Intact | | Toe flexion | Tibial nerve | Intact | **Clinical Pearl:** Foot drop is the hallmark sign of common peroneal nerve injury — the patient cannot clear the toes during the swing phase of gait and must use a steppage gait. ### Sensory Loss Pattern - **Dorsum of foot** — supplied by deep peroneal nerve (sensory branch) - **Lateral leg and dorsum** — supplied by superficial peroneal nerve - **Intact sensation** in this case suggests the nerve injury is motor-predominant or the sensory branches are spared **High-Yield:** The common peroneal nerve splits into **deep peroneal** (anterior tibial) and **superficial peroneal** (fibular) branches just below the fibular neck. Injury at or above this point affects both branches. ### Why Foot Drop Occurs Without dorsiflexion (tibialis anterior), the foot hangs in plantarflexion during swing phase. The patient compensates by **exaggerating hip flexion** (steppage gait) to clear the toes. **Mnemonic:** **LEAF** — **L**ateral leg, **E**version, **A**nterior (dorsiflexion), **F**ibular head (site of injury) — all point to common peroneal nerve.