A 32-year-old man presents with foot drop and inability to dorsiflex the ankle after a knee dislocation injury. Which anatomical feature best distinguishes the nerve injured in this case from the other major branch of the sciatic nerve?

## Distinguishing the Common Peroneal Nerve from the Tibial Nerve ### Anatomical Basis of Injury The patient's presentation of foot drop and inability to dorsiflex the ankle indicates **common peroneal (fibular) nerve injury**. This nerve is vulnerable at the knee due to its superficial course around the fibular neck. ### Key Discriminating Feature **Key Point:** The common peroneal nerve is the only branch of the sciatic nerve that innervates muscles of the **anterior compartment of the leg** (tibialis anterior, extensor digitorum longus, extensor hallucis longus, fibularis tertius). These muscles are responsible for dorsiflexion and toe extension. ### Comparative Anatomy Table | Feature | Common Peroneal Nerve | Tibial Nerve | |---------|----------------------|---------------| | **Motor supply** | Anterior & lateral compartments of leg | Posterior compartments of leg & foot | | **Dorsiflexion** | Yes (via tibialis anterior) | No | | **Plantarflexion** | No | Yes (via gastrocnemius, soleus) | | **Superficial course** | Around fibular neck (vulnerable) | Deep in popliteal fossa | | **Common injury site** | Knee region | Rare in isolation | ### Clinical Pearl **High-Yield:** Common peroneal nerve injury = **foot drop + sensory loss over dorsum of foot and lateral leg**. This is the most frequently injured nerve of the sciatic plexus due to its superficial anatomical course. ### Mnemonic **CLAP** — Common peroneal nerve: **L**ateral compartment (fibularis), **A**nterior compartment (dorsiflexors), **P**eroneal (fibular) course. [cite:Gray's Anatomy 42e Ch 51]