Which clinical finding best distinguishes a common peroneal nerve (CPN) injury from a tibial nerve injury at the level of the popliteal fossa?

Explanation

## Distinguishing CPN vs Tibial Nerve Injury ### Motor Innervation Pattern **Key Point:** The common peroneal nerve innervates the anterior compartment (tibialis anterior, extensor digitorum longus, extensor hallucis longus) and lateral compartment (fibularis longus and brevis) of the leg. The tibial nerve innervates the posterior compartment (gastrocnemius, soleus, tibialis posterior, flexor digitorum longus, flexor hallucis longus). ### Clinical Presentation Comparison | Feature | CPN Injury | Tibial Nerve Injury | |---------|-----------|--------------------| | **Foot position** | Foot drop (inverted, plantarflexed) | Foot in neutral or slight dorsiflexion | | **Dorsiflexion** | **Lost** | Preserved | | **Plantarflexion** | Preserved | **Lost** | | **Eversion** | **Lost** | Preserved | | **Inversion** | Preserved | **Lost** | | **Sensation loss** | Dorsum of foot, lateral leg | Sole of foot (medial + lateral plantar) | | **Gait** | Steppage gait (high-stepping) | Shuffling gait | ### Why Foot Drop Distinguishes CPN Injury **High-Yield:** Foot drop with inability to dorsiflex the ankle is the hallmark of CPN injury because: 1. The tibialis anterior (primary dorsiflexor) is innervated exclusively by CPN 2. Loss of dorsiflexion causes the foot to hang in plantarflexion during swing phase 3. Patient must lift the knee higher (steppage gait) to clear the drooping foot **Clinical Pearl:** In tibial nerve injury, plantarflexion is lost, but dorsiflexion remains intact—the foot does not drop. The patient cannot stand on tiptoes but can walk relatively normally. ### Anatomical Basis **Mnemonic:** CPN = **Foot Drop** (Dorsiflexion lost); TN = **No Plantarflex** (Calf weakness) - CPN: Anterior + Lateral compartments → dorsiflexion & eversion - Tibial: Posterior compartment → plantarflexion & inversion [cite:Clinically Oriented Anatomy 8e Ch 6]