common peroneal nerve

**Origin and course:** The sciatic nerve splits in the distal posterior thigh (near the popliteal fossa) into the tibial nerve (L4–S3) and the common peroneal (common fibular) nerve (L4–S2). The common peroneal nerve runs along the medial border of biceps femoris, crosses the lateral head of the gastrocnemius, and winds around the neck of the fibula. There it is superficial and lies against bone, so it is the most commonly injured nerve in the lower limb. It then pierces the fibularis (peroneus) longus and divides into two branches.

**Branches:** - **Superficial peroneal nerve:** Supplies the lateral compartment (fibularis longus and brevis), which everts the foot. It gives sensation to the lateral distal leg and the dorsum of the foot, sparing the first web space. - **Deep peroneal nerve:** Supplies the anterior compartment (tibialis anterior, extensor hallucis longus, extensor digitorum longus, fibularis tertius), which dorsiflexes the foot and extends the toes. It also supplies extensor digitorum brevis. Its only cutaneous area is the first web space between the great and second toes. - Before dividing, the common peroneal nerve gives off the lateral sural cutaneous nerve (lateral calf), which joins the tibial nerve's medial sural branch to form the sural nerve.

**Mechanisms of injury:** - Fracture of the fibular neck (classic board question) - Lateral knee trauma or knee dislocation - Tight casts, splints, or compression stockings - Prolonged leg crossing, squatting, or lithotomy positioning - Habitual use of a lateral leg brace - Rapid weight loss (loss of the protective fat pad) - Compartment syndrome of the anterior compartment (deep peroneal involvement)

**Clinical presentation of common peroneal injury:** - **Foot drop:** Loss of ankle dorsiflexion from loss of tibialis anterior function. - **Steppage gait:** The patient lifts the knee high so the toes clear the ground. The foot slaps down at heel strike. - **Loss of eversion** (superficial branch). Inversion is preserved because the tibial nerve supplies tibialis posterior. - **Sensory loss:** Anterolateral leg and dorsum of the foot, including the first web space. - Plantarflexion is preserved (tibial nerve).

**Differential for foot drop:** - **L5 radiculopathy:** Weakness of inversion as well, and possibly hip abduction. Pain is common, and sensory loss follows an L5 distribution. - **Sciatic neuropathy:** Also involves plantarflexion and hamstrings. - **Others:** Upper motor neuron lesions (spasticity, hyperreflexia), ALS, and diabetic mononeuropathy. - **EMG/NCS** can help localize the lesion.

**Management:** Remove the compressive cause. Provide an ankle-foot orthosis (AFO) and physical therapy. Surgical decompression or nerve repair/transfer is considered for traumatic or persistent cases. Many compressive neuropathies recover with conservative care.

**High-yield pearls:** - Fibular neck fracture, so think foot drop. - Deep peroneal = dorsiflexion plus first web space sensation. - Superficial peroneal = eversion plus dorsal foot sensation. - Mnemonic: "Peroneal Everts and Dorsiflexes (PED)": the common peroneal nerve's job is the PED, and it is the only nerve of the sciatic trio that can be damaged by a lateral knee hit. Another mnemonic: "TIP" = Tibial Inverts Plantarflexes; "PED" = Peroneal Everts Dorsiflexes.