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Common Peroneal Nerve (CPN)
The Common Peroneal Nerve (CPN), also known as the common fibular nerve, arises from the sciatic nerve (roots L4–S2). It is the most frequently injured lower limb nerve due to its superficial course around the fibular neck. Damage leads to the classic foot drop with compensatory high-stepping or hip-hiking gait.
🔎 Anatomy
- Origin: Branch of sciatic nerve in upper popliteal fossa (sciatic divides into tibial and CPN).
- Course: Runs laterally along popliteal fossa → winds around fibular neck (very superficial) → divides into superficial & deep branches.
- Other branches: Articular (knee), lateral sural cutaneous (upper lateral leg), muscular (short head of biceps femoris).
🧩 Branches & Functions
| Branch | Key Functions |
|---|---|
| Superficial peroneal | Motor: Peroneus longus & brevis → eversion
Sensory: Dorsum of foot (except 1st web space) |
| Deep peroneal | Motor: Tibialis anterior, EHL, EDL, peroneus tertius → dorsiflexion & toe extension
Sensory: 1st web space |
| Lateral sural cutaneous | Sensory only: Upper lateral leg |
⚡ Clinical Features of Injury
- Motor: Foot drop (loss of dorsiflexion), weak eversion; inversion & plantarflexion preserved (tibial intact).
- Gait: High-stepping, foot slap, or hip-hiking/circumduction compensation.
- Sensory: Loss over dorsum of foot + first web space.
- Reflexes: Achilles reflex preserved (S1, tibial nerve).
📊 Causes
| Cause | Notes |
|---|---|
| Compression | Leg crossing, plaster, kneeling → often transient |
| Trauma | Fibular neck fracture, knee dislocation → sudden onset |
| Entrapment | Ganglion cyst, tumour near fibular head → progressive |
| Systemic | Diabetes, weight loss → bilateral neuropathy |
| Iatrogenic | Surgery, poor positioning → perioperative history |
🧠 Differential Diagnosis
| Condition | Key Features |
|---|---|
| L5 Radiculopathy | Back + radicular pain; dorsiflexion & inversion weakness; ↓ ankle reflex |
| Sciatic lesion | Hamstring + plantarflexion weakness; larger sensory loss |
| Peripheral neuropathy | Bilateral, stocking distribution (e.g. diabetes, CMT) |
| Motor neuron disease | Progressive weakness, no sensory loss |
Exam pearl: In CPN lesion, tibialis posterior (inversion, tibial nerve) is spared — unlike in L5 radiculopathy.
🧪 Investigations
- Bloods: Glucose, TFTs, ESR, FBC, U&E, LFTs.
- Electrodiagnostics: NCS localises lesion; EMG shows denervation in tibialis anterior but sparing tibialis posterior.
- Imaging: MRI spine if radiculopathy; MRI/US leg for mass/cyst.
💊 Management
- Conservative: Avoid compression, physiotherapy, AFO for gait.
- Medical: Diabetes optimisation, neuropathic analgesia.
- Surgical: Decompression, nerve repair/graft, tendon transfer if poor recovery.
- Rehab: Long-term physio & OT support.
📌 OSCE Exam Tips
- Inspect gait → foot drop, high-stepping, hip-hiking.
- Test dorsiflexion (tibialis anterior) & eversion (peroneals).
- Check sensation dorsum of foot + 1st web space.
- Differentiate from L5 root: inversion spared in CPN.
- Palpate fibular head for tenderness or compressive masses.
📈 Prognosis
- Compression neuropathy → good recovery (weeks–months).
- Demyelination recovers quicker than axonal injury.
- Axonal damage → incomplete or slow recovery.
🛡 Prevention
- Avoid prolonged leg crossing or pressure at fibular head.
- Padding for thin patients; careful surgical positioning.
- Control systemic risk factors (e.g. diabetes).
📚 References
- Moore KL, Dalley AF, Agur AM. Clinically Oriented Anatomy. 7th ed. LWW; 2013.
- Stewart JD. Foot drop: where, why and what to do? Pract Neurol. 2008;8(3):158-169.
- Kane PM, Oware A. Common peroneal neuropathy. Clin Neurophysiol Pract. 2019;4:112-123.
- Campbell WW. Peripheral nerve injury: evaluation & management. Clin Neurophysiol. 2008;119(9):1951-1965.