Common Peroneal Nerve Palsy
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Direct answer
Common peroneal nerve palsy is injury to the common peroneal nerve, most often at the neck of the fibula, causing foot drop and loss of eversion. It also produces numbness over the lateral leg and dorsum of the foot. The steppage gait compensates for weak ankle dorsiflexion, and most cases are managed with an ankle-foot orthosis and observation, while a tibialis posterior transfer is considered for persistent palsy.
What you must remember
- Anatomy: the common peroneal nerve arises from the sciatic nerve and winds around the neck of the fibula, where it is superficial and vulnerable.
- Causes: fracture of the fibular neck, knee dislocation, pressure from a tight plaster cast, and prolonged compression of the nerve at the fibular head.
- Motor loss: weakness of tibialis anterior, extensor hallucis longus and extensor digitorum longus causes foot drop, while weakness of peroneus longus and brevis causes loss of eversion.
- Sensory loss: the lateral leg and dorsum of the foot, supplied by the superficial and deep peroneal branches.
- Steppage gait: the hip and knee are flexed high in swing to clear the dropped foot.
- Electrodiagnosis: nerve conduction studies and EMG localise the lesion to the common peroneal nerve and distinguish it from an L5 radiculopathy.
- Management: an ankle-foot orthosis, physiotherapy and observation, with nerve exploration for transection and tibialis posterior transfer for persistent loss.
A sequence for assessing foot drop after fibular neck injury
Foot drop after a fibular neck injury needs a sequence that separates a peripheral nerve lesion from a root lesion and then protects the foot. The steps below follow the order in which decisions are usually made, and the timing of surgery depends on the nature of the nerve injury.
- Confirm the drop: test active dorsiflexion and eversion, and look for the steppage gait during walking.
- Map the sensation: the lateral leg and dorsum of the foot indicate the superficial and deep branches involved.
- Look for the cause: palpate the fibular neck and examine for a tight cast, a knee dislocation or a fracture.
- Investigate: nerve conduction studies and EMG localise the lesion and help separate a peroneal nerve lesion from an L5 root lesion.
- Protect the foot: an ankle-foot orthosis prevents toe catching and ankle sprain while walking.
- Plan surgery if needed: explore the nerve for transection or compression, and consider a tibialis posterior transfer for persistent loss of dorsiflexion.
Where the exam tries to catch you
Foot drop questions reward the localisation of the lesion and the restraint to observe before operating.
- Trap: assuming every foot drop is peroneal. Correct: L5 radiculopathy and other causes also produce drop, so the sensory pattern and EMG separate them.
- Trap: operating straight away on a neurapraxic lesion. Correct: many recover spontaneously over months, so observation with an orthosis comes first, unless the nerve is clearly transected.
- Trap: dismissing the nerve injury because no fracture is seen on X-ray. Correct: the nerve can be injured by compression, a plaster cast or stretching, without any fracture visible on radiographs.
Frequently asked questions
What is the steppage gait in common peroneal nerve palsy?
The steppage gait is a compensatory walking pattern in common peroneal nerve palsy, in which the hip and knee are flexed high during swing to lift the dropped foot clear of the ground. It results from weakness of ankle dorsiflexion. The gait is typically seen on the affected side.
Which muscles are weak in common peroneal nerve palsy?
Tibialis anterior, extensor hallucis longus and extensor digitorum longus are weak in common peroneal nerve palsy, causing foot drop. Peroneus longus and brevis are also weak, producing loss of eversion. The tibialis posterior and the calf muscles are spared because they are supplied by the tibial nerve.
Which investigation localises a common peroneal nerve palsy?
Nerve conduction studies and EMG localise common peroneal nerve palsy to the nerve at the fibular head, and they help separate it from an L5 radiculopathy. Nerve conduction across the fibular head shows the level of the lesion. Imaging is used to look for a fracture or other cause at the knee.
What is the tibialis posterior transfer in foot drop?
The tibialis posterior transfer is a tendon transfer in which the tibialis posterior tendon is redirected to the dorsum of the foot to restore active dorsiflexion. It is considered in persistent common peroneal nerve palsy when recovery has not occurred. The procedure aims to reduce the need for an orthosis.