Common Peroneal Nerve Entrapment

Because the nerve is so exposed at the fibular neck, it is vulnerable to compression from the outside — for example habitual leg crossing, prolonged squatting or kneeling, a tight plaster cast or brace, or lying still for a long time during illness or surgery. Rapid weight loss reduces the protective fat over the nerve and can bring it on, sometimes called "slimmer's palsy". Other causes include direct injury or fracture around the knee, a ganglion cyst or other mass pressing on the nerve, and, less commonly, medical conditions affecting the nerves. In some cases the nerve is compressed a little deeper, where it passes beneath a muscle.

The most striking symptom is difficulty lifting the front of the foot, so the foot tends to drop and the toes catch when walking — a "foot drop". People may compensate by lifting the knee higher than usual. There is often numbness or altered sensation over the top of the foot and the outer part of the lower leg. Tapping over the nerve at the outer side of the knee may send a tingle down the leg. Pain is variable and is often not a major feature.

A new foot drop always deserves medical assessment, both to identify the cause and because some causes need prompt treatment. It is important to distinguish peroneal nerve compression at the knee from other causes of foot drop, such as a trapped nerve in the lower back (a lumbar nerve root problem), because the assessment and treatment differ. New, severe or rapidly progressing weakness, or weakness following significant injury, should be assessed urgently.

The diagnosis begins with a careful examination that maps out exactly which movements and sensations are affected, helping to localise the problem to the peroneal nerve at the fibular neck rather than elsewhere. Nerve conduction studies and electromyography — tests of nerve and muscle function — confirm the diagnosis, locate the site of compression, and give information about severity and the likelihood of recovery. Where a mass, cyst or structural cause is suspected, an ultrasound or MRI scan of the area is used.

Where the cause is external compression, the first step is to remove it — for example avoiding leg crossing, prolonged squatting or pressure on the outer knee, and adjusting casts or braces. Many compression-related cases recover once the pressure is relieved. An ankle-foot orthosis — a lightweight brace that holds the foot up — is very helpful in the meantime, preventing tripping and allowing a more normal walking pattern while the nerve recovers. Physiotherapy maintains movement and strength. Any underlying medical condition is treated.

Surgery is considered where there is a structural cause such as a ganglion cyst or other mass compressing the nerve, where the nerve is trapped by scar or by a fracture, or where a nerve that is not recovering needs to be explored and released. The operation aims to relieve the pressure on the nerve, remove any mass, and give the nerve the best chance to recover. The timing and type of surgery depend on the cause and on how the nerve is behaving over time.

The outlook depends heavily on the cause and the severity. Foot drop from simple external compression, caught early, often recovers well once the pressure is removed, though nerves recover slowly, so improvement can take weeks to many months. Severe or long-standing damage, or damage from serious injury, may recover only partially. Throughout recovery, a foot-up brace keeps people mobile and safe, and physiotherapy supports the return of function.

Myth: "Foot drop always comes from the back." Fact: It can come from the back, but compression of the peroneal nerve at the knee is a common and distinct cause that needs to be identified.

Myth: "Crossing your legs is harmless." Fact: For most people it is, but habitual leg crossing can compress the peroneal nerve at the outer knee and contribute to foot drop.

Myth: "A brace is giving up on recovery." Fact: A foot-up brace keeps you safe and mobile while the nerve recovers, and is used alongside, not instead of, treating the cause.

This page is for general education and does not replace an individual assessment. Treatment depends on a person's specific symptoms, examination findings, imaging findings and overall health, and should always be discussed with a suitably qualified healthcare professional.

The common peroneal nerve (also called the common fibular nerve) is a branch of the sciatic nerve that supplies the muscles which lift the foot and turn it outwards, and the feeling over the top of the foot and the outer part of the lower leg. Just below the outer side of the knee, the nerve wraps around the neck of the fibula — the slim outer bone of the lower leg — where it lies very close to the skin with little padding. This makes it the most commonly compressed nerve in the leg. Pressure or injury here can cause weakness in lifting the foot and altered feeling, which this page explains, along with how the condition is diagnosed and treated.

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This information is for general patient education and is not a substitute for an individual consultation. Always seek personalised advice from a qualified spinal specialist.