Common Peroneal Nerve Entrapment

The common peroneal nerve is particularly vulnerable at the fibular head because it runs superficially around the bone with relatively little soft tissue protection. Causes of compression include:

• Habitual leg crossing — crossing the legs at the knee presses the fibular head against a hard surface, compressing the nerve, a common cause of temporary peroneal palsy
• Prolonged squatting or kneeling — occupational or recreational activities requiring prolonged knee flexion with pressure at the fibular head
• Cast, brace, or tight bandaging — external compression from orthopaedic immobilisation
• Rapid weight loss — loss of the protective fat pad around the fibular head increases vulnerability
• Knee surgery or trauma — direct injury to the peroneal nerve at the fibular head during knee procedures, fracture, or dislocation
• Ganglion cyst or fibrous band — intrinsic compression from a structure within the peroneal tunnel at the fibular head
• Prolonged bed rest — pressure on the outer knee during immobilisation

Foot Drop

Weakness of foot dorsiflexion — difficulty lifting the front of the foot — causes foot drop, the most significant and functionally disabling symptom. Patients drag the toes, trip on uneven surfaces, and adopt a high steppage gait to clear the foot during walking. Foot drop is a safety hazard and significantly limits mobility.

Numbness and Tingling

Numbness, tingling, and altered sensation on the outer aspect of the lower leg (supplied by the superficial peroneal nerve) and the dorsum (top) of the foot. The deep peroneal nerve supplies a small area of skin between the first and second toes.

Weakness of Foot Eversion

Weakness of the peroneal muscles (evertors) causes difficulty turning the foot outward and may contribute to ankle instability.

Pain

Aching or burning pain on the outer side of the knee and upper leg, sometimes extending down the leg.

• Clinical examination — assessment of ankle dorsiflexion strength (tibialis anterior), foot eversion, toe extension (extensor digitorum longus), and sensation in the peroneal nerve territory
• Nerve conduction studies (NCS) and EMG — confirm the diagnosis, localise the level of compression, determine severity, and exclude other causes of foot drop (L4/L5 radiculopathy, lumbar plexopathy, peripheral neuropathy)
• MRI of the knee and fibular head — identifies structural causes of compression (ganglion cyst, intrinsic fibrous band, mass); evaluates nerve signal and morphology
• Ultrasound of the peroneal nerve — demonstrates nerve enlargement and dynamic compression at the fibular head; identifies cysts or other structural causes

Distinguishing CPN entrapment from L4/L5 nerve root compression (from lumbar disc herniation or spinal stenosis) is critical, as both can cause foot drop but require entirely different treatment. EMG and MRI lumbar spine are used to make this distinction.

Non-Surgical Treatment

When foot drop is recent and caused by an identifiable extrinsic pressure (leg crossing, cast), removal of the offending compression, combined with physiotherapy and an ankle-foot orthosis (AFO) splint to support the foot during recovery, may allow nerve recovery over weeks to months. Nerve recovery depends on the severity and duration of compression.

Surgical Treatment — Common Peroneal Nerve Decompression

Surgical decompression is recommended when foot drop is persistent despite conservative management, when there is a structural cause identified on imaging (ganglion cyst, fibrous band), when symptoms are severe, or when there is ongoing compression that is unlikely to resolve without surgery.

The procedure involves decompression of the peroneal nerve at the fibular head under general anaesthesia through a short incision just below the outer aspect of the knee. The compressing structure — fibrous tunnel, ganglion, or fibrous band — is released or excised, freeing the nerve from compression. If a ganglion cyst is identified, it is excised to prevent recurrence.

Recovery

• Day case procedure
• Return to light activities — 2 weeks
• Physiotherapy and foot exercises — essential during recovery to maintain joint mobility and retrain muscles
• Ankle-foot orthosis (AFO) — continued until adequate dorsiflexion strength returns
• Nerve recovery — sensory improvement typically begins within weeks; motor recovery (dorsiflexion strength) follows more slowly over months; complete recovery depends on duration and severity of compression before surgery