Common peroneal nerve compression at the knee

Where the peroneal tunnel is

Below the knee, the common peroneal nerve winds around the neck of the fibula, the thin bone on the outer side of the lower leg. Here it is close to the skin, and it passes through a fibro-osseous tunnel between the fibular neck and a tendinous arch of the peroneus longus muscle.^[3] There is little padding, so pressure from outside or a tight tunnel can squeeze the nerve.

The nerve then divides into a superficial branch, which gives feeling to the outer lower leg and top of the foot, and a deep branch, which lifts the foot and toes and gives feeling to the web between the big toe and second toe. For problems that involve only one branch, see superficial peroneal nerve entrapment and anterior tarsal tunnel syndrome.

Symptoms

• numbness or tingling on the outer lower leg and the top of the foot
• pain around the outer knee or fibular head
• weakness lifting the toes and ankle (foot drop), so that the toes catch or the foot slaps down
• a tender spot over the fibular neck; in one series, though, only 12 of 30 people had a positive Tinel sign (tingling when the nerve is tapped) at that site^[1]

Weakness of the tibialis anterior and extensor hallucis longus muscles, along with loss of feeling in the peroneal distribution, is a common presentation.^[1] See foot drop for the full range of causes of weak foot lift.

What causes it

• Outside pressure: habitually crossing the legs, kneeling or squatting for long periods, a tight cast or brace
• Injury and surgery: in a series of 30 people treated surgically, 14 followed trauma to the leg and 12 followed a surgical procedure^[1]
• Knee injury: dislocation of the knee is often accompanied by injury to the common peroneal nerve^[4]
• Masses: a cyst or other swelling near the nerve; two people in the series above had a mass compressing the nerve^[1]
• Other health conditions, including nerve disease that makes the nerve more vulnerable^[5]

Peroneal nerve injuries are the most common nerve injuries of the lower limb in trauma, and outcomes range widely.^[6]

How it is evaluated

Your clinician will ask about knee injuries, surgery, weight loss, prolonged positions and back symptoms, and will test strength in the muscles that lift the foot and turn it outward, along with feeling on the leg and foot. Because a pinched nerve in the lower back can look the same, the back and sciatic nerve are also considered; see back or foot: finding the source of nerve pain and sciatica. All of the patients in the surgical series above had electrophysiological confirmation before surgery.^[1] Nerve studies are arranged with another specialist; MRI or ultrasound can look for a cyst or mass.

Treatment without surgery

Care starts by removing the cause and supporting the foot:

• stop leg crossing, and avoid prolonged kneeling or squatting
• loosen or change a tight brace, cast or garment
• an ankle-foot orthosis to hold the foot up while the nerve recovers
• physical therapy for strength, ankle flexibility and gait

High rates of spontaneous recovery have been reported, which is why many people are observed first.^[3]

Surgery to release the nerve

A subset of people need the nerve released at the fibular neck. In a retrospective series of 20 people (22 operations), 74% with weakness improved, 68% with sensory loss improved, and 69% of those with foot drop improved enough to have no foot drop. Most of the improvement occurred in people operated on within 12 months of symptom onset.^[3] In a prospective study of 15 people with severe idiopathic compression, 13 of 14 who had surgery improved, half regained normal strength, and the one who did not improve had the longest wait before surgery.^[2] A pooled analysis also found meaningful gains after nerve release. The evidence comes mostly from case series, not randomized trials.^[7,6]

When the nerve is cut, stretched beyond recovery or does not improve, other options include nerve grafting, nerve transfer and tendon transfer. See nerve transfer, nerve grafting and nerve decompression. After a knee dislocation, one review recommends coordinated care between the knee surgeon and the nerve surgeon, and a tendon transfer when a foot drop remains.^[4]

Timing

The clock matters. Foot lift recovers best when the pressure is removed early. If you have new weakness in foot lift, do not wait for months to see whether it improves without an evaluation.

When to seek care

Consider an evaluation for numbness or tingling on the outer leg or top of the foot, pain at the outer knee, or any new difficulty lifting the foot.