Common fibular nerve

Key points

The common fibular nerve, also called the common peroneal nerve, is one of the two major branches of the sciatic nerve (the large nerve down the back of the thigh)^1,2.
Its deep branch powers lifting the foot and straightening the toes, and its superficial branch powers turning the sole outward³.
At the neck of the fibula (the thin outer bone of the lower leg), only skin and the tissue just beneath it cover the nerve⁴.
A systematic review (a study that searches for all the research on a question and sums it up) of 43,869 knee dislocations found peroneal nerve injuries in 15.3% of patients⁵.
A systematic review of 214 palsies (losses of muscle power from a nerve problem) after knee dislocation found full recovery in 87.3% of partial palsies. Functional muscle recovery followed 38.4% of complete ones⁶.

Function

Where it starts and what it does

The common fibular nerve and the common peroneal nerve are the same nerve, and “peroneal” is the older name². It is one of the two major branches of the sciatic nerve, the large nerve that runs down the back of the thigh¹. Its fibres come from the spinal levels labelled L4 to S2¹. The sciatic nerve typically splits behind the knee into this nerve and the tibial nerve⁷.

The nerve has two main branches, and each powers different movements³:

the deep branch lifts the foot and straightens the toes³;
the superficial branch turns the sole outward, a movement called eversion³.

Its path around the knee

The nerve leaves the sciatic nerve above the back of the knee¹. It runs down the outer back of the leg under the long head of the biceps femoris, one of the hamstring muscles¹. The biceps femoris is the only muscle it supplies above the knee⁸. It then passes behind the head of the fibula, the bone’s small, knob-like upper end^1,9.

Engraving of the front of the right leg and foot with the muscles pulled apart and the nerves drawn in yellow. At the outer side of the knee the common peroneal nerve curves forward around the top of the fibula and divides in two. The superficial peroneal nerve runs down the outer side of the leg beside the peroneus longus and gives a cutaneous branch that fans out over the top of the foot to the toes. The deep peroneal nerve runs down the middle of the leg beside the red anterior tibial artery, between tibialis anterior and the long toe extensors, passes under the band at the front of the ankle and ends as lateral and medial terminal branches on the foot. The saphenous and sural nerves are labelled at the sides. — Nerves of the front of the leg, with the common peroneal nerve dividing below the kneeAfter Testut, in Henry Gray, Anatomy of the Human Body (1918), Fig. 835. Public domain, via Wikimedia Commons. (source, Public domain)

About 2 cm below the fibular head, the nerve wraps around the neck of the bone³. At the top of the fibula it runs through the fibular tunnel, a passage with bone for its floor⁸. The tunnel’s roof is an arch formed by two lower-leg muscles, the peroneus longus and the soleus⁸. At the fibular neck the nerve divides into its superficial and deep branches¹.

The two branches

The superficial fibular nerve is the motor supply of the peroneus longus and peroneus brevis, two of the peroneal muscles¹⁰. About 12 cm above the ankle joint it leaves these muscles through a gap in the deep fascia, the tough sheet wrapping the leg¹⁰. The branch’s skin fibres supply the front outer leg, the top of the foot and the tops of the toes, apart from one patch¹⁰.

The deep fibular nerve runs down the leg beside the anterior tibial artery, between two muscles¹. It works the four muscles in the anterior compartment, the space at the front of the lower leg⁸. One of them is the tibialis anterior, the muscle at the front of the shin that pulls the foot up⁸. Its skin fibres supply that patch, the first web space, where the skin spans the great and second toes¹¹.

Breakdown

Why the nerve is exposed at the fibula

At the neck of the fibula, only skin and the tissue just beneath it cover the nerve⁴. That leaves it open to being pressed against the bone⁴. Trauma can also stretch it there⁸. An imaging review adds that the nerve has little supporting tissue of its own, which makes compression more likely⁸.

The nerve can be injured anywhere along its course, but compression is most common at the fibular head¹². The usual result is a sudden foot drop, complete or partial¹². One medical reference text defines foot drop as being unable to lift the front of the foot because the muscles that lift it are weak². The same text notes that foot drop has diverse causes, which may include muscle, nerve, spine and autoimmune disorders².

Pressure from outside

Causes of compression without an injury include tight bandages, long spells of leg crossing and positioning under anaesthesia³. A problem in this nerve at the fibular head that follows weight loss is known as slimmer’s paralysis⁴. With weight loss the fat around the fibular head shrinks, so the nerve loses its padding against the bone⁴.

Knee injuries

The common peroneal nerve is the lower limb’s most often injured nerve in people with trauma¹³. Stretch injuries can follow ankle sprains where the sole turns inward, upper fibula fractures and dislocation of the knee⁸. The more the bones and ligaments are damaged, the more likely the nerve is damaged too⁸.

Reported rates after injuries to several knee ligaments at once range from 4.5% to 40%¹⁴. The spread probably reflects how much energy the accidents in each series involved¹⁴. Several studies tie the nerve injury to the posterolateral corner, the ligaments and tendons at the outer back of the knee^15,16. That group includes the lateral collateral ligament.

Group studied	Share with a peroneal nerve injury
43,869 knee dislocations, in a systematic review of 132 studies	15.3%⁵
221 multiligament knee injuries at two centres	19.5%, every one with a posterolateral corner injury¹⁵
61 posterolateral corner injuries that had surgery	26% when first seen¹⁷
100 multiligament knee injuries, in a case series at one centre	31% with a posterolateral corner injury, 4% without one¹⁶

A chart review covered 17 adults with a traumatic dislocation of the joint between the tops of the tibia (shinbone) and fibula¹⁸. Seven had a peroneal nerve deficit before surgery, and six of those had a lasting deficit or an amputation¹⁸.

In sport

One centre reviewed 448 peroneal nerve injuries seen between 1970 and 2010, and 84 were sports related¹⁹. The same review concluded that the injury at the knee in sport is usually a stretch or a contusion (bruise)¹⁹.

Cases among the 84. Bars start at zero; the table below gives each value with its source.

Sport	Cases among the 84
Skiing	42¹⁹
Football	23¹⁹
Soccer	8¹⁹
Basketball	6¹⁹

In competitive athletes, a trapped peroneal nerve is a less common cause of leg pain brought on by exercise²⁰. One sports practice reported seven runners and one soccer player with exercise-related compression of the nerve at the fibular neck²¹.

At the ankle and foot

Studies of ankle sprains give different pictures of how often the nerve is involved^22,23. In 66 patients tested two weeks after a sprain, 17% of grade II and 86% of grade III sprains showed peroneal nerve injury²². The test was electromyography, which picks up a muscle’s electrical signals²². Another paper calls peroneal palsy with an ankle sprain rare, with few reported cases and no known true rate²³.

The deep branch passes under a retinaculum, a band that holds tendons in place, just above the ankle joint²⁴. An anatomy paper lists reasons it gets compressed there²⁵:

direct trauma²⁵;
the foot pointed far down in high-heeled shoes and high boots²⁵;
constant mechanical irritation, such as from tightly laced shoes²⁵;
a clot in the dorsalis pedis artery, an artery on the top of the foot²⁵.

One 1979 report described the tongue of a ski boot pressing on the peroneal nerve at the ankle²⁶. Trapping of the superficial branch is relatively rare²⁷. Its pain and tingling tend to be made worse by walking and exercise²⁷.

Recovery

After pressure on the nerve

A review of nerve tissue injury describes neurapraxia, the mildest grade, as a short-term signal block that clears fully within 12 weeks²⁸. It describes injury to the fibres themselves as followed by slow regrowth, about 1 mm a day, often with incomplete recovery²⁸. Two studies of pressure palsies in this nerve report recovery over weeks to months^29,30:

In 30 patients with sudden palsy after weight loss, a signal block at the fibular head cleared in 29 within three weeks to three months²⁹.
Sixteen farmworkers with palsy from squatting at work all recovered without surgery, in 7.4 weeks on average³⁰.

After knee dislocation

A systematic review of 13 articles covered 214 palsies after knee dislocation⁶. Functional muscle recovery followed 38.4% of complete palsies, and full recovery followed 87.3% of partial ones⁶. In the same review, younger age predicted recovery of the nerve⁶.

In 61 posterolateral corner injuries, 10 complete nerve injuries were stretches, not cuts; all had an operation to free the nerve, and 5 of the 10 regained full function¹⁷. The deep branch often recovers less well than the superficial one after a stretch injury at the knee¹⁴. A review relates this to its closeness to the joint and its tether further down, which give a more severe stretch¹⁴.

What surgery series reported

Decompression is surgery to take pressure off a nerve. Neurolysis is an operation that frees a nerve from the tissue around it. A graft is a piece of tissue moved to rebuild a damaged part. In a systematic review of traumatic injuries, better outcomes went with a shorter wait before surgery, a shorter graft and younger age¹³.

Group studied	What was reported
60 patients who had decompression for a trapped nerve, followed 42 months on average	Muscle recovery was good in 33 of the 38 with both feeling and muscle symptoms beforehand³¹
368 patients, in a meta-analysis (a study pooling earlier studies) of neurolysis	53.9% reached full strength³²
42 studies of traumatic injury, in a systematic review	Useful muscle strength in 81.4% after neurolysis, 78.8% after end-to-end stitching, 49.0% after grafting and 62.9% after nerve transfer¹³
1,577 nerve repairs, in a literature review of 28 studies	Good muscle outcomes in 44% of repairs done within 6 months, against 12% after 12 months³³

A scoping review, which maps what research exists, looked at trapped peroneal nerves with foot drop³⁴. Good outcomes ranged from 0% to 100% without surgery and from 40% to 100% after neurolysis³⁴. It rested mostly on case series, and no study compared the two³⁴.

Connections

Nerves, bones and the knee

The tibial nerve is the other branch of the sciatic nerve⁷. In knee dislocation the common peroneal nerve is injured more often than the tibial nerve¹⁴. A review puts this down to its course around the outer back of the knee, held close to the neck of the fibula¹⁴.

In 303 patients with knee dislocations, the odds of injury to this nerve were 42 times higher when the posterolateral corner was injured³⁵. That corner includes the lateral collateral ligament. In 16 cadaver legs loaded to mimic an ankle sprain, strain (stretch as a share of resting length) in the superficial branch rose with load³⁶.

Muscles and exercises

The superficial branch supplies the peroneus longus and brevis, described on the peroneals page^37,38,39. The deep branch supplies the anterior compartment muscles, including the tibialis anterior, whose tendon has its own page: tibialis anterior tendon⁴⁰. In an electromyography study of 13 runners, the front lower-leg muscles contracted while shortening at first ground contact⁴¹. After 12 weeks of fast foot-lifting training in five people, tibialis anterior motor units switched on earlier in fast contractions⁴². A motor unit is a nerve cell and the muscle fibres it switches on.

Related exercise pages: tibialis raise, running^41,42.