The Anatomy of the Common Peroneal Nerve

The common peroneal nerve, also called the common femoral nerve, is a peripheral nerve that branches off from the sciatic nerve. It and its branches supply sensation and some motor function to the lower leg and top of the foot.

The sciatic nerve's roots leave the spinal cord in the two lowest regions of the spine—the lumbar and sacral regions. The roots they join together and run down through the buttocks and the back of the thigh.

As the sciatic nerve reaches the back of your knee, which you might call the "knee pit" but doctors refer to as the popliteal fossa, it separates into two main branches:

1. Tibial nerve, which is primarily a motor nerve
2. Common peroneal nerve, which is primarily sensory

The tibial nerve continues down the back of the leg while the common peroneal nerve wraps around the outside of the knee to reach the front of your calf.

Structure

The common peroneal nerve has a relatively short run. Soon after branching off from the sciatic nerve, it sends off two cutaneous branches. "Cutaneous" means having to do with the skin; these cutaneous branches provide sensation to the skin of your lower leg. They're called the sural communicating nerve and the lateral sural cutaneous nerve. It also sends a branch, called the genicular branch, to the knee joint.

The common peroneal nerve then divides into its two terminal branches: the deep peroneal nerve and the superficial peroneal nerve.

Location

From its origin just above the popliteal fossa, the common peroneal nerve runs along the inner edge of the biceps femoris muscle, over the head of the gastrocnemius. That's where the two cutaneous branches split off.

The common peroneal nerve then wraps around the neck of the fibula (the calf bone on the outside of your leg), pierces the fibularis longus muscle, and divides into its terminal branches on the outside of the leg, not far below the knee.

Anatomical Variations

There is at least one report of the common peroneal nerve dividing into its two branches in the popliteal fossa instead of at the level of the head of the fibula. This variation could have implications for decompression surgery or in emergency treatment when a regional nerve block is being given to numb the foot.

The common peroneal nerve and its terminal branches have both motor and sensory fibers, but its motor function is limited. The rest of it is purely sensory, sending information about things like temperature and pressure to and from the brain.

Motor Function

The nerve connects to the short head of the biceps femoris muscle. That is part of the hamstring muscles, which are what allow your knee to flex. That's the only portion of the main nerve that serves a motor function.

The superficial peroneal nerve, however, provides movement to the peroneus longus and peroneus brevis muscles down the outside of your calves, which allows you to turn the tips of your feet out.

The deep peroneal nerve, meanwhile, connects to the muscles of the front of your calf, including tibialis anterior, extensor digitorum longus, and extensor hallucis longus. Those muscles allow you to flex your foot and extend your toes.

Sensory Function

The common peroneal nerve and its branches also serve sensory functions:

• The sural communicating nerve joins a branch of the tibial nerve to innervate the skin over the outside rear of your calf and the outer edge of the foot.
• The lateral sural cutaneous nerve provides sensation the muscle on the outside of your calf just below the knee.
• The superficial and deep peroneal nerves have sensory functions, as well. The superficial allows for feeling in the skin on the outer front part of the bottom half of your calf and down across the top of your foot, clear to the tips of your toes.
• The deep peroneal nerve provides feeling to the skin in one small spot between your first and second toes.

The common peroneal nerve can be damaged by injury or disease. That may result in neuropathy, which is pain and abnormal sensations due to nerve damage, as well as impaired function, especially if motor fibers are damaged. Damage to this nerve is the most common cause of neuropathy in the legs.

Common causes of nerve injury include:

• Knee trauma or dislocation
• Fibula fractures
• Prolonged or regular pressure, as from a tight plaster cast, regularly crossing the legs, or regularly wearing tall boots
• Pressure to the knee due to positioning during sleep, surgery, or coma

Diseases that may lead to common peroneal nerve damage include:

• Autoimmune conditions such as polyarteritis nodosa or multiple sclerosis
• Neuroma (tumor on the nerve sheath)
• Other types of tumors or growths
• Diabetes
• Alcoholism
• Charcot-Marie-Tooth disease

Symptoms of neuropathy in the common peroneal nerve may be:

• Decreased sensation, numbness, or tingling in the skin innervated by this nerve or its branches
• Pain in the lower leg or foot
• Weakness in the biceps femoris muscle
• Weakness in the foot and ankle
• Foot drop (inability to flex the foot while walking due to ankle and foot-extensor paralysis)
• The affected foot making a "slapping" noise with each step
• Dragging toes
• Loss of muscle mass due to lack of stimulation by the nerves

Neuropathy in the common peroneal nerve is typically diagnosed using a combination of methods that depend on the specific symptoms and any suspected causes. These can include:

• Physical examination
• Electromyography (EMG), which is a test of electrical activity in the muscles
• Nerve conduction tests, which measure how fast electrical signals move through the nerves
• Magnetic resonance imaging (MRI)
• Nerve ultrasound
• Blood tests
• X-rays
• Other scans

The first step in treating neuropathy in the common peroneal nerve is to treat the underlying cause, whether it's disease or injury. If that's not enough to relieve symptoms, or if symptoms are severe enough to warrant immediate treatment, you have multiple options.

• Physical therapy, to maintain or regain muscle strength
• Occupational therapy, to help you maintain or regain mobility and function
• Braces, splints, orthopedic shoes, or other equipment to help you walk

Over-the-counter pain medications may help ease the symptoms. If they don't provide enough relief, your doctor may prescribe other medications used specifically for nerve pain. These include:

• Gabapentin
• Carbamazepine
• Amitriptyline or other tricyclic antidepressants
• Injected corticosteroids

Surgery may be an option in cases where:

• Movement is seriously impaired
• The neuropathy doesn't go away when the underlying cause is treated
• The nerve axon is damaged
• Something, such as a tumor, is putting pressure on the nerve