The Anatomy of the Deep Peroneal Nerve

The deep peroneal nerve, also called the deep fibular nerve, is a peripheral nerve of the calf. It's a terminal branch of the common peroneal nerve, which is a branch of the sciatic nerve. The deep peroneal nerve contains both motor and sensory fibers.

Anatomy

All of your nerves, except for the cranial nerves, branch off from the spinal cord. The nerves that extend away from the spine and into the limbs are called peripheral nerves. As your peripheral nerves make their way down your arms and legs, they send off branches that connect to various muscles and other tissues to provide them with motor function (movement), sensory function (feeling), or both.

The sciatic nerve's roots leave the spinal cord between the vertebrae in the lumbar and sacral regions of your lower back. The roots then join and become a single nerve that runs through your buttocks and down the back of your thigh.

When the sciatic nerve gets to what's called the popliteal fossa (colloquially known as the "knee pit"), it gives off two main branches:

1. Tibial nerve
2. Common peroneal nerve

The tibial nerve continues down the back of the leg while the common peroneal nerve wraps around the outside of your knee to get to the front of the calf. Just below the knee, the common peroneal nerve separates into two terminal branches:

1. Superficial peroneal nerve
2. Deep peroneal nerve

Structure

The deep peroneal nerve sends off motor branches to several muscles in the calf, including:

• Tibialis anterior
• Extensor hallucis longus
• Extensor digitorum longus
• Fibularis tertius

It also sends a branch to the ankle joint, then puts off two branches into the foot:

1. The lateral branch, which connects to the extensor digitorum brevis and extensor hallucis brevis muscles
2. The medial branch, which is a cutaneous (of the skin) nerve

The lateral and medial are the terminal branches of the deep peroneal nerve.

Location

From where it originates between the fibularis longus muscle and the neck of the fibula (the bone on the outside of the calf), the deep peroneal nerve moves to the front compartment of the calf and runs downward along the anterior tibial artery.

It then passes between the tibialis anterior and the extensor digitorum longus, and then along the extensor hallucis longus, sending out motor branches to connect with these muscles as well as the fibularis tertius in the lower third of the leg.

Continuing downward, it crosses the ankle joint, the divides into its terminal branches along the top of the foot.

Function

The upper portion of the deep peroneal nerve provides motor function to muscles, while the lower portion provides both motor and sensory function to portions of the foot.

Motor Function

By innervating the tibialis anterior, extensor hallucis longus, extensor digitorum longus, and fibularis tertius, the deep peroneal nerve is responsible for pulling the foot back—the opposite motion of pointing the toes. This motion, which is called dorsiflexion, is important for walking. Dorsiflexion is required both when your heel strikes the floor and while your leg is swinging forward.

Via its lateral branch, this nerve allows the muscles to extend the toes.

Sensory Function

The medial branch of the deep peroneal branch is sensory—for one very small spot in the top of the foot. It transmits information about temperature and sensation from the skin between your big toe and second toe. (A terminal branch of the superficial peroneal nerve provides sensory information for the rest of the foot's upper surface.)

Associated Conditions

The main condition associated with the deep peroneal nerve is called foot drop. This condition is the most common mononeuropathy (damage to a single nerve) of the legs.

Foot drop is a loss of the ability to dorsiflex the foot due to nerve entrapment or compression. This can happen at any point along the nerve's journey down the calf or into the foot. Compression is usually due to inflammation from overuse or tight-fitting shoes, especially tight ski boots. It can also be from tumors or other growths that put pressure on the nerve. As well, the nerve may be damaged during knee surgery.

Other medical conditions that can lead to foot drop include:

• Diabetes
• Ischemia (impaired blood flow)
• Motor neuron disease
• Poliomyelitis
• Stroke

Foot drop causes your toes to stay pointed while walking, which can make it difficult for them to clear the ground when you swing your leg. The foot tends to make a slapping sound when it comes down on the floor with each step because you can't control its motion as it's lowered.

Doctors can pinpoint the cause of foot drop through various tests and scans, including:

• X-rays
• Ultrasound
• CT scan
• Magnetic resonance imaging (MRI)
• 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

Rehabilitation

How foot drop is treated depends on what's causing it. In some cases, it may not be treatable, and the associated pain and disability will be permanent.

Treatment can include:

• Physical therapy
• Braces or splints
• Nerve stimulation
• Surgery