The Anatomy of the Anterior Interosseus Nerve

The anterior interosseous nerve (AIN) is a branch of the median nerve, which is in your arm. The nerve supplies function to most of the deep muscles in the front of your forearm, playing a role in both motor (movement) and sensory (feeling) innervation. Those muscles are critical for controlling the motion of your hand.

The anterior interosseous nerve is sometimes called the volar interosseous nerve. (Volar means "palm side.") Some textbooks treat the AIN just as part of the median nerve rather than an actual branch.

Anatomy

The nerves in your body are structured somewhat like trees, with roots coming off of the spinal cord and combining to form trunks. Branches split off from the trunk along the nerve's course, connecting to various muscles, bones, patches of skin and other structures all over your body. All of your nerves except for 12 in your head (the cranial nerves) originate from roots at the spinal cord and pass out of the spinal column between vertebrae.

The AIN’s roots come from the lowest cervical vertebrae (C8) and the highest thoracic vertebrae (T1), which are both at the base of the neck.

Nerves exist as symmetrical pairs, with one on each side of your body. They’re generally referred to, however, as a singular nerve unless there’s a reason (such as an injury) to specify the right or left one.

Structure and Location

At several places along your trunk, you have complex networks of nerves called plexuses. The brachial plexus stretches from the neck down to the armpit. In your shoulder, at the level of the collarbone, several branches of the brachial plexus combine to form the median nerve, which eventually gives rise to the AIN.

The median nerve then divides into several terminal branches, including:

• Palmar cutaneous branch
• Recurrent branch
• Digital cutaneous branches
• Anterior interosseous branch

The AIN splits off at the elbow or slightly below it. There, it passes between the two heads of the pronator teres muscle; runs along the inner-arm surface of the flexor digitorum profundus muscle, which it innervates; continues down along the interosseous membrane between the ulna and radius (bones of your forearm); and comes to an end in the pronator quadratus muscle, which sits just above your wrist.

Anatomical Variations

While the “typical” nerve structure is similar in most people, some people have variations to what’s considered standard anatomy. It’s important for doctors to know about possible anatomical variations as they can make a difference when it comes to diagnosis and treatment—especially surgical procedures.

A known anatomical variation that often involves the anterior interosseous branch is called Martin-Gruber anastomosis (MGA). The MGA is a communicating nerve branch that connects the median nerve and the ulnar nerve in the forearm. Research suggests it’s present in between a quarter and a third of people, and much of the time, it’s the AIN that forms the connection between the two nerves.

In people with an MGA that involve the AIN, damage to the AIN can lead to problems in hand muscles that are typically supplied by the ulnar nerve. Some people with MGA have it only on one side while others have it on both sides.

Function

The AIN is primarily a motor nerve, but it does play a small sensory role and is, therefore, a mixed nerve.

Motor Function

The AIN provides motor function to the muscles of the deep forearm, which are the:

• Flexor digitorum profundus (FDP): The FDP is also innervated by the ulnar nerve. In Latin, the name of this muscle means “deep bender of the fingers.” The FDP runs along the outside half of the back of your forearm and fans out into four sections that connect to the base of your fingers and allow you to flex them. This muscle also assists with flexing the hand.
• Flexor pollicis longus (FPL): The FPL lies along the inside edge of the back of your arm. Its name means “long bender of the thumb.” It’s what gives humans the unique opposable thumb that sets us apart from other primates.
• Pronator quadratus (PQ): The PQ is a square-shaped muscle just above your wrist on the thumb side. It allows you to pronate your hand, which means turning your arm so your palm faces downward. “Quadratus” means square and “pronator” describes its motion.

Sensory Function

In its single sensory role, the AIN sends sensory fibers to a part of the wrist called the volar joint capsule. The capsule forms a sleeve around the joint that secretes fluid to lubricate and nourish the joint. It also acts as a shock absorber. Unlike most sensory nerves, the AIN doesn’t innervate any skin.

Associated Conditions

The anterior interosseous nerve’s course is deep within your arm, which means it’s protected by several structures. That makes damage to it rare.

However, it also means the nerve is subject to compression by nearby structures, including the:

• Struthers ligament
• Fibrous arches between the heads of the pronator teres muscle
• Fibrous arch at the head of the flexor digitorum superficialis muscle
• Veins that cross the nerve
• Abnormal growths such as tumors, cysts, hematomas, and abscesses

Damage to the AIN can cause pronator syndrome or AIN syndrome (also called AIN neuropraxia, AIN palsy, or AIN compression syndrome).

Symptoms of pronator syndrome include:

• Pain in the inner forearm
• Abnormal nerve sensations along the path of the median nerve
• Very little motor involvement

Symptoms of AIN include:

• Weakness in the muscles innervated by the AIN
• Dysfunction or paralysis of the joints in the hand associated with those muscles
• Pain in the elbow, forearm, wrist, and of hands

Rehabilitation

Because damage to the AIN and the associated syndromes are rare, making up less than 1% of arm nerve palsies, standard treatment protocols aren’t yet well established. Treatment may include conservative therapies. Conservative therapy may include rest, anti-inflammatory medications, physical therapy, and activity modification.

If the conservative approach fails, surgery may be recommended to relieve nerve compression. Some research suggests six months of conservative therapy before surgery is considered.