Radial Nerve: What is it? Anatomy, Structure, Functions, Clinical Significance and Associated Lesions

It is the largest branch of the brachial plexus and is the continuation of the posterior cord, with nerve fibers from C6, C7, C8 and occasionally T1.

It innervates the medial and lateral heads of the triceps brachii muscle of the arm, as well as the 12 muscles in the posterior osteofascial compartment of the forearm and the associated joints and overlying skin.

The radial nerve and its branches provide motor innervation to the muscles of the dorsal arm (the triceps brachii and the anconeus ) and the extrinsic extensors of the wrists and hands.

It also provides cutaneous sensory innervation to most of the back of the hand, except for the back of the little finger and the adjacent half of the ring finger (which are innervated by the ulnar nerve).

Anatomy and structure

The radial nerve originates as a terminal branch of the posterior cord of the brachial plexus.

Arm

From the brachial plexus, it travels behind the third part of the axillary artery (part of the axillary artery distal to the pectoralis minor). In the arm, it runs behind the brachial artery and then enters the lower triangular space to reach the radial groove at the back of the humerus.

It travels downward along with the deep brachialis, between the lateral and medial heads of the triceps brachii until it reaches the lateral side of the arm 5 cm below the deltoid tuberosity, where it pierces the lateral intermuscular septum to reach the anterior compartment of the arm .

Then it descends to cross the lateral epicondyle of the humerus where the nerve ends up branching into a superficial and deep branch that continues into the ulnar fossa and then into the forearm.

The radial nerve provides muscular branches to supply the long head, medial head, and lateral head of the triceps brachii muscles before and during their travel in the radial groove.

Above the radial groove, the radial nerve emits the posterior cutaneous nerve of the arm that supplies the skin on the back of the arm.

In the radial groove, it emits the lower lateral cutaneous nerve of the arm and the posterior cutaneous nerve of the forearm. The radial nerve also gives articular branches to supply the elbow joint.

Forearm

In the forearm, it is divided into a superficial branch (mainly sensory) and a deep branch (mainly motor).

The superficial branch of the radial nerve is widely separated from the radial artery in the upper third of the forearm, closely related to the radial artery in the middle third of the forearm, and in the lower third it descends in the forearm below the brachioradialis tendon.

It passes through the brachioradialis to enter the back of the forearm near the back of the wrist and supply the back of the hand.

Provides sensory supply to the dorsal aspect of the hand, dorsal aspect of the thumb, index finger, middle finger, and lateral side of the ring finger, except for the nail beds, which are supplied by adequate digital branches of the median nerve.

The deep branch of the radial nerve (also known as the posterior interosseous nerve by some authors) crosses the supinator muscle.

It wraps it around the radius under the covering of the supinator to reach the back of the forearm, where it again pierces the supinator and is later known as the posterior interosseous nerve.

It pierces the posterior extensor muscles and is located between the superficial and deep muscles of the back of the forearm.

At the lower border of the extensor pollicis brevis, it passes deep into the extensor pollicis longus and then spreads over the posterior interosseous membrane.

It continues to move along with the posterior interosseous artery (a deep branch of the common interosseous artery that is a branch of the ulnar artery) and ends as a pseudoganglion below the extensor retinaculum by supplying the wrist and intercarpal joints.

Clavert et al dissected 30 cadaveric upper limbs to define the anatomical points of the radial nerve and determine the relationship of the trunk and branches of the radial nerve with the bone and peripheral muscular structures in the anterior aspect of the elbow joint, to identify the probable causes of compressive neuropathy

No radial compressive neuropathy was found at the level of the supinator arch and no adhesions were identified between the radial nerve and the joint capsule.

In four cases, Clavert et al noted dense fibrous tissue surrounding the radial nerve supply to the extensor carpi radialis brevis. No fibrous structures or adhesions of the deep branch of the radial nerve were observed along its path through the supinator.

The fibrous arch of the supinator arose semicircularly and was tendinous in 87% of the extremities and membranous in 13%. The average length of the Frohse playroom was 25.9 mm, and the angle formed by the radial arch and the supinator arch was 23 degrees.

Variation

It is commonly believed that the radial nerve also provided motor innervation to the long head of the triceps. However, a 2004 study found that the axillary nerve innervates the long head of the triceps in 20 cadavers without any radial nerve supply.

Features

The following are branches of the radial nerve (including the superficial branch of the radial nerve and the deep branch of the radial nerve / posterior interosseous nerve).

Motor function

The radial nerve innervates the muscles located in the posterior arm and posterior forearm.

In the arm, it innervates the three heads of the triceps brachii, which acts to extend the arm at the elbow. The radial nerve also gives rise to branches that supply the brachioradialis and the extensor carpi radialis longus (muscles of the posterior forearm).

A terminal branch of the radial nerve, the deep branch, supplies the remaining muscles of the posterior forearm. As a generalization, these muscles act to extend into the wrist and finger joints and supinate the forearm.

Note : When the deep branch of the radial nerve penetrates the supinator muscle of the forearm, it is called the posterior interosseous nerve for the remainder of its course.

Sensory function

There are four branches of the radial nerve that provide cutaneous innervation to the skin of the upper extremity. Three of these branches arise in the upper arm:

Lower arm lateral cutaneous nerve: Innervates the lateral aspect of the arm, inferior to the insertion of the deltoid muscle.

Posterior cutaneous nerve of the arm: Innervates the posterior surface of the arm.

Posterior cutaneous nerve of the forearm: Innervates a strip of skin in the middle of the posterior forearm.

The fourth branch, the superficial branch, is a terminal division of the radial nerve. It innervates the dorsal surface of the lateral three and a half digits, and their associated palm area.

The superficial branch of the radial nerve provides sensory innervation to much of the back of the hand, including the web of skin between the thumb and forefinger.

Clinical significance

Injury

Injury to the radial nerve at different levels causes different syndromes with various motor and sensory deficits.

In the armpit

Common Mechanisms of Injury : Saturday night paralysis, crutch paralysis

Motor deficit : loss of forearm extension, supination weakness, and loss of hand and finger extension.

Presence of drooping of the wrist, due to the inability to extend the hand and fingers.

Sensory deficit : loss of sensation in the lateral arm, posterior forearm, the radial half of the back of the hand and the dorsal aspect of 3 1/2 radial digits, excluding their nail beds.

In the middle of the arm

Common mechanisms of injury : humerus fracture in the middle axis.

Motor deficit : supination weakness and loss of extension of the hand and fingers.

Presence of drooping of the wrist, due to the inability to extend the hand and fingers.

Sensory deficit : the same as the previous one.

Just below the elbow

Common mechanisms of injury : Radius neck fracture, elbow dislocation or fracture, tight cast, rheumatoid nodules, injections due to tennis elbow, injury to the deep branch of the radial nerve that pierces the radial head, causing the nerve syndrome posterior interosseous

Motor deficit : weakness in the extension of the hand and loss of extension of the fingers.

Presence of finger drop and partial wrist drop, as the extensor carpi radialis and brachioradialis muscles are working.

Sensory deficit : none, since sensation is supplied by the superficial radial nerve.

Inside the distal forearm

Common Mechanisms of Injury : Wartenberg syndrome, (not to be confused with Wartenberg’s sign), due to retention of the nerve below the brachioradialis tendinous insertion, tight jewelry, and watch bands.

Motor deficit : none.

Sensory deficit : numbness and tingling in the radial half of the back of the hand, and a dorsal aspect of 3 1/2 radial digits, excluding its nail beds.

In Wartenberg syndrome, there is considerable radial pain in the wrist and a strong resemblance to the symptoms in de Quervain’s tenosynovitis. The Finkelstein test can be positive.

Radial nerve entrapment

Radial nerve compression or injury can occur at any point along the anatomical course of the nerve and can have a variety of etiologies.

Radial nerve entrapment is a rare diagnosis that is often not recognized. Compression or entrapment can occur at any location within the course of the nerve distribution, but the most common location of entrapment occurs in the proximal forearm.

This most common location is typically near the supinator and will often involve the posterior interosseous branch. The radial nerve arises from C5 to C8 and provides motor function to the extensors of the forearm, wrist, and fingers.

The superficial radial nerve provides a sensory function to the posterior forearm. Depending on the location of the entrapment, a patient may experience pain, numbness, weakness, and general dysfunction or any combination of these.

Compression or scarring of the radial nerve at different points along its path can cause denervation of the extensor or supinator muscles and numbness or paresthesia in the distribution of the radial sensory nerve (RSN).

The result can be pain, weakness, and dysfunction. The most common compression site is on the proximal forearm in the area of ​​the supinator and involves the posterior interosseous branch.

However, problems can occur proximally in relation to fractures of the humerus at the junction of the middle and proximal thirds, as well as distally on the radial aspect of the wrist.

Pathophysiology of radial nerve entrapment

Nerve injury secondary to compression or traction depends on the intensity and duration. Seddon classified nerve injuries into the following three categories:

Neurapraxia : This is a transient episode of motor paralysis with little or no sensory or autonomic dysfunction; there is no disruption of the nerve or its sheath; With the elimination of the compression force, the recovery must be complete.

Axonotmesis : this is a more severe nerve injury, in which the axon breaks but the Schwann sheath remains; results of motor, sensory and autonomic paralysis; recovery can occur if the compressive force is removed in a timely manner and if the axon regenerates.

Neurotmesis : this is the most serious injury, in which both the nerve and its sheath are affected; although recovery can occur, it is always incomplete, secondary to loss of nerve continuity

Sunderland classified the nerve injury into five categories, as follows:

  • The first is similar to neurapraxia.
  • The second is similar to axonotmesis.
  • The third, fourth and fifth degrees correspond to various degrees of neurotmesis.

Symptoms of radial nerve entrapment

Presentation can certainly vary given various areas of possible entrapment. Symptoms generally develop very slowly. The duration of symptoms often averages many years before a definitive diagnosis is made.

As mentioned above, the symptoms of this type of nerve entrapment are pain, sensory and motor changes, paresthesia, and / or paralysis. Physical examination and / or history often reveal symptoms limited to the dorsoradial aspect of the forearm and distal hand.

Findings of decreased sensation on the dorsoradial aspect of the forearm or hand are helpful in establishing the diagnosis. A positive Tinel sign along the radial aspect of the middle forearm is suggestive of this process.

Wrist flexion, ulnar deviation, and pronation put pressure on the nerve and will often reproduce or exacerbate symptoms. Resistant extension of the middle finger with the elbow extended is another sign of nerve entrapment.

This sign is often used to aid in the diagnosis of lateral epicondylitis, but it is also usually positive in cases of radial nerve entrapment.

Causes of radial nerve entrapment

Radial nerve entrapment is often believed to be the result of overuse, but can occur from other causes, such as direct trauma, fractures, lacerations, compressive devices, or postsurgical changes.

The radial nerve divides into the radial and superficial posterior interosseous nerves at the level of the radiocapitellar joint.

The posterior interosseous nerve eventually emerges from the supinator where it can become trapped in that location, but it also divides into terminal branches that can normally also be compressed at one of the other four sites.

These four sites are the fibrous bands around the radial head, the recurrent radial vessels, the arch of Frohse, and / or the tendon margin of the extensor carpi radialis brevis.

The overuse of actions and exercises that can lead to this condition are often repetitive pronation and supination of the wrist and forearm, and they usually occur in the places mentioned above.

This condition is typically the result of a nerve injury secondary to compression, traction, or direct trauma that causes a process such as local swelling or even fracture.

Compression and / or traction often occur secondary to repetitive movements that cause inflammation or architectural changes in the surrounding tissue.

There are different degrees of severity of nerve damage. In mild cases, compression of the nerve does not cause permanent damage to the nerve and the nerve sheath fully recovers.

The most severe cases can cause permanent damage to the nerve and / or nerve sheath causing persistent deficits.

Evaluation

If entrapment is suspected, an x-ray should be performed to detect or rule out a fracture, colic, or tumor as the cause of the entrapment. Ultrasonography can often provide reliable visualization of injured nerves.

Axonal swelling, hypoechogenicity of the nerve, loss of continuity of a nerve bundle, formation of a neuroma and / or partial laceration of a nerve can be visualized, which can aid in the diagnosis.

Magnetic resonance imaging (MRI) can be helpful in detecting more subtle causes not found on x-rays or ultrasound, such as small tumors, masses, aneurysms, or compressive synovitis.

MRI can also detect nerve changes during acute entrapments. A provider may also consider a diagnostic nerve block to help define the pathology distribution and presentation.

Nerve conduction studies can also be considered, but they are inconsistent and should only be considered if surgery is a possibility. No laboratory work is necessary to establish the diagnosis.

Treatment / management

Most patients respond well to conservative therapy. Consider removing any restrictive or compression devices that are used routinely.

Consider relative rest from offensive activity, such as limiting repetitive pronation, supination, wrist flexion, and ulnar deviation.

Often times, nerve gliding exercises as part of occupation / physical therapy are done in conjunction with rest and activity modification. If symptoms do not resolve with cessation of activity and rest, then consider splinting.

If an area of ​​pathology indicates possible compression and can be visualized with ultrasound, providers may consider ultrasound-guided hydrodissection to release the compressed part of the nerve.

Oral or topical non-steroidal anti-inflammatory drugs (NSAIDs) may be used for pain. Steroids and an anesthetic combination can be injected at the point of greatest sensitivity for symptomatic relief.

The steroid can help decrease any inflammation that contributes to the process.

Surgery should be the last option if this process has become chronic and conservative treatment has failed after six to 12 months. If pain does not respond to conservative measures or if there is rapid progression of paralysis, consider surgical intervention as well.

epidemiology

Among the problems associated with the three main nerves in the upper limb, radial nerve entrapment is the least common.

Carpal tunnel syndrome (compression of the median nerve at the wrist) and cubital tunnel syndrome (compression of the ulnar nerve at the elbow) are much more common.

Prognosis of radial nerve entrapment

The prognosis depends on the type of injury to the affected nerve, as follows:

With a neurapractic injury, whether to the arm, elbow, or wrist, return to normal function is rapid and complete in at least 90% of cases.

With axonotmesis, the results after early launch are not as good, and the full return of function is less reliable; recovery will probably be complete, but it will take longer, depending on the distance the nerve must travel to reach the denervated muscle.

Neurotmesis rarely occurs in an entrapment syndrome, but if the continuity of the nerve has been completely disrupted, full recovery will not occur, and the degree of recovery can only be estimated; the results of neurotmesis treatment, even with surgical repair, are generally unsatisfactory.

Noaman et al studied 36 patients who underwent early surgical exploration for radial nerve injury associated with humerus stem fracture. In general, a narrow dynamic compression plate was used for fixation.

Compression in the lateral intermuscular septum was present in 19 cases, entrapment at the fracture site in nine, and loss of continuity in eight.