Radial Nerve

The radial nerve is a continuation of posterior cord of brachial plexus in the axilla. It is the largest nerve of the brachial plexus. It carries fibres from all the roots (C5, C6, C7, C8, and Tl) of brachial plexus (but T1 fibres are not constant).

Course in The Axilla

In the axilla, the radial nerve lies posterior to the third part of the axillary artery and anterior to the muscles forming the posterior wall of the axilla.

In the axilla, it gives off the following three branches:

• Posterior cutaneous nerve of arm
  (which provides sensory innervation to skin on the back of the arm up to the elbow).
• Nerve to the long head of triceps.
• Nerve to the medial head of triceps.

  Radial nerve enters the arm at the lower border of the teres major. It passes between the long and medial heads of triceps to enter the lower triangular space, through which it reaches the spiral groove along with profunda brachii artery. The radial nerve in the spiral groove lies in direct contact with the humerus.

Boundaries of the Spiral Groove

Anteriorly: Middle one-third of the shaft of humerus.

Above: Origin of the lateral head of triceps.

Below: Origin of the medial head of triceps.

Posteriorly: Fibres of lateral and long head triceps.

Branches in Spiral Groove

In the spiral groove, it gives off the following five branches:

• Lower lateral cutaneous nerve of the arm, which provides sensory innervation to the skin on the lateral surface of the arm up to the elbow.
• Posterior cutaneous nerve of the forearm, which provides sensory innervation to the skin down the middle of the back of the forearm up to the wrist.
• Nerve to lateral head of triceps.
• Nerve to medial head of triceps.
• Nerve to anconeus (it runs through the substance of medial head of triceps to reach the anconeus).

At the lower end of the spiral groove, the radial nerve pierces the lateral muscular septum of the arm and enters the anterior compartment of the arm. Here, it first descends between the brachialis and brachioradialis, and then between brachialis and extensor carpi radialis longus before entering the cubital fossa.

Branches in the Anterior Compartment of Arm

In the anterior compartment of arm above the lateral epicondyle, it gives off the following three branches:

• Nerve to brachialis (small lateral part).
• Nerve to brachioradialis.
• Nerve to extensor carpi radialis longus (ECRL).

At the level of lateral epicondyle of humerus, it terminates by dividing into superficial and deep branches in the lateral part of the cubital fossa.

The deep branch (also called posterior interosseous nerve), in the cubital fossa supplies two muscles, viz.

• Extensor carpi radialis brevis.
• Supinator.

After supplying these two muscles, it passes through the substance of supinator and enters the posterior compartment of the forearm and supplies all the extensor muscles of the forearm. It also gives articular branches to the distal radio-ulnar, wrist, and carpal joints.

The superficial branch (also called superficial radial nerve) is sensory. It runs downwards over the supinator, pronator teres, and flexor digitorum superficialis deep to brachioradialis. About one-third of the way down the forearm (at about 7 cm above wrist), it passes posteriorly,

emerging from under the tendon of brachioradialis, proximal to the styloid process of radius and then passes over the tendons of anatomical snuff-box, where it terminates as cutaneous branches which provide sensory innervation to skin over the lateral part of the dorsum of hand and dorsal surfaces of lateral 3V2 digits proximal to the nail beds.

Clinical Relevance

Injuries of the radial nerve: The radial nerve may be injured at three sites: (a) in the axilla, (b) in the spiral groove, and (c) at the elbow.

Injury of Radial Nerve in the Axilla

In the axilla the radial nerve may be injured by the pressure of the upper end of crutch (crutch palsy).

Wrist-drop: a result of radial nerve palsy

Characteristic in such cases will be as follows:

Motor loss

• Loss of extension of elbow due to paralysis of triceps.
• Loss of extension of wrist due to paralysis of wrist extensors. This causes wrist drop due to unopposed action of flexor muscles of the forearm.
• Loss of extension of digits due to paralysis of extensor digitorum, extensor indicis, extensor digiti minimi, and extensor pollicis longus.
• Loss of supination in extended elbow because supinator and brachioradialis are paralyzed but supination becomes possible in flexed elbow by the action of biceps brachii.

Sensory loss

• Sensory loss on small area of skin over the posterior surface of the lower part of the arm.
• Sensory loss along narrow strip on the back of forearm.
• Sensory loss on the lateral part of dorsum of hand at the base of thumb and dorsal surface of lateral 3.5 digits. More often, there is an isolated sensory loss on the dorsum of hand at the base of the thumb.

Injury of Radial Nerve in the Radial Spiral Groove

• Midshaft fracture of humerus.
• Inadvertently wrongly placed intramuscular injection.
• Direct pressure on radial nerve by a drunkard falling asleep with his one arm over the back of the chair (Saturday night paralysis).

Injury to radial nerve occurs most commonly in the distal part of the groove beyond the origin of nerve to triceps and cutaneous nerves.

Characterstics in such cases will be as follows:

Motor loss

• Loss of extension of the wrist and fingers.
• Wrist drop.
• Loss of supination when the arm is extended.

Extension of the elbow is possible but may be little weak because nerves to long and lateral heads of triceps arises in the axilla i.e., before the site of lesion.

Injury of Radial Nerve at Elbow

Radial tunnel syndrome: It is an entrapment neuropathy of the deep branch of radial nerve at elbow. The compression of radial nerve at elbow may be caused by the following four structures:

• Fibrous bands, which can tether the radial nerve to the radio-humeral joint.
• Sharp tendinous margin of extensor carpi radialis brevis.
• Leash of vessels from the radial recurrent artery.
• Arcade of Frohse, a fibro-aponeurotic proximal edge of the superficial part of the supinator muscle.
  

Characteristic in such cases will be as follows:

• Loss of extension of the wrist and fingers but no wrist drop.
• Pain over the extensor aspect of the forearm.