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Radial Nerve

Key Points:
• Assess radial innervated muscles; check for posterior interosseous nerve branch versus radial nerve proper function:
• Branches of the radial nerve provide elbow extension, occasional elbow flexion (variably through the brachioradialis muscle as well as a branch to the brachialis muscle), supination of the forearm (although the biceps muscle innervated by musculocutaneous nerve is the major supinator) and wrist/finger/thumb extension.
• The posterior interosseous branch of the ulnar nerve specifically provides innervation for the supinator and wrist/finger/thumb extensors.
• Sensory exam includes testing the dorsal aspect of the arm and radial sided fingers.
• Originates from roots: C5-T1.
• Physical examination of the radial nerve includes both motor and sensory examination.
• Radial nerve function is particularly important in that it contributes to the following radial-nerve-only innervated motor functions: elbow, wrist, finger, and thumb extension. 
• The radial nerve sensory component, in particular the superficial radial sensory nerve (or radial sensory branch) is notable for its tendency to be a ‘bad actor’ with significant neuropathic pain resulting from even minor injury. 
Detailed Examination Links:
Proximal Motor Branches
Brachialis (contribution)
Triceps brachii / Anconeus
Extensor carpi radialis longus / Extensor carpi radialis brevis
Distal Branches  (via thePosterior Interosseous Nerve continuation of the radial nerve)
Extensor carpi ulnaris
Extensor digitorum communis
Extensor digiti minimi
Abductor pollicis longus
Extensor pollicis brevis
Extensor indicis proprius
Extensor pollicis longus
Sensory Branches

Posterior brachial cutaneous nerve / Inferior lateral brachial cutaneous nerve

Posterior antebrachial cutaneous nerve

Radial Sensory Nerve (Sensory Branch of the Radial nerve)

Summarized Examination:
• The three heads of the triceps muscle are quickly tested by assessing resisted elbow extension.  Patients with radial nerve dysfunction and intact elbow flexors can flex the elbow and allow gravity to produce the illusion of elbow extension if they are in an upright position so it is critical to assess triceps function by assessing resistance or with gravity eliminated.
• Wrist extension is quickly assessed by resistive activities; this assesses the function of extensor carpi radialis brevis, longus and extensor carpi ulnaris.
• The finger extensors should be assessed by checking for extension at the metacarpal phalangeal joint—this is because the ulnar and median innervated intrinsic musculature contributes to interphalangeal joint extension.  Retropulsion of the thumb is a quick test for extensor pollicis longus function
• The sensory examination involves testing sensation over the dorsoradial aspect of the wrist and hand innervated by the radial sensory nerve.
• Patients with a complete radial nerve palsy have a complete wrist drop whereas patients with a posterior interosseous nerve palsy can still radially extend their wrist (via extensor radialis brevis/longus).
Pathological Findings:
• In patients with isolated radial nerve injury, the most debilitating loss of function is often loss of power grip, because even with intact finger flexion, without the ability to position the wrist in extension, the flexors are suboptimally positioned and powerful grasp is lost. 
• Isolated injury to the radial sensory nerve is problematic because this nerve is a ‘bad actor’.  Iatrogenic injury in the course of routine procedures such as ganglionectomy and DeQuervain’s (first extensor tendon compartment release) should be avoided through the use of tourniquet control and meticulous dissection intraoperatively. Differentiating injury to the radial sensory nerve branches versus the lateral antebrachial cutaneous nerve can be challenging –see Superficial Branch of Radial Nerve (SBR)/Radial Sensory Nerve (RSN) page for further details.
• Known sites of radial nerve compression or entrapment:
• Arm: compression in the arm is rare but can occur within the triceps muscle itself or in the upper arm where it courses the lateral intermuscular septum.
• Elbow/Proximal Forearm: this is referred to as radial tunnel syndrome the radial tunnel is a region that begins just distal to the takeoff of the branches to the brachioradialis, extensor carpi radialis longus and (usually) brevis where the nerve is at the level of the lateral humeral epicondyle.  It continues under the extensor carpi radialis brevis and on top of the radial capitellum and extends distally to the arcade of Frohse and supinator muscle.  The structures that can cause compression include fibrous bands, radial recurrent vessels (leash of Henry), tendinous origin of the extensor carpi radialis brevis, and the arcade of Frohse (which is the tendinous leading edge of the superficial head of the supinator).
• Proximal Forearm: the posterior interosseous nerve can be entrapped at the arcade of Froshse and the supinator muscle; radial recurrent vessels (leash of Henry) may also contribute to compressive forces.
• Forearm: the radial sensory nerve can be entrapped at the level of the forearm (by the brachioradialis and extensor carpi radialis longus muscles between which it passes) and wrist (by external compression such as a tight wrist watch band).
Clinical Relevance:
• Loss of radial nerve function is seen in both open and closed arm level injuries such as humeral fractures.  Reinnervation of the critical functions, wrist and finger extension, is important to restoring normal upper extremity function.  Without wrist extension, patients have diminished ability to grip and hold objects.  Without finger and thumb extension, patients have difficulty in opening the hand to grasp larger objects and do activities such as typing—do note, that the ulnar innervated intrinsic hand musculature provides some finger extension as well.
• In radial nerve injury patterns distal to the triceps branch takeoff, when the denervation time permits (3 to 6 months or less is ideal), wrist and digit extensor function can be restored through use of nerve transfers from expendable branches of the median nerve.  Tendon transfers may also be done but do not provide independent finger/thumb extension as the nerve transfer procedure can.  In chronic injuries, tendon transfers are routinely used and quite effective.
• The nerves to the supinator and the extensor carpi radialis brevis in patients with normal radial nerve function are expendable and can be used for nerve transfer procedures (such as to the median nerve).
Relevant Anatomy
• Roots: C5-T1
• Nerve: Radial nerve.
• Muscles Innervated: Triceps brachii / Anconeus,  Brachioradialis, Extensor carpi radialis longus / Extensor carpi radialis brevis, Supinator, Extensor carpi ulnaris, Extensor digitorum communis, Extensor digiti minimi, Abductor pollicis longus, Extensor pollicis brevis, Extensor indicis proprius, Extensor pollicis longus; Brachialis (contribution; though main supply to brachialis is the musculocutaneous nerve)
• Sensory Distribution Innervated: Posterior brachial cutaneous nerve / Inferior lateral brachial cutaneous nerve, Posterior antebrachial cutaneous nerve, Radial Sensory Nerve (Sensory Branch of the Radial nerve)
• Innervation Route:
• C6, C7, C8, T1 → radial nerve → triceps brachii
• C7, C8 → axillary nerve → triceps brachii branch → medial triceps brachii branch → anconeus
• C5, C6 → radial nerve → brachioradialis branch
• C5, C6, (C7) → radial nerve → supinator branch
• C5, C6, C7, C8 → radial nerve → extensor carpi radialis longus branch
• C5, C6, C7, C8 → radial nerve → variably can come off the radial nerve proper, or even appear to come of the radial sensory nerve branch → extensor carpi radialis brevis branch
• C5, C6, (C7) → radial nerve → supinator branch
• C6, C7, C8 → radial nerve → posterior interosseous nerve → extensor carpi ulnaris branch
 C6, C7, C8 → radial nerve → posterior interosseous nerve → extensor digitorum communis branch
• C6, C7, C8 → radial nerve → posterior interosseous nerve → extensor digiti minimi C7, C8 → radial nerve → posterior interosseous nerve → abductor pollicis longus branch
• C6, C7, C8 → radial nerve → posterior interosseous nerve → extensor indicis proprius
• C7, C8 → radial nerve → posterior interosseous nerve → extensor pollicis longus branch