Posterior Forearm Muscles & Radial Nerve Injury

• Study notes on muscles of the posterior compartment of the forearm

Case Scenario 1

• A 13-year-old injured in a motorbike race presents with cuts and bruises
• The patient is unable to extend the left wrist, fingers, and thumb, but can extend the elbow
• This scenario suggests an injury to the radial nerve in the midhumerus region

Case Scenario 2

• A 21-year-old female athlete with a radial nerve injury in the spiral groove shows symptoms of weakness of thumb abduction and thumb extension

Superficial Group Muscles

• Includes brachioradialis, extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and anconeus

Common Origin and Innervation

• All superficial group muscles originate from the front of the lateral epicondyle of the humerus, except anconeus and brachioradialis
• Anconeus originates from the back of the lateral epicondyle
• Brachioradialis originates from the lateral supracondylar ridge
• All are supplied by the deep branch of the radial nerve (posterior interosseous nerve), except anconeus, brachioradialis and ECRL

Anconeus

• Inserts on the olecranon process
• Assists in elbow extension

Muscles that Attach to the Olecranon Process

• These muscles act to extend the elbow

Brachioradialis

• Inserts on the styloid process of the radius
• Puts the forearm into a mid-prone position (initiates pronation and supination)
• Flexes elbow in the mid-prone position

ECRL

• Inserts on the 2nd metacarpal
• Extends and abducts the wrist

ECRB

• Inserts on the 3rd metacarpal
• Extends and abducts the wrist

Extensor Carpi Ulnaris

• Inserts on the 5th metacarpal
• Extends and adducts the wrist

Extensor Digitorum

• Inserts on the middle and distal phalanges of the medial 4 fingers through the extensor expansion
• Extends all joints of the medial 4 fingers

Extensor Digiti Minimi

• Inserts via the extensor expansion of the little finger
• Extends all joints of the little finger

Deep Group Muscles

• Includes supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis
• All are supplied by the deep radial nerve (posterior interosseous nerve)

Supinator

• Originates from:
• Lateral surface of the olecranon
• Lateral epicondyle
• Upper 1/3 of the radius (posterior and lateral surface)
• Action: supination of the forearm

Abductor Pollicis Longus

• Action: abduction of the thumb

Extensor Pollicis Brevis

• Extends the wrist
• Extends the metacarpophalangeal joint of the thumb

Extensor Pollicis Longus

• Extends the wrist
• Extends all joints of the thumb

Extensor Indicis

• Extends the wrist
• Extends all joints of the index finger

Posterior Interosseous Nerve (PIN)

• Branch of the radial nerve
• Pierces the supinator muscle to reach the posterior compartment of the forearm
• Supplies all muscles of the back of the forearm except anconeus, brachioradialis, and ECRL