Forearm

The forearm is the section between the elbow and the wrist in the upper limb.

Bones:

The forearm’s bony structure consists of the radius and ulna.

• Proximally, the radius and ulna connect with the distal end of the humerus, forming the elbow joint, enabling hinge-like flexion and extension movements.

• Distally, the radius and the articular disc on the ulna’s distal end connect with the carpal bones (scaphoid, lunate, and triquetrum), forming the wrist joint, allowing for flexion, extension, and radial and ulnar movements.

• The radius and ulna also interact at the superior and inferior radioulnar joints and are linked by an interosseus membrane. The superior radioulnar joint facilitates pronation (palm down) and supination (palm up), enabling the rotation of the radius around the ulna.

Flexor muscles:

The anterior (or flexor) muscles of the forearm are divided into superficial, intermediate, and deep groups:

• The superficial muscles originate from the common flexor origin, i.e. the anterior aspect of the humerus’ medial epicondyle. From radial to ulnar, these muscles include the pronator teres, flexor carpi radialis, palmaris longus, and flexor carpi ulnaris. The pronator teres inserts into the mid-shaft of the radius, facilitating pronation. The palmaris longus inserts into the palmar aponeurosis. The two flexors, flexor carpi radialis and flexor carpi ulnaris, insert into the bases of the metacarpals on the radial and ulnar sides, allowing radial and ulnar deviation of the wrist, as well as wrist flexion.

• The flexor digitorum superficialis lies deep to the four above-mentioned muscles. Its four long distal tendons pass anteriorly to the wrist and reach the four fingers (index, middle, ring, and little fingers). Each tendon splits into two, allowing the profundus tendon to pass through. The split tendon then rejoins and inserts onto the anterior surface of the middle phalanx in each of the four fingers.

• The deep group consists of the flexor pollicis longus, flexor digitorum profundus, and pronator quadratus muscles. The four long tendons of the flexor digitorum digitorum profundus lie posterior to the tendons of the superficialis. However, upon entering the fingers (index, middle, ring, and little fingers), each tendon passes through the split of the superficialis tendon and ultimately inserts onto the anterior surface of the base of the distal phalanx. Similarly, the flexor pollicis longus inserts onto the anterior surface of base of the distal phalanx of the thumb.

• Both flexor digitorum supeficialis and the deeply situated flexor digitorum profundus cause wrist flexion, flexion of metacarpophalyngeal and proximal interphalyngeal joints. However, the latter also causes flexion at the distal interphalangeal joints. Finally, the deep muscle known as the pronator quadratus connects the distal anterior surfaces of the radius and ulna and contributes to forearm pronation.

Extensor muscles:

The extensor compartment of the forearm consists of several muscles, some superficial and some deep.

• On the radial side, there are three primary muscles: the brachioradialis, extensor carpi radialis longus, and brevis muscles.

• Additional muscles include the extensor indicis, extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris muscles.

• The three extensor muscles that form the boundaries of the anatomical snuff box are the abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus muscles.

Innervation:

• The median nerve innervates all muscles in the anterior compartment of the forearm except for the flexor carpi ulnaris and the medial half of the flexor digitorum profundus, which are innervated by the ulnar nerve.

• All extensor muscles receive innervation from the radial nerve.

Vasculature:

The brachial artery bifurcates in the apex of the cubital fossa, into its terminal branches: the radial and ulnar arteries.

• The ulnar artery exceeds the radial artery in size. In the proximal forearm, the ulnar artery gives rise to the common interosseous branch, which divides into the anterior and posterior interosseous arteries. These arteries are situated on the anterior and posterior surfaces of the interosseous membrane and eventually anastomose with each other at the wrist.

• The radial artery courses along the lateral aspect of the forearm. It exits the forearm, encircling the lateral aspect of the wrist, and enters the palm by passing between the 1st and 2nd metacarpals. It serves as the primary blood supply to the thumb and the lateral side of the index finger.