Chapter 32. Forearm

Big Picture

The forearm (antebrachium) consists of the radius and ulna. Proximally, the forearm articulates with the humerus through the elbow complex (humeroulnar and humeroradial joints). Distally, the forearm articulates with the carpal bones through the wrist complex, enabling a wide array of actions. The muscles of the forearm that act upon the elbow, wrist complex, and the digital joints are organized into two fascial compartments, similar to those of the arm muscles. The anterior compartment contains flexor muscles and the posterior compartment contains extensor muscles.

Actions of the Wrist

The configuration of the wrist complex allows for motion in two planes (Figure 32-1A):

• Flexion
• Extension
• Radial deviation (abduction)
• Ulnar deviation (adduction)

Forearm Muscles of the Anterior Compartment

The actions produced by the muscles in the anterior compartment of the forearm depend upon which joints the muscles cross. Some muscles cross the elbow, wrist, digits, and perhaps a combination of each. The muscles in the anterior compartment of the forearm have the following similar features:

• Common attachment. Medial epicondyle of the humerus.
• Common innervation. Median nerve with minimal contribution from the ulnar nerve.
• Common action. Flexion.

The vascular supply to the anterior forearm muscles is from branches of the ulnar and radial arteries.

The muscles in the anterior compartment of the forearm are divided into three groups: superficial, intermediate, and deep.

• Superficial group (Figure 32-1B)
  • Pronator teres muscle. Possesses two heads and crosses the elbow complex. The humeral head of the pronator teres muscle attaches to the medial epicondyle and the supraepicondylar ridge of the humerus, and the ulnar head attaches to the coronoid process. Distally, the pronator teres muscle attaches to the midshaft of the radius. The pronator teres muscle primarily produces pronation at the forearm. The median nerve provides innervation (C6–C7) to the pronator teres muscle.
  • Flexor carpi radialis muscle. Attaches to the medial epicondyle and the base of metacarpals 2 and 3. The primary action of the flexor carpi radialis muscle is wrist flexion and radial deviation. The median nerve (C6–C7) supplies innervation to this muscle.
  • Palmaris longus muscle. Attaches to the medial epicondyle of the humerus and courses superficially over the flexor retinaculum to the palmar aponeurosis in the hand. The primary action of the palmaris longus muscle is to resist shearing forces of the palmar aponeurosis; it is also considered a wrist flexor. Innervation is provided by the median nerve (C7–C8). It is important to note that the palmaris longus muscle may be absent on one or both sides in some individuals.
  • Flexor carpi ulnaris muscle. Possesses two heads. A humeral head attaches to the medial epicondyle of the humerus, and an ulnar head attaches to the olecranon process. Both heads come together and attach to the pisiform, hamate and base of metacarpal 5. The flexor carpi ulnaris muscle crosses both the elbow and the wrist complex, producing weak elbow flexion, wrist flexion, and ulnar deviation. It is innervated by the ulnar nerve (C7–T1).
• Intermediate group (Figure 32-1C)
  • Flexor digitorum superficialis muscle. Possesses two heads. A humeral head attaches proximally to the medial epicondyle and a radial head attaches to the radius. The flexor digitorum superficialis muscle attaches distally to the middle phalanges of digits 2 to 5. The muscle primarily produces flexion at the wrist and at the metacarpophalangeal and proximal interphalangeal joints. Innervation is provided by the median nerve (C8–T1). The four tendons of the flexor digitorum superficialis muscle cross under the flexor retinaculum at the wrist and enter the hand through the carpal tunnel.
• Deep group (Figure 32-1D)
  • Flexor pollicis longus muscle. Attaches proximally to the radius and the interosseous membrane and to the distal phalanx of the thumb. The flexor pollicis longus muscle produces flexion at the metacarpophalangeal and interphalangeal joints of digit 1 and is innervated by the anterior interosseous nerve from the median nerve (C7–C8).
  • Flexor digitorum profundus muscle. Attaches proximally to the ulna and interosseous membrane and travels across the wrist complex and attaches distally to the distal phalanges of digits 2 to 5. The flexor digitorum profundus muscle produces flexion at the wrist as well as flexion of the metacarpophalangeal and proximal and distal interphalangeal joints of digits 2 to 5. The lateral half of the flexor digitorum profundus muscle is innervated by the anterior interosseous nerve from the median nerve (C8–T1), and the medial half of the muscle is innervated by the ulnar nerve (C8–T1).
  • Pronator quadratus muscle. Courses horizontally from the distal anterior surface of the ulna to the distal anterior surface of the radius. The pronator quadratus muscle produces pronation, and is innervated by the anterior interosseous nerve from the median nerve (C7–C8).

Forearm Muscles of the Posterior Compartment

The actions produced by the muscles in the posterior compartment of the forearm depend upon which joints the muscles cross. Some muscles cross the elbow, wrist, and digits, and perhaps a combination of each. The muscles in the posterior compartment of the forearm have the following similar features (Table 32-1):

• Common attachment. Lateral epicondyle of the humerus.
• Common innervation. Radial nerve.
• Common action. Extension.

The vascular supply to the muscles of the posterior compartment is from branches of the ulnar and radial arteries.

The muscles in the posterior compartment are divided into superficial and deep groups.

• Superficial group (Figure 32-2A and B)
  • Brachioradialis muscle. Attaches to the lateral supracondylar ridge of the humerus and the styloid process of the radius. The brachioradialis muscle produces elbow flexion (primarily in the midpronated position). It also is important for stabilization of the elbow complex during rapid movements of flexion and extension. The brachioradialis muscle is innervated by the radial nerve (C5–C6).
  • Extensor carpi radialis longus muscle. Attaches to the lateral supracondylar ridge of the humerus and the dorsal surface of the base of metacarpal 2. The extensor carpi radialis longus muscle produces extension and radial deviation of the wrist and is innervated by the radial nerve (C6–C7).
  • Extensor carpi radialis brevis muscle. Attaches to the lateral epicondyle of the humerus and the dorsal surface of the base of metacarpals 2 and 3. The extensor carpi radialis brevis muscle produces extension and radial deviation of the wrist and is innervated by the posterior interosseous nerve (C7–C8).
  • Extensor digitorum muscle. Attaches to the lateral epicondyle of the humerus and the dorsal digital expansions of digits 2 to 5. Intrinsic muscles of the hand, the lumbricals and the dorsal and palmar interossei muscles, also attach to the dorsal digital expansion. Intertendinous connections on the dorsum of the hand may be present, connecting the tendons, but the location and the number of connections are highly variable. The extensor digitorum can extend all of the joints it crosses (wrist and digits 2–5). It is innervated by the posterior interosseous nerve (C7–C8).
  • Extensor digiti minimi muscle. Attaches to the lateral epicondyle of the humerus and the dorsal digital expansion of digit 5. The primary action of the extensor digiti minimi muscle is extension of digit 1, but it will also assist with wrist extension. The muscle is innervated by the posterior interosseous nerve (C7–C8).
  • Extensor carpi ulnaris muscle. Attaches proximally to the lateral epicondyle of the humerus and the posterior ulna and distally to the base of metacarpal 5. The extensor carpi ulnaris muscle produces extension and ulnar deviation of the wrist and is innervated by the posterior interosseous nerve (C7–C8).
  • Anconeus muscle. Attaches to the lateral epicondyle of the humerus and the olecranon. The anconeus muscle contributes to elbow extension as well as controls the ulna during pronation. It is innervated by the radial nerve (C6–C8).
• Deep group (Figure 32-2C)
  • Supinator muscle. Attaches proximally to the lateral epicondyle of the humerus and the supinator crest of the ulna. Distally, the supinator muscle attaches to the lateral surface of the radius and contributes to supination of the forearm. The posterior interosseous nerve from the radial nerve (C6–C7) innervates the supinator muscle. The posterior interosseous nerve travels between the muscle fibers that attach to the humerus and ulna as it travels distally into the musculature of the posterior compartment.
  • Abductor pollicis longus muscle. Attaches proximally to the ulna, radius, and interosseous membrane. Distally, the abductor pollicis longus muscle attaches to the base of metacarpal 1 and abducts the carpometacarpal joint of digit 1. The muscle is innervated by the posterior interosseous nerve (C7–C8).
  • Extensor pollicis longus muscle. Attaches proximally to the posterior surface of the ulna and the interosseous membrane. Distally, the extensor pollicis longus muscle attaches to the base of the proximal phalanx of digit 1. The muscle produces extension of the metacarpophalangeal, carpometacarpal, and interphalangeal joints of the thumb. It is innervated by the posterior interosseous nerve (C7–C8).
  • Extensor pollicis brevis muscle. Attaches to the radius and the interosseous membrane proximally and to the base of the proximal phalanx of digit 1 distally. The extensor pollicis brevis muscle produces extension of the metacarpophalangeal and carpometacarpal joints of the thumb. The muscle is innervated by the posterior interosseous nerve (C7–C8).
  • Extensor indicis muscle. Attaches proximally to the posterior surface of the ulna and interosseous membrane. Distally, the extensor indicis muscle attaches to the dorsal digital expansion of digit 2. The extensor indicis muscle contributes to extension of the index finger, allowing it to be extended independent of the other fingers. It also will assist with wrist extension. The extensor indicis muscle is innervated by the posterior interosseous nerve (C7–C8).

Big Picture

The median, ulnar, and radial nerves provide innervation to the anterior and posterior compartments of the forearm. The median nerve innervates all but one and a half muscles (flexor carpi ulnaris and half of the flexor digitorum profundus muscles) in the anterior compartment of the forearm, which are innervated by the ulnar nerve. The posterior compartment of the forearm is innervated entirely by the radial nerve.

Median Nerve

The median nerve arises from the medial and lateral cords of the brachial plexus and travels with the brachial artery along the medial side of the arm (Figure 32-3A). In the elbow, the median nerve courses through the cubital fossa, deep to the bicipital aponeurosis and between the two heads of the pronator teres, to enter the anterior compartment of the forearm.

• Main branch of median nerve. Courses between the flexor digitorum superficialis and the profundus muscles, supplying the superficial and intermediate muscles of the anterior forearm, with the exception of the ulnar half of the flexor digitorum profundus and the flexor carpi ulnaris.
• Anterior interosseous nerve. Once the main branch of the median nerve exits the two heads of the pronator teres, it gives rise to the anterior interosseous nerve, innervating the flexor pollicis longus, pronator quadratus and radial half of the flexor digitorum profundus muscles.
• Palmar branch. Proximal to the wrist complex, the median nerve gives rise to a palmar branch, which delivers cutaneous innervation to the medial side of the palm.

The median nerve continues distally to travel through the carpal tunnel to enter the hand.

Ulnar Nerve

The ulnar nerve courses posteriorly to the medial epicondyle of the humerus in the osseous groove, into the anterior compartment of the forearm between the two heads of the flexor carpi ulnaris muscle (Figure 32-3B). The ulnar nerve continues through the anterior compartment of the forearm supplying only two muscles, the flexor carpi ulnaris and the ulnar half of the flexor digitorum profundus. Proximal to the wrist, the ulnar nerve gives rise to two cutaneous branches, a dorsal branch and a palmar branch, which provide cutaneous innervation to the dorsal medial side of the hand and the medial side of the palm, respectively. The ulnar nerve continues into the hand superficial to the carpal tunnel and courses through Guyon's canal by the pisiform bone to enter the hand.

Radial Nerve

The radial nerve enters the forearm, anterior to the lateral epicondyle, and travels distally between the brachialis and the brachioradialis muscles, where it bifurcates into a deep terminal branch and a superficial terminal branch (Figure 32-3C). The deep terminal branch becomes the posterior interosseous nerve, and the superficial terminal branch becomes the superficial radial nerve. The posterior cutaneous nerve of the forearm, which branches in the arm, provides sensory innervation to the posterior forearm.

• Posterior interosseous nerve. Pierces between the two heads of the supinator muscle to innervate the muscles in the posterior compartment of the forearm, excluding the brachioradialis and extensor carpi radialis longus, which are innervated by the radial nerve prior to its bifurcation.
• Superficial radial nerve. Courses along the brachioradialis muscle and then through the anatomical snuffbox to provide cutaneous innervation to the dorsum of the hand.

Medial Cutaneous Nerve of the Forearm

The medial cutaneous nerve of the forearm branches from the medial cord, and as its name implies, it supplies the medial skin of the forearm.

Big Picture

The brachial artery extends from the inferior border of the teres major muscle, giving rise to several branches that supply blood to the anterior and posterior compartments of the arm. The brachial artery bifurcates into the ulnar and radial arteries at the radioulnar joint. The radial and ulnar arteries and their tributaries supply blood to the anterior and posterior compartments of the forearm and extend distally into the hand.

Ulnar Artery

The ulnar artery travels through the cubital fossa and continues between the flexor carpi ulnaris and the flexor digitorum profundus muscles, supplying the medial muscles of the anterior compartment of the forearm. Along the way, the ulnar artery gives rise to the following branches (Figure 32-4A and B):

• Superior ulnar recurrent artery. Courses in a superior direction anterior to the medial epicondyle and forms an anastomosis with the inferior ulnar collateral artery.
• Inferior ulnar recurrent artery. Courses in a superior direction posterior to the medial epicondyle and forms an anastomosis with the superior ulnar collateral artery.
• Common interosseous artery. Courses toward the interosseous membrane and bifurcates into the anterior and posterior interosseous branches.
  • Anterior interosseous artery. Travels along the anterior surface of the interosseous membrane, pierces the membrane, and supplies the deep extensor muscles.
  • Posterior interosseous artery. Travels along the posterior surface of the interosseous membrane and supplies the superficial extensors. The posterior interosseous artery contributes to the recurrent interosseous artery, which anastomoses with the vascular network on the posterior side of the elbow.
    • Recurrent interosseous artery. Travels in a superior direction, posterior to the elbow complex, and forms an anastomosis with the middle collateral artery.

The ulnar artery terminates as the deep and superficial ulnar palmar arches of the hand.

Radial Artery

The radial artery travels through the cubital fossa along the lateral side of the forearm, deep to the brachioradialis, and supplies the lateral forearm muscles. In the proximal forearm, the radial artery gives rise to the radial recurrent artery (Figure 32-4A and B).

• Radial recurrent artery. Courses anteriorly to the lateral epicondyle of the humerus to anastomose with the radial collateral artery and supplies the muscles on the lateral side of the forearm.

The radial artery terminates in the hand as the deep and superficial radial palmar arches.

Veins of the Forearm

The veins in the forearm consist of a superficial and a deep venous system. The superficial system consists of the basilic vein, located medially, and the cephalic vein, located laterally. Anterior to the elbow complex, the median cubital vein forms a connection between the basilic and cephalic veins. The deep venous system may consist of two or three veins that course with each artery.

Big Picture

The proximal and distal radioulnar joints form synovial pivot joints that provide pronation and supination of the forearm. The wrist complex is very flexible because of the synovial joint between the radius and the proximal row of carpal bones (radiocarpal joint) and the proximal and the distal row of carpal bones (midcarpal joint).

Distal Radioulnar Joint

The proximal and distal radioulnar joints produce supination and pronation. They are mechanically linked; one joint is unable to move without the other (Figure 32-5A and B). The distal radioulnar joint consists of a synovial pivot joint between the ulnar notch of the radius, the articular disc, and the head of the ulna. The articular disc and its extensive fibrous connections are frequently referred to as the triangular fibrocartilage complex (often referred to as the TFCC).

• Dorsal and palmar radioulnar ligaments. The distal radioulnar joint is supported by two ligaments that originate from the dorsal and palmar aspects of the ulnar notch of the radius and extend to the base of the styloid process of the ulna. These ligaments form the margins for the triangular fibrocartilage complex.
• Interosseous membrane. The interosseous membrane is a wide sheet of connective tissue that connects the radius and ulna and functions to support both the proximal and distal radioulnar joints. The arrangement of the fibers allows for the transmission of forces from the hand and radius to the ulna.

Wrist Complex

The wrist complex consists of the radiocarpal and midcarpal joints that result in wrist flexion and extension and in radial and ulnar deviation (Figure 32-5A and B).

• Radiocarpal joint. Articulation between the radius and the radioulnar disc (triangular fibrocartilage complex) with the proximal row of carpal bones (scaphoid, lunate, and triquetrum).
• Midcarpal joint. Articulation between the proximal row of carpal bones (scaphoid, lunate, and triquetrum) with the distal row of carpal bones (trapezium, trapezoid, capitate, and hamate).

The radiocarpal and midcarpal joints share similar ligamentous and capsular support because most of the structures that support the radiocarpal joint also cross the midcarpal joint. These joints consist of a fairly loose, but strong capsule reinforced with the following ligaments:

• Palmar radiocarpal ligament. Reinforces the anterior capsule and attaches proximally to the distal radius and distally to the scaphoid, lunate, triquetrum, and capitate.
• Palmar ulnocarpal ligament. Attaches proximally to the ulnar styloid process and the triangular fibrocartilage complex and distally to the lunate and triquetrum.
• Dorsal radiocarpal ligament. Reinforces the posterior capsule and attaches proximally to the distal radius and distally to the scaphoid, lunate, and triquetrum.
• Ulnar collateral ligament. Attaches from the ulnar styloid process to the triquetrum and pisiform.
• Radial collateral ligament. Attaches from the radial styloid process to the scaphoid and trapezium.
• Intercarpal ligaments. Interconnects carpal bones within and between rows.