MUSCLES OF FOREARM
The forearm extends from the elbow to the wrist. It is the territory of two bones the radius and the ulna and of many muscles. The muscles are arranged as a flexor mass anteriorly and an extensor mass posteriorly.
There are 19 muscles in the forearm. Eleven are classified as extensor muscles; 8 belong to the flexor group. These designations are simply group characteristics, because certain muscles are primarily rotators of the forearm bones. Eighteen of the muscles can be grouped into six functional groups of 3 muscles each. The muscle excluded from these groups is the brachioradialis,
which is actually an elbow flexor with no action in the digits or at the wrist. All the groups, except for the first, are composed of muscles that move the hand and digits.
1. Rotate the radius on the ulna:
• Pronator teres
• Pronator quadratus
2. Flex the hand at the wrist:
• Flexor carpi radialis
• Flexor carpi ulnaris
• Palmaris longus
3. Flex the digits:
• Flexor digitorum superficialis
• Flexor digitorum profundus
• Flexor pollicis longus
4. Extend the hand at the wrist:
• Extensor carpi radialis longus
• Extensor carpi radialis brevis
• Extensor carpi ulnaris
5. Extend the digits, except the thumb:
• Extensor digitorum
• Extensor indicis
• Extensor digiti minimi
6. Extend the thumb:
• Abductor pollicis longus
• Extensor pollicis brevis
• Extensor pollicis longus
The muscles of the first three groups lie in the anterior compartment of the forearm; those of the last three groups are located in the posterior compartment.
The muscles of the second and third groups and the two pronator muscles of the first group comprise the anterior antebrachial muscles. Five of these belong to a superficial layer, and three belong to a deep layer.
The muscles of this layer are listed in the order in which they lie from the radial to the ulnar side of the forearm; the flexor digitorum superficialis, however, is deep to the other four muscles:
· Pronator teres
· Flexor carpi radialis
· Palmaris longus
· Flexor carpi ulnaris
· Flexor digitorum superficialis
There is a common tendon of origin for these muscles, which is attached to the medial epicondyle of the humerus. The intermuscular septa and the antebrachial fascia also provide partial origins, and certain muscles have additional bony origins.
The pronator teres muscle has both a humeral and an ulnar head. The large humeral head arises in the medial epicondyle via the common tendon and from the adjacent fascia and intermuscular septa. The small, deep ulnar head takes origin from the medial side of the coronoid process of the ulna. It joins the deep aspect of the humeral head, the median nerve descending between them. This obliquely descending muscle ends on the shaft of the radius at the middle of its lateral surface. The insertion is overlaid and obscured by the brachioradialis muscle.
The flexor carpi radialis muscle uses the common tendon of origin. Its tendon accounts for about half of the muscle’s length, passes through the wrist in a compartment formed by a split in the flexor retinaculum, and ends in the base of the second metacarpal (frequently, there is an additional slip to the third metacarpal).
The palmaris longus muscle also uses the common tendon of origin, when present (it is absent in 13% of cases). It terminates in a slender, flattened tendon, crossing the wrist superficial to the flexor retinaculum. It constitutes, by its spreading tendinous fibers, the chief part of the palmar aponeurosis.
The flexor carpi ulnaris muscle has a humeral and an ulnar head, the humeral head coming from the common flexor tendon. The ulnar head springs from the medial border of the olecranon and the upper two thirds of the posterior border of the ulna. The tendon of the muscle inserts on the pisiform of the wrist and, through it by two ligaments, into the hamulus of the hamate and the base of the fifth metacarpal.
The flexor digitorum superficialis muscle arises by a humeroulnar and a radial head of origin; these are connected by a fibrous band that crosses the median nerve and the ulnar blood vessels. The larger humeroulnar head arises from the common tendon, the intermuscular septa, the ulnar collateral ligament, and the medial border of the coronoid process. The radial head is a thin layer arising from the upper two thirds of the anterior border of the radius. The muscle forms two planes; the tendons of its superficial plane pass to the middle and ring fingers, and the deep lamina ends in tendons for digits I and V. These tendons terminate in the palmar aspect of the shafts of the middle phalanges of digits II to V (their relationships are described in the discussion of the wrist and hand).
The deep layer contains the following muscles:
· Flexor digitorum profundus
· Flexor pollicis longus
· Pronator quadratus
The flexor digitorum profundus muscle arises from the posterior border of the ulna (with the flexor carpi ulnaris), the proximal two thirds of the medial surface of the ulna, and adjacent areas of the interosseous membrane. The muscle produces, near the wrist, four discrete tendons that pass side by side under the flexor retinaculum and dorsal to the tendons of the flexor digitorum superficialis muscle. The tendons terminate on the bases of the distal phalanges of digits II to V. In the palm, the tendons give origin to the small lumbrical muscles.
The flexor pollicis longus muscle arises principally from the anterior surface of the radius (just below its tuberosity nearly to the upper border of the pronator quadratus) and from the adjacent interosseous membrane. Its tendon, passing between the two sesamoids of the metacarpophalangeal joint of the thumb, inserts on the base of the distal phalanx of the thumb.
The pronator quadratus muscle is a quadrilateral muscle located just above the wrist and deep to the flexor digitorum profundus and flexor pollicis longus tendons. It arises from the anterior surface of the distal one fourth of the ulna, its fibers running transversely across the wrist and inserting into the anterior surface of the distal fourth of the shaft of the radius.
The muscles of the fourth, fifth, and sixth groups, the supinator muscle of the first group, and the brachioradialis muscle make up the posterior antebrachial muscles. Six of these comprise the superficial layer, and five lie in the deep layer.
The six muscles of the superficial layer are listed in the order in which they lie across the back of the forearm, from the radial to the ulnar side:
· Extensor carpi radialis longus
· Extensor carpi radialis brevis
· Extensor digitorum
· Extensor digiti minimi
· Extensor carpi ulnaris
As for the flexor muscles, there is a common tendon of origin from the lateral epicondyle for all muscles arising below the lateral epicondyle.
The brachioradialis muscle arises from the upper two thirds of the supracondylar ridge of the humerus. Its tendon appears at about the middle of the forearm and descends to insert into the lateral side of the base of the styloid process of the radius.
The extensor carpi radialis longus muscle arises from the lower third of the supracondylar ridge of the humerus.
It has a flat tendon that reaches into the hand to insert on the dorsum of the second metacarpal.
The extensor carpi radialis brevis muscle uses the common tendon of origin for the extensors. Its tendon appears in the lower third of the forearm, closely applied to the overlying tendon of the extensor carpi radialis longus, and inserts on the dorsum of the base of the third metacarpal.
The extensor digitorum muscle also uses the common tendon of origin for the extensors. Above the wrist, it provides four tendons that spread out on the dorsum of the hand, joined side to side in a variable manner by intertendinous connections. Participating in the rather complex “extensor expansion” described in the section on the wrist and hand, these tendons terminate on the bases of the middle and distal phalanges of digits 2 to 5. The extensor digiti minimi muscle is a slender muscle that is sometimes only incompletely separated from the extensor digitorum muscle. Its tendon joins the ulnar side of the tendon of the extensor digitorum muscle to the fifth digit for which it provides independent extensor action.
The extensor carpi ulnaris muscle arises by the common tendon from the lateral epicondyle but also from the middle one half of the posterior border of the ulna. It inserts on the ulnar side of the base of the fifth metacarpal.
The muscles of the deep layer are generally submerged under those of the superficial group, although certain of their tendons and parts of their fleshy bellies outcrop just above the wrist:
· Abductor pollicis longus
· Extensor pollicis brevis
· Extensor pollicis longus
The supinator muscle has a complex origin from the lateral epicondyle of the humerus, the radial collateral ligament, the annular ligament of the radius, and the supinator crest and fossa of the ulna. Its fibers form a flat sheet, directed downward and lateralward, which wraps almost completely around the radius and inserts on the lateral surface of the upper third of this bone. As the muscle courses into the posterior compartment of the forearm, it is separated into superficial and deep laminae by the deep branch of the radial nerve.
The abductor pollicis longus muscle lies immediately distal to the supinator. It arises from the middle third of the posterior surface of the radius and the lateral part of the posterior surface of the ulna below the anconeus muscle. The fibers of the muscle converge onto its tendon, which, with the tendon of extensor pollicis brevis closely applied to its medial side, crosses the tendons of the extensor carpi radialis longus and brevis muscles and inserts on the radial side of the base of the metacarpal of the thumb.
The extensor pollicis brevis muscle, with origins from the radius and the interosseous membrane distal to that of the abductor pollicis longus muscle, inserts on the base of the proximal phalanx of the thumb. It is a specialization of the distal part of the abductor pollicis longus muscle.
The extensor pollicis longus muscle arises from the ulna and the interosseous membrane distal to the abductor pollicis longus muscle. Its tendon passes to the ulnar side of the dorsal tubercle of the radius, then obliquely across the tendons of both radial carpal extensors, terminating on the base of the distal phalanx of the thumb.
The extensor indicis muscle arises just below the extensor pollicis longus from the ulna and from the interosseous membrane. In the hand, the tendon joins the ulnar side of the tendon of the digital extensor muscle for the index finger and participates with it in forming the extensor expansion over that digit.
Knowledge of the cross-sectional anatomy of the forearm is critical to understanding advanced imaging views of the forearm as well as surgical anatomy. Knowing the origins and insertions of the muscles and tendons of the forearm is helpful during operative dissection and exploration of the forearm and wrist and provides the framework to understandi g the functional anatomy of the forearm and wrist.