Posterior Arm, Anterior & Posterior Forearm Anatomy (PDF)

Thieme Anatomy: An Essential Textbook Fig. 14.35A In the limbs, there is a layer of superficial fascia directly beneath the skin. Directly covering muscles there is a thin but stronger sheet of deep fascia that surrounds all of the deep structures. Most major nerves and arteries are contained in a p...

Thieme Anatomy: An Essential Textbook Fig. 14.35A In the limbs, there is a layer of superficial fascia directly beneath the skin. Directly covering muscles there is a thin but stronger sheet of deep fascia that surrounds all of the deep structures. Most major nerves and arteries are contained in a perivascular sheath, while individual muscles are also surrounded by deep fascia. Most limbs are divided into compartments, with a thicker layer of fascia segregating groups of muscles. This is termed an intermuscular septum and it separates the arm into an anterior, flexor compartment and a posterior, extensor compartment. Gilroy 2nd ed. Fig. 21.22A,B The extensor compartment of the arm contains the triceps brachii. As the name implies it has 3 heads: a long head, a lateral head, and a medial head. The long head arises from the infraglenoid tubercle of the scapula, while the lateral and medial heads arise from the shaft of the humerus. Schuenke Vol. 1 2nd ed. Fig. 15.313c,D The medial head is harder to see because it's covered by the long and lateral heads. The radial (spiral) groove separates the origin of the medial head from the lateral head. The radial nerve, which resides in this groove, innervates the triceps as it spirals around the humerus. The triceps is a powerful extensor at the elbow, and the long head can also slightly extend and adduct the shoulder. Gilroy 2nd ed. Fig. 24.11A; Schuenke Vol. 1 2nd ed. Fig 17.369F The radial nerve innervates all of the muscles of posterior compartment of the arm and extensor compartment of the forearm. After arising from the posterior cord of the brachial plexus, it follows the spiral groove near the surgical neck of the humerus. The radial nerve innervates the triceps and sends a branch to the skin of the posterior arm. It appears on the lateral side of the elbow covered by the long forearm extensor muscles. The nerve can be injured lateral to the elbow. Drake 2nd ed. In elbow flexion, the angle between the forearm and arm is decreased. During pronation the distal end of the radius crosses over the distal end of the ulna and the palm faces down. In supination, the distal ends of the radius and ulna uncross and the palm faces up. In wrist flexion, the palm is brought toward the ventral (flexor) side of the forearm. In wrist extension, the palm is brought toward the (extensor) side of the forearm. Radial deviation (abduction) and ulnar deviation (adduction) occur in the frontal plane. In abduction, the wrist moves laterally, away from the body. In ulnar deviation, the wrist moves medially, toward the pinky. Radial and ulnar deviation are used in waving the hand side‐to‐side. Gilroy 2nd ed Fig. 22.1C The bones of the forearm, are the ulna medially and the radius laterally. The ulna is longer and larger proximally while the radius is larger distally. The proximal radius features a head, a neck and a radial tuberosity. The proximal ulna features the olecranon process posteriorly (makes up the bony prominence of the elbow), the trochlear notch (that articulates with the trochlea of the humerus), and the coronoid process and ulnar tuberosity anteriorly. Distally there is a radial styloid process on the lateral side of the forearm and an ulnar styloid (not pictured above) that projects posteromedially from the distal ulna. Netter 436 This figure shows the right elbow joint from the anterior side on the left and the posterior side on the right. Note that the main joint is the humoroular joint. This is between the trochlea of the humerus and the trochlear notch of the ulna. This is a very stable joint. On the lateral side, the head of the radius makes a very shallow joint with the capitulum of the humerus. Gilroy 2nd ed Fig. 22.8, 22.5B, 23.6A The articulation of the humerus with the radius and ulna is actually a complex of three joints: the humeroulnar joint, the humeroradial joint and the radioulnar joint. The main parts of the shaft of the radius and ulna are held together by the interosseous membrane (a “fibrous joint”). The radioulnar joint allows pronation and supination of the forearm. Schuenke Vol. 1 2nd ed. Fig. 14.279Da, Db The proximal radioulnar joint is encircled by the anular ligament. Distally, there is an articular disk that stretches from the ulnar styloid to the distal radius and holds the bones together. This allows for a strong connection while permitting the radius to pivot around the ulna by rotating within the annular ligament proximally. This curvature in the shaft of the radius allows for full rotation (pronation) around the ulna. Rudolph’s Pediatrics 23rd ed. Nursemaid’s elbow is a common injury in young (preschool-aged) children. It usually results from a pull to the arm or wrist of a child causing displacement of the anular ligament of the elbow and subluxation of the radial head. This results in pain and refusal to use the arm. Child classically presents holding the affected elbow close to the body, mildly flexed with pronation of the forearm. The two techniques used for reduction are hyperpronation and supination-flexion. Gilroy 2nd ed. Fig 2449.B; 3rd ed. Fig 29.2 In the forearm there are fascial layers similar to those of the arm. These include an investing fascia surrounding all of the muscles, and intermuscular septae separating the muscles into compartments. The flexor compartment of the forearm is located anteromedially, and the extensor compartment is located dorsolaterally. The flexor compartment contains flexor and pronator muscles, while the extensor compartment contains extensor and supinator muscles. Trauma in the forearm can cause swelling which may be restricted by fascial compartments. This can cause elevated pressure in the compartment and subsequent damage to all structures due to blood flow impairment or nerve compression Gilroy 2nd ed. Fig 22.111A The superficial forearm flexor muscles arise from the common flexor tendon on the medial side of the elbow. These muscles include, from radial to ulnar, the pronator teres, flexor carpi radialis, palmaris longus (not always present), and flexor carpi ulnaris **clinical note: the palmaris longus muscle is absent in up to 25% of the population, suggesting it is not important for proper use of the hand. Accordingly, surgeons can use the palmaris longus tendon in autografts to replace other damaged tendons. It is commonly used in ulnar collateral ligament reconstruction, also known as Tommy John surgery (TJS). This is a surgical graft procedure where the ulnar collateral ligament in the medial elbow is commonly replaced with tendon of the palmaris longus muscle. It is named after the first baseball player to undergo the surgery, major league pitcher Tommy John. Short film about it here for those interested: http://www.espn.com/30for30/film?page=tommyandfrank Gilroy end ed. Fig 22.11B The middle layer of flexor muscles consists of a single, broad muscle, the flexor digitorum superficialis (FDS). This arises from the common flexor tendon and the radius by a very extensive origin. It inserts on the middle phalanx of all fingers except for the thumb, and therefore has 4 tendons distally. Gilroy 2nd ed. Fig. 23.2 The bones of the hand are the carpals, metacarpals, and phalanges. Note that the thumb only has two phalanges (proximal and distal), while the digits have three. The carpal bones exist in two rows of four, a proximal and distal row. The distal row of carpals articulate with the metacarpals of the hand. We will learn the individual names of the carpal bones in the next lecture. Gilroy 2nd ed. Fig. 22.15 The deep layer of flexor muscles arise from the radius and ulna or interosseous membrane. This layer consists of the flexor pollicis longus, flexor digitorum profundus (FDP), and pronator quadratus. Like the flexor digitorum superficialis, the flexor digitorum profundus muscle also has 4 tendons; the tendons, however, extend farther to insert on the distal phalanx of the fingers. Gilroy 2nd ed. Fig. 22.11A Drake 2nd ed. The two pronator muscles, the pronator teres and the pronator quadratus are within this flexor compartment. The supinator muscle is within the extensor compartment. However, the strongest supinator muscle is the biceps. The median nerve gives rise to the anterior interosseous branch, which supplies most of the muscles of the deep compartment of the flexor forearm. The median nerve proper runs between the middle and deep layers of flexor muscles, and supplies most of the superficial and all of the middle layer of muscles. The median nerve then enters the hand and supplies skin over the palm and the radial digits, including the palmar and dorsal tips of those three and a half fingers. The anterior interosseus branch of the median nerve supplies all of the deep group except the medial two heads of the FDP. Damage to anterior interosseus nerve presents with weakness of grip and pinch, specifically thumb, index and middle finger flexion; inability to make OK sign with thumb and index finger (loss of the FDP to index and flexor pollicis longus); pronator quadratus weakness shown with weak resisted pronation with elbow maximally flexed. The median nerve then enters the hand and supplies skin over the palm and the radial digits, including the palmar and dorsal tips of those three and a half fingers. The ulnar nerve enters the forearm by passing through the ulnar groove (posterior to the medial epicondyle). It travels deep to the flexor carpi ulnaris muscle, which it innervates, then continues on the surface of the flexor digitorum profundus. The ulnar nerve innervates the medial half of the flexor digitorum profundus. Before entering the hand, it gives rise to a dorsal cutaneous branch that supplies the medial side of the dorsum of the hand. The ulnar nerve also supplies the medial side of the palm and one-and a half digits. Note that the ulnar nerve does not supply the skin of the medial forearm. This is supplied by independent branches from the medial cord of the brachial plexus, the medial brachial and medial antebrachial cutaneous nerves. Before entering the hand, it gives rise to a dorsal cutaneous branch that supplies the medial side of the dorsum of the hand. The ulnar nerve also supplies the medial side of the palm and one-and a half digits. Note that the ulnar nerve does not supply the skin of the medial forearm. This is supplied by independent branches from the medial cord of the brachial plexus, the medial brachial and medial antebrachial cutaneous nerves. Gilroy 2nd ed. Fig 22.12A The extensor muscles are in two layers, superficial and deep. The longest of these muscles are innervated by the radial nerve, while the shorter ones are innervated by the deep branch of the radial nerve (which becomes the posterior interosseous nerve). For the most part, these muscles extend the wrist and fingers and abduct the thumb. The exceptions are the supinator and the brachioradialis muscle (a flexor of the elbow). The superficial layer of extensor muscles arise from a common extensor tendon, which attaches to the lateral epicondyle and supracondylar ridge. This layer consists of the brachioradialis muscle, the extensor carpi radialis longus and brevis muscles, the extensor digitorum muscle, and the extensor carpi ulnaris muscle. A small muscle, called the extensor digiti minimi (sometimes extensor digiti quinti), branches off the side of the extensor digitorum muscle and goes to the middle phalanx of the small digit only. Gilroy 2nd ed. Fig. 22.17A Lateral view of the superficial extensor forearm muscles. Recall that the extensors are primarily involved in extending the wrist and fingers and abducting the thumb. The exception is the brachioradialis (a flexor of the elbow). Gilroy 2nd ed. Fig 22.12B The deep layer of extensor muscles primarily arise from the radius, ulna and the interosseous membrane (the supinator muscle arises from the common extensor tendon). These deep muscles include the extensor pollicis brevis and longus muscles, the abductor pollicis longus muscle and the extensor indicis muscle. Gilroy 2nd ed. Fig. 23.1 As these muscles crossed the wrist, they’re in compartments under the extensor retinaculum that prevents bow-stringing. There are synovial sheaths that allow for free, sliding movement of the tendons.”(bow-stringing = arching of the tendon away from the underlying bone).” Lateral epicondylitis (aka tennis elbow) is a condition caused by the overuse of the extensor muscles that attach to the lateral epicondyle. This injury is seen in almost 50% of tennis players (hence, the name “tennis elbow”); however, it can affect anyone who participates in repetitive activity. A person with lateral epicondylitis will typically experience pain over the lateral epicondyle. The etiology of the pain is microtears of the proximal attachment of the extensor muscles. A similar condition called “golfer's elbow” occurs at the medial epicondyle and is most commonly seen in golfers The radial nerve leaves the arm by traveling anterior to the lateral epicondyle of the humerus. The nerve is vulnerable to impact or trauma in this location. After crossing the elbow, it divides into an efferent deep branch and an afferent superficial branch. The deep branch pierces the supinator muscle (which it innervates) and then continues as the posterior interosseous nerve to innervate the deep forearm extensor muscles. The superficial branch of the radial nerve innervates skin on the dorsum of the hand. The posterior cutaneous nerve of the arm and the posterior cutaneous nerve of the forearm are branches of the radial nerve that arise proximal to the elbow. The only appearance of the musculocutaneous nerve in the forearm is as the lateral antebrachial cutaneous nerve. Schuenke Vol. 1 , 2nd ed. Figs. 18.18.395Da The radial artery is covered by the brachioradialis muscle on its way towards the wrist, where it can be palpated quite easily next to the flexor carpi radialis tendon. This is the first artery clinicians traditionally palpate of when taking a patient’s pulse. The ulnar artery passes deep beneath the pronator muscle. It gives rise to a common interosseous branch that, in turn, gives rise to a branch called the anterior interosseous artery and another branch that penetrates the interosseous membrane to reach the dorsum of the forearm. Here it is called the posterior interosseous artery. The ulnar artery passes to the hand next to the ulnar nerve. Gilroy 2nd ed. Fig. 24.47; Schuenke Vol. 1 end ed. Fig 18.397Da The anatomical snuff box is an anatomical landmark of the posterolateral hand. The tendons of the extensor pollicis longus laterally, and extensor pollicis brevis and abductor pollicis longus anteriorly create the frame for the snuff box. The radial nerve runs superficial to the snuffbox, while the radial artery pulse can be felt in its depths. The floor of the snuff box is formed by the scaphoid bone is palpable. Gilroy 2nd ed. Fig. 23.6A Distally, the wrist joint consists of the radiocarpal joint, inter-carpal joints between the rows of carpal bones and the carpometacarpal joint (between the carpal bone and metacarpals). Note: the ulna does not articulate directly with carpal bones. Instead, there is a fibrocartilagenous articular disc interposed. The main joint of the wrist is between the radius and the scaphoid and lunate bones (carpal bones). Gilroy 2nd ed. 23.7B We have a couple of different joints represented here. The interphalangeal joints (DIP and PIP) are typical hinge joints that permit flexion and extension only. The joints between the metacarpals and phalanges (MP or metacarpophalangeal) however allow flexion, extension, adduction and abduction (and therefore, circumduction). And finally the carpometacarpal joints b/w carpals and metacarpals. The carpometacarpal joint of the thumb has greater mobility due to its “saddle-shaped” structure. MP, PIP, and DIP joints can be acted on individually in anterior hand digits but because of the arrangement of something called the extensor hood they are all extended at the same time in any single digit when the extensor digitorum contracts Netter’s Atlas The extensor expansion (aka extensor hood) describes the mechanism of how the extensor tendons insert on the phalanges. The tendon splits into a central band and two lateral bands in the vicinity of the proximal phalanx. The central band inserts into the middle phalanx, while the two lateral bands insert into the distal phalanx. The “hood” is a band of transverse fibers that holds the tendon in place, and serves as a site of attachment for some of the intrinsic muscles of the hand (the interosseous and lumbrical muscles).