Definition

The arm also referred to as the upper limb is connected to the chest by the shoulder joint. It is divided into two segments separated by the elbow: the upper arm and the forearm, the latter extending to the wrist and the hand.

The arm and elbow

The arm also referred to as the upper limb is connected to the chest by the shoulder joint. It is divided into two segments separated by the elbow: the upper arm and the forearm, the latter extending to the wrist and the hand.

Bones of the upper arm and elbow

The upper arm is the region of the body located between the shoulder and the elbow. This segment of the arm is comprised of a single, long and large bone named humerus. The anatomy of the humerus consists in the proximal end (head), which articulates into the socket of the shoulder joint, followed by a humerus neck, and a long, cylindrical shaft containing the bone marrow. In the distal portion, the humerus ends with two round extremities, the condyles, which are located in opposition to the bones of the forearm, the radius and ulna. The upper arm is connected to the bones of the forearm through the elbow joint, which is a hinged joint that articulates the humerus with the ulna and radius.

Bones of the forearm

The forearm is made of two bones, the radius and the ulna. The radius is located on the outside of the forearm along the thumb line. It becomes wider at the wrist. The ulna runs on the internal side of the arm, along the little finger. It is felt at the inner tip of the elbow. The radius and ulna are connected with one another by a strong interosseous membrane. In the proximal region, the radius and ulna articulate with the humerus bone of the upper arm to form the elbow joint.

Bones at the wrist

In the wrist joint the radius and ulna articulate with three carpal bones of the wrist: the scaphoid, the lunate, and triquetrum. The radius and ulna allow the rotation of the forearm by turning the palm of the hand up (supination) and down (pronation). In this movement the radius slides around the ulna, which remains in its position.

Elbow joint

The elbow is made by the distal end of the humerus and the proximal end of two smaller bones forming the lower arm, the radius on the outer side, and the ulna on the inner side. These bones allow for the twisting movement of the hand. At the elbow the humerus ends with two processes, the medial and lateral epicondyles that can be felt on each side of the elbow. The head of the radius or radial head is a round concave area, which allows the movement around the opposed end of the humerus (capitellum), whereas the coronoid is the corresponding area of the ulna facing the distal humeral extremity, the trochlea.

Ligaments

Ligaments are strong bands of elastic connective tissue that join bones to other bones. At the elbow they play an important role in stabilising the function of the joint where three bones of the upper extremity articulate with one another. There are three main ligaments at the elbow joint:

Medial Collateral Ligament or Ulnar Collateral Ligament: consists of the anterior and posterior bands. Both originate from the medial epicondyle and cross through the elbow joint. The anterior band attaches at the coronoid process of the ulna, and the posterior band to the olecranon process.

Lateral Collateral Ligament or Radial Collateral Ligament: is a short, thin ligament, which passes from the lateral epicondyle to the annular ligament.

Annular Ligament: is a circular ligament surrounding the radial head that is tightened into the notch of the ulna. It’s role is to stabilise the radio-ulnar joint.

Muscles of the upper arm

Deltoid: is a large and the strongest muscle of the shoulder, connecting to the shoulder blade, the clavicle and the humerus. It is used to lift the arm sideways and rotate it laterally and medially.

Triceps brachii: is the three-headed muscle on the posterior side of the upper arm. It is used to straighten the forearm. It is particularly evident in athletic individuals.

Brachialis: is a strong elbow flexor and helps the biceps brachii to bend the elbow.

Brachioradialis: extends from distal humerus to the lower radius. It is used to flex the elbow and aid pronation and supination of the forearm.

Anconeus: is a small muscle of the elbow with a triangular shape joining the ulna to the lateral humerus. It assists in elbow extension and rotation.

Extensor carpi radialis longus: is a long muscle arising partially at the lateral supracondylar ridge and partially at the lateral epicondyle of the humerus. It runs on the thumb side of the forearm and inserts at the base of the 2nd metacarpal bone (corresponding to the index finger). It is a major muscle used for wrist abduction and extension.

Biceps or Biceps brachii: is the muscle likely used more often when moving the arm. It is connected to the bones of the shoulder, the glenoid and coracoid process, via two tendons, and to the proximal region (tuberosity of the radius) with one tendon. The biceps is essential to bend the arm and turn the hand outward (supination).

Muscles of the forearm

The muscles of the forearm are located on the dorsal and ventral side of the forearm.

Many of them originate from the epicondyles of the upper arm (humerus) and are involved in the movement of the elbow, hand and fingers. Muscles on the dorsal side of the forearm:

Muscles on the dorsal side of the forearm:

Extensor carpi ulnaris: lies on the ulnar side of the forearm. It originates from the lateral epicondyle at the inner elbow and crosses the forearm downward to insert at the base of the 5th metacarpal of the hand. It is used for the extension and adduction of the wrist.

Extensor carpi radialis longus: is a long muscle on the radial side of the forearm used to extend the hand and flex the forearm. It inserts at the lateral humerus to attach distally at the second metacarpal bone of the index finger.

Extensor digitorum: is located on the forearm back side. It arises at lateral epicondyle of the humerus and divides into four tendons to reach the distal and middle phalanges of the fingers. It controls the extension of the medial fingers.

Extensor carpi radialis brevis: originates above the lateral epicondyle of the humerus and inserts into the 3rd metacarpal bone. It functions in synergy with the extensor carpi longus and is used to abduct and extend the hand and wrist.

Extensor digiti minimi: arises at the lateral epicondyle of the humerus and joins with the hand muscle, the extensor digitorium communis. It controls the movements of the little finger (flexion, extension).

Flexor carpi ulnaris: arises at the medial epicondyle of the humerus and the ulnar head. It extends to the wrist where it inserts at the pisiform bone prior to the 5th metacarpal bone. It works together with the extensor carpi radialis to flex or adduct the wrist.

Abductor pollicis longus: together with extensor pollicis muscles, it controls the thumb movement. It begins at the proximal radius and ulna and attaches at the base of 1st metacarpal bone. It assists in thumb abduction and extension and contributes to wrist flexion.

Extensor pollicis brevis: is located on the dorsal forearm. It originates at the distal radius and inserts at the base of the proximal phalanx of the thumb (latin: pollux-pollicis) where it forms the known snuff box at the radial wrist. It works with extensor pollicis longus in the extension and abduction of the thumb.

Muscles of the deep forearm back:

Supinator: is a short muscle arranged in two layers. Its fibres run from the ulna and the lateral end of the humerus to the radius. It supports the biceps brachii in rotating the forearm (supination).

Extensor pollicis longus: originates at the ulna bone and interosseous membrane and inserts at the distal phalange of the thumb. It is used for thumb extension.

Extensor indicis: originates at the interosseous membrane and ulna and travels to the index finger parallel to the extensor digitorum tendon. It assists in the index extension of all phalanges and also of the wrist mid-carpal joints.

Muscles on the ventral side of the forearm:

Pronator teres: inserts proximally with two heads at medial epicondyle of the humerus and ulnar coronoid process and runs diagonally across the forearm to connect to the outer radius. It is used with the pronator quadratus during forearm pronation (palm down)

Flexor carpi radialis: inserts at the humerus epicondyle and runs through the anterior forearm to the base of the 2nd metacarpal bone. It is used during wrist flexion and hand abduction.

Flexor carpi ulnaris: originates with two ends, one at the medial epicondyle of the humerus and the other at the olecranon. It inserts at the wrist carpal bones, the pisiform, hamamate and to the basis of the 5th metacarpal bone. It is used to flex and adduct the hand and wrist.

Palmaris longus: is a thin muscle arising at the medial epicondyle that runs in the middle of the forearm, where it inserts on the palmar aponeurosis on the hand palm. Its function is to flex the hand at the wrist. It can be absent in over 10% of the population or consist of a tendon rather then muscle tissue.

Flexor digitorum superficialis: has two heads one inserting at the medial epicondyle and the other at the radius head. It separates into two tendons that insert at the middle phalanges of the fingers. They are used during flexion of the interphalangeal joints of middle phalanges (2nd to 5th fingers).

Deep muscles of the front forearm:

Flexor digitorum profundus: inserts at the ulna and interosseous membrane and runs to the phalanges of the 2nd to the 5th finger. It controls the flexion of the distal phalanges.

Flexor pollicis longus: originates from the distal radius and runs through the medial forearm to reach the tip of the thumb as a tendon enclosed into the carpal tunnel. It is used for thumb flexion.

Pronator quadratus: is a squared (latin: quadratus) muscle located at the forearm near the wrist. It runs horizontally to connect the ulna with the radius. It assists the pronator teres muscle in forearm pronation (palm facing).

Tendons of the Upper Arm

The tendons of the upper arm connect the muscles to the bones of the forearms. At the proximal side (shoulder) there are two tendons of the biceps: the tendon of the long head inserts to the glenoid, whereas the short head connects with the coracoid process of the shoulder.

The distal tendon of the biceps is connected to the proximal radius and flexes the forearm at the elbow and supinates the forearm. The distal tendon of the triceps is connected to the olecranon and extends the forearm in the elbow joint.

Tendons of the forearm

The forearm tendons (extensors) connect the muscles of the forearm to the lateral epicondyle of the humerus. The extensor carpi radialis brevis is the muscle mostly known for being involved in a pathology named tennis elbow.

Anteriorly - the biceps tendon connects the biceps muscle to the radius. It is used to bend the elbow with strength (see image on previous page).

Posteriorly - the triceps tendon attaches the triceps muscle to the ulna. It is used to straighten the arm with a push-up (see image on previous page).

Elbow capsule

Synovial membrane

The synovial membrane forms a fibrous capsule around the elbow joint. It produces the synovial fluid, which reduces the friction of the bones and tendons by absorbing the pressure during the movement of the joint. It extends from the surface of the humerus, along the ulna and the radius.

Other structures assist in stabilising and protecting the integrity of the elbow joint capsule. The cartilage is a thin layer of connective tissue covering the extremity of all articulating bones (humerus, radius and ulna), which prevents the friction of the bones during the movements of the elbow. In addition, a multitude of ligaments connect the bones with one another to stabilise further the elbow joint (see ligaments).

Blood vessels

Blood vessels are an intricate network of flexible ducts that circulate blood through the body’s tissues. They are divided into arteries and veins. The arteries supply the body with oxygenated blood originating from the heart, while the veins transport carbon dioxide-rich blood from peripheral tissues back to the heart. Together they form the vascular system.

Arteries

The subclavian artery is the largest artery providing blood to the upper limbs. It differs on each body side: the right subclavian artery arises from the brachiocephalic trunk whereas the left subclavian artery branches directly from the aorta. After crossing the lateral side of the first rib, the subclavian artery enters the axilla to become the axillary artery and further down it divides again to form the brachial artery.

The largest artery of the arm is the brachial artery, which is a direct extension of the subclavian artery. Downstream, the brachial artery forms the profunda brachii or, deep artery of the arm, that travels along the posterior surface of the humerus. It supplies the muscles in the posterior part of the arm (e.g triceps brachii), and ends up into a network of vessels at the elbow. As it descends immediately posterior to the median nerve it divides into smaller arteries as it continues towards the arm, wrist and hand. Then it crosses the cubital fossa (or elbow pit, the triangular area on the anterior side of the elbow), it bifurcates into the radial and ulnar arteries. Doctors use this pulsating artery to measure blood pressure. Injury to the brachial artery can seriously affect the hand as it is the only blood supply. At the elbow, the brachial artery divides into:

Ulnar artery with its branches, the superior and inferior ulnar collateral arteries, is located at the medial and anterior side of the forearm to reach the wrist

Radial artery descends in the anterior and lateral side of the forearm. Downstream, it divides into different branches to supply the hand

Interosseous recurrent artery supplies the interosseous membrane between the radius and ulna.

Veins

The main veins of the upper arm, elbow and forearm are the cephalic vein, the basilic vein and the median cubital vein situated on the superficial upper extremities. Their description follows the flow of venous blood, ascending from the lower forearm to the upper arm.

Cephalic vein runs from the forearm through the elbow and arm, up to the shoulder where it merges with the axillary vein in the upper chest

The basilic vein runs at the forearm, it passes the elbow and takes the name axillary vein at the shoulder. It also merges with the subclavian vein. The basilic vein joins with the brachial vein to form the axillary vein

Median cubital vein is a large superficial vein running parallel to the brachial artery. It connects the basilic and cephalic veins. This vein is usually clearly visible through the skin

Median antebrachial vein runs in the anterior aspect of the forearm between the radial and ulnar veins and drains into the basilic vein

Cubital vein is used to draw blood with a needle, or venipuncture, to deliver intravenous infusion of various solutions and medications.

Brachial, radial and ulnar veins are located in the deeper forearm. They take the same names of their corresponding arteries and flow parallel to them. At the elbow, the ulnar and radial veins join to form the brachial vein.

Nerves

The upper limbs are innervated by five nerves: the median, ulnar, radial, axillar and musculocutaneous nerves, each one mediating distinct motor and sensory functions.

Brachial Plexus

The nerves of the arm originate from the cervical spine vertebrae (C5-C8 and T1) and descend along the body to form the brachial plexus from where they interchange and extend along the shoulder, the upper arm and forearm up to the hand and fingers.

Axillary nerve

The axillary nerve or the circumflex nerve is located in the axilla and originates from the brachial plexus. It innervates the upper arm and shoulder region. Differently from other nerves it does not descend along the arm. After leaving the plexus it divides into the anterior and posterior branch.

The anterior branch innervates the deltoid muscle in the upper arm while the posterior branch extends to the upper shoulder, supplying the teres minor (a muscle located in the lateral side of the scapula attaching to the rotator cuff in the shoulder joint) and the posterior area of the deltoid. The axillary nerve provides sensory function to the skin of the shoulder joint and deltoid muscle. 

Nerves of the upper extremities

Three main nerves of the arm are:

Median nerve

Radial nerve

Ulnar nerve

Pathological symptoms of the elbow and forearm can be induced by problems of the nerves. The nerves are enclosed by tunnels and due to the complex arm movements; they are subject to bending and straightening. Alterations of the nerves and tunnels can lead to pain, numbness and weakness in the arm and hand. These nerves can also be injured during trauma and improper surgery.

Median nerve

The median nerve controls the following motor functions: forearm pronation, thumb palmar abduction and thumb/index/long finger flexion. The motor branches of the median nerve and the respective muscles they innervate are listed below:

Proximal extrinsic motor branches (median nerve):

1. Pronator teres

2. Flexor carpi radialis

3. Palmaris longus

4. Flexor digitorum superficialis

Proximal extrinsic motor branches (anterior interosseous nerve)

5. Flexor digitorum profundus (to index and, usually long finger)

6. Flexor pollicis longus

7. Pronator quadratus

Distal intrinsic motor branches (thenar branch)

1. Abductor pollicis brevis

2. Flexor pollicis brevis (superficial head)

3. Opponens pollicis

Distal intrinsic motor branches (common digital branches of the median nerve)

4. First lumbrical (to index and long fingers)

5. Second lumbrical (to index and long fingers)

Palmar branch (to radial base of palm)

Articular branches to the wrist joint (via distal anterior interosseous nerve).

Sensory Branches of the median nerve

These branches control sensation to the illustrated areas of the hand and fingers: volar thumb, index, long, and radial half of ring finger. They also provide sensation to specific areas of the hand - the pinch surfaces of the thumb, index and third fingers. Compression of the median nerve at the wrist is known for causing the carpal tunnel syndrome.

Radial nerve

Motor branches of the radial nerve contribute to the function of the elbow, wrist, finger and thumb.

The muscles Innervated by the radial nerve are:

Triceps brachii / anconeus

Brachioradialis

Extensor carpi radialis longus

Extensor carpi radialis brevis

Supinator

Extensor carpi ulnaris

Extensor digitorum communis

Extensor digiti minimi

Abductor pollicis longus

Extensor pollicis brevis

Extensor indicis proprius

Extensor pollicis longus

Brachialis (contribution; though main supply to brachialis is the musculocutaneous nerve)

Sensory branches

The sensory component of the radial nerve, the superficial radial sensory nerve (or radial sensory branch), is the frequent cause of neuropathic pain following injury.

These are the sensory branches:

Posterior brachial cutaneous nerve/ inferior lateral brachial cutaneous nerve

Posterior antebrachial cutaneous nerve

Radial sensory nerve (sensory branch of the radial nerve)

Ulnar nerve

Motor branches: ulnar nerve contributes to the following motor functions: fine hand movements, coordination of finger motion and pinch strength, flexion of the small and ring fingers.

Proximal extrinsic motor branches (ulnar nerve)

Flexor carpi ulnaris

Flexor digitorum profundus (ulnar half to small, ring +/- long finger)

Distal intrinsic motor branch (superficial ulnar nerve branch)

Palmaris brevis

Distal intrinsic motor branches (deep ulnar nerve branch)

Abductor digiti minimi

Flexor digiti minimi

Opponens digiti minimi

3rd and 4th lumbricals (to small & ring fingers)

Palmar and dorsal interosseous muscles

Adductor pollicis

Flexor pollicis brevis

First dorsal interosseous

First palmar interosseous

Sensory branches

The ulnar nerve mediates parts of the sensation of the palm and of the small finger including the ulnar border of the fourth finger. The ulnar nerve is enclosed in the Guyon canal, where a pathologic nerve entrapment can occur.

These are the main branches of the ulnar nerve:

Dorsal branch

Common and proper digital nerves (superficial branch) to volar small and ulnar half of ring finger

Palmar branch

Articular branches to the elbow joint and carpal and metacarpo-phalangeal joints.

Image shows the areas of sensation of the median radial and ulnar nerves.