Upper limb

The upper limb extends from the shoulder and consists of the arm, forearm, hand, and fingers.

Bony structure:

• The bony structure includes the humerus (arm bone), which connects with the glenoid cavity of the scapula to form the shoulder joint (glenohumeral joint). This ball and socket variety of synovial joint allows various movements such as flexion, extension, adduction, abduction, and circumduction.

• Distally, the humerus articulates with the bones of the forearm to form the elbow joint. The rounded capitulum of the humerus connects with the head of the radius, while the trochlea of the humerus articulates with the trochlear notch of the ulna. The elbow joint functions as a hinge joint, enabling flexion and extension.

• The radius and ulna are connected through the superior and inferior radioulnar joints and the interosseus membrane. Rotational movement of the radius along the ulna’s long axis allows pronation (palm facing dorsally) and supination (palm facing ventrally).

• The hand’s bones consist of carpal bones in the wrist, metacarpal bones in the palm, and phalanges for the five digits (thumb, index, middle, ring, and little finger).

• The distal end of the radius articulates with the carpal bones (scaphoid, lunate, and triquetrum) to form the wrist joint, facilitating flexion and extension.

• Other small joints within the hand include intercarpal, carpometacarpal, metacarpophalangeal, and interphalangeal joints.

Muscles:

• Skeletal muscles cover the bony structure in the shoulder, arm, forearm, and hand regions.

• Several shoulder muscles connect the scapula and clavicle to the trunk, such as the trapezius, levator scapulae, and rhomboids. Other shoulder muscles connect the scapula, clavicle, and bony trunk to the upper end of the humerus, including the pectoralis muscles, deltoid, teres major, latissimus dorsi, and rotator cuff muscles. The rotator cuff muscles (subscapularis, supraspinatus, infraspinatus, and teres minor) provide vital support to the shoulder joint. They originate from the scapula, surround the shoulder joint, and their distal tendons merge with the shoulder joint capsule to form a fibrotendinous rotator cuff before inserting into the upper humerus.

• Arm muscles can be divided into anterior (flexor) and posterior (extensor) compartments. The anterior compartment comprises coracobrachialis, biceps brachii, and brachialis muscles. The latter two cross the elbow joint, facilitating elbow flexion, while coracobrachialis contributes to shoulder flexion. The posterior compartment consists of a single muscle, triceps brachii, which extends the elbow joint.

• In the forearm, most long flexor muscles originate from the common flexor origin (medially), while many long extensor muscles at the back originate from the common extensor origin (laterally). These muscles insert into the carpal bones and the bases of metacarpals, both ventrally and dorsally. Some muscles cross the metacarpophalangeal and interphalangeal joints to insert into the middle and distal phalanges of the index, middle, ring, and little fingers. The long extensor tendons insert into the phalanges through specialized common extensor hoods. The flexor and extensor muscles of the forearm enable flexion and extension at the wrist, metacarpophalangeal, and proximal and/or distal interphalangeal joints.

• Intrinsic hand muscles are exclusively found within the hand and include the lumbricals, interossei, thenar, and hypothenar muscles.

Innervation:

• Somatic innervation of the upper limb arises from the brachial plexus, formed from C5 to T1 spinal nerve roots. Major distal branches of the brachial plexus include the musculocutaneous nerve for anterior arm muscles. The median and ulnar nerves primarily innervate forearm flexors and palm musculature, respectively. The radial nerve innervates both arm and forearm extensor muscles.

Vasculature:

• Vasculature consists of subclavian artery > axillary > brachial > radial and ulnar arteries.

• Deep veins accompany the arteries.

• Superficial veins originating from the venous arch on the dorsum of the hand include the radial cephalic vein and ulnar basilic vein. These veins ultimately drain into the deep veins.

Fascia and skin:

• Deep and superficial facia, along with the skin, enclose the upper limb, similar to other body regions.