# Lecture 7: Upper Limb - Arm and Forearm ## Unit 1.3: Human Gross Anatomy I - Musculoskeletal System --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the bones of the arm (humerus) and forearm (radius and ulna) and their key features 2. Identify the muscles of the anterior and posterior compartments of the arm 3. Describe the elbow joint and its ligaments 4. Identify the muscles of the anterior (flexor) and posterior (extensor) compartments of the forearm 5. Describe the proximal and distal radioulnar joints and their movements 6. Trace the paths of major nerves and vessels in the arm and forearm --- ## Bones of the Arm and Forearm The arm and forearm contain three major long bones that work together to enable the complex movements required for positioning the hand in space. Understanding their bony landmarks is essential for appreciating muscle attachments, joint mechanics, and clinical examination. ### The Distal Humerus The distal end of the humerus is specialized for articulation at the elbow. Two prominent projections, the medial and lateral epicondyles, serve as attachment sites for forearm muscles. The medial epicondyle is larger and more prominent, easily palpable on the medial aspect of the elbow, and serves as the common flexor origin for most anterior forearm muscles. The lateral epicondyle, while smaller, provides the common extensor origin for posterior forearm muscles. The articular surfaces of the distal humerus include the capitulum and trochlea. The capitulum, located laterally, is a rounded eminence that articulates with the head of the radius. The trochlea, a spool-shaped surface located medially, articulates with the ulna's trochlear notch. These two surfaces enable the hinge-like motion of the elbow. Three fossae accommodate the projecting parts of the forearm bones during movement. Anteriorly, the coronoid fossa receives the coronoid process of the ulna during flexion, while the radial fossa receives the radial head. Posteriorly, the deep olecranon fossa accommodates the olecranon process during extension. The supracondylar ridges extending proximally from the epicondyles provide attachment for brachialis and brachioradialis muscles. ### The Ulna The ulna is the medial bone of the forearm and is the primary bone articulating with the humerus at the elbow. Its proximal end features two prominent projections. The olecranon forms the bony point of the elbow and receives the triceps tendon insertion. Anteriorly, the coronoid process projects forward and, together with the olecranon, forms the trochlear notch—a deep concavity that wraps around the humeral trochlea to form the stable humeroulnar joint. The radial notch, located on the lateral aspect of the coronoid process, provides a smooth articular surface for the radial head. Just distal to the coronoid process, the ulnar tuberosity receives the brachialis muscle insertion. The shaft of the ulna features a sharp interosseous border along its lateral edge for attachment of the interosseous membrane. Distally, the ulnar head is a rounded prominence that articulates with the ulnar notch of the radius. The styloid process projects from the posteromedial aspect of the head and serves as an attachment for the triangular fibrocartilage complex. ### The Radius The radius is the lateral bone of the forearm and, unlike the ulna, has its larger end distally where it forms the primary articulation with the wrist. Proximally, the disc-shaped radial head articulates superiorly with the capitulum and medially with the radial notch of the ulna. The neck is a constriction just below the head. The radial tuberosity, located on the anteromedial aspect just distal to the neck, receives the biceps brachii tendon—the most powerful supinator of the forearm. The shaft features a sharp interosseous border for membrane attachment. Distally, the radius expands considerably. The styloid process projects from the lateral aspect and extends further distally than the ulnar styloid. The ulnar notch on the medial surface articulates with the ulnar head at the distal radioulnar joint. The dorsal (Lister's) tubercle on the posterior surface is a palpable landmark around which the extensor pollicis longus tendon passes. Panel A: Anterior and posterior views of distal humerus showing medial epicondyle as common flexor origin, lateral epicondyle as common extensor origin, rounded capitulum, and spool-shaped trochlea. Panel B: Three fossae including coronoid, radial, and olecranon shown in cross-hatching on distal humerus. Panel C: Complete ulna showing olecranon, coronoid process, trochlear notch, radial notch, tuberosity for brachialis, distal head with styloid process. Panel D: Complete radius showing disc-shaped head, neck, radial tuberosity for biceps, interosseous border, expanded distal end with styloid process and Lister's tubercle, inset showing articulated radius and ulna in supination. --- ## Muscles of the Arm The arm is divided into two compartments by the medial and lateral intermuscular septa. The anterior compartment contains the flexors, while the posterior compartment houses the extensors. Each compartment has its own nerve supply. ### Anterior Compartment: The Flexors The anterior compartment muscles are innervated by the musculocutaneous nerve from the lateral cord of the brachial plexus. These muscles primarily flex the elbow, with additional actions at the shoulder and radioulnar joints. The biceps brachii is the most superficial muscle of the anterior arm and, as its name suggests, has two heads. The long head originates from the supraglenoid tubercle of the scapula, its tendon passing through the intertubercular groove of the humerus. The short head arises from the coracoid process alongside the coracobrachialis. The two heads unite to form a single muscle belly that inserts via a strong tendon onto the radial tuberosity. A fibrous expansion called the bicipital aponeurosis extends medially to blend with the forearm fascia and protect underlying structures in the cubital fossa. The biceps is the most powerful supinator of the forearm when the elbow is flexed, and it also contributes to elbow flexion and shoulder flexion. Deep to the biceps lies the brachialis, the workhorse of elbow flexion. Originating from the anterior surface of the distal humerus, it inserts onto the coronoid process and tuberosity of the ulna. Because its insertion is on the ulna (which cannot rotate), brachialis is equally effective regardless of forearm position, making it the primary elbow flexor. A small portion of brachialis may receive innervation from the radial nerve, but its main supply is from the musculocutaneous nerve. The coracobrachialis is a smaller muscle originating from the coracoid process and inserting onto the medial surface of the mid-humeral shaft. It flexes and adducts the arm at the shoulder. This muscle is notable because the musculocutaneous nerve pierces through it—an important anatomical relationship. ### Posterior Compartment: The Extensors The posterior compartment contains the triceps brachii and anconeus, both innervated by the radial nerve. These muscles extend the elbow. The triceps brachii is the only muscle of the posterior arm and has three heads. The long head arises from the infraglenoid tubercle of the scapula and passes between the teres major and minor muscles. The lateral head originates from the posterior humerus above the radial groove, while the medial head arises below this groove. All three heads converge to insert via a common tendon onto the olecranon. The long head, crossing the shoulder joint, also assists in arm extension and adduction. The triceps is the primary elbow extensor. The anconeus is a small triangular muscle on the posterolateral elbow. Originating from the lateral epicondyle, it inserts onto the lateral olecranon and proximal ulna. It assists the triceps in elbow extension and may help stabilize the elbow joint during pronation and supination. Panel A: Anterior superficial view showing biceps brachii with long head tendon from intertubercular groove and short head from coracoid process, merged muscle belly with strong distal tendon inserting on radial tuberosity. Panel B: Bicipital aponeurosis spreading medially, deep dissection revealing brachialis underlying biceps inserting on ulnar coronoid process, coracobrachialis with musculocutaneous nerve piercing through. Panel C: Posterior view showing triceps brachii with three heads color-coded including long head from infraglenoid tubercle, lateral head above spiral groove, medial head below spiral groove. Panel D: All three triceps heads converging on common tendon inserting onto olecranon, small triangular anconeus at posterolateral elbow, nerve pathways indicated. --- ## The Elbow Joint The elbow is a compound synovial joint enclosed within a single joint capsule. It functions primarily as a hinge joint for flexion and extension but also incorporates the proximal radioulnar articulation for forearm rotation. ### Articulations and Structure Three separate articulations share the elbow joint capsule. The humeroulnar joint, between the trochlea and trochlear notch, is the main hinge component and provides stability for flexion and extension. The humeroradial joint, between the capitulum and radial head, is technically a ball-and-socket configuration but functions with hinge and pivot movements. The proximal radioulnar joint, between the radial head and radial notch of the ulna, is a pivot joint allowing pronation and supination. The joint capsule is relatively thin anteriorly and posteriorly but is reinforced on the sides by collateral ligaments. It attaches proximally above the coronoid and olecranon fossae and distally to the margins of the trochlear notch and annular ligament. ### Ligamentous Support The medial (ulnar) collateral ligament is crucial for elbow stability, particularly against valgus stress. It consists of three bands: the anterior band (the strongest) extends from the medial epicondyle to the coronoid process, the posterior band attaches to the olecranon, and the oblique band connects the coronoid to the olecranon. This ligament is particularly important in throwing athletes, where repetitive valgus stress can cause injury (often requiring "Tommy John" surgery for reconstruction). The lateral (radial) collateral ligament extends from the lateral epicondyle to blend with the annular ligament. It resists varus stress but is less commonly injured than the medial collateral ligament. The annular ligament is a unique fibro-osseous ring that encircles the radial head like a collar. It attaches to the anterior and posterior margins of the radial notch of the ulna, forming four-fifths of a ring that holds the radial head in place while allowing it to rotate freely during pronation and supination. The inner surface is lined with synovial membrane and cartilage. ### Movements and Innervation Flexion of the elbow (0 to approximately 150 degrees) is performed by brachialis, biceps brachii, and brachioradialis. Extension, returning the elbow to the anatomical position, is accomplished by triceps and anconeus. The carrying angle—the slight valgus angulation of the extended forearm relative to the arm—is normally 5-10 degrees in males and 10-15 degrees in females. The elbow joint receives innervation from all the major nerves crossing it: musculocutaneous, radial, ulnar, and median nerves. Blood supply comes from a rich periarticular anastomosis formed by branches of the brachial, radial, and ulnar arteries. Panel A: Coronal section showing humeroulnar articulation with trochlea engaging trochlear notch, humeroradial with capitulum articulating with radial head. Panel B: Proximal radioulnar articulation with radial head rotating in radial notch shown with rotation arrow, joint capsule as translucent envelope surrounding all articulations. Panel C: Medial collateral ligament complex with anterior band, posterior band, and oblique band connecting coronoid to olecranon. Panel D: Lateral collateral ligament blending with annular ligament encircling radial head, superior view cross-section showing annular ligament as complete ring with rotation arrows and range of motion markers. --- ## Radioulnar Joints The radius and ulna articulate at two synovial pivot joints—proximal and distal—connected by the interosseous membrane. Together, these structures enable pronation and supination, essential movements for hand positioning. ### Proximal Radioulnar Joint The proximal radioulnar joint is located within the elbow joint capsule. The radial head rotates within the osseofibrous ring formed by the radial notch of the ulna and the annular ligament. During supination, the radius and ulna lie parallel. During pronation, the radius pivots around the relatively fixed ulna, crossing over it. ### Distal Radioulnar Joint The distal radioulnar joint has its own synovial cavity separate from the wrist joint. Here, the movement is opposite: the ulnar head is relatively fixed while the expanded distal radius rotates around it. An articular disc (part of the triangular fibrocartilage complex) separates this joint from the radiocarpal joint and provides an articular surface for the ulnar side of the wrist. ### The Interosseous Membrane The interosseous membrane is a broad fibrous sheet connecting the interosseous borders of the radius and ulna along most of their length. The fibers are oriented obliquely, running inferolaterally from ulna to radius. This arrangement serves multiple functions: it binds the two bones together while permitting the rotation needed for pronation and supination, transmits forces from the radius (which bears most load from the hand) to the ulna, and provides attachment surfaces for forearm muscles. ### Movements of Pronation and Supination Supination rotates the forearm so that the palm faces anteriorly (in the anatomical position) with the thumb directed laterally. The radius and ulna are parallel. The biceps brachii is the most powerful supinator when the elbow is flexed; the supinator muscle works at all elbow positions. Pronation rotates the forearm so the palm faces posteriorly with the thumb directed medially. The radius crosses over the ulna. Pronator quadratus is the main pronator, acting throughout the range; pronator teres assists, particularly with rapid or forceful pronation. Panel A: Anterior view in supinated position showing radius and ulna parallel with palm facing anteriorly, proximal radioulnar joint detail with radial head rotating in annular ligament. Panel B: Anterior view in pronated position showing radius crossing over ulna with palm facing posteriorly, distal radioulnar joint detail with radius rotating around ulnar head. Panel C: Lateral view showing interosseous membrane as diagonal fibrous sheet between radius and ulna with force transmission arrows from radius to ulna. Panel D: Cross-sections at proximal and distal levels showing rotation mechanics, muscles labeled with biceps and supinator for supination, pronator teres and pronator quadratus for pronation. --- ## Muscles of the Anterior Forearm The anterior (flexor) compartment of the forearm contains muscles arranged in three layers. Most originate from or near the medial epicondyle (common flexor origin) and are innervated by the median nerve. These muscles flex the wrist and fingers and pronate the forearm. ### Superficial Layer The superficial layer contains four muscles arranged from lateral to medial. Pronator teres has two heads: the larger humeral head from the medial epicondyle and the smaller ulnar head from the coronoid process. The median nerve passes between these heads. The muscle inserts onto the middle of the lateral radius and serves as a pronator and weak elbow flexor. Flexor carpi radialis originates from the medial epicondyle and inserts onto the bases of the second and third metacarpals. It flexes the wrist and contributes to radial deviation (abduction). Its tendon is a useful landmark at the wrist, passing through its own compartment in the flexor retinaculum. Palmaris longus also arises from the medial epicondyle but has a short muscle belly and a long tendon that inserts into the palmar aponeurosis. It is a weak wrist flexor and tensor of the palmar fascia. Notably, it is absent in approximately 15% of the population—a normal variant often exploited for tendon grafts. Flexor carpi ulnaris has two heads: from the medial epicondyle and from the olecranon. The ulnar nerve passes between these heads to enter the forearm. It inserts onto the pisiform, with extensions to the hamate and fifth metacarpal. It flexes the wrist and produces ulnar deviation (adduction). Unlike other superficial flexors, it is innervated by the ulnar nerve. ### Intermediate Layer The intermediate layer contains only the flexor digitorum superficialis, a powerful muscle that forms the bulk of the anterior forearm. It originates from the medial epicondyle, coronoid process, and anterior radius. Its four tendons pass through the carpal tunnel and insert onto the middle phalanges of digits 2-5. Each tendon splits to allow passage of the flexor digitorum profundus tendon before inserting. The muscle primarily flexes the proximal interphalangeal joints but also assists with metacarpophalangeal and wrist flexion. ### Deep Layer The deep layer contains three muscles that act on the digits and contribute to pronation. Flexor digitorum profundus originates from the proximal ulna and interosseous membrane. Its four tendons pass through the carpal tunnel and the splits of the superficialis tendons to insert onto the distal phalanges of digits 2-5. It is the only muscle capable of flexing the distal interphalangeal joints. Its innervation is split: the lateral portion (to digits 2-3) receives the anterior interosseous nerve (a branch of the median), while the medial portion (to digits 4-5) is supplied by the ulnar nerve. Flexor pollicis longus originates from the anterior radius and interosseous membrane and inserts onto the distal phalanx of the thumb. It is the only flexor of the thumb's interphalangeal joint and is innervated by the anterior interosseous nerve. Pronator quadratus is a flat, quadrangular muscle at the distal forearm, originating from the distal ulna and inserting onto the distal radius. It is the primary pronator of the forearm, active throughout the range of pronation, and also helps hold the radius and ulna together. It receives innervation from the anterior interosseous nerve. Panel A: Superficial layer showing four muscles from lateral to medial including pronator teres with median nerve passing between heads, flexor carpi radialis, palmaris longus marked absent in 15 percent, flexor carpi ulnaris with ulnar nerve passing between heads. Panel B: Intermediate layer with flexor digitorum superficialis dominating view with four tendons and split illustrated showing passage of profundus tendon. Panel C: Deep layer showing flexor digitorum profundus with four tendons to distal phalanges, flexor pollicis longus to thumb, pronator quadratus at distal forearm. Panel D: Common flexor origin at medial epicondyle, nerve distribution color-coded for median, anterior interosseous, and ulnar nerves, cross-section showing layer arrangement and carpal tunnel entrance. --- ## Muscles of the Posterior Forearm The posterior (extensor) compartment contains muscles arranged in superficial and deep layers. Most originate from or near the lateral epicondyle (common extensor origin) and all are innervated by the radial nerve or its deep branch, the posterior interosseous nerve. ### Superficial Layer The superficial extensors are arranged from lateral to medial. Brachioradialis is unique among this group: although it lies in the extensor compartment, it is actually an elbow flexor. Originating from the lateral supracondylar ridge, it inserts near the radial styloid process. It flexes the elbow most effectively from the mid-prone position and is innervated by the radial nerve proper rather than the posterior interosseous branch. Extensor carpi radialis longus arises from the lateral supracondylar ridge and inserts onto the base of the second metacarpal. It extends and abducts the wrist. Like brachioradialis, it receives direct radial nerve innervation. Extensor carpi radialis brevis originates from the lateral epicondyle and inserts onto the base of the third metacarpal. It extends and abducts the wrist. It is innervated by the posterior interosseous nerve and is often affected in lateral epicondylitis. Extensor digitorum is the main finger extensor. From the lateral epicondyle, it gives rise to four tendons that insert into the extensor expansions of digits 2-5. It extends the metacarpophalangeal joints and, through the extensor expansion mechanism, assists extension of the interphalangeal joints. Extensor digiti minimi is a slender muscle that provides independent extension of the little finger. Its tendon joins the extensor expansion of the fifth digit. Extensor carpi ulnaris originates from both the lateral epicondyle and the posterior ulna. Inserting onto the base of the fifth metacarpal, it extends and adducts the wrist. ### Deep Layer (Outcropping Muscles) The deep extensors include the supinator and the "outcropping" muscles of the thumb and index finger. The supinator wraps around the proximal radius, originating from the lateral epicondyle, radial collateral ligament, and supinator crest of the ulna. It inserts onto the lateral, posterior, and anterior surfaces of the proximal radius. As the name suggests, it supinates the forearm and works at all elbow positions, unlike the biceps which is most effective with the elbow flexed. The posterior interosseous nerve passes through the supinator, which has clinical implications for nerve compression. Abductor pollicis longus originates from the posterior surfaces of both the ulna and radius and the interosseous membrane. Its tendon passes to the base of the first metacarpal, abducting and extending the thumb at the carpometacarpal joint. Extensor pollicis brevis arises from the posterior radius and interosseous membrane, inserting onto the base of the proximal phalanx of the thumb. It extends the thumb's metacarpophalangeal joint. Extensor pollicis longus originates from the posterior ulna and interosseous membrane. Its tendon hooks around Lister's tubercle to insert onto the distal phalanx of the thumb. It is the only extensor of the thumb's interphalangeal joint. The anatomical snuffbox is formed between the tendons of extensor pollicis longus (ulnar border) and abductor pollicis longus/extensor pollicis brevis (radial border). Extensor indicis provides independent extension of the index finger, originating from the posterior ulna and inserting into the extensor expansion of the second digit. Panel A: Superficial layer showing brachioradialis marked as flexor not extensor, extensor carpi radialis longus and brevis, extensor digitorum with four diverging tendons. Panel B: Extensor digiti minimi and extensor carpi ulnaris, common extensor origin at lateral epicondyle highlighted. Panel C: Deep layer showing supinator wrapping around proximal radius with posterior interosseous nerve passing through, outcropping muscles including abductor pollicis longus, extensor pollicis brevis and longus, extensor indicis. Panel D: Tendons crossing wrist under extensor retinaculum with six dorsal compartments numbered, anatomical snuffbox inset showing EPL and APL/EPB tendons forming borders. --- ## Neurovascular Structures of the Arm The arm contains major vessels and nerves that course between the shoulder and the forearm. Understanding their paths and relationships is essential for clinical examination and understanding injury patterns. ### The Brachial Artery The brachial artery is the continuation of the axillary artery, beginning at the lower border of teres major. It courses along the medial side of the arm, initially medial to the humerus and then moving anteriorly as it approaches the elbow. It lies medial to the biceps throughout its course. The brachial artery gives off several branches. The profunda brachii (deep brachial artery) is the largest branch, accompanying the radial nerve through the spiral groove to supply the posterior arm. The superior and inferior ulnar collateral arteries contribute to the anastomosis around the elbow. At the cubital fossa, the brachial artery divides into its terminal branches: the radial artery (laterally) and the ulnar artery (medially). This division typically occurs at the level of the radial neck. ### The Musculocutaneous Nerve The musculocutaneous nerve arises from the lateral cord of the brachial plexus (C5-C7). It characteristically pierces the coracobrachialis muscle before traveling distally between the biceps and brachialis. It innervates all three anterior arm muscles. After emerging lateral to the biceps tendon, it continues as the lateral cutaneous nerve of the forearm, providing sensory innervation to the lateral forearm. ### The Radial Nerve The radial nerve, the largest branch of the brachial plexus, arises from the posterior cord (C5-T1). In the arm, it passes posteriorly around the humerus in the spiral (radial) groove, accompanied by the profunda brachii artery. This intimate relationship with the humeral shaft makes the radial nerve vulnerable to injury in humeral shaft fractures. After emerging from the spiral groove on the lateral arm, the radial nerve pierces the lateral intermuscular septum to enter the anterior compartment. It gives branches to the triceps, anconeus, brachioradialis, and extensor carpi radialis longus before dividing in the cubital fossa. ### The Median Nerve The median nerve is formed by contributions from both the lateral and medial cords of the brachial plexus (C5-T1). It accompanies the brachial artery through the arm, characteristically crossing the artery from lateral to medial about midway down the arm. The median nerve gives no branches in the arm—it is solely a conduit to the forearm and hand. ### The Ulnar Nerve The ulnar nerve arises from the medial cord (C8-T1) and initially travels with the brachial artery. At mid-arm, it pierces the medial intermuscular septum to enter the posterior compartment. It then passes posterior to the medial epicondyle in the cubital tunnel—the "funny bone" location where the nerve is palpable and vulnerable to compression or direct trauma. Like the median nerve, the ulnar nerve gives no muscular branches in the arm. Panel A: Brachial artery as central vessel with branches including profunda brachii departing posteriorly to accompany radial nerve, superior and inferior ulnar collateral arteries. Panel B: Musculocutaneous nerve emerging from lateral cord, piercing coracobrachialis, coursing between biceps and brachialis, emerging as lateral cutaneous nerve. Panel C: Radial nerve in posterior view inset passing through spiral groove with profunda brachii, median nerve crossing brachial artery from lateral to medial. Panel D: Ulnar nerve leaving brachial artery midarm passing posterior to medial epicondyle through cubital tunnel, cross-section at mid-arm showing relationships of all structures with color-coded legend. --- ## The Cubital Fossa The cubital fossa is a triangular depression on the anterior aspect of the elbow. It is an important clinical region for venipuncture and blood pressure measurement, and its contents have significant surgical relevance. ### Boundaries The cubital fossa is defined by clear anatomical boundaries. The superior boundary is an imaginary line connecting the medial and lateral epicondyles. The medial boundary is formed by the pronator teres muscle, while the lateral boundary is formed by the brachioradialis. The floor consists of the brachialis muscle proximally and the supinator muscle distally. The roof is formed by the skin, superficial fascia, and the bicipital aponeurosis, with the median cubital vein crossing superficially. ### Contents The contents of the cubital fossa, from lateral to medial, can be remembered using the mnemonic TAN: Tendon, Artery, Nerve. The biceps tendon is the most lateral structure, passing to its insertion on the radial tuberosity. The bicipital aponeurosis spreads medially from the tendon across the other contents, providing some protection. The brachial artery lies medial to the tendon and divides within the fossa into the radial and ulnar arteries. The radial artery passes laterally under the brachioradialis, while the ulnar artery passes medially under the pronator teres. The median nerve is the most medial of the deep contents. It lies just medial to the brachial artery and exits the fossa by passing between the heads of the pronator teres. The radial nerve is not technically within the cubital fossa but lies nearby in the groove between brachioradialis and brachialis. Here it divides into its superficial (sensory) and deep (motor) branches. ### Clinical Significance The median cubital vein, which crosses superficially in the roof of the cubital fossa, is the preferred site for venipuncture. The bicipital aponeurosis partially protects the underlying brachial artery and median nerve from inadvertent puncture. Blood pressure measurement using a stethoscope placed over the brachial artery in this region detects Korotkoff sounds as the cuff is deflated. Panel A: Anterior view of elbow showing cubital fossa boundaries with imaginary line between epicondyles superiorly, pronator teres medially, brachioradialis laterally. Panel B: Floor visible as brachialis and supinator, contents displayed with TAN mnemonic showing biceps Tendon with bicipital aponeurosis spreading medially. Panel C: Brachial Artery bifurcating into radial and ulnar arteries, median Nerve passing between pronator heads. Panel D: Superficial layer showing median cubital vein crossing superficially with venipuncture site marked, cross-section at fossa level showing depth relationships. --- ## Neurovascular Structures of the Forearm The forearm contains the major vessels and nerves that will supply the hand. Their courses through the forearm are clinically important for understanding patterns of injury and for surgical approaches. ### The Radial Artery The radial artery is the lateral terminal branch of the brachial artery. It courses distally under cover of the brachioradialis in the upper forearm, then becomes superficial in the lower forearm where it is easily palpable at the wrist—the common site for pulse assessment. The artery lies lateral to the flexor carpi radialis tendon at the wrist. From there, it passes around the lateral aspect of the wrist into the anatomical snuffbox before entering the palm to form the deep palmar arch. ### The Ulnar Artery The ulnar artery is the medial and larger terminal branch of the brachial artery. It passes deep to the pronator teres and then deep to the flexor digitorum superficialis. In the distal forearm, it becomes more superficial and travels with the ulnar nerve. At the wrist, it passes lateral to the pisiform through Guyon's canal to enter the hand and form the superficial palmar arch. The ulnar artery gives off the common interosseous artery near its origin. This vessel divides into anterior and posterior interosseous arteries that supply the deep forearm muscles and contribute to anastomoses at the elbow and wrist. ### The Median Nerve The median nerve enters the forearm by passing between the two heads of the pronator teres. It then courses distally between the flexor digitorum superficialis and flexor digitorum profundus, adhering to the deep surface of the superficialis. Near its entry into the forearm, the median nerve gives off the anterior interosseous nerve, a pure motor branch that innervates the flexor pollicis longus, lateral half of the flexor digitorum profundus, and the pronator quadratus. The median nerve becomes superficial in the distal forearm and enters the hand through the carpal tunnel. It provides sensory innervation to the palm through the palmar cutaneous branch, which arises proximal to the wrist and passes superficial to the flexor retinaculum. ### The Ulnar Nerve The ulnar nerve enters the forearm by passing between the two heads of the flexor carpi ulnaris, having passed behind the medial epicondyle. It innervates the flexor carpi ulnaris and the medial half of the flexor digitorum profundus in the forearm. The ulnar nerve travels with the ulnar artery in the distal forearm, lying just lateral to the flexor carpi ulnaris tendon. It enters the hand through Guyon's canal (the ulnar canal), located between the pisiform and the hook of the hamate. ### The Radial Nerve The radial nerve divides in the cubital fossa region into superficial and deep branches. The superficial branch is purely sensory, passing distally under the brachioradialis to emerge on the dorsum of the hand and supply sensation to the lateral dorsal hand and proximal digits. The deep branch (posterior interosseous nerve) is purely motor. It winds around the lateral aspect of the radius, passing through the supinator muscle, and then courses on the interosseous membrane to innervate all the posterior forearm muscles. Panel A: Radial artery coursing under brachioradialis then becoming superficial distally with pulse point at wrist, ulnar artery passing deep to pronator teres and traveling with ulnar nerve to Guyon's canal. Panel B: Common interosseous artery dividing into anterior and posterior interosseous arteries, median nerve passing between pronator teres heads coursing between FDS and FDP. Panel C: Median nerve giving off anterior interosseous nerve and entering carpal tunnel, ulnar nerve entering between FCU heads and innervating FCU and medial FDP. Panel D: Radial nerve division with superficial branch to dorsum and deep branch (posterior interosseous) penetrating supinator, cross-sections at forearm levels showing structural relationships. --- ## Clinical Correlations Understanding the anatomy of the arm and forearm enables recognition and treatment of common clinical conditions. The vulnerability of specific nerves and tendons to injury reflects their anatomical relationships. ### Lateral Epicondylitis (Tennis Elbow) Lateral epicondylitis is an overuse injury affecting the common extensor origin at the lateral epicondyle. The extensor carpi radialis brevis is most commonly involved. Patients experience pain at the lateral elbow that worsens with gripping, wrist extension, or lifting with the palm down. Physical examination reveals tenderness at the lateral epicondyle and pain with resisted wrist extension (Cozen's test). Treatment includes rest, anti-inflammatory measures, bracing, and physiotherapy; refractory cases may require surgical intervention. ### Medial Epicondylitis (Golfer's Elbow) Medial epicondylitis affects the common flexor origin at the medial epicondyle. It is less common than lateral epicondylitis. Patients experience medial elbow pain that worsens with gripping, wrist flexion, or pronation. Treatment parallels that of lateral epicondylitis. The proximity of the ulnar nerve must be considered if surgical treatment is required. ### Radial Nerve Injury in the Spiral Groove The radial nerve is vulnerable to injury where it passes in the spiral groove of the humerus, particularly with humeral shaft fractures. The resultant "wrist drop" reflects loss of wrist and finger extension. Patients cannot extend the wrist, fingers, or thumb against resistance. The brachioradialis reflex is lost. Sensory loss occurs over the posterior arm, posterior forearm, and dorsal hand. Recovery depends on the nature of the injury; many patients recover spontaneously with neurapraxic injuries, while more severe injuries may require surgical intervention. ### Median Nerve Injury in the Forearm Proximal median nerve injury (at or above the elbow) affects most forearm flexors. Patients lose pronation, wrist flexion (partially preserved by flexor carpi ulnaris), and finger flexion (except in digits 4-5, preserved by ulnar-innervated flexor digitorum profundus). The "hand of benediction" sign appears when attempting to make a fist: the index and middle fingers remain extended because their flexor digitorum profundus and superficialis muscles are paralyzed. Anterior interosseous syndrome represents isolated injury to this motor branch. It causes weakness of the flexor pollicis longus, lateral flexor digitorum profundus, and pronator quadratus without sensory loss. Patients cannot make an "OK" sign because they cannot flex the thumb IP joint and index finger DIP joint. ### Ulnar Nerve Injury at the Elbow The ulnar nerve is vulnerable where it passes posterior to the medial epicondyle in the cubital tunnel. This is the site of the "funny bone" sensation when the nerve is bumped. Cubital tunnel syndrome causes progressive ulnar nerve dysfunction from chronic compression or repetitive trauma. Patients experience numbness in the little finger and medial half of the ring finger, weakness of intrinsic hand muscles (late finding), and weakness of flexor carpi ulnaris and medial flexor digitorum profundus. In severe cases, a "claw hand" deformity develops (hyperextension at MCP joints with flexion at IP joints in digits 4-5), though this is more pronounced with distal ulnar nerve lesions because the flexor digitorum profundus remains functional. Panel A: Lateral epicondylitis showing inflammation at common extensor origin with pain radiation pattern and resisted wrist extension test arrow, medial epicondylitis on medial side with ulnar nerve proximity indicated. Panel B: Radial nerve injury showing arm with fracture at humeral shaft, spiral groove highlighted, resultant wrist drop with hand dangling and inability to extend fingers. Panel C: Median nerve injury with hand of benediction sign showing attempt to make fist with index and middle fingers extended while ring and small fingers flexed. Panel D: Ulnar nerve at elbow with cubital tunnel highlighted in posterior view, claw hand deformity in digits 4-5 with hyperextended MCP and flexed IP joints, nerve pathways color-coded. --- ## Summary The arm and forearm represent a sophisticated biomechanical system that positions the hand for function. The arm contains two muscular compartments—anterior flexors supplied by the musculocutaneous nerve and posterior extensors supplied by the radial nerve—acting across the elbow joint. The elbow itself is a compound joint containing three articulations within a single capsule: the hinge-like humeroulnar joint, the humeroradial joint, and the proximal radioulnar joint. The forearm continues this compartmental organization. The anterior flexor compartment, primarily innervated by the median nerve (with ulnar nerve contribution to flexor carpi ulnaris and medial flexor digitorum profundus), contains three layers of muscles that flex the wrist and fingers and pronate the forearm. The posterior extensor compartment, innervated entirely by the radial nerve and its posterior interosseous branch, extends the wrist and fingers and includes the supinator muscle. Pronation and supination occur at the proximal and distal radioulnar joints, linked by the interosseous membrane. In supination, the radius and ulna are parallel; in pronation, the radius crosses over the ulna. The major neurovascular structures—the brachial artery dividing into radial and ulnar arteries, and the musculocutaneous, radial, median, and ulnar nerves—course through these regions with predictable relationships that explain their vulnerability to specific injuries. The radial nerve in the spiral groove, the ulnar nerve at the medial epicondyle, and the median nerve in the carpal tunnel represent classic sites of nerve vulnerability. --- ## Key Terms | Term | Definition | |------|------------| | Brachialis | The main elbow flexor, lying deep to the biceps and inserting on the ulna | | Annular ligament | A fibrous ring encircling the radial head, allowing rotation while maintaining stability | | Anterior interosseous nerve | Pure motor branch of the median nerve supplying deep forearm flexors | | Posterior interosseous nerve | Deep motor branch of the radial nerve supplying all posterior forearm muscles | | Cubital fossa | Triangular depression on the anterior elbow containing the biceps tendon, brachial artery, and median nerve | | Interosseous membrane | Fibrous sheet connecting the radius and ulna, transmitting forces and providing muscle attachment | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*