Medical School · Year 1 · Anatomy Msk · includes a quiz and discussion video

Lecture 8: Upper Limb - Hand

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 wrist (carpals) and hand (metacarpals, phalanges)
2. Identify the compartments and muscles of the hand
3. Describe the carpal tunnel and its contents
4. Explain the extensor expansion mechanism
5. Trace the paths of nerves and vessels in the hand
6. Correlate anatomical knowledge with common hand injuries and syndromes

Bones of the Hand

The hand is a marvel of skeletal engineering, containing 27 bones that enable the remarkable dexterity essential for human function. These bones are organized into three groups: the carpal bones of the wrist, the metacarpal bones of the palm, and the phalanges of the digits.

The Carpal Bones

Eight small bones arranged in two transverse rows form the carpus. These bones articulate with each other, with the forearm proximally, and with the metacarpals distally. Understanding their arrangement is fundamental for recognizing injury patterns and performing clinical examination.

The proximal row, from lateral (thumb side) to medial, consists of the scaphoid, lunate, triquetrum, and pisiform. The scaphoid (from Greek "boat-shaped") is the largest bone of this row and spans both carpal rows, making it crucial for wrist mechanics. It is also the most frequently fractured carpal bone, typically from falls onto an outstretched hand. The lunate (crescent-shaped) articulates directly with the radius and is the second most commonly dislocated carpal bone. The triquetrum (three-cornered) articulates with the articular disc rather than directly with the ulna. The pisiform is unique—a small, pea-shaped sesamoid bone that develops within the tendon of the flexor carpi ulnaris and does not participate in the radiocarpal joint.

The distal row, again from lateral to medial, comprises the trapezium, trapezoid, capitate, and hamate. The trapezium features a distinctive saddle-shaped facet for articulation with the first metacarpal, enabling the thumb's remarkable range of motion. The trapezoid is the smallest carpal bone and is wedge-shaped. The capitate is the largest carpal bone, centrally positioned, and articulates with the third metacarpal. The hamate is recognized by its hook-like process (the hamulus), which projects palmarly and serves as an attachment for the flexor retinaculum.

A helpful mnemonic for remembering the carpal bones from lateral to medial, proximal row first then distal row, is "So Long To Pinky, Here Comes The Thumb" (Scaphoid, Lunate, Triquetrum, Pisiform, Hamate, Capitate, Trapezoid, Trapezium).

The Metacarpal Bones

Five metacarpal bones form the skeletal framework of the palm. They are numbered 1 through 5 from thumb to little finger. Each metacarpal consists of a base (proximal end articulating with carpals), a shaft, and a head (distal end articulating with the proximal phalanx). The heads of the metacarpals form the knuckles visible when the fist is clenched.

The first metacarpal is distinctive: it is the shortest and most mobile, set at an angle to the others. This positioning, along with the saddle joint at its base, enables opposition of the thumb—a movement fundamental to the precision grip that distinguishes human hand function. The fifth metacarpal has slightly more mobility than metacarpals 2-4, allowing the palm to cup for grasping.

The Phalanges

The 14 phalanges form the skeleton of the digits. The thumb (pollex) has only two phalanges—proximal and distal—while each of the remaining four digits has three: proximal, middle, and distal. Each phalanx has a base, shaft, and head (trochlea).

This arrangement means the thumb, despite having one fewer joint, retains remarkable functionality because of its unique position and the saddle joint at its metacarpal base. The interphalangeal joints of the fingers (proximal and distal) are pure hinge joints, while the metacarpophalangeal joints allow more complex movements including abduction and adduction.

<image>Panel A: Palmar view of hand skeleton with proximal carpal row in blue gradient (scaphoid, lunate, triquetrum, pisiform) and distal carpal row in green gradient (trapezium, trapezoid, capitate, hamate with hook). Panel B: Metacarpals 1-5 numbered and color-coded showing bases, shafts, and heads (knuckles) with joints indicated at each articulation. Panel C: Phalanges labeled with thumb showing proximal and distal only, digits 2-5 with proximal, middle, and distal phalanges. Panel D: Dorsal view inset for comparison showing all bones from posterior perspective with anatomical position and scale bar.</image>

Joints of the Wrist and Hand

The wrist and hand contain numerous joints of varying complexity, each contributing to the hand's remarkable functional capacity. These range from the nearly immobile intercarpal joints to the highly mobile saddle joint of the thumb.

The Radiocarpal (Wrist) Joint

The radiocarpal joint is an ellipsoid synovial joint formed between the distal end of the radius (and the articular disc covering the distal ulna) and the proximal carpal row (scaphoid, lunate, and triquetrum—the pisiform does not participate). This joint configuration allows movement in two planes: flexion-extension and radial-ulnar deviation (abduction-adduction). The combination produces circumduction. Notably, the ulna does not directly articulate with the carpals; the articular disc of the triangular fibrocartilage complex intervenes.

The Intercarpal and Midcarpal Joints

The intercarpal joints are plane synovial joints between adjacent carpal bones within each row, allowing limited gliding movements. The midcarpal joint is the articulation between the proximal and distal carpal rows. Although these joints individually permit only small movements, together they contribute significantly to wrist motion—approximately 50% of wrist flexion and much of extension occur at the midcarpal level.

The Carpometacarpal Joints

The carpometacarpal (CMC) joints connect the distal carpal row to the bases of the metacarpals. The CMC joint of the thumb (first CMC) is particularly important—it is a saddle joint between the trapezium and first metacarpal. This unique configuration allows flexion-extension, abduction-adduction, and the combined movement of opposition (touching the thumb pad to the fingertips). Arthritis of this joint is common and significantly impairs hand function.

The CMC joints of digits 2-5 are plane synovial joints with minimal movement. However, the fourth and fifth CMC joints have slightly more mobility, allowing the palm to cup around objects during grasping.

The Metacarpophalangeal Joints

The metacarpophalangeal (MCP) joints are condylar synovial joints between the metacarpal heads and the proximal phalanx bases. These joints permit flexion-extension, abduction-adduction, and circumduction. The collateral ligaments of these joints are slack in extension and taut in flexion—this has clinical significance because immobilizing the hand with MCP joints in extension risks ligament shortening and permanent limitation of flexion.

The Interphalangeal Joints

The interphalangeal (IP) joints are hinge synovial joints allowing only flexion and extension. The fingers have two IP joints each: the proximal interphalangeal (PIP) joint between proximal and middle phalanges, and the distal interphalangeal (DIP) joint between middle and distal phalanges. The thumb has a single IP joint between its proximal and distal phalanges.

<image>Panel A: Coronal section through radiocarpal joint showing distal radius and articular disc articulating with proximal carpal row (scaphoid, lunate, triquetrum), ulna not directly contacting carpals. Panel B: First CMC (thumb) saddle joint in detail with trapezium saddle-shaped surface engaging first metacarpal, axes of movement indicated for flexion-extension, abduction-adduction, and opposition. Panel C: MCP joint showing condylar surfaces with collateral ligaments slack in extension and taut in flexion. Panel D: IP joints as simple hinge joints with hinge axis marked, joint capsules in light blue and articular cartilage in white.</image>

The Carpal Tunnel

The carpal tunnel is perhaps the most clinically significant anatomical space in the hand, as it represents a common site of nerve compression.

Boundaries

The carpal tunnel is an osseofibrous channel on the palmar aspect of the wrist. Its floor and walls are formed by the carpal arch—the concave arrangement of the carpal bones. The roof is the flexor retinaculum (transverse carpal ligament), a thick fibrous band that spans the carpal arch.

The flexor retinaculum attaches laterally to the scaphoid tubercle and the trapezium (at its crest), and medially to the pisiform and the hook of the hamate. This unyielding fibrous roof means that any increase in the tunnel's contents—from tendon swelling, fluid accumulation, or other causes—increases pressure on the structures within.

Contents

The carpal tunnel contains ten structures: nine flexor tendons and one nerve. The tendons include the four tendons of flexor digitorum superficialis, the four tendons of flexor digitorum profundus, and the single tendon of flexor pollicis longus. These tendons are invested by synovial sheaths that facilitate gliding.

The median nerve is the most vulnerable structure, lying immediately deep to (posterior to) the flexor retinaculum, typically between the tendons of flexor digitorum superficialis and flexor pollicis longus. Its superficial position within the tunnel makes it susceptible to compression.

It is worth noting that the tendon of flexor carpi radialis, while passing through the region, travels in its own separate compartment within a groove on the trapezium, not through the carpal tunnel proper.

Clinical Significance: Carpal Tunnel Syndrome

Carpal tunnel syndrome results from compression of the median nerve within this confined space. Patients experience pain, numbness, and tingling in the lateral 3.5 digits (thumb, index, middle, and lateral half of the ring finger—the median nerve territory). Symptoms are often worse at night and may be provoked by sustained wrist flexion.

Physical examination may reveal positive Phalen's test (symptoms reproduced by holding the wrist in maximal flexion for 60 seconds) or Tinel's sign (tingling elicited by tapping over the carpal tunnel). In advanced cases, thenar muscle atrophy becomes evident as the recurrent branch of the median nerve, which supplies the thenar muscles, is affected.

<image>Panel A: Cross-sectional view through the carpal tunnel at the distal carpal row showing flexor retinaculum as thick band forming the roof, with carpal bones forming floor and walls. Panel B: Contents arranged within tunnel including FDS tendons (four, in red), FDP tendons (four, deeper, in dark red), FPL tendon (lateral, in orange), and median nerve positioned superficially between FPL and FDS. Panel C: Synovial sheaths around tendon groups with FCR tendon in separate trapezial groove compartment labeled as outside the carpal tunnel, and ulnar nerve and artery passing through Guyon's canal. Panel D: Phalen's test position inset demonstrating wrist flexion provocation for carpal tunnel syndrome diagnosis.</image>

Guyon's Canal

Guyon's canal (the ulnar canal) is a fibroosseous tunnel on the medial aspect of the wrist through which the ulnar nerve and artery pass to enter the hand.

Location and Boundaries

Guyon's canal lies medial and superficial to the carpal tunnel. Unlike the carpal tunnel, its roof is the superficial palmar carpal ligament and palmaris brevis muscle, while its floor is formed by the flexor retinaculum and the hypothenar muscles. The medial wall is formed by the pisiform bone, and the lateral wall by the hook of the hamate.

Contents

The canal contains the ulnar nerve and the ulnar artery. Within the canal, the ulnar nerve divides into its superficial branch (primarily sensory, supplying the medial palm and medial 1.5 digits) and deep branch (primarily motor, supplying most intrinsic hand muscles).

Clinical Significance

Compression of the ulnar nerve at Guyon's canal produces symptoms distinct from cubital tunnel syndrome (compression at the elbow). Because the dorsal cutaneous branch of the ulnar nerve arises proximal to the wrist, sensation on the dorsum of the hand is preserved in Guyon's canal compression but lost in cubital tunnel syndrome—a useful distinguishing feature.

"Handlebar palsy" is a form of ulnar nerve compression at Guyon's canal seen in cyclists who sustain prolonged pressure on the hypothenar region while gripping handlebars.

Intrinsic Muscles of the Hand

The intrinsic muscles of the hand are those muscles that both originate and insert within the hand. They are responsible for the fine motor control that distinguishes human hand function. These muscles are organized into three groups: the thenar muscles (thumb), the hypothenar muscles (little finger), and the muscles of the central compartment (lumbricals and interossei).

The Thenar Muscles

The thenar eminence, the fleshy mound at the base of the thumb, contains four muscles. Three are innervated by the median nerve (via its recurrent branch), while one—the adductor pollicis—is innervated by the ulnar nerve.

The abductor pollicis brevis is the most superficial thenar muscle. It originates from the flexor retinaculum, scaphoid tubercle, and trapezium, inserting onto the lateral aspect of the base of the proximal phalanx of the thumb. It abducts the thumb at the carpometacarpal joint, moving the thumb anteriorly (away from the palm) in a plane perpendicular to the palm. This muscle is often the first to show wasting in carpal tunnel syndrome.

The flexor pollicis brevis lies partly deep to the abductor. Its superficial head originates from the flexor retinaculum and trapezium, inserting onto the lateral base of the proximal phalanx. It flexes the thumb at the MCP joint. While typically innervated by the median nerve, it may have contributions from the ulnar nerve.

The opponens pollicis is the deepest of the median nerve-supplied thenar muscles. It originates from the flexor retinaculum and trapezium and inserts along the entire lateral border of the first metacarpal. Its action, opposition, is a complex movement combining flexion, adduction, and medial rotation at the CMC joint, enabling the thumb pad to touch the pads of the other fingers.

The adductor pollicis is the largest and deepest thenar muscle, with two heads. The oblique head arises from the capitate and bases of metacarpals 2 and 3, while the transverse head arises from the palmar surface of the third metacarpal shaft. Both heads insert onto the medial aspect of the base of the proximal phalanx through a sesamoid bone. The adductor pollicis is innervated by the deep branch of the ulnar nerve, distinguishing it from the other thenar muscles. Its action—adducting the thumb—is tested clinically by asking the patient to hold a piece of paper between the thumb and palm; weakness produces Froment's sign (flexion of the thumb IP joint to compensate using the median nerve-innervated flexor pollicis longus).

<image>Panel A: Superficial thenar layer with abductor pollicis brevis (pink) originating from flexor retinaculum and scaphoid tubercle, inserting on lateral proximal phalanx base, and middle layer flexor pollicis brevis (red) with origin from retinaculum and trapezium. Panel B: Deep layer showing opponens pollicis (maroon) inserting along entire lateral metacarpal 1 shaft with nerve supply color-coded (median recurrent branch in yellow). Panel C: Adductor pollicis (purple) with oblique head from capitate and metacarpal bases and transverse head from metacarpal 3, converging on medial proximal phalanx with sesamoid bone, innervated by ulnar deep branch. Panel D: Froment's sign test inset demonstrating adductor pollicis weakness assessment with movement arrows showing muscle actions.</image>

The Hypothenar Muscles

The hypothenar eminence, the fleshy mound on the medial side of the palm at the base of the little finger, contains three muscles analogous to the thenar muscles. All three are innervated by the deep branch of the ulnar nerve.

The abductor digiti minimi is the most superficial and medial hypothenar muscle. It originates from the pisiform and the pisohamate ligament, inserting onto the medial side of the base of the proximal phalanx of the little finger. It abducts the little finger—spreading it away from the ring finger.

The flexor digiti minimi brevis lies lateral to the abductor. It originates from the flexor retinaculum and the hook of the hamate, inserting onto the medial base of the proximal phalanx. It flexes the little finger at the MCP joint.

The opponens digiti minimi is the deepest hypothenar muscle. It shares origins with the flexor digiti minimi brevis and inserts along the medial border of the fifth metacarpal. Its action cups the palm by rotating the fifth metacarpal toward the thumb, enabling a more secure grip.

<image>Panel A: Superficial hypothenar layer with abductor digiti minimi (light blue) arising from pisiform, inserting on medial proximal phalanx base of digit 5, with action arrows showing abduction. Panel B: Middle layer flexor digiti minimi brevis (medium blue) arising from hook of hamate and flexor retinaculum with flexion action arrows. Panel C: Deep layer opponens digiti minimi (dark blue) inserting along medial fifth metacarpal shaft, all three muscles shown with ulnar nerve deep branch innervation in purple. Panel D: Comparison inset showing thenar and hypothenar eminences on palmar surface with hook of hamate and pisiform bones highlighted as key landmarks.</image>

The Lumbricals

The four lumbrical muscles are unique among skeletal muscles in that they have no bony attachment—they originate from tendons and insert into an aponeurotic expansion. Specifically, they arise from the tendons of the flexor digitorum profundus within the palm.

The first and second lumbricals (to digits 2 and 3) are unipennate, each arising from the lateral side of a single FDP tendon. The third and fourth lumbricals (to digits 4 and 5) are bipennate, each arising from adjacent sides of two FDP tendons. All four lumbricals pass on the lateral side of the MCP joints to insert into the lateral bands of the extensor expansions of digits 2-5.

The lumbricals perform the seemingly paradoxical action of flexing the MCP joints while extending the IP joints. This is possible because they pass palmar to the MCP joints (allowing flexion) and then insert into the extensor expansion, which extends the IP joints. This combined action is essential for fine motor tasks like writing.

The innervation follows a pattern: lumbricals 1 and 2 (to digits 2-3) are innervated by the median nerve, while lumbricals 3 and 4 (to digits 4-5) are innervated by the ulnar nerve. This follows the same pattern as the flexor digitorum profundus tendons from which they arise.

The Interossei

The interossei are the deepest intrinsic muscles of the hand, located between the metacarpal bones. There are two groups: four dorsal interossei and three palmar interossei.

The dorsal interossei are bipennate muscles, each arising from the adjacent sides of two metacarpals. There are four, located in the spaces between all five metacarpals. They insert into the bases of the proximal phalanges and the extensor expansions of digits 2-4. Their action is to abduct the fingers—spreading them away from the middle finger (the axis of the hand). The mnemonic "DAB" (Dorsal ABduct) helps remember this function.

The palmar interossei are three unipennate muscles arising from the palmar surfaces of metacarpals 2, 4, and 5 (there is no palmar interosseous from metacarpal 3, as the middle finger is the axis). They insert into the extensor expansions of the same digits. Their action is to adduct the fingers—drawing them toward the middle finger. The mnemonic "PAD" (Palmar ADduct) applies here.

Like the lumbricals, the interossei also contribute to MCP flexion and IP extension through their attachment to the extensor expansion. All interossei are innervated by the deep branch of the ulnar nerve.

<image>Panel A: Dorsal view showing four dorsal interossei as bipennate muscles between metacarpals, color-coded DI 1-4, arising from adjacent metacarpal sides with arrows showing abduction away from the middle finger axis, "DAB" mnemonic labeled. Panel B: Palmar view showing three palmar interossei as unipennate muscles on palmar surfaces of metacarpals 2, 4, and 5 with arrows showing adduction toward the middle finger axis, "PAD" mnemonic labeled. Panel C: Cross-section at metacarpal level showing position of both groups with insertions into extensor expansions and proximal phalanges, ulnar nerve deep branch innervation in purple. Panel D: Lumbricals arising from FDP tendons with median nerve innervation to digits 2-3 and ulnar nerve innervation to digits 4-5 indicated.</image>

The Extensor Expansion

The extensor expansion (also called the extensor hood or dorsal digital expansion) is a complex aponeurotic structure on the dorsum of each finger that coordinates extension of the MCP and IP joints. Understanding this mechanism is essential for comprehending normal finger movement and recognizing deformities that result from injury.

Structure

The extensor expansion forms a hood-like covering over the dorsum of the MCP joint and the proximal phalanx. The extensor digitorum tendon enters the expansion proximally and divides into three slips: a central slip and two lateral slips.

The central slip inserts onto the base of the middle phalanx. The two lateral slips continue distally, receive contributions from the interossei and lumbricals, and reunite to form the terminal tendon that inserts onto the base of the distal phalanx.

Contributions and Function

The extensor digitorum provides the primary extensor force, with its central slip extending the PIP joint and its lateral slips (via the terminal tendon) extending the DIP joint. However, the extensor digitorum alone cannot fully extend the IP joints when the MCP joint is in certain positions.

The lumbricals and interossei insert into the lateral bands of the expansion. By passing palmar to the MCP joint axis but dorsal to the IP joint axes, they simultaneously flex the MCP joint and extend the IP joints. This "lumbrical position"—MCP flexion with IP extension—is the position of function used in many fine motor activities.

Clinical Considerations

Injuries to the extensor expansion produce characteristic deformities. A boutonnière deformity results when the central slip is disrupted (as from a laceration or inflammatory destruction). The lateral bands displace palmarly, and the PIP joint falls into flexion while the DIP joint becomes hyperextended.

A swan neck deformity shows the opposite pattern: PIP hyperextension with DIP flexion. This can result from various causes including volar plate laxity, intrinsic muscle tightness, or mallet finger (see below).

Mallet finger occurs when the terminal extensor tendon is disrupted, typically from forced flexion of an extended DIP joint ("jammed finger"). The DIP joint remains in a flexed position because there is no extensor force to the distal phalanx.

<image>Panel A: Lateral view of digit showing extensor digitorum tendon entering the expansion and dividing into central slip inserting on middle phalanx base and lateral slips, with interosseous and lumbrical muscles inserting into lateral bands forming terminal tendon at distal phalanx base. Panel B: Hood over MCP joint in semi-transparent overlay with movement arrows showing central slip extending PIP, terminal tendon extending DIP, and lumbricals/interossei flexing MCP while extending IP. Panel C: Deformity insets showing boutonniere (PIP flexed, DIP hyperextended with central slip rupture), swan neck (PIP hyperextended, DIP flexed), and mallet finger (DIP dropped with terminal tendon disruption). Panel D: Dorsal view showing expansion as hood structure over all fingers with color-coded legend for each component.</image>

Fascial Compartments of the Hand

The palm contains several fascial structures that organize its contents and have significant clinical implications.

The Palmar Aponeurosis

The palmar aponeurosis is a thick, triangular sheet of fibrous tissue in the central palm. It is continuous proximally with the flexor retinaculum and the tendon of palmaris longus (when present). Distally, it sends slips to each finger that blend with the fibrous flexor sheaths.

This aponeurosis protects the underlying flexor tendons, nerves, and vessels during gripping. It also anchors the palmar skin, preventing excessive mobility that would impair grip.

Dupuytren's contracture is a fibroproliferative disorder of the palmar aponeurosis. Progressive thickening and contracture of the aponeurosis, forming nodules and cords, gradually draws the fingers into flexion at the MCP and PIP joints. The ring and little fingers are most commonly affected. Risk factors include male sex, northern European ancestry, diabetes mellitus, alcoholism, and smoking.

The Fibrous Flexor Sheaths

Each digit has a fibrous flexor sheath that forms a tunnel extending from the metacarpal head to the distal phalanx. These tunnels contain the flexor tendons (FDS and FDP for digits 2-5; FPL for the thumb) and are lined by synovial membrane.

The sheaths have a series of pulleys—thickenings that hold the tendons close to the bone, maintaining mechanical efficiency. The annular pulleys (A1-A5) are the strongest and most critical; cruciform pulleys (C1-C3) lie between them. The A2 and A4 pulleys, overlying the proximal and middle phalanges respectively, are essential for normal finger function and should be preserved during surgery.

Trigger finger (stenosing tenosynovitis) typically occurs at the A1 pulley at the level of the metacarpal head. Thickening of the tendon sheath or a nodule on the tendon causes catching or locking of the finger during flexion. Treatment ranges from splinting and corticosteroid injection to surgical release of the A1 pulley.

The Synovial Flexor Sheaths

Within the fibrous sheaths, the tendons are invested by synovial sheaths that produce fluid for lubrication. The arrangement of these sheaths has clinical importance for the spread of infection.

In the fingers, each digit has a separate digital synovial sheath extending from the DIP joint to approximately the level of the metacarpal neck. The thumb has its own sheath (the radial bursa) extending from the distal phalanx, through the carpal tunnel, to end in the forearm.

A common flexor sheath (ulnar bursa) surrounds the FDS and FDP tendons through the carpal tunnel. The digital sheath of the little finger typically communicates with this common sheath. The clinical implication: infection in the little finger's sheath can spread through the ulnar bursa, and infection of the thumb sheath can spread through the radial bursa—both potentially reaching the forearm as a flexor tenosynovitis.

<image>Panel A: Palmar aponeurosis shown as triangular sheet in central palm, continuous with flexor retinaculum proximally and palmaris longus tendon, sending slips to each digit distally, with Dupuytren's contracture inset showing nodules and cords causing finger flexion. Panel B: Sagittal section through digit showing fibrous flexor sheath as tunnel containing FDS and FDP tendons with annular pulleys A1-A5 and cruciform pulleys C1-C3, A2 and A4 emphasized. Panel C: Trigger finger inset showing A1 pulley with thickened tendon or nodule catching during flexion. Panel D: Palmar view showing synovial sheaths with individual digital sheaths (light blue), radial bursa (green) extending from thumb through carpal tunnel, and ulnar/common flexor bursa (yellow) with communication to fifth digit sheath and infection spread pathways indicated.</image>

Neurovascular Structures of the Hand

The hand has a rich blood supply from two palmar arterial arches and receives innervation from three major nerves.

Arterial Supply

The superficial palmar arch is formed primarily by the ulnar artery, which enters the palm through Guyon's canal and curves laterally across the palm. It is typically completed by the superficial palmar branch of the radial artery (or by a branch of the princeps pollicis). The arch lies deep to the palmar aponeurosis but superficial to the flexor tendons and digital nerves. It gives rise to common palmar digital arteries, which divide into proper palmar digital arteries supplying the adjacent sides of the fingers.

The deep palmar arch is formed primarily by the radial artery, which enters the palm by passing through the anatomical snuffbox, between the two heads of the first dorsal interosseous muscle. It is completed by the deep branch of the ulnar artery. The deep arch lies deep to the flexor tendons, on the bases of the metacarpals and the interossei. It gives rise to palmar metacarpal arteries, which join the common palmar digital arteries from the superficial arch.

The princeps pollicis artery arises from the radial artery and supplies the thumb. The radialis indicis artery, also from the radial artery, supplies the lateral side of the index finger.

Venous Drainage

The venous drainage of the hand is predominantly through dorsal vessels. Digital veins drain into the dorsal venous network on the back of the hand, which gives rise to the cephalic vein (laterally) and the basilic vein (medially).

<image>Panel A: Superficial palmar arch (dark red) as dominant arc from ulnar artery curving across mid-palm, giving rise to common palmar digital arteries dividing into proper palmar digital arteries to adjacent finger sides, with Guyon's canal entry point marked. Panel B: Deep palmar arch from radial artery entering through first interosseous space, giving palmar metacarpal arteries joining common digital arteries, with princeps pollicis and radialis indicis arteries labeled. Panel C: Anastomoses between superficial and deep arches indicated with connecting vessels, and anatomical snuffbox entry for radial artery shown in separate inset. Panel D: Dorsal venous network shown on dorsal view inset with origins of cephalic and basilic veins labeled, color intensity indicating depth.</image>

Nerve Supply of the Hand

Three nerves provide motor and sensory innervation to the hand: the median, ulnar, and radial nerves. Understanding their territories is essential for clinical examination and localizing lesions.

The Median Nerve in the Hand

The median nerve enters the hand through the carpal tunnel, immediately giving off the recurrent (thenar) branch. This crucial motor branch curves laterally to innervate the abductor pollicis brevis, flexor pollicis brevis (superficial head), and opponens pollicis. Its superficial course makes it vulnerable to injury from lacerations at the thenar crease.

The median nerve continues as palmar digital nerves that provide sensory innervation to the palmar surface of the lateral 3.5 digits (thumb, index, middle, and lateral half of the ring finger) and motor innervation to the first and second lumbricals. The digital nerves also supply the dorsal skin of these digits distal to the nail beds.

The Ulnar Nerve in the Hand

The ulnar nerve enters the hand through Guyon's canal and divides into superficial and deep branches. The superficial branch is primarily sensory, supplying the palmar surface of the medial 1.5 digits (little finger and medial half of the ring finger).

The deep branch is the major motor nerve of the hand. It curves around the hook of the hamate to enter the deep palm, where it supplies the hypothenar muscles (abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi), all interossei (palmar and dorsal), the third and fourth lumbricals, and the adductor pollicis.

The Radial Nerve in the Hand

The radial nerve has no motor function in the hand—all hand extensors are innervated in the forearm by the posterior interosseous nerve. The superficial branch of the radial nerve is purely sensory in the hand, supplying the dorsum of the lateral (radial) part of the hand and the dorsum of the lateral 2.5 digits proximal to the nail beds.

Sensory Distribution Summary

The palmar surface is divided between the median nerve (lateral 3.5 digits) and the ulnar nerve (medial 1.5 digits). The fingertips, including the nail beds, are supplied by the palmar digital nerves regardless of whether the dorsal proximal digit is in radial or ulnar territory.

The dorsal surface shows a different pattern: the radial nerve supplies the lateral side (lateral 2.5 digits proximally), the ulnar nerve supplies the medial side (medial 2.5 digits), and the fingertips are again supplied by the palmar digital nerves.

<image>Panel A: Palmar view showing median nerve territory (yellow shading) covering thumb, index, middle, and lateral half of ring finger, and ulnar nerve territory (purple shading) covering little finger and medial half of ring finger with boundary at midline of ring finger. Panel B: Dorsal view showing radial nerve territory (green shading) covering lateral dorsum and proximal lateral 2.5 digits, ulnar nerve territory (purple) covering medial dorsum and medial 2.5 digits, and fingertips supplied by palmar digital nerves. Panel C: Motor territory schematic showing median recurrent branch to thenar muscles (except adductor) and ulnar deep branch to hypothenar, interossei, lumbricals 3-4, and adductor pollicis. Panel D: Color-coded legend for each nerve with summary of sensory and motor distributions across both surfaces of the hand.</image>

Clinical Correlations

Carpal Tunnel Syndrome

Carpal tunnel syndrome is the most common peripheral nerve compression syndrome. Symptoms include numbness, tingling, and pain in the median nerve distribution, often worse at night and exacerbated by activities that maintain wrist flexion or extension. Patients may describe shaking their hands to relieve symptoms ("flick sign").

Physical examination should include Phalen's test (hold wrist in maximal flexion for 60 seconds; positive if symptoms are reproduced), Tinel's sign (tapping over the carpal tunnel produces tingling), and assessment for thenar atrophy (weakness of abductor pollicis brevis). Electrodiagnostic studies confirm the diagnosis.

Ulnar Nerve Injury at the Wrist

Injury to the ulnar nerve at or distal to the wrist produces a characteristic "claw hand" deformity in the fourth and fifth digits. The MCP joints are hyperextended (due to unopposed action of the extensor digitorum without lumbrical and interosseous counterbalance), while the IP joints are flexed (due to the unbalanced action of the FDP).

Interestingly, the claw deformity is more pronounced with distal lesions than with proximal ones (the "ulnar paradox") because with proximal lesions, the FDP to digits 4 and 5 is also paralyzed, reducing the IP flexion component.

Scaphoid Fracture

The scaphoid is the most commonly fractured carpal bone, typically from falls onto an outstretched hand with the wrist dorsiflexed. Patients present with wrist pain, particularly in the anatomical snuffbox, and may have surprisingly subtle initial radiographs.

The scaphoid's blood supply enters primarily through the distal pole, meaning fractures through the waist or proximal pole can compromise blood flow to the proximal fragment. This creates a significant risk of avascular necrosis and nonunion, requiring vigilant monitoring and often surgical fixation.

Dupuytren's Contracture

Dupuytren's contracture causes progressive, painless flexion contractures of the digits, most commonly the ring and little fingers. Palpable nodules and cords develop in the palmar aponeurosis and its extensions. The condition is treated surgically when contractures impair hand function, though recurrence is common.

Trigger Finger

Trigger finger presents with catching, locking, or snapping of a digit during flexion. Patients may need to passively extend the affected finger. The pathology is at the A1 pulley, where thickening of the tendon sheath or a nodule on the tendon impedes smooth gliding. Treatment options include splinting, corticosteroid injection, and surgical release of the A1 pulley.

Mallet Finger

Mallet finger results from disruption of the terminal extensor tendon insertion, usually from forced flexion of an extended DIP joint during ball sports. The patient cannot actively extend the DIP joint, which remains in a flexed posture. Treatment typically involves continuous splinting of the DIP joint in extension for 6-8 weeks.

<image>Panel A: Carpal tunnel syndrome with cross-section showing compressed median nerve in tunnel, palmar sensory loss pattern over lateral 3.5 digits, and thenar atrophy; ulnar claw hand showing fourth and fifth digits with MCP hyperextension and IP flexion compared to normal. Panel B: Scaphoid fracture with anatomical snuffbox tenderness point marked, plain radiograph showing waist fracture, and blood supply diagram showing AVN risk to proximal pole. Panel C: Dupuytren's contracture in palmar view with nodules and cords visible in palmar aponeurosis drawing ring and little fingers into flexion. Panel D: Trigger finger showing sagittal section of A1 pulley with thickened tendon nodule in locking position, and mallet finger showing lateral view with terminal tendon rupture, dropped DIP joint, and splint treatment position.</image>

The Anatomical Snuffbox

The anatomical snuffbox is a triangular depression on the lateral aspect of the wrist, visible when the thumb is extended. Its name derives from its historical use as a surface for placing snuff before inhalation.

Boundaries

The lateral (anterior) border is formed by the tendons of the abductor pollicis longus and extensor pollicis brevis, which run closely together. The medial (posterior) border is formed by the tendon of the extensor pollicis longus. The floor is formed by the scaphoid and trapezium bones and the radial styloid process.

Contents

The radial artery crosses the floor of the snuffbox, passing from the palmar aspect of the wrist to the dorsum before diving between the two heads of the first dorsal interosseous muscle to enter the palm. This is a potential site for radial artery cannulation.

The superficial branch of the radial nerve and the cephalic vein (at its origin) cross superficially over the snuffbox.

Clinical Significance

Tenderness in the anatomical snuffbox following wrist trauma is concerning for scaphoid fracture. Given the risks of avascular necrosis with delayed treatment, patients with snuffbox tenderness but negative initial radiographs should be immobilized and have repeat imaging in 10-14 days, or undergo early CT or MRI.

<image>Panel A: Posterolateral view of wrist with thumb extended showing triangular depression of snuffbox, lateral boundary formed by APL and EPB tendons, and medial boundary formed by EPL tendon hooking around Lister's tubercle. Panel B: Floor shown in cutaway with scaphoid proximally, trapezium distally, radial styloid at proximal edge, and radial artery crossing floor between bony landmarks. Panel C: Superficial structures showing superficial radial nerve branches (yellow) and origin of cephalic vein (blue) crossing superficially, with thumb extension inset making snuffbox visible. Panel D: Clinical examination inset showing palpation technique for scaphoid tenderness at snuffbox floor with bony floor components labeled in separate bone diagram.</image>

Summary

The hand is a structurally complex and functionally sophisticated region that enables the dexterity defining human capability. Its 27 bones—8 carpals, 5 metacarpals, and 14 phalanges—are organized to provide both stability and remarkable mobility. The saddle joint at the base of the thumb is particularly important for opposition.

The carpal tunnel, an osseofibrous channel bounded by the carpal bones and flexor retinaculum, contains nine flexor tendons and the median nerve. Compression of the median nerve in this space produces carpal tunnel syndrome—the most common peripheral neuropathy.

The intrinsic muscles of the hand include the thenar group (largely median nerve, except the ulnar-innervated adductor pollicis), the hypothenar group (all ulnar nerve), and the lumbricals and interossei (mixed median and ulnar). The lumbricals and interossei produce the combined action of MCP flexion with IP extension through their insertion into the extensor expansion.

The extensor expansion is a complex aponeurotic structure that coordinates digital extension. Understanding its anatomy explains the boutonnière and swan neck deformities that result from its disruption.

The hand receives blood from two palmar arches: the superficial (ulnar dominant) and deep (radial dominant). Sensory innervation follows predictable patterns: the median nerve supplies the palmar lateral 3.5 digits, the ulnar nerve supplies the palmar medial 1.5 digits and much of the dorsum, and the radial nerve supplies the dorsolateral hand proximally.

Key Terms

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