Hand and Wrist Anatomy: Tendons, Nerves, Carpal Tunnel, and the Clinical Exam Findings

The hand and wrist contain 27 bones: 8 carpal bones in the wrist, 5 metacarpals in the palm, and 14 phalanges in the fingers (3 per finger, 2 for the thumb). Three major nerves innervate the hand: the median nerve (enters through the carpal tunnel), the ulnar nerve (enters through Guyon's canal), and the radial nerve (provides sensation to the dorsal hand but no intrinsic hand muscles). The carpal tunnel is a fibro-osseous tunnel on the palmar side of the wrist formed by the carpal bones (floor and walls) and the flexor retinaculum (roof). Contents of the carpal tunnel: the median nerve and 9 flexor tendons (4 tendons of flexor digitorum superficialis, 4 tendons of flexor digitorum profundus, and 1 tendon of flexor pollicis longus). Anything that reduces the space inside the tunnel or increases the volume of its contents (swelling, inflammation, fluid retention, repetitive use) compresses the median nerve, causing carpal tunnel syndrome — the most common peripheral nerve compression syndrome. Carpal tunnel syndrome presents with: numbness and tingling in the median nerve distribution (thumb, index finger, middle finger, and the lateral half of the ring finger — NOT the little finger), worsening at night or with repetitive wrist use, positive Phalen test (wrist flexion for 60 seconds reproduces symptoms), positive Tinel sign (tapping over the carpal tunnel at the wrist reproduces tingling), and eventual thenar muscle weakness (loss of thumb opposition) in advanced cases. The key anatomical fact that determines the clinical presentation: the median nerve innervates specific muscles and sensory areas, so median nerve compression at the wrist produces a predictable pattern. The ulnar nerve is NOT in the carpal tunnel (it passes through Guyon's canal separately), so carpal tunnel syndrome does NOT affect the little finger or the ulnar-innervated muscles. Snap a photo of any hand anatomy question and AnatomyIQ identifies the structures, traces the nerve pathways, and explains the clinical significance of each. This content is for educational purposes only and does not constitute medical advice.

The 8 carpal bones are arranged in two rows of four, articulating with the radius (and disc/ulna) proximally and the metacarpals distally. **Proximal row (lateral to medial)**: Scaphoid, Lunate, Triquetrum, Pisiform. Mnemonic: 'Some Lovers Try Positions...' **Distal row (lateral to medial)**: Trapezium, Trapezoid, Capitate, Hamate. Mnemonic: '...That They Can't Handle.' The full mnemonic for all 8: 'Some Lovers Try Positions That They Can't Handle' — Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate. **Clinically important carpal bones:** **Scaphoid**: the most commonly fractured carpal bone. Fractures typically occur from a fall on an outstretched hand (FOOSH injury). The scaphoid has a blood supply that enters distally and flows proximally — so a fracture through the waist of the scaphoid can disrupt blood supply to the proximal fragment, causing avascular necrosis (AVN). Scaphoid fractures are notorious for being missed on initial X-rays (negative X-ray does not rule out scaphoid fracture — repeat imaging or MRI may be needed). Clinical finding: tenderness in the anatomical snuffbox (the depression between the tendons of the extensor pollicis longus and extensor pollicis brevis when the thumb is extended). **Lunate**: the most commonly dislocated carpal bone. Lunate dislocations can compress the median nerve in the carpal tunnel, causing acute carpal tunnel symptoms after a wrist injury. **Hamate**: the hook of the hamate (hamulus) projects into the palm and is vulnerable to fracture from direct impact (classically from the butt of a golf club, baseball bat, or racquet handle). Hook of hamate fractures are tested on board exams and are often missed on standard X-ray views (requires a carpal tunnel view or CT scan to visualize). **The carpal tunnel anatomy**: the floor of the carpal tunnel is formed by the carpal bones. The roof is the flexor retinaculum (transverse carpal ligament) — a thick, inelastic band of fibrous tissue spanning from the scaphoid and trapezium (laterally) to the pisiform and hook of hamate (medially). The tunnel is RIGID — it cannot expand. Any increase in content volume (tendon swelling, fluid, etc.) compresses the median nerve because there is nowhere for the pressure to go. The contents (10 structures total): median nerve (most superficial, directly beneath the retinaculum — which is why it gets compressed first), 4 tendons of flexor digitorum superficialis, 4 tendons of flexor digitorum profundus, and 1 tendon of flexor pollicis longus. The flexor carpi radialis tendon does NOT pass through the carpal tunnel — it has its own separate channel in the flexor retinaculum. The ulnar nerve and ulnar artery do NOT pass through the carpal tunnel — they pass through Guyon's canal (ulnar tunnel) which is separate and superficial to the flexor retinaculum. AnatomyIQ identifies carpal bones and tunnel contents from labeled or unlabeled diagrams and generates clinical scenario questions around scaphoid fractures, lunate dislocations, and carpal tunnel syndrome.

Three nerves supply the hand, each with specific motor and sensory territories. Knowing these territories is essential for localizing nerve injuries. **Median nerve**: enters the hand through the carpal tunnel. Motor: the LOAF muscles — Lateral two lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis (superficial head). These are thenar muscles responsible for thumb opposition and precision grip. Sensory: palmar surface of the lateral 3.5 fingers (thumb, index, middle, and lateral half of ring finger). Also the dorsal fingertips of the same 3.5 fingers. Median nerve injury at the wrist (carpal tunnel or laceration): loss of thumb opposition (cannot touch thumb tip to little finger tip), thenar atrophy (flattening of the thenar eminence), loss of pinch grip, numbness in the median nerve sensory distribution. The hand posture in severe median nerve injury is called 'hand of benediction' when trying to make a fist — the index and middle fingers cannot flex fully because the lumbricals to those fingers are paralyzed. **Ulnar nerve**: enters the hand through Guyon's canal (ulnar canal), between the pisiform and the hook of the hamate. Motor: most of the intrinsic hand muscles — interossei (all 8, responsible for finger abduction and adduction), medial two lumbricals, adductor pollicis, hypothenar muscles (abductor digiti minimi, flexor digiti minimi, opponens digiti minimi). Sensory: palmar and dorsal surfaces of the medial 1.5 fingers (little finger and medial half of ring finger). Ulnar nerve injury at the wrist: weakness of finger abduction and adduction (cannot spread or close the fingers against resistance), loss of adduction of the thumb (cannot hold paper between thumb and palm — Froment sign positive: the patient compensates by flexing the thumb IP joint using the median-innervated flexor pollicis longus instead of the ulnar-innervated adductor pollicis), hypothenar atrophy, and numbness in the ulnar 1.5 fingers. Severe ulnar nerve injury produces 'claw hand' — hyperextension at the MCP joints and flexion at the IP joints of the ring and little fingers, caused by loss of the interossei and lumbricals. **Radial nerve**: provides NO intrinsic motor innervation to the hand (it innervates the forearm extensors only). The radial nerve provides sensory innervation to the dorsal surface of the lateral 3.5 fingers (proximal phalanges only — the fingertips are median nerve territory even on the dorsal side). Radial nerve injury at the wrist causes sensory loss on the dorsum of the hand but no motor deficit in the hand itself. Radial nerve injury higher up (in the arm — e.g., from a humeral fracture) causes wrist drop (inability to extend the wrist) because the forearm extensors are denervated. **Quick nerve test at the bedside:** - Median nerve: test thumb opposition (touch thumb to little finger) and test sensation on the palmar index finger. - Ulnar nerve: test finger abduction (spread the fingers against resistance) and test sensation on the little finger. - Radial nerve: test wrist extension and test sensation on the dorsal web space between thumb and index finger. **The all-important distinction**: if a patient presents with hand numbness in the little finger, it is NOT carpal tunnel syndrome. The little finger is ulnar nerve territory. Carpal tunnel syndrome affects the thumb, index, middle, and half of the ring finger (median nerve). This distinction appears on virtually every board exam that tests hand anatomy. AnatomyIQ generates nerve injury scenarios and tests the ability to identify which nerve is affected based on the pattern of motor and sensory deficits — the core skill for clinical hand examination.

The tendons that move the fingers pass through the wrist and into the hand. Understanding their anatomy explains common injuries and helps with clinical examination. **Flexor tendons** (palmar side): two major flexor muscles supply the fingers. 1. **Flexor digitorum superficialis (FDS)**: flexes the PIP (proximal interphalangeal) joints of fingers 2-5. Each tendon splits into two slips that insert on the sides of the middle phalanx, forming a tunnel through which the FDP tendon passes (the 'chiasm of Camper'). 2. **Flexor digitorum profundus (FDP)**: flexes the DIP (distal interphalangeal) joints of fingers 2-5. The FDP tendons pass through the split FDS tendons and insert on the distal phalanx. Clinical test: to isolate FDS function, hold all fingers except the one being tested in full extension (this eliminates FDP contribution because FDP shares a common muscle belly). If the patient can flex the PIP of the isolated finger, FDS is intact. To test FDP, hold the PIP in extension and ask the patient to flex the DIP. If they can, FDP is intact. Jersey finger: rupture of the FDP tendon from the distal phalanx, typically from forceful extension against a flexed finger (like grabbing a jersey in football). The patient cannot flex the DIP joint. This is a surgical emergency — the tendon must be reattached. Mallet finger: rupture of the extensor tendon from the distal phalanx, usually from a ball striking the tip of the finger. The DIP is stuck in flexion (cannot actively extend the DIP). Treated with splinting in extension for 6-8 weeks. **Flexor pollicis longus (FPL)**: the dedicated flexor for the thumb IP joint. Its tendon passes through the carpal tunnel. **Extensor tendons** (dorsal side): the extensor digitorum communis provides extension of the MCP joints of fingers 2-5. Its tendons form a complex extensor hood over each finger, with contributions from the interossei and lumbricals. The extensor hood is what allows the interossei and lumbricals to extend the IP joints while flexing the MCP — a critical functional anatomy point. The thumb has its own extensors: extensor pollicis longus (extends the thumb IP joint, forms the medial border of the anatomical snuffbox) and extensor pollicis brevis (extends the thumb MCP, forms the lateral border of the snuffbox). **De Quervain's tenosynovitis**: inflammation of the tendons in the first dorsal compartment of the wrist (abductor pollicis longus and extensor pollicis brevis). Presents with pain on the radial side of the wrist, especially with thumb use. Positive Finkelstein test (pain when the patient makes a fist over the thumb and deviates the wrist toward the ulnar side). Common in new parents (from repeatedly lifting babies) and in people who do repetitive thumb motions. **Trigger finger**: stenosing tenosynovitis where the flexor tendon gets trapped at the A1 pulley (at the base of the finger). The finger 'catches' or 'locks' in flexion and then snaps straight with a click. Common in the ring and middle fingers. Treated with rest, steroid injection, or surgical release of the A1 pulley. **The anatomical snuffbox**: the triangular depression on the radial side of the wrist visible when the thumb is extended. Borders: extensor pollicis longus (medial) and extensor pollicis brevis + abductor pollicis longus (lateral). Floor: scaphoid bone. Tenderness in the anatomical snuffbox after a fall on the outstretched hand is the classic sign of a scaphoid fracture. AnatomyIQ identifies hand tendons and generates clinical scenario questions about tendon injuries, De Quervain's tenosynovitis, trigger finger, and the anatomical snuffbox examination.