Wrist and Hand

Type: Synovial condyloid (ellipsoid) joint.

• Articular surfaces: Distal end of radius and the articular disc of the TFCC (proximally) with the scaphoid, lunate, and triquetrum (distally), the ulna itself does not directly articulate with the carpus, separated by the TFCC.
• Capsule: Loose, attaches proximally to the margins of the radius/TFCC and distally to the proximal carpal row; reinforced by ligaments on all sides.
• Ligaments: Palmar radiocarpal, dorsal radiocarpal, ulnar collateral, radial collateral ligaments, provide stability while permitting a wide arc of motion; palmar ligaments are stronger than dorsal, biasing stability toward preventing hyperextension.
• Movements: Flexion, extension, abduction (radial deviation), adduction (ulnar deviation), and circumduction (combination of these).
• Muscles producing movements:

• Attachments: Laterally, tubercles of scaphoid and trapezium; medially, pisiform and hook of hamate. Converts the concave carpal arch into the carpal tunnel.
• Structures superficial to it: Palmaris longus tendon (inserts into it), ulnar artery and nerve (pass superficial, through Guyon canal), palmar cutaneous branches of median and ulnar nerves.
• Structures deep to it: Median nerve, flexor pollicis longus tendon, flexor digitorum superficialis tendons (×4), flexor digitorum profundus tendons (×4), i.e., the carpal tunnel contents.

• Boundaries: Concave carpal arch (floor and sides, formed by carpal bones) roofed by the flexor retinaculum.
• Contents (9 tendons + 1 nerve):
• Flexor pollicis longus tendon (own synovial sheath).
• Flexor digitorum superficialis tendons ×4 (common synovial sheath, ulnar bursa).
• Flexor digitorum profundus tendons ×4 (same common synovial sheath, ulnar bursa).
• Median nerve: most superficial and anterior structure within the tunnel, hence most vulnerable to compression.
• Median nerve: Gives off the palmar cutaneous branch before entering the tunnel (explains why thenar eminence/palm sensation is often spared in carpal tunnel syndrome). Within the tunnel it lies just deep to the flexor retinaculum, anterior to the flexor tendons. It gives off the recurrent (motor) branch to the thenar muscles immediately after exiting the tunnel.
• Anatomical basis of carpal tunnel syndrome: The tunnel is a rigid, non-expansile osteofascial space; any process increasing its contents' volume or reducing its capacity (tenosynovitis, oedema, pregnancy, hypothyroidism, fracture malunion, repetitive strain) raises pressure and compresses the median nerve. The median nerve is the least tolerant structure to compression.
• Sensory symptoms: numbness/tingling in the lateral 3½ digits, sparing the palm (palmar cutaneous branch exits proximal to the tunnel).
• Motor symptoms: weakness of thenar muscles (abductor pollicis brevis especially), thenar wasting in longer-standing cases.
• Positive Tinel sign (tingling on tapping over the tunnel) and Phalen test (symptoms reproduced on sustained wrist flexion).

• Attachments: Laterally to the distal radius; medially to the pisiform and triquetrum (via the antebrachial fascia).
• Sends septa deep to the tendons, creating six fibro-osseous compartments on the dorsal wrist, each with its own synovial sheath.

Mnemonic: "1, 2, 3: Abducting Policemen (APL, EPB) Rarely Eat Bread (ECRL, ECRB) Except Potato Loaf (EPL). 4, 5, 6: Eat Dog Index (ED, EI), Dinner Menu (EDM), Carpi Ulnaris (ECU)."

• Boundaries:
• Lateral (anterior): tendons of abductor pollicis longus and extensor pollicis brevis.
• Medial (posterior): tendon of extensor pollicis longus.
• Proximal: styloid process of radius.
• Floor: Scaphoid (proximally) and trapezium (distally).
• Contents: Radial artery (crossing the floor obliquely, deep to the tendons, before passing between the two heads of the first dorsal interosseous into the palm).
• The beginning of the cephalic vein, terminal branches of the superficial branch of the radial nerve (superficial to the tendons).
• Radial artery in the snuffbox: palpable and surgically important; provides a route for accessing the deep palmar arch.
• Scaphoid relevance: Tenderness in the floor of the snuffbox is the classic clinical sign of scaphoid fracture, given the bone forms the proximal floor and its precarious retrograde blood supply.

• Boundaries:
• Floor: flexor retinaculum and hypothenar muscles.
• Roof: palmar carpal ligament (superficial to the flexor retinaculum) and palmaris brevis.
• Medial: pisiform.
• Lateral: hook of hamate.
• Contents: Ulnar nerve and ulnar artery (nerve typically medial to artery), each dividing into superficial and deep terminal branches within or just distal to the canal.
• Ulnar nerve in the canal: superficial branch, cutaneous to the palmar surface of the medial 1½ digits and supplies palmaris brevis; deep branch, motor, curves around the hook of hamate to supply hypothenar muscles, adductor pollicis, all interossei, and the medial two lumbricals.
• Ulnar artery: Continues into the deep and superficial palmar arches.
• Clinical significance: Compression here ("cyclist's palsy," ganglion cysts, hook of hamate fractures) causes Guyon canal syndrome: motor and/or sensory ulnar deficits typically sparing the dorsal hand (dorsal cutaneous branch arises proximal to the canal, in the forearm).
• This is a key distinguishing feature from a more proximal ulnar lesion at the elbow, which does affect the dorsum of the hand.

Palmar Aponeurosis

• Triangular fibrous thickening of the palmar fascia; apex continuous with the palmaris longus tendon (when present) and flexor retinaculum; base divides into four longitudinal digital bands to the fingers (not the thumb), which then split for the flexor tendons to pass through.
• Protects underlying neurovascular structures and flexor tendons; anchors the skin, preventing shearing during grip.
• Pathological thickening and contracture causes Dupuytren contracture.

Thenar Muscles

Adductor Pollicis

• Origin: Oblique head, capitate, bases of 2nd/3rd metacarpals; transverse head, anterior surface of 3rd metacarpal shaft.
• Insertion: Base of proximal phalanx of thumb (ulnar side), via the ulnar sesamoid.
• Innervation: Ulnar nerve (deep branch).
• Action: Adducts the thumb toward the palm; important for pinch/key grip strength.

Hypothenar Muscles

Lumbricals

• Origin: From the tendons of flexor digitorum profundus: 1st and 2nd lumbricals arise unipennally from the lateral (radial) two FDP tendons; 3rd and 4th arise bipennally from adjacent FDP tendons.
• Insertion: Lateral aspect of the extensor expansion of digits 2–5 (radial side).
• Innervation: 1st and 2nd (lateral two), median nerve; 3rd and 4th (medial two), ulnar nerve (deep branch).
• Action: Flex the MCP joints while extending the IP joints simultaneously (unique action, via pull on the extensor expansion), essential for coordinated finger movement (e.g., writing grip).

Palmar Interossei (3 muscles)

• Origin: Unipennal, from the metacarpal shaft of the same digit they act on (2nd, 4th, 5th metacarpals, none for the middle finger, which is the axis of reference).
• Insertion: Extensor expansion (and base of proximal phalanx) of digits 2, 4, 5, pulling each toward the midline (3rd digit axis).
• Innervation: Ulnar nerve (deep branch).
• Action: ADduct the fingers toward the middle finger axis (mnemonic PAD: Palmar ADduct).

Dorsal Interossei (4 muscles)

• Origin: Bipennal, from adjacent sides of the metacarpal shafts (between metacarpals 1&2, 2&3, 3&4, 4&5).
• Insertion: Extensor expansion (and base of proximal phalanx) of digits 2, 3 (both sides), 4, abducting them away from the midline (3rd digit axis; the middle finger itself can abduct in both directions via the two dorsal interossei attaching to it).
• Innervation: Ulnar nerve (deep branch).
• Action: ABduct the fingers away from the middle finger axis (mnemonic DAB: Dorsal ABduct).

Interossei and lumbricals together flex the MCP joints and extend the IP joints via the extensor expansion, critical for maintaining the normal resting posture and fine coordinated grip; their loss (ulnar/median nerve injury) produces clawing.

• PAD: Palmar interossei ADduct the fingers (toward digit 3).
• DAB: Dorsal interossei ABduct the fingers (away from digit 3).
• Extensor expansion (dorsal digital expansion/hood): A fibrous sheet over the dorsum of each finger (digits 2–5) formed by the extensor digitorum tendon centrally, joined laterally by the interossei and lumbrical tendons.
• Central slip inserts on the base of the middle phalanx (extends PIP).
• Two lateral bands (receiving interossei/lumbrical contributions) converge to insert on the base of the distal phalanx (extend DIP).
• This arrangement allows the intrinsic muscles (interossei, lumbricals) to extend the IP joints while the extrinsic extensor digitorum primarily extends the MCP joint, division of labour between intrinsic and extrinsic systems.
• Disruption of the central slip produces a boutonnière deformity; disruption of the terminal extensor insertion produces a mallet finger.

• Flexor tendons: FDS tendons split at the proximal phalanx to allow FDP tendons to pass through and insert more distally (Camper chiasm) before FDS inserts on the middle phalanx base; FDP continues to the distal phalanx base.
• Digital synovial sheaths: Each finger's flexor tendons (FDS + FDP) are enclosed in a synovial sheath from the MCP joint to the DIP joint. The tendons are held close to the phalanges by the fibrous flexor sheath (annular and cruciate pulleys: A1–A5, C1–C3), preventing bowstringing during flexion.
• Radial bursa: Synovial sheath surrounding the flexor pollicis longus tendon, extending from the wrist into the thumb; may communicate with the ulnar bursa in some individuals.
• Ulnar bursa (common flexor sheath): Encloses the FDS and FDP tendons for digits 2–5 within the carpal tunnel. It typically extends the full length into the little finger sheath but usually ends at the palm level for digits 2–4 (their digital sheaths beginning separately near the MCP joints).
• Anatomical variation is common, and infection can track proximally through the ulnar bursa into the carpal tunnel/forearm.

• Thenar space: Deep to the flexor tendons of the index finger, lateral to the midpalmar septum; overlies adductor pollicis.
• Midpalmar space: Deep to the flexor tendons of the middle, ring, and little fingers, medial to the thenar space; overlies the interossei and metacarpals 3–5.
• Both spaces are potential sites for the spread of palmar space infections (from penetrating trauma or tendon sheath infection), which can track proximally into the forearm via the carpal tunnel and cause significant morbidity if not drained promptly.

• Opposition: Thumb rotates and moves across the palm to contact the fingertips, enabled by the saddle-shaped 1st CMC joint and the opponens pollicis/opponens digiti minimi; the basis of precision handling and the defining feature of human hand function.
• Precision grip: Object held between the pulps of the thumb and fingers (e.g., pinching a pen), relying on thenar muscles, intrinsic muscles, and fine extrinsic tendon control.
• Power grip: Object clamped between the flexed fingers and the palm, thumb reinforcing the grip, relying primarily on extrinsic flexors (FDS, FDP) with intrinsics stabilising.
• Pinch: Includes tip-to-tip (fine precision), key/lateral pinch (thumb pulp to the lateral side of the index finger, relies heavily on adductor pollicis and FPL), and three-jaw chuck pinch (thumb plus index and middle fingers).
• Intrinsic muscle function: The lumbricals and interossei provide the coordinated MCP flexion/IP extension posture essential for a controlled, non-clawed grip. Loss of intrinsic function (combined median/ulnar nerve injury) produces a flattened, clawed "main en griffe" with loss of fine dexterity even when extrinsic tendons remain intact.