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Carpal bones

The carpal bones are a group of eight small, irregularly shaped bones forming the wrist (carpus). They are arranged in two rows—a proximal row and a distal row—and articulate proximally with the radius and articular disc of the ulna, and distally with the metacarpal bases. Together, they form the carpal arch, which supports the flexor retinaculum and forms the carpal tunnel on the palmar side.

These bones provide flexibility, stability, and shock absorption for hand movement, allowing complex motions such as flexion, extension, radial and ulnar deviation, and circumduction of the wrist.

Synonyms

  • Wrist bones

  • Carpals

  • Carpus

Arrangement and Names

Proximal row (lateral to medial):

  • Scaphoid: Boat-shaped bone articulating with the radius; most commonly fractured carpal.

  • Lunate: Crescent-shaped; articulates with the radius and capitate; prone to dislocation and avascular necrosis.

  • Triquetrum: Pyramidal bone articulating with the articular disc of the distal ulna.

  • Pisiform: Small sesamoid bone within the tendon of flexor carpi ulnaris, lying anterior to the triquetrum.

Distal row (lateral to medial):

  • Trapezium: Saddle-shaped bone articulating with the first metacarpal, forming the thumb carpometacarpal joint.

  • Trapezoid: Small wedge-shaped bone between the trapezium and capitate.

  • Capitate: Largest carpal bone; central keystone of the carpus, articulates with the third metacarpal.

  • Hamate: Has a hook-like projection (hamulus) palpable on the palmar surface.

Articulations

  • Proximally: Scaphoid, lunate, and triquetrum articulate with the radius and articular disc.

  • Distally: Distal row articulates with metacarpal bases.

  • Within the carpus: Intercarpal articulations reinforced by interosseous ligaments, ensuring coordinated movement.

  • Transverse carpal arch: Formed by the distal row concavity supporting the carpal tunnel.

Relations

  • Palmar surface: Covered by flexor retinaculum, flexor tendons, and median nerve (within carpal tunnel).

  • Dorsal surface: Covered by extensor tendons and retinaculum.

  • Medially: Pisiform and ulnar neurovascular structures.

  • Laterally: Scaphoid and trapezium forming the radial border of the wrist.

Attachments

  • Ligamentous:

    • Palmar and dorsal radiocarpal ligaments

    • Ulnar and radial collateral ligaments

    • Interosseous ligaments between adjacent carpals

  • Muscular:

    • Flexor carpi radialis (to trapezium and base of second metacarpal)

    • Flexor carpi ulnaris (to pisiform and hamate)

    • Extensor carpi radialis longus/brevis and extensor carpi ulnaris to metacarpal bases connected via carpal ligaments

Arterial Supply

  • Dorsal carpal arch and palmar carpal arch, formed by branches of the radial and ulnar arteries and their intercarpal anastomoses.

  • Blood supply to the scaphoid is mainly retrograde, predisposing it to avascular necrosis after fracture.

Function

  • Mobility and flexibility: Permit complex wrist and hand movements.

  • Shock absorption: Distribute compressive loads from the hand to the forearm.

  • Stability: Maintain alignment between forearm and hand via intercarpal ligamentous support.

  • Grip efficiency: Adjust wrist angle for optimal hand function.

  • Protection: Provide a concave structure protecting flexor tendons and median nerve in the carpal tunnel.

Clinical Significance

  • Fractures: Scaphoid most frequently fractured; lunate and triquetral fractures less common.

  • Dislocations: Lunate and perilunate dislocations common after falls on an outstretched hand.

  • Avascular necrosis: Particularly affects scaphoid and lunate.

  • Carpal tunnel syndrome: Compression of the median nerve beneath the flexor retinaculum.

  • Arthritis and instability: Degenerative changes, scapholunate dissociation, and carpal collapse patterns.

  • Accessory ossicles: Os centrale and others may mimic fracture on imaging.

MRI Appearance

  • T1-weighted images:

    • Cortical bone: Low signal (dark rim).

    • Marrow: Bright, reflecting fatty content in adults.

    • Cartilage: Smooth, intermediate-to-low signal.

    • Fractures: Appear as linear low-signal lines with adjacent marrow changes.

  • T2-weighted images:

    • Cortex: Dark.

    • Marrow: Bright, though slightly less than fat on T1.

    • Articular cartilage: Intermediate-to-bright signal; effusion or edema appears hyperintense.

    • Pathology: Bone contusion, edema, or osteonecrosis show bright intramedullary signal.

  • STIR:

    • Normal marrow: Intermediate-to-dark signal.

    • Abnormal marrow: Bright hyperintense signal indicating edema, fracture, or osteitis.

    • Useful for detecting early stress injuries or bone marrow edema before visible cortical change.

  • Proton Density Fat-Saturated (PD FS):

    • Normal: Intermediate-to-dark bone signal.

    • Pathologic: Bright hyperintensity in fractures, osteochondral defects, or inflammatory changes.

    • Highlights cartilage thinning and ligament tears.

  • T1 Fat-Sat Post-Contrast:

    • Normal bone: Mild homogeneous enhancement.

    • Osteonecrosis: Patchy or absent enhancement in ischemic areas.

    • Synovitis: Diffuse enhancing synovium; infectious or inflammatory arthritis shows pericapsular enhancement.

CT Appearance

Non-Contrast CT:

  • Cortical bone: High attenuation, sharply defined.

  • Trabecular pattern: Fine, regular in healthy bone.

  • Articular surfaces: Smooth and well-corticated.

  • Fractures: Visualized as lucent lines or cortical step-offs.

  • Osteoarthritis: Demonstrates osteophytes, subchondral sclerosis, cysts, or joint space narrowing.

  • Avascular necrosis: Appears as patchy sclerosis or collapse of bone (most often scaphoid or lunate).

Post-Contrast CT (standard):

  • Normal: Mild enhancement of surrounding soft tissue.

  • Infection or synovitis: Enhancing capsule and pericapsular soft-tissue thickening.

  • Fracture healing: Contrast uptake at fracture margins due to callus or granulation tissue.

CT VRT 3D image

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MRI image

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MRI image

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CT image

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CT image

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