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Trapezium

The trapezium is one of the eight carpal bones, situated in the distal row on the radial side of the wrist, directly beneath the base of the first metacarpal bone. It forms a key component of the thumb carpometacarpal (CMC) joint, which provides the thumb its wide range of motion — including opposition, abduction, adduction, flexion, and extension.

It articulates proximally with the scaphoid, medially with the trapezoid, and distally with the first metacarpal, forming a saddle-shaped joint crucial for thumb dexterity. The trapezium has distinctive features such as the tubercle of the trapezium and a groove that transmits the tendon of the flexor carpi radialis (FCR).

Degenerative and traumatic changes involving the trapezium are common causes of basal thumb arthritis and painful grip dysfunction.

Synonyms

  • Greater multangular bone

  • Os trapezium

Location and Structure

  • Position: Most lateral bone in the distal carpal row, at the base of the thumb.

  • Shape: Irregular quadrilateral with a characteristic saddle-shaped distal surface for articulation with the first metacarpal.

  • Surfaces:

    • Superior (proximal): Concave, articulates with the scaphoid.

    • Inferior (distal): Saddle-shaped, articulates with the base of the first metacarpal.

    • Medial: Flat surface articulating with the trapezoid.

    • Lateral: Features the tubercle of the trapezium and a groove for the FCR tendon.

    • Anterior and posterior: Non-articular surfaces giving attachment to ligaments and joint capsules.

Articulations

  • Superiorly: Scaphoid

  • Medially: Trapezoid

  • Inferiorly (distal): Base of the first metacarpal

  • Occasionally, a small facet may articulate with the base of the second metacarpal

Relations

  • Anteriorly: Flexor carpi radialis tendon in its characteristic groove

  • Posteriorly: Dorsal ligaments of the wrist and radial artery

  • Laterally: Radial border of the wrist capsule

  • Medially: Trapezoid bone and part of the scaphoid

  • Inferiorly: Base of the first metacarpal forming the thumb CMC joint

Attachments

  • Ligamentous attachments:

    • Flexor retinaculum attaches to the trapezial tubercle, forming the lateral boundary of the carpal tunnel

    • Radial collateral ligament of the wrist attaches to its lateral aspect

    • Capsular ligaments of the first carpometacarpal (CMC) joint attach around its distal articular surface

  • Muscular attachments:

    • Occasionally gives origin to part of abductor pollicis brevis and opponens pollicis muscles from the tubercle

Function

  • Joint motion: Enables thumb opposition, flexion, extension, and rotation at the first CMC joint

  • Force transmission: Distributes load from the thumb to the wrist during grasp and pinch activities

  • Stability: Acts as a stable base for the first metacarpal articulation

  • Mechanical leverage: Provides attachment site for flexor retinaculum and contributes to carpal tunnel structure

Clinical Significance

  • Basal joint arthritis (Trapeziometacarpal osteoarthritis): Common degenerative disorder causing thumb pain and weakness

  • Fractures: Usually involve the trapezial ridge or tubercle; often result from axial load on the thumb

  • Dislocation: Rare, but may occur in high-energy wrist trauma

  • Carpal tunnel syndrome: Trapezial ridge forms part of the carpal tunnel margin; deformity can contribute to compression

  • Post-traumatic changes: Include avulsion of the tubercle or degenerative flattening of articular surfaces

  • Imaging role: MRI and CT essential for assessment of fractures, arthritis, and articular cartilage changes

MRI Appearance

  • T1-weighted images:

    • Cortex: Low signal (dark)

    • Bone marrow: Bright signal due to fatty content

    • Articular cartilage: Thin intermediate-to-low signal covering the joint surfaces

    • Adjacent tendons (FCR, thenar muscles): Low signal linear structures

    • Pathology: Fractures or bone marrow edema appear as linear or patchy low-signal regions

  • T2-weighted images:

    • Normal bone marrow: Bright, slightly less than T1 intensity

    • Cartilage: Intermediate-to-bright signal outlining joint contours

    • Joint fluid: Hyperintense, delineating articular margins

    • Pathology:

      • Bone contusions or stress reaction: High-signal intensity

      • Degeneration: Irregular cartilage thinning and subchondral changes

  • STIR:

    • Normal bone: Intermediate-to-dark signal

    • Pathologic marrow: Bright hyperintense in edema, contusion, or fracture

    • Excellent for detecting early inflammatory changes and occult fractures

  • Proton Density Fat-Saturated (PD FS):

    • Normal bone: Intermediate-to-dark signal

    • Pathology: Bright focal or diffuse hyperintensity in cases of marrow edema, stress injury, or synovitis

    • Useful for assessing cartilage defects and joint fluid accumulation

  • T1 Fat-Sat Post-Contrast:

    • Normal bone: Homogeneous mild enhancement

    • Synovial inflammation: Diffuse peripheral enhancement

    • Arthritis or infection: Patchy enhancement of marrow and adjacent capsule

    • Fracture or osteonecrosis: Irregular non-enhancing necrotic core with enhancing periphery

CT Appearance

Non-Contrast CT:

  • Cortex: High attenuation with clearly defined borders

  • Trabecular bone: Fine, uniform internal architecture

  • Articular surface: Smooth, concave proximally and saddle-shaped distally

  • Pathology:

    • Fractures: Clear cortical disruption or ridge avulsion

    • Osteoarthritis: Joint space narrowing, subchondral sclerosis, cysts, and osteophytes

CT VRT 3D image

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MRI image

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CT image

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Trapezium ct coronal