Trapezoid Bone: Anatomy, Articulations, Function, Imaging

Topic

Trapezoid

The trapezoid bone is one of the eight carpal bones of the wrist, located in the distal carpal row. It lies between the trapezium laterally and the capitate medially, articulating distally with the base of the second metacarpal and proximally with the scaphoid.
It is the smallest bone in the distal row and has an irregular wedge-shaped form. The trapezoid plays a vital role in the stability of the central carpal column, transmitting forces from the second metacarpal to the carpus during gripping and weight-bearing.

Synonyms

Lesser multangular bone
Os trapezoideum

Location and Structure

Position: Distal row of carpal bones, between trapezium (lateral) and capitate (medial).
Shape: Irregular wedge-shaped, broader dorsally, tapering toward the palmar surface.
Surfaces:
- Dorsal surface: Rough for ligament attachment.
- Palmar surface: Smaller and smooth, giving attachment to part of the flexor retinaculum.
- Lateral surface: Articulates with trapezium.
- Medial surface: Articulates with capitate.
- Proximal surface: Small oval facet articulating with scaphoid.
- Distal surface: Large, concave facet articulating with the base of the second metacarpal.

Articulations

Proximal: Scaphoid
Distal: Base of the second metacarpal
Lateral: Trapezium
Medial: Capitate

Relations

Dorsally: Extensor tendons of the wrist and hand (notably extensor carpi radialis longus and brevis tendons).
Palmarly: Deep palmar arch and flexor tendons within the carpal tunnel region.
Laterally: Trapezium and tendon of flexor carpi radialis.
Medially: Capitate bone and intercarpal ligaments.

Ligamentous Attachments

Dorsal and palmar intercarpal ligaments connect the trapezoid to the trapezium, capitate, and second metacarpal base.
Radial and ulnar collateral ligaments provide lateral stability across the distal carpal row.

Arterial Supply

Branches from the radial artery, particularly through the dorsal carpal branch and deep palmar arch.

Function

Stabilization: Forms a rigid support between the scaphoid and the second metacarpal.
Force transmission: Transfers compressive loads from the index metacarpal to the carpal bones during gripping and pushing.
Support for wrist movement: Maintains central carpal alignment during flexion, extension, and radial deviation.
Protection: Shields the underlying vascular arch and tendinous structures in the deep palm.

Clinical Significance

Fractures: Rare due to its protected position; when present, often associated with high-impact trauma or crush injuries.
Dislocation: May occur with complex carpal fracture-dislocation patterns.
Arthritis: Degenerative or post-traumatic arthritis can involve the scapho-trapezoid or trapezoid–second metacarpal joint.
Avascular necrosis: Extremely uncommon, but may follow trauma or vascular compromise.
Surgical relevance: Acts as a key stabilizer in carpal reconstructions and intercarpal fusions.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark).
- Bone marrow: Bright due to fatty content in adults.
- Cartilage surfaces: Smooth, thin, intermediate-to-low signal.
- Fracture: Appears as a linear low-signal band through the marrow.
- Avascular necrosis: Focal low-signal region in subchondral area with loss of fatty marrow signal.
T2-weighted images:
- Cortex: Low signal (dark).
- Marrow: Bright, slightly less than on T1 due to partial fat and trabecular components.
- Cartilage: Intermediate-to-bright depending on thickness.
- Pathology:
  - Fracture lines: bright or intermediate depending on fluid content.
  - Marrow edema: hyperintense.
  - Early osteoarthritis: cartilage thinning and subchondral bright foci.
STIR:
- Normal marrow: Intermediate-to-dark signal.
- Pathologic marrow: Bright hyperintense signal in cases of edema, contusion, or osteitis.
- Excellent for detecting occult fractures or bone bruising.
Proton Density Fat-Saturated (PD FS):
- Normal bone: Intermediate-to-dark.
- Pathologic areas: Bright signal indicating marrow edema, inflammation, or early ischemia.
- Useful for evaluating ligament injuries and pericarpal soft-tissue edema.
T1 Fat-Sat Post-Contrast:
- Normal bone: Homogeneous mild enhancement.
- Inflammation or infection: Patchy marrow enhancement.
- Avascular necrosis: Non-enhancing region with surrounding rim enhancement.

CT Appearance

Non-Contrast CT:

Cortex: High attenuation, sharply defined.
Trabecular bone: Fine uniform structure within.
Articular surfaces: Smooth and well-corticated.
Pathology:
- Excellent for detecting fractures, dislocations, and degenerative changes.
- Shows cortical step-offs, sclerosis, and early bone fragmentation in trauma.