First Metacarpal Bone: Anatomy, Structure, Function, Imaging

Topic

First metacarpal bone (metacarpal I)

The first metacarpal bone (metacarpal I) is the shortest, thickest, and most mobile of the five metacarpal bones of the hand. It forms the bony framework of the thumb (pollex) and articulates proximally with the trapezium at the first carpometacarpal (CMC) joint, and distally with the proximal phalanx of the thumb at the metacarpophalangeal (MCP) joint.

Its unique saddle-shaped base and short robust shaft allow a wide range of thumb movements—flexion, extension, abduction, adduction, and opposition—making it essential for fine motor control and grip. The first metacarpal is also a common site of fractures, base dislocations (Bennett’s and Rolando’s fractures), and degenerative joint disease.

Synonyms

First metacarpal
Thumb metacarpal
Metacarpal I

Location and Structure

Position: Lateral (radial) side of the hand, forming the skeletal core of the thumb.
Shape: Short and thick, with a wide base and rounded head.
Base: Saddle-shaped articular surface for the trapezium; allows multi-axial thumb motion.
Shaft: Flattened dorsopalmarly and curved slightly outward.
Head: Rounded, articulates with the proximal phalanx of the thumb.

Articulations

Proximally: With the trapezium (forming the first CMC joint).
Distally: With the base of the proximal phalanx of the thumb (forming the MCP joint).
Laterally: Occasionally with the base of the second metacarpal via a small articular facet.

Relations

Dorsally: Extensor pollicis longus and brevis tendons.
Palmarly: Flexor pollicis longus tendon, adductor pollicis, and thenar muscles.
Laterally: Abductor pollicis longus tendon at the base.
Medially: First dorsal interosseous muscle and second metacarpal.

Muscle Attachments

Base:
- Lateral surface: Abductor pollicis longus.
- Medial surface: First dorsal interosseous muscle.
Shaft: Occasionally gives attachment to adductor pollicis (oblique head).
Head: Joint capsule of the thumb MCP joint.

Ligamentous Attachments

Capsule: Encloses both CMC and MCP joints.
Collateral ligaments: On radial and ulnar sides of the MCP joint.
Volar plate: Reinforces palmar aspect of the MCP joint.

Arterial Supply

Dorsal metacarpal arteries and palmar metacarpal branches from the deep palmar arch.
Small branches from the princeps pollicis artery supply the thumb’s metacarpal head and joint capsule.

Function

Mobility: Allows opposition, abduction, and rotation of the thumb.
Grip and manipulation: Enables pinching and grasping with precision.
Load transmission: Transfers force between the trapezium and proximal phalanx during movement.
Joint stability: Works with surrounding ligaments and thenar muscles for controlled motion.

Clinical Significance

Fractures:
- Bennett’s fracture: Oblique intra-articular fracture at the base extending into the CMC joint.
- Rolando’s fracture: Comminuted Y- or T-shaped intra-articular fracture at the base.
- Shaft fractures: Result from direct trauma or twisting.
Osteoarthritis: Common at the CMC joint, leading to pain, stiffness, and deformity.
Dislocations: Occur at the MCP or CMC joints, often from forced abduction.
Surgical relevance: Landmark in thumb reconstruction, tendon transfer, and hand arthroplasty.
Imaging role: Key in evaluating fractures, joint degeneration, and post-surgical outcomes.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark).
- Bone marrow: Bright, representing fatty marrow in adults.
- Articular cartilage: Smooth intermediate-to-low signal covering the CMC and MCP articular surfaces.
- Pathology: Fractures appear as linear low-signal lines with surrounding bright marrow edema.
- Degeneration: Subchondral sclerosis shows as dark cortical thickening.
T2-weighted images:
- Cortex: Low signal (dark).
- Marrow: Bright, slightly less intense than on T1 but distinct from surrounding muscle.
- Cartilage: Intermediate-to-bright, outlining smooth articular surfaces.
- Joint effusion or synovitis: Hyperintense signal in joint space.
- Fractures or bone contusion: Bright hyperintense regions with cortical disruption.
STIR:
- Normal bone marrow: Intermediate-to-dark signal.
- Pathologic bone marrow: Bright hyperintense signal with poorly defined trabeculae in edema, fracture, or infection.
- Highly sensitive for stress fractures and bone bruises.
Proton Density Fat-Saturated (PD FS):
- Normal marrow: Intermediate-to-dark signal.
- Pathology: Focal or diffuse bright hyperintensity in areas of marrow edema, cartilage defect, or soft-tissue inflammation.
- Useful for detecting subtle intra-articular injuries and ligament tears around CMC joint.
T1 Fat-Sat Post-Contrast:
- Normal bone: Mild homogeneous enhancement of marrow and periosteum.
- Synovitis or inflammatory arthritis: Diffuse enhancement of capsule and pericapsular tissue.
- Osteomyelitis: Patchy marrow enhancement with cortical irregularity.
- Post-fracture healing: Peripheral rim enhancement around granulation tissue.

CT Appearance

Non-Contrast CT:

Cortex: High attenuation, sharply marginated.
Trabecular bone: Fine, dense honeycomb pattern.
Articular surfaces: Smooth concavo-convex contours at trapezium and proximal phalanx articulations.
Pathology:
- Detects fractures, joint incongruity, and subtle cortical disruptions.
- Identifies degenerative sclerosis, subchondral cysts, and osteophytes.
- Essential for preoperative planning in intra-articular fractures.