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Second metacarpal bone (metacarpal II)

The second metacarpal (Metacarpal II) is the bone of the index finger and the longest of all metacarpals. It plays a crucial role in hand stability and precision grip. It lies between the first (thumb) and third (middle) metacarpals and forms strong articulations with the trapezium, trapezoid, capitate, and third metacarpal, making its base one of the most stable in the hand.

It provides anchorage for several muscles, particularly those that control wrist extension, flexion, and fine index finger motion. Because of its rigid carpometacarpal articulation, the second metacarpal forms the axis of hand rotation for the digits.

Synonyms

  • Second metacarpal bone

  • Index metacarpal

  • Metacarpal of the index finger

Location and Structure

  • The second metacarpal lies between the first metacarpal (thumb) and third metacarpal (middle finger).

  • It consists of a base, shaft (body), and head.

  • Length: Longest of the metacarpals.

  • Axis: Serves as the central axis for hand rotation.

Base:

  • Proximal end, quadrilateral and irregular.

  • Articulations:

    • Proximally: With trapezium, trapezoid, and capitate.

    • Laterally: With the first metacarpal (small facet).

    • Medially: With the third metacarpal via small facets.

  • Dorsal surface: Marked by grooves for ligamentous attachments.

Shaft:

  • Slightly curved, convex dorsally, concave palmarly.

  • Cross-section: Triangular.

  • Surfaces:

    • Dorsal: Attachment for extensor carpi radialis longus tendon.

    • Palmar: Gives origin to interosseous muscles.

    • Lateral and medial borders: Sharp ridges for ligamentous and muscular attachments.

Head:

  • Rounded distal end forming a knuckle at the metacarpophalangeal (MCP) joint.

  • Articulates with the base of the proximal phalanx of the index finger.

  • Dorsal surface flattened with grooves for extensor tendons.

Relations

  • Dorsally: Extensor carpi radialis longus tendon, extensor digitorum, and extensor indicis tendons.

  • Palmarly: Lumbrical muscle, flexor tendons, and palmar interosseous muscle.

  • Laterally: First metacarpal and adductor pollicis oblique head.

  • Medially: Third metacarpal and dorsal interosseous muscle.

Attachments

  • Base:

    • Extensor carpi radialis longus inserts dorsally.

    • Flexor carpi radialis tendon attaches partly to its base.

    • Palmar and dorsal carpometacarpal ligaments reinforce the joint capsule.

  • Shaft:

    • Dorsal interosseous muscle (first): lateral side.

    • Palmar interosseous muscle (first): medial side.

  • Head:

    • Collateral ligaments of the MCP joint.

    • Sagittal bands of the extensor mechanism.

Function

  • Framework: Forms the structural axis of the hand and stabilizes the index finger.

  • Grip mechanics: Provides rigid support for precision pinch and fine movements.

  • Muscle leverage: Serves as an insertion site for wrist flexors/extensors and intrinsic hand muscles.

  • Stability: Acts as a stable column for transfer of force from wrist to finger.

Clinical Significance

  • Fractures: Commonly result from direct trauma or punching injuries (“boxer’s variant” involving second or third metacarpal).

  • Base fractures: May involve carpometacarpal dislocations, often seen in high-energy trauma.

  • Stress injuries: Seen in athletes and repetitive manual work.

  • Arthritis: Degenerative changes at the carpometacarpal or MCP joints.

  • Surgical relevance: Landmark in hand reconstruction, tendon graft anchorage, and metacarpal osteosynthesis.

MRI Appearance

  • T1-weighted images:

    • Cortex: Low signal (dark).

    • Bone marrow: Bright signal due to fatty marrow.

    • Articular cartilage: Thin, intermediate-to-low signal lining articular surfaces.

    • Periosteum: Low-signal band; may appear thickened with injury or stress reaction.

    • Fractures: Linear low-signal defect; marrow edema appears intermediate-to-bright.

  • T2-weighted images:

    • Cortex: Dark, low signal.

    • Marrow: Bright signal, though less intense than on T1.

    • Cartilage: Intermediate-to-bright; smooth in normal conditions.

    • Pathology: Marrow edema, stress fractures, or osteochondral injury appear as bright hyperintense zones.

  • STIR:

    • Normal marrow: Intermediate-to-dark.

    • Pathology: Bright hyperintensity in bone marrow edema, infection, or fracture.

    • Highlights soft-tissue swelling and periosteal reaction.

  • Proton Density Fat-Saturated (PD FS):

    • Normal marrow: Intermediate-to-dark signal.

    • Pathology: Bright hyperintensity in stress response or marrow inflammation.

    • Excellent for visualizing subtle bone contusions and joint effusions.

  • T1 Fat-Sat Post-Contrast:

    • Normal bone: Mild homogeneous enhancement.

    • Osteomyelitis: Patchy marrow enhancement with cortical disruption.

    • Synovitis or arthritis: Enhancing synovial lining and pericapsular tissue.

CT Appearance

Non-Contrast CT:

  • Cortex: High-attenuation, sharply marginated outer layer.

  • Trabecular bone: Fine spongy structure visible in detail.

  • Articular surface: Smooth, dense subchondral plate.

  • Pathology:

    • Detects subtle fractures, cortical irregularity, and small bone fragments.

    • Reveals degenerative changes, sclerosis, and erosions at the carpometacarpal and MCP joints.

    • Evaluates fracture alignment and bone healing.

CT VRT 3D image

Metacarpal II (2nd) bone bone CT 3D VRT image -img-00000-00000_00001

MRI image

second metacarpal (Metacarpal II) bone coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

second metacarpal (Metacarpal II) bone coronal ct image