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Third metacarpal bone (metacarpal III)

The third metacarpal bone (metacarpal III) is the central axis of the hand and the second longest of the five metacarpals, following the second metacarpal. It corresponds to the middle finger and provides stability and strength to the carpal–metacarpal arch.

This bone articulates proximally with the capitate, laterally with the second and fourth metacarpals, and distally with the proximal phalanx of the middle finger. The styloid process on its dorsal-lateral base serves as an attachment point for the extensor carpi radialis brevis tendon, distinguishing it from other metacarpals.

Functionally, the third metacarpal acts as the central pillar of the hand, forming a stable reference point for coordinated movements, grip, and fine manual dexterity.

Synonyms

  • Metacarpal of the middle finger

  • Metacarpal III

  • Third ray of the hand

Location and Structure

  • Position: Central metacarpal, located between the second and fourth metacarpals.

  • Shape: Long, slightly curved bone with a cylindrical shaft and expanded ends.

  • Base (proximal end): Articulates with the capitate, and has small articular facets for the second and fourth metacarpals.

  • Shaft (body): Slightly curved with dorsal convexity and palmar concavity.

  • Head (distal end): Rounded, articulating with the base of the proximal phalanx of the middle finger to form the metacarpophalangeal (MCP) joint.

  • Styloid process: Distinct dorsal-lateral projection from the base, unique to this metacarpal.

Relations

  • Proximally: Capitate bone

  • Laterally: Second and fourth metacarpals (via intermetacarpal articulations)

  • Distally: Proximal phalanx of the middle finger

  • Dorsally: Extensor tendons (extensor digitorum, extensor indicis, and extensor carpi radialis brevis)

  • Palmarly: Lumbrical and interosseous muscles, and flexor tendons (flexor digitorum superficialis and profundus)

Attachments

  • At base:

    • Extensor carpi radialis brevis (inserts on the styloid process)

    • Interosseous ligaments connecting the second and fourth metacarpals

  • At shaft:

    • Dorsal and palmar interossei muscles

  • At head:

    • Fibrous capsule of the MCP joint

    • Collateral ligaments on both sides

    • Deep transverse metacarpal ligament connecting adjacent heads

Nerve Supply

  • Sensory innervation over the metacarpal region:

    • Dorsal: Superficial branch of the radial nerve

    • Palmar: Palmar digital branches of the median nerve

Arterial Supply

  • Dorsal metacarpal arteries (from the dorsal carpal arch) supply the dorsal aspect.

  • Palmar metacarpal arteries (from the deep palmar arch) supply the palmar aspect.

Function

  • Support: Central structural axis of the hand, stabilizing carpal and digital motion.

  • Mobility: Allows controlled flexion, extension, and limited rotation at the MCP joint.

  • Grip strength: Acts as a rigid pillar during strong grip and lifting.

  • Load transmission: Transfers forces from the middle finger to the carpus and forearm.

  • Muscle attachment: Provides anchor points for key flexor and extensor tendons.

Clinical Significance

  • Fractures: May occur at the base or shaft, usually from direct trauma; less mobile, hence less prone to displacement.

  • Arthritis: Degenerative or post-traumatic arthritis at the carpometacarpal or MCP joint.

  • Stress injury: Seen in repetitive loading (e.g., athletes, manual laborers).

  • Malalignment: Can cause impaired grip and reduced extension strength.

  • Surgical relevance: Landmark for fixation and hand reconstruction procedures.

  • Imaging importance: Evaluated for fractures, osteochondral lesions, degenerative disease, and inflammatory arthropathy.

MRI Appearance

  • T1-weighted images:

    • Cortex: Low signal (dark).

    • Bone marrow: Bright due to fatty marrow content.

    • Periosteum and ligaments: Low signal linear structures.

    • Pathology: Fractures appear as linear low-signal defects; bone marrow edema appears intermediate-to-bright.

  • T2-weighted images:

    • Cortex: Dark, low signal.

    • Marrow: Bright, slightly less intense than on T1.

    • Cartilage: Intermediate-to-bright over the metacarpal head.

    • Pathology: Marrow edema, subchondral cysts, or bone contusions appear hyperintense.

  • STIR:

    • Normal marrow: Intermediate-to-dark.

    • Pathologic marrow: Bright hyperintense signal in edema, infection, or fracture.

    • Sensitive for early bone and soft-tissue changes.

  • Proton Density Fat-Saturated (PD FS):

    • Normal bone marrow: Intermediate-to-dark.

    • Pathologic: Bright hyperintense areas indicating marrow edema, cortical disruption, or tendon injury.

    • Highlights bone stress, soft-tissue inflammation, or ligamentous injury.

  • T1 Fat-Sat Post-Contrast:

    • Normal bone: Mild homogeneous enhancement.

    • Infection or active inflammation: Patchy, irregular enhancement with cortical involvement.

    • Arthritis or synovitis: Enhancing synovial lining at the MCP or intermetacarpal joints.

CT Appearance

Non-Contrast CT:

  • Cortex: High attenuation, sharply defined.

  • Trabecular bone: Fine honeycomb pattern.

  • Articular surfaces: Smooth, clearly delineated at base and head.

  • Pathology: Identifies fractures, cortical thickening, osteophytes, or erosions.

  • 3D reconstruction: Excellent for visualizing fracture planes, displacements, and surgical planning.

CT VRT 3D image

Metacarpal III (3rd) bone bone CT 3D VRT image -img-00000-00000

MRI image

third metacarpal bone (metacarpal III) coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

Metacarpal III (Third Metacarpal Bone) ct coronal image