Fourth Metacarpal Bone: Anatomy, Structure, Function, Imaging

Topic

Fourth metacarpal bone (metacarpal IV)

The fourth metacarpal bone is one of the five metacarpals forming the skeletal framework of the hand. It lies between the third (middle finger) and fifth (little finger) metacarpals, articulating proximally with the hamate bone, medially with the fifth metacarpal, and laterally with the third metacarpal. Distally, it articulates with the base of the proximal phalanx of the ring finger.

It is shorter than the second and third metacarpals but longer than the fifth, forming part of the flexible ulnar segment of the hand that allows grasping and cupping motions. The base of the bone is quadrilateral, the shaft slightly curved, and the head rounded, providing the bony foundation for the ring finger.

Synonyms

Fourth metacarpal
Metacarpal of the ring finger
IV metacarpal bone

Location and Structure

Base (proximal end): Articulates with the hamate and partially with the third and fifth metacarpals.
Shaft: Slightly concave on its palmar aspect, providing attachment to interosseous muscles.
Head (distal end): Rounded and covered with articular cartilage for articulation with the base of the proximal phalanx of the ring finger.
Length: Approximately 4–5 cm in adults.
Shape: Flattened dorsoventrally at base, narrowing distally.

Relations

Proximally: Hamate bone (via carpometacarpal joint).
Distally: Base of proximal phalanx of ring finger (metacarpophalangeal joint).
Medially: Fifth metacarpal bone.
Laterally: Third metacarpal bone.
Dorsally: Extensor tendons and interosseous muscles.
Palmarly: Flexor tendons and lumbrical muscles.

Attachments

Muscular attachments:
- Dorsal interossei: Third dorsal interosseous originates from its medial side.
- Palmar interossei: Absent on the fourth metacarpal as it shares with adjacent digits.
- Flexor carpi ulnaris and extensor carpi ulnaris: Indirectly related via the fifth metacarpal and hamate region.
Ligamentous attachments:
- Intermetacarpal ligaments: Connect the base to adjacent metacarpals.
- Dorsal and palmar carpometacarpal ligaments: Strengthen proximal articulations.
- Collateral ligaments: Reinforce distal metacarpophalangeal joint capsule.

Nerve Supply

(Supplied indirectly through periosteal and muscular branches of the ulnar and median nerves providing sensory innervation to periosteum and joint capsule.)

Arterial Supply

Dorsal and palmar metacarpal arteries from the deep and superficial palmar arches.
Nutrient artery enters the shaft proximally from the palmar surface.

Function

Structural support: Forms the bony axis for the ring finger and contributes to palmar arch stability.
Mobility: Provides moderate movement at its carpometacarpal joint, allowing cupping of the hand.
Force transmission: Distributes compressive and tensile forces between wrist and fingers during gripping.
Muscle leverage: Serves as a rigid anchor for intrinsic hand muscles.

Clinical Significance

Fractures: Common from punching or crush injuries; may involve the shaft or base (reverse boxer’s fracture).
Dislocations: Rare but possible at the carpometacarpal joint due to high-energy trauma.
Osteoarthritis: Degenerative changes at the base may cause ulnar hand pain and stiffness.
Congenital anomalies: Bifid metacarpal or shortened fourth metacarpal seen in certain syndromes (e.g., pseudohypoparathyroidism).
Imaging importance: Essential in evaluating hand trauma, malalignment, and deformity.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark).
- Bone marrow: Bright due to fatty content in adults.
- Periosteum and ligaments: Low-signal linear structures outlining bone surfaces.
- Muscles: Intermediate signal intensity surrounding the shaft.
- Pathology: Fractures appear as linear low-signal lines; bone marrow edema as intermediate-to-bright signal.
T2-weighted images:
- Cortex: Persistently low signal.
- Bone marrow: Bright (fat and fluid components).
- Cartilage at head and base: Intermediate-to-bright signal with smooth contour.
- Joint fluid: Hyperintense (bright).
- Pathology: Edema, contusion, or cystic changes appear hyperintense relative to marrow.
STIR:
- Normal marrow: Intermediate-to-dark signal.
- Pathologic marrow: Bright hyperintense signal in fractures, contusions, or osteomyelitis.
- Excellent for early detection of marrow edema and soft-tissue inflammation.
Proton Density Fat-Saturated (PD FS):
- Normal bone: Intermediate-to-dark homogeneous signal.
- Pathologic regions: Bright focal or diffuse hyperintensity indicating edema, inflammation, or occult fracture.
- Enhances visualization of periosteal reaction or subtle cortical disruption.
T1 Fat-Sat Post-Contrast:
- Normal bone: Mild homogeneous enhancement.
- Infection or inflammation: Patchy marrow or periosteal enhancement.
- Fracture healing: Linear peripheral enhancement surrounding hematoma.
- Tumor or metastasis: Irregular enhancing mass replacing marrow fat.

CT Appearance

Non-Contrast CT:

Cortex: High-attenuation dense outline with clear trabecular architecture.
Trabecular bone: Fine reticular pattern, uniform density.
Articular surfaces: Smooth at both ends, with joint spaces visible at base (hamate articulation) and head (MCP joint).
Pathology:
- Detects fractures, sclerosis, erosions, and periosteal thickening.
- Identifies subtle malalignment, deformities, or bone resorption.
- Assesses degree of displacement and rotational deformity in trauma.