Capitate Bone: Anatomy, Articulations, Function, Clinical Significance, MRI & CT Imaging

Topic

Capitate

The capitate bone is the largest carpal bone and forms the central axis of the wrist. It occupies the middle of the distal carpal row and articulates proximally with the lunate, distally with the third metacarpal, laterally with the trapezoid, and medially with the hamate. It serves as the keystone of the carpal arch, ensuring stability and coordinated motion between proximal and distal carpal rows.

The capitate plays a vital biomechanical role in wrist flexion, extension, and rotation, transmitting forces from the hand to the forearm. Because of its central position and multiple articulations, it is a common site for fractures, osteonecrosis (capitate avascular necrosis), and carpal instability patterns.

Synonyms

Os capitatum
Carpal capitate
Third carpal bone

Location and Structure

Location: Center of the distal carpal row, between the trapezoid laterally and hamate medially.
Shape: Irregularly shaped with a rounded proximal head and a distal constricted neck that widens to a block-like base.
Surfaces:
- Proximal surface: Convex, articulates with lunate.
- Distal surface: Articulates with the base of the third metacarpal and partially with the second and fourth metacarpals.
- Lateral surface: Articulates with trapezoid.
- Medial surface: Articulates with hamate.
- Dorsal and palmar surfaces: Provide ligamentous attachments.

Articulations

Proximal: Lunate
Distal: Third metacarpal base (primary), also second and fourth metacarpals
Lateral: Trapezoid
Medial: Hamate

Attachments

Ligaments:
- Palmar intercarpal ligaments: Connect to lunate, hamate, and trapezoid.
- Dorsal intercarpal ligaments: Link the dorsal surfaces of neighboring carpals.
- Radiocapitate ligament: Connects capitate to the distal radius.
- Capitohamate and capitotrapezoid ligaments: Reinforce lateral and medial stability.
Muscle/tendon relations:
- Deep to the flexor digitorum tendons and flexor carpi radialis in palmar view.
- Dorsally related to extensor tendons of the fingers and wrist.

Relations

Anteriorly (palmar): Flexor tendons, median nerve, flexor retinaculum
Posteriorly (dorsal): Extensor tendons crossing the wrist
Laterally: Trapezoid and second metacarpal
Medially: Hamate and fourth metacarpal base
Proximally: Lunate and intercarpal ligaments

Function

Central stabilizer: Serves as the axial center of carpal rotation and stability.
Force transmission: Transfers load from metacarpals to radius through lunate.
Kinematic coordination: Maintains synchronous motion between proximal and distal carpal rows.
Support: Contributes to the concavity of the carpal arch forming the carpal tunnel floor.

Clinical Significance

Fractures: Usually occur from axial compression or falls on an outstretched hand; may be associated with perilunate dislocations.
Avascular necrosis (capitate osteonecrosis): Rare but can result from fracture or vascular disruption.
Carpal instability: Injury to capitate ligaments affects midcarpal stability.
Arthritis: Degenerative or post-traumatic changes can limit wrist range of motion.
Surgical relevance: Central landmark during wrist arthrodesis and carpal reconstruction procedures.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark).
- Marrow: Bright signal due to fatty content in normal adults.
- Articular cartilage: Thin, smooth intermediate-to-low signal on articular surfaces.
- Pathology: Fractures appear as low-signal lines; avascular necrosis shows diffuse or focal low-signal marrow changes.
T2-weighted images:
- Cortex: Low signal.
- Marrow: Bright, especially in marrow edema or hyperemia.
- Cartilage: Intermediate-to-bright; thinning or irregularity indicates chondral degeneration.
- Pathology: Fracture edema, cysts, or osteonecrosis manifest as high-signal foci.
STIR:
- Normal marrow: Intermediate-to-dark signal.
- Edema, contusion, or infection: Bright hyperintense signal.
- Useful for early detection of avascular necrosis or bone stress.
Proton Density Fat-Saturated (PD FS):
- Normal marrow: Intermediate-to-dark homogeneous signal.
- Abnormal: Bright signal in edema, bone bruise, or fracture line.
- Provides excellent contrast for subtle trabecular or ligamentous injuries.
T1 Fat-Sat Post-Contrast:
- Normal capitate: Uniform enhancement of marrow.
- Avascular necrosis: Poor or absent enhancement in necrotic regions.
- Synovitis or arthritis: Enhancing capsule and pericapsular soft tissues.

CT Appearance

Non-Contrast CT:

Cortex: High attenuation, sharply marginated.
Trabecular bone: Fine honeycomb structure visible in axial detail.
Articular surface: Smooth and well-corticated.
Pathology: Excellent for detecting fractures, sclerosis, cystic changes, or malalignment; avascular necrosis appears as areas of sclerosis and fragmentation.