Lunate Bone: Anatomy, Relations, Function, and Imaging

Topic

Lunate

The lunate is a crescent-shaped carpal bone located in the proximal row of the wrist, situated between the scaphoid laterally and the triquetrum medially. It plays a crucial role in wrist motion and load transmission between the forearm and hand. The lunate articulates proximally with the radius, distally with the capitate and hamate, and laterally with the scaphoid.

It is the central keystone of the proximal carpal row, contributing to the concavity of the carpal arch and enabling flexion-extension, radial-ulnar deviation, and carpal rotation. The lunate is clinically significant due to its susceptibility to avascular necrosis (Kienböck’s disease) and carpal instability or dislocation.

Synonyms

Semilunar bone
Intermediate carpal bone

Location and Structure

Position: Proximal carpal row, second bone from the lateral side of the wrist.
Shape: Crescentic or semilunar; convex proximally and concave distally.
Surfaces:
- Proximal: Convex, articulates with the lunate fossa of the radius.
- Distal: Concave, articulates with the head of the capitate and sometimes the hamate.
- Lateral: Narrow, articulates with the scaphoid.
- Medial: Broad, articulates with the triquetrum.
- Palmar: Rough surface for attachment of the palmar radiocarpal and ulnocarpal ligaments.
- Dorsal: Rough surface for dorsal radiocarpal ligament attachment.

Articulations

Proximal: Radius (lunate fossa).
Distal: Capitate and sometimes hamate.
Lateral: Scaphoid.
Medial: Triquetrum.

Relations

Anteriorly (palmar): Flexor digitorum profundus tendons and palmar radiocarpal ligaments.
Posteriorly (dorsal): Dorsal radiocarpal ligament and extensor tendons of the wrist.
Proximally: Lunate fossa of the radius.
Distally: Capitate head and hamate.
Laterally: Scaphoid.
Medially: Triquetrum and ulnar collateral ligament.

Arterial Supply

Supplied by palmar and dorsal branches of the radial and ulnar arteries, forming a delicate intraosseous network.
Vascular pattern is variable, with most supply entering dorsally—explaining the lunate’s predisposition to avascular necrosis when dorsal vessels are compromised.

Function

Load transmission: Transfers forces from radius to capitate during wrist movement.
Joint stability: Maintains carpal alignment within the proximal row.
Motion facilitation: Enables wrist flexion, extension, and deviation movements.
Shock absorption: Cushions axial load during gripping and impact activities.
Kinematic pivot: Acts as a central stabilizing bone of the midcarpal joint.

Clinical Significance

Kienböck’s disease: Avascular necrosis of the lunate due to compromised vascularity.
Lunate dislocation: May occur from high-energy trauma or ligament disruption; commonly dorsal.
Perilunate dislocation: Lunate remains aligned with the radius while other carpals displace dorsally.
Fractures: Rare but can be associated with scaphoid fractures or high axial loads.
Carpal instability: Results from ligamentous injury, particularly of the scapholunate or lunotriquetral ligaments.
Radiological importance: Central in diagnosing carpal alignment disorders on wrist imaging.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark).
- Marrow: Bright, homogeneous signal in normal bone (fatty marrow).
- Cartilage: Smooth intermediate-to-low signal outlining articular surfaces.
- Pathology: Kienböck’s disease shows patchy or diffuse low T1 signal due to marrow ischemia; fractures show linear low-signal lines.
T2-weighted images:
- Normal marrow: Bright, slightly less than fat on T1, with homogeneous intensity.
- Cortex: Low signal rim.
- Pathology: Avascular necrosis shows mixed high and low signal (bone edema and sclerosis).
- Joint fluid: Hyperintense around articulations.
- Fracture edema or reactive change: Bright hyperintensity in subchondral bone.
STIR:
- Normal lunate: Intermediate-to-dark signal.
- Pathology: Bright hyperintense areas representing bone marrow edema, infarction, or stress reaction.
- Excellent for early detection of Kienböck’s disease.
Proton Density Fat-Saturated (PD FS):
- Normal bone: Intermediate-to-dark homogeneous signal.
- Pathologic marrow: Bright focal or diffuse hyperintensity indicating edema or inflammation.
- Useful for: Differentiating early ischemia, subtle trabecular injury, and bone contusion.
T1 Fat-Sat Post-Contrast:
- Normal: Homogeneous mild enhancement.
- Avascular necrosis: Non-enhancing or irregularly enhancing zones within lunate body.
- Reactive hyperemia: Peripheral rim enhancement around necrotic core.
- Synovitis or carpal effusion: Enhancing synovium surrounding lunate.

CT Appearance

Non-Contrast CT:

Cortex: High attenuation, sharply defined margins.
Trabecular bone: Fine honeycomb structure; well visualized in thin-section scans.
Articular surfaces: Smooth convex and concave contours, continuous with scaphoid and triquetrum.
Pathology:
- Fractures: Linear lucencies or cortical step-offs.
- Kienböck’s disease: Patchy sclerosis, collapse, or fragmentation of lunate body.
- Instability: Malalignment with adjacent carpal bones (loss of carpal arcs).