Lunate
Os Lunatum
location_on Proximal row of carpal bones, between the scaphoid and triquetrum
The lunate is a crescent or moon-shaped carpal bone in the center of the proximal row, articulating proximally with the radius and distally with the capitate. It is the most commonly dislocated carpal bone and is vulnerable to avascular necrosis (Kienbock disease). The lunate occupies a critical position in carpal mechanics, acting as the keystone of the proximal row.
Key Anatomical Features
- Moon or crescent shape when viewed from the side gives it its name
- Proximal surface is convex and articulates with the lunate fossa of the radius
- Distal surface is deeply concave for the head of the capitate
- Palmar surface is broader than the dorsal surface, contributing to the palmar tilt of the wrist
- May have a medial facet for the hamate (Type II lunate, present in about 50% of people)
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| No direct muscular attachments | The lunate is stabilized entirely by ligaments | N/A |
| Extrinsic wrist ligaments | Short and long radiolunate ligaments | Stabilize the lunate within the proximal carpal row |
| Scapholunate ligament | Between scaphoid and lunate | Maintains proximal row integrity |
| Lunotriquetral ligament | Between lunate and triquetrum | Maintains proximal row integrity |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Radiocarpal joint | Synovial ellipsoid | Lunate fossa of radius |
| Lunocapitate joint | Synovial plane | Capitate |
| Scapholunate joint | Synovial plane | Scaphoid |
Common Pathologies
Kienbock disease
Avascular necrosis of the lunate, causing progressive wrist pain, stiffness, and weakness. Classified by Lichtman staging from I to IV based on radiographic changes.
Lunate dislocation
The most common carpal dislocation. The lunate tilts palmarly out of its fossa and can compress the median nerve in the carpal tunnel. Often missed on initial X-rays.
Perilunate dislocation
The carpus dislocates around the lunate, which remains in its fossa. A high-energy injury often missed on initial radiographs (up to 25% are missed).
Scapholunate dissociation
Disruption of the scapholunate interosseous ligament allowing abnormal motion between the scaphoid and lunate, leading to carpal instability.
Clinical Relevance
On a lateral wrist radiograph, the lunate should sit in the lunate fossa of the radius with the capitate seated in its concavity. In lunate dislocation, the lunate tips palmarly like a spilled teacup. In perilunate dislocation, the lunate remains aligned with the radius but the capitate is displaced dorsally. Kienbock disease is associated with negative ulnar variance (a short ulna relative to the radius).
Development and Ossification
The lunate ossifies from a single center appearing between ages 3 and 4. It is entirely cartilaginous at birth. A Type II lunate (with a medial hamate facet) is associated with a lower incidence of Kienbock disease, possibly due to improved load distribution.
Did You Know?
- Lunate means moon-shaped in Latin (luna = moon), referring to its crescent profile
- The lunate is the most frequently dislocated carpal bone
- Kienbock disease was first described in 1910 by Austrian radiologist Robert Kienbock
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