Lunate bone

The lunate bone is a crescent- (moon-) shaped carpal bone located in the center of the proximal carpal row of the wrist, positioned between the scaphoid (radially) and the triquetrum (ulnarly). It takes its name from its half-moon appearance on a lateral wrist radiograph (Latin luna, moon).

Key anatomical features:

• Articulations: Proximally it articulates with the distal radius (and, on the ulnar side, the triangular fibrocartilage complex) at the radiocarpal joint; distally with the capitate (and variably the hamate) at the midcarpal joint; and on its sides with the scaphoid and triquetrum via the scapholunate and lunotriquetral interosseous ligaments. Its palmar surface is quadrilateral and the lateral (proximal) surface is crescent-shaped.

• Structure: It is composed of a body, a volar (palmar) pole, and a dorsal pole, with multiple stabilizing wrist ligaments attaching to it.

• Functional role: The lunate is regarded as the "keystone" of the proximal carpal row and central carpal column, transmitting substantial axial and torsional loads between the carpus and the radius. As part of the intercalated (proximal-row) segment, it is held in balance between the flexion moment of the scaphoid and the extension moment of the triquetrum.

• Clinical relevance: Its central, load-bearing position makes it pivotal in carpal instability (perilunate dislocation) and it is the bone affected by avascular necrosis in Kienböck disease.

Gray's description:

The lunate bone may be distinguished by its deep concavity and crescentic outline. It is situated in the center of the proximal row of the carpus, between the scaphoid and triangular.

• The superior surface, convex and smooth, articulates with the radius.

• The inferior surface is deeply concave, and of greater extent from before backward than transversely: it articulates with the head of the capitate, and, by a long, narrow facet (separated by a ridge from the general surface), with the hamate.

• The dorsal and volar surfaces are rough, for the attachment of ligaments, the former being the broader, and of a somewhat rounded form.

• The lateral surface presents a narrow, flattened, semilunar facet for articulation with the scaphoid.

• The medial surface is marked by a smooth, quadrilateral facet, for articulation with the triquetrum.