The capitate, also known as the os magnum, is the largest of the carpal bones and sits at the center of the distal carpal row. A distinctive head-shaped bone, it has a protected position in the carpus, and thus isolated fractures are unusual.
The capitate sits in a proximodistal direction with a waist that is proximal to the transverse midline. The capitate head sits in the space allowed by the lunate and scaphoid bones of the proximal carpal bone row. Proximally, the capitate has a rounded surface whilst the distal end has a triangular shape with a palmarly directed apex. The palmar surface is slightly convex with the other surfaces concave being the dorsal, ulnar and radial surfaces. Both dorsal and palmar surfaces are rough to allow for carpal ligament attachment.
Articulations with the capitate include:
- proximal surface: scaphoid and lunate
- distal surface: base of third metacarpal and smaller articulations with base of second and fourth metacarpal bones
- lateral surface: trapezoid
- medial surface: hamate
There is potential for articulation of the capitate with the triquetrum at the proximal medial border with radial deviation of the wrist.
Oblique head of adductor pollicis muscle (also known as adductor pollicis obliquus)
- dorsal and volar carpometacarpal
See “Articulations” above.
The dorsal intercarpal and dorsal basal metacarpal arches provide the majority of the vascular supply. Anastomoses of the ulnar recurrent and palmar intercarpal arches provide additional vascularity. Vessels enter via both the dorsal and palmar bone surfaces, although there may be anastomoses between the dorsal and palmar blood supplies.
On the dorsal surface, vessels enter at the distal two-thirds of the dorsal surface in a proximal and palmar direction. A retrograde course supplies the body and head. Dorsal supply continues through to the palmar surface with terminal vessels to the proximal palmar head.
On the palmar surface, vessels enter the distal surface in a retrograde pattern, returning proximally.
The capitate usually has a small facet with the base of the fourth metacarpal, which is absent in approximately 14% of wrists.
Accessory bones from residual secondary ossification centers may be mistaken as fractures. Accessory bones may also be the result of trauma or synovial tag ossification. See: Accessory ossicles of the wrist.
Due to its protected position, fractures of the capitate are rare. Fractures may be isolated, but are often associated with perilunate injuries and carpometacarpal fracture-dislocation. Fractures may be non-displaced or may demonstrate rotation of the proximal fragment. Proximal pole osteonecrosis may develop.
- fracture of the capitate and scaphoid
- rotated proximal capitate fracture with articular surface displaced
The capitate has one ossification center and is generally the first of the carpal bones to ossify. Ossification usually commences in the second month (however can begin at birth). Accessory bones may be associated if there is a failure of fusion from additional ossification centers.
History and etymology
In Latin, ‘caput’ means ‘head’, and from this, capitate therefore means ‘head-shaped’.
The capitate may be visualized on a number of series of the distal upper limb including:
Capitate injury can be difficult to see on plain film due to projectional overlapping of bones.
CT or MRI imaging will demonstrate the capitate and should be considered if there is clinical suspicion of occult injury.
As with other bones, scintigraphy may demonstrate focal uptake in the setting of fracture.