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The capitate is one of eight irregularly shaped carpal bones in the hand. Situated between the wrist and the long metacarpal bones in the palm in what is referred to as the distal row of carpal bones, it the largest and most centrally located of these bones. The capitate forms a joint with the middle three metacarpals as well as four of the carpal bones surrounding it: the trapezoid or lesser multangular, the scaphoid, the lunate, and the hamate. These joints allow for a variety of movements, including rotation, flexion and extension, and adduction and abduction, or the bringing together and spreading, of the fingers.
Clustered at the base of the hand just beyond the wrist, the carpals join with the radius bone in the forearm to form the radiocarpal or wrist joint. Specifically, the scaphoid and lunate bones join with the bottom of the radius on their superior or upper surfaces, the aspect of the bones facing the wrist. The inferior or lower surfaces of the scaphoid and lunate bones, those facing the fingers, articulate with the superior surfaces of the capitate and hamate bones to form a portion of the midcarpal joint. Resembling a ball-and-socket joint, this articulation permits a small amount of rotation as the capitate and hamate turn vertically within a cup-like socket formed by the adjoining surfaces of the scaphoid and lunate.
On its pinky and thumb sides, the capitate joins with the hamate and trapezoid bones, respectively. Along with the trapezium, also known as the greater multangular, these bones form the row of carpals nearest the fingers. These bones are connected by a series of horizontal ligaments: the volar ligaments on the palm side, the dorsal ligaments on the back side, and the interosseous ligaments between each bone. This connection causes the bones of the distal row to move as a unit, and as such there is not much movement between adjoining bones. That which is allowed between the capitate and the hamate and trapezoid is a slight gliding motion of the bones against each other, classifying these as arthrodial joints.
The inferior surface of the capitate bone features a pair of ridges that subdivide this surface into three distinct articulating surfaces: one for each of the middle three metacarpal bones. Five long bones longitudinally spanning most of the back of the hand, the metacarpals extend from the carpals to the base of the fingers and thumb. The articulations between each metacarpal and the distal row of carpals are referred to as the carpometacarpal joints; the middle three metacarpals, those below the index, middle, and ring fingers, join with the inferior surface of the capitate. Movements allowed at these three joints include very slight flexion and extension, as well as abduction and adduction, or the spreading and closing of the fingers.
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