A trapezoid, also known as a trapezium is the first and most lateral of the distal row of carpal bones is a flat closed shape having 4 straight surfaces or sides, with one pair of parallel sides found within the hand.
The parallel sides of a trapezium are known as the bases, the broad end of the wedge constituting the dorsal, the narrow end the palmar surface; and their non-parallel sides are called legs. A trapezium can also have parallel legs, and parallel sides can be horizontal, vertical, or slanting.
Surfaces
The superior surface, quadrilateral, smooth, and slightly concave, articulates with the scaphoid. The inferior surface articulates with the proximal end of the second metacarpal bone; it is convex from side to side, concave from before backward, and subdivided by an elevated ridge into two unequal facets.
The lateral surface, convex and smooth, articulates with the trapezium. The medial surface is concave and smooth in front, for articulation with the capitate; rough behind, for the attachment of an interosseous ligament.
The dorsal and palmar surfaces are rough for the attachment of ligaments, the former being the larger of the two.
The trapezium bone is bordered medially by the trapezoid bone and superiorly by the scaphoid bone. Inferolaterally, its main articulation is with the first metacarpal bone via a saddle-shaped facet. Inferomedially however, it sometimes also articulates with the second metacarpal bone.
Distally, it forms a stable, relatively immobile joint with the second metacarpal, radially and proximally it forms strong ligaments with the trapezium and the capitate clearly, scaphoid respectively.
However, injury can occur through axial force applied to the second metacarpal base. Subluxations, such as ones caused by delivering a blow, are not uncommon. Direct trauma to the bone can also cause a fracture.
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