Trapezium bone

The trapezium is one of the eight carpal bones, located in the distal carpal row on the radial (lateral/thumb) side of the wrist. It is the carpal bone that supports the thumb, forming the base of the first (thumb) column of the hand.

Key anatomical features:

• Articulations: The trapezium articulates distally with the base of the first metacarpal (forming the trapeziometacarpal, or thumb carpometacarpal, joint), proximally with the scaphoid, medially (ulnarly) with the trapezoid, and with the base of the second metacarpal. This makes it the most mobile bone of the distal carpal row.

• Saddle-shaped articular surface: Its distal (metacarpal) surface has a characteristic saddle configuration—convex in the sagittal plane and concave in the coronal plane of the thumb. This biconcavo-convex geometry permits the flexion–extension, abduction–adduction, and axial rotation required for thumb opposition, the defining feature of the human hand.

• Trapezial ridge (tubercle) and groove: The palmar (volar) surface bears a bony ridge, or tubercle, adjacent to a groove that transmits the tendon of the flexor carpi radialis. The flexor retinaculum (transverse carpal ligament) attaches to this ridge, and the thenar muscles (abductor pollicis brevis, flexor pollicis brevis, opponens pollicis) overlie the volar surface.

• Ligamentous stabilization: The trapezium is the anchor point for the majority of the ligaments stabilizing the thumb base—14 of 16 described ligaments insert on it—with the deep anterior oblique (beak) ligament and dorsoradial ligament being principal stabilizers of the trapeziometacarpal joint.

• Size/relations: Among the carpal bones it ranks fourth in size (after the capitate, hamate, and scaphoid), with reported mean cadaveric dimensions of roughly 23 mm length, 15 mm width, and 15 mm height.

Gray's description:

The trapezium bone (greater multangulate, os multangulum majus) may be distinguished by a deep groove on its volar surface. It is situated at the radial side of the carpus, between the scaphoid and the first metacarpal bone.

• The superior surface is directed upward and medialward; medially it is smooth, and articulates with the scaphoid; laterally it is rough and continuous with the lateral surface.

• The inferior surface is oval, concave from side to side, convex from before backward, so as to form a saddle-shaped surface for articulation with the base of the first metacarpal bone.

• The dorsal surface is rough.

• The volar surface is narrow and rough. At its upper part is a deep groove, running from above obliquely downward and medialward; it transmits the tendon of the Flexor carpi radialis, and is bounded laterally by an oblique ridge. This surface gives origin to the Opponens pollicis and to the Abductor and Flexor pollicis brevis; it also affords attachment to the transverse carpal ligament.

• The lateral surface is broad and rough, for the attachment of ligaments.

• The medial surface presents two facets; the upper, large and concave, articulates with the trapezoid; the lower, small and oval, with the base of the second metacarpal.

The trapezium articulates with four bones: the scaphoid proximally, the first metacarpal distally, and the trapezoid and second metacarpal medially.