First metacarpal

First metacarpal bone
First metacarpal of the left hand (shown in red). Palmar view.
The first metacarpal (Left)
Details
Identifiers
Latin	os metacarpale I
MeSH	A02.835.232.087.319.550
FMA	23899
Anatomical terms of bone []

The first metacarpal bone or the metacarpal bone of the thumb is the first bone proximal to the thumb. It is connected to the trapezium of the carpus at the first carpometacarpal joint and to the proximal thumb phalanx at the first metacarpophalangeal joint.

The first metacarpal bone is short and thick with a shaft thicker and broader than those of the other metacarpal bones. Its narrow shaft connects its widened base and rounded head; the former consisting of a thick cortical bone surrounding the open medullary canal; the latter two consisting of cancellous bone surrounded by a thin cortical shell.

The head is less rounded and less spherical than those of the other metacarpals, making it better suited for a hinge-like articulation.

The distal articular surface is quadrilateral, wide, and flat; thicker and broader transversely and extends much further palmarly than dorsally. On the palmar aspect of the articular surface there is a pair of eminences or tubercles which articulate with the radial and ulnar sesamoid bones of the thumb metacarpophalangeal joint; the lateral eminence is larger than the medial.

The body or shaft is thick and broad — averaging 6 to 11 millimetres (0.24 to 0.43 in). On its dorsal side, the shaft is flat and wide, while the anteroposterior side is less pronounced; usually resulting in an oval-triangular cross-section.

The dorsal surface of the shaft is weakly convex longitudinally, while its palmar, radial, and ulnar surfaces tend to be concave. The palmar and medial surface exhibits a blunt ridge which separates a larger lateral part – the insertion of the opponens pollicis muscle – and a smaller medial part – the origin of the lateral head of the first dorsal interosseous muscle.

The base is significantly different from the bases of the other metacarpals. It is trumpet-shaped and ends in a saddle-shaped articular surface matching that of the trapezial articular surface. The configuration of the thumb carpometacarpal joint plays an important role in the mechanism of opposition. The articular surface is delimited by a thick, crest-like ridge extending around its circumference.