Metacarpal bones

At a glance

English

Metacarpal bones

Latin (TA2)

Ossa metacarpi

System

Bone

Region

Hand

In order

From the first metacarpal bone to the fifth metacarpal bone

About the metacarpal bones

The palm is supported by five miniature long bones, the metacarpals, numbered from the thumb (first) to the little finger (fifth). Each lies between a carpal bone of the distal row at the wrist and the proximal phalanx of its digit, and the knuckles of a clenched fist are their heads. Muscles fill the spaces between them: the palmar and dorsal interossei sit in the gaps, and the thenar and hypothenar muscles cover the first and fifth bones.

Every metacarpal has the same three parts. The base at the wrist end is broad and roughly cuboidal; it meets one or more carpal bones at the carpometacarpal joints and, except for the first, touches the bases of its neighbours at small intermetacarpal joints. The shaft is slim, slightly curved so that it is hollow on the palmar side, and triangular in cross-section. The rounded head at the finger end carries a convex articular surface that extends further onto the palm than onto the back; the proximal phalanx rolls over it at the metacarpophalangeal joint, and small tubercles on each side anchor the collateral ligaments.

The five bones differ in length and in how freely they move. The second is usually the longest and the first the shortest and thickest. The second and third are held almost rigidly to the trapezoid and capitate, making a fixed central pillar for the hand, whereas the fourth and fifth can flex a little at their bases, which lets the palm cup around an object. The first metacarpal is the most mobile of all: it stands apart from the others in a different plane, and its saddle joint with the trapezium allows the thumb to be drawn across the palm in opposition.

Metacarpal fractures are common hand injuries. A break of the neck of the fifth metacarpal from punching a hard surface is known as a boxer's fracture, and a fracture-dislocation at the base of the first, often from a blow along the thumb, is called a Bennett fracture because a fragment stays attached while the shaft is pulled out of place. Each bone ossifies from a primary centre in the shaft and one secondary centre, which lies at the base in the first metacarpal and at the head in the other four.