The metacarpal bones or metacarpals (also known as metacarpus, Latin: os metacarpale, pl. ossa metacarpalia) are five bones that form the middle portion of the hand skeleton. They lie between the carpal bones proximally and the phalanges distally. The metacarpal bones are analogous to the metatarsal bones in the foot.
The five metacarpal bones are numbered from the medial side of the hand to the lateral, and each is associated with a digit:
- first metacarpal (or metacarpal I) - associated with the thumb;
- second metacarpal (II) - with the index finger;
- third metacarpal (III) - with the middle finger;
- fourth metacarpal (IV) - with the ring finger;
- fifth metacarpal (V) - with the little finger.
The spaces between the metacarpal bones are called interosseous metacarpal spaces. The medial and lateral surfaces of the metacarpals are concave, allowing attachment of the interossei muscles.
Each metacarpal bone consists of three main parts:
- base - at the proximal or carpal end;
- body or shaft - the middle part of a bone;
- head - at the distal or digital end;
The transition area between the head and the body is known as the neck of a metacarpal bone.
The base or carpal extremity of each metacarpal bone is cuboidal-shaped and broader behind than in the front. The base articulates with the carpal bones and with the bases of the adjacent metacarpal bones. Its dorsal and ventral surfaces are rough for attachments of ligaments.
The body or shaft of each metacarpal bone is prismoid in form and curved to be concave in front and convex behind. It has three surfaces - medial, lateral, and dorsal. The medial and lateral surfaces are concave for attachment of the interossei muscles and separated from one another by an anterior ridge. The dorsal surface in its distal two-thirds is smooth and covered by tendons of the extensor muscles. Interossei dorsales attach to two sloping surfaces separated by a ridge - located on the dorsal surface of the metacarpals.
The head or digital extremity of a metacarpal bone has an oblong surface, which forms a condyloid joint with the proximal phalanx. On each side of the head is a tubercle. This tubercle serves for the attachment of the collateral ligament of the metacarpophalangeal joint. The dorsal surface is broad and flat, gives attachment to tendons of the extensor muscles. The volar surface of the head has a groove in the middle line for the passage of the flexor tendons.
All metacarpal bones at their distal ends articulate with the proximal phalanges, forming the metacarpophalangeal joints. Proximally, the metacarpal bones form carpometacarpal joints, by articulating with the following carpal bones and with adjacent metacarpal bones:
- I - with the trapezium;
- II - with the trapezium, trapezoid, capitate, and metacarpal III;
- III - with the capitate, and metacarpals II and IV;
- IV - with the capitate, hamate, and metacarpals III and V;
- V - with the hamate, and metacarpal IV.