The metacarpus of the hand is composed of five bony structures known as the metacarpals. They consist of a proximal base, a shaft and a distal head. Together they function as the part of the skeleton which scaffolds and stabilizes the palmar and dorsal regions of the hand. This article will now discuss the anatomical positions, adjacent articulations and landmarks of these individual bones, as well as mention some of the main pathological conditions which may directly involve them.
The metacarpal bones connect the wrist with the fingers. They articulate proximally at their base with the distal row of carpal bones and communicate distally at their head with the proximal phalanges. At these meeting points there are smooth surfaces upon the bones known as articular facets. The palmar surfaces of the metacarpal bones are slightly concave, while the dorsal surfaces are the opposite and have triangular shaped distal heads.
The metacarpal bones are numbered in a medial to lateral direction from a dorsal aspect of the hand and correspond to the fingers whose phalanges they articulate with. This means that if you are looking at the right hand you would start with the thumb and continue as follows:
- 1st Metacarpal: Thumb
- 2nd Metacarpal: Index Finger
- 3rd Metacarpal: Middle Finger
- 4th Metacarpal: Ring Finger
- 5th Metacarpal: Little Finger
The first metacarpal bone is unique in the fact that it is the shortest and the most mobile of all the metacarpals. It is positioned on a more anterior plane and it is also rotated at a right angle medially. Its proximal facet is saddle shaped and it articulates with the trapezium bone of the distal row of carpal bones as well as distally with the first proximal phalanx.
The second metacarpal bone has a notch shaped proximal base which connects it to the trapezium bone, the trapezoid bone and the capitate bone of the distal carpal row, as well as the third metacarpal bone on its lateral aspect. It articulates distally at its head with the second proximal phalanx.
The third metacarpal bone has a complex proximal base, as it articulates with several carpal bones, along with the second metacarpal bone on its medial side and the fourth metacarpal bone on its lateral side, for which there are two articular facets. As a whole, the base is shaped as a styloid process on its dorsoradial aspect and has an articular facet which communicates with the capitate bone of the distal carpal row. Distally, the head of the third metacarpal bone connects it to the third proximal phalanx.
The fourth metacarpal bone has two proximal facets at its base which allow it to articulate with the capitate and hamate bones of the distal carpal row, as well as two small facets medially and laterally for the third and fifth metacarpal bones. Distally it links its head to the base of the fourth proximal phalanx.
Lastly, the fifth metacarpal bone articulates proximally with the hamate bone of the distal carpal row and medially with the fourth metacarpal bone. Distally its head articulates with the fifth proximal phalanx.