The metacarpal bones form the skeleton of the palm, articulating proximally with the carpus, and distally with the digits. They are five in number, one for each digit, and lie side by side and slightly divergent from each other, being separated by intervals called interosseous spaces. Distinguished numerically from the lateral to the medial side, they all display certain common characters; each possesses
- A body or shaft
- A base or carpal end
- A head or phalangeal end.
Body (shaft) of Metacarpal Bones:
The bodies, which are slightly curved towards the anterior aspect, are narrowest towards their middle. The dorsal surface of each is marked by two divergent lines passing distally from the dorsum of the base to tubercles on either side of the head.
Base of Metacarpal Bones:
The bases, all more or less wedge-shaped in form, articulate with the carpus. They differ in size and shape according to the joints they form. Of the five metacarpal bones, the first, i.e. that of the thumb is the shortest and stoutest, the second is the longest, whilst the third, fourth, and fifth display a gradual reduction in length. The medial four bones articulate by their bases with each other, and are united at their distal ends by ligaments. They are so arranged as to conform to the hollow of the palm, being concave from side to side anteriorly and convex posteriorly.
Head of Metacarpal Bones:
The head is provided with a surface for articulation with the proximal phalanx. This area curves farther over its anterior aspect than the posterior. It is convex from before backwards and from side to side and is wider anteriorly than posteriorly. It is notched on its anterior aspect and its edges form two prominent tubercles, which are sometimes grooved for the small sesamoid bones.
First metacarpal bone:
The first metacarpal differs from the others in being free at its distal end. Its proximal end possesses only a carpal articular facet and is not articulated with other metacarpal bones. The first metacarpal bone is the shortest and stoutest of the series. Its body is compressed from before backwards. Its head is of large size and is grooved on its anterior aspect for the sesamoid bones. The base is provided with a saddle-shaped surface for articulation with the trapezius.
Second metacarpal bone:
The second metacarpal bone is recognized by its length and its broad and deeply notched base for articulation with the trapezius. It has a small half-oval facet for the trapezius on the lateral side of its base, while on its medial aspect it presents a narrow vertical strip for the capitate.
Third metacarpal bone:
The third metacarpal bone can usually be recognized by the pointed styloid process which springs from the dorsum of its base towards the radial side. On the proximal surface of the base there is a facet for the capitate. On the lateral side there are two half-oval facets for the second metacarpal. On the medial side there are usually two small oval or nearly circular facets for the fourth metacarpal.
Fourth metacarpal bone:
The fourth metacarpal bone may be recognized by a method of exclusion. It is unlike either the first, second, or third, and differs from the fifth by having articular surfaces on both sides of its base. Proximally there is a quadrilateral surface on its base for articulation with the hamate. On its lateral side there are usually two small oval facets for the third metacarpal. Of these facets the dorsal one not infrequently has a narrow surface for articulation with the capitate. On the medial side there is a narrow articular strip for the base of the fifth metacarpal.
Fifth metacarpal bone:
The fifth metacarpal bone can be recognized by its size and the fact that it has an articular facet only on one side of its base, namely, that on its lateral side for the fourth metacarpal. The carpal articular surface is saddle-shaped, and there is a tubercle on the medial side of the base.
Ossification of metacarpal bones:
The metacarpal bones are developed from primary and secondary centers; but there is a remarkable difference between the mode of growth of the first and the remaining four metacarpals. The body and head of the first metacarpal are developed from the primary center, and its base from a secondary center, in the case of the second, third, fourth, and fifth metacarpals the bodies and bases are developed from the primary centers, the heads in these instances being derived from the secondary centers.