There are three groups of bones in the hand:
The eight carpal bones are the bones of the wrist.
The five metacarpals (I to V) are the bones of the metacarpus.
The phalanges are the bones of the digits—the thumb has only two; the rest of the digits have three (Fig. 7.94).
The carpal bones and metacarpals of the index, middle, ring, and little fingers (metacarpals II to V) tend to function as a unit and form much of the bony framework of the palm. The metacarpal of the thumb functions independently and has increased flexibility at the carpometacarpal joint to provide opposition of the thumb to the fingers.
The small carpal bones of the wrist are arranged in two rows, a proximal and a distal row, each consisting of four bones (Fig. 7.94).
From lateral to medial and when viewed from anteriorly, the proximal row of bones consists of: the boat-shaped scaphoid, the lunate, which has a crescent shape, the three-sided triquetrum bone, and the pea-shaped pisiform (Fig. 7.94).
The pisiform is a sesamoid bone in the tendon of the flexor carpi ulnaris and articulates with the anterior surface of the triquetrum.
The scaphoid has a prominent tubercle on its lateral palmar surface that is directed anteriorly.
From lateral to medial and when viewed from anteriorly, the distal row of carpal bones consists of: the irregular four-sided trapezium bone, the four-sided trapezoid, the capitate, which has a head, and the hamate, which has a hook (Fig. 7.94).
The trapezium articulates with the metacarpal bone of the thumb and has a distinct tubercle on its palmar surface that projects anteriorly.
The largest of the carpal bones, the capitate, articulates with the base of metacarpal III.
The hamate, which is positioned just lateral and distal to the pisiform, has a prominent hook (hook of hamate) on its palmar surface that projects anteriorly.
The carpal bones have numerous articular surfaces (Fig. 7.94). All of them articulate with each other, and the carpal bones in the distal row articulate with the metacarpals of the digits. With the exception of the metacarpal of the thumb, all movements of the metacarpal bones on the carpal bones are limited.
The expansive proximal surfaces of the scaphoid and lunate articulate with the radius to form the wrist joint.
The carpal bones do not lie in a flat plane; rather, they form an arch, whose base is directed anteriorly (Fig. 7.94). The lateral side of this base is formed by the tubercles of the scaphoid and trapezium. The medial side is formed by the pisiform and the hook of the hamate.
The flexor retinaculum attaches to, and spans the distance between, the medial and lateral sides of the base to form the anterior wall of the so-called carpal tunnel. The sides and roof of the carpal tunnel are formed by the arch of the carpal bones.
Each of the five metacarpals is related to one digit:
Metacarpal I is related to the thumb.
Metacarpals II to V are related to the index, middle, ring, and little fingers, respectively (Fig. 7.94).
Each metacarpal consists of a base, a shaft (body), and distally, a head.
All of the bases of the metacarpals articulate with the carpal bones; in addition, the bases of the metacarpal bones of the fingers articulate with each other.
All of the heads of the metacarpals articulate with the proximal phalanges of the digits. The heads form the knuckles on the dorsal surface of the hand when the fingers are flexed.
The phalanges are the bones of the digits (Fig. 7.94):
The thumb has two—a proximal and a distal phalanx.