BONES OF THE HAND
Five metacarpals form the skeleton of the hand. They are miniature “long” bones, comprising a shaft, a head, and a base. They are palpable on the dorsum of the hand and terminate distally in the knuckles, which are their heads (see Plates 4-1 and 4-2).
The shaft is curved longitudinally so as to be convex dorsally and concave on its palmar aspect. The head, the distal extremity, has a rounded smooth surface for articulation with the base of the proximal phalanx. The sides of the head exhibit pits, or tubercles, for the attachment of ligaments. The articular surface of the head is also convex transversely, although less so than dorsopalmarward so that the head fails to be a sphere; however, flexion and extension and abduction and adduction are permitted. The base is cuboidal and broader dorsally than palmarward. Its ends and sides are articular, and the dorsal and palmar surfaces are rough for ligamentous attachments.
The first (thumb) metacarpal is shorter and stouter than the others, and its palmar surface faces toward the center of the palm. It has a proximal saddle-shaped articular surface for contact with the trapezium. Apart from its regular head configuration, it has two palmar articular eminences for the sesamoids of the thumb.
The second (index finger) metacarpal is the longest, and its base is the largest of the metacarpals. There is a deep dorsopalmar groove in the base, which accepts the trapezoid, and the ridges bounding the groove make contact with the trapezium and the capitate. On the ulnar side of the base, there is an incompletely divided facet for the base of the third metacarpal. This complex articulation at the base makes it relatively immobile for power pinch and grip activities.
The third (long finger) metacarpal is distinguished by its styloid process, a dorsally and radially placed proximal eminence. The carpal surface of the bone is concave for the capitate. Subdivided facets exist on the sides of the base for articulation with the bases of the second and fourth metacarpals, also making it relatively immobile.
The fourth (ring finger) metacarpal has a square base that, proximally, has a large facet for the hamate and, laterally, a small facet for the capitate. Two facets on the lateral side of the base make contact with the base of the third metacarpal; and a single, oval facet on the other side faces the fifth metacarpal, allowing for significant motion during power grip.
The fifth (small finger) metacarpal has a single, concavoconvex facet on the proximal surface of its base for articulation with the hamate. A slightly convex facet on the radial side is received into the matching oval facet of the fourth metacarpal that allows for the most movement at the carpometacarpal joint of all the lesser fingers. On the ulnar side of the base there is a prominent tubercle for the attachment of the tendon of the extensor carpi ulnaris muscle.
Ossification of the metacarpals proceeds from two centers one for the body of the bone and one for the distal extremity in each of the four fingers, except for the proximal extremity in the thumb. Ossification begins in the shafts in the eighth or ninth week of fetal life. The centers for the extremity epiphyses appear during the second year, and fusion takes place between ages 16 and 18.
The phalanges are 14 in number, being one short in the thumb (see Plates 4-1 and 4-2). These, too, are miniature long bones, with a shaft and two extremities. The dorsum of the shaft is markedly convex from side to side; its palmar surface is nearly flat. The margins of the palmar surfaces are ridged for attachment of the fibrous flexor sheaths of the digits. The proximal extremity of the first phalanx of each digit is concave and oval and broader from side to side for articulation with the head of the metacarpal. Distally, the proximal extremities of the middle and distal phalanges have two shallow con- cavities separated by an intervening ridge, which articulate with the pulley-like surfaces on the distal ends of the middle and distal phalanges. The distal phalanges exhibit terminal elevated roughened surfaces, which support the pulp of the fingers. The sides of the bases of the phalanges show tubercles for ligaments; the sides of the heads (except on the distal phalanx) exhibit shallow pits for ligamentous attachments.
Ossification of the phalanges proceeds from two centers one for the body and one for the proximal extremity. Ossification in the shaft begins about the eighth week of fetal life and in the epiphysis occurs during the second and third years, with fusion taking place between 14 and 18 years of age.