BONES AND JOINTS OF FOREARM AND WRIST
The distal end of the radius is broadened because its carpal articular surface is the bony contact of the forearm with the wrist and hand. This surface is concave transversely and anteroposteriorly; it is divided by a surface constriction and a slight ridging into a larger triangular portion laterally and a smaller quadrangular part medially, which are for the reception of the scaphoid and the lunate of the wrist, respectively.
The medial surface of the distal extremity of the radius is also concave and articular. As the ulnar notch of the radius, it receives the rounded head of the ulna. Dorsally, the distal part of the radius exhibits its tubercle and is otherwise somewhat ridged and grooved for the passage of the tendons of the forearm extensor muscles. Laterally, the bone ends in a downwardly projecting styloid process.
The distal radius and ulna have a rich vascular supply coming from the radial, ulnar, anterior interosseous, and posterior interosseous arteries. The vessels that supply the dorsum of the distal radius can be described for the corresponding extensor tendon compartments as either intercompartmental or compartmental. Nutrient vessels branch off the main vessels and penetrate the retinaculum and underlying bone. An example is the 1,2 intercompartmental supraretinacular artery. A branch off the radial artery, it gives nutrient branches to the bone between the first and second extensor tendon compartments. Clinically, the vessel and its corresponding branches and underlying bone can be harvested and utilized for vascularized bone grafting procedures in the carpus (for example, for a scaphoid nonunion with avascular proximal pole).
Ossification begins in the distal extremity of the radius at the end of the first year, and fusion takes place at age 19 to 20.
The ulna has a small distal extremity. There is a small, rounded styloid process in line with the posterior border of the bone, and there is also a larger, rounded head. The distal surface of the head is smooth for contact with the articular disc of the distal radioulnar joint; it is continuous with the distal surface of the circumference of the head, which is received into the ulnar notch of the head of the radius.
An ossification center for the distal end of the ulna appears at age 5 or 6 and fuses with the shaft at age 18 to 20.
The skeleton of the wrist consists of eight small bones arranged in two rows, proximal and distal. The bones of the proximal row, from the radial to the ulnar side, are the scaphoid, the lunate, the triquetrum, and the pisiform. Those of the distal row, in the same order, are the trapezium, the trapezoid, the capitate, and the hamate. Fundamentally, these bones may be thought of as cubes, each of which has six surfaces. Their dorsal and palmar surfaces are nonarticular and provide for the attachment of the dorsal and palmar ligaments that hold them closely together.
The other surfaces are articular, except for the sub-sutaneous surfaces of the bones that form the borders of the wrist. These surfaces also mostly lodge ligaments. The proximal articular surfaces are generally convex; the distal surfaces are usually concave. Foramina for the entrance of blood vessels are found on nonarticular areas of each bone.
The boat-shaped scaphoid is the largest bone of the proximal row. Its smooth radial articular surface is convex and triangular. The smooth distal surface is triangular but concave and receives both the trapezium and the trapezoid. The medial surface presents two articular facets one for the lunate and a larger inferior concavity for part of the head of the capitate.
The lunate is crescentic; its proximal convexity is for the more medial of the articular surfaces of the distal end of the radius. The distal surface is deeply concave for the capitate and for a small contact with the hamate. On its radial surface, this bone contacts the scaphoid; medially, it has a surface for the base of the triquetrum.
The triquetrum is pyramidal, with the base of the pyramid toward the lunate and the apex downward and ulnarward on the ulnar border of the wrist. The inferior surface is sinuously curved for articulation with the hamate, and the palmar surface has an oval facet for the pisiform.
The pisiform is small and has been likened to a pea. Its single articular facet is for the triquetrum. The pisiform may be regarded as a sesamoid formed in the tendon of the flexor carpi ulnaris muscle.
The trapezium is the radialmost bone of the distal row of carpals. Its proximomedial surface is concave for articulation with the scaphoid, and its distal surface has a saddle-shaped facet for the base of the first metacarpal. On the palmar surface of the bone is a tubercle and a deep groove, through which passes the tendon of the flexor carpi radialis muscle. The tubercle gives attachment to the superficial lamina of the flexor retinaculum and to several thumb muscles. The medial surface of the trapezium has a large concave facet proximally for articulation with the trapezoid and has a small, flat oval surface at the distal angle of the bone for the second metacarpal.
The trapezoid is somewhat wedge shaped, with the broader base of the wedge dorsally. The quadrilateral proximal surface articulates with the scaphoid, whereas distally there is a large saddle-shaped articular surface for the base of the second metacarpal. The lateral surface is convex for the trapezium, whereas the medial surface has a smooth facet for the capitate.
The capitate is the largest of the carpal bones and occupies the center of the wrist. Its rounded head is received into the concavity of the scaphoid and the lunate. The distal, somewhat cuboidal extremity articulates chiefly with the base of the third metacarpal, but by means of small lateral and medial facets it also makes contact with the bases of the second and third metacarpals. The lateral surface has, distally, a small, smooth facet for the distal extremity of the trapezoid, and the medial surface has an oblong articular surface for the hamate.
The hamate is wedge shaped and has a characteristic hooklike process, the hamulus, or hook. The apical proximal part of the wedge articulates with the lunate; the broad distal surface has two concave facets for the bases of the fourth and fifth metacarpals. Articular surfaces laterally and medially are for the capitate and triquetrum, respectively. The hamulus gives attachment to the flexor retinaculum and the tendon of the flexor carpi ulnaris muscle and provides origin for several small finger muscles.
Ossification takes place from a single center in each bone. It begins first in the capitate and then in the hamate early in the 1st year; in the triquetrum, during the 3rd year; in the lunate, in the 4th year; in the trapezium, trapezoid, and scaphoid, in rather close sequence, in the 4th to 6th years; and in the pisiform, in the 11th or 12th year. Ossification starts earlier in the female and is completed between ages 14 and 16. The hamulus of the hamate may have a separate center. An os centrale, normally part of the scaphoid, may be present between the scaphoid, capitate, and trapezoid.