The posterior surface of the radius is characterized by the presence of a large dorsal tubercle, which acts as a pulley for the tendon of one of the extensor muscles of the thumb (extensor pollicis longus). The medial surface is marked by a prominent facet for articulation with the distal end of the ulna (Fig. 7.80). The lateral surface of the radius is diamond shaped and extends distally as a radial styloid process.
The distal end of the bone is marked by two facets for articulation with two carpal bones (the scaphoid and lunate).
Shaft and distal end of ulna
The shaft of the ulna is broad superiorly where it is continuous with the large proximal end and narrow distally to form a small distal head (Fig. 7.81). Like the radius, the shaft of the ulna is triangular in cross section and has: three borders (anterior, posterior, and interosseous), and three surfaces (anterior, posterior, and medial).
The anterior border is smooth and rounded. The posterior border is sharp and palpable along its entire length. The interosseous border is also sharp and is the attachment site for the interosseous membrane, which joins the ulna to the radius.
The anterior surface of the ulna is smooth, except distally where there is a prominent linear roughening for the attachment of the pronator quadratus muscle. The medial surface is smooth and unremarkable. The posterior surface is marked by lines, which separate different regions of muscle attachments to bone.
The distal end of the ulna is small and characterized by a rounded head and the ulnar styloid process (Fig. 7.81). The anterolateral and distal part of the head is covered by articular cartilage. The ulnar styloid process originates from the posteromedial aspect of the ulna and projects distally.
The distal radio-ulnar joint occurs between the articular surface of the head of the ulna, with the ulnar notch on the end of the radius, and with a fibrous articular disc, which separates the radio-ulnar joint from the wrist joint (Fig. 7.82).
The triangular-shaped articular disc is attached by its apex to a roughened depression on the ulna between the styloid process and the articular surface of the head, and by its base to the angular margin of the radius between the ulnar notch and the articular surface for the carpal bones.
The synovial membrane is attached to the margins of the distal radio-ulnar joint and is covered on its external surface by a fibrous joint capsule.
The distal radio-ulnar joint allows the distal end of the radius to move anteromedially over the ulna.
The interosseous membrane is a thin fibrous sheet that connects the medial and lateral borders of the radius and ulna, respectively (Fig. 7.82). Collagen fibers within the sheet pass predominantly inferiorly from the radius to the ulna.
The interosseous membrane has a free upper margin, which is situated just inferior to the radial tuberosity, and a small circular aperture in its distal third. Vessels pass between the anterior and posterior compartments superior to the upper margin and through the inferior aperture.
The interosseous membrane connects the radius and ulna without restricting pronation and supination and provides attachment for muscles in the anterior and posterior compartments. The orientation of fibers in the membrane is also consistent with its role in transferring forces from the radius to the ulna and ultimately, therefore, from the hand to the humerus.
Pronation and supination of the hand occur entirely in the forearm and involve rotation of the radius at the elbow and movement of the distal end of the radius over the ulna (Fig. 7.83).