32 Systems Anatomy

FIGURE 1.21. (continued) E: Proximal radioulnar joint, with radial head removed, showing

annular ligament. F: Proximal ulna, with proximal radius removed to show annular ligament and

radial notch. G: Right elbow, anterior aspect, showing synovial membrane. The capsule has been

removed and the articular cavity distended. H: Right elbow, posterior aspect, showing synovial

membrane. The capsule has been removed and the articular cavity distended.

E F

G H

and 1.21A,E, and F). The notch is narrow, oblong, and

lined with articular cartilage. The notch articulates with the

circumferential rim of the radial head. The anterior and

posterior margins of the radial notch provide the attachment areas for the annular ligament.

Shaft (Body) of the Ulna

The shaft (or body) of the ulna is triangular in cross-section

in the proximal two-thirds, but becomes round in the distal

third. Longitudinally, the proximal half of the shaft is

slightly convex dorsally and concave anteriorly. The distal

half (and sometimes central portion) becomes longitudinally straight. The distal half of the shaft may be slightly

concave laterally and convex medially. In cross-section, the

triangular shape presents an anterior, posterior, and medial

surface, as well as an anterior border, posterior border, and

interosseous border (each of which is located at the apex of

the triangular cross-sectional shape). The interosseous ligament is attached along the interosseous border apex of the

triangle, and there is no true lateral surface in this region of

the bone. More distally, the bone becomes progressively circular in cross-section. The shaft flares slightly distally as it

enlarges into the ulnar head.

The three borders of the ulnar shaft are the anterior,

posterior, and interosseous borders. The anterior border of

the ulna begins proximal at the prominent medial angle of

the coronoid process and extends distally along the

anteromedial aspect of the shaft to terminate anterior and

medial to the styloid process of the head of the ulna. The

anterior border is best defined in its proximal portion, and

becomes rounder, smoother, and less clearly defined in the

central distal portion as the shaft becomes progressively

circular in circumference distally. In this central portion of

the anterior border, the ulna provides a large surface origin for the flexor digitorum profundus muscle (see Fig.

1.17). The distal one-fourth of the anterior border is

referred to as the pronator ridge and provides origin for the

pronator quadratus (4).

The posterior border of the ulna begins proximally at the

apex of the triangular subcutaneous surface of the olecranon

(see Fig. 1.18). The posterior border extends distally along

the mid-posterior portion of the shaft, to terminate posterior to the styloid process. The posterior border is well

defined along its proximal one-third to three-fourths; however, as the ulna becomes more circular in cross-section distally, the distal portion of the posterior border is more

rounded, smooth, and poorly defined. In the well defined

proximal portion, the posterior border of the ulna gives rise

to the attachments of an aponeurosis, which provides a

common origin for the flexor carpi ulnaris, the extensor

carpi ulnaris, and the flexor digitorum profundus (see Fig.

1.18). The posterior border separates the medial and posterior surfaces of the ulna.

The interosseous border of the ulna is well defined and

can be somewhat sharp in its central portion (see Figs 1.17

to 1.20). The interosseous border actually extends along the

lateral margin of the ulna, beginning at the radial notch and

curving slightly anteriorly as it extends distally. A proximal

portion of the interosseous border is referred to as the

supinator crest, providing a ridge for the attachment of a

portion of the supinator muscle. In the distal one-fourth of

the shaft, the interosseous border is less well defined. The

interosseous ligament attaches along the interosseous border and is thickest at its attachment in the central portion

of the interosseous border. The interosseous ligament provides a partition that separates the anterior and posterior

surfaces of the ulna.