The diaphysis - shaft or body, is the part of the ulna between the ulnar tuberosity and the distal epiphysis. It has a triangular shape in cross-section. Distally, the shaft becomes narrower.

The diaphysis of the ulna has three borders and three surfaces:

Anterior border (volar border) - extends between the coronoid process proximally and the styloid process distally; its upper part is well-defined, while its middle portion is smoother and rounded; this border gives an origin to the flexor digitorum profundus muscle; the distal portion of this border also gives origin to the pronator quadratus muscle;
- Pronator ridge - a short rough line on the anteromedial distal aspect of the diaphysis; it crosses the anterior surface and border; sometimes, this landmark is barely visible; it serves as an origin site for the pronator quadratus muscle;
Posterior border - it is palpable along the entire length of the forearm posteriorly; it starts posterior to the olecranon process and goes down to the styloid process; this border gives attachment to the flexor carpi ulnaris muscle and extensor carpi ulnaris muscle;
Interosseous border (interosseous crest) - a sharp lateral border that faces the radius; it begins at the radial notch and goes down to the ulnar head; it serves as the site for attachment of the interosseous membrane of the forearm, which extends between both forearm bones;
Supinator crest - an oblique ridge on the lateral proximal aspect of the diaphysis; it extends from the posterior aspect of the radial notch and serves as an origin site for the supinator muscle;
Anterior surface (volar surface) - it is found between the anterior and interosseous borders; proximally, it is larger than distally; it serves as an origin site for the flexor digitorum profundus muscle and pronator quadratus muscle;
- Nutrient foramen - a small opening in the anterior surface of the body; usually, it is found more or less at the proximal aspect of the surface; it transmits vessels that supply the bone and are found within its medullary cavity;
Posterior surface - it is located between the posterior and interosseous borders; it gives attachment to several muscles, including the anconeus muscle, supinator muscle, extensor carpi ulnaris muscle, abductor pollicis longus muscle, extensor pollicis longus muscle, and extensor indicis muscle;
Medial surface - it is positioned between the anterior and posterior borders.

Note: 3D model displays the left ulna.

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Ulna (diaphysis)