The ulna is one of the two bones forming the skeleton of the forearm. It is classified as a long bone and it lies medially and parallel to the radius, which is the second bone of the forearm.

Proximally, the ulna articulates with the humerus at the elbow joint, as well as with the radius at the proximal radioulnar joint. Distally, it articulates with the radius at the distal radioulnar joint, and with an articular disc that separates it from the carpal bones at the wrist joint.

Similarly to other long bones, the ulna has three segments - a proximal end, a shaft, and a distal end.

Proximal end of ulna

The proximal end or extremity of the ulna articulates with the trochlea of the humerus. The surface of the proximal ulna has bony prominences for muscle attachments, which enable movement at the elbow joint.

Essential landmarks of the proximal ulna are the:

The olecranon is a large, thick, curved bony projection of the ulna that extends proximally behind the elbow, forming a part of the trochlear notch. It is bent forward at the summit thus presenting a prominent lip, which is received into the olecranon fossa of the humerus in extension of the forearm. The olecranon can be palpated as the most pointed portion of the elbow. The superior surface of the olecranon serves for the attachment of the triceps brachii muscle.

The coronoid process of the ulna is a triangular eminence that projects forward from the anterior proximal part of the bone. It forms a part of the trochlear notch. The apex of this process is received into the olecranon fossa of the humerus during flexion of the forearm. Its medial surface serves for attachment of part of the ulnar collateral ligament, it is also the origin site for one head of the flexor digitorum superficialis, part of the flexor digitorum profundus, and one head of the pronator teres muscle. Frequently, the flexor pollicis longus muscle arises from the lower part of the coronoid process.

The trochlear notch (or semilunar notch) is a large depression in the proximal end of the ulna that is formed by the olecranon and the coronoid process. It is wrench-shaped, and fits the trochlea of the humerus, forming a part of the elbow joint.

The radial notch of the ulna is a narrow, oblong depression located on the lateral surface of the coronoid process, located laterally to the trochlear notch. It receives the circumferential articular surface of the head of the radius at the proximal radioulnar joint.

The tuberosity of the ulna (or ulnar tuberosity) is a rough eminence that is found immediately distal to the coronoid process. A part of the brachialis muscle inserts here.

Shaft of ulna

The shaft of the ulna has a triangular-shape in cross-section. It is also called the ulnar shaft or body. Distally, it becomes narrower. The ulnar shaft has three borders (anterior, posterior, and interosseous) and three surfaces (anterior, posterior, and medial).

The anterior border is also known as the volar border. Its upper part is well defined, while its middle portion is smooth and rounded, giving origin to the flexor digitorum profundus; its lower fourth gives origin to the pronator quadratus muscle;
The posterior border, may also be called the dorsal border, is palpable along the entire length of the forearm posteriorly; it gives attachment to an aponeurosis that gives a common origin to the flexor carpi ulnaris, extensor carpi ulnaris, and flexor digitorum profundus;
The interosseous border is also referred to as the interosseous crest. It serves as the site for attachment of the antebrachial interosseous membrane, which extends between the two forearm bones.
The anterior surface (or volar surface) - serves as the origin site for the flexor digitorum profundus and is covered by the pronator quadratus distally;
The posterior surface (or dorsal surface) of the ulnar shaft - gives attachment to several muscles, including the anconeus, supinator, abductor pollicis longus, extensor pollicis longus, and extensor indicis;
The medial surface - the upper two-thirds of this surface give origin to fibers of the flexor digitorum profundus.

Distal end of ulna

The distal end or extremity of the ulna is the part of the bone near the wrist. It is smaller in diameter than the proximal end. The distal end of the ulna terminates in a rounded head and a distal projection - the ulnar styloid process.

The head of the ulna has an articular surface, part of which is directed downward, and articulates with the upper surface of the triangular articular disk that separates it from the wrist joint. The remaining portion of the head is directed laterally, it is narrow, convex, and received into the ulnar notch of the radius, forming the distal radioulnar joint.
The styloid process of the ulna projects from the medial and posterior part of the bone. Its rounded end gives attachment to the ulnar collateral ligament of the wrist joint.

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Ulna