The ulna is one of the two bones forming the skeleton of the forearm. It is a long bone that lies medially and parallel to the radius, which is the second bone of the forearm.
The ulna mainly acts as a stabilizing bone of the forearm. Proximally, it articulates with the humerus at the elbow joint. It also articulates with the radius at the proximal radioulnar joint. And distally, the ulna articulates with the radius, forming the distal radioulnar joint.
The ulna has three segments: a proximal end, a shaft, and a distal end. Each of these segments presents essential anatomical landmarks.
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 attachment, which enables movement at the elbow joint.
Essential landmarks of the proximal ulna are the olecranon, coronoid process, trochlear notch, radial notch, and the tuberosity of the ulna.
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 ulna. It forms a part of the trochlear notch. The apex of this process is received into the olecranon fossa of the humerus in flexion of the forearm. Its medial surface serves for the 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 significant 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, 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 is a rough eminence located immediately distal to the coronoid process. A part of the brachialis muscle inserts on this tuberosity.
The shaft of the ulna has a triangular-shape when viewed in cross-section. It is also referred to as the ulnar shaft or body. The ulnar shaft has three borders (anterior, posterior, and interosseous) and three surfaces (anterior, posterior, and medial). Distally, the shaft of the ulna becomes narrower.
Borders of the ulnar shaft:
- 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 or 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 interosseus membrane, which extends between the two forearm bones.
Surfaces of the ulnar shaft are as following:
- the anterior or volar surface serves as the origin site for the flexor digitorum profundus and is covered by the pronator quadratus distally;
- the posterior 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 upper three-thirds of the medial surface gives origin to the flexor digitorum profundus.
Distal end of ulna
The distal end or extremity of the ulna is the part of the ulna 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, which separates it from the wrist joint. The remaining portion of the ulnar 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, and its rounded end gives attachment to the ulnar collateral ligament of the wrist joint.