The ulna and radius are parallel bones of the antebrachium or the forearm. The ulna lies medially and consists of a bony tip called the olecranon process at its proximal end. This hook-like projection articulates with the olecranon fossa of the humerus and forms the "hinged" ulnohumeral part of the elbow joint. This joint facilitates forearm extension and flexion while preventing its hyperextension. Similarly, the coronoid process, another bony projection on the proximal/anterior side of the ulna, articulates with the coronoid fossa of the humerus and serves as the insertion point for the muscle brachialis.
Lateral to the coronoid process is a narrow depression called the radial notch that helps attach the radius and forms the proximal radioulnar joint. This joint is essential for the supination and pronation of the forearm. Distally, the ulnar head articulates with the radius forming the distal radioulnar joint. Projecting from the posterior side of the ulnar head is the styloid process of the ulna, a short bony projection. The styloid process attaches triangular fibrocartilage and radioulnar ligaments, stabilizing the wrist joint.
Several muscles attached to the ulna, including the pronator teres, flexor carpi ulnaris, flexor digitorum superficialis, flexor digitorum profundus, and pronator quadratus, facilitate elbow and wrist joints movement such as flexion and extension, pronation, and supination.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 8.2: Bones of the Upper Limb.
