The ulna is a bone in the human forearm. Broader near the elbow and tapering as it approaches the wrist, it is situated alongside the radius bone on the pinky-finger side of the arm. This bone’s structure reflects its function, as it forms three major joints in the arm, two with the parallel radius bone and one with the humerus bone in the upper arm. The latter of these is the elbow joint, and it is the crescent wrench shape of the top of the ulna that makes flexion and extension, or bending and straightening, of the elbow possible as the bone hinges around the end of the humerus.
A long and narrow bone that is more prismatic than cylindrical in shape, the ulna extends the length of the forearm from the olecranon process, the bony protrusion at the proximal end of the bone felt at the elbow, to the styloid process, the smaller bony prominence at the distal end of the bone felt at the wrist on the side of the pinky. The olecranon process acts as a site of attachment for several major muscles of the arm, including the triceps brachii and the flexor carpi ulnaris. It also prevents hyperextension of the elbow. Just below it on the anterior aspect of the ulna is the coronoid process, a smaller, pointy projection that serves as a site of attachment for the brachialis, pronator teres, and flexor digitorum muscles. This process curves into a notch on the humerus, the coronoid fossa, when the elbow flexes.
Other significant anatomical features of this bone include the semilunar notch, which is the crescent-shaped cavity between the olecranon and coronoid processes into which the end of the humerus inserts to form the elbow joint; and the radial notch, a concave surface on the lateral side of the ulna that articulates with the head of the radius to form the proximal radioulnar joint. The proximal radioulnar joint allows the forearm to rotate as the head of the radius, which is bound to the ulna by a ring-shaped ligament known as the annular ligament, turns back and forth against the radial notch. This rotation, which results in the hand turning palm-down and palm-up, is known respectively as pronation and supination.
On the distal end of the ulna near the wrist is the head of the ulna, which forms a joint with the radius along its ulnar notch that mirrors the proximal radioulnar joint. The distal radioulnar joint, like the proximal joint above it, allows the two bones to rotate past each other, turning the hand palm-up or palm-down. Extending slightly beyond the head of the ulna on its medial side is the styloid process, a downward-projecting bony eminence that serves as a site of attachment for the ulnar collateral ligament of the wrist joint. This ligament stabilizes the wrist against bending too far laterally, or toward the thumb side.