Bones of the Upper Limb: Ulna

The ulna and radius are parallel bones that constitute the forearm. These bones are joined at the proximal and distal ends by the radioulnar joints and connected by a flat, flexible interosseous membrane across their length.

The ulna is the longer of the two bones and has two projections at the proximal end — the olecranon  and coronoid processes. A deep trochlear notch between the two processes fits the trochlea of the humerus, forming a hinge joint and allowing the elbow to flex.

During flexing, the coronoid process also slides into the humeral coronoid fossa, forming a V-shaped angle between the forearm and arm.

During arm extension, the olecranon fossa of the humerus holds the olecranon of the ulna preventing the bones of the forearm from sliding beyond a certain anatomical point.

The ulnar shaft ends in a disc-shaped ulnar head that joins the wrist bones. The ulnar head is separated from the wrist bones by a triangular fibrocartilage which stabilizes the distal radioulnar joint and supports the carpal bones of the wrist.