ulnas

The ulna, along with the radius, forms the two long bones of the forearm. In humans it is the medial bone in the anatomical position, running parallel to the radius. The ulna articulates with the humerus at the elbow (humeroulnar joint) and with the radius at the distal radioulnar joint. It is bound to the radius by the interosseous membrane, which provides stability and a surface for muscle attachment, facilitating forearm motion.

Proximally, the ulna features the olecranon, a prominent posterior projection that forms the bony tip of the elbow, and the coronoid process on the anterior aspect. The trochlear notch between these structures articulates with the trochlea of the humerus. The ulna’s shaft has a sharp interosseous border for the interosseous membrane and for muscular attachments.

Distally, the ulna ends in a head, with a concave ulnar notch on its medial side that articulates with the radius, and a styloid process that provides attachment for wrist ligaments.

Functionally, the ulna serves as a stabilizing forearm bone and acts as a lever for forearm muscles. The elbow primarily allows flexion and extension, while rotation of the forearm (pronation and supination) occurs around the radius, coordinated with the ulna via the interosseous membrane and distal radioulnar joint.

Clinical relevance includes common fractures of the ulna, such as isolated midshaft fractures (nightstick fractures) and Monteggia injuries (proximal ulna fracture with radial head dislocation). The ulnar nerve runs along the posterior aspect of the medial elbow and medial forearm, making it susceptible in injuries.