ulna

The ulna is one of the two long bones of the forearm. In the standard anatomical position it lies medially (on the same side as the little finger) and runs parallel to the radius. It participates in the elbow joint and, with the radius, forms the forearm’s distal radioulnar joint, which allows pronation and supination of the hand.

Anatomy and landmarks: The proximal end features the olecranon and coronoid process, together forming the trochlear notch that articulates with the humeral trochlea. The olecranon forms the prominent bony point of the elbow and serves as the insertion of the triceps brachii; the coronoid process helps prevent posterior dislocation and provides muscle attachments, including the brachialis and other forearm flexors. The ulnar tuberosity is another notable projection for muscle attachment. On the medial side lies the radial notch, where the head of the radius articulates with the ulna. The distal end bears the head, which has a styloid process and articulates with the ulnar notch of the radius at the distal radioulnar joint. The interosseous border provides attachment for the interosseous membrane, which binds the radius and ulna along most of their lengths.

Function: The ulna mainly acts as a stable lever for the forearm and as a stabilizer in grip, transmitting loads from the hand to the arm. While the radius primarily enables rotation, forearm rotation (pronation and supination) involves both bones with the interosseous membrane distributing load between them.

Clinical significance: The ulna is susceptible to fractures, especially at the proximal and distal ends. Conditions such as Monteggia fractures (proximal ulna fracture with radial head dislocation) and Galeazzi fractures (distal ulna fracture with distal radioulnar joint disruption) are notable. The ulnar nerve travels near the medial elbow, where it can be irritated or compressed (cubital tunnel syndrome).