Ulna

The ulna forms the primary articulation at the elbow joint. Its olecranon (the bony point of the elbow) fits into the olecranon fossa of the humerus during extension, and its trochlear notch articulates with the trochlea of the humerus throughout the range of elbow flexion and extension. This hinge-like articulation makes the elbow joint inherently stable, with the ulna providing the structural framework for elbow movement.

During forearm rotation (pronation and supination), the ulna remains relatively stationary while the radius rotates around it. The radius, connected to the ulna at the proximal and distal radioulnar joints, swings in an arc when the forearm pronates or supinates, while the ulna serves as the fixed axis of rotation. This is why the ulna is considered the axis bone of the forearm.

An olecranon fracture is a break through the bony process at the proximal end of the ulna that forms the point of the elbow. It most commonly results from a direct fall onto a flexed elbow or from a direct blow. Because the triceps muscle inserts on the olecranon, it often displaces the proximal fragment proximally. Most displaced olecranon fractures require surgical fixation with tension band wiring or plate fixation to restore active elbow extension.

A Monteggia fracture is a combined injury involving a fracture of the proximal ulnar shaft with an associated dislocation of the radial head at the proximal radioulnar joint. It is important to recognize because the radial head dislocation can be missed on plain radiographs if the entire forearm is not imaged. Failure to reduce the radial head dislocation leads to chronic pain, loss of forearm rotation, and ultimately degenerative joint disease.

The ulnar nerve passes through a groove posterior to the medial epicondyle of the humerus—near the proximal ulna—in the cubital tunnel. This superficial location makes it vulnerable to compression from leaning on the elbow, from stretching during prolonged elbow flexion, or from osteophytes and scar tissue in the tunnel. Compression at this site is cubital tunnel syndrome, the second most common peripheral nerve entrapment after carpal tunnel syndrome.