Ulna

The ulna articulates with:

• the humerus, at the elbow as a hinge joint.
• the radius, near the elbow as a pivot joint, this allows the radius to cross over the ulna in pronation.
• the distal radius, where it fits into the ulna notch.

The ulna is broader proximally, and narrower distally.

Proximally, the ulna has a bony process, the olecranon process, a hook-like structure that fits into the olecranon fossa of the humerus. This prevents hyperextension and forms a hinge joint with the trochlea of the humerus. There is also a radial notch for the head of the radius, and the ulna tuberosity to which muscles can attach.

Distally (near the hand), there is a styloid process.