Ulna

Overview
The ulna (elbow bone) is a long bone, prismatic in form, placed at the medial side of the forearm, parallel with the radius.

Articulations
The ulna articulates with:
 * the humerus, at the right side 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 radius along its length via the interosseous membrane that forms a syndesmoses joint.

Proximal and distal aspects
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 ulnar tuberosity to which muscles can attach.

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

Structure
The long, narrow medullary cavity is enclosed in a strong wall of compact tissue which is thickest along the interosseous border and dorsal surface.

At the extremities the compact layer thins.

The compact layer is continued onto the back of the olecranon as a plate of close spongy bone with lamellæ parallel.

From the inner surface of this plate and the compact layer below it trabeculæ arch forward toward the olecranon and coronoid and cross other trabeculæ, passing backward over the medullary cavity from the upper part of the shaft below the coronoid.

Below the coronoid process there is a small area of compact bone from which trabeculæ curve upward to end obliquely to the surface of the semilunar notch which is coated with a thin layer of compact bone.

The trabeculæ at the lower end have a more longitudinal direction.