Bone: Ulna
Upper extremity
Ulna is #2
Gray's	subject #52 214
MeSH	Ulna

The ulna 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.

At the distal end of the ulna 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.

Muscle attachments

See also

• Bone terminology
• Terms for anatomical location
• Ossification of ulna

Additional images

Ulna l. dx. - ant. view	Ulna l. dx. - lat. view	Right posterior human radius and ulna	Human arm bones diagram
Bones of left forearm. Anterior aspect.	The radius and ulna of the left forearm, posterior surface.	Left elbow-joint, showing anterior and ulnar collateral ligaments.	Cross-section through the middle of the forearm.
The Supinator.	Transverse section across distal ends of radius and ulna.

External links

• Ulnar fractures

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained herein may be outdated.