Ulna anatomy: the forearm bone on the little finger side, labelled

What it is

The medial bone of the forearm.

Location

Lies on the little finger side of the forearm. Proximally it forms the main hinge of the elbow with the humerus and articulates with the radial head; distally its head articulates with the radius.

Function

Forms the stable hinge of the elbow and the fixed axis for forearm rotation.

Body system

Skeletal · paired (left and right)

FMA ID

FMA23468 (Foundational Model of Anatomy)

Key facts

• Its olecranon forms the point of the elbow and the trochlear notch grips the trochlea of the humerus.
• Its coronoid process and radial notch complete the elbow joint.

Clinical relevance

A shaft fracture with dislocation of the radial head is called a Monteggia fracture.

The ulna is the medial bone of the forearm, on the same side as the little finger when the palm faces forward. It is the stabilising bone of the forearm: the radius pivots around it to turn the hand, while the ulna stays still. (TeachMeAnatomy: The Ulna.)

Its posterior border lies just under the skin and can be felt along the whole length of the back of the forearm. (TeachMeAnatomy: The Ulna.)

The upper end of the ulna is large. Its olecranon is the curved projection at the back that forms the point of the elbow. Triceps brachii inserts on its upper surface. (TeachMeAnatomy: The Ulna.)

The coronoid process is a ridge that juts forward below the olecranon. Together, the olecranon and coronoid process form the trochlear notch, a deep, wrench-shaped hollow that grips the trochlea of the humerus. (TeachMeAnatomy: The Ulna.)

Just below the coronoid process is the ulnar tuberosity, where brachialis inserts. (TeachMeAnatomy: The Ulna.)

When the elbow is fully straightened, the olecranon slides into the olecranon fossa on the back of the humerus. When it is fully bent, the coronoid process fits into the coronoid fossa on the front. (OpenStax Anatomy and Physiology 2e, 8.2: Bones of the Upper Limb.)

The radial notch is a small, smooth hollow on the lateral side of the coronoid process, beside the trochlear notch. The rim of the radial head turns in it, forming the proximal radioulnar joint. (TeachMeAnatomy: The Ulna.)

The annular ligament wraps around the radial head and holds it against the radial notch. This pivot joint is what allows pronation and supination of the forearm. (OpenStax Anatomy and Physiology 2e, 9.6: Anatomy of Selected Synovial Joints.)

The names are easy to mix up. The radial notch (incisura radialis) is on the ulna and receives the radius. The ulnar notch (incisura ulnaris radii, also called the sigmoid notch) is on the lower end of the radius and receives the head of the ulna at the distal radioulnar joint. (IMAIOS e-Anatomy: Ulnar notch (incisura ulnaris radii).)

The shaft is triangular in cross-section, with three borders and three surfaces, and it narrows toward the wrist. The sharp interosseous border faces the radius and anchors the interosseous membrane, a sheet of dense connective tissue that spans the gap between the two bones. (TeachMeAnatomy: The Ulna.)

The interosseous membrane ties the two shafts together, forming what is sometimes called the middle radioulnar joint. Because it links them so firmly, a fracture of one bone often comes with dislocation of a joint at the other end. (Kenhub: The radius and the ulna.)

The deep head of supinator arises from the supinator crest at the upper end of the interosseous border, and pronator quadratus attaches to the lower part of the anterior surface. (Kenhub: The radius and the ulna.)

The lower end of the ulna is much narrower than the upper end. It ends in a rounded head and a small pointed projection, the ulnar styloid process. The head articulates with the ulnar notch of the radius at the distal radioulnar joint. (TeachMeAnatomy: The Ulna.)

The ulnar head does not touch the carpal bones directly. A triangular fibrocartilage lies between it and the wrist bones, so at the wrist it is the radius that articulates with the carpal bones. (Kenhub: The radius and the ulna.)

The ulna is medial and the radius lateral (thumb side). The ulna is large at the elbow and small at the wrist; the radius is the reverse, with a small disc-shaped head at the elbow and a wide lower end that forms most of the wrist joint. (OpenStax Anatomy and Physiology 2e, 8.2: Bones of the Upper Limb.)

The two bones meet at three places: the proximal radioulnar joint at the elbow, the interosseous membrane along the shafts and the distal radioulnar joint at the wrist. The humeroulnar, humeroradial and proximal radioulnar joints share one joint capsule at the elbow. (OpenStax Anatomy and Physiology 2e, 9.6: Anatomy of Selected Synovial Joints.)

The carrying angle is the slight outward angle of the forearm relative to the arm when the elbow is straight and the palm faces forward. OpenStax gives it as 5 to 15 degrees and notes it is larger in females. (OpenStax Anatomy and Physiology 2e, 8.2: Bones of the Upper Limb.)

An isolated fracture of the ulnar shaft usually follows a direct blow to the forearm. Olecranon fractures are less common and follow a fall on a bent elbow; triceps can then pull the broken fragment upward. (TeachMeAnatomy: The Ulna.)

A Monteggia fracture is a fracture of the upper ulnar shaft with dislocation of the radial head at the elbow, usually forward. It is caused by a force from behind the ulna. The opposite pattern, a fracture of the lower radius with dislocation of the ulnar head at the wrist, is a Galeazzi fracture. (TeachMeAnatomy: The Ulna.)

Because the interosseous membrane transmits force between the bones, fractures of both the ulna and the radius together are not unusual. (TeachMeAnatomy: The Ulna.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Anterior interosseous artery · cardiovascular
• Ulnar head of flexor carpi ulnaris · muscular
• Extensor carpi ulnaris · muscular
• Humeral head of flexor carpi ulnaris · muscular
• Flexor carpi radialis · muscular
• Median antebrachial vein · cardiovascular
• Ulnar artery · cardiovascular
• Ulnar vein · cardiovascular
• Extensor carpi radialis brevis · muscular
• Radial vein · cardiovascular