Ulna

The ulna is the medial bone of the forearm, on the little-finger side, large at the elbow and small at the wrist.

At a glance

English: Ulna
Latin (TA2): Ulna
System: Bone
Region: Upper limb
Side: Paired, left and right

About the ulna

The ulna is the medial forearm bone, on the side of the little finger. It is the stable partner of the pair: thick and hooked where it grips the humerus at the elbow and narrowing to a small rounded head at the wrist, the opposite of the radius alongside it.

Its upper end is shaped like a wrench. The olecranon rises behind as the point of the elbow and receives the triceps tendon, while the coronoid process juts forwards below it. Between the two is the deep, crescent-shaped trochlear notch, which wraps around the trochlea of the humerus to form a hinge. A small radial notch on the outer face of the coronoid process holds the rim of the radial head, and just below the coronoid lies the ulnar tuberosity where brachialis attaches. The shaft is three-sided, with a sharp lateral interosseous border for the membrane that ties it to the radius, and a posterior border that can be felt under the skin along its whole length.

At the lower end the small ulnar head fits into a matching notch on the radius, forming the distal radio-ulnar joint. A triangular fibrocartilage disc separates the head from the carpal bones, so the ulna takes little direct part in the wrist joint itself. Behind and medial to the head projects the ulnar styloid process, which is easily felt.

Taken together, the ulna joins the humerus in a hinge at the elbow, the radius in a pivot near the elbow and again near the wrist, and the radius along its length through the interosseous membrane, a fibrous joint. Important fracture patterns include the Monteggia injury, a break in the upper third of the ulna with dislocation of the radial head, and fractures of the olecranon after a fall on the elbow. An isolated break in the lower two-thirds of the shaft with little angulation, displacement or shortening can often be treated in a cast that extends above the elbow.

Function and supply

Function: Forms the hinge of the elbow for flexion and extension and gives a fixed axis around which the radius turns.
Nerve supply: The ulnar nerve passes behind the medial epicondyle of the humerus and then along the medial side of the ulna into the forearm.
Blood supply: A nutrient artery from the anterior interosseous artery, with branches from the ulnar artery and the anastomoses around the elbow and wrist.

Parts of the ulna

Each number is marked on the model. Press one, here or on the model, to see it named in place.

Anterior border of ulnaMargo anterior ulnaeThe rounded front edge of the shaft, starting at the inner corner of the coronoid process and running down to the front of the styloid process. Flexor digitorum profundus arises on both sides of its upper part.
Articular circumference of head of ulnaCircumferentia articularis capitis ulnaeThe smooth, curved strip around the outer side of the ulnar head. It sits in a matching hollow on the lower radius, which slides round it when the forearm turns.
Body of ulnaCorpus ulnaeThe shaft, three-sided and stout above, then straighter and slimmer as it descends to the wrist. It is the fixed bone of the forearm around which the radius moves.
Coronoid process of ulnaProcessus coronoideus ulnaeA bracket of bone jutting forwards from the front of the upper ulna, forming the lower lip of the trochlear notch. In full flexion it slots into a small hollow, the coronoid fossa, above the humeral trochlea, and helps stop the elbow dislocating backwards.
Head of ulnaCaput ulnaeThe small, rounded lower end of the ulna, which forms the knob on the back of the wrist on the little-finger side. It is separated from the carpal bones by a triangular disc of fibrocartilage.
Interosseous border of ulnaMargo interosseus ulnaeThe sharp outer edge of the shaft that faces the radius. The interosseous membrane attaches along it and spreads force from the radius to the ulna.
Medial surface of ulnaFacies medialis ulnaeThe broad inner face of the shaft, hollowed above and rounded below. Flexor digitorum profundus covers most of it, and its lower part lies just under the skin.
OlecranonThe large, curved block of bone at the top of the ulna that forms the point of the elbow. Triceps brachii inserts on its upper surface, and a bursa lies between it and the skin.
Posterior border of ulnaMargo posterior ulnaeThe back edge of the shaft, running from the olecranon to the styloid process, and felt along its whole length under the skin. Flexor carpi ulnaris, extensor carpi ulnaris and flexor digitorum profundus share an aponeurosis attached to it.
Posterior surface of ulnaFacies posterior ulnaeThe back face of the shaft, between the posterior and interosseous borders. Anconeus covers its upper part, and the deep extensors of the thumb and index finger arise lower down.
Radial notchIncisura radialisA narrow, oblong joint hollow on the outer side of the coronoid process. The rim of the radial head turns in it, held by the annular ligament, at the proximal radio-ulnar joint.
Sublime tubercleTuberculum sublimeA small rise on the inner edge of the coronoid process. The front band of the elbow's ulnar collateral ligament attaches to it, and it can be pulled off in throwing injuries.
Supinator crestCrista supinatoriaA ridge running down from the back of the radial notch on the outer side of the upper shaft. Supinator arises from it and from the hollow in front of it.
Trochlear notchIncisura trochlearisThe deep, crescent-shaped hollow framed by the olecranon above and the coronoid below. It wraps round the trochlea of the humerus to form the hinge of the elbow joint.
Tuberosity of ulnaTuberositas ulnaeA rough patch on the front of the upper shaft just below the coronoid process. Brachialis inserts on it, making it a key point for bending the elbow.
Ulnar styloid processProcessus styloideus ulnaeA short, pointed projection from the back and inner side of the lower end. The wrist's ulnar collateral ligament attaches near its tip, and it is often broken along with the radius in wrist fractures.

Anthromica

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At a glance

About the ulna

Function and supply

Related structures

See the ulna where it really sits, with every layer of the body around it.