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Portrait of Ulna

Anatomy · Bones

Ulna

The little-finger-side forearm bone

Upper limb · forearm

The ulna is the medial (little-finger-side) bone of the forearm, running parallel to the radius. It is broad at the elbow, where its crescent-wrench-shaped trochlear notch grips the humerus and its olecranon forms the point of the elbow, and narrow at the wrist. It is the longer of the two forearm bones.

How to spot them

  • ▸Long forearm bone on the little-finger side, thick at the elbow and slender at the wrist
  • ▸Crescent-wrench-shaped upper end: olecranon behind and coronoid process in front, gripping the humerus
  • ▸Olecranon forms the bony point of the elbow
  • ▸Small rounded head and short styloid process at the wrist end, separated from the carpal bones by a cartilage disc

What to know

The forearm bone on the little-finger side, broad at the elbow and narrow at the wrist

Ulna — the medial forearm bone.

Its upper end resembles a crescent wrench, its C-shaped notch gripping the pulley of the arm bone

Ulna — the trochlear notch.

The pointed bony prominence at the back of the elbow, which slots into a fossa on the arm bone when the elbow is straightened

Ulna — the olecranon.

A blow to the forearm raised in defence can break its shaft in isolation, and is named after a policeman's weapon

Ulna — the 'nightstick' (parry) fracture.

A break of its upper third, together with dislocation of the radial head, carries the name of an Italian surgeon

Ulna — Monteggia fracture-dislocation.

Its end at the wrist never touches the wrist bones directly, being separated from them by a fibrocartilage disc

Ulna — the triangular fibrocartilage complex.

Its Latin name is a cognate of a unit of length, the ell, which was once measured from the elbow to the fingertips

Ulna — Latin ulna, 'elbow' or 'forearm'; related to English 'ell'.

Key ideas

  • In pronation and supination the radius rotates around the ulna, which stays largely still, so the ulna is the stable column of the forearm.
  • The interosseous membrane joins the two bones and transmits force from the hand up the radius to the ulna and then to the humerus.

Quick-fire facts

  • •It is the medial bone of the forearm and is slightly longer than the radius.
  • •The trochlear notch, formed by the olecranon and the coronoid process, articulates with the trochlea of the humerus in the hinge of the elbow.
  • •The radial notch on the side of the upper end receives the head of the radius at the proximal radioulnar joint.
  • •The olecranon is the bony point of the elbow; a direct fall on it can fracture it, and it lies in the olecranon fossa of the humerus when the arm is straight.
  • •The head and styloid process of the ulna are visible and palpable at the back of the wrist on the little-finger side.
  • •Only the radius articulates directly with the wrist bones; the ulna is separated from them by an articular disc.
  • •The ulnar nerve does not run along this bone: it passes behind the medial epicondyle of the humerus, where the 'funny bone' is struck.
  • •A Monteggia fracture, named after Giovanni Battista Monteggia in 1814, is a fracture of the proximal ulna with dislocation of the radial head.

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Sources