Structure
The ulna is large proximally and small distally, the reverse of the radius. Its proximal end forms the elbow hinge; its distal end, the head, sits at the wrist without touching the carpal bones.
Proximal end
- Olecranon: the curved projection that forms the point of the elbow. Triceps inserts on the posterior part of its upper surface.
- Coronoid process: projects forwards below the trochlear notch. Its anterior surface ends in the ulnar tuberosity, where brachialis inserts.
- Trochlear notch: the large C-shaped articular surface between olecranon and coronoid that wraps around the trochlea of the humerus.
- Radial notch: a small facet on the lateral side of the coronoid process for the head of the radius. The annular ligament attaches to its anterior and posterior margins.
- Supinator crest and fossa: a ridge and hollow below the radial notch, where supinator arises.
Shaft
The shaft is triangular and tapers distally. Its lateral interosseous border holds the interosseous membrane. The posterior border is subcutaneous along its whole length and can be felt from the olecranon to the wrist.
Distal end
The head of the ulna is the rounded knob at the wrist. It articulates with the ulnar notch of the radius and is separated from the triquetrum and lunate by the articular disc of the triangular fibrocartilage complex (TFCC). The ulnar styloid process projects distally from its posteromedial side, and a groove between head and styloid carries the tendon of extensor carpi ulnaris.
Articulations and muscle attachments
The ulna forms three joints: the humeroulnar part of the elbow and the proximal and distal radioulnar joints. It also anchors the TFCC, which stabilises the distal radioulnar joint.
| Site | Muscle |
|---|---|
| Olecranon, upper surface | Triceps brachii |
| Lateral olecranon and upper posterior surface | Anconeus |
| Coronoid process and ulnar tuberosity | Brachialis; ulnar heads of pronator teres and FDS on its medial edge |
| Medial olecranon and posterior border (by aponeurosis) | Flexor carpi ulnaris |
| Anterior and medial surfaces, upper three quarters | Flexor digitorum profundus |
| Supinator crest and fossa | Supinator |
| Distal anterior surface | Pronator quadratus |
| Posterior surface | Abductor pollicis longus, extensor pollicis longus, extensor indicis |
The ulnar collateral ligament of the elbow attaches to the medial side of the coronoid process and olecranon. Its anterior band, running to the sublime tubercle on the coronoid, resists valgus stress.
Ossification
The ulna ossifies from a primary centre in the shaft and secondary centres at each end. The distal centre forms the head and styloid. The olecranon ossifies from its own centre in late childhood, sometimes from more than one nucleus, and an unfused olecranon apophysis in an adolescent can be mistaken for a fracture on a lateral radiograph of the elbow.
Clinical relevance
Ulnar fractures are best understood by where they occur and what else is damaged with them.
- Olecranon fracture: from a fall on the point of the elbow. Triceps pulls the proximal fragment upwards, so displaced fractures usually need fixation, commonly by tension-band wiring or a plate.
- Coronoid fracture: occurs with posterior elbow dislocation. Dislocation, radial head fracture and coronoid fracture together form the terrible triad of the elbow.
- Monteggia fracture: a proximal ulnar shaft fracture with dislocation of the radial head. The posterior interosseous nerve can be stretched.
- Nightstick fracture: an isolated ulnar shaft fracture from a direct blow, classically when the forearm is raised to protect the head.
- Ulnar variance: the length of the ulna relative to the radius at the wrist. Positive variance is linked with ulnar impaction syndrome; negative variance is associated with Kienböck's disease of the lunate.
- Olecranon bursitis: swelling of the subcutaneous bursa over the olecranon.