Complete ulnar bone
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ulna bone anatomy complete labeled diagram
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Ulna — Complete Anatomy
The ulna is a long bone of the forearm that runs parallel to the radius, on the medial side. It is the primary load-bearing bone of the forearm, supporting the distal radius as it rotates around it.
Overview
- Located medial to the radius in the forearm
- Broad, hook-like proximal end → articulates with the distal humerus
- Narrow distal end → articulates with the radius and carpal bones
- Primary functions: hinge articulation at the elbow; supports forearm rotation (pronation/supination)
Proximal End
Olecranon
- A large, prism-shaped apophysis on the posterior-superior aspect of the proximal ulna
- Posterior surface: triangular, smooth, covered by the olecranon bursa
- Anterior surface: smooth concave depression merging into the upper semilunar notch
- Superior surface: rough, quadrilateral — insertion for triceps brachii and part of the posterior ligament
- Medial surface: attachment for posterior and oblique bands of the ulnar collateral ligament and flexor carpi ulnaris
- Lateral surface: attachment for the anconeus muscle
Coronoid Process
- Triangular bony projection directed anteriorly and slightly superiorly from the ulnar shaft
- Composed of: tip, body, anterolateral facet, and anteromedial facet
- Superior surface: smooth, concave → forms the inferior portion of the semilunar notch
- Anterior-inferior surface: rough bony imprint — insertion for brachialis
- Lateral surface: radial notch (articular surface for radial head)
- Anteromedial facet (sublime tubercle): insertion for the anterior bundle of the medial collateral ligament
- Medial surface anterior margin: origin for flexor digitorum superficialis
- Medial surface: origin for flexor digitorum profundus and pronator teres
Articular Surfaces of the Proximal Ulna
| Surface | Articulation |
|---|---|
| Semilunar (trochlear) notch | Trochlea of humerus |
| Radial notch | Medial aspect of radial head |
Shaft
The ulnar shaft has a triangular pyramidal shape proximally and mid-shaft, becoming an irregular cylinder distally.
Three Borders
| Border | Description | Muscle Attachments |
|---|---|---|
| Interosseous border | Lateral crest from lateral edge of radial notch | Origin: supinator; attachment: interosseous membrane |
| Anterior border | Medial coronoid process → base of styloid process | Upper ¾: flexor digitorum profundus; lower ¼: pronator quadratus |
| Posterior border | Confluence of medial and lateral olecranon borders → styloid base | Upper ¾: aponeurosis for flexor carpi ulnaris, extensor carpi ulnaris, flexor digitorum profundus |
Three Surfaces
| Surface | Location | Muscle Attachments |
|---|---|---|
| Anterior surface | Between anterior and interosseous borders; concave | Upper ¾: flexor digitorum profundus; lower ¼: pronator quadratus (separated by an oblique ridge) |
| Posterior surface | Between interosseous and posterior borders | Above ridge: anconeus insertion, supinator attachment; lateral portion below ridge: supinator, extensor pollicis longus, abductor pollicis longus |
| Medial surface | Between anterior and posterior borders | Flexor digitorum profundus |
Distal End
The distal ulna is slightly expanded and consists of two parts:
Ulnar Head
- Lateral convex surface: articulates with the radial sigmoid notch (distal radioulnar joint)
- Inferior surface: articulates with the articular disc (triangular fibrocartilage complex, TFCC), which separates the ulnar head from the carpus
Ulnar Styloid Process
- A rounded bony projection from the medial posterior aspect of the distal ulna
- Attachment site for the ulnar collateral ligament
- Morphological variations are common: elongated, angulated, hypertrophic, or unfused separate ossification center
Ulnar Variance
| Type | Definition | Significance |
|---|---|---|
| Neutral | Distal ulnar head cortex level with lunate fossa cortex | Normal; radiocarpal joint bears 82%, ulnocarpal 18% of load |
| Negative (>2.5 mm short) | Ulnar head cone-shaped and small | Associated with avascular necrosis of the lunate |
| Positive | Ulna longer than radius | Predisposes to early joint degeneration; ulnocarpal receives up to 42% more stress |
Biomechanics
- The ulna serves as the fixed axis of forearm rotation
- In pronation: ulnar head rotates styloid process to the palmar side
- In supination: styloid process moves to the dorsal side
- Pronation increases ulnar variance (radius migrates proximally)
- The interosseous membrane (with a thick central band) prevents proximal migration of the radius
- The proximal radioulnar joint is stabilized by the radial notch + annular ligament
- The distal radioulnar joint is stabilized by the TFCC
Summary of Muscle Attachments
| Muscle | Attachment Site on Ulna |
|---|---|
| Triceps brachii | Superior surface of olecranon (insertion) |
| Brachialis | Anterior-inferior surface of coronoid process / ulnar tuberosity (insertion) |
| Anconeus | Posterior surface above ridge (insertion) |
| Pronator teres | Medial surface of coronoid process (origin) |
| Pronator quadratus | Lower ¼ anterior surface and anterior border (origin) |
| Flexor carpi ulnaris | Medial surface of olecranon, posterior border upper ¾ (origin) |
| Extensor carpi ulnaris | Posterior border upper ¾ (origin) |
| Flexor digitorum superficialis | Anterior margin of medial coronoid surface (origin) |
| Flexor digitorum profundus | Anterior and medial shaft surfaces, anterior border (origin) |
| Supinator | Interosseous border, radial notch region (origin) |
| Extensor pollicis longus | Lateral posterior surface below ridge (origin) |
| Abductor pollicis longus | Lateral posterior surface below ridge (origin) |
— Imaging Anatomy: Text and Atlas, Volume 3 — Bones, Joints, Vessels and Nerves, pp. 95–97
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