Ulna
Ulna
location_on Medial (pinky-side) forearm, between the elbow and wrist
The ulna is the medial and longer bone of the forearm, forming the primary bony articulation of the elbow joint. The olecranon process forms the point of the elbow, and the trochlear notch grips the trochlea of the humerus like a wrench. Unlike the radius, the ulna is the dominant bone at the elbow but contributes minimally to the wrist joint. It serves as the stable axis around which the radius rotates.
Key Anatomical Features
- Olecranon process forms the point of the elbow and is the insertion for the triceps
- Coronoid process projects anteriorly and forms the inferior part of the trochlear notch
- Trochlear notch is a deep C-shaped concavity that articulates with the humeral trochlea
- Radial notch on the lateral coronoid articulates with the radial head
- Ulnar styloid process is palpable on the posteromedial aspect of the wrist
- Interosseous border provides attachment for the interosseous membrane
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Triceps brachii | Olecranon process | Extends the elbow |
| Brachialis | Coronoid process and ulnar tuberosity | Flexes the elbow |
| Anconeus | Lateral olecranon and posterior proximal shaft | Assists in elbow extension and stabilizes the elbow |
| Flexor carpi ulnaris | Olecranon and posterior border of ulna | Flexes and adducts the wrist |
| Flexor digitorum profundus (medial part) | Anterior and medial shaft | Flexes the distal interphalangeal joints |
| Extensor carpi ulnaris | Posterior border of ulna | Extends and adducts the wrist |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Humero-ulnar joint | Synovial hinge | Humerus (trochlea) |
| Proximal radioulnar joint | Synovial pivot | Radius (radial head) |
| Distal radioulnar joint | Synovial pivot | Radius (ulnar notch) |
Common Pathologies
Olecranon fracture
Fracture of the elbow tip, commonly from a direct fall onto the elbow. The triceps tendon pull displaces the proximal fragment, usually requiring surgical fixation with tension band wiring.
Monteggia fracture
Fracture of the proximal ulnar shaft with dislocation of the radial head. Always obtain elbow radiographs with ulnar shaft fractures to avoid missing the radial head dislocation.
Nightstick fracture
Isolated fracture of the ulnar shaft from a direct blow, historically from defending against a strike with a raised forearm.
Olecranon bursitis
Inflammation of the bursa over the olecranon causing a visible swelling at the point of the elbow. Can be septic or non-septic.
Clinical Relevance
The ulnar nerve passes behind the medial epicondyle of the humerus in the cubital tunnel and along the medial side of the ulna, making it vulnerable to compression at multiple points. Monteggia fractures are commonly missed injuries; the radiocapitellar line (a line through the radial shaft should always pass through the capitulum on all views) helps detect associated radial head dislocation. Both-bone forearm fractures in adults require open reduction and internal fixation to restore the interosseous space needed for pronation and supination.
Development and Ossification
The ulna ossifies from three centers: one for the shaft (8th fetal week), one for the distal end (around age 5-7), and one for the olecranon (around age 10). The distal epiphysis fuses around age 17-18 and the olecranon around age 16. The olecranon apophysis is a common site of traction injury in adolescent athletes.
Did You Know?
- Ulna derives from the Latin ulna meaning elbow or forearm, which also gave rise to the word ell, an ancient unit of measurement based on forearm length
- The olecranon literally means head of the elbow in Greek (olene = elbow, kranion = head)
- The subcutaneous posterior border of the ulna is palpable along its entire length, the longest palpable bony surface in the body
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