Overview
The ulnar is a long bone in the medial aspect of the forearm. It acts as the stabilising bone.
Structure
Proximal
- Olecranon – large projection of bone that extents proximally, forming part of the trochlear notch. It is the site of attachment for the triceps brachii (muscle of the posterior compartment of the arm)
- Coronoid process – bony ridge projecting anteriorly, forming part of the trochlear notch
- Trochlear notch – wrench-shaped articular surface for trochlea of humerus, formed by olecranon and coronoid process, and contributing to the carrying angle of the arm.. It has sloped medial and lateral portions with a smooth ridge between.
- Radial notch – lateral articulating surface for the radial head, located lateral to the trochlear notch
- Tuberosity of ulnar – roughened anterior surface distal to the coronoid process for attachment of brachialis (muscle of the anterior compartment of the arm)
Shaft
The ulnar shaft has a triangular cross-section, which tapers in width distally, with three borders (anterior, interosseous and posterior) and three surfaces (anterior, medial and posterior).
The three borders:
- Posterior – palpable along the entire length of the forearm posteriorly
- Interosseous – site of attachment for the interosseous membrane, which spans the distance between the two forearm bones.
- Anterior – unremarkable.
The three surfaces:
- Anterior – site of attachment for the pronator quadratus muscle distally.
- Posterior – site of attachment for many muscles.
- Medial – unremarkable.
Distal
- Round head
- Distally and anterolaterally covered by hyaline cartilage
- Ulnar styloid originates posteromedially, projects distally
Ligamentous Attachments
- Anular ligament
- Ulnar collateral of the elbow
- Portion of radial collateral of the elbow
- Quadrate ligament
- Oblique cord
- Interosseous membrane
- Dorsal and palmar radioulnar ligaments
- Ulnar collateral of the wrist
- Palmar ulnocarpal ligaments (ulnotriquetral, ulnolunate, ulnocapitate)
Muscular Attachments
Anterior compartment
- Brachialis
- Flexor carpi ulnaris
- Pronator teres
- Flexor digitorumsuperficialis
- Flexor digitorumprofundus
- Pronator quadratus
Posterior compartment
- Anconeus
- Supinator
- Abductor pollicis longus
- Extensor pollicis longus
- Extensor indicis
Arterial Supply
- Radial artery – supplies the posterolateral aspect of the forearm
- Ulnar artery – supplies the anteromedial aspect of the forearm
- Anterior and posterior interosseous arteries
Venous Drainage
Vena commitantes of the arteries of the same name
Innervation
- Anterior interosseous nerve (of median nerve)
- Posterior interosseous nerve (of radial nerve)
Development
- Shaft (8 weeks gestation)
- Distal (6 years → 16 years)
- Proximal (10 years → 15 years)
Anatomical Variants
- Persistent ossicle of ulnar styloid (osulnostyloideum)
- Ostriangulare
- Lunula