Anatomy of Ulna Bone

Ulna is a long bone in the forearm. It lies medially and parallels to the radius, the other bone in the forearm

It acts as stabilizing the bone, with the radius pivoting to produce pronation and supination movement of the forearm.

The bone participates in the formation of the elbow joint by articulating with the humerus. Distally, the ulna articulates with the radius, forming the distal radio-ulnar joint.

It is the medial bone of the forearm and is homologous with the fibula of the lower limb. Along with other bone called radius connects elbow with radius. Ulna has upper and lower ends and a  shaft.

The Upper End

The upper end of ulna presents the olecranon and coronoid processes, and the trochlear and radial notches.

Olecranon Process

The olecranon process projects upwards from the shaft. It has superior, anterior, posterior, medial and lateral surfaces. The anterior surface is particular. It is concave and forms the upper part of the trochlear notch. The posterior surface forms a triangular subcutaneous area which is separated from the skin by a bursa.

Inferiorly,  it is continuous with the posterior border of the shaft of the ulna. The upper part forms the point of the elbow. The medial surface of the shaft. The lateral surface is continuous inferiorly with the posterior surface of the shaft.

Coronoid Process

Coronoid process projects forwards from the shaft just below the olecranon and has four surfaces, superior, anterior, medial and lateral. The superior surface forms the lower part of the trochlear notch. The anterior surface is triangular and rough. Its lower corner forms the ulnar tuberosity.

The upper part of its lateral surface is marked by the radial notch for the head of the radius. The annular ligament is attached to the anterior and posterior margins of the notch. The lower part of the lateral surface forms a depressed area (to accommodate the radial tuberosity). It is limited behind by a ridge called the supinator crest.

Trochlear Notch

It forms an articular surface that articulates with the trochlea of the humerus to form the elbow joint.

Radial Notch

The radial notch articulates with the head of the radius to form the superior radioulnar joint.

The Shaft

The shaft has three borders and three surfaces.

Borders

The interosseus (or lateral) border is sharpest in its middle two fourths. Inferiorly it can be traced to the lateral side of the head. Superiorly it is continuous with the supinator crest.

The anterior border of the shaft of the ulna is thick and rounded. It begins above on the medial side of the ulnar tuberosity, passes backward in its lower one third, and terminates at the medial side of the styloid process.

Surfaces

Ulna has three borders anterior, posterior and medial.

Distal Portion 

The distal end of the ulna is much smaller in diameter than the proximal end. It terminates in a rounded head which also has a distal projection – the ulnar styloid process.

The head articulates with the ulnar notch of the radius to form the distal radio-ulnar joint.

Attachments

The common tendon of triceps brachii muscle inserts on the posterior part of the superior surface of the olecranon process

Anconeus muscle

Anconeus muscle inserts on the lateral aspect of olecranon process.

Brachialis Muscle

Brachialis muscle inserts on to the interior surface of the coronoid process of the ulna.

Pronator Teres

Pronator teres muscle originates from the medial surface on the middle portion of the coronoid process. It shares the origin with medial epicondyle of the humerus.

Flexor carpi ulnaris muscle

This muscle takes origin from the olecranon process and posterior surface of ulna. It also shares the origin with medial epicondyle of the humerus.

Flexor Digitorum

Flexor digitorum superficialis and profundus muscles take origin from the coronoid process and the anteromedial surface of ulna

Pronator Quadratus

It originates from the distal portion of the anterior ulnar shaft.

Extensor carpi ulnaris

It originates from the posterior border of ulna [and lateral epicondyle of the humerus]

Supinator muscle

Originates from proximal ulna [along with lateral epicondyle of the humerus]

Abductor pollicis longus

This muscle originates from the posterior surface of ulna [and also from the posterior surface of the radius bone]

Extensor pollicis longus

It originates from the dorsal shaft of ulna [and also from the dorsal shaft of the radius and the interosseous membrane)

Extensor Indicis

It originates from the posterior surface of the distal ulna (and interosseous membrane)

Clinical Significance

Knowing and understanding ulnar anatomy provides us with the knowledge to communicate the sites of affection.
An isolated fracture of ulna occurs when ulna is hit by an object. The shaft is the most likely site of the fracture.

Olecranon process gets fractured when patient falls on a flexed elbow. The triceps brachii can displace part of the fragment proximally.

Monteggia’s fracture and Galeazzi’s fracture are other common injury involving ulna.

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