Ulna

From Ganfyd

The ulna is one of the two bones of the forearm, lying medial to the radius in the anatomical position. It articulates with the humerus at the elbow and the carpal bones at the wrist, and also has two points of articulation with the radius.

Surface Anatomy

The posterior border of the ulna is completely subcutaneous and can be felt throughout its length. The olecranon is obvious even with the arm extended, but forms the 'point' of the elbow on flexion. The styloid process can be felt at the wrist, slightly more proximal than its equivalent on the radius. The head is visible, especially in pronation, and can be gripped between the fingers if the hand is flexed.

Ulna
Anterior and posterior views of the ulna
System:	Skeletal system
Function:
Origin:
Branches:
Insertion:
Arterial supply:
Venous drainage:
Lymphatic drainage:
Innervation:
Vertebral levels:
Search for Ulna in Gray's.

Shape

The proximal ulna displays a number of unique features. The olecranon process is the most proximal part of the ulna and is hooked with a concave surface facing distally. The posterior surface of the olecranon is smoother and triangular, coming to the point of the elbow. The anterior articular surface forms the proximal part of the trochlear notch. The trochlear notch articulates with the trochlea of the humerus. It is divided into medial and lateral parts by a central ridge. The coronoid process projects anteriorly at the distal end of the trochlear notch, its proximal aspect also forming the distal surface of the notch. On the lateral surface of the coronoid a shallow radial notch exists which articulates with the head of the radius. The coronoid has a triangular anterior surface, which is raised distally to form the ulnar tuberosity.

The shaft of the ulna is triangular proximally, becoming more cylindrical in its distal portion. It has anterior, medial and posterior surfaces and anterior, interosseous and posterior borders. The interosseous border is most conspicuous as a crest in the middle portion of the lateral aspect, which continues proximally as the supinator crest, which itself merges with the posterior border of the radial notch. The nutrient foramen is proximal to the midpoint of the anterior surface, angulated proximally, and contains a branch of the anterior interosseous artery. An oblique line, often faint, continues the supinator crest more posteriorly to the distal quarter as the vertical ridge.

The distal end of the ulna is only a little expanded from the diameter of the shaft. The head has a convex lateral surface which articulates with the ulnar notch of the radius, and a smooth distal surface where it is in contact with the triangular articular disc. This disc is attached to a rough area adjacent to the styloid process. The styloid process of the ulna is a short posterolateral projection of the medial border. A posterior vertical groove separates the head and styloid.

Attachments

Origins

• Flexor digitorum superficialis has three heads, the ulnar head arising from the proximal medial border of the coronoid (from the 'sublime tubercle')
• Pronator teres has two heads, the ulnar head arising from the distal medial border of the coronoid
• Supinator has two origins, the deep part arising from the supinator crest
• Flexor digitorum profundus arises from a large area on the upper two thirds of the anterior, medial and posterior aspects as far as the posterior border
• Abductor pollicis longus arises from a large area on the posterior forearm including the upper posterior surface of the ulna
• Extensor pollicis longus arises from the middle third of the posterior ulna, distal to abductor pollicis longus
• Extensor indicis arises from the lower posterior shaft distal to extensor pollicis longus
• Pronator quadratus arises from the lower quarter of the anteromedial shaft.

An aponeurosis arises from the posterior border of the ulna, common to extensor carpi ulnaris, flexor carpi ulnaris and flexor digitorum profundus. This also continues as the deep fascia of the forearm.

In addition, flexor pollicis longus occasionally has a small head arising at the base of the coronoid process above the origin of flexor digitorum superficialis.

Insertions

• Triceps inserts into the posterior part of the upper surface of the olecranon process
• Anconeus inserts into the lateral surface of the olecranon and the upper quarter of the posterior surface
• Brachialis inserts into the coronoid process and ulnar tuberosity.

The interosseous membrane of the forearm is attached to the interosseous border.

The ulnar collateral ligament attaches to the ulnar styloid.

Joints

• The humeroulnarjoint at the elbow
• The proximal and distal radioulnar joints.

The ulna does not directly contribute to the wrist joint, being separated from the carpal bones by the triangular articular disc. The ulnar styloid is separated from the triquetral by a small meniscus.

Direct Relations

The ulna is either subcutaneous or surrounded by muscles for the majority of its length.

• The ulnar nerve lies medial to the coronoid process at the elbow.

Ossification

The ulna ossifies from four centres: two in the olecranon and one each in shaft and distal end. Midshaft ossification begins in the eighth week. The proximal epiphysis unites in the fourteenth to sixteenth years, the distal in the seventeenth to eighteenth years.

Clinical Relevance

• May be fractured
• May be dislocated as in dislocated elbow or Galeazzi fracture
• Patients with clubhand show ulnar shortening.