Musculoskeletal Imaging

Ulna

1. Anatomy

one of the long tubular bones of the forearm, which articulates with the humerus, with the head of the radius and with the carpal bones. Also, see ulna.

2. Pathology

Developmental defects of the ulna include absence (aplasia) or incomplete formation (hypoplasia). In some cases these abnornalities may be combined with hypoplasia or absence of the radial carpal bones or thumb.

Translocation of the ulna may occur when the carpus shifts in an ulnar direction. This anomaly sometimes occurs in patients with severe capsular injury of the carpus or joint disorders such as rheumatoid arthritis.

The ulna may also be the site of various types of fracture and dislocation. A greenstick fracture or bowing fracture typically occurs in long bones such as the ulna and radius. Growth plate injuries may also involve the ulna.

Fractures of the olecranon of the ulna may result from direct injury, indirect injury or a combination of the two. Isolated fractures of the coronoid process of the ulna may occur, but more often these fractures are associated with posterior dislocations of the elbow.

The ulnar diaphysis is frequently the site of fracture; however, it forms part of a forearm ring structure, and in common with other ring structures in the body (pelvis, mandible), disruption of the ring at one site is usually accompanied by disruption at a second (and even third) site. Both injuries may be fractures or the injuries may consist of a fracture and a dislocation, as in Monteggia fracture dislocation. Concomitant fractures of the radius and the ulna are common and usually result from direct injury. Fractures of the diaphysis of the ulna may occur as in isolation (nightstick fracture) or as part of a Monteggia fracture  dislocation. In addition, fractures of the distal end of the ulna most commonly occur in a fall on the outstretched hand causing dorsiflexion of the wrist. Fractures of the styloid process of the ulna are usually a component of a Colles fracture, but an injury isolated to the ulnar styloid process is observed occasionally.

The combination of an ulnar fracture and radial head dislocation is termed a Monteggia fracture  dislocation. Various types are recognized (Table 1).

Ulna, Table 1. Types of Monteggia fracture  dislocation.

Type I:Fracture of the middle or upper third of the ulna with anterior dislocation of the radial head and anterior angulation of the ulna.
Type II:Fracture of the middle or upper third of the ulna with posterior dislocation of the radial head and posterior angulation of the ulna.
Type III:Fracture of the ulna just distal to the coronoid process with lateral dislocations of the radial head.
Type IV:Fracture of the upper or middle third of the ulna with anterior dislocation of the radial head and fracture of the upper third of the radius below the bicipital tuberosity.

The Monteggia fracture - dislocation is a common injury in adults; therefore, multiple views of the elbow should be obtained in all patients with fractures of the proximal half of the ulna.

Elbow dislocation commonly involves the ulna or both the radius and the ulna. In adults, a hyperextension injury may be complicated by fracture of the coronoid process of the ulna. A divergent dislocation of the elbow, in which the radius and ulna move in different directions, may occur but is rare. Complications of elbow dislocations include heterotopic ossification or calcification and damage to blood vessels and nerves.

DR

DR