118 CLINICAL APPLIED ANATOMY. 



Dislocations at the Elbow- Joint. The elbow- joint is anato- 

 mically a fairly secure articulation. The greater sigmoid cavity of 

 the ulna very closely embraces the trochlear surface of the 

 humerus, which is limited internally by a prominent margin. 

 While the shallow cup-shaped facet on the upper surface of the 

 head of the radius cannot be said to have much co-aptation with 

 the capitellum of the humerus, yet the ligaments adjacent, par- 

 ticularly the orbicular and the external lateral, together with the 

 supinator brevis muscle, serve to keep the bony surfaces securely 

 in contact. 



The smallness of the lip of the coronoid process, the liability 

 of the upper extremity to severe violence, and the complicated 

 movements of pronation and supination all tend to neutralise the 

 anatomical peculiarities making for safety. Thus it comes about 

 that the elbow is more frequently the site of dislocation than any 

 other joint except the shoulder, and the temporo-mandibular 

 joints. 



The bones of the forearm entering into the elbow-joint may be 

 dislocated together or separately. There are, however, only two 

 common dislocations, one where both the ulna and radius are 

 displaced backwards, and the other where the radius alone 

 has its head dislocated forwards. The backward displacement is 

 probably most usually brought about by a fall on the extended 

 and pronated hand, with the elbow slightly flexed. The anterior 

 ligament and the anterior portions of the lateral ligaments are 

 suddenly put upon the stretch, they snap, and allow the coronoid 

 process to slip around beneath the trochlea into the olecranon 

 fossa, dragging the head of the radius surrounded by the orbicular 

 ligament with it. At the same time the lower end of the humerus 

 is forced forwards, completing the laceration of the anterior 

 ligament and the displacement of the bones. Occasionally the tip 

 of the coronoid process is broken off, but it does not carry with 

 it the attachment of the brachialis anticus. The fracture of 

 the process however leads almost necessarily to a recurrence 

 of the displacement after reduction, owing to the want of an 

 anterior lip to the sigmoid cavity to hold the ulna in position. 



