The most common form of dislocation at the elbow is one of both bones backwards. The bones generally go together, owing to the firm connection between them. They tend to be displaced back or forwards rather than laterally, owing to the weakness of the capsule, slight muscular support antero-posteriorly, and to the narrowness of the articular surface in that direction, while a forward displacement is rendered almost impossible by the large curved olecranon, the coronoid process rendering much less resistance to a backward displacement. The cause is generally a fall on the hand with the arm fully extended, the force producing first a hyperextension, which brings the olecranon into contact with the humerus, so as to act as a fulcrum, and lever the coronoid process away from the humerus. The upward acting force then comes into play, pushing the two bones up behind the humerus. Dislocation may also be produced by wrenching inwards of the forearm with the elbow semiflexed. The internal lateral ligament is torn, and the coronoid turned down under the articular surface of the humerus. When the dislocation is complete, the coronoid is opposite to, but not in, the olecranon fossae ; the head of the radius lies behind the outer condyle ; the anterior and lateral ligaments are torn ; the brachialis anticus much stretched and generally torn ; the anconeus rendered very tense, as are likewise the ulnar and median nerves ; and the biceps is markedly stretched over the lower end of the humerus. In such cases the tip of the olecranon lies above the intercondylar line, notwithstanding the flexion of the limb, which generally coexists, while the head of the radius can generally be detected posteriorly behind the external condyle, and the rounded extremity of the humerus felt anteriorly. Even in the not infrequent incomplete dislocation of both bones backwards, the relationship of the olecranon to the condyles is a good guide to the condition.
Lateral displacements of both bones are much less common, are generally incomplete, and are more frequently outwards than inwards, owing to the marked projection downwards of the humeral articular surface on the inner side. A slight lateral deviation frequently accompanies the backward displacement. Dislocation of both bones forwards is very rare. When a single bone is dislocated, it is generally the radius which is affected, owing to its slight connection to the humerus, its greater mobility, and its greater connection with the hand. It is most often displaced forward by jerks of the forearm, the anterior ligament of the elbow-joint and the orbicular ligament giving way. The radial head then lies in front of the external condyle, producing an abnormal fulness, and flexion and supination are both interfered with.
A sprain of the elbow, or ' pulled elbow,' is a somewhat similar condition frequently met with in young children, where, owing to a sudden jerk of the arm, the radius is pulled out of the orbicular socket. The limb then lies in a position of slight flexion and pronation, and supination is impossible. The radius may also be displaced backwards or outwards. When the ulna is dislocated alone, it is always backwards. The condition is very rare.