THE REGION OF THE ELBOW 61 



of the humerus slips forwards out of the semilunar notch. The 

 coronoid process comes to rest in the olecranon fossa ; the head 

 of the radius lies behind the lateral epicondyle ; and the distal 

 end of the humerus sinks into the cubital fossa. The arm and 

 forearm meet at an angle of about 120,, but in old-standing 

 cases the angle becomes less pronounced owing to the constant 

 traction of the triceps. The relations of the epicondyles to one 

 another are unchanged, but their relations to the other bony 

 points undergo considerable alteration. 



(b) Anterior. This rare dislocation results from violence, 

 applied to the olecranon from behind when the elbow is flexed. 

 The triceps is ruptured and the radius and ulna pass forwards, 

 the forearm being flexed by the brachialis and the biceps. All 

 the ligaments except the annular (orbicular) are torn, and the 

 muscles arising from the epicondyles may be injured. Compared 

 with the sound limb, there is an increase in the distance between 

 the lateral epicondyle of the humerus and the styloid process 

 of the radius. 



An incomplete variety of this dislocation occurs when the 

 injury is complicated by fracture of the olecranon. The small 

 fragment remains in contact with the trochlea, but the remainder 

 of the ulna and the radius pass forwards in front of the distal 

 end of the humerus. 



(c) Lateral and (d) Medial dislocations occur from falls 

 upon the pronated, outstretched hand. The direction of the 

 displacement depends on whether the line of force is lateral or 

 medial to the mid-point of the transverse axis of the elbow. 



The lateral dislocation may be complete, and, if so, all the 

 ligaments, except the annular, are ruptured. More commonly 

 the semilunar notch does not entirely leave the trochlea, and 

 the dislocation is consequently incomplete. 



The medial dislocation is always incomplete, as the head of 

 the radius remains partially in contact with the capitulum, 

 though at a different angle, as the long axis of the bone is now 

 directed distally and medially. 



Divergent dislocations can occur only after rupture of the 

 annular (orbicular) ligament. They may be either antero- 

 posterior, the radius passing forwards and the ulna backwards, 

 or they may be horizontal, the radius being displaced laterally 

 and the ulna medially. 



Reduction of the posterior dislocation may be performed 

 by traction on the forearm and counter-extension of the arm. 



