LUXATION OF THE SHOULDER 75 



DISEASES OF THE SHOULDER JOINT 



LUXATION OF THE SHOULDER 



It will readily be gathered that the combined action of the 

 tendons which play over the joint, and which have previously been 

 described, will materially assist in keeping the articular head of 

 the humerus in position in the glenoid cavity, and from their manner 

 of disposition round the joint will, unless the tendons be ruptured, 

 effectually prevent internal or external dislocation. Dislocation in the 

 posterior direction is prevented by the action of the triceps extensor 

 cubiti muscle, the large head of which suspends the humerus in such a 

 manner that its long axis forms an acute angle with that of the scapula. 

 Since the angle included by these two axes is directed backwards, the 

 greater the pressure on the front of the shoulder the more tightly does 

 the articular head of the humerus become wedged in the glenoid cavity 

 of the scapula. Thus it comes about that although at first sight the 

 shallowness of the glenoid cavity, and the fact that the joint only possesses 

 one ligament which, again, is quite loosely attached to the bones, would 

 make it appear that dislocation of the shoulder joint would be a matter 

 of common occurrence, it is, for the anatomical reasons given, extremely 

 rare, and fracture of the scapula at the glenoid cavity is more likely to 

 occur than dislocation in the posterior direction, in cases of abnormally 

 great pressure in front of the joint. 



In such cases of dislocation which do occur, therefore, we usually have 

 luxation in the anterior direction. It will readily be understood that if 

 the biceps muscle be in a contracted or tense state, its tendon will prevent 

 the head of the humerus from slipping forwards out of the glenoid cavity. 

 Luxation thus occurs when this muscle is relaxed, and the elbow joint 

 involuntarily flexed. Another contributory condition is extreme flexion 

 of the shoulder joint itself. A sudden fall with the accompanying 



