20 THE SUPERIOR EXTREMITY 



Dislocation of the Shoulder. In the child, the capsule 

 is stronger than the bond of union between the diaphysis and 

 epiphysis, and consequently the common injury is a separation 

 of the proximal epiphysis. In the adult, the shoulder-joint is 

 ligamentously and osseously weak, and is strong only by virtue 

 of its muscular support. Stability is sacrificed to the necessity 

 for mobility, and, as a result, dislocation of the joint is exceedingly 

 common. 



The violence which causes the dislocation is unexpected, 

 and the supporting muscles are consequently relaxed at the 

 time of the accident. The humerus is generally in the abducted 

 position and forms the long arm of a lever, whose short arm is 

 formed by the head of the bone. The acromion acts as a fulcrum, 

 and when the long arm of the lever is thrust upwards, the head 

 is tilted downwards upon the inferior part of the capsule, which 

 is the only area unsupported by muscular insertions. This 

 gives way, and the head of the bone passes through the tear 

 and lies below the glenoid cavity. 



The head may remain in this position the sub-glenoid 

 dislocation. The lower margin of the glenoid rests in the groove 

 of the anatomical neck, and the head lies between the long head 

 of the triceps behind and the subscapularis in front. 



Though all dislocations are, in the first instance, sub-glenoid, 

 the flexors and adductors of the shoulder usually draw the head 

 of the humerus upwards, forwards, and medially. This is the 

 anterior displacement, and the head of the bone may occupy 

 any of the following positions : 



1. The Sub-coracoid. This is the commonest dislocation. 

 The posterior part of the anatomical neck rests against the 

 anterior margin of the glenoid cavity, and the head lies behind 

 the subscapularis. The articular surface is directed somewhat 

 forwards, and the humerus is laterally rotated owing to the 

 stretching of the infra-spinatus and the teres minor. The 

 posterior part of the capsule is stretched across the articular 

 surface of the glenoid cavity. 



2. The Intra-coracoid. In this dislocation the anatomical 

 neck rests on the anterior surface of the neck of the scapula. 

 The lateral rotators are over-stretched, and if they do not rup- 

 ture, a part or the whole of the greater tubercle is torn away, 

 and the subscapularis, being unopposed, rotates the humerus 

 medially. 



3. The Sub-clavicular. The head of the humerus rests 



