GLENOHUMERAL JOINT

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The glenohumeral joint (sometimes also called the scapulohumeral joint) is formed by the articulation of the head of the humerus with the glenoid fossa of the scapula.
 

This articulation is also considered a ball and socket synovial joint.  If offers the greatest range of motion (ROM) and movement potential of any of the body's joints because of the limited ligamentous support.  For the same reason it is also the most frequently dislocated joint

The roof of the shoulder is formed by the end of the clavicle and the acromion from the scapula.  The four rotator cuff muscles pass under this roof (also known as the coracoacromial arch) to reach the humeral head.
 
 

The structures forming this arch include the acromion, the acromioclavicular joint (AC), the coracoid process of scapula and the coracoacromial ligament.  The architecture of the roof is dependent on the anterior slope of the acromion.  This sloping can reduce the space available for the rotator cuff to reach the humeral head, possibly leading to impingement syndrome.
 
 

In the above picture you see three types of anterior slopes of the acromion.  Type I is a flat acromion, Type II is curved and Type III is hooked.  Full thickness tears of the of the rotator cuff appear to be correlated with a Type II and Type III acromion.