The shoulder joint is made up of from the articulation of the ball of the head of the arm bone called the humerus into the cup of the shoulder blade called the scapula. The cup makes only 1/8th of the circumference of the shoulder and hence the shoulder joint is seemingly unstable, especially if compared to other large ball and socket joint of the body that is the hip joint.
The cup of the shoulder joint is deepened secondary to the presence of a fibrocartilaginous rim around the cup of the shoulder in the form of a gasket called the labrum. This provides some degree of stability to the shoulder joint. The stability of the shoulder joint is further strengthened by the capsule of the shoulder joint as well as the surrounding muscles which keep the head or the bulb in place over the cup.
Due to the attributes of certain degree of instability of the shoulder joint, there is increased mobility and hence more functionality of the shoulder. There are multiple muscles around the shoulder joint. The major muscles include the deltoid which is on the outer aspect of the shoulder and helps in elevation of the arm. Another group of important shoulder muscle are called rotator cuff muscles which essentially include four muscles, the subscapularis which is present in the front, the supraspinatus which is present on the top, the infraspinatus and teres minor which are present on the back of the shoulder joint.
All these four muscles merge to each other to form a continuous tendinous cuff that arises from the shoulder blade and inserts itself around the head of the humerus. This confluent tendinous portion of these muscles is called the rotator cuff.
The rotator cuff is important in stabilizing the ball onto the socket as well as in elevation of the arm and inside and outside rotation of the arm. The rotator cuff is present in a space between the bulb or the head of humerus and a projection of bone coming out of the scapular bone called Acromion. Normally, there is enough space for the tendon to glide smoothly.
There is enclosed lining on the outside of the cuff called the subacromial bursa which helps in smooth gliding of the cuff underneath the acromion. The acromion on the inner side in the front also articulates with collar bone or the clavicle and forms the acromioclavicular joint. This joint is stabilized by the capsule as well as ligaments attaching the clavicle to the shoulder blade.