Injury or fatigue of the rotator cuff muscles can lead to poor control of the humeral head and wear of the supraspinatus against the acromion. This is appreciated as pain over the outside of the shoulder through an arc when the arm is loaded away from the body. Repetitive use of the arm at and above chest height is a common cause.
The cornerstone of treatment of this condition is to understand the mechanics and avoid loading the arm with the elbow away from the body. The irritation to the tendon will settle with time but may require a cortisone injection. At this stage, it will be possible to build control of the shoulder blade on the chest wall and then strengthen the short muscles of the rotator cuff selectively through a carefully supervised program of exercises. Whilst this sounds simple, if the exercises are performed inexpertly, they may aggravate your condition. Your physiotherapist will tailor a specific program for you individually.
Occasionally it is not possible to restore good control to the ball and socket. In this case, surgery may be appropriate. Your surgeon may recommend an arthroscopic acromioplasty.