The shoulder region includes the glenohumeral joint and the acromioclavicular joint. The glenohumeral joint is a ball and socket joint in that the head of the humerus sits into the shallow “socket” of the scapula (or shoulder blade) known as the glenoid cavity. Ball and socket joints allow for movement in all directions including rotation. The shallow socket does make the joint less stable though and relies on the musculature and ligaments around it for stability.
The acromioclavicular joint is the joint between the clavicle (collarbone) and the acromion process of the scapula. The acromion process is the highest point of the shoulder. It is one of the joints that allows full range of movement of the glenohumeral joint
Previous research has suggested that you can have anatomical variance in the shape of the acromion however it is now thought that the variances are a result of how the surrounding muscles function and ossification of the superior end of the coracoacromial ligament which passes between the coracoid process and the acromion process. If an abnormal tension is placed on the ligament through muscle dysfunction then there is a pull on the acromion process and may result in a hook shape forming. (Edelson, The Journal of Bone and Joint Surgery, Vol 77-8, No 2, March 1995). Those who have a hook shaped acromion are more susceptible to shoulder injuries such as rotator cuff tears and impingement.
The muscles surrounding the shoulder region included the rotator cuff (supraspinatus, infraspinatus, subscapularis and teres minor), teres major and deltoid. The primary function of these muscles is to stabilise the glenohumeral joint and to keep the head of the humerus centred in the glenoid fossa. When the muscles are not functioning properly due to strength imbalance, overactivity or underactivity then the humeral head will be off centre and eventually result in wear and tear of the ligaments and tendons surrounding it.