The shoulder is arguably the most complex joints in the body. Three bones make up the shoulder joint; the scapula (the shoulder blade), the humerus (the bone in the upper arm) and the clavicle (the collar bone). The scapula has many protuberances and fossas (hollows or depressions) to allow for tendons and ligaments to attach to the scapula, and one very large fossa called the glenoid fossa that the head of the humerus glides across.
Although these three bones make up the shoulder, they connect to each other and the sternum (the bone in the centre of the chest) at four distinct joints. These are the glenohumeral joint (where the humerus meets the scapula at the glenoid fossa), the scapulothoracic joint (where the scapula meets the rib cage at the back), the sternoclavicular joint (where the sternum meets the clavicle) and the acromioclavicular joint (where the clavicle meets a protuberance on the scapula called the acromion).