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Source: Shoulder Pain Info
The shoulder complex is made up of three bones, which are connected by muscles, ligaments, and tendons. The large bone in the upper arm is called the humerus. The shoulder blade is called the scapula and the collarbone is called the clavicle. The top of the humerus is shaped like a ball. This ball sits in a socket on the end of the scapula. The ball is called the head of the humerus and the socket is called the glenoid fossa, hence the term “glenohumeral” joint. The glenoid fossa has a rim of tissue around it called the glenoid labrum. The glenoid labrum makes the glenoid fossa deeper. The glenohumeral joint is the most mobile joint in the body.
Articular cartilage is a smooth shiny material that covers the humeral head and the glenoid fossa of the glenohumeral joint. There is articular cartilage anywhere that the bony surfaces come into contact with each other. Articular cartilage allows these bones to slide easily over each other as the arm moves.
The glenohumeral joint is just one of the joints in the shoulder complex. The other two joints are the sternoclavicular joint and the acromioclavicular joint. The sternoclavicular joint allows a small amount of movement to occur between the inner (medial) part of the clavicle and the breastbone (sternum). The acromioclavicular joint allows a small amount of movement to occur between the outer (lateral) part of clavicle and a projection on the top of the scapula called the acromion process. The scapula sits on the back of the ribs and moves as the arm moves.