The healthy shoulder is a complex structure of bones, muscles, ligaments (connect bone to bone), tendons (connect muscle to bone), and cartilage. The bones that form the shoulder joint are the humerus (upper arm bone) and the scapula (shoulder blade). The top end of the humerus has a large round surface called the humeral head, which rides against the scapula in a small socket called the glenoid. Think of this connection as a golf ball on a tee. The limited contact area is what allows near unlimited range of motion in a healthy shoulder.
The humeral head and the glenoid are covered with a cushioning tissue called cartilage. Cartilage is a tough lubricating tissue that allows the bones to painlessly glide against each other. The shoulder joint is encapsulated by a group of muscles and ligaments called the rotator cuff. The rotator cuff helps stabilize the shoulder joint and provide movement.