The shoulder is a ball-and-socket joint and has the greatest range of motion of any joint in the body. Because of this mobility, it is at risk for injury or degenerative problems. The bones of the shoulder are the humerus (upper arm bone), clavicle (collar bone), and scapula (shoulder blade). The head of the humerus bone (the ball) is lined with cartilage that glides over the shoulder socket (also known as the "glenoid cavity"). The clavicle attaches the shoulder to the rib cage and holds the shoulder out from the body. The scapula is a large triangular bone located on the back side of the upper body, and it is connected to the clavicle through the acromioclavicular (AC) joint.
In the shoulder socket, the humerus sits like a golf ball on a tee, supported by a complicated arrangement of muscles, tendons, and ligaments. The rotator cuff is a group of tendons that attaches four shoulder muscles to the upper arm. These tendons help keep the humerus bone in place within its shallow socket and ensure that the arm moves freely within the joint.