In some cases, the unstable shoulder actually slips out of the socket. If the shoulder slips completely out of the socket, it has become dislocated. If not treated, instability can lead to arthritis of the shoulder joint. The rotator cuff connects the humerus to the scapula. Tendons attach muscles to bones.
Muscles move bones by pulling on tendons.
The muscles of the rotator cuff also keep the humerus tightly in the socket. A part of the scapula, called the glenoid , makes up the socket of the shoulder. The glenoid is very shallow and flat. Surrounding the shoulder joint is a watertight sac called the joint capsule . The joint capsule holds fluids that lubricate the joint.