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. The walls of the joint capsule are made up of ligaments.
Ligaments are soft connective tissues that attach bones to bones.
The joint capsule has a considerable amount of slack, loose tissue, so that the shoulder is unrestricted as it moves through its large range of motion. If the shoulder moves too far, the ligaments become tight and stop any further motion, something like a dog coming to the end of its leash. Dislocations happen when a force overcomes the strength of the rotator cuff muscles and the ligaments of the shoulder. Ninety-seven percent of dislocations are anterior dislocations, meaning that the humerus slips out of the front of the glenoid. Only three percent of dislocations are posterior dislocations, or out the back.