What is instability?
Instability means the humerus (ball) slips out of the glenoid (socket). This can occur from an injury and is then called a dislocation (dislocated shoulder). If the dislocation occurs repeatedly, it will lead to arthritis unless the instability is corrected. A less severe form of instability is called subluxation,

 

where the ball slips part way out and then goes back in. Subluxation can be subtle but if untreated will usually lead to chronic strain on the rotator cuff tendon. Dislocation or sublaxation can also cause injury of cartilage in the shoulder.