Shoulder instability refers to a condition where the humeral head, or ball, does not maintain adequate contact with the socket of the shoulder. This can occur after an acute traumatic dislocation as the labrum and the ligaments that link the ball and socket are torn. Some patients may avoid recurrent dislocation if the ligaments and labrum heal adequately. In many cases, however, incomplete or improper healing leads to recurrent dislocation and instability requiring surgical repair. Instability most often occurs in the anterior or inferior direction, meaning that the ball feels as though it is slipping out the front. Classic findings within the labrum and ligaments are seen in most MRI scans following recurrent dislocation and can provide useful information guiding surgical repair.

Instability can also occur in situations where patients naturally possess and abnormally high degree of laxity or looseness in their ligaments, affording too much movement to the ball and the feeling that ball is slipping excessively even in the absence of a specific injury. This is referred to as multidirectional instability or multidirectional laxity and generally responds favorably to an appropriate strengthening program aiming to recruit the surrounding muscles to increase their support of the loose ligaments. Only in cases where a prolonged period of physical therapy fails to provide improved stability to the shoulder is surgery considered for multidirectional laxity.

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