MULTIDIRECTIONAL INSTABILITY (MDI)

The shoulder is meant to offer a wide range of motion. In order to maintain that safely, the muscles in the joint need to be strong and balanced. When a person has too much looseness in the shoulder, and cannot maintain the position of the joint, it’s called multidirectional Instability.

MDI can occur from any number of issues within the shoulder structure. The ball and socket joint is made up of the humeral head (the ball) and the glenoid fossa (the socket). The humeral head fits loosely into its socket, and so the joint relies on the integrity of the surrounding ligaments and muscle structures to provide balance and stability. If any one of these ligamentous or muscle structures is damaged, it can cause MDI.

There are two ways that shoulder instability can occur, either by atraumatic onset or traumatic onset. With a traumatic instability onset, the instability is caused by a sudden injury. MDI is considered an atraumatic instability. Patients with MDI typically have an overall looseness in the shoulder joint that leads to instability.

MDI usually develops from repetitive overhead motions, although someone can be born with excessive laxity in the shoulder. We see MDI occur in sports like gymnastics, swimming, and baseball, where the humerus is repeatedly abducted and extended. These sports call for extreme range of motion in the shoulder, which can stress the ligaments and cause the shoulder to dislocate.