Shoulder instability

The shoulder owes its wide range of motion to the structure of the joint. The shoulder joint functions as a ball-and-socket joint: the head of the upper arm fits into the socket of the shoulder. In a stable shoulder, the ball and socket align well.

With shoulder instability, the ball can partially or completely slip out of the socket. This may happen during certain movements or even spontaneously.

To keep the ball and socket together, the joint is surrounded by tissues such as the joint capsule, ligaments and other connective structures. These structures provide passive stability to the shoulder.

When shoulder instability is present, these tissues have usually been overstretched in the past. As a result, they are no longer able to keep the joint fully stable. This increases the likelihood of the shoulder partially or completely dislocating (shoulder luxation).