The shoulder is a ball and socket joint where the ball is formed by the head of the upper arm bone or humerus and the socket is formed by a shallow cavity in the shoulder blade called the glenoid. The glenoid is surrounded by a raised ridge of fibrous cartilage called the labrum which provides some depth to the socket increasing the stability of the joint. Some people are born with a loose shoulder capsule making them prone to shoulder instability. The shoulder capsule may also loosen due to repetitive overhead activities including sports such as tennis and swimming. Severe injury or trauma to the shoulder may lead to dislocation and injury to the labrum.