Recurrent shoulder dislocation

Breaking down of the rim of the cup (labrum) of the shoulder joint, as described under cartilage (labrum) tear, is a key factor in recurrent shoulder dislocation or subluxation (slight dislocation).

In a healthy state, the labrum contains your arm bone (humerus) in its place while a degraded labrum makes your shoulder unstable. Shoulder joint instability may be augmented by rotator cuff injury. 

The front and back of the shoulder cup (where the biceps and triceps muscle attach respectively) are more susceptible to shear forces as it is not as deep as the upper and lower parts of the cup. 

Dislocation can occur when a strong force is applied to the joint such as a blow in sports, falling on an outstretched arm, pulling or pushing strongly or they are known to occur with trivial movements, even when sleeping.