Habitual, strenuous activity can wear away the protective, cartilagenous lining which reduces friction in joints. Ball and socket joints present a special risk because they allow greater mobility thereby increasing the risk of injury to the soft tissues and bone.
The head of the humerus and glenoid cavity shown in this photograph were in complete contact for many years prior to this individual's death. The surfaces are smooth and shiny, indicating that the joint capsule and cartilage had worn away, allowing bone on bone contact in the cavity.