Osteoarthritis

Osteoarthritis normally affects older people: it is a disease where joints wear out. As the joint surface wears away, it sheds particles which stimulate the joint lining to produce fluid. This causes the joint to swell. When the joint "cartilage" wears away, the core of the bone becomes exposed. The exposed bone rubs against other exposed bone. 
Symptoms.
Joints will be stiff and painful, and may be swollen. The pain may be worse after exercise. It may get harder to move the joint.