The exact cause isn't known. A person may be at increased risk of osteoarthritis because it runs in the family. Osteoarthritis seems to be related to the wear and tear put on joints over the years in most people. But wear and tear alone don't cause osteoarthritis. Normally, a smooth layer of cartilage acts as a pad between the bones of a joint.
Cartilage helps the joint move easily and comfortably.
In some people, the cartilage thins as the joints are used. This is the start of osteoarthritis. Over time, the cartilage wears away and the bones may rub against one another. Bones may even start to grow too thick on the ends where they meet to make a joint, and bits of cartilage and bone may loosen and get in the way of movement. This can cause pain, joint swelling and stiffness.