Animals supported by water, such as dolphins and whales, can develop osteoarthritis, but the two animals that hang upside down, bats and sloths, don't develop it. Because the disorder is so widespread in the animal kingdom, some authorities think that osteoarthritis may have evolved from an ancient method of cartilage repair. Although osteoarthritis is more common in older people, it's not caused by the simple wear and tear that occurs with aging. Normally, joints have such a low friction level that they don't wear out unless they are used excessively or are injured. Next, the cartilage may grow too much, but it eventually thins and develops cracks on the surface.
Tiny cavities form in the marrow of the bone beneath the cartilage, weakening the bone.
These bumps may interfere with normal joint function, causing pain. Ultimately, the smooth, slippery surface of the cartilage becomes rough and pitted, so that the joint can no longer move smoothly. Some people who repetitively stress their joints, such as foundry workers, coal miners, and bus drivers, are particularly at risk. However, trained long-distance runners aren't at higher risk of developing the disorder. Obesity may be a major factor in the development of osteoarthritis, but the evidence is inconclusive.