The surface layer of cartilage breaks down and wears away in osteoarthritis (sometimes called wear-and-tear arthritis). The breakdown of cartilage causes the bones under the cartilage to rub together (bone-on-bone).
Pain, swelling, and loss of motion result. The joint may lose its normal shape over time and become deformed. Bone spurs (osteophytes) can develop along the edges of the joint. Bits of bone or cartilage can break off and float inside the joint space, causing even more pain and damage.