Osteoarthritis used to be called wear-and-tear arthritis because it was thought that age-related wear caused the tissues to break down. However, researchers are learning that the causes are more complex.
A few different factors cause osteoarthritis. Stress and joint mechanics, sometimes combined with injuries or other conditions, cause inflammation that leads to gradual destruction of the cartilage, tendons, ligaments and synovium (a membrane that lines your joints).
As these tissues wear down, thickening occurs at the bone ends, bony growths known as bone spurs can develop, and the once-smooth surface that allowed your joint to move easily becomes rough. Bone and cartilage fragments may float within the joint space, and fluid-filled cysts can form in the bone. Over time, you have a harder time moving your joints.