Osteoarthritis is the gradual wearing away of joint cartilage. Cartilage acts like a cushion on the end of your bones. Every time you take a step, it absorbs some of the pressure and transfers the force of the step through to the bone. Once this cartilage is gone there is no layer to act as the shock absorber. The two bones rub against each other and this causes pain. Osteoarthritis is common in the large weight bearing joints such as the knee and the hip.