What is arthritis and why does my knee hurt?

In the knee joint, there is a layer of smooth cartilage on the lower end of the femur( thigh bone), the upper end of the tibia (shin bone) and the undersurface of the kneecap (patella). This cartilage serves as a cushion and allows for smooth motion of the knee. Arthritis is wearing away of this smooth cartilage. Eventually it wears down to the bone. Rubbing of bone against bone causes pain, swelling and stiffness.