 Hi, I'm Dr. Sunil Kini working as a concert and orthopedic surgeon at Manipal Hospital's Bangalore. The cause of arthritis again depends on what the condition is. In terms in cases such as the most common arthritis which is osteoarthritis, the cause is mainly related to wear and tear of the cartilage, which is the natural protective layer over the bone. In inflammatory conditions such as rheumatoid arthritis or systemic lupus erythromatosis, the cause of pain is actually related to the inflammation of the synobium which lines the knee, so mainly related to the soft tissue structures. In other conditions such as gout, this is mainly related to deposition of crystals in the joint.