Arthritis is a severe disease of the joint that occurs when you have cartilage loss in your joint. The cartilage in your joint is a smooth surface that allows for easy movement of your joints. Unfortunately, when you lose cartilage, the joint begins to grind together and cause severe pain. A “joint” is the connection between two bones. The most common areas where arthritis occurs are your knees (between the femur and tibia) and hips (between the pelvis and the femur).