The knee joint joins the thigh to the lower leg and is formed by the distal end of the femur, the proximal end of the tibia, the patella or kneecap and the soft tissues that surround them. Between the femur and the patella is the patellofemoral joint, and between the femur and the tibia, the femorotibial joint.
In healthy knees, the bone ends are covered in cartilage, a cushioning and elastic tissue that prevents direct friction between the bones. This allows free and painless movement at the joint. The muscles and ligaments provide the knee joint with mobility and stability, respectively, and the synovial tissues produce a fluid that acts as a lubricant for it.
The knee is one of the joints of the human skeleton in which osteoarthritis develops most commonly. The reason osteoarthritis is more common in the knee, compared to other joints in the body, is because it is a «load-bearing» joint: it has to support the weight of the body as well as the weight of objects we carry when we stand, walk, run, or go up and down stairs.