There are many causes of knee pain, and arthritis is one of the leading causes. In the United States, more than 40 million people suffer from various types of arthritis, and in China, more than 100 million people are affected. Osteoarthritis is the most common type of arthritis that causes knee pain. The bone structure of the knee consists of three parts: the distal thigh bone (femur), the proximal lower leg bone (tibia) and the knee bone in front (patella). These bones are covered with cartilage, and when the cartilage is damaged and the bone surfaces are directly exposed and worn away from each other, it can lead to pain. This is where osteoarthritis causes pain. Often, this pain is exacerbated during activities that put increased weight on the knee, such as going up and down stairs and squatting activities, and in severe cases, the knee can feel pain at rest and even interfere with sleep.