Many people have experienced swollen and painful knees, especially when walking up and down stairs. What is wrong with the knee joint? What do doctors say about arthritis, synovitis, joint effusion, and osteophytes? In fact, these problems are very closely related. As the diagram shows, the knee joint is a very complex large joint in the human body, consisting mainly of three bones, the femur (above the knee), the tibia (below the knee) and the patella (directly in front of the knee), and the meniscus and ligaments in between them, surrounded by synovium, joint capsule and muscles. It carries almost all of the body’s weight, and standing, walking, running, and jumping in daily life cannot be done without the movement of the knee joint. The knee joint is like a precision bearing and has a certain lifespan because of the friction that occurs at the various bone joints during operation. In order to reduce friction, the synovial membrane that covers the joint secretes fluid to lubricate the bones; the surface of the patella is covered with a layer of articular cartilage, which has a smooth surface, while the surface of the head of the femur and the head of the tibia is also covered with a layer of smooth articular cartilage, which prevents the two bones from touching each other and reduces friction between them; in addition, there is a layer of “cushion” between the femur and the tibia, called the meniscus. In addition, there is a “cushion” between the femur and tibia, called the meniscus, which also acts as a cushion and reduces friction. Just as people age, the “parts” of the knee joint also age. As we age, the cartilage on the surface of the joint and the meniscus in the middle of the bone undergo countless hours of friction and become rough and uneven. In addition, during the process of wear and tear, small particles fall out, and these particles become free in the spaces of the joints, increasing the wear and tear of the joint surfaces and causing pain; the free particles also stimulate synovitis in the synovial membrane and secrete more joint fluid, which “floats” the patella and makes the whole joint look swollen (joint effusion). The cartilage surface of the joint is not innervated, so the friction between cartilage and cartilage does not produce pain. When the cartilage on the surface of the joint wears away, it exposes the deeper bones, which are innervated by nerves, and friction between the bones can cause pain. In addition, the friction between bone and bone will stimulate the excessive production of bone around the joint, which is called osteophyte, also known as bone spur or osteophyte. Because the bone growth is irregular, it can become more worn and painful during joint movement. This wear and tear, destruction and proliferation of knee cartilage is a natural aging phenomenon that manifests itself in painful swelling of the knee, especially when walking up and down stairs, and is collectively known as degenerative knee disease, or osteoarthritis of the knee. It is not just the cartilage that ages, but the entire joint, including the ligaments, cartilage, meniscus, and synovium.