Diseases of the knee can be traumatic, inflammatory, tumors and many others. Common conditions that cause severe pain include the following: The knee joint carries the weight of the body on a daily basis and has to be moved around, so prolonged overuse can lead to gradual wear and tear of the cartilage. In more severe cases, the bone underneath the cartilage also wears away. The joint surface becomes uneven, sliding movement is impaired, and the resulting inflammation leads to pain, which is called osteoarthritis. In Asia, there are many people whose calves tend to bend inward, and the direction of weight gain causes the cartilage on the inner side to wear down and decrease, resulting in the gradual development of the O-shaped leg deformity, which is very characteristic. Rheumatoid arthritis Rheumatoid arthritis is a disease that causes inflammation of joints throughout the body, including the fingers, wrists, elbows, knees, ankles, and spine. Early symptoms include swelling and pain associated with arthritis. As the disease progresses, the cartilage of the joints and the bone underneath the cartilage are damaged, leading to dislocation and deformation of the joints. When rheumatoid inflammation of the knee progresses, the joint cannot be straightened, and the deformity and pain make it difficult to walk.