The knee is a hinge joint, formed where the thigh bone and shinbone meet. When the knee is healthy, the joint moves freely. This is because the joint is covered with slippery tissue and powered by large muscles.
When one or more parts of the knee are damaged, joint movement suffers. Over time, cartilage starts to crack or wear away. Because cartilage cannot fully repair itself, the damage may keep increasing. At first, your knee may just be a little stiff. But as the bones of the joint begin rubbing together, you’re likely to feel pain.
Years of normal use can cause cartilage to crack and wear away (osteoarthritis). As exposed bones rub together, they become rough and pitted. The joint grinds. Being overweight or having an alignment problem, such as knocked or bowed knees, puts extra force on the joint. This may speed up the damage.
A chronic disease, such as rheumatoid arthritis or gout, can cause swelling and heat (inflammation) in the joint lining. As the disease progresses, cartilage may be worn away and the joint may stiffen.
A bad fall or blow to the knee can injure the joint. If the injury does not heal properly, extra force may be placed on the joint. Over time, this can cause the cartilage to wear away (traumatic arthritis).