The most common cause of knee pain is knee arthritis. The pain may come and go, get worse over time, and be accompanied by other symptoms, such as knee stiffness.
Knee osteoarthritis is defined by degeneration of the knee’s articular cartilage—the flexible, slippery material that normally protects bones from joint friction and impact. The condition also involves changes to the bone underneath the cartilage and can affect nearby soft tissues.
Other types of knee arthritis
Knee osteoarthritis is by far the most common type of arthritis to cause knee pain, and often referred to as simply knee arthritis. Many other less common types of arthritis can also cause knee pain, including rheumatoid arthritis, pseudogout, and reactive arthritis.
The Damage and Loss of Knee Cartilage
The defining feature of osteoarthritis is the breakdown and loss of articular cartilage. In the knee, articular cartilage covers the top of the tibia (shinbone), the bottom of the femur (thigh bone) and the back of the patella (kneecap). Articular cartilage protects the surfaces of these bones where they meet at the joint. During the development of osteoarthritis:
- Cartilage weakens. The composition of the articular cartilage changes, becoming weaker. This change in composition is especially common in older age.
- Cartilage becomes damaged. The weakened articular cartilage thins or wears away.
- Cartilage may attempt to heal. The damaged cartilage may attempt to produce new cells, but it is not enough to replace the missing cartilage. In addition, these new cells may grow in irregular, bumpy patterns rather than the smooth pattern of the original cartilage.
- Meniscus damage is likely. Damage to the knee’s meniscus is likely to also occur.1 Because meniscal cartilage is different from articular cartilage, meniscus damage is considered a separate but related condition to knee osteoarthritis.
Cartilage does not contain nerves, so damaged cartilage does not necessarily cause pain. Instead, the damaged or missing knee cartilage causes other problems, such as friction between bones and changes to bone tissue, which can cause pain.
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Changes to the Knee Joint’s Bones
When cartilage is damaged, bones experience more friction and impact. This can cause the bone to undergo changes. For example:
- Bone spurs. Abnormal bony growths, called osteophytes or bone spurs, develop on the bone at the knee joint. It is believed that bones produce bone spurs to compensate for deteriorated or missing cartilage by redistributing weight loads. The bone spurs can create more friction in the knee joint, leading to discomfort and pain. It is important to note that bone spurs are a normal sign of aging, and the presence of them alone is not a cause for concern.
- Subchondral bone sclerosis. The surfaces of the tibia and femur that lie just beneath knee cartilage can change in composition and harden. (“Chondral” means cartilage; subchondral bone is located just beneath cartilage. “Sclerosis” means hardening.)
- Cysts and bone marrow lesions. The bone underneath damaged or missing knee cartilage may develop cysts (typically referred to as “subchondral cysts”) and areas of abnormal swelling called bone marrow lesions. These lesions may be associated with knee pain.2-3
Subchondral bone sclerosis and bone marrow lesions can occur at any time but are particularly common in the later stages of knee arthritis.
Changes to the Knee Joint’s Fluid
The knee joint contains fluid, called synovial fluid. Typically, this fluid promotes healthy cartilage and lubricates the joint. In an arthritic knee, both the composition and the amount of joint fluid in the knee joint can change.4
Changes to the Knee Joint’s Soft Tissues
As the body attempts to compensate for arthritic changes in the knee joint, the surrounding soft tissues undergo stress. For example, the nearby tendons and ligaments may become strained and/or the meniscus may become injured, leading to further loss of stability and function in the knee.
See Knee Anatomy