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What happens to the joint?

knee
The illustration shows a healthy knee joint to the left and an osteoarthritis-affected knee joint to the right. Illustration by: Andrea Dell'Isola

Osteoarthritis (OA) affects not only the cartilage but the entire joint, i.e., the synovium, the joint capsule, the synovial fluid, the menisci, the ligaments, the muscles, and the bones. The exact order in which the joint is affected is not clear and can most likely differ from case to case. However, the main sign of the disease is cartilage degeneration which impairs joint function.

A healthy joint consists of a relatively thin layer of articular cartilage coating the surfaces of the two or more bones that meet inside the joint. These cartilage surfaces are lubricated with the help of synovial fluid, which allows for them to glide smoothly against each other without friction to facilitate joint mobility and distribute joint load evenly. Joint cartilage is important as it acts as a “shock absorber” between the bones. Therefore, when the cartilage gradually starts to degenerate, the bones in the joint are less protected. This results in an increased mechanical load on surrounding bones and bone marrow. Bone marrow contains a lot of nerve endings that can trigger pain. The bones may start to deform and grow bone spurs (osteophytes).

The whole joint is affected

When the synovial membrane becomes inflamed, it starts to produce more synovial fluid which causes the joint to swell and hurt. Both the joint capsule and the ligaments can thicken and extend. As the disease progresses, the muscles around the joint often weaken, leading to decreased joint mobility. In later stages of the disease, misalignment of the joint may occur. For example, some people with knee OA have knees that collapse inwards (knock knees).

The most common changes in an OA-affected joint are cartilage degeneration, bone spurs, and an inflamed joint capsule. A damaged meniscus is also common in people with knee OA. However, most signs of OA are not visible on X-rays until the disease is in its later stages, which is the reason why OA primarily is a clinical diagnosis, meaning that the diagnosis is based on medical history, presence of risk factors, and symptoms.

Basing the diagnosis mainly on symptoms

It is not uncommon to want to get an X-ray if suffering from joint pain. However, it is important to keep in mind that what is seen on an X-ray or MRI often doesn't match patient symptoms. An X-ray or MRI can thereby mislead both doctor and patient. Radiographic findings that are not related to OA symptoms risk triggering unnecessary anxiety and treatment.