What are the cardinal signs of Osteoarthritis of the knee on plain radiographs? How (mechanistically) do they appear?
The four cardinal signs of osteoarthritis seen on x-rays are the following:
- Asymmetric joint space narrowing
- Subchondral sclerosis
- Subchondral cysts
Figure 1: Examples of the four cardinal signs of osteoarthritis of the knee on x-ray, contrasted with normal: osteophyte (orange arrow), asymmetric joint space narrowing (because it is harder to see what is not there, this is best appreciated by mentally contrasting with the NORMAL joint space, shown by red arrow on the figure to the left), subchondral sclerosis (the white line at the surface green arrow), and subchondral cyst (blue star). (Images modified from Radiopedia)
Asymmetric joint space narrowing represents a loss of articular cartilage, which separates the bones. Recall that x-rays do not show cartilage directly; rather, the presence of cartilage is inferred: to the extent that two bones that are pushed toward each other do not touch, something interposed must be stopping it.
In that regard, if the patient is not standing when the x-ray is taken, the images may be falsely negative: the bones may not be close together, despite the loss of cartilage, merely because there is no force pushing them.
The narrowing is osteoarthritis asymmetric because the cartilage loss is also asymmetric. In osteoarthritis, the medial side is often more severely affected than the lateral (leading to the characteristic varus, or bow-legged, deformity).
Subchondral sclerosis is defined as hardening (sclerosis) of that which is under the cartilage (subchondral). It is manifest in Osteoarthritis as a white line under the surface of the tibial plateau. Here’s why: As cartilage is lost in osteoarthritis, load is not spread evenly. Some areas are excessively loaded. The increased pressure in areas that are excessively loaded stimulates bone formation. (The formation of bone in response to load is known as Wolff’s law). Because x-rays measure density, the newly formed bone appears as a white line on the images: more bone is present, so more of the x-ray beam is blocked and attenuated before striking the receptor, leaving it white.
Osteophytes are bone spurs. In arthritis, there are bony overgrowths that form off the side of the joint surface. It’s not clear exactly why the bone grows adjacent to the joint (as contrasted with subchondral sclerosis, where the growth is directly under the area that is loaded), but these are a reliable marker of arthritis nonetheless.
Subchondral cysts form when synovial fluid in a joint affected by osteoarthritis seeps through cracks in the cartilage and into the bone. If this fluid escapes into soft tissue behind the knee, a so-called baker’s cyst may develop (Figure 2). A baker’s cyst is at times discovered as an incidental finding if a patient has an ultrasound of the leg as part of work-up for a possible thrombosis. The figure highlights two facts: 1) there is fluid normally found in the joint, and 2) the fluid of a subchondral or baker’s cyst is normal fluid, just in an abnormal place. The fluid is able to enter this abnormal space because of breach in the cartilage. In that sense, the discovery of a baker’s cyst is a diagnostic clue to the presence of some arthritis - just as much as the discovery of a subchondral cyst.