What are the cardinal signs of Osteoarthritis of the knee on plain radiographs? How (mechanistically) do they appear?
Cardinal signs of osteoarthritis of the knee on plain radiographs are:
asymmetric joint space narrowing (short arrow)
subchondral sclerosis (long arrow), and
subchondral cysts (hard to see here).
Asymmetric joint space narrowing occurs as articular cartilage is lost is areas of abnormal load. Notice above that the lateral space is wide (though not necessarily disease-free; it may be wide because the joint is tilted to the medial side. That is, if the bones are touching, there is no cartilage; if they are not touching, maybe there is cartilage, maybe there isn't.)
Osteophytes typically develop. Why? Maybe as a (futile/foolish) reparative response. Maybe because of abnormal loads stimulating bone. "Peaking of the tibial spines", ie higher than expected, may be considered a form of osteophytosis.
Subchondral sclerosis is simply the deposition of bone in area under areas of stress—Wolff's law. This makes the bone stiff and less compliant, and more prone to further damage.
Cysts form when joint fluid seeps through the cracks in the cartilage and get into bone. When the fluid escapes into the soft tissue, a BAKERS CYST may develop.