A subchondral cyst forms during the erosive phase of osteoarthritis and occurs when the cartilage slowly lost as part of the disease’s progression attempts to repair itself, according to eMedicineHealth. As the cartilage is lost, the bone remodels, and bone underneath hardens, forming subchondral cysts.
The erosive phase of osteoarthritis follows a stationary phase during which the joint spaces narrow and osteophytes form, reports eMedicineHealth. It progresses to the obliteration of the joint spaces before the appearance of subchonddral cysts. As of 2015, more than 25 million Americans suffer from osteoarthritis, a painful condition marked by functional or structural failure of the joints.