To better understand the explanation of the lesions, it is important to understand the mechanisms of healthy joint function. If this is not your case, you can look under the definitions in the osteoarthritis section and learn about the different anatomical elements of a healthy joint.
Osteoarthritis is mainly a disease of the cartilage. This is indeed the first structure of the joint injured in osteoarthritis. But other components will also suffer from the consequences of it.
The mechanisms of lesion formation are similar for each affected joint.
Osteoarthritis occurs when there is more or less severe destruction of cartilage following various mechanical and biological phenomena. It becomes predominant and lesions appear in the cartilage, in the form of fissures or patches of cartilage that become detached. We talk about chondropathy (chondro = cartilage and pathy = disease).
The cartilage will try to heal in the form of fibrocartilage, but it is of lower quality. Thus, sooner or later it will be unable to compensate for the imbalance.
The thickness of the cartilage decreases, causing a narrowing of the joint space (the two bone ends move closer together) and fissures appear in the cartilage tissue. They can extend to the subchondral bone. The latter, in response to the narrowing, will condense. Following the same principle, bone proliferation can occur from the cartilage towards the exterior of the joint, causing the appearance of osteophytes.
Lastly, the synovial membrane also plays a significant role in osteoarthritis. The breakdown of cartilage releases substances into the joint that irritate the synovial membrane. This will then produce the joint fluid in greater quantities leading to the classic synovial effusion.
This liquid is of lower quality than normal, with a lower concentration of hyaluronic acid. It affords less protection to the cartilage and thus contributes to its damage.