Figure 2.

Illustration of structural and microenvironment transformations during OA progression. Compared with normal joints, OA joints will undergo different degrees of changes with the OA progression. In early-stage OA, the surface contour of cartilage is lightly unregulated, the superficial zone of articular cartilage splits slightly, the subchondral bone plate is thinner yet more porous, and a few vessels erode into the avascular cartilage. In late-stage OA, the whole cartilage structure is completely destroyed, which is usually accompanied with subchondral bone sclerosis, joint space narrowing, osteophyte formation, and more vessels erosion into the cartilage.