Figure 1.

Alterations of subchondral bone during OA progression. (A) Located at the ends of bones, articular cartilage provides a low friction surface for weight bearing and joint movement, and is made up of four zones: superficial zone, middle zone, deep zone, and calcified zone. In the healthy joint, the thin layer of calcified zone/cartilage (yellow) present at the bottom of the articular cartilage is separated from the upper three zones by the histological marker termed tidemark. Subchondral bone is the bone tissue lying beneath the calcified cartilage, and includes both the subchondral cortical plate and subchondral trabecular bone, although there is no precise distinction for the differences between these two structures. (Inset) Depiction of the mineralized rod and plate structures in the trabecular bone. (B) In early OA, when the cartilage is still intact, the thickness of subchondral cortical plate is decreased due to elevated rate of bone remodeling. At the same time, bone loss also occurs in the subchondral trabecular bone. (Inset) A drastic loss of rod-like trabeculae and mild thickening of plate-like trabeculae is also detected in early OA. (C) In late OA, when degenerative changes are evident in articular cartilage, the thickness of the subchondral plate is increased, and the subchondral trabecular bone becomes sclerotic. Other periarticular bone changes, such as the formation of osteophytes (blue arrows) also occurs at this stage. The amount of calcified cartilage expands and penetrates into the upper hyaline articular cartilage, with the tidemark duplicated or disrupted. (Inset) The drastic loss of rod-like trabeculae and thickening of plate-like trabeculae continue in late OA. Inset figures depicting structure of trabecular plate and rod are reprinted with permission from Chen et al. (2018).