Avascular necrosis and subchondral insufficiency fracture

Case One. This image shows the femoral head. Here is the articular cartilage, and you can see a defect between the articular cartilage and the underlying bone. Notice how the underlying bone itself looks unusual. It has this grey-white lesion that extends into the underlying cancellous bone. You can see that very spongy appearance, which is classic for avascular necrosis. Remember, the overlying cartilage remains alive because it's fed by the synovium. The bone underlying it is dead, and there's a sharp interface, typically wedge-shaped, between the dead bone and the adjacent viable bone. Occasionally, the cartilage will get ripped off and float on the surface.
Case Two. Here, you can see the articular cartilage on the top. Below that, there's cancellous bone, which appears somewhat osteoporotic. But there is also this grayish-bluish, cartilage-like material right under the articular surface, or sub-articular cartilage. Notice how this material is arranged in a somewhat linear pattern. This is not avascular necrosis; this is a subchondral insufficiency fracture, which is a linear fracture right under the articular cartilage. In this particular case, there is an overwhelming amount of fracture callus that has developed along the line of the fracture, indicating a subchondral insufficiency fracture."