In Ficat and Arlet classification, there are 5 stages of the femoral head’s avascular necrosis, including stage 0 when the disease process is silent with no symptoms and no changes visible on plain radiographs. However, an MRI can find AVN changes even in stage 0. In stage 1, the patient may experience symptoms with AVN changes only visible on an MRI.
With an absent blood supply, a segment of bone is necrosed. The surrounding bone with blood supply tries to revascularize the dead bone. Vascular tissue lays down new bone in the area which increases the density (visible as increased whiteness in an X-ray in stage 2a.)
A small fracture or break develops between the new bone and the dead bone. The space in between the break is filled by scar tissue. Overlying cartilage without nourishment softens and decays. The weight-bearing on the joint leads to small dents or breaks in the cartilage, leading to collapse (visible as a crescent sign-in stage 2b).
The smooth gliding is now lost, and the ball grinds with the socket in the area (segmental arthritic changes in stage 3). The process becomes widespread, leading to severe arthritis and the destruction of the entire joint surface (stage 4).