Fig. 3

Dotted lines divide the femoral head into medial, central and lateral pillars. The gray dashed line indicates approximately the middle of the lateral pillar. Its dashed outline is the necrosed area. The zig-zag line represents the subchondral fracture size (fs). a Healthy femoral head. b Cavity with 25% of total area lost; its discontinuous outline is a necrosed area and there is a subchondral fissure (fs). c Loss of ~ 50% of total area, increased necrosed area, increased size of fs and loss of lateral abutment height of < 50%. d Loss of > 50% of total area, increased fs and loss of lateral abutment height of ~ 50%. d Total cavity with loss of nearly 100% of area, maximum subchondral damage and damage of > 50% of lateral abutment