avascular necrosis (AVN) staging

  Stage                CT/xray                    bone scan
  -----                -------                    ---------

   I                   -                          cold/hot spot

   II                  sclerotic focus with
                         osteopenic ring

   III                 "crescent sign"
                         (subchondral lucency)

   IV                  articular collapse
                       flattening of femoral head



sensitivity            86%                        78%
specificity            79%                        75%