Stage 0. Stage 0 indicates that a hip is suspected of having ON although radiographs and MR are normal or nondiagnostic, such as a painful hip in a patient with proven ON in the opposite hip. Stage 0 would include hips evaluated too soon after a precipitating event for changes to appear or hips with too small an area of involvement to be detected with current modalities. The diagnosis might be made some time after the original evaluation when the lesion appears on imaging studies. A definitive diagnosis in stage 0 will also most likely be possible in the future with further refinements in our diagnostic techniques. Thus, inclusion of stage 0 will extend the useful life of this classification.

Stage I. Plain radiographs are normal, but MRI and/or technetium scans indicate the presence of ON.

Stage II. Radiographs now show definitive abnormalities consistent with ON. These consist of radiolucent and/or sclerotic regions, at times demarcated from normal bone by a sclerotic border. Rarely, the earliest radiographic findings will include generalized osteopenia of the femoral head.

Stage III. Stage III is diagnosed by the appearance of a radiolucent crescent line just beneath the subchondral end plate, but without flattening of the femoral head. This indicates collapse of the cancellous trabeculae beneath an intact articular surface. A crescent sign does not always appear as the head progresses from earlier to later stages and may be seen in only one radiographic projection. A relatively small percentage of hips will fit the criteria for stage III. However, we feel that they should be assigned a separate stage rather than being grouped together with hips which show gross flattening of the articular surface because clinical experience documents a better outcome for hips in stage III than for hips in stage IV, where flattening is already present [1, 15–18]. The hip is still spherical at this stage and it is theoretically possible to preserve its normal anatomy by bone grafting or other procedures.

Stage IV. There is now definite flattening of the femoral head which appears in either the AP, lateral, or both views. Occasionally small areas of collapse cannot be seen on plain radiographs but can be detected with CT. The acetabulum appears radiographically normal at this stage. However, early changes in the acetabular cartilage have been identified in the majority of stage IV hip patients whose pain and disability were sufficient for them to require arthroplasty. Stage IV has serious implications, because irreversible changes have taken place and a “normal” femoral head cannot be anticipated despite appropriate treatment.

Stage V. In addition to flattening of the femoral head, there is now clear radiographic evidence of joint line narrowing and later sclerosis and cystic changes in the acetabulum. Progressive degenerative changes follow.

Stage VI. Degenerative changes in both the femoral head and acetabulum have progressed to the point where the joint line is virtually obliterated and marked deformity appears. Other staging systems have grouped stages V and VI together; however, a small but definite number of hips develop changes so advanced that they can be distinguished from stage V. We thus feel that they deserve a separate designation and this allows us to follow progression more accurately.