The ARCO Classification Of Osteonecrosis

By the ARCO-Committee on Terminology and Staging

Bone in an organ that consists of mineralized and non-mineralized tissue. Bone necrosis is a disease which causes death of bone and is called “OSTEONECROSIS”

Diagnostic criteria:

The goal has to be to make to diagnosis “true osteonecrosis” as early as possible in the course of the disease.

The diagnostic criteria that are available at this moment are:



-Functional Bone investigation




Staging is a method of following the development of the disease and a way to start to treat the patient. Staging has to include the onset of the disease, Stage “0”, and extend till the final end, the complete joint destruction.

Quantitation of MRI and X-ray is needed for study of the development of the disease and prospective study concerning the different forms of treatment.


Stage 0: All present diagnostic techniques are normal or non-diagnostic. Future diagnostic techniques will at some point enable us to make the diagnosis in this Stage “0”. Diagnostic techniques are beyond the plain X-rays, the use of scintigraphy and MRI.

Stage 1: Plain X-ray and CT-scan are normal.
At least one of the following techniques is positive: scintigraphy and MRI.
An open biopsy will confirm the diagnosis.

This stage has to be subdivided in 3 categories according to the LOCATION of the lesion and the extension of the lesion under weightbearing dome of the acetabulum.

These 3 categories are: MEDIAL, CENTRAL and LATERAL (see table 1). For further study and follow-up on the results of different forms of treatment and the development of the disease, QUANTITATION can be added. This quantitation is a calculation of the area of femoral head involvement: minimal     < 15%
moderate    15-30%
extensive    > 30%

Stage 2 : There is NO subchondral fracture known as the CRESCENT SIGN. Radiography shows areas of abnormalities: a mottled aspect, sclerosis, osteolysis and focal porosis.

The femoral head remains spherical on AP and lateral views on X-ray and CT-scan. Again scintigraphy and MRI are positive. Subclassification is important, as described in stage 1.

Stage 3: CRESCENT SIGN visible on the X-ray. The femoral head fails mechanically.

The axial X-ray shows a fine radiolucent subchondral fracture line, usually referred to as the “crescent sign”. Progressive flattening of the femoral dome will occur. The spherical configuration starts to deteriorate and finally the dome will collapse. Late radiographs show the articular surface of the femoral head to be flattened, but there is no evidence of joint line narrowing or acetabular involvement.

CT-scanning laminograms might be helpful if there is no evidence of collapse on the plain X-rays.

Subclassification and Quantitation is added, but Quantitation can also be done by the calculation of the amount of flattening of the Femoral Dome.

First it is determined whether the crescent sign appears more prominent in the AP or lateral view. After selection of the most prominent view the length of the crescent is expressed as a percentage of the entire articular surface:
Minimal = < 15% involvement or a depression of <2 mm.
Moderate = 15-30% involvement or a depression of 2-4 mm.
Extensive = > 30% involvement or a depression of more than 4 mm.

Stage 4: Progression to OSTEOARTHRITIS.

Radiographically the femoral articular surface is flattened and the joint-space starts narrowing.

This is progressively associated with changes on the acetabular side of the joint and with signs of a beginning osteoarthritis with areas of sclerosis, cyts and marginal osteophytes. Later on the radiographic examinations show advanced degenerative changes and finally a complete joint destruction is seen.

Subclassification and Quantitation is not needed anymore. (see table 1)

The Committee suggests strongly that the minimum requirements for reporting results on clinical and scientific work must included the following:
1 : Staging according to this “International Classification”, in which the classification is considered to be a minimum requirement.
2 : A clinical quantitation by means of any hipscore-rating, e.g. the Harris Hip Score.
3 : Radiolographical and MRI quantitation to be reported separately from the clinical quantitation.
4 : Therapeutic results on survival of the femoral head.

A more detailed or more specified subdivision of any stage and all other diagnostic techniques or examinations can be added freely but separately.

The above mentioned requirements are again a minimum to be able to really compare all the different reports on osteonecrosis.

Histology should be done at every possible opportunity. It remains a “golden standard” although it is an invasive method, but it can also be a curative method in the early stage of the disease. A descriptive histologic classification of the disease is not yet available but is urgently needed.

Avascular Necrosis of Bilateral Femoral Heads

Avascular Necrosis of Bilateral Femoral Heads (Coronal T1) showing bilateral avascular necrosis of differing age

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