Stage each hip from the highest directly supported structural finding. Fracture is the stage-II/III boundary; lesion size, location, symptoms and treatment choice remain explicit separate inputs.
{
"categories": [
{
"outcome_code": "1",
"radiograph": "normal",
"MRI": "low_signal_band_on_T1_weighted_images",
"bone_scintigraphy": "positive_is_an_alternative_qualifier",
"subchondral_or_necrotic_zone_fracture": "absent",
"femoral_head_depression": "absent",
"osteoarthritis": "absent"
},
{
"outcome_code": "2",
"radiograph": "abnormal_sclerosis_focal_osteoporosis_or_cystic_change",
"subchondral_or_necrotic_zone_fracture": "absent",
"femoral_head_depression": "absent",
"osteoarthritis": "absent"
},
{
"outcome_code": "3",
"radiograph_or_CT": "subchondral_or_necrotic_zone_fracture",
"collapse": "may_be_present_or_absent",
"subdivisions": [
{
"code": "IIIA",
"femoral_head_depression": "less_than_or_equal_to_2_mm"
},
{
"code": "IIIB",
"femoral_head_depression": "greater_than_2_mm"
}
],
"osteoarthritis": "not_category_defining"
},
{
"outcome_code": "4",
"radiograph": "osteoarthritis_with_joint_space_narrowing_acetabular_change_and_or_joint_destruction",
"collapse": "usually_present"
}
],
"applicability": {
"intended_use": "Stage nontraumatic osteonecrosis of one femoral head using the 2019 revised ARCO system.",
"classification_unit": "one_hip_at_one_imaging_timepoint",
"bilateral_rule": "Stage the right and left hips independently and preserve laterality on every finding.",
"outside_scope": [
"traumatic_femoral_head_necrosis",
"other_skeletal_osteonecrosis_sites",
"lesion_size_or_location_classification",
"collapse_probability",
"symptom_grade",
"treatment_selection_by_stage_alone"
]
},
"imaging_gate": {
"MRI": "Preferred diagnostic test when radiographs are normal or suspicion remains; the canonical stage-I feature is a band-like low-signal lesion on T1-weighted images.",
"radiograph": "Required for the radiographic stage-II changes and stage-IV osteoarthritis descriptors but may miss an occult fracture.",
"CT": "CT or MRI is preferred to radiography for detecting a subchondral fracture. CT may show a fracture not evident on radiography or MRI and can move a hip from stage II to III.",
"scintigraphy": "A positive bone scan can support stage I in the 2019 definition, but current diagnostic guidance favors MRI.",
"adequacy_rule": "If fracture status is unresolved on available imaging, do not force stage II; return stage II versus III and identify CT or appropriate MRI review as the missing discriminator."
},
"staging_algorithm": [
{
"step": 1,
"if": "radiographic_osteoarthritis_with_joint_space_narrowing_acetabular_change_or_joint_destruction",
"output_code": "4"
},
{
"step": 2,
"if": "subchondral_or_necrotic_zone_fracture_on_radiograph_or_CT",
"output_code": "3",
"then": "measure_maximum_head_depression_to_assign_IIIA_or_IIIB"
},
{
"step": 3,
"if": "radiographic_sclerosis_focal_osteoporosis_or_cystic_change_without_fracture_flattening_or_osteoarthritis",
"output_code": "2"
},
{
"step": 4,
"if": "radiograph_normal_and_MRI_T1_low_signal_band_or_positive_bone_scan",
"output_code": "1"
},
{
"step": 5,
"if": "no_supported_osteonecrosis_or_key_imaging_missing",
"output": "not_classifiable_or_adjacent_stage_uncertainty"
}
],
"decisive_boundaries": {
"stage_1_2": "Any qualifying radiographic abnormality changes stage I to II; MRI lesion conspicuity or extent alone does not.",
"stage_2_3": "A subchondral or necrotic-zone fracture is stage III even before visible flattening. Do not require collapse to call stage III.",
"stage_3A_3B": "Measure maximal femoral-head depression against the expected contour: exactly 2 mm is IIIA; more than 2 mm is IIIB.",
"stage_3_4": "Collapse alone remains stage III. Stage IV requires secondary osteoarthritis such as joint-space loss, acetabular change or joint destruction.",
"worst_supported_feature": "Use the highest stage-defining feature in the same hip while retaining the modality and measurement that established it."
},
"revision_identity": {
"removed_stage_0": "The 2019 revision deleted historical stage 0. Do not emit 0 or map an all-imaging-negative hip to stage I.",
"stage_III_split": "The revision divides stage III at 2 mm of femoral-head depression into IIIA and IIIB.",
"separate_size_and_location": "The prior ARCO size/location subclassification was not incorporated into the revised stage. Lesion size, location and extent must be captured separately and must not change the I-IV code.",
"version_rule": "Return 2019 revised ARCO staging even though the primary article was published in 2020; the 2026 care guideline adds management context without changing the stage codes."
},
"required_separate_descriptors": {
"lesion_extent": "Report lesion size or percentage/volume only when measured with a named method; MRI can quantify extent and follow response.",
"lesion_location": "Report weight-bearing or lateral extension and any named JIC, modified Kerboul or other system separately; never translate it into an ARCO stage.",
"symptoms_and_function": "Record pain, functional limitation and symptom laterality independently.",
"patient_context": [
"age",
"etiology_and_risk_factors",
"duration",
"prior_treatment",
"contralateral_hip_status",
"comorbidity",
"patient_values"
]
},
"current_2026_care_context": {
"diagnosis": "MRI is recommended for diagnosis; CT or MRI may be used to identify a subchondral fracture, and MRI may quantify lesion area or extent.",
"asymptomatic_stage_I_or_II": "Intervention may be considered according to collapse risk, lesion size and extent, using low-morbidity options when appropriate; evidence is very low and the stage alone is insufficient.",
"symptomatic_stage_I_or_II": "Consider joint-preserving alternatives to total hip arthroplasty before fracture, with shared decision-making. Core decompression may be considered, and bone-marrow-cell augmentation may reduce collapse, but evidence does not establish one universally superior strategy.",
"selected_joint_preservation": "Vascularized fibular grafting or osteotomy may be considered for selected stage-I or II hips; recommendations are weak and patient selection matters.",
"stage_III": "Joint-preserving procedures have smaller and less predictable functional improvement than total hip arthroplasty after fracture/collapse, yet shared decision-making remains essential, especially for younger patients.",
"stage_IV": "Secondary osteoarthritis commonly leads to arthroplasty consideration, but candidacy still depends on symptoms, function, age, comorbidity and patient goals.",
"safety_rule": "Never generate decompression, grafting, osteotomy or arthroplasty as an automatic command from the ARCO stage. Integrate fracture, lesion size/location, symptoms, age, lifespan and patient preference."
},
"risk_and_interpretation": {
"ordinal_direction": "Later stage indicates structural progression from occult disease to radiographic change, fracture/collapse and secondary osteoarthritis.",
"no_per_stage_calibration": "The revised staging paper does not provide a calibrated individual collapse or arthroplasty probability for each code.",
"dominant_separate_predictors": "Lesion size and location are major determinants of collapse risk but are intentionally separate from the stage; a small stage-II lesion and a large stage-II lesion do not have identical risk.",
"serial_rule": "Store each hip's stage by examination date. A later higher stage must not overwrite the earlier baseline or imply the contralateral hip progressed."
},
"agent_output_contract": [
"right_and_left_hip_staged_separately",
"exact_ARCO_stage_and_IIIA_or_IIIB_substage_when_applicable",
"modality_and_finding_that_established_the_stage",
"fracture_status_and_CT_or_MRI_adequacy",
"maximum_head_depression_in_mm_for_stage_III",
"joint_space_acetabular_and_joint_destruction_findings",
"lesion_size_and_location_as_separate_named_descriptors",
"symptoms_age_etiology_prior_treatment_and_patient_goals",
"uncertainty_or_missing_discriminator_without_forced_stage",
"care_context_without_stage_only_treatment_command"
],
"supporting_sources": [
{
"role": "current_staging",
"citation": "Yoon et al. J Arthroplasty. 2020;35:933-940",
"doi": "10.1016/j.arth.2019.11.029",
"pmid": "31866252"
},
{
"role": "current_evidence_based_care_context",
"citation": "Cheng et al. JBJS Am. 2026;108:903-906",
"doi": "10.2106/JBJS.25.01616",
"pmid": "41961916"
}
],
"source_locator": "Yoon et al. 2019 revised ARCO staging paper, J Arthroplasty 2020, DOI 10.1016/j.arth.2019.11.029, abstract and staging table for stages I-IV, deletion of stage 0 and the 2 mm IIIA/IIIB split. ARCO/AAOS international evidence-based CPG 2026, DOI 10.2106/JBJS.25.01616, diagnostic imaging and stage-group treatment recommendations for the current care boundary."
}