- Tonnis grades hip osteoarthritis on plain radiographs from 0 (normal) to 3 (advanced) using four parameters: subchondral sclerosis, joint-space narrowing, subchondral cysts, and femoral head deformity.
- Grade 0 is normal; grade 1 is doubtful or very mild change. Only grades 2 and 3 satisfy most epidemiological definitions of radiographic hip OA.
- The grade guides the surgical decision. Grades 0 and 1 are the window for joint-preserving procedures (osteotomy, arthroscopy, biological interventions). Grade 3 usually means total hip replacement is the most reliable option.
- Assess on an AP pelvis film, not just a dedicated hip view. The pelvis film allows comparison with the contralateral side and is the standard for grading.
A Tonnis grade of 2 or greater defines radiographic hip osteoarthritis in most epidemiological and clinical studies. Grade 1 represents questionable or minimal change that may not meet OA criteria. Grade 3 represents end-stage disease where joint-preserving surgery has a limited role and total hip arthroplasty is the standard operative option. Examiners expect you to state the grade, explain each parameter, and use it to justify your operative plan.
The Tonnis Grading System
Each hip is graded 0, 1, 2, or 3 on a standardised AP pelvis radiograph by assessing four radiographic parameters. The highest applicable grade is assigned.
| Parameter | Grade 0 (Normal) | Grade 1 (Doubtful) | Grade 2 (Moderate OA) | Grade 3 (Severe OA) |
|---|---|---|---|---|
| Subchondral sclerosis | None | Possible slight sclerosis | Definite sclerosis of femoral head or acetabulum (or both) | Marked sclerosis, broad zone |
| Joint-space narrowing | Normal (2 mm or greater superiorly) | Possible slight narrowing | Definite narrowing, less than the normal minimum | Severe narrowing, near obliteration |
| Subchondral cysts | None | None | Possible small cysts | Definite large subchondral cysts |
| Femoral head deformity | Spherical, normal contour | Normal contour | Possible slight flattening or oval deformation | Definite flattening, mushroom deformity, or large osteophytes |
Sclerosis β’ Space β’ Cysts β’ ContourThe four parameters
0 None β 1 Maybe β 2 Definite OA β 3 End-stageThe grade thresholds
0β1 Preserve β 2 Personalise β 3 ReplaceWhat the grade decides
Superolateral narrowing is the single most sensitive early radiographic sign. The hip OA pattern is typically superolateral eccentric, and narrowing appears here first. Medial or concentric narrowing patterns raise differentials including inflammatory arthropathy, protrusio acetabuli, or chondrocalcinosis. Always state the pattern of narrowing when you grade.
Grade Interpretation and Clinical Decision-Making

| Grade | Radiographic Meaning | Non-operative Options | Joint-Preserving Surgery | Replacement |
|---|---|---|---|---|
| 0 | No OA features | Lifestyle modification, activity modification, analgesia as needed | Not applicable | Not indicated |
| 1 | Doubtful or minimal β does not meet radiographic OA criteria | Weight loss, physiotherapy, analgesia, activity modification | Consider for associated pathology (femoroacetabular impingement, dysplasia) if symptoms warrant intervention | Not indicated |
| 2 | Definite radiographic OA β moderate | Non-operative measures still first-line in older or low-demand patients | Periacetabular osteotomy (if dysplasia), hip arthroscopy (if impingement with limited OA), surgical dislocation with osteochondroplasty in carefully selected younger patients | Total hip arthroplasty if joint-preserving surgery is unsuitable or has failed; reasonable in patients over 55β60 with significant symptoms |
| 3 | Advanced radiographic OA β end-stage | Non-operative for patients unsuitable for surgery or who decline operation | Very limited role; only in exceptional circumstances (very young patient refusing replacement, severe comorbidity limiting anaesthesia) | Total hip arthroplasty is the standard β the most predictable and durable option at this grade |
The Tonnis grade describes the radiograph, not the patient. Symptomatic hip pain correlates poorly with radiographic grade, especially at grades 0 and 1 β patients with minimal radiographic change can have severe symptoms from labral pathology, femoroacetabular impingement, chondral damage, or early cartilage loss that is not yet visible on plain film. Always correlate the grade with the clinical picture, MRI findings, and the patient's functional demands before deciding on surgery.
In young patients (under 40) with Tonnis grade 2 and dysplasia or femoroacetabular impingement, a periacetabular osteotomy or hip arthroscopy with correction of the bony impingement may still provide years of good function before conversion to replacement. The key patient selection factors are: young age, high activity demands, preserved range of motion, correctable bony deformity, and a motivated patient who understands the possibility of future revision. Examiners expect you to articulate these selection criteria.
Limitations and Modern Context
- Plain radiograph is a surrogate for cartilage loss. The Tonnis grade measures bone reaction to cartilage depletion, not the cartilage itself. Early-grade OA may have significant chondral damage that is radiographically silent β MRI with dGEMRIC or T2 mapping detects this but is not routinely used for grading.
- Inter-observer agreement is moderate. Studies report kappa values around 0.5 to 0.7 for Tonnis grading, with the greatest disagreement at the grade 1 to grade 2 boundary. Joint-space narrowing is the most reproducible parameter; cysts and sclerosis show more variation.
- It does not capture clinical symptoms or functional status. The grade must always be read alongside the patient's pain pattern, range of motion, and functional limitation. A Tonnis grade 1 hip can be devastatingly symptomatic if the pain originates from a labral tear or femoroacetabular impingement.
- The lateral centre-edge angle of Wiberg is a companion measurement, especially in dysplasia. Tonnis grade and LCEA together provide a more complete picture: a grade 2 hip with a LCEA less than 20 degrees has a different surgical pathway (periacetabular osteotomy) than a grade 2 hip with a normal LCEA (consider arthroscopy or replacement).
- Alternative grading systems exist. The Kellgren and Lawrence (K&L) system is more widely used in knee OA and in large epidemiological studies of the hip, but Tonnis is the standard in adult hip reconstruction and joint-preserving surgery literature. The Croft grade is another alternative used in some European centres.
- Weight-bearing radiographs are essential. Grading must be performed on a standardised AP pelvis taken weight-bearing. A supine film underestimates joint-space narrowing and may down-grade the severity. The false-profile view (Lequesne) adds information about anterior acetabular coverage but does not substitute for the AP grading view.
Evidence Base
Defining osteoarthritis of the hip for epidemiologic studies
- Established standardised radiographic criteria for defining hip osteoarthritis in epidemiological research
- Demonstrated that Tonnis grade 2 or greater and Kellgren-Lawrence grade 2 or greater were broadly comparable for defining radiographic hip OA
- Mild (grade 1) change was common and did not reliably correlate with symptoms
The American College of Rheumatology criteria for the classification and reporting of osteoarthritis of the hip
- ACR classification criteria for hip OA combined clinical and radiographic findings
- Radiographic OA (equivalent to Tonnis grade 2 or Kellgren-Lawrence grade 2) plus hip pain or restricted internal rotation provided the highest specificity for clinical hip OA
- Clinical criteria alone had lower specificity without radiographic confirmation
The reliability of the Tonnis grading system in patients undergoing hip preservation
- Evaluated inter- and intra-observer reliability of Tonnis grading among hip preservation surgeons
- Tonnis grading showed moderate inter-observer reliability, with the greatest disagreement at the grade 1 to grade 2 boundary
- Intra-observer reliability was higher than inter-observer reliability
Radiographic evaluation of the hip has limited reliability
- Comprehensive evaluation of inter- and intra-observer reliability for common radiographic parameters of adult hip structural anatomy
- Tonnis grade showed moderate inter-observer agreement, with joint-space narrowing being the most reproducible individual parameter
- Overall radiographic evaluation of the hip had limited reliability β grade boundary decisions are the weakest point
Exam Viva
Practise clinical reasoning and management decisions out loud
βA 34-year-old woman presents with a two-year history of right groin pain exacerbated by running and prolonged sitting. Her AP pelvis radiograph shows definite superior joint-space narrowing of the right hip to approximately 1.5 mm, definite subchondral sclerosis, no cysts, and a normal femoral head contour. The lateral centre-edge angle is 18 degrees. What is the Tonnis grade and how would you manage her?β
βA 58-year-old man with a 10-year history of left hip pain presents with worsening symptoms despite physiotherapy and analgesia. AP pelvis radiograph shows marked superolateral joint-space narrowing with near obliteration, severe subchondral sclerosis of both the femoral head and acetabulum, multiple subchondral cysts, and a flattened femoral head with superior osteophyte formation. The right hip has normal joint space and no sclerosis. What is the grade, what type of OA pattern is this, and what are your surgical options?β
The four parameters assessed on AP pelvis radiograph
- Subchondral sclerosis: none (0), possible slight (1), definite (2), marked broad zone (3)
- Joint-space narrowing: normal (0), possible slight (1), definite below 2 mm (2), near obliteration (3)
- Subchondral cysts: none (0), none (1), possible small (2), definite large (3)
- Femoral head contour: normal (0), normal (1), possible slight flattening (2), definite flattening and osteophytes (3)
Clinical decision thresholds
- Grade 0β1: non-operative management; consider joint-preserving surgery for associated pathology (impingement, dysplasia)
- Grade 2: the decision zone β joint preservation possible in young, selected patients; replacement reasonable in older patients
- Grade 3: total hip arthroplasty is the standard β joint preserving surgery has no reliable role
- Always correlate the radiographic grade with symptoms, function, MRI findings, and patient demands
Key exam points
- Grade 2 or greater defines radiographic hip OA in epidemiological and ACR criteria
- Superolateral narrowing is the earliest and most sensitive sign β state the pattern
- Tonnis grade predicts joint-preserving surgery survival (Steppacher data: grade 2 good, grade 3 poor)
- Weight-bearing AP pelvis is mandatory β supine films underestimate narrowing and under-grade severity
- Companion the grade with the lateral centre-edge angle of Wiberg in dysplasia assessment