High-yield overview
Grades 0–3radiographic hip osteoarthritis severity
4 parameterssclerosis, joint-space narrowing, cysts, head deformity
Grade 2+radiographic OA threshold — joint-preserving surgery still an option
Grade 3advanced OA — drives the conversation toward total hip replacement
Critical Must-Knows
- 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.
The threshold examiners want
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 |
How to grade
Grade each hip independently on an AP pelvis radiograph centred over the symphysis pubis. Compare with the contralateral side. Use the strictest parameter that applies: if sclerosis is grade 3 but narrowing is grade 2, the hip is grade 3.
Normal joint space
The superior joint space of a normal adult hip measures at least 2 mm on AP radiograph. Measurement is taken at the narrowest point of the superolateral weight-bearing zone. A threshold less than 2 mm constitutes definite narrowing (Tonnis grade 2 or higher).
Mnemonic
Sclerosis • Space • Cysts • ContourThe four parameters
Mnemonic
0 None — 1 Maybe — 2 Definite OA — 3 End-stageThe grade thresholds
Mnemonic
0–1 Preserve — 2 Personalise — 3 ReplaceWhat the grade decides
Clinical Pearl
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 — 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 — 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 55–60 with significant symptoms |
| 3 | Advanced radiographic OA — 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 — the most predictable and durable option at this grade |
Caution
The Tonnis grade describes the radiograph, not the patient. Symptomatic hip pain correlates poorly with radiographic grade, especially at grades 0 and 1 — 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.
Clinical Pearl
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 — 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
Evidence
Defining osteoarthritis of the hip for epidemiologic studies
Croft P, Cooper C, Wickham C, Coggon D • Am J Epidemiol (1990)
Key Findings:
- 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
Clinical implication: Supported the use of Tonnis grade 2 or higher as the epidemiological threshold for radiographic hip OA.
Limitation: Single-region UK population study; inter-observer data were limited.
Source: Am J Epidemiol 1990;132(3):514-22
Evidence
The American College of Rheumatology criteria for the classification and reporting of osteoarthritis of the hip
Altman R, Alarcón G, Appelrouth D, et al. • Arthritis Rheum (1991)
Key Findings:
- 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
Clinical implication: Radiographic grading (Tonnis 2 or equivalent) is a required component of formal hip OA classification — clinical findings alone are insufficient.
Limitation: Criteria derived from a mixed rheumatology and orthopaedic referral population.
Source: Arthritis Rheum 1991;34(5):505-14
Evidence
The reliability of the Tonnis grading system in patients undergoing hip preservation
Pullen WM, Carreira DS, Wong I, et al. • Am J Sports Med (2023)
Key Findings:
- 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
Clinical implication: Tonnis grading is reliable enough for clinical use and research but should be interpreted with awareness of the grade 1/2 boundary uncertainty.
Limitation: Surgeon-grade observers in a hip preservation context; may not generalise to all settings.
Source: Am J Sports Med 2023;51(2):476-480
Evidence
Radiographic evaluation of the hip has limited reliability
Clohisy JC, Carlisle JC, Trousdale R, et al. • Clin Orthop Relat Res (2009)
Key Findings:
- 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 — grade boundary decisions are the weakest point
Clinical implication: Tonnis grading is reliable enough for clinical decision-making but surgeons should recognise the limitations, particularly at grade boundaries where treatment decisions change.
Limitation: Academic hip surgeons at high-volume centres; reliability may be lower in general practice.
Source: Clin Orthop Relat Res 2009;467(3):666-675
Exam Viva
Practise clinical reasoning and management decisions out loud
Viva scenarioStandard
Clinical prompt
“A 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?”
Practical approach
The Tonnis grade is 2. She has definite sclerosis and definite joint-space narrowing below the 2 mm normal minimum, without cysts or head deformity. The LCEA of 18 degrees confirms acetabular dysplasia. At 34 years old, the priority is joint preservation. I would discuss a periacetabular osteotomy to improve acetabular coverage and redirect the weight-bearing surface, which can delay or avoid the need for total hip replacement. I would also assess for femoroacetabular impingement with an MRI arthrogram to evaluate the labrum and articular cartilage, as combined dysplasia and impingement pathology may need both osteotomy and femoral-sided osteochondroplasty. I would counsel her that periacetabular osteotomy has good survivorship at 10 years in Tonnis grade 2 dysplasia, but that conversion to replacement remains possible long-term.
Key clinical points
State the grade explicitly — Tonnis grade 2 (definite OA, moderate)
Identify dysplasia by the lateral centre-edge angle of Wiberg (less than 20 degrees)
Joint preservation is the priority in a 34-year-old — periacetabular osteotomy is the operation of choice for dysplasia-associated OA at this grade
MRI arthrogram to assess labral and chondral status before planning surgery
Common pitfalls
Calling it Tonnis grade 1 because there are no cysts or head deformity — grade 1 does not have definite narrowing
Offering total hip replacement as first-line in a 34-year-old with correctable dysplasia
Missing the dysplasia by not checking the lateral centre-edge angle
Further questions
“What are the contraindications to periacetabular osteotomy?”
“At what Tonnis grade would you stop recommending joint-preserving surgery?”
“How does the Tonnis grade compare with the Kellgren-Lawrence grade?”
Viva scenarioAdvanced
Clinical prompt
“A 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?”
Practical approach
This is a Tonnis grade 3 left hip with a typical primary superolateral eccentric osteoarthritis pattern. The four parameters are all grade 3: marked sclerosis, severe near-obliteration of joint space, definite subchondral cysts, and definite femoral head flattening with osteophytes. The right hip is Tonnis grade 0. At 58 with grade 3 disease and failed non-operative management, total hip arthroplasty is the standard and most reliable option. Joint-preserving surgery has no role at this grade of degeneration. The key surgical decisions are: cemented versus uncemented fixation (I would favour uncemented for bone ingrowth given his age and likely good bone quality), bearing surface (ceramic-on-polyethylene or ceramic-on-ceramic depending on his activity level and institutional preference), and approach (posterior versus direct anterior, each with specific complication profiles). I would discuss the risks of dislocation, leg-length discrepancy, infection, and periprosthetic fracture, and explain the expected recovery timeline and restrictions.
Key clinical points
State the grade and tick off all four parameters — all are grade 3
Identify the OA pattern as superolateral eccentric (the most common primary OA pattern)
Total hip arthroplasty is the only reasonable operative option at Tonnis grade 3
Discuss fixation choice, bearing surface, and approach — examiners want to hear surgical reasoning
Common pitfalls
Suggesting any form of joint-preserving surgery at grade 3 (no evidence to support it)
Failing to describe the OA pattern (superolateral versus medial versus concentric)
Not mentioning the contralateral hip — it is Tonnis 0 now but needs long-term monitoring
Further questions
“What would change your management if the contralateral hip were also Tonnis grade 3?”
“How would you counsel a 38-year-old patient with the same radiographic findings?”
“What bearing surface would you choose and why?”
Exam day cheat sheet
Tonnis Grade — Exam Cheat Sheet
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 0–1: non-operative management; consider joint-preserving surgery for associated pathology (impingement, dysplasia)
- Grade 2: the decision zone — joint preservation possible in young, selected patients; replacement reasonable in older patients
- Grade 3: total hip arthroplasty is the standard — 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 — state the pattern
- Tonnis grade predicts joint-preserving surgery survival (Steppacher data: grade 2 good, grade 3 poor)
- Weight-bearing AP pelvis is mandatory — supine films underestimate narrowing and under-grade severity
- Companion the grade with the lateral centre-edge angle of Wiberg in dysplasia assessment