Clinical calculator
Tönnis Angle
Tönnis Angle (Acetabular Index of the Weight-Bearing Zone)
The Tönnis angle measures the slope of the acetabular weight-bearing zone on an anteroposterior pelvis radiograph, where 0 to 10 degrees is normal, above 10 degrees indicates a steep upsloping roof associated with dysplasia and instability, and a negative angle indicates overcoverage.
What the Tönnis Angle score means
Tönnis Angle scores run from -10 to 30, where a lower score is better. The bands below are conventional reporting categories, not diagnostic cut-offs.
- Overcoverage-30–-0.1
A negative Tönnis angle means the sourcil slopes downwards laterally, indicating acetabular overcoverage and an association with pincer-type femoroacetabular impingement.
- Normal0–10
A horizontal or slightly upward-sloping sourcil. This is the normal range and implies a stable weight-bearing zone.
- Acetabular dysplasia / instability10.1–60
A Tönnis angle above 10° indicates a steep, upsloping weight-bearing zone. This is a marker of structural instability and is one of the strongest predictors that a borderline hip will behave as dysplastic.
Published criteria and reliability
The published cut-offs that define this instrument, and the measured agreement between observers using it. Each is reported with the population it was derived in.
| Type | Value | Subscale | Population | Follow-up | Source |
|---|---|---|---|---|---|
| Criterion | 0–10° — normal | — | Adults, AP pelvis radiograph | — | Tönnis D 1987 |
| Criterion | > 10° — acetabular dysplasia or structural instability | — | Adults, AP pelvis radiograph | — | Tönnis D 1987 |
| Criterion | < 0° — acetabular overcoverage, associated with pincer impingement | — | Adults, AP pelvis radiograph | — | Tönnis D 1987 |
| Criterion | A Tönnis angle above 10° in a borderline LCEA hip predicts instability | — | Borderline dysplasia, LCEA 20–25° | — | Wyles CC 2017 |
Criterion is a published cut-off, definition or performance figure belonging to a classification or decision rule — it is not a change score and should not be read as one.
How the score is calculated
Stated in full so the calculation can be independently checked, and reproduced by hand if needed.
A horizontal reference line is drawn connecting the inferior aspects of the two acetabular teardrops. A second line is drawn from the medial point of the acetabular sourcil to its most lateral point. The Tönnis angle is the angle between these two lines. It is recorded as positive when the sourcil slopes upwards laterally, which indicates a steeper and less containing weight-bearing surface, and as negative when it slopes downwards laterally, which indicates overcoverage.
Instrument profile
- Developer
- Dietrich Tönnis
- Institution
- Dortmund, Germany
- First published
- 1987
- Items
- 1
- Response format
- Single angular measurement in degrees
- Score range
- -10 to 30
- Direction
- Lower score = better
- Completion time
- About 1 minutes
- Age range
- Adults and skeletally mature adolescents
- Regions
- Hip
- Conditions
- Hip dysplasia, Femoroacetabular impingement, Hip instability, Acetabular undercoverage
- Procedures
- Periacetabular osteotomy, Hip arthroscopy
- Observer agreement
- Interobserver ICC typically 0.80–0.93
- Validity
- Correlates with hip instability and with the risk of progression to osteoarthritis, and is one of the parameters used to decide whether a borderline hip should be treated as dysplastic.
- Reliability
- Reliability is slightly lower than for the lateral centre-edge angle, because identifying the medial point of the sourcil is more subjective than identifying its lateral edge. The measurement is sensitive to pelvic tilt and rotation.
Normative values
Reference values from published cohorts, for comparison against the score you calculated.
| Population | Age | Mean | SD |
|---|---|---|---|
| Asymptomatic adults | — | 4–6 | 4 |
| Symptomatic dysplasia | — | 15–20 | — |
When to use the Tönnis Angle
Well suited to
- Alongside the lateral centre-edge angle when assessing acetabular morphology, particularly in borderline dysplasia.
- Pre-operative planning for periacetabular osteotomy, where correcting the sourcil slope is an objective of surgery.
Limitations and poor fits
- On poorly centred or rotated radiographs.
- In skeletally immature patients, where the acetabular index measured to the lateral bony rim is the conventional parameter.
- In isolation — its value comes from being read together with the LCEA and the impingement signs.
Tönnis Angle compared with other instruments
Instruments measuring a related construct, with the practical difference between them.
| Instrument | Items | Range | What it adds |
|---|---|---|---|
| LCEA | 1 | 0–60 | The lateral centre-edge angle measures how far the acetabulum covers the femoral head laterally on an anteroposterior pelvis radiograph, where below 18 degrees indicates frank dysplasia, 18 to 25 degrees is borderline, 25 to 39 degrees is normal, and 40 degrees or more indicates overcoverage. |
| Alpha Angle | 1 | 30–90 | The alpha angle quantifies loss of sphericity at the femoral head-neck junction, the imaging marker of cam-type femoroacetabular impingement, with published thresholds for cam morphology ranging from 50 to 60 degrees depending on the imaging view used. |
| Crowe | 2 | 1–4 | The Crowe classification grades developmental dysplasia of the hip in adults by how far the femoral head has migrated proximally, expressed as a percentage of the femoral head height and estimated from the pelvic height, giving four types: under 50%, 50–75%, 75–100% and over 100% subluxation. |
Copyright and permitted use
Published openly because it is the question every clinician and researcher asks before using an instrument, and almost nobody answers it.
A published radiographic measurement method. Freely reproducible; no licence is required.
References
- Tönnis D. Congenital Dysplasia and Dislocation of the Hip in Children and Adults. Springer-Verlag. 1987.
- Wyles CC, Heidenreich MJ, Jeng J, Larson DR, Trousdale RT, Sierra RJ. The John Charnley Award: redefining the natural history of osteoarthritis in patients with hip dysplasia and impingement. Clinical Orthopaedics and Related Research. 2017.PMID 27830486
- Clohisy JC, Carlisle JC, Beaulé PE, et al.. A systematic approach to the plain radiographic evaluation of the young adult hip. Journal of Bone and Joint Surgery (American). 2008.PMID 18978043
Editorial status
This page has not yet been signed off by a named clinical reviewer. The scoring logic is stated in full above so it can be independently verified, and every threshold is traceable to its source. See our editorial methodology.