Clinical calculator
Alpha Angle
Alpha Angle for Cam Morphology
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.
What the Alpha Angle score means
Alpha Angle scores run from 30 to 90, where a lower score is better. The bands below are conventional reporting categories, not diagnostic cut-offs.
- Normal head-neck offset0–49.9
Below roughly 50° the femoral head-neck junction is spherical enough that cam impingement is unlikely.
- Borderline cam morphology50–59.9
The 50–60° range is where published thresholds disagree. Whether this is pathological depends on symptoms, the view used, and whether there are corroborating findings such as a labral tear or an anterior offset deficiency.
- Cam morphology60–180
Above 60° cam morphology is present on essentially every published definition, and the association with labral and chondral damage is consistent.
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 | > 50.5° — original threshold for cam morphology | — | Nötzli derivation cohort, radial MRI | — | Nötzli HP 2002 |
| Criterion | ≥ 60° — threshold with the most consistent association with hip pathology | — | Systematic review across imaging modalities | — | van Klij P 2020 |
| Criterion | 55° is the most commonly applied clinical cut-off | — | Clinical practice, Dunn and cross-table lateral views | — | Griffin DR 2016 |
| Criterion | Cam morphology is present in roughly 15–25% of asymptomatic adults | — | Asymptomatic general population | — | Frank JM 2015 |
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 circle is fitted to the femoral head. A line is drawn along the femoral neck axis through the centre of that circle. A second line is drawn from the centre of the head to the point at which the anterosuperior head-neck contour first departs from the fitted circle. The alpha angle is the angle between these two lines. A larger angle means the head-neck junction departs from sphericity earlier, which is the morphological basis of cam impingement.
Instrument profile
- Developer
- Hubert Nötzli and colleagues
- Institution
- University of Bern, Switzerland
- First published
- 2002
- Items
- 1
- Response format
- Single angular measurement in degrees
- Score range
- 30 to 90
- Direction
- Lower score = better
- Completion time
- About 1 minutes
- Age range
- Adolescents and adults
- Regions
- Hip
- Conditions
- Femoroacetabular impingement, Cam morphology, Labral tear, Hip pain, FAI syndrome
- Procedures
- Hip arthroscopy, Femoral osteochondroplasty
- Observer agreement
- Interobserver ICC typically 0.75–0.94, best on Dunn views and radial MRI
- Validity
- Correlates with labral and chondral damage at arthroscopy and with the development of hip osteoarthritis in longitudinal cohorts.
- Reliability
- The dominant limitation is view dependence: the same hip can differ by 10° or more between an AP pelvis and a 45° Dunn view, because a cam lesion that sits anterosuperiorly is under-represented on some projections. Published thresholds differ partly for this reason, so the view must always be recorded.
Normative values
Reference values from published cohorts, for comparison against the score you calculated.
| Population | Age | Mean | SD |
|---|---|---|---|
| Asymptomatic adults | — | 42–47 | 6 |
| Asymptomatic male athletes in impact sports | — | 55–65 | — |
| Symptomatic cam-type FAI referred for arthroscopy | — | 62–72 | — |
When to use the Alpha Angle
Well suited to
- Assessing for cam morphology in a young adult with hip or groin pain and a positive impingement test.
- Pre-operative planning for femoral osteochondroplasty, and post-operative assessment of correction.
Limitations and poor fits
- As a diagnosis of FAI syndrome on its own — the Warwick Agreement requires symptoms and clinical signs alongside the imaging finding.
- Without recording the imaging view, since thresholds are view-specific.
- In skeletally immature patients, in whom the cam lesion typically develops during the growth spurt and the morphology is still changing.
Alpha 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. |
| Tönnis Angle | 1 | -10–30 | 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. |
| HOOS | 40 | 0–100 | The HOOS is a 40-item patient-reported questionnaire that scores hip function as five separate subscales — Symptoms, Pain, Activities of Daily Living, Sport and Recreation, and Hip-related Quality of Life — each from 0 (extreme problems) to 100 (no problems). |
| HOS | 26 | 0–100 | The Hip Outcome Score is a 28-item patient-reported questionnaire for non-arthritic hip conditions that yields two independent percentage scores — an Activities of Daily Living subscale, administered as 19 questions of which 17 are scored, and a 9-item Sports subscale — each running from 0% (unable to perform the listed activities) to 100% (no difficulty at all). |
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 and MRI measurement method. Freely reproducible; no licence is required.
References
- Nötzli HP, Wyss TF, Stoecklin CH, Schmid MR, Treiber K, Hodler J. The contour of the femoral head-neck junction as a predictor for the risk of anterior impingement. Journal of Bone and Joint Surgery (British). 2002.PMID 12211678
- Griffin DR, Dickenson EJ, O’Donnell J, et al.. The Warwick Agreement on femoroacetabular impingement syndrome (FAI syndrome): an international consensus statement. British Journal of Sports Medicine. 2016.PMID 27629403
- Frank JM, Harris JD, Erickson BJ, et al.. Prevalence of femoroacetabular impingement imaging findings in asymptomatic volunteers: a systematic review. Arthroscopy. 2015.PMID 25636988
- van Klij P, Reiman MP, Waarsing JH, et al.. Classifying cam morphology by the alpha angle: a systematic review on threshold values. Orthopaedic Journal of Sports Medicine. 2020.PMID 32426403
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.