Hip
Femoroacetabular impingement
Femoroacetabular impingement syndrome requires symptoms, clinical signs and imaging findings together — an abnormal alpha angle alone is a morphological finding present in a large share of asymptomatic people, not a diagnosis.
What you are actually deciding
The 2016 Warwick Agreement defines FAI syndrome as a triad. Measure the alpha angle for cam morphology and the centre-edge and Tönnis angles to exclude coexisting dysplasia, then confirm with a hip-specific PROM that the patient is actually symptomatic.
Patient-reported outcome measure2
- HOOS Calculator40 itemsHip disability and Osteoarthritis Outcome ScoreThe 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 Calculator26 itemsHip Outcome ScoreThe 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).
Clinical calculator3
- Alpha Angle1 itemsAlpha Angle for Cam MorphologyThe 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.
- LCEA Calculator1 itemsLateral Centre-Edge Angle of WibergThe 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 Angle1 itemsTönnis 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.
Thresholds that apply here
Published change thresholds across the instruments above. Check the population matches your patient before using a value — a threshold derived in a different cohort will mislead you.
| Instrument | Type | Value | Subscale | Population |
|---|---|---|---|---|
| HOOS | MCID | 10 points | All subscales | Total hip arthroplasty |
| HOOS | MCID | ~9 points | Pain | Hip osteoarthritis, non-surgical |
| HOOS | PASS | ~73 points | Pain | Hip arthroscopy for FAI |
| HOOS | MDC | 10–16 points | Varies by subscale | Hip osteoarthritis |
| HOS | PASS | ≈ 87% | Activities of Daily Living | Arthroscopic surgery for femoroacetabular impingement |
| HOS | PASS | ≈ 75% | Sports | Arthroscopic surgery for femoroacetabular impingement |
See the full threshold database for every instrument on the site.