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Femoroacetabular impingement: can the alpha angle be estimated?
Mohamed R Nouh1, Mark E Schweitzer, Leon Rybak
1Department of Radiology, Hospital for Joint Disease, New York University, New York, NY, USA. mragab73@yahoo.com
AJR. American Journal of Roentgenology
|April 24, 2008
Summary
Subjective assessment of the alpha angle for femoroacetabular impingement is unreliable. Visual appraisal is a poor predictor of abnormal angles, even for experienced observers.
Area of Science:
- Orthopedic surgery
- Radiology
- Biomedical engineering
Background:
- Femoroacetabular impingement (FAI) is a significant clinical condition.
- Cam-type FAI involves femoral head-neck dysplasia, quantified by the alpha angle.
- Previous reports suggest subjective assessment may suffice, obviating direct measurement.
Purpose of the Study:
- To scientifically evaluate the accuracy of subjective alpha angle assessment.
- To compare subjective opinions with mathematically measured alpha angles (gold standard).
Main Methods:
- 50 consecutive hip MRIs (1.5 T) were reviewed.
- Two musculoskeletal radiologists provided subjective alpha angle assessments (1-5 confidence scale).
- A third observer performed direct mathematical measurement of the alpha angle.
Main Results:
- Significant variability observed in subjective alpha angle assessments.
- Area under the ROC curve (AUC) of 0.606 indicates poor prediction of wide alpha angles (>55 degrees).
- Over half of abnormal cases were subjectively deemed normal, and nearly half of normal cases were subjectively deemed abnormal.
Conclusions:
- Subjective assessment of the alpha angle is suboptimal for diagnosing FAI.
- Visual appraisal is unreliable unless a significant bone abnormality is evident.
