Wide variations in the alpha angle reporting of the hip in asymptomatic individuals—a systematic review

The alpha angle is commonly used to quantify cam-type morphologies in hips. However, the threshold values that define a pathological alpha angle remains disputed. To identify the threshold values for the alpha angle measured on X-ray, computed tomography (CT), and magnetic resonance imaging (MRI) in asymptomatic individuals in different radiological modalities. Using the Preferred Reporting Items for Systematic reviews and Meta-Analysis guidelines and checklist, a literature search was conducted in May 2024 by two independent reviewers. The PubMed, Embase, Cochrane, and CINAHL databases were searched using the following search terms: ('Alpha angle') AND (('FAI') OR 'femoroacetabular impingement') AND ((((Imaging) OR CT) OR MRI) OR X ray). Of the studies identified, those measuring the alpha angle using either X-ray, CT, or MRI in an asymptomatic population were included. Study population and, if stated, demographics like gender, ethnicity, and activity level were then compiled using Excel as well as number of individuals and hips, imaging modality and view, method of evaluating symptoms, mean alpha angle, and 95% reference interval. Twenty-five articles were included in the review. Threshold values varied depending on what radiological method was used, with thresholds for X-ray ranging from 61.0° to 122.9°, thresholds for CT ranging from 50.0° to 95.3°, and thresholds for MRI ranging from 52.7° to 103.5°. According to current literature, threshold values indicating increased risk of FAI syndrome vary physiologically and depend on imaging view and modality. Therefore, no single cut-off reliably defines cam morphology and measurements should be interpreted within their clinical context.