What is the recommended X-ray (XR) view for evaluating Femoroacetabular Impingement (FAI)?

Recommended X-ray Views for Femoroacetabular Impingement (FAI)

For evaluating Femoroacetabular Impingement (FAI), the recommended X-ray views should include anteroposterior (AP) pelvis and lateral femoral head-neck radiographs, with the lateral view specifically being a Dunn view, frog-leg lateral view, or cross-table lateral view. 1

Primary Radiographic Protocol for FAI

The optimal radiographic evaluation for FAI should follow this algorithm:

1. Initial views (required):

   • Anteroposterior (AP) pelvis view
   • Lateral femoral head-neck view, which can be one of:
     • Dunn view (45° Dunn view preferred)
     • Frog-leg lateral view
     • Cross-table lateral view

2. Advanced imaging (when indicated):

   • MRI/MRA or CT scan for three-dimensional morphological assessment
   • Particularly useful when surgical intervention is being considered

Comparative Value of Different Lateral Views

Different lateral views offer varying advantages for visualizing cam-type FAI:

• 45° Dunn view: Shows highest sensitivity (84%) for detecting cam deformities, particularly in the superior-anterior aspect of the femoral head-neck junction 2
• Frog-leg lateral view: Better for visualizing anterior aspect deformities (62% sensitivity) 2, and provides accurate visualization of femoral head-neck offset 3
• Meyer lateral view: When combined with 90° Dunn view, provides the most effective prediction of 3D shape of the proximal femur and cam lesion region 4

Clinical Considerations

When interpreting radiographs for FAI:

• The alpha angle is a key measurement, with values >55° generally indicating cam-type FAI
• Head-neck offset measurements are also important, with decreased values suggesting impingement
• Gender-specific thresholds may be considered: alpha angle values >58° in females and >63° in males indicate cam-type femoral geometry 5

Important Caveats

• Radiographic findings alone should never be used to diagnose FAI without correlation to clinical symptoms and examination findings 1
• Incidental findings are common in asymptomatic individuals and should be interpreted with appropriate clinical context 1
• Even the best radiographic views can describe no more than half of the overall variation in the shape of the proximal femur and cam lesion 4
• For patients with persistent symptoms where surgical intervention is being considered, advanced imaging with MRI/MRA or CT is recommended for better surgical planning 1

Alternative Imaging Modalities

• Ultrasound has shown comparable reliability to plain radiographs for diagnosing cam-type FAI, with similar correlation to MRI findings (r = 0.77 for ultrasound vs. r = 0.73 for frog-leg view) 6
• This can serve as an alternative or additional method in initial imaging, especially in settings where radiation exposure is a concern

The diagnostic accuracy of radiographs for FAI is maximized when using both AP pelvis and appropriate lateral views, with the 45° Dunn view offering the best overall sensitivity for cam-type deformities.