X-rays

Plain radiographs remain the first-line imaging modality in the pre-operative workup for hip pathologies, used to screen candidates, characterize bony morphology and rule out contraindications like advanced osteoarthritis or severe dysplasia. A complete radiographic series for femoroacetabular impingement syndrome (FAIS) typically includes some of the following five views.

1. Anteroposterior Pelvic View

A weight-bearing, standing view centered on the pubic symphysis used to assess femoral head coverage, joint space and screen for both CAM and PINCER morphology bilaterally.

2. Cross-Table Lateral View

This view is obtained with the patient supine with the contralateral hip flexed making it useful to measure the alpha angle at the anterior head-neck junction.

3. 45° or 90° Dunn View

The hip is flexed to 45 or 90 with 20° abduction while the patient is supine and is used to evaluate femoral head sphericity and contour of the femoral head-neck junction. This view is considered highly sensitive for detecting anterolateral cam lesions, with reported sensitivity of 91% and specificity of 88% for cam deformity.

4. Frog-leg Lateral View

Both hips are flexed and abducted with soles of feet together which offers good visualization of the anterosuperior head-neck junction.

5. False-profile view

This is a standing lateral view with the pelvis rotated 65* relative to the film with the foot parallel to cassette used to assess anterior acetabular coverage via the anterior center-edge angle.

Cam Morphology

The alpha angle is the cornerstone measurement for cam deformities. It is constructed by drawing a best-fit circle around the femoral head, then measuring the angle between (1) a line from the center of the femoral neck's narrowest point through the center of the best-fit circle, and (2) a line from that center to the point where the head-neck contour first extends beyond the circle (loses sphericity).

Normal and pathologic thresholds vary somewhat by source and by the projection used, since different views capture different regions of the head-neck junction (AP for lateral junction, frog-leg/Dunn views for anterolateral junction). However the most common thresholds are a normal angle of <50-55 and >50-55 is considered pathologic.

Pincer Morphology

Pincer-type impingement and acetabular overcoverage are assessed with a different set of angles:

1. Lateral center-edge angle (LCEA) of Wiberg: angle between a vertical line through the femoral head center and a line from that center to the superolateral acetabular margin. Normal range is roughly 25°–40°; dysplastic if <20°, borderline dysplastic 20–25°, and overcovered if ≥40°

2. Tönnis angle (acetabular index): The acetabular index is the slope of the weight bearing surface of the acetabulum relative to a level horizontal line. Normal range is −10° to 10°; values >10–15° indicate dysplasia, while values near or below 0° suggest overcoverage and pincer risk

3. Anterior center-edge angle: measured on the false-profile view, assesses anterior femoral head coverage, complementing the LCEA's lateral assessment. As with the LCEA a normal value is between roughly 25°–40°.

Contraindications Visible on X-ray

In addition to characterizing FAIS morphology, X-rays also help identify relative contraindications to hip arthroscopy. Common contraindications are outlined below:

1. Advanced osteoarthritis: Tönnis grade 3 or higher, or absent/near-absent joint space on weight-bearing films, favors arthroplasty over arthroscopy.

2. Severe dysplasia: LCEA <20° (or Tönnis angle >15°, anterior center-edge angle <20°) suggests the patient needs a periacetabular osteotomy rather than arthroscopy alone, since arthroscopy should not be the sole treatment for structural instability .

3. Borderline dysplasia: LCEA 20–25° combined with Tönnis angle 10–15° defines a clinically important gray zone requiring careful surgical judgment .

4. Joint ankylosis: prevents safe arthroscopic distraction and instrument access

Conclusion

Plain radiography remains the essential first step in the evaluation of any patient being considered for hip arthroscopy. A well-executed series allows the clinician to characterize the type and severity of femoroacetabular impingement and other bony deformities. Just as importantly, these same films screen out patients for whom arthroscopy is inappropriate: advanced osteoarthritis, structural dysplasia, and joint ankylosis all redirect management toward arthroplasty or osteotomy rather than arthroscopic intervention. What plain radiographs cannot do, however, is show the soft-tissue structures that often generate a patient's symptoms: the labrum, articular cartilage, and ligamentum teres. Once bony morphology has been characterized and contraindications excluded, the workup must therefore advance to a modality capable of resolving these structures directly. This is the role of MRI and the MR arthrography.