Femoroacetabular Impingement (FAI): Key X-Ray Views for Diagnosis

What is the xray view for FAI?

Diagnosing Femoroacetabular Impingement (FAI) typically involves a series of specific X-ray views designed to evaluate the complex bony morphology of the hip joint, including the femoral head-neck junction and the acetabulum, to identify characteristic impingement-related abnormalities.

Understanding Femoroacetabular Impingement (FAI)

Femoroacetabular Impingement (FAI) is a condition characterized by abnormal contact between the femoral head/neck and the acetabular rim, particularly during hip movements like flexion and internal rotation. This repetitive abnormal contact can lead to damage to the articular cartilage and labrum, causing pain, restricted motion, and potentially contributing to early-onset osteoarthritis of the hip. FAI is broadly categorized into three types:

• Cam Impingement: An abnormally shaped femoral head (often described as "pistol grip deformity") that jams into the acetabulum.
• Pincer Impingement: Over-coverage of the femoral head by the acetabulum, often due to a deep socket or acetabular retroversion.
• Mixed Impingement: A combination of both cam and pincer features.

The Role of Imaging in FAI Diagnosis

While a thorough clinical examination, including specific provocative tests, is crucial, imaging plays a definitive role in confirming FAI and characterizing its specific type and severity. X-rays are typically the first-line imaging modality due to their ability to clearly visualize bone morphology, measure key angles, and identify bony abnormalities indicative of FAI. They are cost-effective and readily available.

Key X-Ray Views for Diagnosing FAI

To adequately assess the hip joint for FAI, a combination of specific X-ray views is necessary. No single view can capture all the necessary anatomical information.

• Anteroposterior (AP) Pelvis View: This view provides a broad overview of both hips and the pelvis. In the context of FAI, it allows for the assessment of:

  • Joint Space: To check for signs of osteoarthritis.
  • Overall Hip Morphology: Including the sphericity of the femoral head and the depth of the acetabulum.
  • Acetabular Coverage: Evaluation of the lateral center-edge angle (LCEA) to assess the degree of lateral acetabular coverage.
  • Acetabular Retroversion: Identified by the "crossover sign" (the anterior wall of the acetabulum crosses over the posterior wall) and the "posterior wall sign" (the posterior wall does not reach the center of the femoral head), both indicative of pincer-type impingement.

• Lateral Views (e.g., Dunn View, Cross-Table Lateral): Lateral views are critical for evaluating the anterior aspect of the femoral head-neck junction, which is often affected in cam impingement.

  • Dunn View (45° Hip Flexion, 20° Abduction): This is considered one of the most sensitive views for detecting cam deformity. The hip is flexed to 45 degrees and abducted 20 degrees, which brings the anterior femoral head-neck junction into profile. This view is excellent for measuring the alpha angle, a key diagnostic parameter for cam impingement. An elevated alpha angle (typically >50-55 degrees) indicates a loss of sphericity at the femoral head-neck junction.
  • Cross-Table Lateral View (or Modified Cross-Table Lateral): Performed with the patient supine and the affected hip abducted and externally rotated (often 15 degrees), this view also profiles the anterior femoral head-neck junction and allows for alpha angle measurement. It is often used when the Dunn view is difficult to obtain due to pain or limited mobility.

• False Profile View (Lequesne and de Sèze View): This specialized view is designed to assess the anterior acetabular coverage and the anterior joint space. The patient stands with the affected hip rotated 65 degrees externally from the sagittal plane. It helps identify subtle pincer impingement features, particularly anterior over-coverage.

• Frog-Leg Lateral View: While less frequently used as a primary diagnostic view for FAI compared to the Dunn view, the frog-leg lateral view (hips flexed and externally rotated, knees bent) can provide an additional perspective on the femoral head-neck junction and overall hip joint alignment. It can highlight certain cam deformities that might be missed on other views, though it often foreshortens the femoral neck, making precise alpha angle measurement less reliable than the Dunn view.

What Radiologists Look For

Beyond specific bony abnormalities, radiologists and orthopedic surgeons evaluate several key measurements and signs on these X-ray views:

• Alpha Angle: Measured on lateral views (especially Dunn view), this quantifies the sphericity of the femoral head-neck junction. An angle greater than approximately 50-55 degrees is suggestive of cam impingement.
• Lateral Center-Edge Angle (LCEA): Measured on the AP pelvis view, this assesses the lateral coverage of the femoral head by the acetabulum. A low angle (<20-25 degrees) indicates hip dysplasia (under-coverage), while a high angle (>40 degrees) can suggest pincer over-coverage.
• Acetabular Index (Tönnis Angle): Also from the AP view, it measures the inclination of the acetabular roof.
• Crossover Sign and Posterior Wall Sign: As mentioned, these are indicators of acetabular retroversion, a common cause of pincer impingement.

Beyond X-Rays: Other Diagnostic Tools

While X-rays are foundational, additional imaging modalities may be used for a comprehensive assessment:

• Magnetic Resonance Imaging (MRI) or MR Arthrography (MRA): Crucial for evaluating soft tissues, including the labrum, articular cartilage, and surrounding musculature. MRA, with contrast injected into the joint, is particularly effective at detecting labral tears and cartilage damage.
• Computed Tomography (CT) Scan: Provides detailed 3D bony anatomy, which can be invaluable for surgical planning, especially in complex cases or when assessing rotational deformities.

Conclusion

For fitness professionals, trainers, and student kinesiologists, understanding the specific X-ray views used in FAI diagnosis is vital. It underscores the precision required in identifying this complex condition and highlights why a multi-planar imaging approach is necessary. Recognizing the types of bony abnormalities these views reveal helps in comprehending the underlying biomechanical issues contributing to FAI symptoms, guiding appropriate exercise modifications, and facilitating effective communication with medical professionals.