In cases of suspected FAI, on the AP radiograph of the pelvis, the physician should look for signs of acetabular retroversion, including the crossover, posterior wall, and ischial spine signs. The crossover sign is positive when any part of the anterior acetabular wall is more lateral than the posterior wall in the proximal region of the acetabulum (Figure 1). The posterior wall sign is present when the posterior acetabular wall is more medial than the center of the femoral head, indicating reduced posterior wall coverage. The ischial spine sign is positive when the ischial spine is visible medial to the pelvic inlet or iliopectineal line. Furthermore, coxa profunda is when the floor of the acetabular fossa contacts or overlaps the ilioischial line on an AP view. Protrusio acetabuli is present when the femoral head crosses the ilioischial line medially. Dunn views obtained with the hip abducted 20° and flexed either 45° or 90° provide the best assessment of the anterosuperior femoral head-neck junction. Additionally, the head-neck offset ratio can be assessed on these views. The alpha angle used to quantify the degree of head asphericity and cam impingement is used with cross-table lateral radiographs² and MRI,³ and it is defined as the angle subtended by a line down the middle of the femoral neck and the point at which the excess bone deviates from the normal sphericity of the head (Figure 2).

Advanced imaging techniques can be helpful to further delineate bony and soft-tissue anatomy and corresponding pathology. Low-dose CT scans can be used to assess femoral and acetabular version, to better characterize the bony anatomy of the pelvis in planning for periacetabular osteotomy, or to help map the shape of the acetabular rim and femoral head-neck junction prior to osteoplasty. MRI techniques provide valuable information regarding the condition of the periarticular soft tissues, including the labrum, articular cartilage, ligamentum teres, capsule, and the surrounding musculotendinous structures. Magnetic resonance arthrography has been shown to be more sensitive in identifying labral tears and chondral lesions than conventional MRI, although it has limitations in detecting undetached chondral separations.⁴ Other magnetic resonance sequences, such as delayed gadolinium-enhanced MRI of cartilage and T1rho mapping, are increasingly being used more in both clinical and research settings because these sequences provide more information regarding the health of a joint by providing an objective measure of the proteoglycan content of the articular cartilage.⁵

On the AP radiograph of the pelvis, measurements can be made of acetabular coverage, including the degree of inclination (the Tönnis angle) and the lateral center-edge (LCEA) angle of Wiberg, with normal values being between 0° and 10° from the horizontal for the Tönnis angle and 25° to 35° for the angle of Wiberg (Figure 3, A and B). A break in the Shenton line is indicative of subtle subluxation of the hip. The false-profile view, which highlights the anterior center-edge angle, anterior acetabular coverage, and anterior joint space narrowing, is taken with the patient standing with the affected hip against and the ipsilateral foot parallel to the cassette with the pelvis rotated 65° (Figure 3, C).

The acetabular labrum is a protective ring of fibrocartilage that contributes to hip stability and the hip suction seal.¹⁰ Labral tears commonly occur in the setting of FAI (Figure 4, A) and can also occur in the setting of hip dysplasia.¹¹ The acetabular labrum normally attaches to the articular cartilage of the acetabulum through a histologic transition zone called the chondrolabral junction, and this region is vulnerable to injury in FAI.¹² With cam-type impingement, the bony prominence of the femoral head-neck impacts the labrum and chondrolabral junction and can lead to outside-in chondral injuries, as described in a 2019 study.¹³

There is a high prevalence of labral tears in asymptomatic patients. In one study, 45 asymptomatic volunteers underwent hip MRI.¹⁴ The mean age of the patients was 37.8 years of age, and 60% were men. Labral tears were identified in 69% of hips. Those older than 35 years were more likely to have a chondral defect and subchondral cyst.

Classification systems have been developed to describe labral tears and acetabular cartilage injury that occur as a result of FAI. One commonly used classification system, the Beck classification, was originally described for hips undergoing surgical hip dislocation; it has since been adapted to be used in hip arthroscopy.¹⁵ Table 1 describes the Beck classification of cartilage damage, and Table 2 shows the Beck classification of labral damage.

Nonsurgical treatment of FAI includes NSAIDs, physical therapy for core and gluteal muscle strengthening, and intra-articular hip corticosteroid injection. The UK FASHIoN Trial, a multicenter randomized clinical trial, investigated nonsurgical treatment of FAI with physical therapy compared with hip arthroscopy.¹⁶ A total of 348 patients were enrolled, and after 12 months, both groups had significant improvements in their primary outcomes and hip-related quality of life, as measured by the 33-item International Hip Outcome Tool. Although both groups showed improvement, the group undergoing hip arthroscopy had greater improvement than those in the physical therapy group.

Modern surgical treatment of FAI includes hip arthroscopy and surgical dislocation of the hip, both of which may involve labral repair (Figure 4, B), acetabuloplasty (for those with pincer lesions), and femoral osteochondroplasty (for those with cam lesions, Figure 5), with or without capsular closure. Hip arthroscopy is performed on a traction table to facilitate joint distraction and safe access to the hip joint.