Cam, pincer, and mixed types of impingement can occur between the proximal femur and the acetabulum. Cam-type FAI occurs when a nonspherical femoral head leads to an abnormal contour of the head-neck junction. This morphologic abnormality can impinge with the acetabulum and labrum when the hip is placed in positions of flexion, adduction, and internal rotation. Cam-type impingement may result from a variety of causes including distortion of the physis during growth, Legg-Calvé-Perthes syndrome, previous trauma, or slipped capital femoral epiphysis.

Preoperative planning with a 3D CT protocol can help tailor the operative approach (Table 1). Cam lesion location is commonly defined by using clockface terminology. Typically, 12 o’clock represents the most superolateral aspect, 3 o’clock represents the most anterior aspect, and 6 o’clock represents the most inferomedial aspect of the femoral head-neck junction. The cam is most commonly found between 12 o’clock and 4 o’clock [9]. Cam lesions that are beyond 12 o’clock (i.e., 11 o’clock) toward the posterior aspect of the hip are difficult to address arthroscopically and may be more amenable to an open procedure.

An objective method to quantify the size of the cam lesion is the alpha angle. The alpha angle is measured at the maximal offset where the abnormal bony contour of the femoral head-neck junction extends beyond the spherical confines of the femoral head. Using CT, the alpha angle can be measured on multiple planes. The measurement is an angle from the axis of the femoral neck to the point where the head-neck junction loses its spherical contour [10] (Fig. 2). An alpha angle greater than 42° is considered abnormal [10]. In a retrospective, comparative study, Hetsroni et al. found that the alpha angles for men and women presenting with hip pain and labral tears were 63.6 ± 10.8° and 47.8 ± 9.5°, respectively [11].

The neck-shaft angle (NSA) is measured on the coronal view. This NSA angle is the angle subtended by the femoral neck and diaphyseal axes (Fig. 3). The neck axis is defined by the line connecting the center of the femoral head and the center of the femoral neck at its isthmus. The diaphyseal axis is defined by the line connecting the midpoint of two lines drawn perpendicularly across the diaphysis of the femur. A normal NSA is between 126° and 139°. Coxa vara is defined as a NSA <126° and is more consistent with impingement. Coxa valga is defined as NSA >139° and is more consistent with instability and dysplasia [12].

Pincer-type FAI occurs when the abnormal morphology is in the acetabulum. Several possible causes of abnormal acetabular morphology include coxa profunda, protrusio, rim prominence, and retroversion. Acetabular version angle is formed by the intersection of the line perpendicular to the line between the posterior pelvic margins and the line connecting the anterior and posterior rim of the acetabulum (Fig. 4) [13]. This measurement is conducted at 1 o’clock, 2 o’clock, and 3 o’clock with normal being approximately 5°, 10°, and 15°, respectively. Patients with retroverted acetabulums are associated with pincer-type FAI.

Subspine impingement occurs when there is direct impingement of the inferior femoral neck along with the labrum, capsule, and indirect head of the rectus femoris against the anterior inferior iliac spine (AIIS). This type of impingement occurs in positions of straight hip flexion. AIIS morphology can be typed and subtyped based on the direction of slope of the AIIS as well as the presence or absence of a clear subspine space (Figs. 5 and 6) (Tables 2 and 3). AIIS type I has an upsloping morphology, which does not contribute to impingement. AIIS type III has a downsloping (crossing the rim) morphology that is more likely to cause impingement. AIIS subtype A has a clear subspine space, which does not contribute to impingement. However, AIIS subtype B has a subspine bone prominence or rim level-based AIIS that is more likely to cause impingement. Therefore, AIIS types II and III and subtype B have a higher predilection for subspine impingement [14]. The AIIS is also assessed to determine the degree of subspine impingement. The distance from the distal base of the AIIS to the acetabular rim should be ≥0.5 cm. A measurement of <0.5 cm is consistent with subspine impingement. The width of the AIIS should be about 1 cm.

Acetabular dysplasia is defined as an abnormality with the version, volume, or inclination of the acetabulum [15]. The morphologic deformity places a greater concentration of weight-bearing forces on a smaller area of articular cartilage, resulting in degenerative soft tissue injuries and osteoarthritic symptoms. Patients with subtle dysplasia are difficult to identify, particularly as a spectrum of disease involve both dysplasia and FAI, which often have overlapping symptoms [16]. Morphologically, acetabular dysplasia is one of the most complex 3D deformities. CT reconstruction circumvents the limitations of traditional 2D radiographic screening, allowing for more accurate estimations of the severity of dysplasia. Measurements such as center-edge angle (CEA), acetabular index (AI), and acetabular sourcil angles can be used to evaluate for dysplasia.

The CEA corresponds to the degree of coverage of the femoral head by the acetabulum. A higher CEA indicates a greater degree of coverage, whereas a lower CEA indicates a lesser degree of coverage. The center-edge angle can be measured on the sagittal or coronal views. The specific cuts chosen to measure these angles are through the center of the femoral head. The coronal CEA is measured by first drawing a line through the center of the femoral head perpendicular to the transverse pelvic axis. Next, a line is drawn through the center of the femoral head to the superolateral aspect of the acetabular roof. The angle between these lines defines the coronal CEA (Fig. 7a). This corresponds to the lateral CEA on radiographs [17], where a CEA <20° is considered to be dysplastic [18]. The average CEA is approximately 41° [19]. The sagittal CEA is measured by first drawing a vertical line through the center of the femoral head. Next, a line is drawn through the center of the femoral head to the most anterior aspect of the acetabular rim. The angle between these lines defines the sagittal CEA (Fig. 7b). This corresponds to the anterior CEA on radiographs where an anterior CEA <20° is considered to be dysplastic [20]. The average anterior CEA angle is approximately 31° [19]. In patients with a CEA <20°, a procedure such as a periacetabular osteotomy (PAO) may be indicated.