The AP pelvic view is most helpful in obtaining a general sense of acetabular bony morphology. One can also get an appreciation of acetabular depth. Using this view, the hips can be classified as being globally “overcovered” or as having a “deep socket” if they fall into two general categories: “coxa profunda,” [4] if the floor of the acetabular fossa lies at or medial to the ilioischial line (ICC = 0.02; range = −0.72–0.44) [17], or “protrusio acetabuli,” if the femoral head sits medial to the ilioischial line (ICC = 0.10; range = −0.57–0.49) [17]. In a recent study, Nepple et al. [18] found that the presence of coxa profunda can be a normal finding and has a limited role in diagnosing pincer-type FAI. To further assess femoral head overcoverage, they recommend investigating the following parameters: crossover sign, posterior wall sign, lateral center-edge angle, anterior center-edge angle, and acetabular inclination. These parameters help to further distinguish global overcoverage from localized areas where the acetabular margin may be prominent.

The Tönnis angle [19] is used to calculate the degree of acetabular inclination. It represents the horizontal orientation of the weight-bearing zone of acetabulum on an AP pelvic radiograph. It is measured by calculating the angle between a horizontal line at the most inferior aspect of the sclerotic acetabular sourcil parallel to the teardrop line and a line extending to the most lateral edge of the sclerotic acetabular sourcil [19]. The normal range for this angle measurement is 0–10°. Values of >10° and <0° are considered to have increased and decreased inclination, respectively. In general, acetabuli with increased Tönnis angles are usually dysplastic and may be subject to structural instability, whereas those with decreased Tönnis angles are at risk for pincer-type femoroacetabular impingement [8] (ICC = 0.70; range = 0.48–0.83) [17].

The lateral center-edge angle (LCEA) of Wiberg [20] is the most common measure of acetabular coverage. Specifically, it is used to quantify the superolateral acetabular coverage and is best measured on an AP pelvic view. It is the angle between a line drawn perpendicular to the transverse axis of the pelvis and a line drawn from the center of the femoral head extending to the most superolateral point of the sclerotic acetabular sourcil (weight-bearing zone). An LCEA of <20° is considered as femoral head undercoverage or, traditionally, acetabular dysplasia [21–24]. An LCEA of >40° is found to be abnormal and defined as acetabular overcoverage or profunda, seen specifically in pincer-type FAI [21, 25–28]. When analyzing the reliability to interpret common radiographic findings of the adult hip by various observers, Carlisle et al. found that the LCEA was the most consistently assessed value between readers, with an excellent intra-rater observer (ICC = 0.88; range = 0.85–0.91) and interobserver value (ICC = 0.64; range = 0.52–0.75) [29].

On a false-profile lateral view, the anterior center-edge angle of Lequesne [14] is calculated to assess the anterior femoral head coverage. It is the angle between a vertical line through the center of the femoral head and a line extending to the most anterior portion of the sclerotic acetabular sourcil. An angle of <20° can be indicative of anterior undercoverage, seen in entities like dysplasia [8] (ICC = 0.38; range = 0.26–0.53) [29].

Acetabular version can also be investigated on the AP pelvic view. Acetabular anteversion is appreciated on the AP pelvic view, when the anterior portion of the anterior acetabular rim is superior and medial to the posterior rim and does not cross the posterior portion of the rim before reaching the lateral aspect of the sourcil. Less commonly, acetabular retroversion is seen when the anterior portion of the acetabular rim does cross the posterior portion of the rim before reaching the lateral edge of the sourcil. This has been described as the “crossover” sign [9] (ICC = 0.29; range = −0.25–0.59) [30]. True acetabular retroversion is characterized by global anterior overcoverage with corresponding posterior undercoverage and may result in isolated anterior impingement or combined anterior impingement with posterior coverage deficiency, leading to posterior instability. This morphology is different from focal cranial retroversion, which is characterized by localized overcoverage only at the cranial aspect of the acetabulum with normal posterior wall coverage. The presence of a posterior wall sign (the posterior wall of the acetabulum sits medial to the center of the femoral head [10]) (ICC = 0.20; range = −0.40–0.54) [17] and an ischial spine sign [31] (exaggerated size of the ischial spine projecting medial to the pelvic inlet (ilioischial line)) (ICC = 0.55; range = 0.20–0.74) [17] are radiographic findings on the AP pelvic radiograph that are suggestive of acetabular retroversion [31].

Zaltz et al. [32] demonstrated that acetabular retroversion remains difficult to identify and cannot be definitively diagnosed based on the presence of a “crossover” sign or ischial spine sign alone, even on a well-aligned pelvic radiograph with acceptable tilt and obliquity. Furthermore, Larson et al. [33] demonstrated in their CT-based study that the presence of a crossover sign (53 %; 95 % CI, 46–60 %) and a positive posterior wall sign (20 %; 95 % CI, 15–26 %) were frequent findings in a young asymptomatic cohort and may very well be a normal variant rather than pathologic.

On AP and different lateral views, the femoral head sphericity and offset should be assessed. A Mose template [34] is a template, where concentric circles are used as reference for measuring head sphericity. As a rudimentary guideline, if the femoral epiphysis extends beyond the reference circle margin by >2 mm, the head is considered aspherical. If the femoral epiphysis does not extend beyond 2 mm, then the femoral head is considered spherical [34, 35]. Deviations in head sphericity may be observed not only in FAI but also in avascular necrosis (secondary to segmental collapse) and as sequelae of residual childhood hip conditions such as Legg-Calve-Perthes disease and slipped capital femoral epiphysis (SCFE).

On all the views, one can appreciate the femoral head-neck junction and analyze the relationship of the radius of curvature anteriorly versus posteriorly. Clohisy et al. [8] described that a head-neck junction is said to have symmetric concavity, when both the anterior and posterior concavities are symmetric. Otherwise, if the concavity at the anterior aspect of the head-neck junction has a radius of curvature that is greater than that at the posterior aspect of the head-neck junction, the hip is considered to have a moderate decrease in terms of head-neck offset. Finally, if the anterior aspect of the head-neck junction has a convexity, as opposed to a concavity, the head-neck junction is considered to have a prominence (i.e., a “cam” lesion). Peelle et al. [36] calculated the head-neck offset ratio, which can be measured on lateral radiographs. It is the ratio of three lines: the first is through the center of the long axis of the femoral neck; the second is parallel to the first line, through the most anterior aspect of the femoral neck; the third line is parallel to the second line, through the most anterior aspect of the femoral head. The distance between the second and third line is then divided by the diameter of the femoral head, the normal being an absolute value of ≥9 mm or a ratio of the head diameter of ≥0.17 [37]. A ratio of <0.17 indicates that a cam deformity is likely present [36] (ICC = 0.86; range = 0.76–0.92) [17].

Nötzli et al. [38] described the alpha angle, which is a measurement of femoral head-neck dysplasia, in other words, cam-type impingement. Originally it was measured on magnetic resonance imaging (MRI) axial views, but can also be calculated on a lateral-type radiograph. It is calculated by measuring the angle between a line drawn from the center of the femoral head to the point of the anterolateral aspect of the head-neck junction where the contour of the femoral head loses its sphericity and the prominence starts (i.e., where the radius of the femoral head begins to increase beyond the radius found more centrally in the acetabulum where the head is more spherical). Originally, the reported average value was 42° (range = 33–48°) in normal controls (ICC = 0.84; range = 0.72–.091) [17], compared with 74° (range = 55–95°) in patients with symptomatic FAI [38–40]. Several threshold values have been suggested to describe when the alpha angle indicates a pathologic entity that may benefit from surgery [8, 41–43]. The most widely accepted threshold angle is 55° and is considered to be indicative of cam impingement [25] (ICC = 0.19; range = −0.43–0.54) [17]. Inter- and intra-rater reliability with FAI parameters measured on conventional radiographs is reportedly poor in several studies [8, 29, 44]. Lohan et al. [45] found in their retrospective analysis of MR arthrographic studies that the alpha angle measurement was statistically of no value in suggesting the presence or absence of cam-type FAI with an up to 30 % of the mean value intra-observer variability between the first and second alpha angle measurements for each of their 78 subjects (mean sensitivity = 39,3 %; mean specificity = 70.1 %).

The Tönnis OA grade can be used to quantify the degree of OA in the impinging hip and can be seen on all views. The scale ranges from 0, which is normal (no signs of OA), to 1, which is mild (increased sclerosis, slight joint space narrowing, no or slight loss of head sphericity), to 2, which is moderate (small cysts, moderate joint space narrowing, and loss of head sphericity), to 3, which is severe (large cysts, severe joint space narrowing, and loss of head sphericity) [19] (Table 4.1).

Intraoperative fluoroscopy has been advocated by many and proven to be extremely valuable. It is an essential tool to direct osteochondroplasty intraoperatively. It aids in quantifying the location, configuration, and extent of the cam lesion prior to the resection and in judging the adequacy of the resection thereafter. Unfortunately, it is the senior author’s experience that the same concept does not often apply for pincer lesions, as a true AP radiograph can be difficult to replicate fluoroscopically on the operating table.

Larson and Wulf [46] described a reproducible and systematic intraoperative fluoroscopic evaluation of the hip for the management of cam and pincer deformities during arthroscopic treatment of FAI. Ross et al. [47] found that their six (6) intraoperative fluoroscopic views allowed further confirmation of bony resection and helped avoid inadequate resections with resultant impingement. They stated that their intraoperative fluoroscopic views are reproducible and could prove to be critical in the absence of a preoperative 3D CT scan.

Although recent studies have demonstrated that fluoroscopy-assisted hip arthroscopy entails safe levels of radiation [48, 49], some may argue that our fluoroscopic views – in addition to preoperative radiographs and CT – may generate summative doses of radiation that could be avoided. Budd et al. [48] determined on 50 consecutive hip arthroscopies that the mean total fluoroscopy time was 1.10 min and the mean dose area product value was 297.2 cGycm² and concluded that a low maximum dose of radiation was achieved and supports its safe use. Gaymer et al. [49] calculated the maximal theoretical risk to a fetus on 166 hip arthroscopies in women of childbearing age. They found that the maximal theoretical dose was 2.99 mGy to the fetus, which places the procedure as low-risk category.

The diagnosis and treatment of cam-type FAI rely on the radiographic identification of deformity and correction of the 3-dimensional (3D) asphericity and loss of offset at the femoral head-neck junction, respectively. Advanced imaging allows for a 3D understanding of the correction needed, but does not necessarily facilitate the intraoperative localization in the absence of navigated instrumentation [38]. Although a considerable ionizing radiation exposure risk is to be taken into account, high-resolution computed tomography (CT) has allowed for increased precision and better definition of osseous morphology of the hip.

Magnetic resonance imaging (MRI) is the preferred modality for the investigation of intra-articular hip pathology [50]. Several studies have demonstrated evidence of MR findings in acetabular labra in asymptomatic volunteers. In 200 asymptomatic hips, Lecouvert et al. [51] found a homogenous low-intensity signal in 44 % of labra, which seemed to decrease significantly with age. Conversely, they also found that the frequency of heterogeneous signal intensities increased with age in 42 % of cases. Cotten et al. [52] later showed in 52 asymptomatic hips that intralabral regions of intermediate or high signal intensity were found in 57 % of hips. Abe et al. [53] detected similar findings, where in 56 % of their labral segments of 71 asymptomatic hips, homogenous low signal intensity was detected.