Anteroposterior Pelvic Radiograph

To correctly analyze hip joint morphology, an entire pelvic view—not only imaging of the affected hip—is mandatory. The AP pelvic radiograph is taken as a standard in our department, with the patient lying supine (Figure 3-4, A). This can be directly compared with intraoperative AP pelvic radiographs while the patient is under general anesthesia or with follow up radiographs during the early rehabilitation period. The legs have to be internally rotated to adjust for femoral antetorsion, and the film-focus distance should be 1.2 m. The central beam should be directed to the midpoint between the superior border of the symphysis and a line that connects both of the anterior superior iliac spines; these landmarks can easily and reproducibly be palpated by the radiology technician. It is extremely important for the correct interpretation of the radiographs that all radiographs are obtained with the same protocol.

Figure 3–4 The technique of the basic radiographic projections of the hip is shown. A, The anteroposterior pelvic radiograph is taken with the patient in a supine position. The central beam is directed to the midpoint between the pubic symphysis and a line that connects the anterior superior iliac spines. B, The axial cross-table view of the proximal femur is taken with a 45-degree angle and the contralateral hip flexed. C, The false profile of the hip is taken with the patient standing. The index hip is in contact with the radiographic table, the patient is rotated 25 degrees backward, and the long axis of the foot remains parallel with the x-ray table.

Axial Cross-Table View of the Proximal Femur

The axial cross-table view of the proximal femur visualizes the anterior and posterior contour of the femoral head–neck junction, which cannot be seen on the AP pelvic radiograph. This image is taken with the patient in the supine position with the contralateral hip flexed and the ipsilateral leg internally rotated (Figure 3-4, B). Again, the film-focus distance is 1.2 m, and the central beam is directed to the inguinal fold with an angle of 45 degrees. As an alternative to this view, a Dunn-Rippstein view or a frog-leg lateral radiograph may be obtained.

False Profile

The false profile of the hip is taken with the patient standing (Figure 3-4, C). The index hip is in contact with the radiographic table, and the patient is rotated 25 degrees backward while the long axis of the foot remains parallel with the x-ray table. This view is used to quantify anterior acetabular coverage, the narrowing of the joint space posteroinferiorly or anterosuperiorly, and the decentration of the femoral head into the defect.

Abduction View

The abduction view is an analogous view to the AP pelvic radiograph, with the patient’s legs positioned in approximately 20 degrees of abduction and with the internal rotation of both legs. This view is used to simulate the appearance—particularly the congruency—of the hip after a planned hip reorientation osteotomy (Figure 3-5).

Figure 3–5 The abduction view is used for patients with developmental dysplasia of the hip for the detection of joint congruency after acetabular reorientation procedures and to simulate postoperative coverage. A, Preoperative anteroposterior pelvic radiograph with an incongruent hip joint on the right and a congruent hip joint on the left side. B, Abduction/internal rotation view. A persistent joint incongruency is visible on the right side, which represents a relative contraindication for joint-preserving surgery.

Acetabular Depth

Acetabular depth is important to evaluate, because it is often correlated with an increased acetabular coverage that can lead to symptomatic pincer FAI. A normal depth is defined as the acetabular fossa being lateral to the ilioischial line (Figures 3-6, A, and 3-7, left; see Table 3-1). A coxa profunda is characterized as the acetabular fossa touching or crossing the ilioischial line (Figures 3-6, B, and 3-7, middle). In a protrusio acetabuli, which represents the deepest form of acetabular depth, the even femoral head crosses the ilioischial line (Figures 3-6, C, and 3-7, right). These joint configurations are primary pathologies and should not be mixed up with secondary acetabular protrusion in patients with end-stage osteoarthritis.

Figure 3–6 Overview of the most often used coxometric hip parameters. A-D, Parameters that describe acetabular depth; E-L, parameters that describe acetabular coverage; M-P, parameters that describe femoral asphericity; Q,R, parameters that describe joint congruency; S,T, additional findings.

Figure 3–7 Evaluation of acetabular depth. Left, Normal acetabular depth with the acetabular fossa (F) lying lateral to the ilioischial line (IIL). Middle, With a coxa profunda, the acetabular fossa is touching or overlying the ilioischial line. Right, With a protrusio acetabuli, even the femoral head is touching the ilioischial line.

Care must be taken if an AP view of the hip (rather than the entire pelvis) is evaluated. Here, because of the particular x-ray beam centering, a false-positive coxa profunda can be created with a normal-appearing AP radiograph that is centered over the entire pelvis (see Figure 3-3). The only quantitative parameter that describes acetabular depth is the ACM angle (Figure 3-6, D; values given in Table 3-1). This view is rarely used in daily clinical practice.

Acetabular Coverage

Acetabular coverage has to be judged laterally, anteriorly, posteriorly, and in relation to each other.

Lateral coverage can be characterized by the lateral center edge (LCE) angle, the acetabular index, and the extrusion index (values given in Table 3-1). Normally, the LCE angle varies between 25 degrees, which defines a deficient acetabular coverage, and 39 degrees, which defines excessive coverage (Figure 3-6, E).

The acetabular index, which is also called the acetabular roof angle, is formed by a horizontal line and a line through the medial edge of the sclerotic zone and the lateral edge of the acetabulum (Figure 3-6, F). In patients with hips with coxa profunda or protrusio acetabuli, this index is typically 0 degrees, or it may even be negative (see Figures 3-6, C, and 3-7, B). The femoral head extrusion index is defined as the horizontal portion of the femoral head in a horizontal direction that is uncovered by the acetabulum (Figure 3-6, G). Although there is a maximum extrusion of 25%, which indicates dysplasia, no study has defined a minimum extrusion index.

Anterior coverage can be quantified on a false-profile view. The anterior center edge angle (Figure 3-6, H) is used for quantification. It is regarded as normal when it is more than 25 degrees, as borderline when it is between 20 and 25 degrees, and as pathological when it is less than 20 degrees.

The posterior acetabular rim is judged on the AP pelvic radiograph. A deficient posterior rim is present when its projected line runs medial to the femoral head center (i.e., posterior wall sign; Figure 3-6, J).

The relationship between anterior and posterior coverage is judged by carefully tracing the anterior and posterior acetabular rim on the AP pelvic radiograph. In a patient with normal hip morphology, the posterior rim typically lies laterally to the anterior rim. In addition, the posterior rim runs more vertically (see Figure 3-6, J). Sometimes, distinguishing between the two portions of the acetabular rim can be difficult. The posterior rim line can always be readily identified when starting from the inferior edge of the acetabulum. A too-prominent anterior acetabular rim is present when its projected line runs laterally to the posterior line in the cranial portion of the acetabulum, thus causing a “figure-of-8” sign (Figures 3-6, J, and 3-8, A). This relative anterior overcoverage can cause the pincer type of FAI, and it is occasionally seen in hips with DDH. Because it is assumed that the entire affected inferior hemipelvis is rotated, these “retroverted” hips usually show an ischial spine that protrudes into the true pelvis (i.e., ischial spine sign; Figure 3-6, K).

Figure 3–8 A 27-year-old male patient with mixed femoroacetabular impingement. The figures show images of the hip taken from an anteroposterior pelvic radiograph. A, An acetabular retroversion is seen with the projected anterior acetabular wall (AW) crossing (arrow) the posterior wall (PW). As an indirect sign, the ischial spine (IS) protrudes into the true pelvis. B, The axial cross-table view shows a typical aspherical anterior femoral head–neck junction.

Care has to be taken with regard to the interpretation of the projected acetabular rim, because it strongly depends on the position of the pelvis on the x-ray table. An acetabular retroversion can be created by an increased pelvic tilt or rotation to the ipsilateral side of the patient (Figure 3-9). Similarly, other radiographic parameters (e.g., the LCE angle) can substantially change with different pelvic orientations (Figure 3-10). Neutral pelvic rotation around the longitudinal axis is a given when the tip of the coccyx points toward the middle of the symphysis. As an indicator of neutral pelvic tilt, the distance between the upper border of the symphysis and the middle of the sacrococcygeal joint should be 3.2 cm ± 1 cm for men and 4.7 cm ± 1 cm for women. To accurately determine the individual pelvic tilt of the patient, a strong lateral pelvic radiograph is very useful (Figure 3-11). A neutral pelvic tilt is defined by 60 degrees of pelvic inclination, which is determined by a horizontal line and a line that connects the symphysis with the sacral promontory (see Figure 3-11). Specific software called Hip²Norm is available to correct the individual acetabular morphology to a standardized neutral orientation (see Figure 3-11); this will allow for differences between anatomic and functional differences in acetabular pathomorphology. The further use of this software in clinical practice is currently being evaluated.

Figure 3–9 The influence of individual pelvic malpositioning on the projected acetabular morphology is shown. A, In a hip with a normal tilt, no retroversion is visible. B, In a hip with increased pelvic tilt, a crossover can be created. C, The same effect can be obtained with an ipsilateral rotation around the longitudinal axis.

Figure 3–10 The influence of pelvic tilting on the lateral center edge (LCE) angle is shown. A, X-ray of a cadaver pelvis with a wire-marked acetabular rim in a neutral pelvic orientation. The LCE angle is 28 degrees. B, With excessive pelvic tilt, the same pelvis shows an LCE angle of 48 degrees. A retroversion with a positive ischial spine sign is created.

Figure 3–11 A, An anteroposterior pelvic radiograph of a 26-year-old patient that shows a bilateral acetabular retroversion. B, The strong pelvic lateral radiograph shows an increased pelvic inclination of 69 degrees. C, After computerized correction to the neutral pelvic orientation of 60 degrees of pelvic inclination with Hip²Norm software (University of Bern, Switzerland), a more or less normal acetabular configuration without a crossover sign is present.

Sphericity of the Femoral Head

The assessment of sphericity of the femoral head is important because it can lead to the symptomatic cam type of FAI. A nonspherical, oval-shaped femoral head is often seen in the hips of patients with DDH, and this is routinely treated with an intraoperative capsulotomy and osteochondroplasty during acetabular reorientation procedures. Head asphericity may become especially clinically relevant when it is located in the lateral or anterior aspect of the femoral head–neck junction.

Lateral asphericity is also called the pistol grip deformity, and it can be quantified with the triangular index (see Figure 3-6, L; see Table 3-1). Therefore, on the femoral neck axis, half of the radius r of the femoral head is measured, and a perpendicular line is drawn. A new radius R is defined as the distance between the femoral head center and the intersection point of the perpendicular line with the superior femoral head–neck contour. It is considered to be pathological if R ≥ r + 2 mm (with an estimated magnification of 1.2).

Anterior asphericity is clinically relevant in end-range flexion and internal rotation because it can cause the cam type of FAI. It is invisible on an AP pelvic radiograph, and it can be better visualized on a lateral cross-table projection. Three parameters are measured for assessment: the alpha angle, the femoral offset, and the femoral offset ratio. The alpha angle is formed by the femoral neck axis and a line that connects the femoral head center with the point of beginning asphericity of the anterosuperior femoral head contour (Figure 3-6, M). An alpha angle that exceeds 50 degrees is an indicator of an abnormally shaped femoral head–neck contour. The anterior offset is defined as the difference in radius between the anterior femoral head and the anterior femoral neck (Figure 3-6, N). In asymptomatic hips, the anterior offset is 11.6 mm ± 0.7 mm; hips with cam impingement have a decreased anterior offset of 7.2 mm ± 0.7 mm. The offset ratio is defined as the ratio between the anterior offset and the diameter of the head (Figure 3-6, O). It is 0.21 ± 0.03 in asymptomatic patients and 0.13 ± 0.05 in patients with hips with relevant asphericity that causes the cam type of FAI.

The triangular index has a better reproducibility than the alpha angle because it is defined by clearer geometric landmarks, whereas the alpha angle can sometimes be difficult to pinpoint. In addition, the triangular index is more independent of femoral rotation.

Joint Incongruency

Joint congruency is difficult to assess, and it is more relevant in patients with DDH than in those with FAI. Because there are no parameters that describe the three-dimensional incongruency of the joint rotation centers, joint congruency can only be assessed indirectly by the degree of joint subluxation. Subluxation in the vertical direction can be objectified with the intactness of the Shenton line, which consists of the medial border of the femoral head and the superior border of the obturator foramen (see Figures 3-6, P, and 3-12, C). Subluxation of the femoral head in the horizontal direction can be measured as the shortest distance between the medial aspect of the femoral head and the ilioischial line (see Figure 3-6, Q).

Additional Findings

The centrum collum diaphyseal angle is different in patients with hips with dysplasia as compared with those with impingement (see Figure 3-6, R). Hips with the global pincer type of FAI (i.e., coxa profunda, protrusion acetabuli) often have a varus type of morphology (Figure 3-12, A). In these hips, the horizontal line through to the tip of the greater trochanter runs cranial to the center of the femoral head. In a normal hip, the horizontal line runs approximately through the joint center (Figure 3-12, B). Patients with DDH frequently present with a valgus configuration of the proximal femur (Figure 3-12, C), which is defined as when the horizontal trochanteric line lies inferior to the head center. Subsequently, the fovea capitis femoris lies more cranial than it does in the normal hip morphology (see Figure 3-12, C). This so-called fovea alta can further reduce the weight-bearing zone between cartilaginous joint surfaces, thus causing the condition to worsen and leading to the early degeneration of the joint.

Figure 3–12 The trochanteric line (TL) runs inferior to the femoral head center (C). This leads to a fovea alta (F), which is in contact with the weight-bearing zone of the acetabulum. If a subluxation is present, the Shenton line (SL) is interrupted. A, In a hip with the pincer type of femoroacetabular impingement, more varus configurations are seen. As a result, the trochanteric line runs cranial to the femoral head center. B, In a normal hip, the Shenton line is intact, and the trochanteric line runs approximately through the femoral head center. C, Dysplastic hips typically have a coxa valga.