Hip Imaging

Standing Anteroposterior Pelvic View

A proper AP view of the pelvis should be taken with the patient standing. The x-ray tube-to-film distance should be approximately 120 cm, and the x-ray tube should be aimed perpendicular to the film. Both lower extremities should be internally rotated by 15 degrees to account for normal anatomic femoral neck anteversion. The crosshairs of the beam should be centered on a point half the distance between the superior border of the pubic symphysis and on a line drawn connecting the anterior superior iliac spine (ASIS). The coccyx should be centered in line with the pubic symphysis. The radiographic teardrops, iliac wings, and obturator foramina should be symmetrical in appearance. A 1- to 3-cm gap should be seen between the apex of the coccyx and the superior border of the pubic symphysis for proper pelvic inclination. A standing rather than a supine AP radiograph is obtained because acetabular roof obliquity, center edge angle, and minimum joint space width may vary between weight-bearing and supine positions.

The standing AP pelvis radiograph assesses (1) functional leg length inequalities, (2) neck shaft angle (NSA), (3) femoral neck trabecular patterns, (4) lateral center edge angle, (5) acetabular inclination, (6) joint space width, (7) lateralization, (8) head sphericity, (9) acetabular cup depth, and (10) anterior and posterior wall orientation ( Figs. 78.1 and 78.2 ).

Cross-Table Lateral View

For the cross-table lateral view radiograph, the patient should be supine on the x-ray table. The contralateral hip and knee should be flexed out of the way of the x-ray beam (typically >75 degrees). The hip of interest should be rotated internally 15 degrees to help accentuate the anterolateral surface of the femoral head-neck junction ( Fig. 78.3 ). The x-ray beam should be parallel to the table and oriented at a 45-degree angle to the limb of interest, with the crosshairs aimed at the center of the femoral head.

45-Degree or 90-Degree Dunn View

The Dunn view is commonly used for assessment of femoral head sphericity in patients believed to have cam-type femoroacetabular impingement (FAI). It was originally described as a technique to measure femoral neck anteversion in children.

The 90-degree Dunn view assesses the patient with 90-degree hip flexion, whereas the 45-degree Dunn view (“modified Dunn view”) assesses the patient with 45 degrees of hip flexion ( Fig. 78.4 ). For both views, the film cassette is placed beneath the pelvis and the tube is centered over the upper border of the pubic symphysis. Each leg should be abducted 15 to 20 degrees from the midline, and the pelvis and tibia should be parallel to the long axis of the body (neutral rotation). The crosshairs of the beam should be directed at a point midway between the ASIS and the pubic symphysis, and the tube-to-film distance should be approximately 40 inches in a line directed perpendicular to the table.

Frog-Leg Lateral View

To obtain the frog-leg lateral view, the patient should be positioned supine on the x-ray table with the hip of interest abducted 45 degrees, the ipsilateral knee flexed 30 to 40 degrees, and the ipsilateral heel resting against the contralateral knee ( Fig. 78.5 ). The cassette is positioned so that the top of the film rests at the ASIS. The crosshairs of the beam are directed at a point midway between the ASIS and the pubic symphysis, with an x-ray tube-to-film distance of approximately 40 inches.

This view permits assessment of another view of medial and lateral joint space width, femoral head sphericity, congruency, head-neck offset, alpha angle, and bone morphology.

False-Profile View

The false-profile view is helpful for evaluation of the anterior acetabular coverage of the femoral head. The view is obtained with the patient standing, the affected hip against the cassette, and the pelvis rotated 65 degrees from the plane of the cassette ( Fig. 78.6 ). The beam is centered on the femoral head and perpendicular to the cassette. The tube-to-film distance should be approximately 40 inches.

Functional Leg Lengths

Functional leg lengths may be assessed on an AP pelvis radiograph by constructing a line horizontally off the superior most portion of the iliac crest. Ideally this line should be symmetrical to the contralateral hemipelvis. A discrepancy greater than 2.0 cm corresponds with a functional leg length discrepancy and can have an adverse effect on the kinematic function of the hip.

Neck Shaft Angle

The NSA is defined by the angle formed by the longitudinal axes of the femoral neck and the proximal femoral diaphyseal axis. One line is drawn down the anatomic axis of the femoral neck, and the other is drawn down the anatomic axis of the femur. The angle formed represents the NSA. This angle is normally between 125 and 140 degrees. An angle less than 125 degrees is classified as coxa varus. An angle greater than 140 degrees corresponds to coxa valgus. The NSA dictates the load transfer from the femur to the acetabulum.

Trabecular Pattern

The trabecular pattern is influenced directly by the NSA and reflects the compressive and tensile forces within the femoral neck. For cases of coxa varus, tensile trabeculae are more prominent. For cases of coxa valgus, compressive trabeculae are more prominent.

Lateral Center Edge Angles

The lateral center edge angles can be used to assess the superolateral coverage of the femoral head by the acetabulum. The lateral center edge angle is calculated by measuring the angle between two lines of an AP pelvis radiograph: (1) a line through the center of the femoral head, perpendicular to the transverse axis of the pelvis, and (2) a line through the center of the femoral head, passing through the most superolateral point of the sclerotic weight-bearing zone of the acetabulum. Normal values range from 22 to 42 degrees in adults, although values less than 26 degrees may indicate inadequate coverage of the femoral head.

Anterior Center Edge Angles

The anterior center edge angles are created through use of the false-profile view. This angle assesses the anterior coverage of the femoral head. It is calculated by measuring the angle between the vertical line through the center of the femoral head and a line connecting the center of the femoral head and the most anterior point of the acetabular sourcil. Values of less than 20 degrees can be indicative of structural instability.

Acetabular Inclination

Also known as the Tonnis angle, acetabular inclination is best appreciated on the AP view. It is formed by drawing a horizontal line and a tangent from the lowest point of the sclerotic zone of the acetabular roof to the lateral edge of the acetabulum. Acetabular inclination can be classified into three different groups based on degree of inclination :

• 1.

  Normal = Tonnis angle of 0 to 10 degrees

• 2.

  Increased = Tonnis angle greater than 10 degrees, subject to structural instability (increased inclination)

• 3.

  Decreased = Tonnis angle less than 0 degrees, subject to pincer-type FAI (decreased inclination)

Joint Space Width

Joint space width is described as the shortest distance between the surface of the femoral head and the acetabulum. Joint space width is examined using the standing AP radiograph because the effects of position influence the joint space observed. Evidence of joint space narrowing can be classified using the Tonnis grade for osteoarthritis.

Hip Center Position

For hip center position, the distance between the medial aspect of the femoral head and the ilioischial line is measured. If the distance is greater than 10 mm, then the hip is classified as lateralized. The distance of 10 mm should serve as a general reference point, and film magnification and patient body habitus must be taken into account and may influence this measurement.