Athletic injuries of the hip often require radiographs and advanced imaging for diagnosis. Plain radiographs evaluate for osseous injury, provide a structural context behind an athlete’s symptoms and examination, and offer a backdrop for interpretation of advanced imaging. An understanding of normal anatomy, imaging findings, and radiographic measurements allows for recognition of pathoanatomy and ability to diagnose accurately. Advanced imaging modalities, including magnetic resonance imaging, computed tomography, and ultrasonography, each play a role in evaluation of the athlete’s hip. Although MRI and CT provide high-resolution imaging of the hip, ultrasonography offers the unique ability to perform dynamic imaging and guided injections.
The ability to critically interpret plain radiographs is essential in the evaluation of the athlete’s hip. Radiographs augment physical examination findings and provide context for advanced imaging.
An understanding of normal anatomy, imaging findings, and radiographic measurements allows for the recognition of pathoanatomy and the ability to diagnose accurately.
A thorough understanding of the strengths and limitations of various advanced imaging modalities helps guide imaging choice.
Magnetic resonance imaging and computed tomography provide high-resolution imaging of the hip, whereas ultrasonography offers the unique ability to perform dynamic imaging and guided injections.
Initial assessment of an athlete’s hip involves a thorough physical examination and detailed history, which should allow for formation of a differential diagnosis and guide next steps in obtaining imaging studies. Radiographic imaging of the pelvis provides structural context, which augments clinical examination findings and can provide insight into disruptions in normal hip function. Radiographs help to establish baseline anatomy and provide a necessary backdrop for interpretation of advanced imaging. Advanced imaging with computed tomography (CT), magnetic resonance imaging (MRI), and ultrasonography (US) provides high-resolution characterization of a patient’s anatomy, allowing for accurate diagnosis and surgical planning.
Radiographs are indicated in most, if not all, patients with athletic injuries of the hip to evaluate for acute osseous injury in addition to gaining insight into the underlying structural morphology. A standard series, including anteroposterior (AP) pelvis, Dunn lateral, and false-profile views, provides an excellent overview of hip anatomy from which an abundance of information can be gathered. The following sections detail techniques for obtaining various pelvic radiographic views and descriptions of key measurements, which have important clinical implications.
Anteroposterior Pelvic Radiograph
The AP radiograph can be obtained with a patient standing or supine. Bilateral lower extremities should be internally rotated 15°, which maximizes the length of the femoral neck by offsetting for normal anteversion ( Fig. 1 ). The optimal x-ray tube-to-film distance is 120 cm, with the tube oriented perpendicular to the film. The coccyx should be centered in line with the pubic symphysis, and the radiation beam should be centered between the 2 horizontal lines connecting anterior superior iliac spines (ASISs) bilaterally and superior aspects of the pubic symphysis, respectively. Some providers prefer standing AP images, asserting that weight bearing may affect joint positioning and certain hip angles for evaluation.
A well-centered, well-tilted, and well-rotated image is essential for the interpretation of apparent abnormalities. A well-centered AP image aligns, along a vertical axis, the pubic symphysis with the midline of the sacral vertebrae. Offset of the symphysis from the sacral midline suggests a rotated image, which also may be indicated by asymmetry in the appearance of obturator foramina, ischial spines, and radiographic teardrops. The pubic symphysis and tail of the coccyx should be approximately 1 cm to 3 cm in distance for an appropriately inclined hip image. These markers of image quality should be screened routinely prior to more in-depth evaluation and interpretation of hip radiographs.
Frog-Leg Lateral View
The frog-leg lateral view is obtained with the patient placed supine, the hip of interest abducted to 45°, and the ipsilateral knee flexed to 30° to 40° ( Fig. 2 ). The ipsilateral heel should rest comfortably against the opposite knee. The cassette is placed with the superior edge of the film aligning with the ASIS and the beam aimed at the midway point between the ASIS and pubic symphysis. The tube-to-film distance should be approximately 40 in (102 cm).
Dunn Lateral View, 90° or 45°
The Dunn lateral radiograph is obtained with the patient supine and hips flexed to 90° (for 90° Dunn) or 45° (for 45º or modified Dunn). The Dunn radiograph provides a view of the elongated neck for better characterization of proximal femoral morphology. For both the 90° and 45° Dunn views, the hip is abducted 15° to 20°, with the pelvis and tibia aligned in neutral rotation, parallel to the long axis of the body ( Fig. 3 ). The tube-to-film distance is approximately 40 in (102 cm) from the table, with the tube perpendicular to the table and the x-ray beam centered at the midpoint of the ASIS and pubic symphysis. An AP radiograph of the pelvis with both hips positioned for the Dunn Lateral view provides a comparison of the proximal femoral anatomy of each hip.
The false-profile view is obtained from the standing patient, with the hip of interest placed closest to the cassette film. The ipsilateral foot should run parallel with the plane of the cassette, and the pelvis is angled 65° from the plane of the cassette ( Fig. 4 ). The x-ray beam is centered on the femoral head, perpendicular to the film, with a tube-to-film distance of approximately 40 in (102 cm).
Acetabular version describes the AP orientation of the acetabulum in the horizontal plane. Normal version is considered to be 15° to 20° anteversion. It is essential to evaluate acetabular anatomy on an AP pelvis radiograph so that pelvic tilt and rotation can be taken into account. On the AP pelvis film, version can be assessed by evaluation of the relative positions of the anterior and posterior walls. To assess version, outline the anterior and posterior acetabular rims extending to the lateral sourcil. In the anteverted acetabulum, the anterior and posterior rim lines do not cross when traced to the lateral sourcil. If the outline of the posterior rim crosses with the anterior rim before either extends to the lateral sourcil, this is known as the crossover sign , and may signify retroversion of the acetabulum ( Fig. 5 ). Specifically, crossover sign on AP radiograph represents focal retroversion, or anterior overcoverage. By comparison, global retroversion is demonstrated by presence of the posterior wall sign, in which the center of the femoral head is lateral to the posterior wall. Also associated with retroversion is the presence of a prominently visible ischial spine on AP film. It is important to consider that inappropriately tilted hips can introduce artificial crossover signs bilaterally, and a rotated pelvis can induce a unilateral crossover sign with an excessively anteverted appearance of the contralateral side.
Acetabular depth is delineated in reference to the ilioischial line. Normal depth on AP pelvic radiograph demonstrates the medial aspect of the femoral head positioned lateral to the ilioischial line. If an aspect of the acetabular fossa is touching or crossing medial to the ilioischial line, it is considered coxa profunda. If the medial aspect of the femoral head crosses medial to the ilioischial line, this is termed, protrusio acetabuli ( Fig. 6 ).
Acetabular angles are useful metrics for assessing structural stability. In determining these angles, it is helpful to establish the center of the femoral head and demarcate the horizontal pelvic plane, which in a well-positioned AP pelvis radiograph can be obtained by drawing a line between the 2 teardrops.
The lateral center-edge angle (angle of Wiberg) describes superolateral coverage of the femoral head. The angle is created by drawing 2 lines: (1) a vertical line through the femoral head center, perpendicular to the horizontal pelvic plane, and (2) a line connecting the femoral head center to the lateral sourcil ( Fig. 7 ). The normal range is between 25° and 40°. An angle greater than 40° is overcovered whereas an angle less than 20° is considered dysplastic; an angle 20° to 25° is considered borderline dysplasia.
Acetabular inclination, measured by the Sharp angle, is determined by the following 2 lines: (1) the horizontal pelvic plane drawn between the teardrops and (2) the line connecting the teardrop to the lateral sourcil ( Fig. 8 ). The normal range is 40° to 45°, with angles greater than 45° suggesting dysplasia.
Acetabular index, or Tonnis angle, is another measure of acetabular coverage. It is defined as the angle between a horizontal line through the medial sourcil and a line that connects the medial sourcil to the lateral edge of the sourcil ( Fig. 9 ). A normal Tonnis angle is between 0° and 10°. An upsloping Tonnis angle greater than 10° may be structurally unstable, whereas a downsloping angle (less than 0°) increases the risk for femoroacetabular impingement (FAI) by creating pincer deformity.
The extrusion index describes the percentage of femoral head that is uncovered by the acetabulum on AP imaging. Two distances are identified: (1) the horizontal distance between the lateral-most aspect of the femoral head and the lateral-most aspect of the acetabulum and (2) the total head diameter ( Fig. 10 ). The first distance is taken as a percentage of the second to determine the extrusion index. An index less than 25% is normal, whereas a hip with greater than 25% is undercovered.