A multitude of structural hip disorders can occur in athletes with hip pain. Although the history and physical examination play a critical role in determining the diagnosis, it is also important to have a systematic approach to help diagnose these disorders radiographically. This chapter describes the key imaging studies used when examining a skeletally mature patient with a pathologic hip, as well as a systematic approach to interpretation of these studies.
Radiographic Techniques
Traditionally, the lateral hip radiograph demonstrates details of the femoral neck and helps identify cam impingement pathology, whereas the anteroposterior (AP) view demonstrates the acetabular version. Several radiographic views are important for proper evaluation of the hip. Among these, the most commonly referenced include the AP view of the pelvis (AP pelvic view), a cross-table lateral view, a 45-degree or 90-degree Dunn view, a frog-leg lateral view, and a false-profile view. All views are technique dependent, and each demonstrates a different anatomic perspective of the hip joint. Descriptions of each view are provided in the following sections.
Standing Anteroposterior Pelvic View
A proper AP view 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 anteversion, and this position helps maximize the view of the femoral neck. 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. 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 radiograph assesses (1) functional leg length inequalities; (2) neck shaft angle (NSA); (3) femoral neck trabecular patterns; (4) lateral and anterior center edge angles; (5) acetabular inclination; (6) joint space width; (7) lateralization; (8) head sphericity; (9) acetabular cup depth; and (10) anterior and posterior wall orientation ( Figs. 81-1 and 81-2 ).
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. 81-3 ). 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. 81-4 ). 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 anterior superior iliac spine (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. 81-5 ). 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. 81-6 ). The beam is centered on the femoral head and perpendicular to the cassette. The tube-to-film distance should be approximately 40 inches.
Interpretation of Plain Radiographic Images
Interpretation of both the false-profile and AP pelvic views generally helps to characterize the acetabular morphology, whereas the other views better describe the proximal femoral anatomy. By combining all views, we should be able to define the following parameters for each patient: leg length inequalities, NSA, femoral neck trabecular patterns, lateral and anterior center edge angles, acetabular inclination, joint space width, lateralization, head sphericity, acetabular cup depth, and anterior and posterior wall orientation.
Functional Leg Lengths
Functional leg lengths may be assessed on an AP pelvis radiograph by constructing a line horizontally off the superiormost 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: (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. Three classifications are used for acetabular 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 femoroacetabular impingement (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.
Head Sphericity
Assessment of head sphericity should be performed with both the AP and frog-leg lateral views because a patient may have an apparently spherical head on one view and not on the other. The femoral head is classified as spherical if the epiphysis does not extend beyond the margin of reference circle by more than 2 mm, and it is classified as aspherical if it extends beyond this 2-mm margin. The margin of reference is known as a Mose template (concentric circles).
Acetabular Cup Depth
The acetabular cup depth is a relationship between the floor of the fossa acetabula and the femoral head relative to the ilioischial line. If the floor of the fossa acetabula touches or is medial to the ilioischial line or the posterior wall extends lateral to the center of axis of rotation, the hip is classified as coxa profunda. If the medial aspect of the femoral head is medial to the ilioischial line, the head is classified as protrusion. Additionally, the inner acetabular wall thickness, medial wall shape, and inferior cup orientation should be recognized.
Acetabular Version
Normally, the acetabulum is anteverted by approximately 20 degrees. Acetabular version can be defined as either being retroverted or anteverted by identifying the presence or absence of a crossover sign on the AP view. The acetabulum is considered to be anteverted if the line of the anterior aspect of the rim does not cross the line of the posterior aspect of the rim before reaching the lateral aspect of the sourcil, and it is considered to be retroverted if the line of the anterior aspect of the rim does cross the line of the posterior aspect of the rim before reaching the lateral edge of the sourcil. It is important to note that true acetabular retroversion is associated with a deficient posterior wall, whereas a hip with a crossover sign but no posterior wall deficiency refers to anterior overcoverage—cranial acetabular retroversion or anterior focal acetabular retroversion.
Head-Neck Offset
Head-neck offset can be evaluated from the frog-leg lateral radiograph and is classified on the basis of the gross appearance of the relationship between the radius curvatures of the anterior aspect of the femoral head with the posterior aspect of the head-neck junction. If the anterior and posterior concavities of the head-neck junction are symmetrical, then the head-neck offset is classified as symmetrical. If the anterior concavity has a radius of curvature greater than that at the posterior aspect of the head-neck junction, it is classified as having a moderate decrease in head-neck offset. If the anterior aspect has a convexity, as opposed to a concavity, the head-neck junction is classified as having a prominence (cam-type FAI).
Alpha Angle
The alpha angle is classically described for use with axial magnetic resonance imaging (MRI) scans; however, its use may be extrapolated to lateral radiographs. The angle is formed between a line connecting the center of the femoral head to the anatomic axis of the femoral neck and a second line running from the center of the femoral head to the prominence of the head-neck junction where the head sphericity ends. The angle between these two lines is known as the alpha angle . Values of more than 42 degrees are suggestive of head-neck offset deformity.
Computed Tomography Evaluation of the Hip
Computed tomography (CT) is an ideal modality for characterization of the osseous structures in the area of the hip. Three-dimensional (3D) multiplanar reformatted CT images have been used to assess hip pathology. Although the CT images and the 3D reformatted images that they create are an excellent noninvasive method of demonstrating osseous morphology, the disadvantages of this technique are the utilization of ionizing radiation in the young patient population and the fact that CT does not allow direct visualization of intraarticular structures. As a result, the workup of a painful hip should progress to MRI evaluation, which is discussed in the following section.
Magnetic Resonance Evaluation of the Hip
Although conventional and 3D CT scanning of the hip has been described for assessment of acetabular version and FAI, magnetic resonance arthrography (MRA) of the hip has gained popular acceptance for the diagnosis of these conditions. Compared with conventional MRI, magnetic resonance (MR) arthrograms can more readily identify intraarticular abnormalities such as cartilage defects, loose bodies, and labral tears. The purpose of this section is to help the reader identify some of the most common pathologies within the hip joint with the use of MRA, including acetabular labral tears, FAI, hip sprains and dislocations, and stress fractures.
Labral Conditions
Labral pathology can often be stratified by patient age. In young patients, pathology is more likely to be caused by a traumatic mechanism, ranging from an acute twist injury of the hip during sport all the way to a hip dislocation. For middle-aged patients (<50 years), the mechanism is often FAI. More senior patients often experience degenerative tears associated with osteoarthritis.
Labral Variant
The acetabular labrum is a fibrocartilaginous rim that deepens the socket of the hip joint, although its role in hip stability remains unclear. It encompasses approximately three fourths of the circumference of the acetabulum and is absent along its inferior aspect. It is here where the transverse acetabular ligament extends from the anterior to the posterior aspect of this inferior acetabular fossa.
Several clinically insignificant labral variations have been described in asymptomatic patients. The posterior inferior sublabral sulcus should not be misinterpreted as a posterior labral tear on axial images. Additionally, an anterosuperior cleft may be seen as a normal variant in the presence of a normal lateral acetabular labrum. On anterior coronal or sagittal images, this cleft is seen as a partial undercutting of the labrum on a single image. A transverse ligament labral junction sulcus is a normal sulcus found between the transverse ligament and the labrum either anteriorly or posteriorly. Finally, the perilabral sulcus resembles the normal space between the acetabular labrum and the capsule on coronal images ( Figs. 81-7 and 81-8 ).
