Bryan D. Springer
Over the past several decades there have been significant technologic advances in the field of radiology including computed tomography, magnetic resonance imaging, and nuclear medicine. Plain radiography, however, remains the initial diagnostic imagining study of choice and should serve as the first step in the radiographic evaluation of the hip.
The goal of any imaging study is to confirm or provide a diagnosis of a disorder. For any imaging modality to be useful, it must be performed and interpreted accurately. This chapter outlines the screening radiography of the hip along with special views and additional modalities that may be useful in evaluating a patient with suspected hip pathology.
Plain Radiography (X-Ray View)
Every patient with suspected hip pathology should undergo a screening radiographic examination. For evaluation of the pelvis and hips, this should include anterior-posterior pelvis (AP pelvis) and cross-table or frog-leg lateral views. The findings on screening radiography allow for identification of initial pathology and enable the physician to make a more directed radiographic evaluation with special views or other imaging modalities.
AP Pelvis
The AP pelvis radiograph (Fig. 2-1) is performed with the patient supine on the x-ray table. The legs are internally rotated 15 degrees to compensate for normal femoral anteversion. The beam should be directed centrally to view the entire pelvis. This view allows for imaging of the iliac bones, sacrum, pubis, ischium, femoral heads, and acetabulum and the proximal aspect of the femur including the greater and lesser trochanter.
Cross-Table Lateral
The cross-table lateral or groin lateral view (Fig. 2-2) is obtained with the patient supine on the examination table and the opposite hip flexed and abducted. The x-ray cassette is placed on the outside of the affected hip, and the beam is angled from the opposite side toward the patient's groin. This view provides a lateral image of the femoral head allowing for assessment of anteversion angle of the femoral neck, which can range from 25 to 30 degrees. It also allows for visualization of the ischial tuberosity and the anterior and posterior margins of the acetabulum.
Frog-Leg Lateral
The frog-leg lateral view (Fig. 2-3) is performed with the patient supine on the x-ray table with the knees flexed and the thigh maximally abducted. The beam is directed either vertically or with 10 to 15 degrees of cephalad tilt. This projection demonstrates the lateral aspect of the femoral head and both trochanters.
Special Views
Ancillary views for the pelvis and hips can often provide essential additional information to screening radiography.
Pelvis
The inlet and outlet views of the pelvis are used in addition to the AP pelvis for evaluation of the bony pelvis. The inlet view (Fig. 2-4) projects the rings of the pelvis and allows for evaluation of rotational alignment of the pelvis. The outlet view (Fig. 2-5) projects parallel to the pelvic rim and perpendicular to the sacrum and allows for evaluation of vertical translation or malalignment.
Acetabulum
Judet oblique views of the pelvis and acetabulum are obtained by rotating the patient 45 degrees from the supine position with the beam directed anteroposteriorly. They allow for visualization of the columns and walls of the acetabulum and are used in evaluation and classification of acetabular fractures as well as in the evaluation of bone loss and osteolysis in patients with a failed acetabular component. The two views are as follows:
Anterior oblique.
The anterior oblique or “obturator oblique'' view (Fig. 2-6) demonstrates the anterior column and the posterior wall of the acetabulum.
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Posterior oblique.
The posterior oblique or “iliac oblique'' view (Fig. 2-7) demonstrates the posterior column and the anterior wall of the acetabulum.
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Figure 2-1 Anterior-posterior (AP) view of the pelvis. A, ilium; B, ischium; C, sacrum; D, acetabulum; E, femoral head; F, greater trochanter; G, pubic symphysis. |
Radiographic Landmarks of the Pelvis and Femur
Pelvis
Radiographic landmarks of the pelvis (Fig. 2-8) are as follows:
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Figure 2-2 Cross-table lateral view of the pelvis and femur. A, greater trochanter; B, femoral head; C, femoral shaft; D, ischium; E, anterior margin of acetabulum; F, posterior margin of acetabulum; G, angle of femoral anteversion. |
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Figure 2-3 Frog-leg lateral view of the pelvis and femur. A, greater trochanter; B, lesser trochanter; C, femoral head; D, ilium; E, ischium. |
Femur
Radiographic landmarks of the femur (Fig. 2-9) are as follows:
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Figure 2-4 The inlet view of the pelvis. A, pubic symphysis; B, ilium; C, sacral promontory; D, acetabulum. |
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Figure 2-5 The outlet view of the pelvis. A, pubic symphysis; B, ilium; C, sacrum/sacroiliac (SI) joints; D, acetabulum. |
Radiographic Measurement of the Hip
Radiographic measurement of the hip (Fig. 2-10) involves examination of the following:
Femoral offset: Horizontal distance from the center of the femoral head to the center of the femoral shaft.
Femoral neck shaft angle: Angle measured between the central line of the femoral neck and shaft. Normal 125 degrees with a range of 120 to 140 degrees.
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Figure 2-6 The anterior oblique or obturator oblique Judet view of the pelvis. A, anterior column of pelvis; B, posterior wall of acetabulum. |
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Figure 2-7 The posterior oblique or iliac oblique Judet view of the pelvis. A, posterior column of pelvis; B, anterior wall of acetabulum. |
Shenton line: Observed as a confluent arch between the inferior border of the superior pubic ramus and the medial border of the femoral neck. Helpful in evaluating the relationship of the femoral head to the acetabulum. A break in the Shenton line indicates migration of the femoral head.
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Figure 2-8 Radiographic landmarks of the pelvis. A, iliopectineal line; B, ilioischial line; C, acetabular teardrop; D, anterior acetabular rim; E, posterior acetabular rim. |
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Center edge angle of Wiberg: A line is drawn from the center of the femoral head to the edge of the acetabulum. A second line is drawn vertically to the center of the femoral head to form the incident angle.
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Figure 2-9 Radiographic landmarks of the femur. A, greater trochanter; B, lesser trochanter; C, calcar femoris; D, primary compressive trabecular bands; E, primary tensile trabecular bands. |
Kohler Line: A line is drawn from the medial border of ischium to the medial border of ilium. Penetration medial to this line indicates protrusion.
Radiographic Characteristics of Osteoarthritis of the Hip
The hip and knee are the most common sites of osteoarthritis (Fig. 2-11). Symptoms may vary from stiffness to pain to difficulty walking. The severity of symptoms may not always correlate with radiographic severity. The classic radiographic features of osteoarthritis include the following:
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Figure 2-10 Radiographic landmarks of the femur. A, femoral offset; B, femoral neck shaft angle; C, Shenton line; D, center edge angle of Wiberg; E, Köhler line. |
Imaging of the Prosthetic Hip
Evaluation of a patient with a prosthetic hip requires a thorough and complete radiographic assessment. Adequate radiographs should show the entire prosthesis and any surrounding cement. In most cases, plain radiographs will provide adequate information for the diagnosis, and ancillary
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imaging (CT scan, bone scan) is needed only in selected circumstances.
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Figure 2-11 Radiographic characteristics of osteoarthritis of the hip. |
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Figure 2-12 Anteroposterior pelvis radiograph demonstrating a dislocated total hip arthroplasty. |
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Figure 2-13 Anteroposterior pelvis radiograph demonstrating appropriate acetabular abduction. A, abduction angle of acetabular component. |
Some common complications of arthroplasty often identified on plain radiographs include the following:
Component dislocation may occur at any time after surgery (Fig. 2-12). A common cause of early dislocation is component malposition. AP radiograph should demonstrate approximately 45 degrees of abduction (Fig. 2-13). Assessment of acetabular anteversion can be obtained from a true lateral radiograph. Appropriate acetabular component anteversion is 10 to 30 degrees. Occasionally CT scan to directly demonstrate femoral and acetabular component position may be required.
Aseptic loosening (Fig. 2-14) of either the femoral or acetabular components is a common cause of failure of the prosthetic hip. Radiographic changes can be subtle and
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often require the evaluation of serial radiographs taken over a period of time. Different mechanisms and patterns of failure exist for loosening of cemented and uncemented prosthesis. Table 2-1 lists the common radiographic findings of aseptic loosening of a total hip arthroplasty.
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Figure 2-14 Anteroposterior radiograph of the femur demonstrates aseptic loosening of the femoral component. |
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TABLE 2-1 Radiographic Criteria of Aseptic Loosening |
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Figure 2-15 Anteroposterior radiograph of the femur demonstrates periprosthetic bone resorption secondary to infection. |
Infection (Fig. 2-15) continues to be one of the most devastating complications for the patient and surgeon following prosthetic hip replacement. Infection may be seen as an acute process or may be chronic. Radiographic findings in the acute setting are often absent and in the chronic setting may be subtle. The chronically infected prosthetic joint may present with periprosthetic bone resorption, frank bony destruction, and mechanical failure of the prosthesis.
Periprosthetic fractures (Fig. 2-16) are often the result of a fall or trauma resulting in a fracture of the bone around the prosthesis. The pattern is often described as being around the implant, at the tip of the implant, or distal to the implant. The prosthesis may continue to be well fixed despite the fracture or be loose as a result of the fracture of the surrounding supportive bone.
Ancillary Imaging
The cause of hip pain can be occult. Additional information not available on plain radiographs oftentimes is required to properly diagnose and treat certain disorders. In these situations, ancillary imaging techniques may be helpful. The most commonly used techniques include computed tomography, magnetic resonance imaging, and bone scintigraphy.
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Figure 2-16 Periprosthetic fracture around the stem of a total hip arthroplasty. |
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Figure 2-17 Computer tomography 3D reconstruction of the pelvis with components in place (A) and with prosthesis subtracted (B) demonstrating severe protrusion defect of acetabular bone but intact anterior and posterior columns. |
Computed Tomography
Computed tomography is most commonly used to evaluate primary disorders of the hip, assess pelvic bone quality, and aid in the postoperative evaluation of prosthetic component positioning. The latest generation of CT scanners use multiple detectors that allow for improved resolution. Reconstruction algorithms allow the generation of reformatted and three-dimensional images (Fig. 2-17A, B). These techniques are particularly useful in regions with complex anatomy such as the pelvis.
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Figure 2-18 Preoperative radiograph of a 68-year-old man with an infected left total hip arthroplasty (A) with retained intrapelvic cement confirmed on CT scan (B) that required removal during surgery. |
For patients who present for complex revision of a failed acetabular component, CT scans can aid in determining the adequacy of remaining bone stock in patients with protrusio defects or pelvic discontinuity. In addition, CT allows for localization of intrapelvic cement and retained hardware in relationship to intrapelvic vasculature (Fig. 2-18A, B). A CT scan may also be useful for assessing component position in the clinical setting of recurrent prosthetic instability.
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The evaluation of acetabular or femoral component ante- version in relationship to fixed bony landmarks may provide useful information prior to revision surgery.
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Figure 2-19 Radiographs (A) and T1-weighted magnetic resonance imaging (B) of a 50-year-old man with bilateral avascular necrosis of the hips. |
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) has gained wide acceptance in the evaluation of the painful total hip. Pathologic conditions in which MRI may be useful include: avascular necrosis, labral pathology, occult fractures, infection, and tumors. Multiplanar capability combined with superior soft tissue resolution allows for accurate visualization of bone and soft tissue structures surrounding the hip joint.
MRI is the diagnostic modality of choice for early detection and evaluation of osteonecrosis (ON). MRI has proven to be the most sensitive and specific test for the diagnosis of ON. It is useful in the detection of early disease, asymptomatic disease, and bilateral disease present in approximately 80% of patients. The characteristic MRI appearance is a focal, segmental signal abnormality in the subchondral bone of the femoral head (Fig. 2-19A, B). In contrast, transient osteoporosis of the femur, a clinical and diagnostic entity often confused with avascular necrosis, has signal abnormality involving the head and neck diffusely without discrete signal abnormality.
The addition of paramagnetic intravenous contrast agents such as gadolinium to MRI increases the signal in vascular structures around the hip. In the setting of avascular necrosis, this may help to distinguish between areas of reparative and necrotic tissue. Intravenous (IV) contrast may also help in identifying focal fluid collections. After contrast administration, rim enhancement of a nonvascularized abscess can be differentiated from diffusely enhancing inflammatory tissue. MRI arthrography, consisting of the direct installation of gadolinium into the hip joint, has proven valuable in the diagnosis of labral and chondral pathology of the hip (Fig. 2-20).
Magnetic Resonance Imaging and Prosthetic Evaluation
Previously, large artifacts and noise prevented useful magnetic resonance imaging of the total hip prosthesis. Recently, new imaging protocols have allowed for less artifact and more accurate depiction of soft tissue and bone around the prosthesis. One area where this technology may prove particularly useful is in the early detection and assessment of periprosthetic osteolysis. Preliminary data suggests that metal suppression MRI techniques may be more sensitive and specific than plain radiographs in determining the presence and volume of periprosthetic osteolysis.
Nuclear Imaging
Nuclear imaging studies (bone scintigraphy) remain useful tools for detecting areas for abnormal metabolic activity
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in bone. The bone scan is typically performed in three phases after administration of technetium 99:
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Figure 2-20 MRI arthrography of the hip demonstrating anterior labral tear. |
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Figure 2-21 Tech 99 bone scan demonstrating symmetric uptake of tracer around the femoral component of a right total hip arthroplasty consistent with loosening of the prosthesis. |
Bone scintigraphy may be useful in evaluating the suspected loose or infected total hip prosthesis when other clinical modalities (laboratory values, x-ray views) are equivocal. Although this technique has high sensitivity, it also has poor specificity. Increased tracer uptake can be seen surrounding a normal hip prosthesis fo ≤24 months after surgery. A loose hip prosthesis will commonly show abnormal tracer uptake at the trochanters, tip of the prosthesis, and acetabulum (Fig. 2-21).
Although infection at the site of prosthesis will generally show diffuse uptake, bone scintigraphy is not specific enough to differentiate infection from loosening. The addition of indium-111—labeled leukocyte scan that accumulates in regions of infection by chemotaxis has been shown to improve both the sensitivity and specificity of identifying periprosthetic infection, especially when combined with technetium-99 bone scan. Because indium-111 may also accumulate at the site of bone marrow distribution, the addition of a sulphur colloid bone marrow scan may also aid in improving the sensitivity and specificity. If abnormal uptake of white blood cells on an indium scan is not matched by congruent uptake on a bone marrow scan, the findings more likely correlate with infection.
Suggested Readings
Garvin KL, McKillip TM. History and physical examination. In Callaghan JJ, Rosenberg AG, Rubash HE. The Adult Hip. Philadelphia: Lippincott-Raven Publishers; 1998:315–332.
Greenspan A. Orthopaedic Imaging: A Practical Approach. Philadelphia: Lippincott Williams and Wilkins; 2004.