Applied Radiology

Ms. Kwong is a medical student at the University of California, San Diego School of Medicine, San Diego, CA.
ÝDr. Sartoris is deceased.

The hip joint is one of the major articulations that is often involved with numerous pathologic conditions and injuries. With recent technological advances in imaging, many new methods are available for radiologic evaluation of the hip and pelvic region. In general, conventional radiography remains the initial imaging technique in almost all cases, while computed tomography (CT) and magnetic resonance imaging (MRI) often serve as secondary or subordinate imaging evaluations. However, the accuracy, cost-effectiveness, and convenience of each technique in evaluating different conditions of the hip should be taken into account. This article reviews the most effective imaging techniques for evaluation of hip abnormalities, including trauma, infection, osteonecrosis, arthropathies, neoplasm, and developmental abnormalities.

Trauma

Fractures of the hip are very common in victims of motor vehicle accidents or postmenopausal women with severe osteoporosis. Usually, the initial evaluation involves frontal radiography (figure 1). Further views are recommended for additional evaluation (i.e., oblique/Judet for the evaluation of the acetabulae, and inlet and outlet views for information regarding the sacrum and sacroiliac joints [figure 2]). For the evaluation of severe hip fractures, CT is superior to simple radiography. ¹ It provides better demonstration of the size, location, and displacement of fractures, especially in relation to intra-articular loose bodies, pelvic hematomas, sacral fractures, and sacroiliac joint diastasis. Advances in spiral (helical) CT scanning allow for rapid, high-quality, two- and three-dimensional imaging with decreased interference from patient motion. Three-dimensional CT imaging is best used to evaluate hip fractures in conjunction with axial CT scans, since it does not clearly demonstrate displaced fractures <2 mm. For minimal fractures of the hip, pelvis, or proximal femur, radionuclide bone scintigraphy is sensitive, but often nonspecific, and may be negative within the first 24 hours of injury. MR imaging is superior to CT in detecting insufficiency fractures, due to its high sensitivity to alterations within the bone marrow and its accuracy in evaluating soft tissues around the hip and pelvis. ²

Infection

Osteomyelitis, bone infection usually caused by bacteria (often Staphylococcus , but sometimes a Ringus), is evaluated readily with conventional radiography (figures 3 and 4), CT, and MR imaging. According to Bolton et al, ³ CT changes indicative of osteomyelitis are cortical destruction, periosteal reaction, and increased attenuation within the marrow space. CT is also capable of demonstrating surrounding soft-tissue inflammation. However, MRI is superior to CT for identifying acute osteomyelitis and areas of active and chronic osteomyelitis, although CT is capable of detecting sequestra, an indication of active chronic osteomyelitis. ⁴ CT and MRI are both effective in evaluating subacute osteomyelitis, such as a Brodie's abscess, as a focal intraosseous fluid collection with surrounding sclerosis. CT and MRI are also very useful in identifying anatomically localized fluid collections involved in soft-tissue infection around the hip joint or muscles. In general, localized soft-tissue infection around the hip should be excluded when focal fluid collections are not indicative on CT imaging. Another important use of CT and MRI is that they can readily distinguish nonlocalized changes of cellulitis from the focal fluid mass of a soft-tissue abscess, which is important since cellulitis can be treated medically, while abscess requires drainage.

Avascular necrosis and transient osteoporosis

For avascular necrosis (AVN) of the femoral head, the most sensitive and specific radiologic evaluation is MRI (figure 5A), ⁵ specifically, coronal MR images. Occasionally, however, MRI of AVN may show diffuse marrow edema without focal defects, which is similar to transient osteoporosis. ⁶ The use of intravenous contrast may help to distinct between the two. It is important to remember that there is a time lag between the occurrence of histologic changes and the detection of AVN on MRI. On CT, the earliest sign of AVN is a star-like condensation of the trabeculae within the femoral head called the asterisk, which can be central, peripheral, or sclerosis. CT is also excellent in detecting which femoral surface is intact, which can guide the planning and surgical treatment of this disease.

Sometimes one may be confused by AVN on MRI, which may only show diffuse marrow edema without focal defects with transient osteoporosis. ⁷ This problem can be solved by using intravenous gadolinium contrast to distinguish the two. ^8,9 For transient osteoporosis, which can be related to or be a variant of AVN, radiographs (Figure 5B) and MRI are the best methods of evaluation. ⁹ Radiographs generally show osteopenia, while bone scanning demonstrates activities within the femoral head region. MRI usually shows diffuse marrow edema with decreased signal on T1-weighted scans and more intense signal on T2-weighted scans. Along with the above methods, dual-energy X-ray absorptiometry is also a good method to quantitatively assess bone density (figure 6) and fracture risk of the proximal femur. ⁷

Arthropathies

Pelvic and hip joints that are more commonly involved with arthropathies include the sacroiliac joints and the symphysis pubis. According to Hayes and Balkissoon, ⁸ in general, imaging techniques are very useful in providing information for primary diagnosis, preoperative planning, and follow-up treatments for suspected arthritis. Usually, conventional radiographs are the primary imaging modality for suspected arthritis involving the hip and the pelvis (figures 7 and 8). Indications of joint-space narrowing, osteophytosis, sclerosis, and subchondral cyst formation signify the existence of osteoarthritis (figure 7). For inflammatory arthritis, joint-space narrowing and articular erosions are involved (figure 9). According to Jelinek et al, ¹⁰ MR imaging and CT are valuable in evaluating unusual arthritides, such as pigmented villonodular synovitis (PVNS) and synovial chondromatosis. However, conventional radiography is not an appropriate method for assessing sacroiliac joint disease, mainly due to the curvature of the joint and complications caused by bowel gas and stool. Therefore, CT, bone scintigraphy, and MR imaging are usually used as secondary tests. Several recent studies have suggested the use of MR imaging to assess early sacroiliac joint disease. ¹¹ Murphey et al ¹² suggests that MR imaging is more sensitive than CT in detecting cartilage loss, small erosion, and edema.

Primary and secondary tumors (neoplasms)

Generally, conventional radiography is the preliminary evaluation for suspected primary bone neoplasm (figures 10 and 11). This is a critical step in identifying the initial formation of any potential tumors. However, additional imaging is usually needed to prevent misinterpretation due to other overlapping structures, and to assess the extent of the lesion for preoperative or other treatment plans. Both CT and MR imaging are excellent in assessing the characteristics of a lesion; however, they are not capable of distinguishing between benign and malignant tumors. ³ In general, no one technique is recommended for the evaluation of bone metastases. Bone scintigraphy suffers from low specificity and low sensitivity. MR imaging usually presents heterogeneity in the pelvic and proximal femoral region, especially for older or obese patients. Hayes and Balkissoon ⁸ suggested the use of CT (figures 12 and 13) along with a percutaneous biopsy to confirm suspected metastases on radiographs or scintigraphy.

Congenital and developmental abnormalities

Developmental dysplasia of the hip (DDH), also known as congential hip dysplasia, is generally evaluated by radiography (figures 14A and B, and 15), ultrasonography, and MRI. ¹³ MRI is especially useful because it does not expose patients to radiation, and it is able to evaluate surrounding soft-tissue structures clearly. CT arthrography, with the injection of contrast medium to the joint, is very useful in evaluating DDH (figure 14C) after intraoperative reduction. ¹¹

For proximal femoral focal deficiency (PFFD), a cogenital partial absence of the upper femur can be detected by MRI or CT. It is often hard to evaluate with radiography, due to the variable nature of the disease. In general, MRI is superior to CT for the evaluation of PFFD due to its improved contrast resolution and multiplanar imaging capability.

Conclusion

Pain of the hip and pelvic region is generally the first sign of clinical abnormality involving this major articulation of the body. Usually, radiography is the first step in the evaluation of patients with hip pain; however, CT and MRI are also crucial diagnostic tools in the accurate assessment of hip disease. The indications or imaging findings of these different abnormalities can provide efficient and specific evaluation of the hip. AR