The imaging workup of the patient with hip pain should begin with conventional radiographs of the pelvis and hips. The acetabular lines should be scrutinized carefully. The addition of oblique views of the affected hip may be necessary to evaluate the anterior and posterior columns of the acetabulum. If an acetabular fracture is identified, computed tomography (CT) is suggested to assess the position of the fracture fragments and to exclude intraarticular loose bodies. If a hip fracture is suspected and radiographs are negative, magnetic resonance imaging (MRI) is recommended for confirmation. Magnetic resonance imaging is the imaging examination of choice for the evaluation of patients with chronic atraumatic hip pain when osteonecrosis or bone marrow edema syndromes are a concern and for the evaluation of muscle injury. 12 Magnetic resonance arthrography is indicated for the evaluation of intraarticular disorders, particularly acetabular labral tears.
The diagnosis of an occult hip fracture can be elusive. The initial radiographs often are normal, especially in elderly patients with osteoporosis. In the past, radionuclide imaging was the first advanced imaging technique used. When done properly, scintigraphy is sensitive and specific in the diagnosis of hip fracture. 11 However, it now is clear that MRI should be the initial imaging modality after routine radiographs of the hip and pelvis because of its high specificity and the additional clinically important information, which MRI can provide (Fig 1). Magnetic resonance imaging with limited T1-weighted coronal images is 100% accurate in detecting occult hip fractures. 15 Scintigraphy is sensitive but often is nonspecific and may be negative immediately after the injury or fall, especially in the elderly. When MRI and clinical outcome were used as the standard of reference, the prospective accuracy of MRI in the diagnosis of the presence or absence of hip fracture was 100%. 7 Although a limited MRI is adequate for the diagnosis of occult hip fractures, a more comprehensive multiplanar examination including fat-suppressed T2-weighted sequences will elucidate additional pelvic fractures and soft tissue injuries. In one study, 80% of patients referred for MRI because of radiographically suspected occult fracture had a bone or soft tissue abnormality detected with MRI. 5
Stress or fatigue fracture of the femoral neck in a young patient represents abnormal stress applied to normal bone. These stresses, none of which are individually capable of producing a fracture, lead to mechanical failure with time. Stress fracture of the femoral neck tends to remain asymptomatic until advanced.
Two types of femoral neck stress fracture have been described on the basis of their precipitating strain patterns. The compressive variety occurs along the lower, medial border, displays a sclerotic appearance on plain films, and tends not to displace. 1,16 Distraction fractures along the superior portion of the femoral neck typically are radiolucent and are prone to become displaced because of tensile forces that act to pull the fracture margins apart.
Magnetic resonance imaging facilitates the early diagnosis of stress fracture. Compressive side medial femoral neck fracture in athletes often presents with bone marrow edema best depicted on fat-suppressed T2-weighted scans or short tau inversion recovery (STIR) images (Fig 2). The increased water content of the associated medullary edema or hemorrhage results in high-signal intensity against the dark background of suppressed fat. A low-signal fracture line often will be seen in the midst of the edema.
Developmental dysplasia of the hip refers to a developmental anomaly of the hip regardless of its etiology. The femoral head, acetabulum, or both may be dysplastic. Hip dysplasia in adults can result from multiple causes, including neuromuscular diseases, cerebral palsy, slipped capital femoral epiphysis, Perthes’ disease, injury, and epiphyseal dysplasia. 6 The presence of hip dysplasia can result in hip pain and premature osteoarthritis. Subtle cases of dysplasia in adults are being recognized more commonly. It is important to evaluate the slope of the acetabulum, the amount of femoral head that is uncovered by the acetabulum, and it is valuable to measure the center edge angle of Wiberg as reported by Delaunay et al. 6 This angle is formed by measuring an angle off the vertical from the center of the femoral head to the lateral margin of the acetabulum. A normal center edge angle measures greater than 25°. Twenty to 25° is borderline and less than 20° is diagnostic of acetabular dysplasia. 6 Acetabular labral abnormalities may be associated with hip dysplasia in adults; these are shown best on MR arthrography.
Bone Marrow Edema Syndromes
Another unusual but very important cause of hip pain in the adult is the transient bone marrow edema pattern recognized by MRI. 10 In many cases, this represents transient osteoporosis of the hip, an unusual but distinct syndrome characterized by self-limited pain and radiographically evident osteoporosis of the affected hip that can be distinguished from other causes of the bone marrow edema pattern on the basis of clinical findings and radiographically evident focal osteopenia developing within 8 weeks after the acute onset of hip pain. 17 (Fig 3). The term transient bone marrow edema syndrome can be used to describe a patient in whom a reversible bone marrow edema pattern is seen on MRI scans. This pattern of bone marrow edema manifests on MRI scans as low signal intensity on T1-weighted sequences. The area of abnormal signal typically involves the entire femoral head, neck, and even may extend into the subtrochanteric region. 4 Fat suppressed T2-weighted sequences or STIR sequences will show very high signal in the affected areas.
Initially described in pregnant women, this entity actually is more common in middle-aged men. The results of laboratory tests are normal, there is a joint effusion of the affected hip, osteopenia of the femoral head and neck, and the radionuclide bone scan shows increased uptake in the involved hip. Symptoms generally resolve in several months and may affect the contralateral hip in the future. 17 Bone marrow edema also may accompany osteonecrosis of the femoral head. In contrast to transient marrow edema, patients with avascular necrosis typically manifest a serpiginous, low signal intensity line deep to the articular surface of the femoral neck. This line demarcates infarcted from noninfarcted bone marrow.
In young adults, osteoid osteoma of the hip frequently causes pain and may be difficult to diagnose on radiographs because these intraarticular lesions may not evoke bone sclerosis or periosteal new bone apposition (Fig 4). The two most common radiographic findings are juxtaarticular osteoporosis and widening of the medial joint space. Thin section CT is the best imaging technique for the identification and localization of the nidus of an osteoid osteoma. 2 Computed tomography scanning is more specific than MRI in the detection of the nidus. Magnetic resonance imaging may be misleading because of the marked bone marrow edema or soft tissue mass, which may accompany this lesion. 3,18 In this setting, an osteoid osteoma may be falsely mistaken for a more aggressive pathologic process.
Pigmented Villonodular Synovitis
Pigmented villonodular synovitis is a localized monarticular neoplastic process characterized by a proliferation of synovial fibroblasts and histiocytes. It usually presents in adults between the ages of 20 and 50 years. There often is pain, swelling, and a limp, and the hip is the second most common site affected after the knee. The lesion presents with either a focal nodular or diffuse pattern characterized by multinucleated giant cells with pigmentation caused by intracellular and extracellular hemosiderin deposition. 8 The lesion is highly vascular and may invade both sides of the joint causing well-defined erosions with sclerotic margins in the femur and the acetabulum (Fig 5). The addition of MRI has improved the diagnostic accuracy, preoperative assessment, and postoperative followup in these patients. Magnetic resonance imaging of the hip in these patients shows characteristic foci of low signal intensity on T1 and T2 imaging sequences related to the hemosiderin deposition.
This benign entity manifests as multiple small osteocartilaginous bodies in the hip. This diagnosis should be considered in any patient with greater than five or six intracapsular bodies in the hip. Synovial osteochondromatosis represents a metaplasia of the synovial lining. Despite the benign nature of this condition, the multiple bodies within the joint capsule can cause damage to the articular surfaces if left untreated. The number of these loose bodies may cause bone erosion while still preserving the joint space and the bone density of the hip (Fig 6). Computed tomography scanning, MRI and CT or MRI arthrography all are potentially useful for operative planning.
Magnetic Resonance Arthrography
Tears of the acetabular labrum may result in potentially disabling mechanical symptoms limiting the daily activities of individuals or their competitive participation in athletics. The importance of early diagnosis and treatment of these labral abnormalities may prevent the onset of osteoarthritis and also may provide significant relief for patients with debilitating hip pain. 9 Unfortunately, tears remain an elusive clinical diagnosis. Symptoms often are vague and nonspecific. The history and physical findings may suggest other etiologies such as osteoarthritis, synovitis, juxtaarticular soft tissue abnormalities, osteonecrosis, and stress fracture.
Conventional MRI is limited for evaluating the labrum because of lack of joint distention. 13 Magnetic resonance arthrography should be reserved for the preoperative assessment of patients with a suspected labral tear and negative results on conventional imaging studies including MRI. In MR arthrography, the hip is injected with fluid before an MRI scan is done. By distending the capsule the labrum is outlined with contrast and tears are identified more readily. In addition to labral tears, MR arthrography readily depicts articular cartilage, allowing for the diagnosis of early cartilage degenerative changes.
Magnetic Resonance Arthrography: Technique and Interpretation
To avoid excessive synovial resorption of fluid, the MRI study should begin no later than 30 to 45 minutes after completion of the injection. Magnetic resonance arthrography should be done with a dilute gadolinium solution in sterile saline or saline alone. In general, gadolinium MRI is preferable because the injected fluid is bright on T1-weighted images, allowing differentiation from noncommunicating paralabral cysts, bursae, and other preexisting extraarticular fluid collections.
With the patient in the supine position on the fluoroscopy table, the hip is localized. The hip is maintained in the neutral position, the puncture site is selected, and the overlying skin is marked, the skin is cleansed with an iodine solution, and sterile drapes are applied. Superficial and deep local anesthetic (1% lidocaine mixed with sodium bicarbonate) is infiltrated along the expected needle course. The anterior joint capsule extends down to the trochanteric ridge. To avoid the femoral vessels, the capsule is punctured overlying the outer half of the femoral neck.
A 9-cm long, 20-gauge spinal needle is advanced down to the cortical surface of the femoral neck and intracapsular location is confirmed with the injection of 2 cc of iodinated contrast material. Approximately 10 cc of a very dilute solution of gadolinium in nonbacteriostatic sterile saline (1:150–1:200) then is instilled into the hip. The procedure is done under direct fluoroscopic control to ensure that there is no inadvertent extracapsular injection. The patient then is transferred to the MRI scanner. To optimize detail, a small, phased-array coil is placed directly over the symptomatic hip. Imaging is done in the coronal, sagittal, oblique sagittal, and axial planes (Fig 7). The oblique sagittal plane is of particular value in evaluating the anterior acetabular labrum. This sequence is obtained in an axis approximately parallel to the superior labrum and lateral femoral neck. On fat saturated T1-weighted images, the gadolinium solution will be intensely bright whereas the adjacent soft tissue signal will be suppressed. This heightens the conspicuity of labral tears. Extracapsular fluid collections including noncommunicating bursitis, soft tissue and bone marrow edema, and muscle edema, will not be visible on the T1-weighted fat suppressed images. Therefore, at least one T2-weighted sequence is necessary to identify disorders of bone and extracapsular soft tissue.
On MR arthrography, labral tears manifest as an abnormal, linear extension of high signal intensity gadolinium solution into the labrum, labral blunting or detachment of the labrum from the underlying bone (Figs 8, 9). Paralabral cysts may accompany labral tears. The normal labrum has a uniform very low signal intensity, appearing black on MRI scans. With labral degeneration, there is abnormal intermediate (gray) signal within the labrum, but labral contours are preserved, and there is no imbibing of gadolinium into the substance of the labrum. 14 Tears are identified most frequently in the anterior labrum, but also may be seen in the superior labrum. Lesions of the posterior labrum are much less common.
Normal hyaline cartilage manifests intermediate to bright signal on MRI scans, although typically less intense than that of gadolinium solution. The articular cartilage should be evaluated carefully on all imaging planes. The gadolinium solution will outline the articular margins, defining any cartilage defects (Fig 10). Other features of osteoarthritis, such as subchondral cysts also will be shown. Osteochondral bodies are represented by low signal intensity filling defects within the gadolinium solution (Fig 11).
In the setting of acetabular dysplasia, labral degeneration and tears are frequent. The anterior labrum, superior labrum, or both may be elongated. Early degenerative joint disease may be observed with articular cartilage loss and subchondral cyst formation.
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Joseph C. McCarthy, MD—Guest Editor© 2003 Lippincott Williams & Wilkins, Inc.