MR imaging of the hip and pelvis: Current concepts

One of the well-known indications for MR imaging of the hip is for detection of vascular insults to the femoral head, the most common of which is avascular necrosis (AVN). In the early stages of this process, bone scans and radiographs may be normal.¹ The MR appearance of AVN includes diffuse bone marrow edema, which becomes more focal in the femoral head with a central area of fatty marrow surrounded by a serpiginous line of low signal between the 10 o’clock and 2 o’clock positions (Figure 1).² Less commonly, the “double-line sign” of a low signal area surrounding an inner line of high signal on T2-weighted images is seen. The stages of AVN visible on conventional radiography do not parallel those seen on MR imaging.³ MR is recommended when AVN is suspected clinically and the conventional radiographs are normal, and in patients with known AVN in one hip to assess for contralateral involvement, which is not an infrequent occurrence. Treatment includes core decompression, rotational osteotomy, and free vascularized fibular grafting.

The current etiologic theory of idiopathic transient osteoporosis of the hip (ITOH) is that it is caused by a vascular insult.⁴ This abnormality has also been called transient painful bone marrow edema. Although more common in middle-aged men, it also can occur in pregnant women. Patients present with severe pain but no history of trauma.⁵ Conventional radiographs may show osteopenia, while MR imaging shows low T1 signal and high T2 signal in the femoral head extending to the inter-trochanteric region (Figure 2). There is often an associated moderate joint effusion. This pattern is nonspecific and can mimic early AVN and infection but resolves spontaneously, generally in 6 to 8 months, thus allowing distinction between AVN and ITOH. A follow-up MR is occasionally necessary to make the diagnosis.^6-8 This process can also migrate to other joints. Treatment is protective weight-bearing.

Stress fractures and insufficiency fractures have been reported in all bones around the pelvis. Often, the initial radiographs are normal and, therefore, MR imaging is strongly indicated if there is suspicion of fracture, as it is sensitive and specific for identifying stress fractures. These fractures should be diagnosed in a timely fashion, especially when the femoral neck is involved, as patients should be protected to prevent progression to complete fracture, which can occur with continued unlimited weight-bearing. In such cases, MR imaging shows a linear low signal fracture line on T1- and T2-weighted images with associated surrounding edema (Figure 3).⁹

Sacral stress fractures have recently been described in long-distance runners. The patient’s symptoms mimic that of disk disease, for which they are often erroneously treated. Findings on MR examination show low signal on T1-weighted images and high T2 signal paralleling the sacroiliac joint (Figure 4). The treatment is rest with no further running until symptoms have resolved.¹⁰

Insufficiency fractures are most common in patients with osteoporosis and those who have undergone radiation therapy. These fractures can be radiographically occult. Since the patient’s bones are osteopenic, even incidental trauma can result in a fracture. Therefore, lack of history of trauma should not dissuade physicians from considering fracture as a possible explanation for hip pain. Similarly, since weight-bearing is sometimes tolerated by the patient when the fracture is nondisplaced, a history of “weight-bearing” can be misleading. Bone scintigraphy was formerly the imaging modality of choice to diagnose radiographically occult insufficiency fractures, with delayed imaging often necessary in these osteopenic patients. With MR imaging, the fracture line is readily visible as linear low signal on T1- and T2-weighted images with surrounding edema (Figure 5).

The most common locations for insufficiency fractures of the pelvis are subcapital, intertrochanteric, sacral, supra-acetabular, and pubic bones; often multiple fractures co-exist. Supra-acetabular insufficiency fractures are diagnosed by recognition of a curvilinear low signal fracture line that parallels the acetabular roof with adjacent edema, while sacral insufficiency fractures show the low signal fracture line paralleling the sacroiliac joint (Figures 6 and 7).¹¹ Most commonly, the adjacent edema in sacral fractures does not cross the midline unless the fracture is bilateral. This can mimic metastatic disease when edema without a distinct fracture line is present. A search for surrounding soft-tissue extension and bone destruction supports a pathologic process.

Salter-Harris fractures can occur at the hip and are most common in adolescence. Slipped capital femoral epiphysis (SCFE) affects males more often than females and has a higher incidence in overweight children. Radiographs may be equivocal and MR can be utilized to show the widened physis with increased signal on T2-weighted images as well as the medial and posterior displacement of the femoral head. Early AVN,which affects 15% of children with this process, can also be detected and can be a result of the injury to the physis during the Salter injury or as a result of the treatment.

Herniation pit is a commonly seen aperture in the cortex of the femoral neck along its anterior-lateral surface. It is the result of ingrowth of fibrocartilaginous elements through a perforation in the cortex. Herniation pits can change in size due to mechanical forces and can be unilateral or bilateral, but are asymptomatic. MR shows a well-defined focus of low signal on T1-weighted images and increased signal on T2-weighted images in the typical anterolateral location.¹²

Osteochondral defects of the femoral head are yet another entity that can cause hip pain in the setting of normal plain radiographs. These are more common in athletes and there may or may not be a history of a specific traumatic event. MR imaging shows a wedge-shaped signal abnormality along the medial femoral head from the 10 o’clock to 11 o’clock positions (Figures 15 and 16). This area is smaller than that seen for AVN, and does not show serpiginous low signal as is seen with AVN. Several of these patients have also been shown to have delamination injuries at surgery.²⁰

The more well-recognized etiologies of hip pain, such as vascular abnormalities and fractures, are often radiographically occult. Other potential sources of hip pain, such as labral pathology, muscle and tendon injuries, inflamed bursa, and osteochondral lesions, are diagnosed exclusively with MR imaging. Therefore, MR imaging is the modality of choice for evaluating pathology in and around the hip and pelvis. A patient with hip pain and negative plain films should undergo MR imaging to assess the source of pain, as the lesions presented are radiographically occult and management of the patient can be affected.  AR