MODALITIES

AVASCULAR NECROSIS OF THE HIP (Figs. 11-1 through 11-6)

A number of predisposing conditions can result in AVN (the most common are listed in Box 11-1). Often, however, no cause is found (idiopathic is common). Patients present with groin pain, which becomes worse with weight-bearing.

Figure 11-1 Radiographic signs of treatment for avascular necrosis. Core decompression (drilling through the femoral neck into the head) is commonly performed in early stage (i.e., stage 1 or 2) avascular necrosis. Some surgeons prefer to insert a vascularized fibular strut graft; the nutrient vessels of the autograft are connected to the circumflex femoral artery and vein, and blood supply is theoretically restored to the subchondral bone in addition to providing added structural support against collapse. It remains controversial whether treatments affect the ultimate course.

Figure 11-2 Early avascular necrosis on MRI shows diffuse marrow edema.

Figure 11-3 Subacute avascular necrosis on MRI. FSE fat sat, fast spin-echo fat saturation. Consolidation of marrow edema in subchondral bone. Note anterosuperior location, typical of avascular necrosis of the hip.

Figure 11-4 Avascular necrosis: double-line sign on MRI. Zone of hyperintense signal represents granulation tissue at the margin of dead and living bone.

Figure 11-5 Dynamic contrast-enhanced MRI (DCEMRI). Diagnosis of acute avascular necrosis.

Figure 11-6 Avascular necrosis: collapse. When the articular surface collapses, the patient becomes acutely symptomatic. On MRI, the articular surface is flattened. Often a linear hyperintensity represents the fracture line. Marked surrounding edema and moderate joint effusion are present and can suggest a differential diagnosis of early avascular necrosis or transient osteoporosis. OA, osteoarthritis.

In the early stages of AVN, radiographs are negative. MRI will show diffuse marrow edema of the femoral head, possibly with some subchondral crescentic signal. At this stage, the differential diagnosis includes subchondral stress fracture or transient osteoporosis of the hip (TOPH). AVN does not show bone resorption because there is no blood flow. Therefore, to differentiate these entities the radiologist can perform a dynamic contrast-enhanced MRI looking for lack of enhancement (AVN) versus hyperemia (stress fracture, TOPH). Alternatively, noncontrast CT may show relative lucency of the femoral head in TOPH but not in the other conditions. However, care should be taken when comparing with the opposite side in patients who may have AVN bilaterally.

The Ficat staging system is a radiographic staging system, but it is still the most widely used. It has since been modified by the Subcommittee of Nomenclature of the International Association on Bone Circulation and Bone Necrosis; Table 11-1 summarizes the stages. The key is that appearance and progression are similar no matter what bone is affected. In other words, the disease progresses from radiographically occult to an increase in density, to collapse and fragmentation, to osteoarthritis. Treatment is based on the modified Ficat stage. However, in radiologic reports it always better to be descriptive rather than to list the stage because some surgeons may refer to the original Ficat staging, which is numbered differently.

Table 11-1 Modified Ficat Staging System for Avascular Necrosis of the Hip

In later stages, it is easier to document the finding as AVN. In stage 2 in which increased radiographic density is seen, MRI demonstrates a focal subchondral geographic signal abnormality, usually centered anterosuperiorly. The central signal is variable, often high on T1 (trapped/mummified fat) but occasionally bright on T2 or dark on all sequences (fibrotic). The margins demonstrate the classic “double line sign” of Mitchell, representing the interface between living and dead bone with a rim of granulation tissue. Typically, the surrounding marrow edema has resolved. At this point, patients may be asymptomatic.

Patients may go on for many years at the prior stage. When a patient with established AVN presents with acute pain, the suspicion and imaging should be directed toward finding articular collapse, which is often very subtle. On radiographs, a subchondral lucent line may be seen, representing a fracture. Flattening of the normally spherical femoral head indicates collapse. If no radiographic signs of collapse are seen, CT with sagittal and coronal reformats is very useful, especially when using multidetector CT with thin cuts. MRI can also be used, although CT has higher resolution. The advantage of MRI is visualization of the diffuse marrow edema that results from collapse, edema that extends to the intertrochanteric region simulating early AVN. If established AVN is seen in addition to this finding, certainly acute-on-chronic AVN is a possibility, but subtle articular surface collapse should be sought. Like AVN, the collapse is usually anterosuperior. TOPH is in the differential for diffuse femoral head edema, but underlying established AVN (or AVN on the other side) helps exclude this possibility.

Osteoarthritis is the last stage of AVN, but distinction should be made between patients with AVN, collapse and subsequent osteoarthritis and those with incidental AVN, no collapse, and osteoarthritis from some other cause. These patients may be treated differently.

TRANSIENT OSTEOPOROSIS OF THE HIP

Transient osteoporosis of the hip is a painful condition that was first described in patients in the third trimester of pregnancy (Figs. 11-7 through 11-9). Radiographs and CT show asymmetric osteopenia of the femoral head. MRI shows diffuse edema of the femoral head extending to the intertrochanteric region, an appearance that mimics early AVN. However, unlike AVN, TOPH resolves spontaneously. One theory is that TOPH is ischemia that does not go on to frank necrosis; another theory is that TOPH represents a subchondral stress fracture, much like spontaneous osteonecrosis of the knee. Appearance in late pregnancy supports a stress-related etiology. Also supporting a stress origin is the presence of a subchondral crescentic low-signal region in many cases. CT (showing osteopenia) or dynamic contrast-enhanced MRI (showing early contrast enhancement) should be able to differentiate TOPH from AVN.

Figure 11-7 Transient osteoporosis of the hip. Osteopenia of the femoral head is seen on radiographs and CT. This suggests lack of avascular necrosis, since blood flow is necessary to resorb bone. Intense marrow edema is seen on MRI, often with a subchondral line. Proposed etiology is that this actually represents a subchondral stress fracture, like spontaneous osteonecrosis of the knee. If the diagnosis remains unclear, MRI can be performed with dynamic contrast enhancement to differentiate marrow edema related to hyperemia (transient osteoporosis) from ischemia (avascular necrosis).

Figure 11-8 Transient osteoporosis. Follow-up shows improvement.

Figure 11-9 Dynamic contrast-enhanced MRI (DCEMRI) is useful for distinguishing transient osteoporosis of the hip (most likely a stress fracture and should result in hyperemia) from avascular necrosis (which shows a region of nonenhancement).

STRESS FRACTURE (Figs. 11-10 through 11-13)

Stress fracture is divided into two categories: fatigue (normal bone undergoing abnormal stress) and insufficiency (abnormal bone undergoing normal stresses). Stress fracture is a relatively common source of hip pain in certain populations, such as teens or young adults undergoing new physical activity (fatigue) and older patients with radiation therapy to the pelvis or with underlying osteoporosis and alteration of activity (e.g., recent total knee arthroplasty with shifting of weight-bearing to the other side) (insufficiency).

Figure 11-10 Classic stress fracture of the hip in a young patient at the medial aspect of the base of the femoral neck. Fractures at the medial aspect (compressive side) are usually treated conservatively, whereas fractures at the lateral aspect (distractive side) are often treated aggressively with pinning.

Figure 11-11 Stress fracture on MRI. The subchondral density on radiographs corresponds to a low-signal subcortical line on T1 and fluid-sensitive sequences. This represents trabecular callus and is surrounded by marrow edema. This line is very specific for stress fracture, and MRI is the test of choice to exclude other etiologies such as tumor. Follow-up MRI can be useful for tracking therapeutic response, but radiologic findings lag behind clinical improvement.

Figure 11-12 Stress fractures around the hip in young patients often occur at the pubic rami. Whenever a fracture is seen at the pelvic ring, other fractures of the ring should be sought, whether the cause is acute trauma or stress. In older patients, they are seen more frequently at the sacrum and supra-acetabular region; in this older population, they may be mistaken for tumor. The finding of a dark line with surrounding edema is highly specific.

Figure 11-13 Acetabular insufficiency fracture. On T1-weighted images, the fracture and surrounding edema can resemble a malignancy. However, on fluid-sensitive images the low-signal fracture line is seen, set apart by surrounding edema. MRI with in-phase and out-of-phase imaging can be used to document residual marrow fat if the diagnosis remains unclear on routine sequences.

If MRI is ordered for a stress fracture, which is suspected either based on the clinical history or hip pain in a susceptible population, it is recommended to use a large field of view because these injuries often are multiple and bilateral in expected locations around the pelvic ring.

In young patients, common sites for stress fracture around the pelvis include the superior and inferior pubic ramus and the femoral neck. In older patients, the sacral alae and supra-acetabular region are more common.

Stress fractures at the femoral neck tend to be solitary and unilateral. They tend to occur most frequently by far at the base of the medial femoral neck (the compressive aspect, where each step pushes the fracture sides together). Since bony apposition is essential for healing, stress fractures in this location have a good prognosis for healing. If the fracture occurs laterally, it is more unstable (this is the tensile aspect; weight-bearing tends to pull the fracture apart). The latter may be prophylactically treated with pin fixation.

Stress fracture has a characteristic appearance on most modalities and should not be confused with tumor or another pathology. Despite this, sacral stress fracture is often referred for biopsy or more imaging, based on concern over the patient’s age and the presence of a sclerotic lesion.

Radiographs show a sclerotic region, which is linear and extends from the cortex. Sacral fractures are vertically oriented at the ala and often cross the midline, leading to the classic H sign on bone scan. Sacral stress fractures can be nonspecific on radiographs and present with ill-defined sclerosis. CT or MRI is the next test to confirm the diagnosis. Bone scan is useful if the injury is bilateral or if unilateral uptake in one sacral ala is nonspecific. CT shows sclerosis, which, if inspected closely or with reformatted images, shows a linear pattern. In the sacrum, inspection of the foraminal cortices often shows very subtle cortical step-off that can help confirm the diagnosis. MRI shows a subcortical low signal line on T1- and T2-weighted images, representing microcallus, surrounded by marrow edema.

TRAUMA

Occult Fracture (Figs. 11-14 through 11-17)

Hip and/or pelvic fractures may be undetectable (“occult”) on radiographs, even when they are reviewed after learning of the precise location. This situation is especially common in elderly patients who suffer a nondisplaced fracture through osteoporotic bone during a fall. Usually, a more severe injury is suspected clinically because the patient often cannot bear weight on the affected side, and additional imaging is requested. The best modality for definitive diagnosis of occult fracture is MRI. In fact, this exam is considered one of the few “musculoskeletal emergencies” (outside of spine imaging) that would prompt calling in the night technologist. During the day, an abbreviated hip survey exam (refer to protocols) can be performed in approximately 10 minutes, sandwiched between routine exams.

Figure 11-14 Occult fracture. CT (especially multidetector CT) can be very useful for diagnosis of clinically suspected fracture. However, nondisplaced fractures (presenting on CT as a small asymmetric dent or cortical step-off) can still be difficult to detect.

Figure 11-15 Occult fracture of the hip. MRI is generally preferred over CT and can be performed as an abbreviated, rapid protocol. There is a high association of muscle strain and fracture. When edema of the hip adductors, obturators, and/or gluteus muscles is seen in the setting of trauma, careful inspection should be made for underlying fracture.

Figure 11-16 Occult fracture on MRI. The fracture line may be obscured on fluid-sensitive images, since the fracture line and surrounding marrow are both bright. T1-weighted images are essential for visualizing the fracture line (this is the opposite of a stress fracture in which the dark fracture line is best seen on fluid-sensitive images).

Figure 11-17 Occult femoral neck fracture with impaction. Trabecular compression can also result in a low signal line similar to stress or subacute fracture. History and other findings (e.g., adjacent soft tissue edema) point to acute injury.

The survey exam is acquired using a large field of view to detect associated soft tissue injury and contralateral injury to the rigid pelvic ring. A coronal STIR sequence is used to provide maximal fluid conspicuity while limiting artifact from field heterogeneity. Bone marrow edema is easily seen, as are muscle tears and traumatic bursitis. Muscles that commonly tear include the adductors, glutei, and obturator externus and internus. In the acute stage, wherever they occur, fractures can be hard to distinguish from bone bruises on fluid-sensitive sequences (STIR or fat-suppressed T2-weighted imaging) because the fracture line is bright and so is the surrounding marrow. Although this is less of an issue in the pelvis after acute trauma, acquisition of a coronal T1-weighted sequence facilitates visualization of the fracture line itself. Location, whether complete or partial, and associated soft tissue injury should be reported. Just as on trauma radiographs, when one fracture is found, others should be sought.

CT can be used as an alternate modality, but this is less sensitive than MRI, especially for nondisplaced fractures that may have only slight cortical indentation or step-off or impacted femoral neck fractures that can be confused for a healed fracture or ring of osteophytes. Comparison from side to side is useful, as is close inspection of commonly fractured sites (femoral neck, acetabulum, superior/inferior pubic rami, sacral alae) and close inspection of bone adjacent to traumatic soft tissue infiltration.

Nuclear medicine bone scan is too insensitive in the acute phase of occult fracture. A traditional low-cost alternative to advanced imaging, that being conservative therapy with follow-up radiographs, is not an option in this setting because the fracture can displace and become more complicated in addition to pulmonary embolism and other dreaded complications resulting from prolonged immobilization of the elderly.

Avulsion Fracture (Figs. 11-18 through 11-21)

Avulsion fractures are seen with sudden, forceful muscle contraction; adolescents are susceptible to avulsion of the sartorius from the anterior superior iliac spine (ASIS), and the rectus femoris from the anterior inferior iliac spine (AIIS). Hip pain in a young patient should always draw attention to these attachment sites, regardless of the modality. MRI of the hip with a small field of view can miss the ASIS, and if included, near field brightening or artifact can simulate pathology. Again, a large field-of-view STIR sequence is very useful as a survey for this and other pathology. In high-performance adolescent athletes, hamstring avulsion can be seen. Avulsions acquired during youth may heal back to the underlying bone leaving a bony prominence that can be mistaken for an osteochondroma. Often the avulsion remains separate from the native bone, becoming rounded and proliferative over time.

Figure 11-18 Avulsion fracture. Avulsions at the hip are common in adolescent athletes, especially at the anterior superior iliac spine (ASIS; from the sartorius or tensor fascia lata origin, referred to clinically as a “hip pointer”) or the anterior inferior iliac spine (AIIS; rectus femoris origin).

Figure 11-19 Avulsion of the anterior inferior iliac spine—acute and chronic appearance.

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