Hips and Pelvis

Osteonecrosis (Avascular Necrosis) ( Box 14.2 )

One of the major indications for MRI of the hip is for detection of osteonecrosis of the femoral head. The presumed mechanism of mechanical failure of the femoral head in AVN is accumulated stress fractures of necrotic trabeculae that are not repaired. MRI is capable of detecting the early stages of ischemic necrosis, which is important clinically so that therapy can be instituted before the onset of femoral head collapse, fragmentation, degenerative change, and hip replacement. Early diagnosis can lead to joint-sparing techniques, such as core decompression, rotational osteotomy, or free vascularized fibular graft.

Numerous causes of AVN have been cited, including trauma, steroids (anabolic and catabolic), hemoglobinopathies, alcoholism, pancreatitis, Gaucher’s disease, and radiation. AVN is considered idiopathic when no cause can be identified.

Radiographs are relatively insensitive for detecting AVN, especially in the early stages of the disease. MRI is much more sensitive for its detection and is also more sensitive than computed tomography (CT) or radionuclide bone scintigraphy. Additionally, MRI can provide useful information regarding articular cartilage, marrow conversion, joint fluid, and associated insufficiency fractures, which also are common in patients taking catabolic steroids.

AVN is bilateral in 40% of hips, so both hips should be imaged simultaneously for this occurrence. The radiographic classification scheme for AVN cannot be used for MRI. Proposed classification utilizing a combination of plain radiographs, MRI, and clinical features includes Ficat and Arlet and the Steinberg staging systems.

The Ficat and Arlet system includes four stages (0 normal):

MRI is most useful when plain films are negative. The Steinberg classification differs from other classification systems by quantifying the involvement of the femoral head. There are seven stages of involvement. Stage IV quantifies the flattening of the femoral head (A, < 2 mm, mild; B, 2-4 mm, moderate; C, > 4 mm, severe).

The appearance of AVN on MRI is variable. A diffuse pattern of bone marrow edema may be observed very early in the condition with bone marrow edema potentially extending from the subchondral bone of the femoral head through the intertrochanteric region (similar in appearance to transient bone marrow edema), subsequently becoming more focal in the femoral head; a serpiginous border of low signal intensity surrounding an area of fatty or more heterogeneous marrow in the femoral head between the 10-o’clock and 2-o’clock positions on coronal images (usually in the anterosuperior quadrant of the femoral head) is characteristic for AVN ( Figs. 14.2 and 14.3 ). This is the typical geographic pattern of AVN that is seen most often. Collapse of bone and sclerosis as noted on conventional radiography may result in a focal area of low signal on T1W images that demonstrates variable signal on T2W images—in other words, not a fatty center. Low T1 and low T2 signal corresponds to necrosis, and low T1 and increased T2 signal can be seen with evolving necrosis and the presence of granulation tissue.

Important additional findings to include in the report are the volume of the head that is involved with AVN and the amount of articular collapse, as well as any evidence of associated degenerative disease such as joint space narrowing and osteophytes. The presence and size of a joint effusion often correspond to the severity of clinical symptoms and should also be commented on in the report.

Misinterpretation of osteonecrosis may occur if one is unfamiliar with a few potential pitfalls ( Box 14.3 ). Occasionally, normal hematopoietic marrow can be identified in the subchondral femoral head; however, this typically lacks the classic, irregular low signal intensity border seen with AVN. A synovial herniation pit, which represents a tiny defect in the bone that allows joint fluid to fill this space, can erroneously be interpreted as an area of AVN ( Fig. 14.4 ). The fovea centralis, a normal anatomic finding, inadvertently may be diagnosed as a site of AVN with subchondral collapse ( Fig. 14.5 ). Knowledge of the presence of the fovea and its location will avoid this mistake. Subchondral cysts from degenerative joint disease of the hip may appear similar to AVN, but the low signal margins of the cyst are smooth and regular rather than serpiginous, as with AVN. Also, abnormalities on both sides of the joint usually are present, with evidence of degenerative cysts, cartilage loss, and osteophytes. Metastases occasionally may have an appearance similar to AVN that is diffusely low signal on T1W images. However, the appearance on T2W images may help with the identification of a more masslike lesion.

Legg-Calvé-Perthes disease refers to idiopathic AVN of the growing femoral epiphysis that results in a progressive deformity and outward displacement of the femoral head; it occurs in children 4 to 10 years old ( Box 14.4 ). MRI has been reported to be useful for assessing Legg-Calvé-Perthes disease ( Fig. 14.6 ). Diffuse low signal in the femoral head on T1W and T2W images is the most common finding. Collapse of the femoral epiphysis also can occur. Jaramillo and associates showed that physeal bridging and deformity of the femoral head with abnormal low signal in the head on MRI were statistically significant predictors of growth arrest. Abnormalities of the epiphysis alone did not relate to growth arrest. Depiction of physeal bridging at MRI was the best predictor of growth arrest in their study. MRI allows earlier diagnosis and diagnosis of coexistent contralateral disease and monitoring of therapy.

Idiopathic Transient Osteoporosis of the Hip (Transient Painful Bone Marrow Edema) ( Box 14.5 )

Idiopathic transient osteoporosis of the hip (ITOH) also has been referred to as transient painful bone marrow edema . The first well-documented description was reported in three women during the last trimester of pregnancy. ITOH is an uncommon and usually self-limited clinical entity of unknown cause that often affects middle-aged men (40-55 years old), though it may be seen in women and at other ages. The male-to-female ratio is 3:1. Although the underlying cause is uncertain, a vascular basis almost certainly exists. As mentioned, when women are affected, it is usually during the third trimester of pregnancy. There are anecdotal reports of ITOH occurring in early pregnancy with resolving hip pain after spontaneous or therapeutic abortions. This condition is rare in children, and there are no known predisposing factors except pregnancy. Generally, only one hip is affected at a time, though it may occur bilaterally. Recurrence in the same hip can occur. This entity is known as regional migratory osteoporosis when it migrates to other joints.

Clinically, patients present with disabling pain without a history of trauma. Conventional radiography may show osteopenia isolated to the affected hip but often appears normal. ITOH generally resolves spontaneously in 6 to 8 months after protected weight bearing and symptomatic support. Insufficiency fractures of the hip may occur without protected weight bearing because of the severity of the localized osteoporosis. MRI in ITOH shows diffusely decreased signal on T1W images and increased signal on T2W images extending from the femoral head to the intertrochanteric region ( Fig. 14.7 ). These findings have been attributed to bone marrow edema and have been reported within 48 hours after the onset of symptoms of ITOH. The signal intensity in the acetabulum is normal.

Because early AVN can have a similar appearance on imaging and the treatments are different, it is important to differentiate ITOH from AVN. There is some controversy as to whether ITOH represents a very early, reversible stage of AVN. VandeBerg and colleagues developed criteria to increase sensitivity and specificity of irreversible changes of ITOH that would lead to a diagnosis of AVN. The presence of a low signal subchondral area measuring more than 4 mm in thickness and greater than 12.5 mm in length on T2W images or postcontrast T1W images suggests an irreversible lesion (AVN).

An intra-articular osteoid osteoma of the hip can cause marrow edema in a similar distribution as ITOH, and a cortically based, small, round lesion should be carefully searched for to exclude this; however, different age groups generally are affected by these two disease processes. Infection of the proximal femur with a septic joint also should be considered in the differential diagnosis when bone marrow is identified in the presence of a joint effusion. Clinical history is important in making the distinction.

Fatigue Fractures

These fractures occur commonly around the hips and in the pelvis. Fatigue fractures result from increased or abnormal stress applied to normal bone. MRI is superb for evaluating this abnormality and may be positive when conventional radiography is negative. Bone scintigraphy is sensitive but not specific for stress fractures or stress reactions. MRI is sensitive and specific because it can show the linear, low signal fracture line surrounded by bone marrow edema or, as in the instance of a stress reaction, the presence of bone marrow edema without a visible fracture line ( Figs. 14.8-14.10 ). Because of the exquisite contrast and intrinsic spatial resolution of MRI, this abnormality can be diagnosed early, leading to treatment sooner rather than later.

In the femoral neck, stress fractures most commonly occur along the medial aspect (compressive surface) but may involve its superior lateral aspect (tensile surface). Given the potential for displacement due to the tensile forces in this region, these lateral femoral neck stress fractures are considered “high risk.” The early diagnosis of any femoral neck stress fracture can thwart progression of the fracture and the need for surgical intervention.

Additional locations of stress fractures in the pelvis include the sacrum, supra-acetabular region, and the pubic rami (superior and inferior). Sacral stress fractures are most often seen in athletes, especially runners. Patients often present with low back pain, and a lumbar spine MRI is ordered. The sacrum should be inspected for this abnormality (T1W and fat-suppressed images) when examining a lumbar spine MRI, particularly in athletic individuals ( Fig. 14.11 ). The diagnosis can be overlooked because the bone marrow edema may not be conspicuous on non–fat-suppressed T2W images

Insufficiency Fractures

Insufficiency fractures occur from normal stresses applied to abnormal bone weakened by osteoporosis, radiation therapy, or other factors. In the clinical setting of a painful hip in an osteoporotic patient (even with no significant history of trauma, and despite the fact that the patient may be able to bear weight), a radiographically occult fracture may exist. MRI is the fastest, most cost-effective, and most sensitive and specific method for making the diagnosis in this setting. A limited MRI examination consisting of coronal and axial images of the entire pelvis using T1W and fat-suppressed T2W sequences can be done to evaluate for this entity. Any marrow sequence would suffice but because the fracture lines may be more conspicuous on T1W images in some cases and on fat-suppressed, fluid-sensitive sequences in others, both are recommended for screening.

MRI shows linear low signal with surrounding bone marrow edema on T1W images ( Fig. 14.12 ). The fracture remains low signal on T2W images, but surrounding bone marrow edema is high in signal intensity. Images of the entire pelvis with larger field of view are recommended because hip pain may be referred from abnormalities outside of the hip, such as sacral insufficiency fractures and pubic rami fractures. Multiple fractures often coexist. The most common locations for insufficiency fractures around the pelvis are subcapital, intertrochanteric, sacral, supra-acetabular, pubic bones, and superior or inferior pubic rami. In the setting of what appears to be a nondisplaced greater trochanter fracture on radiographs, MRI should be recommended because it has been demonstrated that the greater trochanteric fracture is often associated with radiographically occult extension of the fracture into the intertrochanteric region.

A sacral insufficiency fracture has a pathognomonic appearance on MRI. T1W images show linear low signal (representing the fracture), usually paralleling the sacroiliac joint with surrounding bone marrow edema. Generally, the bone marrow edema is confined to the sacral ala with the fracture and does not extend across the midline, unless the fracture is bilateral. If the linear component of the fracture is not evident, the bone marrow edema of the sacral marrow may mimic metastatic disease. Masslike encroachment upon sacral foramina and expansion of the sacrum are features that help distinguish a sacral tumor from the bone marrow edema associated with an insufficiency fracture.

Supra-acetabular insufficiency fractures are reliably diagnosed with MRI by noting a curvilinear (eyebrow-shaped) low signal fracture line that parallels the roof of the acetabulum, accompanied by surrounding bone marrow edema (low signal on T1W images that becomes high signal on T2W images) ( Fig. 14.13 ). These fractures are seen in the same patient population susceptible to sacral insufficiency fractures (i.e., patients with osteoporosis and especially patients who had previous pelvic radiation, which significantly weakens the bone). The presence of a curvilinear, low signal fracture within the area of bone marrow edema allows for accurate differentiation of an insufficiency fracture from the edema related to a tumor, which may also demonstrate an associated soft tissue mass and destruction of the acetabulum.

Salter Fractures ( Box 14.7 )

Traumatic epiphyseal slip (Salter Harris type I) can occur in skeletally immature individuals as a result of birth trauma or accidental or nonaccidental trauma. If the femoral head ossification center is not mineralized, conventional radiography may suggest developmental dysplasia of the hip (DDH) (due to lateral displacement of the femoral shaft). A T2W sequence, particularly fast spin echo with fat suppression or (fast) STIR imaging, shows edema and hemorrhage through the physis and is diagnostic of a shear injury seen in type I fractures. Physeal widening can also be identified more readily on MRI than it can on conventional radiography.

Slipped capital femoral epiphysis occurs predominantly in adolescents, typically in boys 10 to 17 years old and in girls 8 to 15 years old. Boys generally are more frequently affected than girls, and this entity is more common in blacks than in whites. The incidence is especially high in overweight children. Proposed causes include adolescent growth spurt, hormonal influences, increased weight, and increased activity, all of which result in repetitive stresses, resulting in a Salter type I fracture of the proximal femoral growth plate.

If conventional radiography is equivocal, MRI can be used to assess the relationship of the femoral head and neck. MRI shows a widened growth plate with abnormal high signal on the T2W image through the growth plate and medial and posteriorly located femoral epiphysis with respect to the metaphysis. On the coronal T1W image, the growth plate appears wider than normal, as evidenced by increased width of the low signal physis ( Fig. 14.14 ). These findings can aid the surgeon with operative planning. An additional important advantage of MRI is its ability to identify early osteonecrosis, which may be present in 15% of children with this entity due to associated disruption of the epiphyseal blood supply.

Benign Osseous Lesions ( Box 14.8 )

The hip is not a unique site for any particular bone tumor. More commonly encountered lesions with their characteristic appearance will be mentioned in this section. An enchondroma is a benign lesion that can be found incidentally. As in other long bones, it is well defined, lobular in contour, and low to intermediate in signal intensity on T1W images (depending on how much calcification is present) and high in signal intensity on T2W images, with curvilinear and punctate low signal foci representing calcified components ( Fig. 14.16 ). There is no associated bone marrow edema.

Another lesion that can be seen in the epiphysis, trochanters (apophysis), or flat bones of the pelvis is a giant cell tumor. The appearance on conventional radiographs is usually characteristic. MRI is helpful if conventional imaging is not diagnostic. Giant cell tumor is low signal on T1W images and intermediate on T2W images. This tumor usually does not get very high in signal on T2W images. MRI is also useful for defining the local extent of the lesion and can show areas of cortical destruction and any extraosseous soft tissue component. The presence of hemosiderin seen as low signal foci on T1W and T2W images within the lesion is pathognomonic.

In children and young adults, a chondroblastoma may be encountered around the hip. As with giant cell tumor, chondroblastoma also has a predilection for the epiphysis or apophysis. MRI shows a round lesion with low signal on the T1W image; the T2W image also shows low to intermediate signal intensity, generally with a large area of surrounding marrow edema that may cross to the metaphysis. Calcified chondroid matrix may be identified as punctate areas of low signal within the lesion, but is better appreciated on CT or conventional radiography than with MRI. Periosteal elevation also can be seen occasionally with this lesion. It is not unusual to have a small soft tissue component with this benign lesion. The bone marrow edema associated with this primary lesion of bone may be very pronounced and cross into the metaphysis of the bone ( Fig. 14.17 )

Occasionally, a subchondral cyst (low signal intensity on T1W images that becomes high in signal [fluid-like] on T2W images) in the femoral head or acetabulum can become very large, mimicking an aggressive lesion or AVN. Subchondral cysts may enlarge over time and are seen in entities such as osteoarthrosis, rheumatoid arthritis, and in association with AVN. A search for associated abnormalities, such as cartilage loss with joint space narrowing, osteophyte formation (osteoarthrosis), pannus formation (rheumatoid arthritis), or curvilinear subchondral low signal (AVN) can help confirm the diagnosis of a subchondral cyst.

Malignant Osseous Lesions

No primary malignant tumor of bone is site specific for the hip or pelvis. Chondrosarcoma occurs more commonly in this location than do other malignant tumors of bone. As mentioned in Chapter 7 , it can be difficult sometimes to differentiate an enchondroma from a chondrosarcoma. A soft tissue mass, bone marrow edema, and periosteal reaction are helpful signs that suggest a chondrosarcoma. Metastatic lesions and myeloma (plasmacytoma) are the most common malignant entities that affect the hip and pelvis and are discussed in detail in Chapter 2 .