Guidelines Summary

Radiographic findings are unremarkable in early stages of AVN. Nevertheless, the American College of Radiology (ACR) considers radiographs of the pelvis and hips the most appropriate initial imaging study in patients at risk for AVN who present with hip pain.If radiographs are normal or show femoral head lucencies suspicious for osteonecrosis, magnetic resonance imaging (MRI) of the hips without contrast is most appropriate.^[5]

The ACR advises that MRI is the most sensitive and specific imaging modality for diagnosis and provides optimal evaluation of the likelihood of articular collapse. Involvement of greater than 30-50% of the femoral head, often in the sagittal plane, indicates significantly increased risk of articular collapse.

Additional ACR recommendations include the following^[5]:

Contrast-enhanced MRI may be needed to detect early osteonecrosis of the hip in pediatric patients, which is indicated by hypoperfusion
In patients with a contraindication for MRI, alternative imaging modalities are computed tomography (CT) or bone scintigraphy with single-photon emission CT (SPECT)

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Media Gallery

Axial computed tomography scan in a patient without avascular necrosis of the femoral head shows prominent and thickened but normal trabeculae (arrow) within the femoral head. Note the delicate, sclerotic, raylike branchings emanating in a radial fashion from the central dense band. This is the asterisk sign. See also the next image.
Coronal multiplanar reconstructed computed tomography image in a patient without avascular necrosis of the femoral head. Thickened trabeculae are seen at the medial aspect of the junction of the femoral head and neck and extend to the subchondral region. The physis (arrowhead) is a transverse white line located between the lateral and medial aspect of the femoral head.
Coronal T1 MRI of the pelvis and hips in a patient without avascular necrosis of the femoral head. A thin low-signal line, representing the long cortex, surrounds the femur and iliac lines. The fovea is a small indentation along the medial aspect of the femoral head and is the site of penetration of the artery of the ligamentum teres into the femur. High signal is present within the medullary space of the proximal femur, representing normal fatty marrow. The physis is a thin line of low signal extending from the lateral to the medial aspect of the femoral head.
Avascular necrosis of the femoral head. Coronal T1-weighted magnetic resonance image in a patient showing hypointense signal within the proximal femoral neck and intertrochanteric regions (arrows) representing hematopoietic marrow. Increased signal is present within the greater trochanters and femoral capital epiphysis representing normal fatty marrow. See also the next image.
Avascular necrosis of the femoral head. Coronal T2-weighted magnetic resonance image in a patient showing hypointense signal within the proximal femoral neck and intertrochanteric regions, representing hematopoietic marrow. Increased signal is present within the greater trochanters (single arrowhead) and femoral capital epiphysis (stars), representing normal fatty marrow. The normal hip joints (double arrowheads) contain a small amount of fluid.
Avascular necrosis of the femoral head. Illustration of the normal circulation of the femoral head, viewed from the posterior approach. The posterior-superior retinacular arteries provide the major blood supply to the epiphysis. They traverse the femoral neck and are contained within the joint capsule and give rise to the lateral epiphyseal vessels at the junction of the femoral head and neck. From there, they penetrate the femur and supply the femoral epiphysis. A. = artery.
Avascular necrosis of the femoral head. Illustration demonstrating that the blood supply to the femoral head is compromised by subcapital femoral fractures or slipped capital femoral epiphysis. As the epiphysis or femoral neck separates from the femoral head, the femoral metaphysis displaces superolaterally and the femur rotates externally. This causes the distal posterior-superior retinacular arteries and proximal lateral epiphyseal vessels to kink or rotate, compromising the blood flow to the epiphysis. If this condition were to persist, the femoral head would be at high risk for developing avascular necrosis.
Avascular necrosis of the femoral head. Anteroposterior view of the pelvis shows flattening of the outer portion of the right femoral head from avascular necrosis (arrow), with adjacent joint-space narrowing, juxta-articular sclerosis, and osteophytes representing degenerative joint disease. See also the next image.
Frogleg lateral view of the right hip in a patient with avascular necrosis shows the crescent sign, indicating subchondral fracture. Therapeutic interventions are less likely to halt progression of the disease once this sign appears. The frogleg lateral view is better than anteroposterior (AP) projection for demonstrating this sign, because the anterior and posterior margins of the acetabulum on the AP projection are superimposed over the superior portion of the femoral head, the usual location of the sign.
Coronal fat-saturated T2-weighted magnetic resonance image in a patient with avascular necrosis shows increased signal within the femoral head and neck (arrow), representing edema. Short tau inversion recovery (STIR) and fat-saturated imaging are excellent for demonstrating this finding. See also the next image.
Axial fat-saturated T2-weighted magnetic resonance image in a patient with avascular necrosis demonstrates a joint effusion (arrow) and the extensive abnormal increased signal representing edema within the femoral head. A fracture in the anterior aspect of the femoral head is present. See also the next image.
Coronal T1-weighted magnetic resonance image (MRI) in a patient with avascular necrosis shows decreased signal within the femoral head (arrow), representing edema. This is an MRI class C lesion. The subchondral abnormality can also indicate a fracture. Multiplanar reconstructed computed tomography scanning would be helpful for further investigation.
Axial computed tomographh scan of a patient with avascular necrosis of the femoral head shows clumping and distortion of the central trabeculae representing the asterisk sign (arrowhead) and an adjacent low-density region (arrow) representing the reparative zone. See also the next image.
Axial computed tomography (CT) scan of a patient with avascular necrosis of the femoral head shows a fracture to the anterior aspect of the femoral head. This finding was demonstrated on an axial fat-saturated T2-weighted magnetic resonance image, but it is delineated more clearly using CT scanning because of the superior resolution of CT scanning. See also the next image.
Axial computed tomography scan in a patient with avascular necrosis. This image shows joint space narrowing, juxta-articular sclerosis, and osteophyte formation (degenerative joint disease) around the anteromedial and posterolateral aspects of the right hip.
Bone scan of a patient with avascular necrosis of the femoral head shows increased uptake in the superolateral aspect of the right femoral head, indicative of avascular necrosis but providing little information concerning the structural integrity of the hip.
Anteroposterior view of the left hip in a patient with avascular necrosis demonstrates alternating regions of the sclerosis and lucency within the superior aspect of the left femoral head (arrows), representing the reparative stage (stage 2) of avascular necrosis. The lucent areas represent the site of resorption of necrotic marrow and trabecular. The sclerosis represents apposition of new bone on dead trabeculae.
Coronal T1-weighted magnetic resonance image of the pelvis in a patient with avascular necrosis of the left femoral head shows a mixed pattern of abnormal signal within the femoral head. The medial aspect consists predominantly of high signal representing blood. The superior portion demonstrated low signal, representing edema. In this patient, the dominant signal is edema.
Coronal T1-weighted magnetic resonance image (MRI) of the pelvis in a patient with avascular necrosis of the femoral head shows increased signal within the superior aspect of the femoral head, representing fat. This is an MRI class 1 hip. These patients may have more than one type of signal within the abnormal avascular area. In this situation, the most dominant signal is used for MRI classification.
Anteroposterior view of the left hip in a patient with avascular necrosis obtained 6 months after presentation shows that the patient has undergone core decompression but has developed mild flattening of the femoral head, indicating progression of disease despite treatment.
Anteroposterior view of the pelvis in a patient with bilateral avascular necrosis of the femoral head. Mild flattening to the superior aspect of the right femoral head (open arrow) indicates stage 3 disease. The left femoral head has a normal contour, indicating stage 2 disease. The black arrows indicate the margins of the reparative zone, representing new bone formation on dead trabeculae. When avascular necrosis is bilateral, it usually occurs in each hip at different times, and the staging of disease in each hip can be, and often is, at different stages. See also the next image.
Coronal T1-weighted magnetic resonance image of the pelvis in a patient with bilateral avascular necrosis of the femoral head demonstrates decreased signal within the right femoral head, representing fluid, and increased signal within the left femoral head, representing blood. Differences in signal intensity between the femoral heads in patients with bilateral avascular necrosis are common and reflect the fact that, although prone to bilateral disease, onset in each hip occurs separately and progresses at a different rate. See also the next image.
Coronal T2-weighted magnetic resonance image of the pelvis in a patient with bilateral avascular necrosis of the femoral head shows decreased signal within the right femoral head, representing fluid, and increased signal within the left femoral head, representing blood. The right hip has a class C lesion, and the left hip has a class B lesion.
Planar bone scan of the pelvis in a patient with bilateral avascular necrosis of the femoral head shows marked increased uptake of radiopharmaceutical agent in both hips.
Plain film finding in a patient with bilateral avascular necrosis of the femoral head 6 months after presentation demonstrates that subtle flattening of the left femoral head (open arrow) has occurred. This indicates progression of disease from stage 2 to stage 3 despite conservative treatment. The patient underwent bilateral core decompression and bone grafting. The defects within the proximal femurs represent removal of the dead bone, and the tubular densities within each femur represent bone grafts in an attempt to revascularize the region of avascular necrosis.
Plain film findings in a patient with bilateral avascular necrosis of the femoral head who underwent bilateral core decompression and bone grafting. Despite treatment, an anteroposterior plain film of the pelvis obtained 6 months later shows further flattening of the right femoral head (black arrows). Note the progressive increase in the size of the lucency within the right femoral head, which resulted in further weakening of the femoral head. This lucency represents removal of dead bone. Once fracture of the femoral head occurs, operative attempts to stabilize the femur usually fail. Disease usually progresses rapidly, requiring total joint replacement.
Coronal T2-weighted magnetic resonance image in a patient with avascular necrosis of the femoral head shows the double-line sign within the left femoral head, which is pathognomonic for avascular necrosis. The inner margin of the border of the abnormality is believed to represent a chemical shift artifact.
Anteroposterior plain film of the pelvis in a patient with avascular necrosis of the femoral head shows asymmetric joint-space narrowing (arrow), juxta-articular sclerosis, and subchondral cyst formation around the right hip secondary to degenerative joint disease. This may be confused with avascular necrosis, but the intimate localization of all of the bindings and the absence of femoral head collapse makes a diagnosis of avascular necrosis highly unlikely.
A coronal T2-weighted magnetic resonance image of a patient who complained of a short onset of right hip pain. The study is normal. See also the next image.
A coronal T1-weighted magnetic resonance image of a patient who complained of a short onset of right hip pain. There is now diffuse low signal involving the lateral aspect of the right femoral head. See also the next image.
A coronal FS T2-weighted magnetic resonance image (MRI) taken 2 years later from a patient who complained of a short onset of right hip pain. There is diffuse marrow edema (bright white signal) in the right femoral neck and head. There is also a focus of decreased (dark) signal in the superomedial aspect of the right femoral head, probably related to fibrous tissue. There is now a well-defined focus of dark signal indicating avascular necrosis of the left femoral head (arrow). There is no double line sign, which is often the case on FS T2-weighted MRIs. A frequency selective pulse was not used.
An anteroposterior pelvic radiograph of a patient who complained of a short onset of right hip pain showing flattening of the superolateral aspect (the weightbearing portion) of the right femoral head. There is a crescent-shaped density (arrow) representing the femoral head surface on an underlying zone of decreased density underneath the crescent-shaped bone representing demineralization. Such findings indicate advanced disease and are associated with poor outcomes.