Approach Considerations
No laboratory test findings specifically suggest or confirm the presence of avascular necrosis (AVN). Plain radiographic findings are unremarkable in early stages of AVN. Nevertheless, the American College of Radiology (ACR) considers x-ray the most appropriate initial imaging study in patients at risk for AVN. This includes chest, pelvis, hip, femur, knee, tibia/fibula, ankle, foot, shoulder, humerus, elbow, forearm, wrist and hand. [27] If radiographs are normal or show findings suspicious of osteonecrosis, magnetic resonance imaging (MRI) without contrast is most appropriate. [27]
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. The ACR found insufficient medical literature to conclude if patients would benefit from CT without IV contrast but it may be appropriate. [27]
For more information, see Imaging in Avascular Necrosis of the Femoral Head.
Histology is the criterion standard for diagnosis of AVN. However, bone biopsy is not routinely performed because of the availability of sensitive noninvasive tests such as MRI.
Histologic Findings
Histology is the criterion standard for diagnosis of AVN, although it is usually unnecessary. The histologic specimen is usually obtained during surgery, although it is occasionally obtained during diagnostic bone biopsy. Histologic changes are observed in both cortical bone and bone marrow.
In the early stage, interstitial edema and plasmostasis is confined to the bone marrow. As the severity progresses, osteocytes continue to disappear and necrotic tissue fills the medullary spaces. New living bone is laminated onto dead trabeculae with partial resorption of dead bone. In the subchondral trabeculae, bone resorption exceeds formation leading to the net removal of bone, loss of structural integrity of trabeculae, subchondral fracture, and joint incongruity. [8]
Staging
Several different staging systems have been developed and continue to be used. Ficat initially developed an AVN staging system based on radiologic findings. This staging system was revised after the widespread use of MRI in the workup of AVN. The staging system presented in the below table is based on the consensus of the Subcommittee of Nomenclature of the International Association on Bone Circulation and Bone Necrosis (ARCO: Association of Research Circulation Osseous). [28] The most important consideration is collapse of the femoral head cortex. Repair and complete recovery may be possible prior to collapse. Afterward, the collapse is irreversible.
Staging of Avascular Necrosis (Open Table in a new window)
Stage |
Clinical and Laboratory Findings |
Stage 0 |
Patient is asymptomatic. Radiography findings are normal. Histology findings demonstrate osteonecrosis. |
Stage I |
Patient may or may not be symptomatic. Radiography and CT scan findings are unremarkable. AVN is considered likely based on MRI and bone scan results (may be subclassified by extent of involvement [see below]). Histology findings are abnormal. |
Stage II |
Patient is symptomatic. Plain radiography findings are abnormal and include osteopenia, osteosclerosis, or cysts. Subchondral radiolucency is absent. MRI findings are diagnostic. |
Stage III |
Patient is symptomatic. Radiographic findings include subchondral lucency (crescent sign) and subchondral collapse. Shape of the femoral head is generally preserved on radiographs and CT scans. Subclassification depends on the extent of crescent, as follows:
|
Stage IV |
Flattening or collapse of femoral head is present. Joint space may be irregular. CT scanning is more sensitive than radiography. Subclassification depends on the extent of collapsed surface, as follows:
|
Stage V |
Radiography findings include narrowing of the joint space, osteoarthritis with sclerosis of acetabulum, and marginal osteophytes. |
Stage VI |
Findings include extensive destruction of the femoral head and joint. |
Radiography
Plain radiographic findings are unremarkable in early stages of AVN. Nevertheless, the American College of Radiology considers x-ray of the pelvis the most appropriate initial imaging study in patients at risk for AVN who present with hip pain.Both an anteroposterior view of the pelvis and a frog-leg lateral view of the hip are necessary, as articular collapse or cortical depression may be seen on only one of those projections. [29]
In children, the earliest radiographic findings of AVN include the following [29]
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Smaller ossific nucleus
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Increased radiodensity
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Subchondral fracture
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Metaphyseal radiolucencies.
Subsequentl findings in pediatric patients include fragmentation, resorption, reossification, and remodeling of the femoral head and neck. [29]
In mild-to-moderate AVN, radiographs demonstrate sclerosis and changes in bone density. In advanced disease, bone deformities, such as flattening, subchondral radiolucent lines (crescent sign), and collapse of the femoral head, are evident (see images below).
Avascular necrosis of the shoulder showing subchondral radiolucent lines (crescent sign).
Magnetic Resonance Imaging
MRI is the most sensitive and specific imaging procedure for AVN, of the hip with an overall sensitivity that exceeds 90%. The specificity of MRI is also very high. The use of gadolinium is particularly useful in early detection.
MRI findings of AVN include decreased signal intensity in the subchondral region on both T1- and T2-weighted images, suggesting edema (water signal) in early disease. This relatively nonspecific finding is often localized in the medial aspect of the femoral head. This abnormality is observed in 96% of cases.
The next stage is characterized by a reparative process (reactive zone) and shows low signal intensity on T1-weighted scans and high signal intensity on T2-weighted scans. This finding is diagnostic for AVN (see images below).
Avascular necrosis of both femoral heads. This T1-weighted image shows decreased signal intensity in both femoral heads.
MRI of the distal femur and proximal tibia. This T2-weighted image shows increased signal intensity in the marrow.
Advanced AVN is characterized by deformity of the articular surface and by calcification, which are also easily detected with radiography and CT scanning.
Radionuclide Bone Scan
In early AVN, osteoblastic activity and blood flow are increased; thus, the sensitivity of radionuclide bone scan is better than that of plain films at this stage.
The central area of decreased uptake is surrounded by an area of increased uptake. This phenomenon is known as the doughnut sign and indicates the reactive zone surrounding the necrotic area.
Limitations of bone scan include the following:
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In early AVN, bone scan is less sensitive than MRI
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Findings are nonspecific
-
Results are difficult to interpret if disease is bilateral; in unilateral disease, the healthy side can be used for comparison
Single-photon emission CT scanning
SPECT scanning provides images of the radioactivity within the target organ in 3 dimensions. With this modality, overlying and underlying areas of radioactivity may be separated into sequential tomographic planes, thus providing increased image contrast and improved lesion detection and localization, as compared with planar scintigraphy. SPECT scanning is used as an alternative to MRI when MRI cannot be performed or when the results of MRI are indeterminate. [30]
A meta-analysis of seven studies of SPECT in patients with avascular necrosis (AVN) of the femoral head reported a pooled sensitivity and specificity of 94% (95% confidence interval of 87-97%) and 75% (95% confidence interval of 68-81%) respectively. [31]
Computed Tomography
Computed tomography (CT) is not commonly used for assessment of osteonecrosis in pediatric patients. In adults, CT is used principally to provide information for surgical planning, by determining the severity and location of articular collapse and providing evidence of early secondary degenerative joint disease. [29]
CT scans show sclerosis in the central part of femoral head as an asterisk sign. Changes in the anterior part of the femoral head are easily observed
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Avascular necrosis in the femoral head resulting from corticosteroid therapy.
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Avascular necrosis of the shoulder showing subchondral radiolucent lines (crescent sign).
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Avascular necrosis of both femoral heads. This T1-weighted image shows decreased signal intensity in both femoral heads.
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MRI of the distal femur and proximal tibia. This T2-weighted image shows increased signal intensity in the marrow.
Tables
Stage |
Clinical and Laboratory Findings |
Stage 0 |
Patient is asymptomatic. Radiography findings are normal. Histology findings demonstrate osteonecrosis. |
Stage I |
Patient may or may not be symptomatic. Radiography and CT scan findings are unremarkable. AVN is considered likely based on MRI and bone scan results (may be subclassified by extent of involvement [see below]). Histology findings are abnormal. |
Stage II |
Patient is symptomatic. Plain radiography findings are abnormal and include osteopenia, osteosclerosis, or cysts. Subchondral radiolucency is absent. MRI findings are diagnostic. |
Stage III |
Patient is symptomatic. Radiographic findings include subchondral lucency (crescent sign) and subchondral collapse. Shape of the femoral head is generally preserved on radiographs and CT scans. Subclassification depends on the extent of crescent, as follows:
|
Stage IV |
Flattening or collapse of femoral head is present. Joint space may be irregular. CT scanning is more sensitive than radiography. Subclassification depends on the extent of collapsed surface, as follows:
|
Stage V |
Radiography findings include narrowing of the joint space, osteoarthritis with sclerosis of acetabulum, and marginal osteophytes. |
Stage VI |
Findings include extensive destruction of the femoral head and joint. |
