Avascular Necrosis Workup
- Author: Jeanne K Tofferi, MD, MPH, FACP; Chief Editor: Herbert S Diamond, MD more...
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 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.
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 :
- 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)
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.
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.
Necrosis of cortical bone is followed by a regenerative process in surrounding tissues. Increased osteoclastic activity occurs and removes necrotic bone and increased osteoblastic activity as a reparative process.
Bone marrow lesions are usually large. Edema, hemorrhage, fibrilloreticulosis, and hypocellularity are present. Adipocytes in marrow are replaced by eosinophilic debris.
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). 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.
Table. Staging of Avascular Necrosis (Open Table in a new window)
|Stage||Clinical and Laboratory Findings|
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.
In children, the earliest radiographic findings of AVN include the following :
- Smaller ossific nucleus
- Increased radiodensity
- Subchondral fracture
- Metaphyseal radiolucencies.
Subsequentl findings in pediatric patients include fragmentation, resorption, reossification, and remodeling of the femoral head and neck.
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).
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).
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:
- In early AVN, bone scan is less sensitive than MRI
- Findings are nonspecific
- Results are difficult to interpret if disease is bilateral; in unilateral disease, the healthy side can be used for comparison
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.
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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|Stage||Clinical and Laboratory Findings|