Laboratory Studies
Routine laboratory studies are of little value in the evaluation of femoral head AVN other than to rule out other conditions that may cause hip pain (eg, rheumatoid arthritis). (See also the Medscape Reference articles Rheumatoid Arthritis and Juvenile Idiopathic Arthritis.)
Hematologic studies may reveal sickle cell disease, if clinically suspected. (See also the Medscape Reference article Sickle Cell Anemia.)
Subtle coagulation disturbances (eg, hypofibrinolysis, thrombophilia) are frequent findings, but the significant cost and limited availability of the sophisticated coagulation tests that are necessary for these diagnoses argue against routine screening.
Imaging Studies
Plain radiographs
Obtain anteroposterior and frog-leg lateral views of both hips. The high incidence of bilaterality (>60%) and occult disease in cases of femoral head AVN warrant imaging of the unaffected leg.
Early radiographic findings include femoral head lucency and subchondral sclerosis.
With disease progression, subchondral collapse (ie, crescent sign) and femoral head flattening become evident radiographically. Joint space narrowing is the end result of untreated femoral head AVN.
Radiographic staging of AVN was first proposed by Ficat and Arlet in the 1960s and later amended in the 1970s. [12] This 4-stage system delineates the natural history of AVN from normal radiographs (stage I) to cystic changes and sclerosis (stage II), to subchondral collapse or femoral head flattening (stage III), and finally to joint space narrowing (stage IV). However, this system does not differentiate among certain phases in disease progression (eg, subchondral vs femoral head collapse), nor does it quantify the size and extent of the lesion.
Steinberg proposed the following staging system, known as the Steinberg Classification System, which is concise and delineates the progression and extent of AVN involvement more accurately. [13, 14] This staging system has gained increasing acceptance in the orthopedic community. The stages are as follows:
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Stage I – Normal radiographs; abnormal MRI or bone scan
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Stage II – Abnormal lucency or sclerotic site in femoral head
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Stage III – Subchondral collapse (ie, crescent sign) without flattening of femoral head
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Stage IV – Flattening of the femoral head; normal joint space
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Stage V – Joint space narrowing, acetabular changes, or both
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Stage VI – Advanced degenerative changes
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Stages I-IV are further subdivided according to the percentage of femoral head involvement: A (< 15%), B (15-30%), or C (>30%).
MRI
MRI is the study of choice in patients who demonstrate signs and symptoms that are suggestive of AVN but whose radiographs are normal.
MRI is the most sensitive and specific means of diagnosing AVN. MRI may detect disease as early as 5 days subsequent to an ischemic insult.
Characteristic MRI findings for AVN of the hip include a low signal intensity band (seen on T1 and T2 images) that demarcates a necrotic anterosuperior femoral head segment. The extent and location of femoral head necrosis on MRI have been studied as predictors of femoral head collapse. Smaller lesions (less than one fourth the diameter of the femoral head) and more medial lesions (away from primary weight-bearing areas) predict a better outcome. [14]
Bone scanning
Abnormalities may show up on a bone scan before they do on plain radiographs. Bone scan findings should be supplemented with MRI findings.
The presence of a photopenic area that is surrounded by increased tracer uptake is the typical scintigraphic picture for radionuclide imaging.
Bone scans are considerably less sensitive and less specific than MRI, but the images may be useful if the use of MRI is contraindicated.
Computed tomography (CT) scanning
CT scans confer significant radiation exposure to the patient and are less sensitive than MRI in diagnosing AVN.
CT scanning may help delineate early subchondral collapse because the resolution of bony architecture with this modality is unsurpassed.
Angiography is an invasive mean of diagnostic confirmation of AVN; it is most useful as an investigational modality.
Procedures
Biopsy, angiography, and measuring bone marrow pressure are invasive measures of confirming the diagnosis of AVN, but these procedures are most useful as investigational modalities.
