Knee Osteonecrosis Workup

Updated: Jan 23, 2018
  • Author: Amar Rajadhyaksha, MD; Chief Editor: Thomas M DeBerardino, MD  more...
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Workup

Laboratory Studies

Lab tests are limited for osteonecrosis. Some tests that may be helpful include the following:

  • Sickle-cell screening, especially in African Americans

  • Lipid profile

  • Screening for coagulopathies (eg, protein S and protein C deficiencies, factor V Leiden disease)

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Imaging Studies

Plain radiographs

  • Always obtain anteroposterior (AP), lateral, and tunnel view plain radiographs when entertaining the diagnosis of osteonecrosis. See image below.

    Plain radiograph of a knee affected by osteonecros Plain radiograph of a knee affected by osteonecrosis; note the cystic and sclerotic lesions in a serpiginous pattern.
  • Initially, no abnormalities may be seen, but as the disease progresses, flattening of the weightbearing portion may occur.

  • A radiolucent area forms in the subchondral bone, surrounded by a rim of sclerosis. Later in the disease course, the subchondral bone collapses, leading to secondary arthritic change and, possibly, a valgus or varus deformity.

  • In spontaneous osteonecrosis of the knee (SPONK), these lesions usually are seen in the medial femoral condyle, whereas in secondary osteonecrosis, both the medial and lateral sides may be affected. [13]

MRI

  • MRI can depict osteonecrosis before it is visible on plain radiographs. See image below.

    MRI confirming diagnosis of osteonecrosis; bilater MRI confirming diagnosis of osteonecrosis; bilaterality suggests secondary osteonecrosis.
  • MRI can also reveal the extent of disease more precisely than plain radiographs. [14]

  • In SPONK, lesions are isolated to a single condyle (usually medial) or plateau.

  • In SPONK, on T1-weighted images, osteonecrosis is seen as a discrete area of low signal intensity, replacing the high-intensity signal normally produced by marrow fat.

  • The T2-weighted image shows an area of low signal intensity surrounded by a high-intensity signal caused by edema.

  • The T1- and T2-weighted images in secondary osteonecrosis are similar to those in SPONK but are larger, are more serpiginous, and may be multifocal. Lesions usually are seen in the epiphyseal region of the distal femur or proximal tibia.

Technetium-99m scans

  • Bone scans have been used to diagnose SPONK and may show osteonecrotic lesions before plain radiographs do.

  • In SPONK, bone scans usually show a localized area of radioisotope uptake in the medial femoral condyle.

  • Bone scans are less effective for diagnosing secondary osteonecrosis than for diagnosing SPONK.

  • Studies have reported that bone scans provide a correct diagnosis of secondary osteonecrosis in only 40-70% of cases. [15]

  • Because secondary osteonecrosis commonly presents bilaterally, bilateral symmetric uptake may be read incorrectly as degenerative changes or as a negative study.

  • Bone scans generally are unreliable. Therefore, plain radiographs remain the initial imaging study of choice, with MRI for confirmation.

Because specificity and sensitivity of MRI are 98% in osteonecrosis, MRI is the diagnostic study of choice.

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Other Tests

The following methods are outdated and are mentioned for the sake of completeness. Core biopsy, however, can be useful for pathologic diagnosis of osteonecrosis of the knee.

  • Venography

  • Interosseous pressure measurements

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Diagnostic Procedures

Core biopsy may be useful for pathologic diagnosis of osteonecrosis of the knee.

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Histologic Findings

Macroscopic pathology

Early in osteonecrosis of the knee, there may be slight discoloration and flattening of the articular cartilage. As the disease progresses, a line of demarcation becomes evident, and an osteochondral flap overlies the area of osteonecrosis. Late in the disease course, secondary arthritic changes occur, leading to a cartilage defect filled with necrotic debris and to signs of osteoarthritis (eg, osteophyte formation, eburnated bone). [16]

Microscopic pathology

On microscopic examination, the osteonecrotic bone shows empty lacunae and fatty degeneration within the center of the lesion. The surrounding area shows evidence of osseous healing, including osteoblastic activity, fibrovascular granulation tissue, and cartilage formation.

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Staging

Aglietti devised the following classification system for spontaneous osteonecrosis of the knee (SPONK), which was a modification of an earlier classification by Koshino [17] :

  • Stage I: Plain radiograph findings are normal. Diagnosis must be made from MRI or bone scan.

  • Stage II: Radiographs show flattening of the weightbearing portion of the condyle

  • Stage III: Radiographs show a radiolucent area surrounded by sclerosis

  • Stage IV: Radiographs show a more defined ring of sclerosis and subchondral bone collapse forming a calcified plate, sequestrum, or fragment

  • Stage V: Narrowing of the joint space, osteophyte formation, and/or femoral and tibial subchondral sclerosis is shown

For secondary osteonecrosis, Mont and Hungerford developed the following staging system, which is a modification of the Ficat and Arlet staging of osteonecrosis of the hip [18, 19, 20, 21, 22, 23, 24] :

  • Stage I - Plain radiographs reveal no change, but MRI scan findings are positive

  • Stage II - Radiographs reveal cystic and sclerotic changes in the distal femur and/or proximal tibia

  • Stage III - Subchondral collapse is seen as the crescent sign

  • Stage IV - Evidence of degenerative changes is present on both sides of the joint (eg, joint space narrowing, osteophytes); at this stage, it may be difficult to distinguish osteonecrosis from osteoarthritis of the knee on plain radiographs

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