Knee Osteonecrosis Workup
- Author: Amar Rajadhyaksha, MD; Chief Editor: Thomas M DeBerardino, MD more...
Lab tests are limited for osteonecrosis. Some tests that may be helpful include the following:
- Sickle-cell screening, especially in African Americans
- Lipid profile
- 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.
- MRI can also reveal the extent of disease more precisely than plain radiographs.
- 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.
- 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.
- 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.
The following methods are outdated and are mentioned for the sake of completeness.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.
- Interosseous pressure measurements
Core biopsy may be useful for pathologic diagnosis of osteonecrosis of the knee.
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).
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.
Aglietti devised the following classification system for spontaneous osteonecrosis of the knee (SPONK), which was a modification of an earlier classification by Koshino :
- 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[14, 15, 16, 17, 18, 19, 20] :
- 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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|Physical Characteristic||SPONK||Secondary Osteonecrosis|
|Age||Typically >55 y||Typically < 55 y|
|Sex (male-to-female ratio)||1:3||1:3|
|Associated risk factors||None||Corticosteroids, alcohol, SLE, sickle cell disease, caisson disease, Gaucher disease, fat emboli, thrombus formation|
|Other joint involvement||Rare||Approximately 75%|
|Laterality||99% unilateral||Approximately 80% bilateral|
|Condylar involvement||One (usually medial femoral condyle or either tibial plateau)||Multiple|
|Location||Epiphyseal to the subchondral surface||Diaphyseal, metaphyseal, epiphyseal|
|Symptoms||Commonly sudden onset of pain and increased pain with weightbearing, stair climbing, and at night||Usually long-standing insidious pain; patient may have symptoms and signs of an underlying disorder, such as SLE|
|Examination||Pain localized to affected area; small synovitis or effusion may occur; ligaments are stable; range of motion may be limited by pain or effusion||Pain is difficult to localize; ligaments are stable; range of motion is grossly intact but may be limited by pain|