Spontaneous Osteonecrosis of the Knee (SONK)

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What is Spontaneous Osteonecrosis?

Spontaneous osteonecrosis of the knee used to be felt to be due to an idiopathic cause, which meant we just did not know what caused it. It was generally found in middle-age women and often there would be rapid development of osteoarthritis of the medial femoral condyle, often with an insufficiency fracture. In the past, many of these patients went on to need total knee replacements within months to a few years.

What Causes Spontaneous Osteonecrosis?

We now know that almost all cases of spontaneous osteonecrosis of the knee are due to previously unrecognized medial meniscal root tears.  In these circumstances, often with a squatting-type position, such as when cleaning floors, doing gardening, or with deep flexion with sporting activities such as skiing, the meniscus root would tear off its attachment in the back of the knee. In many patients, the meniscus root tear would then proceed to slip out of the joint, which we call extruding, and that basically removes the main cushion to prevent arthritis on the inside part of the knee. It has been found biomechanically that when one does have a meniscus root tear, that it is equivalent to having all your medial meniscus taken out.

How to Treat Spontaneous Osteonecrosis

The treatment in patients who have fairly normal cartilage surfaces still remaining and who have spontaneous osteonecrosis of the knee is to repair the meniscus root tear if possible. In these circumstances, restoring the cushion to the knee, getting the meniscus root tear to heal, and the period of nonweightbearing on crutches which is required for the root tear to heal, almost always results in the healing of the spontaneous osteonecrosis of the knee, even when there was an insufficiency fracture.

Thus, it is currently felt that most cases of spontaneous osteonecrosis of the knee that are caught early enough can be treated.  No longer do patients need to require a partial or total knee arthroplasty because the diagnosis is in doubt. Most cases are due to a posterior horn medial meniscus root tear with extrusion and early diagnosis and treatment is recommended to slow down the progression of arthritis in these patients.

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