Spontaneous osteonecrosis of the knee (SONK) is a relatively uncommon condition. It occurs when a part of the bone in the knee dies, usually for unknown reasons.
This can cause severe pain in the knee, often with no changes on X-rays in the early stages. A Magnetic Resonance Image (MRI) will usually show up the problem in the early stage.
Later in the disease the dead section of bone can collapse. An injection of a special type of drug called a bisphosphonate can sometimes be helpful in preventing this bone collapse if the problem is picked up early enough.
Once the bone has started to collapse knee replacement surgery is usually needed.