Over the past 10 years from the age of 50 I have had the spontaneous, transient osteonecrosis in both knees and ankles. It is transient in the sense that the outbreaks last about a year or so and it is spontaneous in the sense that the problems come on spontaneously (though a minor ligament injury in the knee or ankle is the prelude). The symptoms are that it is very painful to walk; there is no treatment apart from minimising walking. MRI scanning is the main tool used in diagnosis.
Osteonecrosis and avascualar necrosis (AVN) appear to be labels for related diseases.
Not much appears to be known about the diseases. I would be interested in hearing from anyone else who has had spontaneous osteonecrosis.
The other thing that might be of interest is that Oxford University's NDORMS MSK Research department is running an epidemiological study called Rudy that is looking into rare bone diseases including Vasculitis, XLH, AVN. The epidemiological study will provide case studies for their research. It appears that they might be planning to do research into osteonecrosis if they can find enough case studies (one of the reasons for this posting).
The link to the study can be found here: research.ndorms.ox.ac.uk/rudy/
Hope this study proves useful to other people with rare bone diseases.