There are two major forms of ON, post-traumatic and non-traumatic.  Minor trauma is not believed to cause ON.  Even major injury does not often result in ON.  Certain kinds of fracture, where the blood vessels to part of the bone have been physically damaged, may result in ON.

Non-traumatic ON has been associated with a wide variety of diseases including gout, lupus, sickle cell disease, kidney or liver disease, and clotting disorders.  In addition, high dosage steroid (cortisone) use is sometimes associated with ON, as well as high alcohol consumpution.  Finally, as many as 30% of all patients with osteonecrosis are otherwise completely healthy with no associated risk factor.  This is called "idiopathic," a medical term meaning "of unknown cause."

WHO'S AT RISK?
If a person is completely healthy, the risk of getting osteonecrosis is quite small, probably less than one in 100,000.  Another way to understand this is that most of the people who get ON probably have an underlying health problem.  Children, as young as 4 and extending to the teens, get a form of ON which is called Legg-Calve-Perthe's disease (Perthe's for short) after the doctors who first described it.  Most patients are between 30 and 50 with an average age of 38.  Patients over the age of 50 are likely to have developed ON either by a fracture of the hip or more rarely in  association with disease of the major blood vessels to the lower leg.  Although the specific cause of the bone death is not precisely known except in the case of fracture, a number of conditions have been associated with ON.  The most common includes a history of high dose steroid treatment for some medical condition (including Lupus, Chronic lung disease, an organ transplant, etc).  Low dose steroids (cortisone, prednisone, etc) commonly used for bee stings, poison ivy and acute allergies are not thought to cause ON.  The next most common associated condition is a history of alcohol intake.  The higher the intake the higher the risk.

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