Avascular necrosis is when bone breaks down because of a lack of blood supply. The most common place this happens is your thigh bone, in particular the ball shaped head of it that connects to the hip.
The insides of our bones are filled with honeycomb like structure that gives them strength. However, if they don’t receive enough oxygen and nutrients from a good blood supply they can breakdown. Deep-sea divers can experience this as a result of pressure building up in there bones. Other common causes are injury and alcohol abuse or in your case, long-term use of corticosteroids to help treat arthritis. Let me reassure you though, if bone destruction has occurred there are surgical options available to help restore it. If the damage is too great, then joint replacement may be needed.
But if caught earlier enough, before the bone breaks down too much, the condition can be improved with pain killers and bed rest. Gentle, regular exercise, and keeping a healthy body weight will also help.