What Causes Avascular Necrosis On The Hip?

I have avascular necrosis of the right hip finally diagnosed Jan./10....doctors cannot tell me how I got it. I have been taking Symbicort 200 Turdo inhaler as needed before I exercise for about 3 years. I am also on Atacand Plus 16mg/12.5mg once a day and Amlodipine Besylate 5mg. once a day for blood pressure. Could any of these meds cause or worsen the necrosis. I am taking Arthrotec 75mg. twice a day for pain. What can I do to improve my bone health and maybe get off the anti-inflammatories?
Thank you.