Is Power Yoga Advisable When Suffering From Avascular Necrosis?

I am 27years old, I have Avascular necrosis (right hip), doctor has asked me to reduce wieght. I am thinking that going to power yoga would reduce my wiegth, increase blood flow to the joints , increase in flexibility etc..... the teacher in yoga class is little worried - does yoga make avascular nicrosis even worst?
Can you please let me know if yoga will really hurt and worsten the Avascular nicrosis?