I saw a presentation by an OS at an event held by the Arthritis Society in Canada. He said his hospital, a major Canadian one at the front of this research, was looking at why both hips get OA in some people and an others only one hip gets OA. Also research is underway to see why the second hip replacement tends to require revision before the first, even where the second hip was installed years after the first.
In other words, no one knows.
At least one cause should be structure of the hips. I got OA in my left hip first and now my right as well. Dr. de Smet said the x-rays indicate coxu profunda shape of hip.
Had no FAI on left hip though by MRI.
Interestingly, my right hip I diagnosed by symptoms. It started to have pain less than a year after the left. For a year or two after that, I could side split all the way when my right leg was extended to the side. I wounder, was this evidence of FAI?
Anyway, right is pretty much pain free now, so might have righted itself.
I bet OA in both hips is related to bone structure.