Unknown. Most common in post menopausal women with an autoimmune disorder such as diabetes or thyroid problem. i.e. not me but I got it anyway.

You "recover" when your body stops recognizing the soft tissue around your shoulder as the enemy and it has a chance to heal. For most it takes 9-18 months (per shoulder) and stops when it does regardless of physical therapy. The one thing that seems to help is a cortisone shot _early_ which seems to knock about three months off the the disease's course. Shots later do not have the same effect.

Logically it is an auto immune response, to what I have no clue. Starts like any strain you don't quite remember how you got and just keeps getting worse.

If you get it in one shoulder you are very likely to get it in the other. But you are more likely to recognize it early and get that cortisone shot.

In my case, the recovery required physical therapy. My shoulders have also improved somewhat on their own over time. From what I’ve read, it’s not genetic. My shoulder injuries were each caused by a specific physical event, and then gradual loss of range occurred along with increasing pain.