This is what I found out re: arthritis, and it's relationship with CF. Arthritis is an auto-immune disorder. When the immune system is weakened, arthritis flares. I had arthritis for many years, from 12-20 years old. I do not have flare-up's anymore. What I have now, is residual cartilage loss, and really bad knees.
The pattern that I noticed is, when one has an infection or anything is making the immune system work harder, I got an arthritis flare-up. Because my immune system was low, the arthritis was set in motion. When I keep my immune system up, no arthritis.
My arthritis was, i believed caused by many years of severe depression. I beat up my immune system with depression. But, it also was toppled with lung infections. It was this crazy circle of events.
Because I do not suffer from chronic depression anymore, and learned how to be much healthier, arthritis has gone away.
Anything that challenges the immune system with cause an effect with arthritis, unfortunatley. I have heard people being free of arthritis after seeking a therapist, I don't know if that works for everyone.
I emphasize with your child's pain, I could barely walk countless times. I missed school 'cause it took to long to get to class, hurt to much to sit in class, and couldn't barely walk period.
I very sorry your daughter has arthritis.