I have a 6 yr old mare with a stifle joint hellp!!?

True rose (Rosie) is a beautiful blue roan mare she is so friendly and we love her to bits she has had a foal in the recent past about 2yrs ago. My mum bought her for me and my sister but didn't realise she was in foal she kept bucking us off and we couldn't understand why so we sent her in for some training at the riding school and she is not bombproof on the road so we tried to straighten that out first. Along came a tractor and Rosie frightened to death, jumped backwards into the dike and came out with a stifle joint she had the foal and we didn't realise about her leg until it kept clicking in and out we got the vet a leg specialist we tried magnets in-fa red the treadmill thing the swimming acupuncture and all sorts nothing worked. It obviously is annoying for her but it doesn't hurt we can't ride her anymore due to this leg but we want to do inhand with her as she is so pretty and moves beautifully when her leg is clicked in but what can we do to stop it clicking out help! we don't want her to go to waste or to sell her do you have any ideas