You might try an equine chiropractor, just to get another opinion and some options. I don't really know of anything else; we had to put our horse down that broke her stifle (this was like 18 years ago or so, before there were many alternative options in the area where I lived). You also might try googling "horse stifle injuries" and see what you run into, but you've probably already done that. I briefly checked it out and a couple of the sites said rest. Good luck and let us know how it turns out.