If it’s a stifle issue a lot of the time it won’t show up on X-rays. They need to do more of an ultra sound on the joint.
My older gelding had a stifle injury a few years ago(before I got him). The vet said it was as if a football player got tackled in the knee.
The biggest thing if it is a stifle is the proper rehab. So many people don’t rehab their horses properly. Because mine wasn’t rehabbed properly he can no longer trot for long periods of time or canter for more than a few steps, he can’t do small circles either and no more dressage.
They recommend hill work because it will strengthen that joint up.
I’ve never heard of stifle injections, only hock injections. Maybe get a second opinion and even have a body adjuster come out, maybe that will help too.
Good luck. I know how frustrating things can be when we can’t figure out how to help our horses.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."