Oh dear. So sorry to hear she is getting worse.
Our beloved horse, Neopolitano Alea VI-2 ( AKA Storm) is now 21 and his stifle problem didnt begin until he was in his teens due to weakness in his hind quarters from very bad riding prior to us buying him aged 12 and then his hocks becoming arthritic making it impossible to do much retraining for two years.
He still has an odd time when his stifle gets close to catching, and he goes slightly unlevel at the back but he can usually walk this off if he is ridden well and he hasnt locked up completely for a very long time now.
He is noticeably more prone to it catching when the muscles are tired, but as his fitness has improved his muscles take longer to tire and we can now walk, trot and canter him most days working him in lateral work quite hard in the school most days. he also hacks out and is great fun - (completely mad) as a hack across bridleways.
He still isnt completely reliable however even so, and I cannot risk entering him to compete in any dressage tests as he might not be perfectly sound at the exact right moment for the test. I chose not to have him operated on having discussed the various options and outcomes with my vet.
I dont pretend to be an expert on this, but this is a list of what we have been doing for Storm, maybe it will be helpful:
*Using all of the exercises in Straightness Training in the Work in Hand sequence - taught and then worked in the following order, Quality circles with good LFS - (Lateral bending, Forwards and down, Stepping under), Straight with Bending, Shoulder fore, Shoulder In, Quarters In, Renvers, Half Pass and finally Working Pirouette - all first in walk and then in trot with softness, balance and suppleness as the core elements and concentrating on the essences of each movements rather than just doing the movements.
*Always keeping him searching into the bridle for a good forwards- down neck and head at first, (yes a bit on the forehand, but a price worth paying) and now he is moving more to forwards forwards and a little more collection, but its hard to collect this horse without him hollowing (he is a pure bred Lipizzaner and he can do funny swan shaped neck things and hollows his back downwards all too easily).
*If he drops his back and doesnt use his hind quarters stepping under properly, his stifle will slip almost immediately. If a rider who does not have supple hips rides him he goes lame almost immediately too.
*We have used raised poles to step him over in a walk, which was especially useful if his stifles were making him slightly lame as I found this stepping action he had ot make with his hind legs helped recover him from an incident very well, but dont need them usually now.
*We have asked him to quietly back up - but the head must be lowered - no hollow backs allowed.
*We found small hills to walk him up and down. Excitement is an issue with hacking as his head raises and his back dros when he is excited, also his stifles when they stuck made him unable to move so that was difficult to handle out hacking and I had to gallop home on my horse Fox, leaving Storm with Rosie and get the trailer to rescue him once or twice.
*We make sure he has lots of variations in direction - dont stick to either the left or the right bending for too long, make changes of direction very smooth and ensure there are plenty of variations in his paces and within the paces from more forwards down to more collected.
*He always has to have a very long walk warm up every time he is ridden or worked in hand - and that is true for him even now.
*A warm rug really helps his muscles remain functional and a thick woolly exercise rug - (even though he is not clipped) in cold weather helps him for sure. If he is cold he is more likely to have an incident. A heated rug by Blazewear is something we have used on him and one of our other horses too. Very effective. Ideally I think Storm would like to live under some infra red lights (!) but sadly we dont have those!
*We have a fantastic deep muscle massage therapist who did a degree in this for human massage then studied with Mary Bromily then apprenticed with a Mary Bromily ex pupil and she is absolutely fabulous and visits when we think he needs help. Not all massage therapists are the same.
*He has Turmeric in his diet, but only since this summer, so not sure how effective this might be yet.
*He has no pain killers and never has had any as the pain is only present when the stifle catches and not all the time.
*He is out for 20 hours a day normally - but in bad weather/wet times of the year this is reduced as our fields are clay.
*He is NOT ALLOWED to lift up his head and make a bulge come under his neck and drop his back while he is being ridden or worked in hand. We bought him with a mahoussive under neck, but it has been reduced a lot by our hard work to change his habitual balancing techniques. He still wants to default back to his natural tendency or learned behaviour to raise his neck and drop his back sometimes.
*I dont like gadgets, but I did use a cheap De-gogue a few years ago on him, with it clipped back to itself and not taken through the bit. I just wanted it to work like a height limiter to stop him raising his head above a certain height, but I didnt want to draw in his head as a draw rein would as I knew the raised head action always caused him to always drop his back and shortening his neck would also tend to do that too. He always had an incident of a catching stifle when he dropped his back either due to a shortened neck or a raised neck or usually both Keeping his head and neck stretched out and held forwards and down (without rollkur of course so not btv noses) put him on his forehand - yes - but it kept his back arched upwards more. For that reason also, the work in hand was better for him as the downwards pressure of a rider is removed.
This is an example of the work In Hand Sequence I have been using for Storm and in this clip is is ony being shown in a walk, demonstrated here by the lovely Sunny Boy.
I am no expert on stifle slipping/catching, but I actually think in this case for our horse, this work has probably saved Storms life.