Got him down to my vet, and a bunch of nerve blocks and xrays later, he's got a bone spur on a small joint within his hock, that accounts for approximately 5% of the horse's movement. The joint is so tiny that there is no margin for error, so when something goes wrong, it makes the horse rather sore! Joint fluid has all but leaked out of the joint, so now it is bone on bone thus his showing arthritic symptoms.
He is now on Pentosan injections once a week for a month, which act as anti-inflammatories and a joint guard, as well as being on joint supplements. All we can do for him is hope that the joint will now fuse, which will relieve the pain he's getting and allow him to continue a sound, functional life as a competitive dressage horse. If the joint does not fuse in the next 6 months, it will be in his best interest to be retired to a paddock ornament or light pleasure horse.
In the meantime, light work is the go, and also stimulates new bone tissue to form and help the joint fuse. So it is strictly walk and trot work on straight lines and nothing under a 20m circle for no more than one circle at a time. No cantering, no laterals other than a few steps of shallow leg yield at a time, and not bringing him up to where I had him before his spell, beginning very light collection. This has to continue for another few months yet.
Now to the point of my post. He has been very stiff to the left for some time now, and now that this injury has come up, and after consulting my vet, it is evident as to WHY he's so stiff on the left rein. The injured hock is his right hock, hence his compensating and stiffening the left side of his body to take more weight than the right. His left side is now effectively 'locked'.
Usually, with a stiff horse, I would use a lot of lateral work, transitions, changes of rein etc. But I'm stuck for solutions here as I cannot do any of these things. Transitions have to be very gradual and having him sit down on his hocks into a downwards, or push off firmly in an upward is too much pressure on the joint at this point in time.
At the moment, I am flexing him left and right for a few strides each. To the right he is lovely, I can put him anywhere and he softens to the rein immediately. To the left however, I just cannot un lock his neck. He will turn his body for an hour if I let him, rather than unlock that neck of his. Obviously, riding him in a tight turn is not going to help either of our problems! I'm at my wits end here, I've always been so heavily reliant on laterals to supple a horse, and now I can't use them!