If the horse is being ridden gently and slowly, walking and trotting on straight lines and large figures (no cantering, no jumping, no tight turns), the exercise will help keep the horse fit while encouraging the fusion to proceed as quickly as possible. If the horse is being worked hard, asked to canter or gallop or perform small, intricate ring figures or tight turns, the exercise can cause even more damage
Depending on the degree of cartilage degeneration present, your vet may choose to use several different forms of treatment. The most common treatment nowadays involves chondroitin sulfate (ask about Adequan); your vet may also suggest injecting the joints, usually with Hyaluronic acid, sometimes with steroids. Or he may advise against this: Some vets prefer not to do any joint injections at all, because any joint injection is invasive and could result in an infection. If he does suggest injections, this is how they work: The steroids (typically cortisone) injections serve to reduce the inflammation, allowing the horse to move more comfortably. The steroids also simultaneously contribute to the further destruction of cartilage, thus tending to speed up the process of fusion.
I pulled that from a good article I found on it.