After much thought, I have decided that the best approach to managing Star's arthritis is to give her an oral supplement, increase her daily MSM (which she was already getting in her hoof supplement), and start her on monthly Adequan injections.
Monthly injections are an off-label use of Adequan; it is actually marketed for a loading dose of 7 injections over the course of a month, or for intrarticular injection (in the specific joint). However, many people have been using Adequan as an intramuscular injection monthly with success, and this is what my vet recommended. The hope is that we can extend the time needed between knee injections.
In the end, I chose the monthly IM injections not only because that was what the vet recommended, but for a few other reasons, as well. The loading dose was skipped because A) Essentially, the knee injections were a loading dose for that joint, B) Star isn't expressing discomfort to the point of needing a systemic loading dose, and C) The loading dose is cost-prohibitive. An IM injection will also have a systemic effect, unlike an IA injection, which will affect only that joint. If Star has significant arthritis in one knee, it is not unlikely that she has issues elsewhere, as well, so choosing something with a more systemic effect seemed like a good idea.
I do believe I made the right choice. I've only ridden Star twice since she received her first Adequan injection. One of those times was prior to starting on her oral joint supplement, and both of those times were post-knee injection. The difference has been amazing. I honestly do not remember the last time my horse felt so good. Even the times I rode her after her knee injections, she didn't feel like this.
My usually-somewhat-fast-and-definitely-strong-and-on-the-forehand-at-the-canter horse can now canter at a reasonable pace, in a snaffle, without getting strong, heavy, on the forehand, or unbalanced. She can use her back end like she's supposed to use it. Her upward transitions to the trot, while still not a prompt as I would like them, are no longer draggy-shuffley-I-don't-really-want-to-trot transitions.
What I had, for years, chalked up to a training issue and some proper foundation work a bit lacking, has probably not been that at all. My horse was uncomfortable, and doing the best she could to do what I asked while minimizing her discomfort.
My horse has heart - a LOT of it. And even if we can't make it to the Grand National this year and bring home another title for it, that is what makes a true champion.