A client vetted a horse recently, a delightful horse, exactly what we wanted. Unfortunately, the horse had really unfortunate X-rays, showing lots of damage to multiple joints. The horse is in his mid-teens, has been showing at FEI for years, rides great, and has never had a joint injection in his life.
My vet said something I'd never thought of before: while there are lots of reasons to inject a joint, one of them is that in a joint with a normal quantity of lubricating fluid, that fluid takes the beating of training. Without that fluid, the structures in the joint are the ones taking the hits. And the cartilage that covers the bones wears down, causing (among other things) arthritis and skunky x-rays.
I'm not saying that injections completely prevent arthritis, of course, nor am I saying we should inject every joint willy-nilly. I did not go to vet school, and as such I put my trust in some extremely wonderful and competent veterinarians to help guide my decisions. But because I did not go to vet school, I take my upper-level horses to see said vet every six months, whether I'm feeling an issue or not.
My rule-of-thumb, which is based on nothing remotely scientific, is that once a horse is working on Third Level stuff, they get added to the list. Every six months, I load the trailer up and head off to Virginia Equine Imaging, home of Dr. Kent Allen, former USET vet and head of the USEF Drugs & Medications Committee, and his merry (wo)men. It's really important to me to use an experienced sports medicine practice, vets who focus on athletic performance.
Each horse is examined on a hard surface, on a straight line and on a specially-designed lunging circle, and then flexed. We also take regular balance films, to keep an eye on their farrier care, because any good farrier will tell you that the outside of a foot can sometimes belie the inside.
Anything that shows up in the exam as abnormal, we keep exploring. Sometimes we'll take action on something, sometimes we won't, and will just keep an eye on things.
I do this for a bunch of reasons. I like having extensive documentation of what "normal" looks like, because inevitably, no horse is perfect. One of my horses has a chip in a stifle that's been there, totally benign, for as long as I've known that horse. Another is super snatchy with the hind legs during flexions, as if it's sore, but it's been that way for its whole career with me. The third has weird lumps at the top of the cannon bone, under the hocks. Been there for as long as I've known that one, too. By bringing them to a credible veterinarian many times over their careers, I have countless records, should I ever decide to sell a horse, or if something scary does pop up.
I also like preventing problems before they start. A mild-to-moderate flexion might be six months away from a problem I feel under saddle; catching it early lets me adjust my management and training plans accordingly.
VEI also has excellent snacks in their waiting room. This is important stuff.
Mostly, though,I want a longstanding relationship with a top class team of vets, and I want them on my side. When things go wrong - and it's a when, not an if, like when your top horse gets to take a little vacation because the jerk is so food aggressive he can't stop kicking the bloody wall during feeding time, to the point where he's strained something. Delightful. - the team in my corner is incredible. It takes a village, and my village has an awful lot of PhDs.