Missy was quite sound when she came to me this past January. I noticed as we were working together over the next several months that she had some difficulty bending left and was often slightly stiff tracking left when we were warming up at the trot. But she always warmed up out of it and remained sound. This made me think that she probably had some arthritis, most likely in her hocks, which is quite common in horses in their teens - Red also has some hock arthritis and starts out stiff in trot but loosens up as he works. My general rule for this sort of thing is: if the horse improves during a work session and ends up moving sound, then the work is likely helping the horse stay mobile with some degree of arthritis.
But, if the horse either comes out worse off the next day, or doesn't improve during a work session, then it's time for a rethink. Over the past several months, Missy's soundness at the trot was becoming questionable. We'd have a good day, but sometimes she'd come out stiffer the next day. Or she'd trot fine tracking right, but then be somewhat off tracking left, and not warm up out of it. I'd give her a few days off, with some bute, and she'd be fine again for a bit, but would be off again once we started working again.
There was never any swelling or heat nor any signs of footsoreness. It was pretty clear that the issue was somewhere in the right front/left hind pair, but it was hard for me to determine precisely what it was. So it was time to have the vet out for a look, to determine exactly what was going on, so that we'd know what we should be doing or not doing. My vet is really excellent at lameness evaluations. She takes her time and is very systematic and thorough. For example, when she does flex tests, she does all the major joints, not just the one that seems to be the problem.
On the lunge, Missy was slightly off at the trot in both directions on the right front/left hind pair, more so when tracking left. Cantering on the lunge was very difficult for her. On the left lead, she would tend to "bunny hop" behind, bringing both hinds down very close in time, and would counterbend to the outside - to protect the left hind. Tracking right, she wanted to stop cantering or switch leads behind. This was consistent with my experience in the canter work we'd done under saddle.
Flex tests showed soreness in the left hock, some slighter soreness in the right hock, and on palpation she had some soreness in her right neck and shoulder and sternal area - due to compensating for the left hind. (Fortunately, we were able to get the chiropractor out within a few days, and she's a lot less muscle sore.) She was also positive for hock soreness using the Churchill test, which is more specific for hock pain than flex tests. We agreed that hock x-rays to confirm what we suspected were the next thing to do - the left hock was clearly the bigger issue.
For reference, here's an x-ray of a normal hock:
The hock is a complex joint, with many small bones, and is anatomically equivalent to our ankle. Look at the stacked bones in the middle. There are three joints there running from front (left) to back (right) of the hock. From a functional point of view, only the topmost of the three horizontal joints is necessary for the horse to flex the hock joint - the lower two joints are what are known as low-motion joints. These lower joints are the ones most prone to arthritis.
Here's Missy's right hock (the one giving her only a little bit of trouble) - the second view is frontal:
When the vet saw these images, she said "yep, hock arthritis". The right hock's middle joint is basically 100% fused - there's no joint space left and bone has filled in. The lower joint is also pretty much fused as well. In the frontal image (the second one of the two above), the lump on the right overlapping the bottom joint is a bony bridge - a bone spavin - this lump has always been there, which means this joint has probably been fused for quite some time. The shadow on the x-ray indicates some irritation of the bone, due to strain on this hock in compensation for the left one.
And here's the troublesome left hock joint:
Both lower joints on the left hock are well on the way to fusing as well - my vet says they're about 75% fused, with the middle joint further along than the lowest one. The fact that Missy's fairly recently developed obvious soreness means that the arthritic changes are progressing, probably to full fusion in this hock as well.
Many horses have some hock arthritis, but not all horses progress to fusion - many don't. Hocks fusing can be a result of a combination of genetics, conformation and how the horse is used. Fusion can happen quickly, or can take quite a while - even years.
Now this may sound bad, but it's actually good news. If and when the left hock fuses, the pain and inflammation should go away and it's quite likely she'll be completely sound in both her hocks, losing only about 10% of the flexion in the hocks. It's also good news that she's only slightly off and not seriously gimpy (some horses apparently become severely lame during hock fusion) - this means that she can stay in light work so long as she doesn't get worse. Joint injections in the two lower joints are not recommended due to no joint space existing or being so minimal that getting a needle in would be too difficult. The upper of the three joints of both hocks are in good shape, which is good news.
So our objective is to keep her moving but also keep her as comfortable as possible, while waiting for the fusion to progress. Sometimes fusion happens quickly, and sometimes it takes a long time, but it's far enough advanced that it should happen at some point, and her soreness indicates the process is active. My vet will come back in 6 months or so to take another set of x-rays to see how things are doing, and in the meantime Missy stays in full all-day herd turnout - she moves around just fine out there - and we'll do light work. I've been riding her bareback (to reduce excess weight) for about 15 minutes at walk and a few minutes of trotting. We do 5 minutes of "ambling around" walk, and then 10 minutes of marching walk. Then we trot a few lengths of the arena - no turns. We're also doing topical anti-inflammatories (Surpass ointment) on both hocks for a few weeks, and then she'll go back on daily aspirin. If she doesn't seem uncomfortable, we can slowly add a bit more straight-line trotting. I'm also keeping an eye on the height of the inner (medial) edges of her hind hooves and slightly rasping them down if they start to get longer to take some of the pressure off the inside surfaces of her hocks.
And she's also on a diet - she's a hay-eating machine and has gotten a bit heavy lately (no, we won't use the "fat" word . . .).