Here's some things I've learned over the years about soundness, and how to hopefully maintain it - please remember that these are just my ideas and opinions, nothing more:
1. It starts with the feet. If you don't have good feet, you've got nothing. The feet affect the legs, which affect the body and back, etc. . . you get the idea. The best feet ultimately come from good breeding - unfortunately in many breeds and lines, good feet are the last thing on anyone's mind. That's where horses with feet like Maisie's, or some QH lines bred for halter with massive bodies and little, itty, bitty, upright feet come from. When you get or breed a horse, consider foot structure and shape and the relationship between foot size and the horse's size and weight - many modern horses have feet too small for their size. All that said, even good feet need proper farrier care and marginal feet can be improved to a certain point by good farrier care. If you're looking to economize on horse care, one of the last places you should skimp is on good, timely farrier care, whether you decide to keep your horse in shoes or barefoot.
2. Next comes the mouth. Horses with dental problems often spend time moving their mouths, heads and necks, or even their whole bodies, to avoid pain. This can even affect horses who are ridden bitless - if a horse has TMJ issues, or cannot chew properly due to dental problems, it will have problems with using its head and neck correctly. There are major ligaments that run from the poll all the way to the tail - so if the head and neck are affected, everything is. Good, regular dental care is very important. That said, any major corrections need to be done gradually and some horses are just plain over-floated. If you use a bit, find one that is comfortable for your horse - horses have differently shaped palates and larger or smaller tongues, all of which affect bitting.
3. Good nutrition is important. A horse that is properly nourished with quality hay - forage always comes first - and feed is a horse that can build and maintain muscle, bone and supporting structures. But a horse that is over-nourished, or fed inappropriate types of feed for its metabolism and level of work, will at best will become fat - putting strain on its body - and at worst can develop serious health and soundness problems, such as laminitis.
4. As much turnout as possible. Horses were made to move. A horse that receives as much turnout as possible, preferably with other horses, will stay sounder and will be much healthier mentally. Horses that stand in a stall all but a short time every day, and then are asked to do strenuous exercise, are more prone to injury, and standing around can worsen many soundness issues such as the joint stiffness arising from arthritis.
5. Consider your horse's age. One of the biggest contributors to unsoundness is starting horses too young and then doing strenuous work with them before their bodies are mature. This is a problem in many horse disciplines - people are in a hurry, and the horses are the ones who suffer for it - many horses are used up and discarded before they are even grown up. Different breeds, and different horses, mature at different rates. If you're in doubt whether your horse is fully grown, have x-rays of a few major joints such as the knees - it is possible to see if the growth plates have fused.
6. Beware "weekend warrior" syndrome and take care of your horse in light of its condition. A horse that stands around all week and then is asked to do strenuous work one or two days a week is a horse that is prone to injury and unsoundness. Horses need to be fit if they're going to do strenuous work. Be sure to do proper warm-ups - it's amazing to me how many people get on and the next minute they're trotting and cantering and doing exercises with the horse. When your horse is showing signs of fatigue, stop - most serious injuries happen when muscles, joints and structures are fatigued. Don't repetitively do the same exercise over and over - this can also lead to fatigue injuries and repetitive motion disorders - mix up your exercises - your horse will also mentally be much happier. Do cross-training - if you ride Western and do trails, do some dressage, if you do the hunters or dressage, do some trails.
7. Supplements and joint injections? I'm a skeptic on these. Many people use joint supplements and like them - I've never found them particularly useful and they certainly won't solve problems if the basic principles of soundness aren't met. Although I started in the hunter/jumper world, where joint injections are routine, I don't do them. If my horse needs joint injections to stay sound enough to do the work I'm asking for, in my opinion I need to go back and reexamine my care of the horse and what I'm asking the horse to do - it may be time for rest and rehab, or the horse may no longer be able to do work at as high a level. Joint injections are invasive, carry the risk of infection, can cause degenerative joint changes and are at best band-aids - they don't correct unsoundness due to joint issues, they simply treat it on a short-term basis until the next injections are needed. Joint injections also only treat the joint, and do nothing to help soreness arising from muscular or soft tissue problems. I do believe in making horses comfortable if nothing else can be done - I have an elderly horse who gets Aspirease for arthritis and I wouldn't hesitate to use joint injections to keep an uncomfortable horse comfortable if and only if I had already made all the other changes I could to help with unsoundness.
8. Saddle fit is critical. You know how you feel if your shoes don't fit? There are a lot of sore horses out there with poorly fitting saddles - it's amazing to me that they put up with it. In my experience, many people who claim to know about saddle fit don't know very much and in fact are often interested in selling you a saddle. Most people know about making sure that there is spinal clearance and trying to find a saddle with a tree size that fits the horse, but there are many horses with saddles that "bridge" or fit unevenly or that pinch in the shoulders when the horse moves. When you're trying a saddle, put your hand between the front edge of the saddle just above where your knee lies, and ride the horse - if your hand is squeezed tightly, your horse's shoulders will be pinched and not able to move properly. Finding a saddle that fits can be difficult and some horses are much harder to fit than others - we tried over 20 saddles that we thought might fit Maisie before finding the right one, and it's made all the difference.
9. How you use your own body makes a difference. A lot of horses who carry themselves in odd ways or have trouble doing certain work are compensating for their riders - either their riders are unfit and having trouble maintaining proper body position, need some help learning how to use their bodies properly, or have their own "soundness" issues that directly affect their horse's way of going. A rider's issues can contribute to or even cause soreness or unsoundness in the horse. Even if you don't take lessons, have other people watch you ride - are you leaning, sitting harder on one seat bone, twisting or dropping a shoulder, carrying one hand higher than the other without meaning to? All of these things will affect your horse's ability to move - it's very hard for a horse to travel straight if the rider is crooked. Work on yourself and many of these issues will come right.
10. Find the right professionals to help. You need good professionals - farrier, dentist and vet for sure. In my experience, many vets are really good at structural issues - bones, joints, foot structure, tendons and ligaments, but are not always that good at detecting or treating soreness and the resulting subtle unsoundness that is due to muscular or other soft tissue issues. I used to be a huge skeptic about chiropractic, but am now a big believer - but with a qualification - you have to find a good chiropractor and there are a lot of mediocre ones out there. One place to start is the American Veterinary Chiropractic Association (for those of you Stateside) - they require extensive training and testing and their members are all vets. A good chiropractor can also teach you how to do some basic massage on your horse to loosen tense muscles or even scar tissue, and will educate you about the horse's anatomy and how the different joints and muscles work and what is needed to maintain soundness. When you start chiropractic, expect it to cost some money and take some time to get serious issues resolved - and different horses take different levels of ongoing maintenance. Some people have had good experience with other alternative treatments, like acupuncture - I've never used them.
11. Learn about, and learn to see and feel, soundness and unsoundness. Don't just leave it all up to professionals, your trainer or your friend at the boarding barn to detect when your horse is having a soundness issue - these things often come on gradually and if you can nip them in the bud that's a good thing. Read and learn about horse anatomy and physiology - there are many good resources out there, including books, magazines and the Internet. Learn as much as you can (without pestering them to death!) from your vet, farrier, dentist and chiropractor. And get people who are good at it to show you how to see unsoundness - learn to see not only gross lameness in the trot on a hard surface, but subtle lameness in the walk on a soft surface. (See my post here for some ways to detect subtle unsoundness.) Learn how to feel what's going on with your horse from the saddle so you can get ahead of problems before they get worse. Watch horses, and particularly your horse, moving in the pasture and at liberty - this will tell you a lot about how they are feeling from day to day. As you're leading your horse, listen to their footfalls - you can hear a lot of things like uneven stride or uneven weighting of a foot.
12. Know your horse and listen to what your horse tells you. Know your horse's body - run your hands over the horse and learn to feel areas of soreness or tension. Know your horse's legs - run your hands up and down each one and learn where the little normal lumps and bumps are so you can easily feel anything new or different. Pay attention to what your horse is saying. If your horse complains about grooming a particular spot, or is girthy, pay attention. If your horse develops an odd gesture - like Dawn's head shake of a few days ago - notice it. If your horse struggles to do something that was easy before, or can't do something that you ask, take a hard look to see if there is a soundness issue - or you getting in the horse's way! - before assuming the horse is being stubborn. If your horse's mouth is gaping or there's bit-chomping going on, don't tie your horse's mouth shut with a flash - find out what the problem is and fix it. If a horse head-tosses, don't use a tie-down or a martingale, figure out what's causing the issue and fix it. If a horse has a serious behavioral problem like bucking, rearing or bolting, first rule out soundness issues or pain - such as from ulcers - before treating it as a training issue. Many serious misbehaviors arise from pain or the fear of pain - and some don't. You own it to your horse to rule out all the physical issues first.
13. Help your horse to carry itself properly, with roundness and softness. My objective with my horses is engagement from behind, use of the core and relaxation of the top line, and ultimately, true, soft self-carriage. Gadgets such as draw-reins and tight tie-downs do nothing to produce this - they're just short cuts that force the horse into position without developing the muscles and habits of going that promote roundness, engagement and softness. A horse that is able consistently to carry itself correctly, without force and without gadgets, is a horse moving in the way which will best promote correct movement and continued soundness. Even a horse with serious conformational defects or who has developed an incorrect way of moving due to prior unsoundness or poor training can make good progress down this road - it doesn't come quickly, but is a road well worth taking.
I hope everyone has a Happy Thanksgiving!