Drumrunner posted excellent information on polos. I very much agree that polos offer good protection against concussive injuries, yet almost no actual structural SUPPORT. In fact, few boots can be said to offer much SUPPORT either.
The study that was offered by a company promoting polos should be looked at pretty closely. There is a definite conflict of interest. All independent I have read have said that they offer little to no support.
Here is further info from the Sustainable Dressage information site;
Bandages & Polo Wraps
Hard shell leg protection.
There are all kinds of leg protection devices out there in the tack shops. Most of them are made to protect the horse's legs from either his own hooves, or from poles and obstacles hitting the leg and causing bruises, inflamation, woulds, pain, etc. This is all good. Even for the dressage horse this is good, since an unbalanced horse simply trying to move along in regular gaits very easily can hit his own legs with his hoofs because he is unbalanced. Now, I hope you understand that the hoof does not bend up and hit the leg it is attached to. That would be terribly unnatural. But the left foreleg's hoof can hit the right foreleg. Or it can hit the left hindleg. Or even the right hindleg. It is more common for the hindlegs' hooves to hit the forelegs or each other, or for the frontlegs to hit each other. Muscular imbalance can cause the legs to slant inwards and the hooves to be placed tighter together on the ground and not on two separate tracks. Their line of travel can also be affected by muscular imbalance and move the trajectory towards the other side instead of in a vertical bow. But I will not go into that here.
Soft wrap leg protection.
Polo wraps or bandages can protect from that too, although not as well as for example a shell made from leather or plastic lined with neoprene or fleece. Those kinds of splint boots work like a helmet on the horse's legs. In a bandaged leg, you do not have the outer shell that spreads the impact for the lining to absorb the shock.
Bandages can protect from brushing injuries which are small cuts in the skin. But splint boots are sometimes better, at least if they fit. If they don't fit, sand and gravel can sneak in between the leg and the wrap and cause abrasions and blisters.
Bandages and polo wraps can cause blisters as well. If you wrap them too tightly, or the pads are misaligned, or they are too loose and shafe. It is an art to wrap correctly. Wraps can also cause serious accidents if they come undone. The loose end can scare the horse, or he can step on it and trip. He can also get caught in fencing or other things and tear the whole place down, fall and land on top of you, etc, etc. Horrible.
Those accidents are however rare and keep getting rarer because the fastening devices on bandages are getting better and better. 40 years ago, they were two little cotton strings that one tied in a bow. Neat, but hardly safe. Now we have velcro. Amen.
The stress on the fetlock joint.
But, to be honest, most dressage riders don't use polo wraps to protect the leg from the other legs. They usually use them to support the leg that is heavily loaded. Now, what is it that is being protected, here? Well some say it is the tendons at the back of the leg. The wraps are supposed to keep them in place and support their position. The thought might be nice, but it does not work that way. The tendons are kept in place by strong sheaths of ligament and other tissue. This is doing all the work. If it needs support from some flimsy fabric on the outside, the horse is desperately lame and needs operation. Think about it - horses land from 5 feet obstacles, usually on one leading leg. Imagine the impact of the entire warmblood horse plus the rider's weight. The tendons take this, and the sheaths keep them im place. As the horse lands and loads the legs like this several times a week, the tendons and sheaths are reinforced becoming stronger and stronger. If a horse can withstand this, what extra can relatively loosely wrapped fabric do to aid it? The answer is - Nothing.
The dressage horse is not under such strain, ever. He is also supposedly more meticulously trained, and his tendons are really strengthened over the years.
Tight, tough neopren wrap.
Some say that the bandages support the fetlock joint. The wrapping will stop the fetlock joint from overbending as the leg is loaded. But again we are talking about much greater forces than fabric can handle. It will simply stretch or compress the skin and the padding to allow the give that the weight of the horse will cause in the fetlock. Science has even tried to construct devices that can be scientifically proven to lessen the give in the fetlock. These devices are made from 1/3 inch thick neoprene, reinforced with nylon webbing and velcro fastenings, and are wrapped around the leg with fantastic effort. The wide velcro fastenings have to be extremely firmly tightened in a precise crossing position under the fetlock, to have any effect at all. So what will some semi-loose bandages do in comparison? Nada.
So do they have really no effect whatsoever?
Well, no. They do have one effect that is positive. And when you think of the origins of wrapping horses' legs, you see why. The whole wrapping tradition comes from English thoroughbred racing. They always wrap their horses legs, during work or at night in the stables and so on. Why?
Not because it supports the tendons or the fetlock, but because it warms the leg and promotes circulation. The racing industry is a fast one. Horses are not exactly warmed up with 10 minutes of walk and then a jogging trot to get the juices going. They are taken out to the track, the jockey is shoved up and away we go!
Dressage horses are not ridden that way, or at least they shouldn't be. Dressage horses don't need wraps, because they are supposedly constructively trained, and are warmed up slowly to get the synovial fluids going.