There are two main reasons why horses with broken legs are often euthanised and horse owners might say that the first reason is more important than the second, (1) is unviable to repair a broken leg and (2) economics come into play i.e. the cost of repair and the prognosis. The more cyncial of us would say the second reason plays the major role in deciding euthanasia.
Attitude of owner
It would appear that the attitude of the horse’s owner also comes into play as to whether they choose the former or the latter. Sometimes there is no choice but I suspect that when there is a choice economics come into play. Some people would say economics don’t come into play because there are occasions when champion horses have been euthanised after a broken leg even though they would have earned millions as a stud. That is surely good evidence that a repair was unviable.
Racehorse legs are lighter than the legs of regular horses. Because they are light they shatter making it unviable to repair them. Also, they have little soft tissue covering the bone. This makes healing of a repaired bone more difficult. Further, the repair of nerves, blood vessels and tendons might be unviable because of a lack of blood supply to these tissues (because of a thin soft tissue covering and poor blood supply after the injury).
The 500 kg weight of a horse is distributed evenly throughout the four legs and hooves. If you take one away because it has been repaired and is healing, the other three legs have to take the weight and this causes a condition called laminitis. This refers to an inflammation of the fibrous tissue in the hoof. It can be treated with painkillers but it can develop into a very severe case causing a lot of pain to the horse. The quality of life is much reduced. Quality of life is a factor in deciding euthanasia.
You could put a horse into a sling to support their weight but this causes problems as well. The horse won’t be used to it and it might take weeks for the broken leg to heal which creates a welfare issue for the horse. If the horse is heavy there is compression of the intestinal tract. It seems that putting a horse into a sling for a long time is probably unviable but there may be economic issues as well.
Sometimes people question why horses are put down so quickly on the racecourse. The argument is that a veterinarian can decide within the timeframe allowed whether the horse should be euthanised or treated. If the horse can be treated it would be taken away in a horse ambulance.
There have been improvements in treatments such as better anaesthetics, better pain relief and better technology to hoist horses. There is a better understanding of bone biology and how they heal. Also diagnosis is better nowadays. This results in a greater chance of survival for a horse with a broken leg.
I wonder whether wild horses have anatomies and legs which are more robust and more repairable than the legs of racehorses. The point I’m making is that the way racehorses are bred may predispose them to un-repairable broken legs. If that is a fair comment then some of the responsibility for euthanising a horse with a broken leg can be put down to breeding for speed rather than robustness. I suspect that the breeders are a main reason why horses are euthanised rather than undergo surgery for a broken leg.
Clearly, the type of break is a major factor in deciding whether a horse is euthanised or not. This is veterinarians’work. I can’t discuss it here. I think that the bottom line, based on my research, is that it is much harder to treat a horse with a broken leg than a person and I suspect that if the horse does not have a stud value then for economic reasons the horse will be put down if the owner foresees a difficult repair over a long period of time.