Why do horses get put down if they break a leg?
Racehorse injuries
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Tendinitis is inflammation of a tendon. Tendons connect muscles to bone and normally have an elastic property so that they can stretch. Tendinitis occurs when the tendon is overstretched or overloaded, which causes straining (tearing) of individual or multiple fibers and the formation of a lesion.[49] A lesion is strained tendon fibers with associated hemorrhage (bleeding) and edema (fluid retention).[50] Often strained tendons go undetected or may be subclinical before the clinical signs of tendonitis are observed.[51]  Tendinitis is characterized by heat, swelling, and pain. Racehorses often experience tendinitis when transitioning from sedentary activity to conditioning work. Thus young horses and those returning to exercise from lay-off are those most expected to suffer from tendinitis.[52] The incidence of tendon injuries is approximately 30% among Thoroughbred racehorses in training, mostly in the SDFT of the forelimbs.[53] Horsemen frequently refer to tendonitis as bowed tendon due to the bowed appearance of the SDFT.[54]  Bowed tendons force 25% of racehorses to retire and are the most common non-fatal career-ending injury.[37]  Racehorses are so often retired after tendinitis because as many as 70% will not be able to return to their previous level of performance and more than 66% of them will have a recurrence of injury.[55][56]