Jockey Terry Thompson broke his right leg in a three-horse spill in Tuesday's second race at Prairie Meadows.
The incident occurred when pacesetter Soriano Cat, with jockey Kelly Murray aboard, broke down entering the turn of six-furlong race.
Thompson and his mount, Brilliant Beau, fell over the top of Soriano Cat, sending Thompson to the ground. Precocious Builder and jockey R.D. Williams were unable to avoid the fallen horses and also went down, with Precocious Builder appearing to clip Thompson as he fell.