By Racinguk.com staff
The Mark Johnston-trained colt had to be put down when the three-year-old son of Teofilo sustained a broken leg in the mile-and-a-quarter Grade One at the Illinois circuit, won by Oscar Performance.
Permian unseated Buick soon after the partnership crossed the line in last place.
Buick was taken to Northwest Community Hospital for further treatment.
Charlie Johnston, who is also the Middleham handler's son, said: "We've lost the horse. He suffered a catastrophic fracture of his left fore.
"I was with him on the track for 30 seconds. He was in the process of running a very disappointing race and I believe it was when he was winding down that it happened.
"I am with William and he is conscious, but complaining of back pain."
Buick was taken to Northwest Community Hospital. The Daily Racing Form in America reported that Buick had suffered a fractured T12 vertabra.
Permian was one of the big success stories of this year's Classic generation, having won the Dante Stakes at York and the King Edward VII Stakes at Royal Ascot.
He earned over £400,000 in prize-money and won six of his 13 career starts.
It was a third Grade One success for Oscar Performance after victories in the Breeders’ Cup Juvenile Turf in October and the Belmont Derby Invitational.
The Aidan O’Brien-trained Taj Mahal rallied from fifth of a sextet to finish second by one and a half lengths under Ryan Moore, while Al Shaqab Racing’s Afandem overcame a slow break to rally and snag third by three-quarters of a length under Frankie Dettori.
Dettori said: “Everything went smooth like I thought. He’s a young horse, and he hasn’t encountered these kinds of bends. You have to race into them and race off them and he’s not used to those kinds of things and lost all his momentum on the turns and that cost me.”
British and Irish representation failed to land a blow in the Arlington Million, the feature event at the Chicago track.
Joel Rosario and Beach Patrol denied Daniel Muscutt and Fanciful Angel, while Moore had to settle for third aboard Deauville.
Beach Patrol's owner Sol Kumin highlighted the Million had been his sole aim all season.
“This was the spot and the race we’ve been waiting for a year," he said.
"He loves it here, Joel gave us a great ride, obviously Chad and his team had the horse ready and we’re super happy. It’s funny I’ve seen the trophy and Chad always talked about this is the best trophy in horse racing.
"It’s always been on the bucket list and I’m super fortunate to be here with my kids and dad. It doesn’t get much better than this.”
Muscutt was happy with Marco Botti's charge: “We had a good spot on the rail and he cruised around," the jockey said. "Got space at the right time, they were just one better at the end. I’m delighted.”
Moore believed the 10 furlongs stretched Deauville, who has won over the distance only at Chester and Belmont Park.
“I think he ran a very good race," he said. "He had his chance, maybe the trip stretched him.”
Chad Brown's Dacita won the Beverely D. Stakes, while Edward Graham's Postulation impressed in the American St Leger.