The Dante Stakes and Royal Ascot winner was running in the Secretariat Stakes at Arlington and had finished a disappointing last of six when shortly after crossing the line he shattered a leg, firing William Buick into the turf.
A decision was taken by Johnston's son, Charlie, a qualified vet, to have Permian put to sleep but some people contacted the trainer to criticise that choice rather than have him put in a horse ambulance, however, Johnston reiterated there was no other alternative.
"I think 'stunned' is the best word to describe the way I felt when I realised that he had broken his leg and would not survive," Johnston wrote on his website, www.markjohnstonracing.com.
"It is the horse that never needs a lay off that is least at risk of injury. Permian was such a horse. He never had to miss a day's exercise due to lameness. He was always sound and didn't even show any sign of stiffness or pain in our routine, post-race, inspections.
"That is why the news that he had suffered a catastrophic fracture was such a shock to us all. It is a horrendous blow to a racing yard to lose its best horse but it was particularly bad because we were so unprepared for it. We never thought it would happen to him. We were already making plans for next year.
"The phone was ringing before I had walked from my seat by the television to the door of the room and it was Charlie who said, 'he'll have to be put down, it's not even a grey area'.
"It never ceases to amaze me that animals, and particularly horses, can suffer horrendous injuries and show little or no sign of being in pain. The effect of endorphins and adrenalin on the flight animal – nature's survival mechanism – is verging on miraculous but it doesn't last long and Charlie did exactly as I would have done in ensuring that the horse was spared any suffering. A good decision.
"The number of negative comments was in single figures, amongst hundreds of genuine condolences, but they hurt nonetheless and not because there was any truth in them.
"I find it sad that people jump to the conclusion that an accident like this happens because of one too many runs. We all know that there is a risk of injury if you run and, if you run very fast the risk is increased. But we do not know that the risk on the eighth start is any greater than the risk on the first.
"I have no complaints about anyone's actions. I would employ the same team tomorrow in the full knowledge that nobody could do a better job."