Scotland loose forward David Denton has been forced to retire from professional rugby at the age of 29 on medical advice after an 11-month battle with concussion.
Denton, capped 42 times for Scotland, suffered a blow to the head while playing for Leicester Tigers in a Premiership match against Northampton Saints at Twickenham last October, with concussion ultimately dashing his hopes of appearing at the World Cup starting Friday.
"My actual reaction at the time my neurologist told me it was no longer a good idea to play rugby... to be honest, there was a bit of relief," Denton said in a statement.
"This had been building up inside of me for four to five months. By the time I got to it, I had been through all the emotional highs and lows, so I was prepared for it."
In August, World Rugby agreed trials of new laws which will limit tackle height to waist in a bid to reduce head injuries, with new rules potentially being in place by the 2023 World Cup.
"Personally, I think World Rugby is doing as much as it can to try and limit the number of incidents," said Denton.
"Controlling tackles above the shoulders has been a positive step but I think making anything above the waist illegal would cause more trouble than good.
Denton, who played his last test against Argentina in June 2018, had been expected to return from injury and fight for a place in Gregor Townsend's squad for Japan.
"We're really disappointed that someone who still had a lot to offer the game both at club level and for Scotland hasn't been able to do that," Townsend said.