In reply to Removed UserGRUMPY MONKEY:
It does exist, it's mainly modified/lighter weight versions of mountain biking body armour, which, in turn, is modified/lighter weight motocross body armour.
Dainese are possibly the market leaders, but not the only ones - I wear a Dainese snowsports helmet and padded undershorts and a 661 MTB back protector when snowboarding, but have increasingly worn them skiing. I also tend to wear elbow/forearm guards (unbranded), as I already have damaged elbows from a MTB accident on tarmac and don't wish to repeat that on ice.
Specific knee braces aside, none of it is really going to prevent ligament injuries - it's saved me from a few bumps on ice and crashing into a suicidal novice snowboarder, though.