The definition of "sprain
" is a tear. When someone tells you that you sprained your ankle, it means that you tore the ligament that stabilizes the ankle.
It scars over and heals, so we recover quite well. But the fact is it has torn.
For the knee, a sprain means you tore something. It could be the mcl, which heals on its own. Or it could be the acl, which doesn't heal, or somewhere in between.