I talked to some serious lifter friends of mine...and they said that the lunge is the worst and most dangerous exercise for the knees and that using 40 pound dumbells in each hand can have the pressure of 300+ pds on your knees because of the awkwardness lunges can cause.
Nevertheless..that is still a freak accident.
I would definitely stay on the GH and maybe let us know how you progress from the injury.