i too have a torn acl and a bad meniscus. i find that the best ab exercises for me are regular crunches six inches planks and leg lifts. i would stay away from the bicycle crunches but that's the only ab exercise that has caused me knee pain.
husband's acl and meniscus in right knee are tore. i need some ab exercises he can do without moving his legs please. he is in the military and will have to continue doing pt which includes sit-ups pushups and running after the surgery and recovery. he needs some exercises he can do before and after the surgery to keep his abs as well as arm strength up.