from what I've gathered from websites and a little research,
boots used to be lower on the leg and in the 70s and 80s, people broke their ankles and lower legs a lot, then they made boots go higher on the leg and those injurys went down, unfortunately, while higher boots give more ankle support and better performance, they put more twisting stress on the knee. I am not sure of any new boots that claim to reduce knee injuries, and missing the slider piece in a binding definitely isnt good, but I dont think it is the only reason why your knee was hurt, your din and foward pressure could also have contributed to your injury. Acls are super scary and sometimes you just fall the wrong way. there are a few small steps in falling correctly that can help avoid dangerous situations, here is a broshure thing that you can order from vermont ski safety
http://www.vermontskisafety.com/faq_skiers/faq_skiers_tips.html