Knee pain after skiing wipeout.

Patient: I’ve been having problems with my left knee for quite a while now (aching after running). I tweaked my knee skiing about a week ago in the Alps (I’m a soldier stationed in Germany). I felt a sharp pain on the outside of my knee when it twisted wrong. I can walk fine, but going up and down stairs is painful so I have to take one step at a time. The pain is primarily on the outside of the knee, but the other day I got down on one knee (the hurt one), and when I stood back up it also hurt on the inside of the knee. My wife bought me an Ace bandage and a brace, but they haven’t helped. It is most painful when bent and putting weight on it (hence the trouble going up and down stairs). It’s fine when my leg is straight. Lateral movement, primarily when I bend my left knee inside toward the right one, is the most painful. Any idea what it might be?