Knee injury - please give opinions?

Alright, I tore my ACL completely through in 1992. I didn't have anything done about it until 1999, when my orthopedist discovered that the ACL had actually healed back together, though not smoothly, and that there was scar tissue giving me problems. He removed the scar tissue and cleaned up the wound. It healed very well. Then, in 2003, I fell on a flight of concrete stairs and tore my miniscus badly. I opted not to have surgery, and did physical therapy and ultra sound treatments whenever I got a Baker's Cyst. Now, something else entirely is wrong with the same knee. While I was grocery shopping, I twisted to my left to lift up a case of Diet Coke and something tore. It didn't hurt very much at the time, but I felt something tear. Now, a month later, my knee is VERY unstable. The symptoms are as follows: (1) the joint is very swollen, to the point that the knee cap is displaced, (2) there is pain below the right side of the knee cap (this is the Left leg) and in the back of the knee joint, (3) sometimes the joint won't support my body and just gives out, and (4) when I walk or move the leg with any weight on it, it feels like the Tibia is twisting outward to the left and sliding backwards.

I know I should see an orthopedist, but I'd like to have some ideas of what to expect as far as this injury before I go. I am certain that I will need an MRI. Any ideas as to the results of said MRI? Have you had similar experiences, and were you able to avoid surgery after so many injuries to the same joint?

Thanks!