Knee feels like it pops out of place?

the outside of my right knee feels like it comes out of place and hurts so bad I could scream. For a while I can't even stand to move it and then when I can it feels like bone popping over bone. Most of the time it is extremely sore afterward. I've had xrays and MRI's and they show nothing. I am supposed to have arthroscopic surgery on the 19th. Just wondering if anyone else has had this problem and if they found out what it is. I have arthritis and the other knee will soon be candidate for knee replacement but this seems completely different. I fell back in April and had xrays of my knees then but they said it didn't show anything but degenerative changes.

your problem is ligiments..
had tis problem and i also have arthritis.. small amounts of fluid an push your knee up adn when that happens your ligiments move alittle and lock your knee this happened to me for year or two then one of my knee done it and i found out i had ripped ligiments inmy right knee..