Sorry to hear about the LP. It can be caused by injury, but more than likely it was just not diagnosed when he was young and it has progressed to the point of having pain. Lets hope there is not joint degeneration now because of it, because surgery can't fix that. I am going through bilateral LP surgery with Gracie. Here is a link to the thread. I have several other posts on LP as well. There is a wealth of info, just do a search on luxating patella.
http://www.yorkietalk.com/forums/sho...d.php?t=113462