I don't know that we have any vets here on the board but I can speak from a pet owner's point of view. My toy poodle, Pumpkin, (RB) suffered from this. It started one day when he jumped up at the door to go outside and then there was a yelp. I rubbed his leg, and in so doing I actually caused it to slide back in place. The vet then showed me how to do it the proper way in case it should happen in the future. Well it became a chronic problem, and eventually he did have to have surgery and a pin was inserted in his knee. He had no problems after that. I was told that it is a problem in smaller breed dogs. Apparently there is a hereditary factor also, and possibly a more responsible breeder would not have bred his parents if they suspected this problem in one of them. That said, I wouldn't have wanted to not have had the chance to have this little guy in my life!