That is just a fancy name for a kneecap that won't stay in place. Our dog Lexie, who will be 2 in June had this problem in both of her back legs. Genetics are probably to blame thanks to the bow legs from the pug side of her. She had surgery last Friday to fix them both. She is home now. I know that some would say "she's just a dog" blah blah and to that's okay if people feel that way but I don't so be nice if you decide to comment.
She is a part of our family. Would I ever put her over the human members of my family? No. But, if there is something I can do then I will. She loves our family unconditionally and makes us laugh daily.
Hopefully in 4 to 6 weeks she'll be as good as new.
Both legs look like this one.