it is not normal but it is common, gosh i know the technical name but i just cant think of it now. I believe you can have surgery to help correct it but it is very costly & time consuming. If it does not happen often Most vets just reccomend limited activity & of course keep the dog on the slim side. If your interested in treating it i would reccomend some more info from your vet &/or a second opinion as well as a reccomendation to a specialist.
Ill try to look into it for you when i get a moment.