I was reading this and was going to say why don't you try glucosomine, My moms horse had 3 bad injuries when she was between 2-4yo and now has joint problems. She is also very young still and will probably need to be on glucosomine for the rest of her life. Also, When my mom went into the health store to talk to them about getting in glucosomine the man told her she needs to add garlic and vitamin C also. She spoke to her vet about this and she also said the same thing. They help absorb the glucosomine, Also the vitamin C promotes the growth of connective tissue.
This is the case for people and horses so i'm sure it is the same for dogs. You might want to ask your vet about this.....