Not real sure where you are, or even what vet you have already used, but 2 vets i would highly recomend are Bruce Thompson in Orange (Springside Vets) and Ruth Thompson at Blayney, we had a dog we bred quite a few years back had a ligament problem in her front feet, Bruce gave us some tablets, no idea what they were now, we joked and called then elastic tablets, as thats what her feet and legs looked like at the time, so came home gave her them twice a day crated her for 2 weeks, and it worked miricales, she has never had a problem since and is now 6 years old.
However the owners who we took her from in regards to the problem had also been feeding her nothing but cooked food, as that what they ate, so the dog can too, was thier reasoning.
We never feed puppy food to our pups, always a good quality dry adult dry food.
and as many are saying loose gravel, or big stones, we have blue metal dust in our runs, and it really helps with flat feet and pastern problems, bought a dog with bad feet, and in about a month of having on the blue metal dust his feet were nice again.
My pet hate i am sorry is flat feet!!