I agree with Julie. Although they can develop tendonitis also. My pup grew up big and fast, also limped occasionally but grew out of it when his legs caught up with his body Whatever that meant, it wasn't serious.
You don't say what size your dog is but the bigger they are the more weight-bearing they have to do on their poor legs. I mentioned early an Xray did not show soft tissue (ligament) injury. Remember that altho they may not yelp it is a very painful condition and can cause further damage if left untreated. Your vet should recommend a surgeon who is experienced at this type of surgery -there's no other way-sorry-also a catscan or MIR...expensive, yes, but i would have remortgaged my house if I had to