It could be a skin problem and skin problems are usually related to digestive problems.
If your dog has an unhealthy gut, small food particles will make their way into the blod and settle out into various parts of the body, including the joints.
The immune system will then attack these foreign particles are create skin conditions and arthritus over time.
You should consider changing your dogs diet off a grain based food if that is what you have been feeding.
You might consider a raw meat diet which I have heard good things about.
If your dog has an unhealthy gut, small food particles will make their way into the blod and settle out into various parts of the body, including the joints.
The immune system will then attack these foreign particles are create skin conditions and arthritus over time.
You should consider changing your dogs diet off a grain based food if that is what you have been feeding.
You might consider a raw meat diet which I have heard good things about.