Orijen is the answer!
So sorry to hear about your dog. I know how much it pained me to see my little guy suffering. My dog Neno (I didn't name him, I adopted, I know better than to name a dog something that ends with a command "no"!!) came down with early-onset arthritis in both back legs, at only 5! It was heartbreaking! He's not a bit overweight, and gets a healthy amount of exercise. It's just his breed (pitbull/american bulldog). I did an extensive amount of research on the topic and found out exactly what type of vitamin/mineral/food regiment is best because I was determined not to kill his liver and kidneys with pain medication at only 5. ORIJEN!! ORIJEN!! ORIJEN!! It literally saved his life. The food is top-notch quality and contains massive amounts of naturally derived, omega 3 & 6 fatty acids, gloucosimine and chondroite, all beneficial to any mammals suffering arthritis. Within two weeks of feeding him Orijen, Neno was no longer on any medication, no longer limped, and was back to his regular spunky, fun-loving self. I'm not made of money, but the food is worth every penny. When you consider that I no longer need to pay for medication, the food pays for itself. Two years later, and Neno still loves Origen. I rotate between the 6 fish formula, the regular adult formula, and the Senior type now. Orijen is a high protein diet, and I understand your concern in that respect, but all versions of Orijen have limited amounts of calcium and phosphorus. That's the real danger in high protein diets, so no need to worry there. Once I ran out of Orijen accidentily. Just two weeks of Blue Buffalo Wilderness, and Neno was limping again!! Broke my heart! I'll never let that happen again. I wish you and your little lady years of happiness and pain-free fun.