Get a 2nd opion today...

My guess: Luxated patella, really common in chis and many small dogs. It Doesn't usually show as a puppy, but happens later when the tendon slips out of the groove in the knee that it is supposed to 'ride" in. The groove is so shallow (genetic) that it is easy for the tendon to jump ship. Our little Poquita lived with it for 15 years, no surgery. I learned how to put it back in place (per vet) and we got along like that for her whole life. At 7 yrs of age, we took her to an ortho surgeon vet who took x-rays and said if the tendon is in place at time of x-ray, the problem is not viewable. Knee was in place. That vet advised us to leave it alone but told us she would eventually develop arthritis, which she did. We just accommodated to her at that point and she rode in a stroller for walks the last few years and also was on pain meds as needed. I could tell by looking at her eyes if she need the pain meds. The associated pain/discomfort was so much "her normal" that she still "helped" us by jumping a little when we picked her up until that final trip to the vet. Look up luxated patella on the web. Many dogs lead happy lives in spite of it.