Lexi came up lame about 3 years ago during agility and was diagnosed by one vet with a luxating patella. She told me that she would need to have surgery and may not be able to do agility again. I was devastated.
We took her to two other vets--neither of which could find anything except maybe the most minor of slips.
Regardless, we took her to a top rehabilitation program in Chicago (called TOPS) and they checked her out, felt that it was so minor it would never be an issue, and sent us home with a great exercise regimen. They also felt that seeing a chiropractor on a regular basis would help her (she went every week for month, then every other week, and now she just goes every month).
Strengthening the muscles around the joint is super helpful, and knock on wood, we have had no issues since.
My recommendation would be to find a very experienced sports vet and get a regimen to follow.