Melanie - one of the risks is when dogs 'launch' themselves with their back legs. Flyball would create just that kind of risk as the dog tenses in anticipation of the release - and wants to launch themselves off the starting line quickly. That being said, I am sure that Nitro is in very good physical condition - and that reduces the risk quite a bit. Keeping the dog at a good weight is very important - and as we all know, 'good weight' means overweight to alot of people.
While there are things you can do to prevent the injury, it is sadly the most common knee injury in dogs, and moreso for active dogs. Do you have him do some warm up exercise before running a flyball course?
Livin in a Newfie Drool Zone