Nyx was only 18 months old when she chipped a molar and had to have it sealed, as I mentioned in another thread. As part of the process they cleaned all of her teeth. And the first thing the Vet said afterward was "you let her play with tennis balls don't you?". He went on to say that there was noticible wear on her canines, at only 18 months!
Since then I've kept her away from tennis balls as much as is possible. I've used chuckit, nerf (they make rubber balls), ruffwear, and brite-ize balls.
Chuckit and ruffwear balls have held up the best.
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain