tooth fractures don't happen from decay. they must be chewing on somethings hard. of course decay can soften the enamel (outer layer).
dogs that have breaks in the front normally get them from chain link fences, table legs, etc. dogs that have breaks in the back might be chewing on rocks or sticks.
get rid of anything hard and replace with some durable rubber toys.
i would also see the vet to have them checked. any exposed nerves will be extremely painful.