Someone here who knows about such things once said that Friskies Dental
Diet is the only one with pieces big enough to force the cat to chew,
hence scraping the teeth. I fed this as a treat to my cat who lived to
be 20. She had all her teeth and her mouth was not smelly, even
when she was old.

I have been thinking about this with regard to my cat Boo. She has
a heart and thyroid condition, and I do not want to risk anesthesia.
She has no obvious problems but every now and then I pick up a
"sour dishrag" smell when I pill her, not the usual nasty cat breath
that is like tuna and butt, you know? So I think problems may
be brewing.

You know what would be neat? Something tasty they would
have to gnaw on to get piece of,. almost like a bone for a dog.
But I don't even know if cats would do that. They might just try
to lick it to oblivion. :)

My vet said that hard food does not clean the teeth - cats usually
don't even chew the hard food that much.  I guess the thing is not so
much the toothbrush, but the toothpaste for cats.  There is an enzyme
in it that helps prevent/get rid of the tartar that builds up.  So, if
you could get the toothpaste on the teeth cheekside every week or so,
that would be all you need.

Hills offers a prescription diet food called T/D that is suppose to help
with dental care.  Not sure if it works because I haven't really used it.
The kibble is bigger than normal dry cat food.  You can get it from your
vet.

Sue

Science Diet Oral Care kibble

There is also a water additive that fights placque, available at Petsmart

I let my kitties lick kitty toothpaste off my finger (because they
won;t let me use a toothbrush)