My wife is 92 with dementia. We use a Water Pic to floss her teeth and a regular tooth brush to brush her teeth. Advice?

I don't know that you can.

Sometimes we just have to let whatever be good enough.

You could try to help her reach the back teeth and the back of her teeth by using an electric brush that you stand behind her to hold and move around.

This is a big issue with dementia.

Some dentists encourage their elderly patients to use electric toothbrushes. They do a better job in a shorter time. However, not everyone likes them.

Also they make 'travel' toothbrushes that have a spot of toothpaste on them. So you CAN'T spit it out. I don't know that they can really be used daily, but on the days that she's just not cooperative, they'd do in a pinch.

The waterpik is amazing, but it can really leave a mess--but if you do use it, put a small amount of fluoride rinse in the water and that would be OK.

I know we should brush after every meal, but that's not practical for most folks. As long as you don't go to bed with a 'dirty mouth'...you'll probably be fine.

Maybe try Teepa Snow's hand under hand technique

https://www.youtube.com/watch?v=6gLrH8mioCw

Ask her dental  doctor or hygienist about a rinse.  When my jaw was broken, my teeth were wired and I couldn't brush them. My denntist scripted for a rinse, used after every meal.

Actually, she's not all that wrong in spitting out toothpaste, if she's likely to swallow any of it.

You would have to do that for her. I think using a water pik gets rid of the stuff in between. Brush will remove the plaque on the teeth. Just need to watch and see if it looks like she is getting everything.