I worked for a place that recruited people to test stuff like toothpaste, mouthwash, and other personal care products way back in mid 00s. We worked with a lot of big-name companies, and one of them was trying to get approval for a very similar product. The criteria were very similar to what's described here, where the person had to be perfectly healthy and also wear dentures. I think it took us about a year to recruit like 3 people before they just gave up and dropped the whole thing.