How dentists got it all wrong when it comes to flossing.

Last week The Associated Press ( AP) did a fascinating investigative story looking into the evidence favouring routine use of dental floss. In the lede to his summary of the data — which, as you have probably already heard, is feeble — the AP’s Jeff Donn pointed out that there are legal stakes here. Flossing is recommended by the U. S. surgeon general under guidelines that are, by statute, supposed to be evidence-based. So what is the evidence?

The AP put that question to professional colleges and the government, and also did its own survey of a decade’s worth of studies on flossing. It found that most of the studies were tiny, comically small, and often indirect, using “warning signs” like bleeding or detectable inflammation as a proxy for gum health. “Some (studies) lasted only two weeks, far too brief for a cavity or dental disease to develop,” noted Donn. “One tested 25 people after only a single use of floss.”

Donn’s reporting reached social media in the usual way — as a headline (“Medical benefits of dental floss unproven”), which everyone immediately treated as a declaration of freedom from an ancient chore. I for one would not have guessed that people hated flossing so much. I suppose the real issue is that nobody flosses as often as they are told to, and what they hate is the guilt they associate with the mere thought of it.

North Americans live under constant pressure from pervasive professional marketing, so it is natural to cry “aha” when one learns that interdental flossing became a “scientifically” recommended habit in the 19th century, mostly because some sharp American had invented dental floss, and that no serious effort at testing has ever been made.

If you did not check, what would you guess the evidence in favour of flossing consisted of ? You might suppose there were large longitudinal studies comparing flossers with mere brushers. Someone must, you would think, have looked for inter- regional or international comparisons between populations. Or asked about flossing in a mass household survey. Or inquired into correlations between dental-floss sales volume and general oral health. The companies that make dental floss might have tried flooding a little town with those little F- and Y- shaped flossing implements to see if it made any difference.

Nothing of this sort, it turns out, has been tried. Flossing was established as a moral imperative long before anyone had the idea of “evidence- based dentistry.” Consider that water fluoridation is still a little controversial, and then realize: the quality and variety of scientific inquiry into fluoridation is much, much better — 100 times better — than it is for flossing.

When I read Donn’s story, I got curious about older research. The AP’s investigation focused on newer efforts, but older studies sometimes have their merits (they didn’t bother much about the ethics of experimenting on humans back in the day), and I thought perhaps there could have been meritorious investigations that had been used to establish the effectiveness of flossing in the first place. Moreover, flossing was a big part of the regimen I was taught as a child, at the dental clinic and at school. Were all those sober authority figures simply disguised shills for Johnson & Johnson?

Well, if you search for “dental floss” on Google Scholar and specify a date before 1980, you will get three published scientific studies of any kind, and one of them is actually by employees of Johnson & Johnson, so make of that what you will. I found a 1973 crossover trial that was intended to compare the effects of waxed and unwaxed dental floss; this involved following just 36 adults for 98 days, and the investigators found that the experiment induced improved toothbrushing, which “eventually caught up with the advantages initially provided by flossing” in gum health and plaque removal.

In 1977, community dentists from the University of Western Ontario did a study with 66 firstgrade children in the town of Dorchester, Ont., which was chosen for low natural levels of fluoride in the water supply, and found that flossing seemed to have the potential to prevent tooth decay. This study had the interesting wrinkle that the children were flossed daily at school by dental technicians, and only in two of the four quadrants of their mouth; each child thus served as his own control. The findings were positive, but also discouraging, since the study passed over a summer break and the apparent initial benefits of flossing dissipated quickly.

In the era of evidence- based dentistry, it would not be any researcher’s first thought to depend on a test of such an artificial version of a health intervention. Few people, after all, can afford to have their teeth flossed every morning by a professional. By now, you will have guessed that the Johnson & Johnson study from 1979 had a small sample ( 50 adults), that the flossing was done by a hygienist and that the experiment did not last long (14 days).

There is a detectable spirit of incuriosity in these early, patchy studies, and the incuriosity becomes explicit in the AP’s contemporary interviews with dentistry authorities. In the AP article, the president of the American Academy of Periodontology says, sure, the evidence for flossing is lame, but failing to floss would be “like building a house and not painting two sides of it.” I don’t wish to discourage anyone from flossing their teeth, but I do not think I have ever heard anything that sounds so much like a literal old wives’ tale. Science by metaphor?

The periodontology guy passes the buck to the American Dental Association, whose spokesman gripes that maybe “research participants didn’t floss correctly.” Sooo … your profession has been recommending a procedure to everybody for decades, and your defence for the lack of an empirical foundation is that nobody does it right anyway?

YOU’VE BEEN RECOMMENDING THIS FOR DECADES AND YOU HAVE NO EVIDENCE TO SUPPORT IT?