Stringing Us Along?

Grant Ritchey

“Ok, Larry. You take an 18” strand of floss, then you wind the ends around your middle fingers like this. No, your middle fingers. No, I don’t think it matters if it’s clockwise or counter-clockwise. Ok, got it? Now grasp the floss with your thumbs and index fingers, leaving about 3-4” of floss between them. No, it doesn’t have to be exactly 3-4”; whatever is comfortable for you. Ok, now you….”

This is where Larry’s eyes start to glaze over. I’m boring him to tears and we both know that when he leaves, he’s more interested in finding Pokémon creatures than he is than cleaning bacterial colonies out from between his teeth. But we in the dental profession still feel compelled to lecture, cajole, encourage, and sadly sometimes even shame our patients into learning the critical importance of flossing. We say cute aphorisms like “Only floss the teeth you want to keep” to drive home our point.

Why do we dental professionals have such a fanatical devotion to flossing? Certainly it’s because of overwhelming scientific evidence that flossing prevents cavities and gum disease, right? Why else would we all go through the inconvenient time consuming gymnastics of trying to slide that little piece of string between all of our teeth (or lying about doing same)? Sure, it’s a hassle, but ostensibly the evidence tells us that by doing so, we’ll get fewer cavities and our teeth won’t fall out, right? Looking at the dental literature, one would think that good studies about flossing and brushing would be abundant, but surprisingly this isn’t the case. Who knows why, but I assume that it’s one of those topics where efficacy was always presumed from experience – both the patient’s and the dentist’s. To be fair, there is robust biological plausibility for brushing and flossing;  it is unequivocal that the complex bacterial biofilm (commonly called “plaque”) found on teeth is necessary for both dental caries (decay) and gum disease to begin. There is a tight chain of events that leads to these diseases: bacteria colonize on tooth surfaces, they then metabolize the carbohydrates we consume in our diet, and the byproducts of this metabolism (chiefly acids and enzymes) erode tooth enamel as well as the tissues surrounding our teeth. This is a vast oversimplification of a complex process of course, but the take home message is that if the biofilm can be thoroughly removed, then the risks of cavities and gum disease are greatly reduced if not eliminated.

But some old news has been resurrected recently that casts doubt on the efficacy of flossing. Like Jason in the Friday the 13th movie franchise, this story keeps coming back to life from time to time. The most recent iteration appeared in the New York Times on August 2nd in a story by Catherine Saint Louis entitled “Feeling Guilty About Flossing? Maybe There’s No Need.” This article had a new twist, because it pointed out that “(t)he latest dietary guidelines for Americans, issued by the Departments of Agriculture and Health and Human Services, quietly dropped any mention of flossing without notice.” These dietary guidelines are just that, guidelines, evidence based recommendations for maintaining a healthy lifestyle and making wise dietary choices. Almost comically, this “quiet dropping” of flossing is framed in the article in a way that leads the reader to believe it’s the federal government that issues flossing regulations to its subjects. “For decades, the federal government — not to mention your dentist — has insisted that daily flossing is necessary to prevent cavities and gums so diseased that your teeth fall out” the article states. No, the federal government has “insisted” no such thing; they just make evidence based recommendations gleaned from other organizations like the American Dental Association. This mindset was further catalyzed in an article two days later, where an article entitled “How An AP Reporter Took Down Flossing” made the internet rounds. I won’t go into it too much (you can read it yourself), but in it, AP reporter Jeff Donn was interviewed about a story he did about the flossing “controversy.” His article pretty much mirrored the Times article mentioned earlier, but with one absurd twist. In a ploy that would make Kevin Folta’s foes proud, Mr. Donn filed a formal Freedom of Information Act (FOIA) request for the documentation behind the departments of Health and Human Services and Agriculture’s original recommendations that flossing is a good idea. Alas, the conspiracy to foist flossing upon the American public by government was thwarted by Mr. Donn’s crack investigative skills and FOIA request (so he claims), and the flossing recommendation was removed from the next dietary guideline release.

The purpose of this article is not to ponder how much time and money were wasted filing and responding to Mr. Donn’s FOIA request, or to even question it, as he was within his rights to do so. Nor do I want to impugn his news story or his abilities as a reporter; from his perspective as a non-scientist, this looked like just the sort of juicy fodder that would make a great piece. Rather, as a dentist, my focus is to address the misconceptions regarding the referenced flossing research and my critiques on how the literature is consistently framed and presented to the public by journalists.

The Meta-Analyses

In an attempt to crystalize the data on the matter, a couple of systematic reviews were done, one in the Journal of Dental Research, a high quality, peer reviewed dental journal, and the other by the Cochrane Collaboration, also a well-respected resource for such endeavors. Both of the reviews have been out for a while (one was published in 2006, the other in 2011), but have recently popped up again in the lay press and presented as breaking news. As you might be able to guess, much of the “science journalism” is misleading or downright wrong.

The first systematic review I will cover is “Dental flossing and interproximal caries: a systematic review” by Hujoel, et. al. In this systematic review, the investigators assessed the effect of flossing on interproximal (between the teeth) caries (tooth decay) risk. Toothbrushing doesn’t reach between the teeth, therefore flossing is the logical solution to preventing cavities there. The authors identified six trials involving 808 subjects aged 4 to 13. In one of the reviewed studies, children were professionally flossed every school day for 1.7 years, which resulted in a 40% reduction of caries risk. Professional flossings every three months for three years as well as self-performed flossing for two years did not statistically alter caries risk. Again, they looked only at tooth decay reduction, not gum disease. The take home message from this review is that:

  1. Flossing when done by professionals five days per week works! That means if everything goes just right, if you know what you are doing (e.g., you are a dental hygienist, a dentist, or just really anal), you have good visualization of the teeth from outside the mouth, you are motivated and do the job regularly, you can in fact reduce caries. This is the definition of efficacy.
  2. If you don’t do it five days per week but just occasionally (even if you are good at it) there is no benefit in terms of lessening your chances of getting a cavity.
  3. If you ask people (adolescents) to do it themselves, they can not achieve a good result. Why? They probably can’t technically do it right, don’t do it often enough, or some other unknown reason. This is the definition of effectiveness.
  4. So the conclusion for caries prevention is that flossing has some efficacy but not good effectiveness when left to patients to be the flosser.

In the Cochrane review, “Flossing for the management of periodontal diseases and dental caries in adults,” Sambunjak et. al. searched multiple data bases in all languages, going back to at least 1950, for studies which assessed the effects of flossing in addition to toothbrushing (as compared with toothbrushing alone) in the management of periodontal diseases and dental caries in adults. Even with these broad search criteria, only twelve studies involving 582 participants were included in the systematic review. Seven of the trials were assessed as “unclear risk of bias”, while the remaining five were at “high risk of bias.” Plaque and gingivitis were the measured outcomes of the studies. While there was not much difference in the amount of plaque found on flossers and toothbrushers vs. toothbrushers alone, there was a statistically significant decrease in gingivitis in the flossing group at one, three, and six months. While this is a good thing, keep in mind that gingivitis is merely inflammation of the gums: the redness, puffiness, and bleeding you see in gums that aren’t healthy. This is to be distinguished from periodontitis, which is when the infection actually penetrates into the jaw bone surrounding the teeth, causing tooth mobility and ultimately tooth loss. So while this systematic review did show benefit in gingivitis reduction, there was no real benefit in terms of outcomes like tooth retention or other quality of life issues over the duration of the study. In a nutshell, the investigators concluded that “(t)here is some evidence from twelve studies that flossing in addition to toothbrushing reduces gingivitis compared to toothbrushing alone. There is weak, very unreliable evidence from 10 studies that flossing plus toothbrushing may be associated with a small reduction in plaque at 1 and 3 months.” They also pointed out that in this review, “(n)o studies reported the effectiveness of flossing plus toothbrushing for preventing dental caries” in adults.

The take home message from this review is that flossing might make your gums less puffy, and likely makes your mouth feel better without those hunks of shredded chicken stuck in that annoying space between your molars, but the study didn’t show any major benefit as to disease prevention or tooth loss.

But the real question to be asked of the second systematic review is: Did the study not show a decrease in disease incidence because flossing really doesn’t do anything, or was it because it wasn’t properly designed to do so? Let me explain. Periodontitis (gum disease that progresses into the degradation of the jaw bone, loss of attachment between the tooth and supporting ligament between the tooth and bone, and deepening of the pockets between tooth and gum) is usually a chronic condition, often taking years or decades to develop and manifest its signs and symptoms. Most of the studies included in the meta-analysis were of relatively short duration; thus, they weren’t able to truly ascertain if flossing had an effect on periodontitis or not.

The other point I want to make is that since we know that most people don’t know how to floss properly, were these studies of flossing standardized enough so that we know that the participants were Grade A flossers? Did they all floss the same way? For the same amount of time? Same frequency? Did they lie about flossing like everyone else does?

CONCLUSIONS

At the end of the day, here’s what we can conclude about the two systematic reviews and the news articles that were spawned from them: first and foremost, the sensationalist news articles are right. The scientific research does not show that flossing is effective against tooth decay or gum disease. But then again, it doesn’t show it’s not. To say things like “flossing may be unnecessary” or “is your dentist lying to you” is a bit of a stretch and taking the data where it should not go. An AP reporter did not “take down” flossing. There are no high level conspiracies wherein the government and its minions (i.e. dentists and hygienists) are withholding information from an unsuspecting public.

Is flossing beneficial? When done properly and regularly, the evidence shows that it is beneficial. But I think we can all agree that better studies are needed before we can state unequivocally that flossing reduces cavities and gum disease. I don’t think it’s unscientific to tentatively accept that flossing’s health benefits are true, but it can’t be claimed as absolute based upon our available evidence. Remember too that there are other ways to clean the plaque from between your teeth. Interdental cleaners with tiny brushes, high quality electric toothbrushes, and gadgets like the Waterpik® are adjunctive aids to keep your mouth clean and healthy.

So when someone asks me “Does flossing work?”, my response is “It doesn’t not work.”

Grant Ritchey

Grant Ritchey has practiced general dentistry in Tonganoxie, Kansas since 1986 and has been a strong advocate for evidence based health care policies and practices his entire career. He received his Bachelors Degree in Human Biology from the University of Kansas in 1982 and his Doctor of Dental Surgery Degree from the University of Oklahoma in 1986. He also recently completed the certification program in Science and the Public through the CFI. A contributor to the Science Based Medicine blog, Grant also hosts the popular Prism Podcast, a podcast that explores the spectrum of science, medicine, and critical thinking. He can be found on Twitter as @skepticaldds, and can be reached at skepticaldentist@gmail.com