AESTHETICS, NOT MEDICINE:

The uneasy relationship between dentist and patient is further complicated by an unfortunate reality: Common dental procedures are not always as safe, effective, or durable as we are meant to believe. As a profession, dentistry has not yet applied the same level of self-scrutiny as medicine, or embraced as sweeping an emphasis on scientific evidence. "We are isolated from the larger health-care system. So when evidence-based policies are being made, dentistry is often left out of the equation," says Jane Gillette, a dentist in Bozeman, Montana, who works closely with the American Dental Association's Center for Evidence-Based Dentistry, which was established in 2007. "We're kind of behind the times, but increasingly we are trying to move the needle forward."

Consider the maxim that everyone should visit the dentist twice a year for cleanings. We hear it so often, and from such a young age, that we've internalized it as truth. But this supposed commandment of oral health has no scientific grounding. Scholars have traced its origins to a few potential sources, including a toothpaste advertisement from the 1930s and an illustrated pamphlet from 1849 that follows the travails of a man with a severe toothache. Today, an increasing number of dentists acknowledge that adults with good oral hygiene need to see a dentist only once every 12 to 16 months.

Many standard dental treatments--to say nothing of all the recent innovations and cosmetic extravagances--are likewise not well substantiated by research. Many have never been tested in meticulous clinical trials. And the data that are available are not always reassuring.

The Cochrane organization, a highly respected arbiter of evidence-based medicine, has conducted systematic reviews of oral-health studies since 1999. In these reviews, researchers analyze the scientific literature on a particular dental intervention, focusing on the most rigorous and well-designed studies. In some cases, the findings clearly justify a given procedure. For example, dental sealants--liquid plastics painted onto the pits and grooves of teeth like nail polish--reduce tooth decay in children and have no known risks. (Despite this, they are not widely used, possibly because they are too simple and inexpensive to earn dentists much money.) But most of the Cochrane reviews reach one of two disheartening conclusions: Either the available evidence fails to confirm the purported benefits of a given dental intervention, or there is simply not enough research to say anything substantive one way or another.

Fluoridation of drinking water seems to help reduce tooth decay in children, but there is insufficient evidence that it does the same for adults. Some data suggest that regular flossing, in addition to brushing, mitigates gum disease, but there is only "weak, very unreliable" evidence that it combats plaque. As for common but invasive dental procedures, an increasing number of dentists question the tradition of prophylactic wisdom-teeth removal; often, the safer choice is to monitor unproblematic teeth for any worrying developments. Little medical evidence justifies the substitution of tooth-colored resins for typical metal amalgams to fill cavities. And what limited data we have don't clearly indicate whether it's better to repair a root-canaled tooth with a crown or a filling. When Cochrane researchers tried to determine whether faulty metal fillings should be repaired or replaced, they could not find a single study that met their standards.

"The body of evidence for dentistry is disappointing," says Derek Richards, the director of the Centre for Evidence-Based Dentistry at the University of Dundee, in Scotland. "Dentists tend to want to treat or intervene. They are more akin to surgeons than they are to physicians. We suffer a little from that. Everybody keeps fiddling with stuff, trying out the newest thing, but they don't test them properly in a good-quality trial."

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