Root-canal therapy is less painful than people think it will be, but dentists are not very adept at estimating patients' pain during the procedure.

A recent study asked 333 adults to use a 100-point scale immediately before and after a root-canal procedure to predict how painful the procedure would be and to describe how painful it actually was. Dentists were asked to estimate patients' pain levels during the procedures.

During a root-canal procedure, a dentist removes the nerve and blood vessels from inside a tooth and replaces them with a rubber-like compound. The procedure is done under local anesthesia.

Patients' predictions of pain averaged 38.3 on the 100-point scale. The level of pain they reported experiencing averaged 22.7. People who predicted higher pain levels (40 or above) were more than twice as likely as other people to actually experience higher levels of pain.

The study also found that although women tended to predict significantly more pain than men, both sexes experienced the same levels of pain. Dentists in the study misjudged all patients' pain during the procedure, although they were better at estimating women's pain.

"Men may not be expressing their pain adequately for whatever reason, but regardless of gender, dentists are underestimating patients' pain," said Catherine Watkins, DDS, PhD, assistant professor in the department of preventive and community dentistry at the University of Iowa and first author of the study.

In a surprising result, people who had received root-canal therapy in the past did not predict they would feel less pain than people having their first root-canal treatment. "It doesn't seem to matter if this is your first or fifth root canal," Watkins said. "It's like having children ? every one is different. Dentists need to recognize that they should prepare patients no matter how many times they've gone through it."

In addition, she said, "We typically think that if the [tooth's] nerve is totally dead, the person won't experience as much pain, but we found that didn't seem to matter." The group also found that root-canal procedures on molars were not more painful than those on smaller teeth.

One week after the procedure, the researchers contacted each person and asked him or her how they remembered their pain then, compared with directly after the procedure. "Some remembered it higher than they'd reported it and some remembered it much lower," Watkins said. "I don't know what happens to people; maybe some people like to blow it out of proportion and others like to forget about it."

The study was published in the January issue of the Journal of the American Dental Association.