Along with getting a driver's license, voting for the first time and reaching legal drinking age, having your wisdom teeth removed is a familiar milestone en route to adulthood.

"I remember that it was just something I had to do, like a rite of passage," says Rebecca Saxer, 25, of Alexandria. Saxer had her wisdom teeth pulled at age 19 because her dentist "said I should get them out before they started to grow in and mess up my other teeth. Getting the wisdom teeth out seems to be a universally shared experience among people I know."

The American Academy of Oral and Maxillofacial Surgeons (AAOMS) generally recommends removal by age 25 for any impacted wisdom tooth -- one that's blocked from surfacing through the gum into a normal position. Even teeth that show no signs of disease or aren't causing obvious problems should be extracted in most cases, the group holds, to prevent future problems like infection, gum disease, tooth decay or damage to adjacent teeth.

But there is little good research to support the practice of extracting wisdom teeth for prophylactic reasons -- and little reliable data on the associated medical risks and costs.

In a 1996 review on the issue, Thomas Daley, an associate professor of oral pathology at the University of Western Ontario in Canada, wrote that wisdom teeth "are sacrificed . . . like no other human tissue, in the name of preventive health care." Daley estimated that no more than 12 percent of adults over age 30 will develop problems with impacted wisdom teeth requiring extraction and that the risk of surgical complications in these patients increases by only about 10 percent from what it is in adolescents.

In Britain last year, an advisory group to the government's National Health Service recommended against the extraction of disease-free wisdom teeth.

No one tracks the number of wisdom teeth extracted each year in the United States; estimates range from the hundreds of thousands to about 2 million. One of the most serious surgical risks is permanent nerve damage, estimated to occur in about 1 percent of cases. Less dire surgical complications, including temporary nerve damage, impaired healing, infection, pain and excessive bleeding, occur more frequently.

But keeping one's wisdom teeth also has risks. In one extreme case, says Kevin Schwartz, an oral surgeon with offices in Olney, Rockville and Mount Airy, a large cyst developed around a patient's unerupted wisdom tooth and ate away a portion of the bone in the jaw. The patient required extensive surgery to rebuild the tissue, using bone grafts from the hip. Schwartz says he also sees emergency cases where infections caused by impacted wisdom teeth have closed off a patient's throat -- a potentially fatal event.

Assessing your own odds can be difficult. "What makes this such a hard case is that we don't have predicting factors that let us know who's going to be at high risk of developing problems with wisdom teeth and who's not," says E. Preston Hicks, an orthodontist and assistant professor in the department of oral health at the University of Kentucky in Lexington. Nor do dentists have good numbers on how often these problems occur overall.

A study being sponsored by AAOMS, the first of its kind in the United States, may provide definitive answers. The study, in its fourth year, is examining what happens when people keep their wisdom teeth. Research leader Ray White, a professor of oral and maxillofacial surgery at the University of North Carolina in Chapel Hill, says its goal is to "fill in the gaps in our knowledge" and "give information to patients who have to make a decision" about wisdom tooth removal. Longitudinal studies like this one, which follow a group of people over time, are more reliable than earlier "snapshot" studies or studies that analyzed dental records.

Prehistoric Souvenirs

Wisdom teeth, formally known as third molars, are the four teeth farthest back in the mouth -- one on each side of the upper and lower jaw. Not everyone has wisdom teeth. About 85 percent of people between the ages of 15 and 20 have all four, some or all of which may be impacted, and 95 percent have at least one, according to Scandinavian studies. (No comparable U.S. data exist, says White.)

Scientists say third molars are probably an evolutionary leftover from prehistoric humans, who had larger jaws. Today, many people's jaws are too small to comfortably accommodate a third set of molars. The result is that a developing wisdom tooth's path is often blocked by bone, gum or another tooth -- in which case the tooth is considered to be impacted.

Some teeth are fully impacted, failing to break through, or "erupt," at all, while others are partially impacted, with part of the tooth poking out through the gums. Sometimes an impacted tooth lies in the jaw at an angle instead of sitting straight up.

Oral and maxillofacial surgeons (dentists with special training in surgery of the mouth and face) perform roughly 90 percent of all wisdom tooth removals; general and pediatric dentists perform the rest. The cost of surgery generally ranges from about $100 to $350 per tooth, depending on whether a tooth is covered by soft tissue or bone; more complicated extractions can cost more. Add at least another $150 if the surgery is done under general anesthesia. Dental insurance typically covers at least part of the cost of surgery.

Dentists who favor removal of all or most impacted teeth say these teeth will almost certainly cause problems in the future, and should be taken out when a person is young and risk of surgical complications is minimal. The AAOMS advises that when surgery is warranted, it should be done by age 25, or before the roots of these teeth have fully developed. Delaying extraction, it says, makes surgery more difficult, prolongs healing and increases the risk of complications.

Also helping to tip the balance toward extraction may be parents' concerns -- unwarranted, according to several studies, but still supported by many dentists -- about the cosmetic effects of their children's unerupted wisdom teeth. "A lot of parents have spent thousands of dollars on orthodontia for their kids, and the last thing they want to happen is to have their kids' teeth misaligned because of wisdom teeth coming in," says Debbie Lichtblau of Reston, whose teenage daughter recently had her wisdom teeth removed.

Less Ambiguous Cases

The most common clearcut indications for removing third molars, says James Hupp, professor and chair of oral-maxillofacial surgery at the University of Maryland in Baltimore, are problems causing pain or pressure. Pain most often signals inflammation or infection of tissues around a partially erupted tooth. Decay in the tooth itself can also cause pain and swelling, and on-and-off pressure in the back of the mouth can occur when wisdom teeth are trying unsuccessfully to erupt.

Problems warranting extraction can also exist in the absence of symptoms; only an X-ray would reveal the cause of concern. In some cases, a cyst forms around a tooth and may grow and eat away surrounding bone. A wisdom tooth coming in at an angle can generate enough pressure to erode the bone that supports and protects the back of the adjacent second molar. Less often, a third molar tries to erupt into the side of the root of the second molar and creates enough pressure to wear away the back of the root. Wisdom teeth may also be removed to facilitate orthodontic or surgical procedures.

Hupp stresses that dentists should weigh the risks and benefits of removing teeth for each patient -- a position also held by the AAOMS.

A major risk of third molar surgery is temporary or permanent damage to the inferior alveolar nerve, which runs next to the lower wisdom teeth; damage causes numbness and tingling of the lip or chin. (X-rays can help predict the risk of injury.)

Less often, surgery damages the lingual nerve, which provides sensation to the tongue and floor of the mouth, resulting in a numb tongue and impaired taste, either temporarily or permanently.

All told, nerve damage has been reported to occur in up to 20 percent of wisdom tooth extractions, and is permanent in an estimated 0.5 to 1 percent. In some cases, an expert surgeon can repair the damaged nerve and restore at least partial feeling. In the worst cases, patients with permanent lingual nerve damage have chronic pain and describe feelings resembling an electric shock when they touch certain areas in the mouth. They may bite their tongue or burn their mouth often and have trouble speaking or eating.

The main risk of surgery on the upper wisdom teeth is creation of an opening into the maxillary sinus, which can result in sinus pain or infection. This occurs less often than nerve damage and can be repaired or may even heal on its own. Wisdom tooth surgery can also result in a painful though treatable condition called dry socket, reported to occur in up to 35 percent of extractions, in which healing of the empty tooth socket is delayed.

Less frequent complications include problems opening the jaw, damage to adjacent teeth or surrounding bone, a broken jaw or an obstructed air passage. Other possible complications, which can occur with any surgery, are excessive bleeding, swelling or pain; infection; and reactions to anesthesia.

Preliminary Findings

White's study on third molars has already offered some insights on the likelihood that problems will develop and the kinds of problems most likely to occur if symptom-free wisdom teeth are not removed.

The data are coming from more than 300 study participants aged 14 to 40 who have all four wisdom teeth and no symptoms. Researchers enrolled these people over a 2 1/2-year period, and hope to follow each patient for at least four years.

Results so far suggest that roughly two out of three people will develop problems with their wisdom teeth over their lifetime. "You can look at that several ways," White says. "If you're going to Las Vegas with those odds, you'd make money." But, he says, "if you were running an insurance company or you were an employer paying for dental insurance, you'd be paying for 30 percent of people who didn't need their wisdom teeth out."

When the study is completed, White says, "hopefully we'll be able to predict better than we can now who's going to be in the two-thirds" who will develop problems.

Problems that occur most often, says White, are periodontal (gum) disease and wisdom tooth decay, both of which he estimates will occur in more than 20 percent of those who keep their wisdom teeth, and localized infection, which he projects will occur in 8 to 10 percent. White called the rate of periodontal problems "much higher than anyone would have [previously] estimated." Another surprise, he said, is that "the teeth that are causing the most problems are those that are vertical and in the mouth at the biting surface," not those that are impacted and recommended for prophylactic removal.

White is leading several other studies as part of the AAOMS-funded research on third molars, including one on factors affecting recovery after wisdom tooth surgery. "We're not going to answer all the questions with one clinical trial, but we're moving in the right direction," he says.

Searching for Consensus

In the absence of conclusive data, sometimes a dentist and patient agree to disagree.

"I still have my wisdom teeth, despite having been told approximately once a year for the past 10 years to get them removed," says Lila Guterman, 27, a Washington science writer. She recently chose to fill cavities in two partially impacted wisdom teeth rather than have them removed.

Steven Fields, a dentist in Gaithersburg, says he tries to be conservative. "If a wisdom tooth seems to be erupting correctly, and there looks like there's going to be room, then I would monitor it." But he's more likely to recommend removal "if it looks like there's going to be a problem with brushing, if they don't take care of their teeth and don't come in to see me regularly."

At present, dentists rely on X-rays and physical examinations to monitor wisdom teeth, knowing that problems can often precede symptoms. But White says more specific monitoring guidelines are badly needed. A decade from now, he says, dentists may have access to more sophisticated screening techniques, including tests that identify bacteria in the third molar area or check for substances known to play a role in inflammation.

Meanwhile, they're waiting to see if the data finally being collected will offer better guidance. "We're still struggling with trying to be an evidence-based model of practice," says dental professor E. Preston Hicks. "We're becoming better at it, but we have more improvements to make."

Elia Ben-Ari is a Washington area freelance writer.

A panoramic X-ray of a jawbone shows two impacted wisdom teeth on each side of the patient's mouth.Simplified illustrations show the three most common types of wisdom tooth impaction.