The hottest new orthodontic device in America was invented not by doctors but by a couple of MBAs. It can cost half again as much as the conventional treatment, and it's backed by a multimillion-dollar ad campaign -- but not (so far, anyway) by much clinical data. Yet despite its curious pedigree, Invisalign, a clear-plastic, removable and, as the name suggests, nearly invisible alternative to traditional braces, has become the treatment of choice for thousands of maloccluded adults.

Introduced in 1999 by Align Technologies Inc., a Silicon Valley start-up launched by Stanford business school classmates Zia Chishti and Kelsey Wirth (a D.C. native), Invisalign is now straightening teeth in some 15,000 mouths nationwide. The company says that nearly three-quarters of North America's 8,500 orthodontists have been trained to use the system, in which patients are fitted with a series of disposable, custom-made aligners that gradually move teeth into the desired position. The training consists of a single five-hour program, which includes viewing a videotaped presentation and attending a question-and-answer session.

Invisalign has its roots in the decades-old concept of using a series of removable devices to straighten teeth. But earlier attempts proved time-consuming and ultra-expensive, as every individual aligner had to be painstakingly calibrated and handcrafted. Invisalign's genius lies in its use of computer imaging and fabrication processes to to manufacture the aligners without all the handiwork.

Silicon Valley technology may have made invisible braces practical, but it hasn't made them cheap. Align says its system can ultimately cost patients anywhere from $2,000 to $8,000, depending on the complexity of the treatment; Align estimates that the typical patient pays $4,000 to $6,000 to straighten both the upper and lower teeth.

Included in the fee is the $1,000 to $3,000 per patient the orthodontist pays Align for fees and lab work; beyond that, the premium patients pay is at the orthodontist's discretion. Checkups and supervision are far less time-consuming than with traditional orthodontia. Since pricing is set by the orthodontist, it pays to shop around.

"The average cost for [conventional] braces is $4,000," Wirth explains, adding that most doctors using Invisalign add about a $1,000 premium to that fee.

That math doesn't quite jibe with Peter Greeves's experience. "For me, it's been about twice as expensive" as traditional braces would have been, he says. Greeves, the 34-year-old owner of a D.C. real estate business, says he's shelling out $7,000 for a 17-month Invisalign treatment rather than the $3,000 he says he would have paid for metal braces.

Even if you can afford Invisalign and choose to pay the additional money for the relative invisibility of its treatment, it's not the answer for everyone.

"Invisalign is very good at straightening teeth within an arch," says Greeves's orthodontist, Wayne Hickory, who practices in Silver Spring, Ellicott City, Clarksville and Bethesda. Hickory, currently Align's biggest customer, says he often starts patients whose needs are complicated or severe with traditional braces and switches to Invisalign once the major corrections are made.

"About a third of my Invisalign cases don't meet the criteria, so we start them with pre-Invisalign treatment, which can include retainers with springs or expansion mechanisms. We have dozens and dozens of removable appliances that are very powerful," Hickory says.

Richard Levy, an orthodontist with offices in the District and Gaithersburg, agrees that the aligners are best at fixing crowding or adjusting the amount of space between teeth. "With Invisalign, there's not a lot of correction of malocclusion caused by a mismatched bite" or with correcting problems caused by the skeletal structure, he says.

Still, even in the absence of published research on the product (at least four Invisalign-related studies are in the works, and Align has published some 30 case studies on its Web site), Michael Rennert, president of the American Association of Orthodontists, says he is "confident that it works in specific cases."

"I suspect [Invisalign] does work in carefully selected cases, defined as adults with minor corrections," Rennert says. "Invisalign is just one more appliance that orthodontists can choose."

An Align spokesperson disagrees, saying that Invisalign can treat whatever traditional braces can treat, with the exception of cases requiring extractions other than lower incisors. "One thing that traditional braces can treat better is missing teeth, because it's harder to anchor the aligner when there are lots of missing teeth." Finally, as with braces, conditions such as active gum disease, untreated cavities and severe TMJ (temporomandibular joint disorder) prohibit orthodontic work.

The Invisalign consumer message board (at www.invisible.com) is full of missives from teenagers desperate to use Invisalign instead of being sentenced to a mouth full of metal. While Invisalign received FDA clearance to market its product to patients of all ages and degrees of malocclusion (crooked teeth) in 1998, the company has opted for now to restrict its marketing to adults and teens (generally age 15 or older) whose teeth are fully grown in.

As Hickory explains, "You cannot factor a variable like growth" into the process for generating a course of treatment using Invisalign, "so it's not for growing children." And even in adults, Hickory says, "if there's a major bite problem, if the upper and lower jaws don't meet, Invisalign isn't the right treatment."

In the end, though, teeth straightened by Invisalign get just as straight as those treated with metal braces, and in about the same amount of time. (The average for either treatment is six to 18 months.) Though it's too soon to tell whether Invisaligned teeth are more likely than their traditionally adjusted counterparts to slip back into their old places, Hickory reminds us that even metal braces don't always achieve permanent correction.

"A lot of adults who had braces as kids need work again," Hickory says. Align and orthodontists who use Invisalign note that people whose teeth have been straightened by any method often must wear retainers after the treatment is completed to keep teeth in place.

Those who have worn metal braces know that the regular orthodontist visits for "tightening" are literally wrenching experiences, as the doctor manipulates the wires and rubber bands that pull the teeth into place. With Invisalign, there is no tightening: Every six weeks or so, the orthodontist just peeks in to make sure the teeth are going where they're supposed to go and hands the patient his next few sets of aligners.

But the pain remains. One of the reasons the Washington-based orthodontist Avadis Abrahamian doesn't use Invisalign (besides the system's price, which he considers prohibitive) is that he's heard that the aligners are painful to wear. But orthodontist John H. Jones III of Sterling, who started offering Invisalign to his patients last fall, says, "You have to define pain. Different people have different thresholds."

"But even with metal braces, you get used to the discomfort," Jones says, adding that whenever you straighten teeth, by any method, "you're moving a solid substance through a solid substance. The teeth have to get a little loose before they'll move."

So far, patients seem willing to tolerate that discomfort. While the American Association of Orthodontists keeps no records of customer complaints, the FDA does, and so far the agency has received none about Invisalign. In Align's filing with the Securities and Exchange Commission in anticipation of its initial public offering in February, the company noted that the customer complaints it has fielded so far have to do with shipping delays (patients can expect to wait a frustrating month or more -- sometimes many more -- between diagnosis and delivery of the first set of aligners) and "minor manufacturing irregularities."

And while some of the messages on the Invisalign Web site include questions about whether the soreness they experience is normal, many more messages have to do with the product's availability and cost -- and whether insurance will cover that cost. (Align suggests that those companies that cover traditional braces are likely to cover Invisalign, since insurers don't discriminate among methods of orthodontic work. And for those without coverage, Align recently began offering its own financing plan.)

Despite the cost, Greeves is an Invisalign booster. "I was always just a little self-conscious about my teeth," which he describes as having been "slightly crooked," "but never to the point of putting on wire braces." When he saw Hickory's newspaper ad for invisible braces, Greeves says, he was "sold instantly." With just a few months left before graduating to a retainer, Greeves has remained thrilled with the product and the difference it has made in his appearance.

"I used to take the aligners out for important business meetings, but not anymore," Greeves says. Although he says he initially was conscious of slightly slurring his speech, "either that stopped or I just got used to it." And while he says "your tongue messes around with them at the beginning," he now claims he "wouldn't even take them out for kissing." Greeves adds that, as a side benefit, his oral hygiene has improved since he started using Invisalign. "You have to take them out when you eat, and you have to brush and floss after every meal" before reinserting the aligners, he explains.

But even Greeves admits that Invisalign isn't perfect. In addition to the cost, he cites other, minor drawbacks. "When you take them out, there's always a string of spit that comes out with them," he says, and he has opted to wrap the aligners in a napkin or stick them in his shirt pocket while he eats, rather than use the carrying case supplied by Invisalign. Greeves says the case "is easier for a woman to use, because she can just stick it in her purse."

Finally, Greeves concedes that, despite his hygiene efforts, the aligners have given him "slightly bad breath. It seems like saliva stagnates in them," he says.

Invisalign's founders have worked hard to distinguish their product from traditional orthodontics through a $40 million ad campaign. The orthodontic community welcomes the campaign as a rising tide that will float all appliances: The universe of maloccluded adults represents a huge, untapped market for orthodontists, and Invisalign provides a perfect entree. Align notes that of the 200 million Americans who have "some form of malocclusion," only 2 million currently enter orthodontic treatment each year. Hickory says the ad campaign might bring adults who have been wary of braces into orthodontists' offices at last. Even if Invisalign turns out to be inappropriate for all comers, Hickory notes, "While it's not a bait and switch operation, it is wonderful if people come in and find out about another option."

And Invisalign stands to make orthodontic practices more profitable: While Hickory notes that he reduces the cost to patients a bit by subtracting the price of the technical work associated with traditional braces, doctors generally can spend less "chair time" with Invisalign patients -- while charging them more.