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When John Hofford, a 27-year-old digital media specialist, wanted to straighten his teeth, he wanted invisible plastic aligners that would correct his crowding, instead of conspicuous metal braces. But he couldn’t afford the clear aligners sold at his local orthodontist in Atlanta.

So in September, he paid $124.95 for a kit to take his dental impressions at home. After watching a how-to video, he mixed putty, put it in mouth-guard-type trays and took impressions of his upper and lower teeth, which he mailed back to be turned into a three-dimensional model.

A dentist he had never met or spoken to suggested a series of clear aligners to correct his crowded bottom row and one slightly forward tooth on top.

“You’re saving money by not talking to someone face to face,” said Mr. Hofford, who is on his third set of aligners, which are progressively shaped to shift teeth. He has paid about $600, roughly half of the total cost.

Invisible aligners are usually prescribed by an orthodontist or dentist after an in-person examination and discussion of treatment options. Invisalign and ClearCorrect are two of several companies that make aligners that entail hands-on supervision at repeat office visits, at a typical cost of $4,000 to $6,000.

Aiming to disrupt the status quo, two companies, SmileCareClub and CrystalBraces, the company that Mr. Hofford is working with, now offer aligners remotely so adults and older teenagers can straighten their teeth at home for $900 to $2,100, depending on the complexity of their teeth-straightening needs.

“It shouldn’t have to cost a small fortune to straighten your teeth,” said Doug Hudson, one of four entrepreneurs who founded SmileCareClub. He disputes that their aligners are “do-it-yourself” braces and said patients “are guided all along the way” — in this case by customer representatives rather than the prescribing dental professional.

Critics of the fledging phenomenon argue that prescribing aligners to patients sight unseen risks overlooking oral health issues that can compromise orthodontic treatment. And, experts say, aligners are not appropriate for everyone, even if companies suggest they can correct advanced crowding, in which teeth overlap or are twisted, for example.

“You can waste your time and money trying to do something that’s physically impossible,” said Dr. DeWayne McCamish, an orthodontist in Chattanooga, Tenn., with 42 years of experience. He said he would never rely on a patient’s “selfie picture” to determine whether the teeth within the bone would align properly when in-office X-rays could be taken.

The Food and Drug Administration considers aligners prescription devices and has cleared more than 10 kinds, the agency said. But it does not regulate the practice of dentistry or orthodontics, leaving it to states and their dental boards to decide what is appropriate care.

Last month, the American Association of Orthodontists, a trade group, warned consumers against moving teeth without an initial exam and continuing supervision by an orthodontist. “Our concern is that patients who don’t see an orthodontist for regular checkups and/or for a complete diagnosis are more likely to be harmed,” said Dr. Rolf Behrents, a spokesman for the association.

Dr. Behrents says he is also concerned about people using elastic hair bands to close a gap between their front teeth, a practice inspired by do-it-yourself YouTube videos made by people with no dental training. In September, he wrote an editorial alerting people that these so-called “gap bands” can work themselves under the gum and lead to tooth loss.

“Minors are teaching minors how to do this,” he said. “It’s disturbing, and the outcome could be devastating.”

Many orthodontists argue that an in-person exam is critical to looking for untreated cavities and underlying gum infections before any teeth-straightening is done. “Orthodontic treatment exacerbates gum disease,” said Dr. Scott Hamilton, an orthodontist in Topeka, Kan. “You really can’t look at impressions to determine if there’s any periodontal disease.”

But Dr. Son Tran, the creator of CrystalBraces and a Dallas dentist who is the sole reviewer of that company’s mailed-in impressions, disagrees. He said he could tell from a snapshot or a dental impression if a patient had swollen gums that would require the patient to visit a dentist before getting CrystalBraces.

Dr. Tran insists that prescribing an aligner to a remote patient is not delivering a lesser standard of care. “I’m looking at all the models and trays coming in,” he said, though he has spoken to just a handful of the 3,600 patients he has treated since 2013. “Quality of care is upheld through the whole proceedings.”

For its part, SmileCareClub has signed up more than 90 orthodontists and dentists in 43 states to review photographs taken by patients and a 3-D model mostly made from at-home impressions. (They are recruiting providers in the remaining seven states.)

Still, the company relies on a patient’s honesty to confirm oral health, with patients required to sign a form that their dentist has checked for decay and gum disease and taken X-rays in the last six to 18 months..

Mr. Hudson said about 20 percent of cases required referrals for in-person care, and that patients must be at least 15, because younger teenagers may need “the hand-holding of repeat visits to make sure they are adhering to treatment.”

Dr. Damon Barbieri, an orthodontist in Nashville, says he uses SmileCareClub aligners for “minor” issues and applauds that an “affordable option” exists. However, he said, “The ideal is still to be seen by the orthodontist for treatment, no matter how small your case, if you can afford it, and have the time for it.”