For a glimpse of the future of orthodontics, consider a peek inside Motor City Lab Works, a Birmingham, Mich., office filled with 3D printers and other equipment that create custom retainers and clear plastic aligners.

In the laboratory, technicians use digital imaging and printers that spray resins to not just straighten people’s teeth, but also potentially save patients thousands of dollars and the discomfort of metal braces.

“This is like the Terminator coming out of the metal,” lab co-owner Christian Groth said, describing how one of the big printers works. “You know when the Terminator came out of the liquid? This is kind of what this printer is like. You’ve got this liquid resin and the model is rising out of it.”

In the science fiction film, it took seconds for the liquid-like substance to take solid form. In real life, it takes minutes and hours to make orthodontic models.

Still, dentists and orthodontists say it is only a matter of time before even more sophisticated technology makes it possible to make plastic appliances during patients’ appointments and for entrepreneurs, such as those behind SmileDirectClub, to sell more do-it-yourself treatments.

The advances already are setting off some big legal — and medical — clashes.

The broader question in the medical community — not just among dentists and orthodontists — is what limits, if any, should be put on entrepreneurs as innovations give patients treatment options that eliminate face-to-face visits with licensed professionals.

“One of the struggles that we are having in this field right now is getting both sides of the story,” Groth said. “The two sides of the story are the SmileDirectClub side and the other side is the orthodontists and dentists.”

The dental industry has been transformed in the past 20 years by computers that can design a sequence of clear aligners — plastic appliances that fit over the teeth and nudge them into place — to replace metal wires and brackets.

Moreover, 3D printer technology, which has helped create the molds for the aligners, has become more sophisticated and affordable, giving rise to companies such as SmileDirectClub — started in Detroit by two entrepreneurs who grew up in Michigan — that sell the aligners directly to patients.

SmileDirectClub emphasizes convenience and cost, calling its service “the secret to affordable braces for your teen” and saying that it “eliminates the middlemen who mark up the cost and cuts out all those unnecessary monthly office visits.”

SmileDirectClub says that braces can run $5,000 and more, but with the company’s aligners, children 12 and older can have their smiles transformed for $1,850.

With the click of a mouse, SmileDirectClub customers can set up a free, 30-minute appointment to book a scan of their mouth at a shop and order a kit to make their own teeth molds to mail back to get “a digital rendering of how great your new smile will look following treatment.”

But, Kevin Dillard, general counsel for the St.-Louis-based American Association of Orthodontists, argues that orthodontists are essential and that self-treatment, such as that offered by SmileDirectClub, can in the long run, do more harm — and cost more — than good.

“If what they mean by bypassing the middleman is taking the specialist or the professional out of the treatment plan process or follow-up process, that’s a bad thing,” Dillard said. “These people are specialists who make sure the teeth and jaw fit together properly.”

The idea of using braces to straighten teeth dates back thousands of years, the evidence of which comes from archaeologists who report finding ancient mummies with what appear to be metal bands around their teeth.

Modern orthodontics took root in the 18th and 19 centuries, and by the late 1990s, two Stanford University business students — Zia Chishti and Kelsey Wirth — found they could use 3D computer imaging to make clear plastic aligners to replace wires and brackets.

Chishti and Wirth had been orthodontic patients but were not orthodontists.

They took their innovation to market, forming the now-public San Jose, Calif.-based company Align Technology. The company sold the clear plastic appliances, which they called Invisalign, to dental and orthodontic practices.

Groth, whose father is a dentist, said he was his dad’s first Invisalign patient.

“This is moving teeth with plastic,” Groth said of the aligners. “Before that, it was impossible to take a digital model, sequentially move the teeth on the computer and take it into the real world and make physical models.”

In recent years, Groth and other orthodontists have realized that they can invest in their own labs to affordably make their own appliances for patients and even have a small business manufacturing them for other practices.

Makers of 3D printers see dental offices as an opportunity to sell their products.

And entrepreneurs have figured out that they can now sell appliances directly to patients by mail, offering customers a lower-cost alternative to traditional treatment that cuts the expense of visiting an orthodontist.

They say that if you can buy contact lenses and eyeglasses online, you should be able to buy orthodontia.