How clear aligners work, from the first scan to the last tray

Clear aligners work by moving teeth in small planned steps. A clinician scans your teeth, plans the finished position in software, and the plan is cut into a series of plastic trays, each shaped a fraction closer to the end result. You wear each tray for one to two weeks for 20 to 22 hours a day, then move to the next, with check-ups along the way and a retainer at the end.

That is the process the US Food and Drug Administration describes on its consumer page, and the one the makers describe for their own systems, as read on 18 Oct 2026. The trays are the visible part. What decides whether treatment goes well is the examination before them, the plan behind them, and the checking in between.

The five steps, as the FDA sets them out

The FDA's page on clear dental aligners breaks treatment into five stages. They are worth reading in order, because each one says who does what.

Notice how much of that table is the clinician's work and how little is the plastic's. That balance is the honest answer to "how does it work".

Small steps: what a tray actually does

Each tray is made to the position the teeth are meant to reach next, not to where they are now. Pressed onto the teeth, the slightly-wrong shape pushes; worn for a week or two, the teeth follow; then the next tray asks for the next small step.

The makers describe it in their own words. Align Technology says your Invisalign aligners are designed to apply the right amount of force to the right place at the right time based on a digital plan that maps out the exact movements of your teeth. Ormco describes Spark as a series of doctor-prescribed, custom-made, removable aligners that move teeth incrementally with gentle force.

How often you change trays is set by the plan, and the published intervals vary by system. Align says patients typically switch to a new set weekly or every two weeks, depending on the provider's instructions. Ormco says Spark patients usually get a new set every one to two weeks. AlignerCo says each of its sets is typically worn for around 10 days, and NewSmile says about every two weeks.

The scan and the plan

Everything is built from a model of your mouth. In a practice, that is usually an intraoral scan; Align describes its iTero scanner producing a 3D scan from which the doctor maps out the treatment plan. Mail-order sellers start from impressions you take at home or a scan at a partner location; AlignerCo describes taking impressions or scans, receiving a projected result, approving the plan and then receiving the aligners.

The plan itself is made in the maker's software. Align names ClinCheck, Ormco names its Approver software, and ClearCorrect names ClearPilot. All three let the clinician set the movements, stage them and preview the finish. The preview is a simulation of a planned result, not a promise of an outcome, which is why the before-and-after pictures deserve the scepticism they get here.

Two things should happen before any of that: an examination and X-rays. The American Association of Orthodontists points out that X-rays show what a scan of the tooth surfaces cannot, including the roots and the bone around them, and that mail-order options usually skip this step.

Attachments and IPR

A smooth tray on a smooth tooth can tip it, but some movements need something to push against. That is what attachments are: small tooth-coloured bumps bonded to certain teeth, with matching pockets in the tray. Ormco calls them handlebars that help move teeth in sync with the aligners; ClearCorrect calls its version Engagers. The detail is in our guide to attachments.

The other tool is interproximal reduction, usually shortened to IPR: a clinician removes a small amount of enamel between certain teeth to make room for them to move. ClearCorrect's guide for dentists tells them to explain the purpose of Engagers and IPR to the patient before proceeding, if the plan calls for either. If yours does, ask where, how much, and why — it is a step that cannot be undone, and you are entitled to understand it.

Tray changes and check-ups

Align describes a series of regular, face-to-face check-ups at which the doctor hands over each new batch of aligners and monitors progress. Ormco says the frequency of visits depends on each patient's orthodontist. The FDA says periodic in-person visits help make sure teeth are moving safely and as planned.

What a check-up looks for is simple to describe: whether the teeth are where the plan expected them to be at this tray, whether each tray is seating fully, whether attachments are intact, and whether the gums and bite are healthy. A tray that has stopped fitting is information. It means teeth and plan have drifted apart, and only somebody who can see the teeth can decide what happens next.

When the plan needs a second pass

Teeth do not always land exactly where the software put them. When that happens, the usual answer is a new scan and an additional series of trays, often called refinements. Angel Aligner's own history describes one of its materials as designed to reduce refinements, which tells you they are an expected part of aligner treatment rather than a sign that something has gone wrong.

AlignerCo's FAQ says treatment can take longer than expected, and gives the two reasons: inconsistent wear time, or more complex tooth movement. The first is in your hands. Before you agree a price, ask whether refinements are included and how many, because that is where a quoted course and a finished course most often part company. The factors that lengthen treatment are covered in how long clear aligner treatment takes.

What an examination rules out first

Aligners move teeth in bone and gum, so the bone and gum have to be fit to be moved. Ormco's FAQ says patients with severe periodontal disease should not start aligner treatment until the disease is managed. The FDA says an in-person examination can help lower the risks of an uneven bite, unintended tooth movement, pain and tooth loss.

Decay, failing fillings and crowns matter too. The FDA lists among the limitations of aligners that existing crowns, fillings and implants may loosen or become dislodged and need repair or replacement. Those are things a dentist looks at before the first tray, not after the fifth.

Where aligners struggle

The FDA says clear aligners are commonly used to treat mild to moderate orthodontic problems, including crowded teeth, gaps and bite problems, and that more complex conditions may require braces or other treatment. It also lists, among the limitations, that aligners may not effectively treat more complex conditions.

Makers' claims run wider. Ormco says orthodontists use Spark for open bites, deep bites, overbites, underbites, crossbites, crowding and spacing. Align's published treatment-time figures are based, by its own footnote, on mild to moderate cases, which it defines as crowding and spacing up to 6mm. Both statements can be true at once: a system may be capable of a movement, and your case may still be one where another appliance is the better choice. Which one it is gets decided by somebody looking at your records.

Who is supervising

The FDA's position is direct. It reviews and authorises clear aligners before they are sold in the United States, and it has not authorised clear aligners sold directly to consumers without a prescription and supervision by a dentist or orthodontist. Clear aligners are prescription devices, it says, only safe to use under the supervision of a licensed practitioner.

The FDA also adds a test anyone can run. You can ask your clinician which product is being used, or search the FDA's 510(k) database for product code NXC. It warns that "FDA-registered" is not the same as "FDA-authorized": registration means a manufacturer listed its facility and products, while authorisation means the agency reviewed the specific product.

Remote monitoring is not ruled out. The FDA says virtual visits can be a useful part of care, but that periodic in-person visits help reduce the risks it lists. The history of the companies that tried to do without them is on what happened to Byte, SmileDirectClub and Candid.

After the last tray

Treatment does not end with the last aligner. Align says that, like every teeth-straightening treatment, Invisalign is followed by a retainer to keep teeth straight, either its own Vivera retainers or another brand depending on the doctor. The FDA's fifth step says the same. Retainers are a cost and a habit that last far longer than the trays; the retainer page prices them.

What to ask before you start

1. Who is my treating clinician, and will they examine me in person with X-rays before anything is made?
2. Which system will you use, and is it authorised for sale in my country?
3. Will my plan need attachments or IPR, and where?
4. How often will I change trays and how often will you see me?
5. Are refinements and retainers included in the price?

Those five answers tell you more about how your aligners will work than any animation of moving teeth. Every published price, and what it includes, is on what clear aligners cost.