What clear aligners can and cannot fix

Clear aligners move teeth. With the right case, they can straighten crowded teeth, close mild gaps and correct many bite problems, often with help from small tooth-colored bumps called attachments, rubber bands called elastics, or a little reshaping between some teeth. They cannot reposition an adult's jaws, replace a missing tooth, move a dental implant, or treat decay, gum disease or tooth color. Whether aligners suit your teeth depends on what is causing the problem and how severe it is, and an in-person assessment works that out.

In the United States, aligners are prescription devices. The FDA says they are most commonly used for mild to moderate crowding, gaps and bite problems, and that more complex conditions may need braces or other treatment.

Problem by problem

The table sets out the general pattern described by the American Association of Orthodontists and the FDA. It is a guide for the conversation with a dentist or orthodontist rather than a diagnosis, because the same label can describe a small tooth problem or a large jaw problem.

Some of those movements are harder for aligners than for braces. A 2019 systematic review in BMC Oral Health, which compared the two, found aligners less effective at widening the arch, which matters for a narrow upper jaw, at controlling the angle of tooth roots, and at getting the back teeth to meet firmly. Turning teeth was the least accurate movement in the study described further down.

Why the cause matters more than the label

An overbite, underbite, crossbite or open bite can come from where the teeth sit, from the size and position of the jaws, or from a mix of both.

Aligners work on the first. They push teeth through the bone that holds them, and they leave the jaw bones themselves where they are.

When the problem is mainly in the teeth, aligners can often do the job, sometimes with extra help. Attachments give the trays something to grip. Elastics hooked between the upper and lower teeth help correct how the teeth meet. The American Association of Orthodontists says aligners may not work as predictably for severe bite problems, significant rotations or large gaps, where braces may be needed, and that some plans combine aligners with elastics or other appliances.

When the jaws are the main cause, the treatment changes. The American Association of Orthodontists says some bite problems cannot be solved by braces or clear aligners alone, and that the jaws can then be repositioned by an oral and maxillofacial surgeon. For problems caused by growth, that surgery is usually done once growth has finished, with orthodontic treatment before and after it. A good assessment will say which kind of problem you have before any trays are made.

What reshaping between teeth is for

Interproximal reduction, sometimes called slenderizing or filing, removes a thin layer of enamel from the sides of some teeth. It makes the teeth slightly narrower so that crowded teeth have room to line up, and the British Orthodontic Society says it can also be used to improve the shape of teeth. It is used when only a small amount of space is needed, and removing too much enamel can harm the teeth; ask whether your plan includes it, on which teeth and why. Gaps are usually closed by moving teeth together rather than by reshaping.

How closely teeth follow the plan

Before treatment starts, you are usually shown a digital model of where your teeth should end up. Real teeth often fall short of that model.

A 2020 study in the American Journal of Orthodontics and Dentofacial Orthopedics followed 38 Invisalign patients, with an average age of 36, who all wore their aligners as instructed. It measured how far each tooth actually moved compared with the movement planned for that set of aligners. On average, teeth achieved about half of the planned movement. Tipping teeth toward the tongue or cheek was the most accurate, at 56%; turning teeth was the least accurate, at 46%; and some movements were lower still, such as turning the upper canines (37%) and moving the lower front teeth deeper into their sockets, called intrusion (35%).

That figure measures the accuracy of individual movements, not whether treatment worked: it does not mean half of treatments fail. When 19 of the finished cases were graded against a standard orthodontic scoring system, 14 of them (74%) reached a passing score.

That was one aligner system in one study, and newer systems may perform differently. Extra sets of aligners, called refinement aligners, are a common way of closing the gap.

The practical point is that a plan is a target. Before you start, ask how many refinements are included and what happens if teeth still do not move as planned.

What aligners do not treat

Decay and gum disease. These are checked at the assessment and dealt with before teeth are moved. The American Association of Orthodontists says aligners can trap liquids and food against the teeth, increasing the risk of cavities and gum disease, and ordinary dental checkups should continue throughout.

Missing teeth and implants. Where a tooth is missing, there are two different plans. Aligners can sometimes close the space by moving the neighboring teeth together, in which case no replacement may be needed. Or they can open or hold the space so that a bridge or implant can fill it. Your clinician should say which plan you are on before treatment starts. An existing implant is fused to the bone and does not move under orthodontic force, which is why orthodontists sometimes use one as a fixed anchor for moving other teeth. The plan has to work around it. The FDA also lists loosening of existing crowns, fillings or implants among the risks of aligner treatment.

Tooth color and shape. Whitening is a separate treatment, and worn, chipped or small teeth may need bonding or veneers. Some people combine treatments, straightening first and then changing shape or color. That sequence is covered in aligners, veneers and bonding.

Keeping the result. Teeth tend to move back after treatment. The American Association of Orthodontists says you will likely need to wear retainers for the rest of your life. What that looks like day to day is in care after aligner treatment.

Children and teenagers

In a growing child, a jaw problem can sometimes be guided while it is still developing, and teeth that have not come through yet change what the plan can do. That makes the decision different from an adult with mild crowding. The details are in alignment treatment for children and adolescents. An age printed on a provider's website is a product condition. The clinical decision comes from the examination.

What an assessment should include

The American Association of Orthodontists advises an in-person examination and appropriate X-rays before orthodontic treatment, because they are needed to diagnose the problem and plan it safely. The FDA says in-person evaluation by a licensed dental professional is preferred and that it has not authorized aligners sold directly to consumers without a prescription and supervision. Being registered with the FDA is not the same as being authorized; if you want to check a product, the FDA's 510(k) database lists aligners under product code NXC.

Questions that make the assessment useful:

• Is my problem mainly in the teeth, the jaws, or both?
• Which teeth will the plan move, and which parts of my bite will it leave as they are?
• Will I need attachments, elastics, reshaping between teeth, or braces for part of the treatment?
• Do I have decay, gum disease or dental work that should be dealt with first?
• What happens if my teeth do not follow the trays, and are extra aligners included?
• How long will I need to wear retainers, and are they included in the price?

How a clinic plan and a remote plan differ is covered in Invisalign and at-home aligners. The practical sequence of a remote plan, for people who have already been told they are suitable, is in the process of straightening teeth at home.