Clear aligners straighten teeth by applying controlled pressure through a series of custom-fitted plastic trays, each one slightly different from the last. You wear each tray for one to two weeks, then swap it for the next in the sequence. Every new tray is shaped to move your teeth a fraction of a millimeter closer to their final position, and over months, those tiny shifts add up to a complete realignment.
How Each Tray Moves Your Teeth
When you snap in a new aligner, it doesn’t quite fit your teeth as they are right now. That small mismatch is intentional. The plastic pushes against specific teeth, creating gentle force that triggers your body’s natural bone-remodeling process: the bone on the pressure side gradually breaks down, while new bone fills in behind the tooth as it shifts. This is the same biological mechanism that traditional braces use, just delivered through plastic instead of wires.
The key difference is that each aligner can only produce a limited amount of movement before the plastic settles into place and stops applying force. That’s why you cycle through so many trays. A typical course of treatment for moderate crowding, spacing, or mild bite issues runs 6 to 12 months, with anywhere from a dozen to 50 or more individual trays depending on complexity.
The Digital Planning Behind the Scenes
Before a single tray is made, your orthodontist or dentist takes a 3D scan of your teeth. Software then separates each tooth into an individual digital object that can be moved independently, complete with estimated root positions and gum contours. Your provider maps out where every tooth needs to end up, and the software calculates the path each one will take to get there.
This is where the real engineering happens. The software determines how fast each tooth can safely move based on its crown shape, root size, and position in the arch. It sequences the movements so that teeth don’t collide with each other along the way, resolving potential contact-point conflicts step by step. The result is a frame-by-frame animation of your entire treatment, and each frame becomes a physical tray.
What the Plastic Actually Does
Most aligners are made from one of two types of thermoplastic: a rigid material called PETG or a slightly more flexible variety called high-hardness thermoplastic polyurethane. Some brands, including Invisalign, use multilayer designs that sandwich different plastics together. Multilayer trays produce gentler forces, roughly two to three times lower than single-layer PETG trays, which tends to make them more comfortable while still moving teeth effectively.
Plastic behaves differently than the metal wires in braces. Metal springs back with consistent force over time, but plastic is viscoelastic, meaning it loses some of its push as it settles. The force is strongest when you first put a new tray in and gradually decreases. Engineers account for this by designing each tray to slightly overshoot the needed force so that, even after the initial drop-off, enough pressure remains to keep teeth moving. Some newer materials use a shape-memory approach: the tray is manufactured in your target tooth position, then heat-set to match your current mouth shape. Body temperature slowly triggers it to return toward its original form, providing a more gradual, sustained force.
How Attachments Help With Difficult Movements
For anything beyond simple crowding, you’ll likely need attachments. These are small, tooth-colored bumps of composite resin bonded directly to certain teeth. Think of them as tiny handles that give the aligner something to grip. Without them, the smooth surface of a tooth doesn’t provide enough leverage for the plastic to rotate, tilt, or pull a tooth in a specific direction.
The shape, size, and placement of each attachment are customized to your treatment plan. A rectangular attachment on a canine, for instance, might help the aligner push that tooth’s root into a new position, while a beveled attachment on a premolar could help rotate it. Once treatment is over, your provider removes them by polishing the composite off, leaving no permanent mark.
Why 22 Hours of Wear Matters
The standard recommendation is 22 hours of daily wear, leaving only about two hours for eating, drinking, and brushing. That number isn’t arbitrary. Teeth need sustained, consistent force to remodel bone safely. If you wear your trays for only 12 hours a day, the force cycles on and off, essentially pushing teeth in one direction, then letting them drift back before pushing again. This “jiggling” effect can slow treatment and may increase the risk of root damage.
Compliance is the single biggest variable patients control. Falling short on wear time is one of the most common reasons treatments take longer than planned or require extra rounds of trays.
Where Aligners Have Limits
Clear aligners handle mild to moderate crowding, spacing, and some bite corrections well, but they struggle with certain complex movements. Root positioning (torque) is a consistent weak spot. One study found that when aligners were programmed to tip the roots of upper front teeth by more than 10 degrees, the actual movement achieved was less than half of what was planned. Deep bite correction fares similarly: research has measured overbite reduction accuracy at around 61%, and curve-of-Spee leveling at about 62%.
Across the board, studies show a roughly 50% gap between what the digital plan predicts and what actually happens in a patient’s mouth. That doesn’t mean half your teeth won’t move. It means some movements fall short of their targets, particularly posterior torque, overjet correction, and molar repositioning. Fixed braces, with their ability to bend wires and apply torque directly, still outperform aligners for these specific challenges.
Refinements Are Normal
Because of that prediction gap, most aligner patients need at least one round of refinement trays. Orthodontists report that 70% to 80% of their aligner patients require some form of mid-course correction. After you finish your initial set of trays, your provider rescans your teeth, evaluates what moved as expected and what didn’t, and orders additional trays to close the remaining gaps. Some people need only a handful of refinement trays. Others go through two or three rounds.
Common reasons for refinements include a bite that hasn’t fully corrected, a stubborn tooth that didn’t track with its tray, gaps that didn’t close completely, or small rotations that fell short. This isn’t a failure of the system. It’s built into how aligner treatment works, and most comprehensive treatment plans include refinements at no extra cost. In a small percentage of cases, the remaining movements are complex enough that patients switch to fixed braces to finish.