Invisible aligners move your teeth by applying gentle, constant pressure through a series of custom-fitted plastic trays. Each tray is slightly different from the last, nudging your teeth a fraction of a millimeter at a time toward their final position. The process works because bone is living tissue that reshapes itself in response to sustained force, allowing teeth to shift gradually over months.
The Biology Behind Tooth Movement
Your teeth aren’t fused directly into your jawbone. They’re held in place by a thin ligament that connects the tooth root to the surrounding bone. When an aligner pushes on a tooth, it compresses this ligament on one side and stretches it on the other. That mechanical signal triggers your body’s bone remodeling process.
On the compressed side, specialized cells called osteoclasts break down bone to make room for the tooth to move into. On the stretched side, bone-building cells called osteoblasts lay down new bone to fill the gap left behind. This cycle of breakdown and rebuilding is constant and coordinated. It’s the same biological process your body uses to heal a fracture, just happening in slow motion under controlled pressure. The entire treatment depends on this remodeling staying in balance. If bone breaks down too fast or too slow, teeth either loosen or resist movement.
What the Aligners Are Made Of
Clear aligners are made from medical-grade thermoplastic polymers, most commonly thermoplastic polyurethane (TPU) or polyethylene terephthalate glycol (PETG). These aren’t interchangeable materials. They have different strengths that affect how treatment feels and how predictably your teeth move.
PETG is valued for its transparency, impact resistance, and stiffness. It delivers higher initial force but can lose that force more quickly over a wear cycle. TPU, on the other hand, is more elastic. Major aligner brands favor it because its flexibility allows it to apply lighter, more consistent pressure over the days you wear each tray. That consistency matters: all aligner polymers lose a significant amount of their pushing force within the first eight hours of wear, then level off at a steady plateau for the rest of the tray’s lifespan. A material that starts with a gentler force and holds it steadily tends to produce more predictable tooth movement than one that starts strong and drops off sharply.
The manufacturing process itself also changes the material. When flat sheets of plastic are heated and vacuum-formed over a model of your teeth, they thin out and become less stiff. So the final aligner on your teeth behaves differently from the raw material it started as.
How Each Tray Moves Your Teeth
Before treatment starts, your orthodontist or dentist takes a detailed 3D scan of your teeth. Software maps out the entire movement sequence from start to finish, then divides that journey into tiny increments. Each aligner tray represents one increment, typically moving teeth about 0.25 to 0.3 millimeters per stage.
When you snap in a new tray, it doesn’t quite fit your current tooth positions. That slight mismatch is intentional. The tray pushes your teeth toward the shape it was molded to, and over the next several days, your teeth settle into that new position as bone remodels around them. Once they’ve moved enough, the tray fits comfortably and it’s time to switch to the next one.
You’ll wear each tray for 20 to 22 hours a day, removing them only to eat, drink anything other than water, and brush your teeth. The switching interval varies. For years, the standard was changing trays every 14 days. In 2016, some manufacturers introduced 7-day protocols, and many providers now use 7 to 10-day changes depending on the complexity of the case and how your teeth are responding.
Attachments and Complex Movements
Aligners work well for straightforward movements like closing gaps or correcting mild crowding, but some tooth movements are harder to achieve with a smooth plastic shell alone. Rotating a tooth, pulling a tooth downward (extrusion), or moving roots rather than just crowns all require more grip than a slippery tray can provide on its own.
This is where attachments come in. These are small tooth-colored bumps of composite resin bonded directly to specific teeth. They act as handles, giving the aligner something to grab onto so it can deliver force in directions that would otherwise be impossible. For example, rotating a round premolar is difficult because the aligner tends to slide over the smooth surface. An attachment on that tooth creates a ledge the tray can push against, converting sliding pressure into rotational force.
Not everyone needs attachments, but most moderate to complex cases use them. Your provider plans their placement digitally before treatment begins, positioning each one to optimize a specific movement. Without them, unwanted side effects can occur, like a tooth drifting sideways when it was only supposed to rotate. Attachments are removed at the end of treatment by polishing them off the tooth surface.
How Long Treatment Takes
Treatment duration depends almost entirely on how far your teeth need to move. Mild cases, like minor spacing or slight crowding, typically take 6 to 9 months. Moderate issues such as a noticeable overbite or overlapping front teeth generally require 12 to 18 months. Complex cases involving significant bite correction or severe crowding can take 18 to 24 months or longer.
These timelines assume full compliance with wear time. Every hour the aligners spend out of your mouth is time your teeth aren’t receiving the pressure they need, and bone remodeling slows or reverses. Falling below 20 hours of daily wear is the most common reason treatment takes longer than planned. Some patients also need refinement trays at the end of treatment: additional sets of aligners to fine-tune teeth that didn’t move exactly as the software predicted. Refinements can add a few weeks to a few months.
What Aligners Can and Cannot Do
Clear aligners handle tipping movements well. Tipping means tilting the visible part of the tooth (the crown) in a new direction, and it’s the easiest motion for a plastic tray to produce. They’re also effective at closing small gaps, relieving mild to moderate crowding, and making minor bite adjustments.
Root movements are harder. Moving the entire root through bone requires extensive remodeling and more force control than aligners naturally provide. Extrusion, or pulling a tooth further out of the gumline, is another challenging movement because it relies on a pulling force rather than a pushing one, and aligners are fundamentally push-based devices. These movements are possible with attachments and careful planning, but they’re less predictable than with traditional braces, where a wire threaded through brackets can deliver pulling and torquing forces directly.
For severe skeletal discrepancies, where the issue is jaw position rather than tooth position, aligners alone typically aren’t sufficient. These cases may require braces, jaw surgery, or a combination approach.
The Day-to-Day Experience
New trays feel tight for the first day or two, sometimes with mild soreness similar to a dull ache. This fades as your teeth begin to settle. Most people adjust to the feeling within the first week of treatment and barely notice subsequent tray changes.
Speech can feel slightly different at first. Some people notice a minor lisp for a day or two with a new tray, but the tongue adapts quickly. The trays are thin enough (usually under a millimeter) that most people around you won’t notice you’re wearing them unless they’re looking closely.
You’ll visit your provider every 6 to 8 weeks for progress checks. These appointments are usually quick, since there are no wires to tighten. Your provider checks that teeth are tracking with the trays, replaces any attachments that have come loose, and may use interproximal reduction (gently polishing between teeth to create small amounts of space) if the treatment plan calls for it. Between visits, you’re managing treatment largely on your own by wearing your trays consistently and switching them on schedule.