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How Clear Aligners Move Teeth

The aligner is a spring: each tray is molded to where the tooth should be, so it pushes the tooth toward that target. An illustrated look at distance-based movement, per-stage limits, and why aligners default to tipping.

How does a thin sheet of plastic actually move a tooth? Understanding this one thing changes how you plan every case.

The aligner is a spring

Each tray is molded not to where the tooth is now, but to where it should be at the end of that stage. So when you seat it, the tray is slightly deformed — and it pushes the tooth toward that target shape. That built‑in difference is the activation:

  1. The aligner is made to the target position, so its shape doesn’t match the current tooth.
  2. Once seated, the plastic deforms and presses at the points of mismatch — a gentle, continuous force.
  3. The tooth moves, and as the shapes come together, the force fades. Movement, then rest.

Distance‑based, not force‑based

This is why aligners are distance‑based. Each stage prescribes a small, fixed amount of movement, and the tray delivers whatever force that shape happens to produce. Ask for too much in a single stage, and the tooth simply can’t keep up — you lose tracking.

Each type of movement has its own ceiling per aligner:

Rotation · ~2°
Crown tip · ~1°
Root torque · ~2°
Bodily · ~0.3 mm
Vertical · ~0.1 mm

And these scale with root area — a big molar moves slower than a slender incisor.

It only moves while it’s worn

The tooth moves only while the aligner is worn — about 22 hours a day to express each stage. Wear it less, and the tooth lags behind the tray: you lose tracking, the aligner stops fitting, and the plan drifts. Compliance isn’t optional here; it’s the engine.

Where the force lands: the default is tipping

The tray grips the crown — and only the crown. That means the force is applied well above the tooth’s center of resistance. A force away from the center of resistance creates a moment, so the aligner’s natural, default movement is tipping: the crown goes one way, the root the other.

Force at the crown, above the center of resistance → a moment → the tooth tips (crown one way, root the other).

Tipping is easy; everything else takes control. To get bodily movement, rotation, or torque, you add a second force — a couple — to counter that tipping. That’s what attachments and auxiliaries like elastics are for.

Key takeaways

The aligner is a spring, movement is distance‑based (with a strict ceiling every stage), it needs ~22 hours a day, and its default is tipping around the center of resistance. Keep those four in mind and the rest of biomechanics falls into place.

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Frequently Asked Questions

How do clear aligners actually move teeth?

Each tray is molded not to where the tooth is now, but to where it should be at the end of that stage. When you seat it, the tray is slightly deformed and presses at the points of mismatch — a gentle, continuous force that pushes the tooth toward the target and fades as the tooth catches up.

How much can a clear aligner move a tooth per stage?

Small, fixed amounts. Roughly: rotation ~2° per stage, crown tipping ~1°, root torque ~2°, bodily movement ~0.3 mm, and vertical movement (intrusion/extrusion) just ~0.1 mm. Aligners are distance-based, not force-based — ask for more than the ceiling in one step and the tooth can't keep up.

Why do you have to wear clear aligners 22 hours a day?

The tooth only moves while the aligner is worn. It needs about 22 hours a day to express each stage. Wear it less and the tooth lags behind the tray — you lose tracking, the aligner stops fitting, and the plan drifts.

Why do clear aligners tend to tip teeth?

The tray grips only the crown, so the force lands well above the tooth's center of resistance. A force away from the center of resistance creates a moment, so the aligner's default movement is tipping. Controlled movements (bodily, rotation, torque) need a second force — a couple — from attachments or auxiliaries.