Clear aligners move teeth by applying gentle, sustained pressure that triggers your jawbone to reshape itself around each tooth. Each aligner tray is manufactured slightly different from your current tooth positions, and that mismatch between the plastic and your teeth creates the force that drives movement. The process is the same biological mechanism behind traditional braces, just delivered through a different device.
The Mismatch That Creates Movement
Every aligner tray is made from a thermoplastic material molded to a shape your teeth haven’t reached yet. When you snap the tray over your teeth, the elastic material bends slightly to fit, and that deformation creates pressure against specific teeth. Each tray is designed to move teeth about 0.25 mm, a fraction of a millimeter, before you switch to the next one in the sequence. That tiny increment is deliberate: it keeps forces light enough to be biologically safe while still triggering the bone remodeling process.
The specific direction and amount of pressure depend on several factors, including the thickness and stiffness of the plastic, the shape of the tray, and whether small attachments have been bonded to certain teeth. These variables make aligner mechanics more complex than they appear from the outside.
What Happens Inside Your Jaw
The real action takes place in the periodontal ligament, a thin layer of tissue connecting each tooth root to the surrounding bone. When an aligner pushes a tooth in one direction, it compresses the ligament on one side and stretches it on the other. Those two sides respond very differently.
On the compressed (pressure) side, blood flow decreases and some cells in the ligament die off in a process called hyalinization. Your body sends in specialized cells called osteoclasts, which dissolve and absorb the bone in the path of the moving tooth. This clears the way for the tooth to shift.
On the stretched (tension) side, blood flow increases. That triggers a different set of cells, osteoblasts, to start depositing new bone on the surface the tooth is moving away from. The original fibers connecting the tooth to the bone become embedded in this new bone tissue, which gradually hardens. New bone keeps forming until the ligament returns to its normal width. So your tooth isn’t just sliding through bone. The bone in front of the tooth is being removed while new bone fills in behind it, essentially rebuilding the socket in a new position.
Cell activity on the tension side typically begins within 30 to 40 hours after force is applied, which is why consistent wear matters so much in the early days of each new tray.
How Force Changes Over Time
Aligner plastics don’t push with perfectly constant force from day one to day seven. These materials are viscoelastic, meaning they behave somewhere between a rubber band and a stiff sheet of plastic. When stretched into a fixed position (over your teeth), the force they deliver drops over time, a phenomenon called stress relaxation.
That drop can be significant. Some aligner materials lose roughly 50% of their initial force within the first 24 hours. Others retain force better: materials like F22 Evoflex and Erkoloc-Pro show less decay, maintaining more consistent pressure over a full wear cycle. The ideal material would deliver a light, steady force throughout the entire period you wear each tray, but in practice, most of the active tooth movement happens in the first few days when forces are highest.
This force decay is one reason different brands and orthodontists prescribe different wear schedules. Some trays are designed for 7-day changes, others for 10 to 14 days, depending on the material properties and the complexity of the movements planned.
Tipping vs. Bodily Movement
Not all tooth movements are created equal. The two main types are tipping (where the crown of the tooth tilts in one direction while the root stays mostly in place) and bodily movement, or translation (where the entire tooth, root included, shifts as a unit).
Aligners are naturally better at tipping. Because the plastic only grips the visible crown of the tooth, force is applied above the tooth’s center of resistance, which sits partway down the root. That leverage point tends to tip the crown rather than push the whole tooth. Research consistently shows that expansion achieved with aligners comes more from crown tipping than from true bodily movement.
Translation is harder because it requires controlling the root, which the aligner can’t directly touch. To get around this limitation, orthodontists use attachments.
What Attachments Do
Attachments are small tooth-colored composite bumps bonded to the surface of specific teeth. They change the way the aligner grips the tooth, creating leverage points that make more complex movements possible.
For bodily movement, rectangular or ellipsoid attachments on the outer surface of a tooth allow the aligner to generate a counterbalancing force. Instead of just pushing the crown, the aligner can push against the attachment in a way that creates a pair of opposing forces (a “couple”), which moves the root along with the crown.
Rotation is one of the trickiest movements for aligners. Round teeth, like canines, don’t give the plastic a flat edge to push against. Without a grip point, the aligner tends to slide around the tooth rather than turning it. Attachments placed on the front and back corners of these teeth give the aligner the surface area it needs to apply rotational force effectively.
Extrusion, pulling a tooth downward out of the bone, presents a similar challenge. Without an attachment, the aligner can slip over the tooth like stacking traffic cones: it moves over the tooth without actually pulling it. An attachment near the gum line gives the aligner something to grab onto, converting what would be wasted sliding motion into genuine vertical force.
Why 22 Hours a Day Matters
Current protocols call for wearing aligners 20 to 22 hours per day, removing them only for eating and brushing. That requirement isn’t arbitrary. It’s tied directly to the biology of bone remodeling.
The pressure side of a moving tooth needs sustained force to keep osteoclasts actively removing bone. If you remove the aligner for extended periods, the biological signals that drive resorption weaken, and the tooth can begin to drift back toward its original position. The tension side also needs continuous stimulation to keep depositing new bone in the right location. Inconsistent wear doesn’t just slow treatment down. It can make individual tooth movements less accurate, requiring additional trays (called refinements) to correct the shortfall.
Each tray is typically worn for about 7 days before switching, though some treatment plans extend that to 10 or 14 days depending on the difficulty of the planned movements and the material’s force retention. The cumulative effect of dozens of trays, each moving teeth a quarter of a millimeter, produces the total correction over months of treatment.
Limits of What Aligners Can Do
Because aligners rely on crown contact and plastic elasticity rather than rigid wires anchored to brackets, certain movements remain less predictable. Large rotations on round teeth, significant vertical movements, and substantial root repositioning are all areas where aligners underperform compared to traditional braces, even with attachments.
Orthodontists compensate by “overcorrecting” in the digital treatment plan, programming slightly more movement than needed so that even if the tooth only achieves 70 or 80 percent of the planned displacement, the clinical result is acceptable. For complex cases, some practitioners use a combination of aligners and short phases of fixed appliances to handle the movements aligners struggle with most.