How Teeth Actually Move: The Biomechanics Behind Every Aligner

A tooth moves when a light, steady force presses it against the bone that holds it. The bone gives way slowly on the side being pressed and rebuilds on the other side, and the tooth travels with it. An aligner is simply a way of delivering that force. What decides the result is how much force each tooth receives, in which direction and for how long, which is the field of orthodontic biomechanics.

The biology: bone that remodels

Each tooth sits in a socket of bone, held by a thin cushion of fibres called the periodontal ligament. When a force pushes the tooth, the ligament is squeezed on one side and stretched on the other. Cells on the squeezed side remove bone, and cells on the stretched side lay new bone down. Over weeks, the socket itself moves.

That is why force is measured in small amounts and kept steady. Too little, and nothing happens. Too much, and the squeezed ligament can lose its blood supply, which stalls movement until the body clears the damaged area.

There is no single “right” force for every tooth. A systematic review screened 466 animal and human studies; only 29 met its standards, and the authors concluded that no evidence on the optimal force level could be extracted from the literature (Ren et al., The Angle Orthodontist). Among the reasons they gave were large differences between people and studies that did not control the type of movement. The force a tooth needs depends on the person, the tooth and the movement planned.

The mechanics: where the force lands

A tooth is anchored by its root, so a push on the crown does not simply slide it sideways. Pushed at the crown alone, a tooth tips, like a fence post leaning in soft ground. To move the whole tooth, root and crown together, the force has to be paired with a turning effect called a moment, which keeps the root travelling with the crown.

The balance between that push and that turning effect decides the movement:

• Tipping. The crown moves and the root stays close to where it was. This is the easiest movement to produce.
• Bodily movement. Crown and root move together. This needs a carefully balanced force and moment.
• Root movement (torque). The root moves more than the crown. This is the hardest movement for an aligner, and studies report losing up to half of the planned root movement (Nucera et al., Materials).

Force also changes with time. In a 2024 laboratory study of five aligner materials, starting up-and-down forces ranged from 0.6 to 3.0 newtons, about the weight of 60 to 300 grams. Every material lost a significant share of its force or turning effect within the first two days, in most directions measured, with no further significant drop to day 14 (Elshazly et al., Journal of Orofacial Orthopedics).

What this means for aligners

Your result depends on the plan more than on the plastic. A tray can only deliver what was designed into it: the force, its direction and the turning effect on each tooth. Planning starts from that physics, then the trays are made to match.

Measuring those forces was long a gap in the field. Most earlier lab work measured one or two teeth at a time. Dr. Hisham Badawi originated OSIM, an orthodontic simulator that measures the forces and turning effects on all 14 teeth of an arch at once, in three dimensions, and led its development as his PhD work at the University of Alberta. The first results were published in the American Journal of Orthodontics and Dentofacial Orthopedics in 2009, and research groups in several countries have since cited the work.

At Smile Elements, that biomechanics is the foundation of treatment:

• The 3D plan, the 3D printing and the finished aligners all stay with us, on one technology platform built on orthodontic biomechanics.
• Each movement is planned for the force it needs, so tipping, bodily movement and root movement are each set up the way the physics requires.
• The clinician leads every decision. Planning support includes machine-learning clinical decision support, with the clinician in the loop as the sole author of every prescription.
• Diagnosis comes first. Some movements are better done with braces, and the consultation decides which tool suits your case. How We Choose Your Treatment explains how.

See the plan for your own teeth

Every case starts with a diagnosis. Book a consultation and Dr. Badawi will examine your teeth and bite and explain which movements your treatment needs.

References

1. Ren Y, Maltha JC, Kuijpers-Jagtman AM. Optimum force magnitude for orthodontic tooth movement: a systematic literature review. The Angle Orthodontist. 2003;73(1):86-92.
2. Nucera R, Dolci C, Bellocchio AM, et al. Effects of composite attachments on orthodontic clear aligners therapy: a systematic review. Materials. 2022;15(2):533.
3. Elshazly TM, Keilig L, Nang D, et al. Effect of thermomechanical aging on force system of orthodontic aligners made of different thermoformed materials. Journal of Orofacial Orthopedics. 2025;86(5):337-346.
4. Badawi HM, Toogood RW, Carey JP, Heo G, Major PW. Three-dimensional orthodontic force measurements. American Journal of Orthodontics and Dentofacial Orthopedics. 2009;136(4):518-528.
5. Badawi H. The use of multi-axis force transducers for orthodontic force and moment identification. PhD thesis, University of Alberta, 2009.

Related: Clear aligners for adults