The biology of tooth movement
Teeth are not rigidly fused to the jawbone. Each tooth is suspended in its socket by a network of fibres called the periodontal ligament (PDL). These fibres connect the root surface to the surrounding bone and act as a shock absorber during chewing — but also as the biological mechanism that allows orthodontic movement.
When a brace or aligner applies a gentle, sustained force to a tooth, the PDL on the compression side (the direction the tooth is being pushed towards) becomes compressed. On the tension side (opposite), the fibres are stretched. This mechanical signal triggers a cascade of cellular activity:
On the compression side: Osteoclasts (bone-removing cells) are activated and begin dissolving the bone in front of the moving tooth, creating space for it to move into.
On the tension side: Osteoblasts (bone-forming cells) deposit new bone behind the tooth, filling the space the tooth has left.
This continuous cycle of bone removal and bone formation — known as bone remodelling — is what allows teeth to move predictably through the jaw. It is the same process that heals fractures and allows the jaw to respond to changes in bite load throughout life.
The components of a brace and what they do
| Component | What it does |
|---|---|
| Bracket | Bonded to the tooth surface; holds the archwire and transfers force to the tooth |
| Archwire | The main wire that runs through all brackets; provides the sustained force that moves teeth |
| Ligature / module | Ties or clips that hold the archwire into the bracket slot |
| Elastics / rubber bands | Added by the patient; apply inter-arch force to correct the bite relationship |
| Molar bands | Metal rings cemented on back molars; provide anchorage for the system |
| Coil springs | Open or close spaces between specific teeth |
How do clear aligners move teeth?
Clear aligners work on exactly the same biological principle. Each aligner tray is manufactured very slightly differently from the current position of the teeth — usually a fraction of a millimetre — and worn 20–22 hours per day. The pressure from the aligner is transmitted to the PDL and initiates the same bone remodelling process.
The key differences compared to fixed braces are that aligners exert forces primarily via surface contact rather than via brackets and wires, and force can only be applied when the aligner is in the mouth — which is why compliance (wearing them consistently) is critical to treatment success.
Attachments — small composite tooth-coloured bumps bonded to teeth — are often added to improve the grip the aligner has on the tooth and to allow more precise control of tooth movement, particularly for rotations and vertical movements.
Why retention matters
Bone remodelling continues for months after braces are removed. During this period, the newly formed bone is immature and less stable, and the stretched PDL fibres on the tension side retain a 'memory' — they want to pull teeth back to where they came from. Without retainers, teeth will relapse.
The British Orthodontic Society (BOS) advises that retainers should be worn indefinitely — lifelong nightly wear — to maintain results. This is not a sign of treatment failure; it is simply because the forces that cause teeth to drift (lip pressure, tongue pressure, late jaw growth) do not stop after braces come off.
Frequently Asked Questions
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