A clear aligner has no wires and no visible mechanism, yet it moves teeth using the same biological process that braces use. The force simply comes from a different source.
The tray is deliberately the wrong shape
Each aligner is manufactured to the shape the teeth should reach, not the shape they currently have. The difference is small, typically a fraction of a millimetre per tray.
Seating that tray forces the plastic to deform slightly around the existing teeth. Being elastic, it tries to return to its manufactured shape, and that recovery presses on the teeth.
The tooth then responds as it would to any sustained pressure: bone resorbs ahead of the movement and rebuilds behind it, and the tooth migrates into the space.
Smooth plastic cannot grip a smooth tooth
Some movements need the aligner to pull rather than push, and a slippery tray on a rounded tooth has nothing to hold. It simply lifts off.
Small tooth-coloured composite shapes are therefore bonded to selected teeth. These attachments give the plastic a defined surface to engage, converting a general squeeze into a directed force.
Their position and shape are planned rather than decorative, which is why attachments differ between people and between stages of the same treatment.
Some movements are harder than others
Tipping a crown sideways is straightforward, because the force is applied where the tray grips best and the tooth pivots readily.
Moving a whole root bodily, rotating a rounded tooth or extruding one from its socket demands more control, and these are the movements that most often need attachments or extra stages.
Case selection reflects this. The system suits many situations well, but not every problem, and that judgement requires records rather than photographs alone.
Wear time is not a suggestion
Force only produces movement when it is sustained. Intermittent pressure allows the tooth to settle back between periods of wear, and progress stalls.
This is why wear schedules are specified in hours per day. The number is doing real mechanical work rather than acting as encouragement.
Trays that no longer seat fully usually indicate that the teeth have fallen behind the plan, and that is a reason to contact the supervising clinician rather than skip ahead.
Supervision does more than dispense trays
Someone has to check that teeth are tracking, that gums and bone are healthy enough to accept force, and that the bite is not being left worse than it started.
Tooth movement is not risk free. Existing gum disease and untreated decay both alter what should be attempted, and neither is visible in a smiling photograph.
An in-person examination is what establishes whether movement is appropriate at all, and it is the part of the process that no scanning app performs.