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BACKGROUND	The exact force systems as well as their progressions generated by removable thermoplastic appliances have not been investigated .
BACKGROUND	Thus , the purposes of this experimental study were to quantify the forces and moments delivered by a single aligner and a series of aligners ( Invisalign ; Align Technology , Santa Clara , Calif ) and to investigate the influence of attachments and power ridges on the force transfer .
METHODS	We studied 970 aligners of the Invisalign system ( 60 series of aligners ) .
METHODS	The aligners came from 30 consecutive patients , of which 3 tooth movements ( incisor torque , premolar derotation , molar distalization ) with 20 movements each were analyzed .
METHODS	The 3 movement groups were subdivided so that 10 movements were supported with an attachment and 10 were not .
METHODS	The patients ' ClinCheck ( Align Technology , Santa Clara , Calif ) was planned so that the movements to be investigated were performed in isolation in the respective quadrant .
METHODS	Resin replicas of the patients ' intraoral situation before the start of the investigated movement were taken and mounted in a biomechanical measurement system .
METHODS	An aligner was put on the model , the force systems were measured , and the calculated movements were experimentally performed until no further forces or moments were generated .
METHODS	Subsequently , the next aligners were installed , and the measurements were repeated .
RESULTS	The initial mean moments were about 7.3 Nmm for maxillary incisor torque and about 1.0 N for distalization .
RESULTS	Significant differences in the generated moments were measured in the premolar derotation group , whether they were supported with an attachment ( 8.8 Nmm ) or not ( 1.2 Nmm ) .
RESULTS	All measurements showed an exponential force change .
CONCLUSIONS	Apart from a few maximal initial force systems , the forces and moments generated by aligners of the Invisalign system are within the range of orthodontic forces .
CONCLUSIONS	The force change is exponential while a patient is wearing removable thermoplastic appliances .

