In vitro force generation of aligners in relation to attachment geometry and 3D printing technology: A pilot study

Objective: To evaluate in vitro forces and moments generated by clear aligners in relation to attachment geometry and 3D printing technology.
Materials and methods: Five different movements of a premolar were simulated in a virtual model. Combined with four attachment geometries, horseshoe models were printed using fused filament fabrication and digital light processing 3D printing. On these models, 120 aligners (100 active and 20 passive) were produced with 0.6- and 0.8-mm foils (Erkodur, Erkodent Erich Kopp, Pfalzgrafenweiler, Germany). Using a modular 3D printable orthodontic measurement apparatus, forces and moments were measured for all active aligners. Statistical analysis, including the intraclass correlation coefficient and a generalised linear model, was conducted to obtain information about the influence of model fabrication technology, attachments, movement and aligner foil thickness on force, which was calculated as a vector.
Results: Error measurement showed an excellent intraclass correlation coefficient, greater than 0.93 for all directions. The printing technology had no significant influence on force development of the thermoformed aligners (P = 0.7123), and the aligner material showed borderline significance (P = 0.0531). The presence or absence of attachments was also not significant (P = 0.5153). In the generalised linear model, type of movement, aligner material and vertical rectangular attachments 1 mm in size were identified as predictors of the amount of force generated.
Conclusion: Printing technology does not influence the amount of force generated by aligners. The presence or absence and variation in shape of attachments may be influential but do not show consistent behaviour.