Forces and moments delivered by PET-G aligners to an upper central incisor for labial and palatal translation

Aligners made of polyethylene terephthalate glycol (PET-G) were tested in an experimental study for labial and palatal translation of an upper central incisor to quantify the forces and moments thus delivered and to biomechanically evaluate the capability of bodily movement.

Using a resin model of the upper dentition, tooth 21 was separated and connected to a 3D force/moment (F/M) sensor to record the forces and moments delivered by aligners for labial and palatal displacement. An impression was taken with tooth 21 in its neutral position to obtain casts for standardized thermoplastic fabrication of aligners varying in make and foil thickness (Duran® 0.5/0.625/0.75 mm; Erkodur® 0.5/0.6/0.8 mm; Track-A® 0.5/0.63/0.8 mm). Upon placing each aligner over the teeth of the resin model, the separated tooth was subjected to 0.01 mm increments of labial and palatal translation by 0.25 mm in either direction.

The mean forces delivered by the thinnest (0.5 mm) aligners for 0.25 mm of palatal displacement of tooth 21 were 3.01 ± 0.07 N (Duran®), 5.31 ± 0.89 N (Erkodur®), and 3.69 ± 0.81 N (Track-A®). The thickest (0.75 or 0.8 mm) aligners delivered 4.49 ± 0.16 N (Duran®), 7.22 ± 0.45 N (Erkodur®), and 5.20 ± 0.68 N (Track-A®). The mean forces for palatal as compared to labial displacement were higher by a mean of 48 % with the Erkodur® and by 23 % with the Track-A® aligners but were smaller by 37 % with the Duran® aligners. The moment-to-force (M/F) ratios, calculated in relation to the center of resistance of the separated measurement tooth, ranged from − 9.91 to − 12.22 mm, thus, approaching the value of − 8.80 mm for uncontrolled tipping of this tooth.

Manufacturers of PET-G aligners have recommended setup increments of 0.5–1 mm, which appears excessive based on our results. PET-G aligners not featuring modifications (e.g., reinforcing ribs or composite attachments bonded to the teeth) are unsuitable for bodily movement of upper central incisors in labial or palatal directions.