Effect of different attachment geometries on the mechanical load exerted by PET­G aligners during derotation of mandibular canines : An in vitro study.

AIM: Derotation of rounded teeth has proved difficult for aligners to achieve. In this study, we investigated the effect of aligner attachment geometry on the three-dimensional (3D) force and moment (F/M) values exerted during derotation of a mandibular canine. MATERIALS AND METHODS: The experiment setup comprised an acrylic mandibular arch model with a separated right canine (tooth 43) mounted on a hexapod via a 3D F/M sensor. Polyethylene terephthalate glycol (PET­G) aligners with thicknesses of 0.5, 0.625, and 0.75 mm were tested in combination with quarter-sphere, vertical-ellipsoid, and pyramidal attachments bonded to tooth 43. The experimentally measured movement consisted of mesio- and distorotation of tooth 43 in 1° steps up to ±15° in each direction. RESULTS: Compared with no attachment, vertical-ellipsoid and quarter-sphere attachments increased the rotational moment by a median factor of 1.5-12.3. Moment increases for pyramidal attachments were significantly smaller (Mann-Whitney U­test, p < 0.05). Quarter-sphere attachments inhibited the intrusive forces up to 6.07° distorotation, whereas the intrusion prevention range for most aligner attachment combinations was significantly smaller (2.95° for vertical-ellipsoid and 2.88° for pyramidal attachments; Mann-Whitney U­test, p < 0.05). None of the attachment geometries could completely prevent intrusive forces during mesiorotation. CONCLUSION: The quarter-sphere geometry had the best overall mechanical properties because it induced relatively high rotational moment increases and counteracted unwanted intrusive forces most effectively of all three geometries. The determined maximum attachment dislodgement and intrusion prevention angles of approximately 6° provide a guide to determining setup increments for mandibular canine derotation.

Mechanical load exerted by PET-G aligners during mesial and distal derotation of a mandibular canine : An in vitro study.

INTRODUCTION: The six force-moment (F/M) components exerted by aligners of different thickness during simulated mesiorotation and distorotation of a mandibular canine were studied. MATERIALS AND METHODS: An acrylic mandibular model with a separated right canine mounted on a hexapod via a 3D F/M sensor was used. Duran+® aligners (Scheu Dental, Germany) of thickness 0.5, 0.625, and 0.75 mm were fabricated on plaster models with the measurement tooth in its neutral position. The F/M values were recorded during progressive mesiorotation or distorotation of tooth 43 in 1° steps up to ±15°, corresponding to 0.5 mm displacements of the tooth's interdental contacts. Each rotation step included renewed seating of the aligner on the acrylic model. Three aligners were tested three times each for each thickness and direction of rotation. RESULTS: The median rotational moments for the 0.5 mm aligner and 15° distorotation of tooth 43 was 27.49 Nmm (interquartile range, IQR 1.45 Nmm). The corresponding values for the 0.625 and 0.75 mm aligners were 41.04 Nmm (IQR 5.62 Nmm) and 42.48 Nmm (IQR 2.17 Nmm), respectively. The average rotational moments for distorotation were 15% higher than for mesiorotation (p = 0.01). Relatively high collateral F/M components, specifically an intrusive force and labiolingual and mesiodistal tipping moments, were observed. CONCLUSION: To avoid overloading of periodontal structures, derotation of lower canines should be limited to 10° per setup step, leading to rotational moments of about 15 Nmm. The mechanical behavior of the 0.625 and 0.75 mm aligners were similar; thus, it may be omitted from the aligner sequence. Further studies are required to investigate specific aligner modifications or attachments for minimizing collateral F/M components or unwanted movements, respectively, during canine derotation.