Torque movement of the upper anterior teeth using a clear aligner in cases of extraction: a finite element study

Abstract

Background Clear aligner treatment has become popular over recent years. It is necessary to identify methods by which we could avoid the bowing effect in extractions with clear aligner. The present study was to identify the appropriate method to design torque movement involving the upper anterior teeth of extraction cases, in order to maintain or improve the axis and torque of the upper anterior teeth with a clear aligner during movement and closure of the extraction space. Results As the height of the power ridge increased, the rotation angle of the upper central incisor in the sagittal direction decreased gradually and the location of the rotation center changed significantly; the rotation center moved in the apical direction and then changed to the crown side. The highest von-Mises stress of the upper central incisor root, periodontal ligaments, and alveolar bone, showed little change as the power ridge height increased. When the axial inclination of the upper central incisor was normal (U1-SN = 105°), the tendency of movement for the upper central incisor approached translation with a power ridge height of 0.7 mm (corresponding distorted angle: 5.8415). When the axial inclination of the upper central incisor was oversized (U1-SN = 110°), the axial inclination of the upper central incisor reduced to normal following completion of the anterior segment retraction with a power ridge of 0.4 mm (corresponding distorted angle: 3.4265). Conclusion Analysis indicates that pure palatal tipping movement of the upper anterior teeth is generated without torque control, thus resulting in the bowing effect. The required torque control of the upper anterior teeth with oversize axial inclination is weaker than that of the upper anterior teeth with normal axial inclination because limited torque loss is expected for oversize axial inclination teeth. Variation sensitivity of the rotation center should be considered carefully due to biological problems when designing translation of the upper anterior teeth with normal axial inclination.

The three-dimensional displacement tendency of teeth depending on incisor torque compensation with clear aligners of different thicknesses in cases of extraction: a finite element study

“…From the perspectives of biomechanics, Hahn et al considered that controlling the torque of an upper central incisor requires the creation of effective couples: a tipping force was required, evoked by reversible deformation of the appliance near the gingival margin and the resulting force in the opposite direction produced by movement of the tooth against the inner opposite surface of the appliance near the incisor edge [ 33 ]. However, the force couple generated at the cervical and incisal regions of the aligners is not large enough to generate an adequate counter moment due to limitations imposed by material properties [ 17 , 34 ]. This situation could eventually be improved by selective modifications to aligners by means of mechanical reinforcement of the cervical area [ 34 ], such as the power ridge of the Invisalign system.…”

“…In essence, the presence of a power ridge in the tooth neck produced a counter moment because it generated an angle between the inner face of the aligner and the tooth surface; this led to further deformation of the aligner. From mechanical knowledge, this angle was determined by the height of the power ridge and reflected the magnitude of torque control [ 17 ]. This indicated that the 0.75 mm-thick aligner generated less torque loss and required less torque compensation, that is, the thicker aligner magnified the effect of the selective modification.…”

“…Clear aligner and composite attachments: We assumed a linear elastic isotropic and homogeneous behavior. Mechanical properties were inferred from previous studies [ 15 – 17 , 24 , 25 ].…”

“…In another study, Liu constructed a FEM for maxillary dentition and demonstrated that appropriate overtreatment, by placing attachments on the canines, should be designed to ensure the bodily retraction of anterior teeth in cases of extraction [ 16 ]. Our previous work showed that the required torque control for upper anterior teeth with oversize axial inclination was weaker than that for upper anterior teeth with normal axial inclination [ 17 ]. Although many studies have analyzed the biomechanics of aligner-attachments and torque control of anterior teeth, the thickness of aligner has not been investigated in full detail, even though this factor is closely associated with other important features such as the forces and moments they generate and the displacement tendency of teeth [ 18 , 19 ].…”

Effects of upper-molar distalization using clear aligners in combination with Class II elastics: a three-dimensional finite element analysis

“…we employed SOLID187, a 3D 10-node tetrahedral structural solid. The material properties were extracted from previous research works [ 14 , 16 – 18 ] (Table 1 ). A linear viscoelastic model was employed for biomechanical modelling of the disc, which was implemented using a generalized Maxwell model via an optimized Prony series [ 15 ].…”