| BackgroundRecent years,clear aligner therapy(CAT)has developed rapidly as the development of artificial intelligence and materials.Clear aligners,a series of orthodontic operations that incorporate all of the teeth and the keratinized gingiva,involves a form of tooth movement that features a predetermined “mismatch” between the aligner and the teeth.Due to the limitation of the materials property,the “roller-coaster effect” is more likely to occur in extractions with CAT than with conventional fixed orthodontics.Therefore,scholars need to explore more to improve the biomechanical system of CAT.Finite Element Analysis(FEA)refers to the use of a mathematical dentition model derived from by computer aided design and estimates the stresses generated within different tissues,such as alveolar bone,the peridental membrane,and teeth,and can also determine the loading and displacement patterns of all structures.This form of analysis can contribute to our biomechanical understanding of orthodontic devices.An increasing body of evidence demonstrates that scholars are applying this technology to explore the biomechanical principles underlying the orthodontics.ObjectivesThis study aimed to use FEA to explore the relevant biomechanical principles about the torque compensation for the anterior teeth in extraction cases from the perspective of the inclination degree of incisors and the thickness of aligners,with the power ridges of different height as medium,in order to provide scientific guidance in clinic practice.1.To provide a biomechanics platform by establishing three-dimensional(3D)finite element models of retracting the anterior teeth in extraction cases.2.To identify the appropriate torque compensation involving the upper anterior teeth with different axial inclination 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.3.To identify the appropriate torque compensation for anterior teeth under different thickness of clear aligners and assess the effects of incisor torque compensation with different thicknesses of clear aligners on the 3D displacement tendency of teeth in cases of extraction.Methods1.The CBCT data of an adult volunteer was imported into Mimics 20.0 software in format of DICOM for threshold setting,artifact erasure and hole filling,and 3D reconstruction instructions were used to obtain the 3D model of triangular surface.The model was further optimized using the Geomagic Studio 2014 software.The attachments and clear aligners were established using the NX1911 software,and the aligners were deformed inward to form power ridges at the neck part of the central incisor.After assembling the obtained model,material attribute definition,mesh division and boundary constraints were carried out via Ansys Workbench 2019 software to form the final 3D finite element models.2.Based on the model of the experiment I,configuration 1 was constructed: the thickness of aligner was set as 0.5mm,and the value of U1-SN was set as 105°.Power ridges of different heights were applied to the central incisors to mimic torque compensation,then,the ANSYS Workbench 2019 software was used to analyze the initial displacement of teeth and the power ridges producing the translation of the central incisors were selected.Based on the models of the experiment I,configuration 2 was constructed:the thickness of aligner was set as 0.5mm,and the value of U1-SN was set as 110°.The same method as configuration 1 was used to find the height of the power ridge under the optimal torque compensation that made the central incisor controlled tipping movement,and the results were compared with those in configuration 1.3.Configuration 3: Based on the models of the experiment I,the thickness of aligner was set as 0.75 mm,and the value of U1-SN was set as 105°.The same torque compensation method was used to find the height of the power ridge under the optimal torque compensation value that made the central incisor translation.The results were compared with those in configuration 1,to assess the effects of incisor torque compensation with different thicknesses of clear aligner on the 3D displacement tendency of teeth in cases of extraction.Results1.3D finite element models of the maxillary without the first premolar in clear aligner was constructed with high fidelity.2.In configuration 1 and configuration 2,as the height of the power ridge increased,the transient displacement tendency of the central incisor ranged from pure tipping movement,controlled tipping movement,translation to torque movement.In configuration1,the central incisor exhibited pure tipping movement without torque compensation,the value of torque loss was 0.1688°,the central incisor approached translation with a power ridge height of 0.7 mm(corresponding torque compensation angle: 5.8415°).In configuration 2,when the power ridge height was 0mm,the value of torque loss was0.1630°,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 torque compensation angle: 3.4265°)in 60 steps.3.Configuration 3 showed that a power ridge of 0.25 mm could cause translation of the central incisors with the 0.75mm-thick aligner,and the corresponding torque compensation angle was 2.9435°,which was smaller than that in Condition 1.Without torque compensation,distal tipping,lingual tipping and extrusion of the incisors,distal tipping and extrusion of the canines,and mesial tipping and intrusion of the posterior teeth were all generated by clear aligner therapy.The displacement tendency of intrusion for the central incisors increased as the power ridge height increased.Proper torque compensation created by the power ridges generated palatal root torque and intrusion of the incisors,intrusion of the canines,mesial tipping and the intrusion of the second premolar;these effects were more significant with a 0.75mm-thick aligner,but it has little influence on lateral incisor teeth.After torque compensation,the stress placed on the periodontal ligament of the incisors was distributed more evenly with the 0.75mm-thick aligner.Conclusions1.It is feasible to establish 3D finite element models with high fidelity for this study via Mimics 20.0,Geomagic Studio 2014,NX1911 software and Ansys Workbench 2019.2.Analysis indicates that the varying degrees of torque compensation can be realized by power ridges of different heights,and proper torque compensation is beneficial to the translation of the anterior teeth.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.3.For the influence of aligner thickness on the torque of anterior teeth in extraction cases,0.75mm-thickness aligners have stronger torque control and vertical intrusion control on anterior teeth compared with 0.5mm-thickness aligners,causing less torque compensation for anterior teeth.In cases of extraction,appropriate torque compensation with a thicker aligner should be designed to ensure torque control of anterior teeth and minimize root resorption,but more attention should be paid to the retention of lateral incisors and the anchorage control of the posterior teeth.This study provided valuable information for the design of torque movement for upper anterior teeth with clear aligners in extraction cases.There existed some limitations in this study.One of the limitations of the present study was that FEA could not take the biological events into consideration,such as root length,root morphology,aligner material property,and so on.Another limitation was that the current FE model lied in the difficulty associated with simulating the large displacements on the tooth as a result of the long-term application of orthodontic forces.That was,FEA ignored the events of resorption–apposition present in variable time frames.Therefore,our findings must be clinically validated to support clear aligners with solid evidence. |
PDF Full Text Request