Objective To investigate the biomechanical characteristics of clear aligners combined with Class Ⅱelastics during retraction of upper anterior teeth,and compare the differences between two traction methods.Methods A case with a molar distal relationship and extraction of four first premolars was selected.The finite element method was applied to analyze tooth displacement,force distribution,and periodontal ligament(PDL)stress during 0.2 mm en-masse retraction of the anterior teeth.Three working conditions were defined:en-masse retraction without elastics(Condition 1),120 g Class Ⅱ elastics with aligner-cut hooks on upper canines and lower first molars(Condition 2),120 g Class Ⅱ elastics with aligner windows and bonded buttons on upper canines and lower first molars(Condition 3).Results Class Ⅱ elastics significantly enhanced lingual movement of the upper anterior teeth and mesial movement of the lower posterior teeth,while reducing mesial movement of the upper posterior teeth and lingual movement of the lower anterior teeth.In the transverse direction,the forces exerted on the teeth in all three conditions were minimal.In the sagittal direction,in Condition 2,the mesial force of the upper posterior teeth was effectively reduced by an average of 0.13 N,and the mesial force of the lower posterior teeth was increased by an average of 0.31 N.In Condition 3,the distal force of the upper canine teeth and the mesial force of the lower first molar were significantly increased by 0.40 N and 1.14 N,respectively.In the vertical direction,In condition 2,the average extrusive force of the upper teeth and the extrusive force of the lower molars were increased by 0.22 N and 0.20 N,respectively.In Condition 3,the upper canine extrusive force was increased by 0.91 N,while the lower molar intrusive force and the second molar extrusive force were reduced by 0.27 N and 0.25 N,respectively.The PDL stress distribution in the three conditions was generally similar.In Condition 3,the maximum principal stress distribution area on the lower first molars expanded slightly,but the magnitude did not increase significantly.Conclusions Condition 2 optimized the lower posterior teeth mesialization through balanced force distribution and protected the upper posterior teeth anchorage.Condition 3 significantly increased extrusive and distal forces on the upper canines and mesial forces on the lower first molars but did not substantially elevate periodontal risks for these teeth.

Objective To investigate the biomechanical characteristics of clear aligners combined with Class Ⅱelastics during retraction of upper anterior teeth,and compare the differences between two traction methods.Methods A case with a molar distal relationship and extraction of four first premolars was selected.The finite element method was applied to analyze tooth displacement,force distribution,and periodontal ligament(PDL)stress during 0.2 mm en-masse retraction of the anterior teeth.Three working conditions were defined:en-masse retraction without elastics(Condition 1),120 g Class Ⅱ elastics with aligner-cut hooks on upper canines and lower first molars(Condition 2),120 g Class Ⅱ elastics with aligner windows and bonded buttons on upper canines and lower first molars(Condition 3).Results Class Ⅱ elastics significantly enhanced lingual movement of the upper anterior teeth and mesial movement of the lower posterior teeth,while reducing mesial movement of the upper posterior teeth and lingual movement of the lower anterior teeth.In the transverse direction,the forces exerted on the teeth in all three conditions were minimal.In the sagittal direction,in Condition 2,the mesial force of the upper posterior teeth was effectively reduced by an average of 0.13 N,and the mesial force of the lower posterior teeth was increased by an average of 0.31 N.In Condition 3,the distal force of the upper canine teeth and the mesial force of the lower first molar were significantly increased by 0.40 N and 1.14 N,respectively.In the vertical direction,In condition 2,the average extrusive force of the upper teeth and the extrusive force of the lower molars were increased by 0.22 N and 0.20 N,respectively.In Condition 3,the upper canine extrusive force was increased by 0.91 N,while the lower molar intrusive force and the second molar extrusive force were reduced by 0.27 N and 0.25 N,respectively.The PDL stress distribution in the three conditions was generally similar.In Condition 3,the maximum principal stress distribution area on the lower first molars expanded slightly,but the magnitude did not increase significantly.Conclusions Condition 2 optimized the lower posterior teeth mesialization through balanced force distribution and protected the upper posterior teeth anchorage.Condition 3 significantly increased extrusive and distal forces on the upper canines and mesial forces on the lower first molars but did not substantially elevate periodontal risks for these teeth.