Objective:To simulate the instantaneous distal moving trend of the maxillary first molar based on the three-dimensional finite element mode(l3-FEM)of Forsus distalizing the maxillary first molar with some visco-elastic-plastic characters and explore the favorable loading mode of molar distal movement in Forsus in order to provide biomechanical basis for clinical practice.Methods:A male patient with ClassⅡmalocclusion was selected.The 3-FEM of Forsus distalizing the maxillary first molar was built through spiral CT scanning

Mimics8.0

and Abaqus6.5 software.This study simulated instantaneous distal moving process of the maxillary first molar under three dif-ferent loading modes(loading force was 300 g

point of force was on the center of the crown’s buccal surface

loading time was 1s)

which included a distalizing force

a distalizing force+counterrotation moment

a distalizing force+countertipping moment+counterrotation moment.Results:When Forsus distalized the maxillary first molar

different loading modes lead to different PDL stress distributions.When the model was added by a distalizing force

the maxillary first molar would be relatively seriously inclined;when added by a distalizing force +counterrotation moment(Mt/F=9)

the maxillary first molar would be relatively seriously rotated and when added by a distalizing force+ countertipping momen(tMt/F=9)+counterrotation momen(tMr/F=5)

the distal surface of the PDL would bear the lightest stress

and the first molar would achieve approximate translation movement.Conclusion:When mandible is guided forward by Forsus

the maxillary first molar would achieve better molar translation effect if added certain countertipping and counterrotation moment