The Effects On Three Types Of Appliances Of Rapid Maxillary Expansion With Protraction-a Three-dimensional Finite Element Analysis

Objective: Three-dimensional finite element models of cranial-maxillary complex and three kinds of rapid maxillary expansion with protraction appliances was established to evaluate the displacement trend and stress distribution of maxilla and teeth under the action of three kinds of appliances,to provide reference for clinical treatment.Methods: A male patient of 12 years old with maxillary deficiency was selected for research and underwent spiral CT scan.After obtaining the scan data,Mimics software was used for preliminary modeling.Then the Geomagic Studio2014 software was used for model optimization and the model of dentition and periodontium was created.The three-dimensional finite element model of Hyrax with tooth-borne protraction appliance,MSE(Maxillary Skeletal Expander)with toothborne protraction appliance,MSE with bone-borne protraction appliance was established using Unigraphics NX 12 software and was assembled with cranialmaxillary complex,which were then imported into Ansys workbench 19.2 software.Finally,three three-dimensional finite element models of cranial-maxillary complex and rapid maxillary expansion with protraction appliance were established.The expander screw was activated by 0.25 mm and the traction force of 500 g per side was applied to the traction site of the appliance.The direction of the force was 30°downward and forward to the occlusion plane.The displacement trend and stress distribution of maxilla and teeth under the three kinds of appliances were evaluated.Results:The stress distribution of the maxilla and the sutures and the displacement trend of the maxilla were similar,but the lateral displacement of the maxilla under the two kinds of MSE with protraction appliance was 2.7 times as much as that of the Hyrax’s;2.6 times in sagittal displacement;and its stress value was much higher than that of the Hyrax’s.The tooth displacement trend of the three models is similar,but the displacement values of teeth under the two kinds of MSE with protraction appliance was higher than the Hyrax’s.The stress on the first premolars under the action of Hyrax with protraction appliance was the greatest,while the stress on the first molars under the action of the two kinds of MSE with protraction appliance was the greatest.MSE with bone-borne protraction appliance still affect the anchored molar.Conclusion:(1)The displacement of maxilla under two kinds of MSE with protraction appliance is greater than that of Hyrax’s under the same expansion and traction force.MSE with protraction appliance is a better choice for adults or patients who need a large amount of maxilla movement.(2)Although the expansion and protraction force of the MSE with bone-borne protraction appliance is directly applied to the palatal,the appliance still has an effect on the first molar,and it brings more than bone effect.Moreover,the protraction force weakens the expansion force when it was applied to the expander screw.MSE with protraction appliance needs to be improved.