Effects of two rapid expansion methods combined with protraction on the treatment of skeletal class Ⅲ malocclusion in adolescents: a three-dimensional finite element analysis

Abstract

Objective·To compare the effects of bone-anchored rapid expansion and tooth-borne rapid expansion combined with protraction on craniofacial sutures, skeletal points, bones and maxillary dentition using three-dimensional finite element analysis, and provide guidance for the clinical selection of appropriate traction methods and sites.Methods·A cone beam computed tomography (CBCT) image of one adolescent with skeletal class Ⅲ malocclusion and maxillary hypoplasia during the mixed dentition period was selected to establish a three-dimensional finite element model of the maxillary complex (including craniofacial sutures, skeletal points, bones and maxillary dentition). Based on this, the three-dimensional finite element models of bone-anchored and tooth-borne rapid expansion combined with protraction were respectively established. Then, the aforementioned models were assembled into a three-dimensional finite element model of maxillary complex with bone-anchored rapid expansion combined with protraction (Model 1), and a three-dimensional finite element model of maxillary complex with tooth-borne rapid expansion combined with protraction (Model 2). According to the different expansion methods and protraction sites, the following conditions were set up: ① Based on the expansion methods, Model 1 was set as Group A, and Model 2 was set as Group B. ② Based on the protraction sites, Group A and B were further divided into experimental group Ⅰ(protraction hooks were placed buccally on both sides of the maxillary canines), experimental group Ⅱ(protraction hooks were placed buccally on both sides of the maxillary first premolars) and experimental group Ⅲ (protraction hooks were placed buccally on both sides of the maxillary second premolars), respectively. Additionally, as a control, Group A0 used bone-anchored rapid expansion alone without protraction, while Group B0 used tooth-borne rapid expansion without protraction. The stress distribution characteristics of craniofacial sutures in groups A and B at different protraction sites, as well as the displacement trends of craniofacial skeletal points, craniofacial bones and maxillary dentition were analyzed by using charts and tables.Results·In terms of stress distribution characteristics of craniofacial sutures, pterygomaxillary suture′s equivalent strain was maximal in both groups A and B, and it increased when protraction hooks were placed backwards. The maximum principal strain value of each suture in Group AⅠ was larger than that in Group BⅠ. In terms of the displacement trend of craniofacial bones, as the protraction sites shifted posteriorly, both the nasal bones and maxilla in the horizontal direction moved rightward with decreasing displacement trends in both groups A and B. In the sagittal direction, the nasal bones moved posteriorly with decreasing displacement trends, while the maxilla moved anteriorly with increasing displacement trends in groups A and B. In the vertical direction, the nasal bones moved downward with decreasing displacement trends, and the maxilla moved upward with decreasing displacement trends in groups A and B. In terms of displacement trends of craniofacial skeletal points (ANS, PNS), the maxillary plane (ANS-PNS plane) in Group A underwent clockwise rotation, with the clockwise rotation trend decreasing as the protraction sites shifted posteriorly, while the maxillary plane (ANS-PNS plane) in Group B underwent counterclockwise rotation, with the counterclockwise rotation trend becoming more apparent as the protraction sites shifted posteriorly. In terms of the displacement trend of the maxillary dentition, the displacement of the central incisors in the horizontal, sagittal and vertical directions in the experimental groups A and B was all negative, presenting a tendency to move distally, labially and extrusively. The displacement of the first molar in the horizontal direction was also negative, indicating a trend of buccal displacement. Additionally, as the protraction site shifted posteriorly, the labial movement trend of the central incisors′ crown increased, and the crowns of the first molars changed from mesial to distal movement.Conclusion·Clinically, placing protraction sites posteriorly is beneficial for the anterior movement of the maxilla. Adolescent with skeletal class Ⅲ malocclusion can choose different rapid expansion with protraction to achieve maxillary anterior displacement while realizing favorable rotation of maxillary plane.

Keywords

• bone-anchored rapid expansion
• tooth-borne rapid expansion
• protraction
• three-dimensional finite element analysis
• skeletal class ⅲ malocclusion