Three-dimensional mandibular changes associated with Class II elastics and extractions treatment in adult patients

Abstract Backgroud: The apllication of Class II elastic can induce the rotation of mandible and condylar response. The purpose of this study was to assess the positional and morphological alterations of mandible after orthodontic treatment with Class II elastic by using of 3-dimensional superimposition of pretreatment (T1), posttreatment (T2) and after retention (T3) CBCT data. Methods: With sequential 3D superimpositions that combined cranial base superimpositions with regional mandibular superimpositions, the virtual reference mandibles were placed to distinguish morphological changes from positional changes. The morphological changes and positional changes of mandibles were measured by comparing multiple combinations of reference and original mandibles respectively, and the corresponding paired t test was performed using SPSS (IBM SPSS version 23). Results: During orthodontic treatment (T1-T2), the mandibular molars were extruded, and the condyles were revealed apparent adaptive remodeling and upward/backward displacements. The mandible rotated backwardly and drifted backwardly/downwardly. The condylar growth and displacements were significantly different. The condylar growth did not induce isometric displacements and the forward rotation was restricted. Two years after retention (T2-T3), continuous morphological and positional changes occurred. The condylar remodeling and forward/downward displacements induced forward mandibular rotation and neutralized the backward rotation during treatment period (T1-T2). The overall positional changes (T1-T3) were translations with slight rotation. Statistically significant differences were found between the treatment and overall variables regarding the 3D rotation, condylar displacement, Pog displacement. And with the continuous morphological and positional changes, the condyle moved back to the initial position after the retention stage.Conclusion: Conclusions: the sequential 3D superimposition method can produce the reference mandibles and distinguish the morphological changes from positional alterations. The class II elastics induced mandibular complex changes including condylar adaptive remodeling and 3D rotation and translation. The condyles morphologically adapted to resist the rotation effects of Class II elastic.