Different sliding mechanics in space closure of lingual orthodontics: a translational study by three-dimensional finite element method.

Lingual orthodontics have become popular in modern society as they do not cause aesthetic impairment. From the translational medicine point of view, the use of biomechanical analysis to solve a clinical problem has rarely been reported. Here, we combined the clinical trial and 3-D finite element (FE) method to translate the clinical problem to the FE analysis and back to clinic. Twenty upper premolar extraction cases treated with customized lingual appliances were recruited in this study. Cephalometric films and cast records analysis showed that the "bowing effect", which is a major side effect in lingual orthodontics, occurred during the first treatment stage with single lingual cable retraction. In order to translate the problem to biomechanical research, we introduced the 3-D finite element (FE) model of a customized lingual orthodontic system. The 3-D FE model including the maxilla, periodontal ligament (PDL), and dentition was constructed from human computed tomography data. The tendency of tooth movements in three dimensions and stress distribution in the PDL were analyzed by different mechanical loading methods. 3-D FE analysis confirmed the "bowing effects" and unexpected tooth movements with application of single lingual retraction force. Interestingly, we found that applying forces on both buccal and lingual sides, called "double cable" mechanics, could prevent the "bowing effect". For the clinical trial, we applied the "double cable" force during space closure stage for 4 months, and confirmed "double cable" mechanics could correct and prevent the "bowing effect" clinically. Based on our results, both buccal and lingual forces should be used during space closure in lingual orthodontics to prevent and correct the "bowing effect". Moreover, the magnitude of buccal force should not be lower than the force on the lingual side.