Overall stress in periodontal ligament under orthodontic movement during a periodontal breakdown.

Abstract

INTRODUCTION\nThis research aimed to assess qualitatively and quantitatively the overall stress in the periodontal ligament during gradual periodontal breakdown (0-8\xa0mm) under orthodontic movements. Correlations between the applied forces, the level of bone loss, the decrease of force magnitude, and the increase of stress were also assessed.\n\n\nMETHODS\nOn the basis of cone-beam computed tomography examinations (voxel size, 0.075\xa0mm), nine 3-dimensional models of a mandibular second premolar with intact periodontium were created and then individually subjected to various levels of horizontal bone loss. Orthodontic forces (intrusion at 0.2\xa0N; extrusion, rotation, and tipping at 0.6\xa0N; translation at 1.2\xa0N) were applied on the brackets. Finite elements analysis was performed, and von Mises (VM) stresses were quantitatively and qualitatively determined.\n\n\nRESULTS\nRotation and translation induced the highest stress apically and cervically, whereas intrusion determined the lowest. Apical stress was lower than cervical stress. In intact periodontium, VM stress was under maximum hydrostatic pressure (MHP) and maximum tolerable stress (MTS). In reduced periodontium, VM stress was lower apically than MHP, whereas cervically, the rotation, translation, and tipping exceeded MHP.\n\n\nCONCLUSIONS\nA force of 1.2\xa0N seemed safe to be used in the intact periodontium. Forces higher than 0.6\xa0N could produce stresses exceeding MHP and MTS endangering the reduced periodontium. VM stress failure criterion (despite its limited use) seemed to be more adequate for accurate quantitative results. An overall correlation between the applied force, VM stress increase, and periodontal breakdown applicable to all 5 movements could not\xa0be established. This was possible only for individual movements.