Compressive stress in periodontal ligament under orthodontic movements during periodontal breakdown.

Abstract

INTRODUCTION\nThis analysis aimed to assess quantitatively and qualitatively the compressive stress (S3) in periodontal ligament in a gradual periodontal breakdown (0-8\xa0mm) under orthodontic movements. Correlations between the applied forces, the level of bone resorption, the decrease of force magnitude, and S3 increase were also conducted.\n\n\nMETHODS\nOn the basis of cone-beam computed tomography examinations (voxel size, 0.075\xa0mm), nine 3-dimensional models of the second mandibular premolar with intact periodontium were created and then individually subjected to various levels of horizontal bone loss. Orthodontic forces (intrusion: 0.2\xa0N; extrusion, rotation, tipping: 0.6\xa0N; translation: 1.2\xa0N) were applied on the brackets. Finite elements analysis was performed, and S3 stresses were quantitatively and qualitatively determined.\n\n\nRESULTS\nTranslation and rotation induced the highest stress apically and cervically, whereas intrusion determined the lowest. Apical stress was lower than cervical stress. In intact periodontium, only intrusion and extrusion exhibited S3 stresses lower (apically and cervically) than maximum hydrostatic pressure (MHP) and maximum tolerable stress (MTS). In reduced periodontium, S3 stress (except for intrusion) exceeded MHP and MTS.\n\n\nCONCLUSIONS\nIn reduced periodontium, forces of 0.2\xa0N seems safe to be used. Forces of 0.6-1.2\xa0N may produce stresses exceeding both MTS and MHP, endangering the periodontium. S3 failure criterion (despite its widely use) seems not to be adequate for accurate quantitative results when evaluating the stress in the periodontal ligament while remaining\xa0adequate for qualitative results. An\xa0overall correlation between the applied force, S3 increase, and periodontal breakdown applicable to all 5 movements could not be established-this was possible only for sole movements.