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

INTRODUCTION: This analysis aimed to assess quantitatively and qualitatively the compressive stress (S3) in periodontal ligament in a gradual periodontal breakdown (0-8 mm) under orthodontic movements. Correlations between the applied forces, the level of bone resorption, the decrease of force magnitude, and S3 increase were also conducted. METHODS: On the basis of cone-beam computed tomography examinations (voxel size, 0.075 mm), 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 N; extrusion, rotation, tipping: 0.6 N; translation: 1.2 N) were applied on the brackets. Finite elements analysis was performed, and S3 stresses were quantitatively and qualitatively determined. RESULTS: Translation 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. CONCLUSIONS: In reduced periodontium, forces of 0.2 N seems safe to be used. Forces of 0.6-1.2 N 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 adequate for qualitative results. An overall 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.

Finite element analysis of the dental pulp under orthodontic forces.

INTRODUCTION: To evaluate the stress at the apical third of the pulp and neurovascular bundle (NVB) during 5 types of orthodontic movement at different levels of bone loss. Furthermore, correlations among bone loss, orthodontic appliances, and stress increase were assessed. METHODS: Based on cone-beam computed tomography datasets, 10 models of the mandibular second premolar were created. Each of these models was subjected to a gradual horizontal bone loss simulation (0-8 mm). Orthodontic forces of 20 g, 60 g, and 120 g were applied during the finite element analysis (FEA). For each bone loss level, stress values were evaluated with the use of Abaqus at the apical third of the pulp and the NVB. RESULTS: The stress manifested at the apical third of the pulp was smaller than that at the NVB. The highest apical NVB stress was found for rotation (0.000546 N/mm2 for 8 mm bone loss) whereas the lowest stress resulted after translational movements (2.35E-04 MPa for 8 mm bone loss). The FEA showed that Proffit's indicated orthodontic forces did not significantly disturb the pulpal blood flow and damage the apical NVB. Up to a doubling of the NVB stress, bone loss correlated with the force reduction to obtain similar stress levels compared with teeth with no bone loss. CONCLUSIONS: The present findings indicate that the stress manifested at the apical third of the pulp is smaller than that at NVB. Rotational movements induce the highest stress and translational forces develop the lowest stress related to the physiologic capillary blood pressure. Furthermore, in situations with reduced periodontium, lower forces are needed to reach the maximum tolerable stress compared with teeth with intact periodontium.