Validation of finite element models for orthodontic aligners.

PURPOSE: Clear thermoplastic aligners have become popular in orthodontics, but the biomechanics of these devices is not well understood. Neither is the tooth movement induced by such devices. The aim of this study was to develop and validate finite element (FE) models for clear thermoplastic teeth aligners for orthodontic force prediction. METHODS AND MATERIALS: FE models were created from Micro-CT scans of an aligner and a model arch of teeth with one of the incisors tipped buccal-lingually by 2.4°. The models were uniformly meshed with 0.3-mm long elements. Linear-elastic mechanical properties provided by the material manufacturers were used. Fitting of the two components was simulated using Abaqus's interference fit, followed by frictional surface-to-surface interaction. The assembled FE model was validated by comparing its prediction for the teeth-aligner gaps and aligner surface strains with experimental data. The experimental teeth-aligner gaps were obtained from the Micro-CT scans whereas the aligner surface strains were measured using a 2-camera digital image correlation (DIC) system. RESULTS: Good agreement between prediction and measurement was obtained for both the teeth-aligner gaps and aligner surface strains. The linear regression between prediction and measurement for teeth-aligner gaps sampled at different positions had a R2 value of 0.99. The mean difference between prediction and measurement for the aligner surface strains (von Mises) over 1544 nodes on the labial side and 1929 nodes on the lingual side was 0.07% and 0.01%, respectively, both being lower than the mean background noise. CONCLUSION: A FE model for clear thermoplastic teeth aligners has been successfully developed and validated. The model can therefore be used with confidence to predict the forces and moments applied to teeth by the aligners, thus improving our understanding of the biomechanics of such devices and the tooth movement they induce.

Laboratory simulation of longitudinally cracked teeth using the step-stress cyclic loading method.

AIM: To simulate in a laboratory setting longitudinal cracking in root filled premolar teeth, using cyclic mechanical fatigue. METHODOLOGY: Mesial-occlusal-distal (MOD) cavities were prepared in twenty root filled, single-rooted, mandibular premolars restored with fibre posts and resin composites. The samples were randomly divided into two groups based on the loading approaches: static loading with a crosshead speed of 0.5 mm/min and step-stress cyclic loading (1 Hz) with increasing amplitude. The loads and numbers of cycles to failure were recorded. Micro-CT was also used to identify the fracture modes. Statistical analysis was performed using Student's t-test. The level of significance was set at 0.05. RESULTS: The mean fracture loads for the static loading and cyclic loading groups were 769 ± 171 N and 720 ± 92 N, respectively. There was no significant difference between the two groups (P > 0.05). The proportions of longitudinal, cuspal and mixed-mode fractures under cyclic loading were 50%, 20% and 30%, respectively. Longitudinal fractures occurred with larger numbers of cycles and higher average loads per cycle compared with the other fractures. Static loading produced only cuspal fractures. CONCLUSIONS: Longitudinally cracked premolar teeth with root fillings were successfully produced using the step-stress cyclic loading method. This provides a more clinically representative methodology for studying cracked teeth in a laboratory setting.

Elastic properties and apparent density of human edentulous maxilla and mandible.

The aim of this study was to determine whether elastic properties and apparent density of bone differ in different anatomical regions of the maxilla and mandible. Additional analyses assessed how elastic properties and apparent density were related. Four pairs of edentulous maxilla and mandibles were retrieved from fresh human cadavers. Bone samples from four anatomical regions (maxillary anterior, maxillary posterior, mandibular anterior, mandibular posterior) were obtained. Elastic modulus (EM) and hardness (H) were measured using the nano-indentation technique. Bone samples containing cortical and trabecular bone were used to measure composite apparent density (cAD) using Archimedes' principle. Statistical analyses used repeated measures ANOVA and Pearson correlations. Bone physical properties differed between regions of the maxilla and mandible. Generally, mandible had higher physical property measurements than maxilla. EM and H were higher in posterior than in anterior regions; the reverse was true for cAD. Posterior maxillary cAD was significantly lower than that in the three other regions.