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Introduction: The orthodontic correction of periodontally compromised dentitions constitutes a huge challenge in the clinical practice of adult orthodontics. The biological and physical distinct features of these conditions require a carefully designed mechanical plan for the successful treatment of these complex cases., Setting and Sample Population: A segment of a human maxilla containing the central and lateral incisors, obtained from autopsy, was scanned with microcomputed tomography, and a finite element (FE) model was generated to represent an intact periodontal dentition. Based on this model, three additional models simulating a mild, moderate and severe bone alveolar loss were created as well., Materials and Methods: Two loading scenarios for the application of intrusive and retraction mechanics with a three-piece base arch appliance were evaluated in a series of FE analyses. The tooth displacements and strains in the periodontal ligament (PDL) were calculated and compared for the four FE models., Results: The periodontal reduced dentitions exhibited a similar axis of resistance for intrusive mechanics, but the axis of resistance for retraction movements was significantly dependent on the degree of alveolar bone loss. The tooth displacements and PDL loads were higher in the reduced dentitions for both intrusive and retraction mechanics., Conclusions: A reduction in the force levels applied to periodontal reduced dentitions is indicated, and a customized selection of appropriate points of force application is needed according to the specific amount of alveolar bone loss., (© 2020 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Objective: To analyze whether the decreased bone density due to the manipulation of bone remodeling rate has an influence on the type of the planned tooth movement., Materials and Methods: A finite element model of a lower incisor has been developed. The density of the alveolar bone surrounding the tooth has been assumed to simulate the one occurring after corticotomy to increase tooth movement rate. Moment-to-force ratios corresponding to three different types of movements have been simulated; uncontrolled tipping, translation, and root movement. The three tooth movements have been analyzed in both corticotomized and non-corticotomized simulations, and the final effects on the amount and type of tooth movement analyzed. The stress and strain levels in the periodontal ligament have been analyzed too., Results: The amount of tooth movement obtained in case of lower bone density is higher in all types of movement simulations. The centre of rotation of the movement shifts more apically in case of translation, controlled and uncontrolled tipping. In the corticotomy simulations, the compressive stresses in the periodontal ligament decreased while the tensile stresses increased., Conclusion: A decreased bone density influences not only the amount of tooth movement, but also its type. This study suggests that the moment-to-force ratios used in conventional orthodontics should be modified in case of techniques that decrease bone density to enhance tooth movement rate., (© The Author 2017. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: journals.permissions@oup.com)

Objective: To evaluate the force systems produced by transpalatal arches (TPAs) activated according to the six classes of geometries described by Burstone and Koenig., Materials and Methods: Sixty appliances were tested for first-order activations using a mechanical force testing system. The TPAs were first checked for passivity in sagittal, transverse, and vertical planes at the measuring machine. Then 10 appliances per group were activated using a millimeter template to obtain the six classes of geometries, and the activated appliances were inserted into lingual tubes of the Force System Identification machine that recorded the deactivation forces and moments delivered by both terminal ends of the TPAs., Results: The overall force system with the actual values of forces and moments recorded by each type of activation was illustrated and compared with the mathematical model reported by Burstone and Koenig. Although a great consistency of the direction of forces and moments were observed, the theoretically feasible force systems could not be fully accomplished by the TPA activated for the six classes of geometries., Conclusion: The first-order activations of the TPA can deliver predictable force systems in respect to the direction of forces and moments attainable, but some unexpected forces and moments are also produced. Careful clinical monitoring is, therefore, strongly recommended when using this statically indeterminate system.

Introduction: Changes in the stress/strain distribution in the periodontium after the application of orthodontic forces trigger remodeling processes that make tooth movement possible. The type of orthodontic tooth movement is linked to the force system applied to the bracket. By combining moments and forces, often expressed as the moment-to-force (M/F) ratio, it is possible to determine the prescribed type of tooth movement. According to classical theory, fixed values for M/F are associated with specific tooth movements., Methods: A segment of a mandible containing the canine and the first premolar obtained from autopsy was scanned with microcomputed tomography, and a finite element model was generated. In a series of finite element analyses simulating teeth subjected to various orthodontic loading regimens, the influences of the M/F ratio and the force magnitude were examined., Results: By applying a range of values of M/F, different types of tooth movement were generated, although the classic prescription of the M/F ratio suggested in the literature could not be confirmed. Due to the nonlinear behavior of the periodontal ligament, loading modes with a constant M/F ratio, yet varying the force magnitude, resulted in different types of tooth movement., Conclusions: The material properties of the periodontal ligament, the morphology of the root, and the alveolar bone are patient specific. Therefore, the M/F values generally advocated to obtain orthodontic tooth movement should be used only as guidelines. To be effective and accurate, the force system selected for a specific tooth movement must be monitored and the outcome compared with the predicted tooth movement.

Microcracks and microdamage have been associated with bone remodelling. The aim of this study was to investigate the role of microcracks as a trigger for alveolar bone remodelling after the application of an orthodontic load. In 25 3-month-old male Danish land-race pigs, the lower right first molar was moved buccally with a force of 130 cN. The contralateral molar was not treated and was used as an internal control. After 1, 2, 4, 7 and 15 days of treatment, the regions containing the right and left molars were excised and en bloc stained in basic fuchsin. The presence of microcracks on the buccal and lingual sides of both treated and untreated teeth was detected and expressed as crack density (number/mm2). The buccal treated side showed significantly more cracks than the buccal untreated side at day 1. This difference was significantly larger than that observed at days 2, 7 and 15. The same side showed significantly more microcracks than the lingual treated side at day 1, and this difference was larger compared with that observed at days 4 and 15. The presence of more microcracks on the treated side than on the untreated side suggests a role for microcracks in the initiation of bone remodelling after orthodontic loading. The increased presence of microcracks on the side towards which the tooth was moved, and where bone resorption is usually observed, suggests that microcracks could represent the first damage induced by orthodontic force that has to be repaired by bone remodelling. Finally, the strain levels in the alveolar bone during the orthodontic load transfer in the experiment were examined by finite element (FE) analysis. Although this showed that the strains were very low (10-100 microstrain), it should be noted that occlusal loading was not taken into account. In addition, high-resolution microtomography of the alveolar bone/periodontal ligament (PDL) interface revealed that the actual surface of the alveolar bone was very rough, predisposing it to high local stress/strain peaks.

Objectives: To examine whether the transition temperature of Cu-NiTi archwires has an effect on the tooth movement during the alignment phase of orthodontic treatment., Design: 'Split mouth' design in randomly selected patients., Setting and Sample Population: The Department of Orthodontics, School of Dentistry, University of Aarhus. Fifteen randomly selected patients with identical level of irregularity in the alignment phase of their treatment. Experiment Variable - Specially manufactured Cu-NiTi archwires for the upper arch were inserted. These consisted of two separate halves, each with its own transition temperature, respectively 27 degrees and 40 degrees C, and clamped together in the middle., Outcome Measure: The tooth movement, expressed as two translations and a rotation, in the occlusal plane was measured from the patients' intraoral photographs taken upon insertion of the archwires and again after 1 month., Results: Tooth movements tended to be larger on the 40 degrees C-side, however only in case of the total translation of the premolars was this difference significant. In general, patients had not noticed any difference between the two sides of the archwire, although one patient stated the 27 degrees C-side to be more comfortable as the 40 degrees C-side had bothered her when drinking hot beverages., Conclusion: The transition temperature of Cu-NiTi archwires has indeed an effect on the amount of tooth movement during alignment. However, the differences are so small though that it is the question whether they can be noticed clinically. The study corroborates the trend towards the use of lower forces within orthodontics.

The aim of this study was to determine, in a rat model, whether a state of high or low bone turnover had an effect on the rate and type of tooth movement and on the incidence of root resorption induced by orthodontic treatment. The maxillary left first molar was moved mesially for 21 days in 52 6-month-old Wistar rats. They were divided into three groups: group 1 (n = 19) with normal bone turnover, group 2 (n = 16) with high bone turnover, and group 3 (n = 17) with low bone turnover. The contralateral side was left untreated to act as a control. The different metabolic rates were created by inducing hyper- and hypothyroidism. The amount of tooth movement was measured using an electronic calliper and the location of the centre of rotation (CRot) was determined after microcomputer tomographic scanning and subsequent three-dimensional reconstruction. Histomorphometric evaluation of root resorption was performed on undecalcified 7 microm thick sections of the maxilla and the differences between treated and untreated sides were evaluated. The results showed that high bone turnover increased the amount of tooth movement compared with the normal or low bone turnover state. There was no statistical difference in the location of the CRot. The treated side presented more root resorption than the untreated side, but this difference was not influenced by the metabolic rates. On the contrary, the untreated side in the low bone turnover group showed more root resorption, suggesting that in subjects where a decreased bone turnover rate is expected, the risk of root resorption could be increased.

The aim of this study was to evaluate intramaxillary molar movement after 8 months of cervical traction and posttreatment displacement 7 years later. The total molar displacements in relation to stable intraosseous reference points were compared with those observed in an untreated control group that also had intraosseous reference indicators inserted. During the headgear period, the type of molar displacement could be predicted by the direction of the force system acting on the teeth. It was noted, however, that the variation in the vertical development was related more to each patient's growth pattern than to the force system applied. After cessation of the headgear, intramaxillary displacement of the molars was noted, and the total displacement of the molars did not differ from that of the untreated group. The indication for intramaxillary displacement of the molars by means of extraoral traction is therefore questioned.