Biomechanical finite-element investigation of the position of the centre of resistance of the upper incisors

An Evaluation of the Rate and Type of Orthodontic Tooth Movement When Injecting Platelet-Rich Plasma During Mini-Implant-Based Segmented en-Masse Retraction of Upper Anterior Teeth

INTRODUCTION

The study aimed to determine the influence of palatal injection of platelet-rich plasma (PRP) on the rate and type of orthodontic tooth movement (OTM) during the en-masse retraction of upper anterior teeth.

MATERIALS AND METHODS

Two-arm parallel-group trial, in which 30 class II division 1 adult patients (7 males and 23 females) aged 16 to 27 years were recruited. The sample was randomly divided into two groups: the experimental group, in which PRP was injected in the palatal mucosa of the maxillary six anterior teeth immediately before starting the en-masse retraction of upper anterior teeth, whereas in the control group, traditional treatment was employed. Following the first premolar extraction, space closure was accomplished using frictionless mechanics for the en-masse retraction of upper anterior teeth. In both groups, a rigid segmented arch made of stainless steel with a diameter of 0.021 x 0.025 inch and an 8-mm power arm was used for the upper anterior teeth, and mini-implants were inserted between the upper second premolar and first molar at 8 mm apical from the archwire line. NiTi coil springs were used for retraction. Measurements were recorded at the onset of space closure (T0) and every 40 days till the middle of the en-masse retraction of upper anterior teeth (T1).

RESULTS

Thirty patients completed the trial, and no patients were lost to follow-up in both groups. The OTM rate in the PRP group was similar to that of the control group (P = 0.596). The upper anterior teeth in the two groups were retracted mostly by controlled tipping and partially by translation. Statistically insignificant differences were observed between the two groups regarding the rest of the cephalometric variables. No serious harms were observed in either group.

CONCLUSIONS

PRP was ineffective in accelerating the OTM rate during the en-masse retraction of upper anterior teeth and it did not affect the type of tooth movement.

Effect of occlusal hypofunction on centre of resistance in maxillary central incisor using the finite element method

OBJECTIVES

To determine differences in the location of centre of resistance (Cres) between functional and hypofunctional teeth and to evaluate the relationship between the pulp cavity volume and locations of the Cres, using the finite element (FE) method.

DESIGN

Retrospective cohort study.

PARTICIPANTS

FE models of right maxillary central incisor, derived from cone-beam computed tomography (CBCT) images of 46 participants, were divided into normal function (n = 23) and hypofunction (n = 23) groups using anterior overbite and cephalometric measurements.

METHODS

Measurements of the tooth and pulp cavity volume were made from the CBCT. Cres levels were presented as percentages of the root length from the root's apex. All data were analysed and compared using the independent t-test (P < 0.05). The relationship between the location of Cres and volume ratios were evaluated statistically.

RESULTS

The means of the pulp cavity/tooth volume and root canal/ root volume ratio of the maxillary central incisor in the anterior open bite group were significantly greater than those in the normal group. The average location of Cres in the anterior open bite group was 0.6 mm (3.7%) apically from the normal group, measured from root apex. The difference was statistically significant (P < 0.01). There was a significant correlation between root canal/root volume ratio and locations of Cres (r = -0.780, P < 0.001).

CONCLUSIONS

The Cres in the hypofunctional group was located more apical than the functional group. As the pulp cavity volume increased, the level of Cres shifted apically.

Biomechanical Simulation of Orthodontic En-Bloc Retraction Comparing Compound Technique and Sliding Mechanics Using a HOSEA Robotic Device

En-bloc retraction is a common procedure in orthodontic therapy. The application of palatal root torque moments is required to control incisor inclination during retraction, yet studies comparing forces and moments with respect to different mechanics are lacking. This study aimed to investigate the forces and moments during orthodontic en-bloc retraction using a robotic biomechanical simulation system, comparing two distinct approaches: (I) compound technique [stainless steel (SS) combined with nickel-titanium (NiTi)] using industrially pretorqued retraction-torque-archwires (RTA) in combination with NiTi closed coil springs; (II) conventional sliding mechanics using SS archwires with manually applied anterior twist bends in combination with elastic chains. Two dimensions (0.017" × 0.025" and 0.018" × 0.025") and ten archwires per group were investigated using 0.022" slot self-ligating brackets. Kruskal-Wallis tests with a significance level of α = 0.05 were conducted. The biomechanical simulation showed that en-bloc retraction was characterized by a series of tipping and uprighting movements, differing significantly regarding the examined mechanics. Collateral forces and moments occurred in all groups. Notably, RTA exhibited fewer extrusive forces. The most bodily movement was achieved with the compound technique and the 0.018" × 0.025" RTA. Sliding mechanics exhibited maximum palatal root torque moments of more than 20 Nmm, exceeding recommended values.

Force-Controlled Biomechanical Simulation of Orthodontic Tooth Movement with Torque Archwires Using HOSEA (Hexapod for Orthodontic Simulation, Evaluation and Analysis)

This study aimed to investigate the dynamic behavior of different torque archwires for fixed orthodontic treatment using an automated, force-controlled biomechanical simulation system. A novel biomechanical simulation system (HOSEA) was used to simulate dynamic tooth movements and measure torque expression of four different archwire groups: 0.017″ x 0.025″ torque segmented archwires (TSA) with 30° torque bending, 0.018″ x 0.025″ TSA with 45° torque bending, 0.017″ x 0.025″ stainless steel (SS) archwires with 30° torque bending and 0.018″ x 0.025″ SS with 30° torque bending (n = 10/group) used with 0.022″ self-ligating brackets. The Kruskal-Wallis test was used for statistical analysis (p < 0.050). The 0.018″ x 0.025″ SS archwires produced the highest initial rotational torque moment (My) of -9.835 Nmm. The reduction in rotational moment per degree (My/Ry) was significantly lower for TSA compared to SS archwires (p < 0.001). TSA 0.018″ x 0.025″ was the only group in which all archwires induced a min. 10° rotation in the simulation. Collateral forces and moments, especially Fx, Fz and Mx, occurred during torque application. The measured forces and moments were within a suitable range for the application of palatal root torque to incisors for the 0.018″ x 0.025″ archwires. The 0.018″ x 0.025″ TSA reliably achieved at least 10° incisal rotation without reactivation.

Effect of trimming line design and edge extension of orthodontic aligners on force transmission: A 3D finite element study

OBJECTIVES

To investigate in a numerical study the effect of the geometry and the extension of orthodontic aligner edges and the aligner thickness on force transmission to upper right central incisor tooth (Tooth 11).

METHODS

A three-dimensional (3D) digital model, obtained from a 3D data set of a complete dentulous maxilla, was imported into 3-matic software. Aligners with four different trimming line designs (scalloped, straight, scalloped extended, straight extended) were designed, each with four different thicknesses (0.3, 0.4, 0.5, and 0.6 mm). The models were exported to a finite element (FE) software (Marc/Mentat). A facial 0.2 mm bodily malposition of tooth 11 was simulated.

RESULTS

The maximum resultant force was in the range of (7.5 - 55.2) N. The straight trimming designs had higher resultant force than the scalloped designs. The resultant force increases with increasing the edge extension of the aligner. The normal contact forces were unevenly distributed over the entire surface and were concentrated in six areas: Incisal, Mesio-Incisal, Disto-Incisal, Middle, Mesio-Cervical, and Disto-Cervical. The resultant force increases super linearly with increasing thickness.

CONCLUSIONS

The design of the trimming line, the edge extension, and the thickness of the aligner affect significantly the magnitude of the resultant force and the distribution of normal contact force. The straight extended trimming design exhibited better force distribution that may favor a bodily tooth movement.

CLINICAL RELEVANCE

A straight extended trimming design of an orthodontic aligner may improve the clinical outcomes. In addition, the manufacturing procedures of the straight design are much simpler compared to the scalloped design.

Effect of Trimming Line Design and Edge Extension of Orthodontic Aligners on Force Transmission: An in vitro Study

OBJECTIVES

To investigate how the stress distribution and forces transmitted from orthodontic aligners to the tooth surface are affected by the geometry and extension of the trimming line.

MATERIALS AND METHODS

Thirty-six aligners were thermoformed from Zendura FLX sheets (0.75 mm thick) and divided into four groups based on the design of the trimming line: Scalloped, Scalloped extended, Straight and Straight extended. Fuji pressure-sensitive films were used for pressure measurement. The pressurized films were scanned and evaluated. Pressures and forces were measured over the entire facial surface of an upper right central incisor (Tooth 11) and at 7 different locations [cervical, middle, incisal, mesio-incisal, mesio-cervical, disto-cervical, and disto-cervical]. In addition, the thickness of the aligners at these 7 sites was measured with a digital caliper.

RESULTS

The active force ranged from (2.2 - 6.9) N, and the average pressure was (1.6 - 2.7) MPa. The highest values were recorded for the (straight extended) design, while the lowest values were recorded for the scalloped design. The forces and stresses were not uniformly distributed over the surface. When the values in each area were compared separately, significant differences were found between the different designs in the cervical area, with the scalloped design transmitting the lowest cervical forces. Aligner thickness was drastically reduced (60-75% thinning) over the entire tooth surface after thermoforming.

CONCLUSIONS

The straight extended design of aligner's trimming line exhibited more uniform force transfer and stress distribution across the surface than the other designs.

CLINICAL RELEVANCE

The trimming line design could have a significant impact on the clinical outcome of orthodontic aligner treatment.

Does pulp cavity affect the center of resistance in three-dimensional tooth model? A finite element method study

OBJECTIVES

To compare the center of resistance (Cres) of the maxillary central incisor in models with and without the pulp cavity and to evaluate the association of pulp cavity/tooth volume ratio and difference in Cres position between the two models.

MATERIALS AND METHODS

CBCT images of the right maxillary central incisor were collected from 18 subjects. Pulp cavity/tooth volume ratio was measured, and finite element models of teeth and periodontal structures were generated. Cres location was presented as a percentage of root length measured from the root apex. Differences in Cres positions between models were compared using the paired t-test, while the correlation between pulp cavity/tooth volume ratio and a difference in Cres was evaluated by Pearson's correlation coefficient.

RESULTS

For the pulp cavity model, the average location of the Cres measured from the apex of the root was 58.8% ± 3.0%, which resulted in a difference of 4.1% ± 1.1% (0.5 mm) apically, when compared with the model without pulp cavity. Differences in Cres between the models were statistically significant (P < 0.01), while the correlation between pulp cavity/tooth volume ratio and a difference in Cres between models was significantly positive (r = 0.709, P = 0.001).

CONCLUSIONS

In the pulp cavity model, the Cres was located in a more apical position. The difference in Cres between models increased as the pulp cavity/tooth volume ratio increased.

CLINICAL RELEVANCE

The line of force must be applied more apically in the pulp cavity model to achieve the desired orthodontic tooth movement.

An analysis of the optimal intrusion force of the maxillary central incisor with root horizontal resorption using the finite element method and curve fitting

There are no studies on the optimal intrusion force in orthodontic patients with the existing root resorption (RR). The study aimed to analyze the optimal intrusion force for central incisors with existing horizontal root resorption using the finite element method (FEM). We calculated the optimal intrusion force using the finite element method and curve fitting. We found that with the increase of the maxillary central incisor's root horizontal resorption length, the optimal intrusion force interval's median gradually increases. If the resorption length is more significant than 1/2 of the root length, it is not recommended to use intrusion force theoretically.

Where is the center of resistance of a maxillary first molar? A 3-dimensional finite element analysis

INTRODUCTION

The center of resistance (C_Res) is regarded as the fundamental reference point for predictable tooth movement. Accurate estimation can greatly enhance the efficiency of orthodontic tooth movement. Only a handful of studies have evaluated the C_Res of a maxillary first molar; however, most had a low sample size (in single digits), used idealized models, or involved 2-dimensional analysis. The objectives of this study were to: (1) determine the 3-dimensional (3D) location of the C_Res of maxillary first molars, (2) evaluate its variability in a large sample, and (3) investigate the effects of applying orthodontic load from 2 directions on the location of the C_Res.

METHODS

Cone-beam computed tomography scans of 50 maxillary molars from 25 patients (mean age, 20.8 ± 8.7 years) were used. The cone-beam computed tomography volume images were manipulated to extract 3D biological structures via segmentation. The segmented structures were cleaned and converted into virtual mesh models made of tetrahedral triangles having a maximum edge length of 1 mm. The block, which included the molars and periodontal ligament, consisted of a mean of 7753 ± 2748 nodes and 38,355 ± 14,910 tetrahedral elements. Specialized software was used to preprocess the models to create an assembly and assign material properties, interaction conditions, boundary conditions, and load applications. Specific loads were applied, and custom-designed algorithms were used to analyze the stress and strain to locate the C_Res. The C_Res was measured in relation to the geometric center of the buccal surface of the molar and the trifurcation of the molar roots.

RESULTS

The average location of the C_Res for the maxillary first molar was 4.94 ± 1.39 mm lingual, 2.54 ± 2.7 mm distal, and 7.86 ± 1.66 mm gingival relative to the geometric center of the buccal surface of the molar and 0.136 ± 1.51 mm lingual (P <0.01), 1.48 ± 2.26 mm distal (P <0.01), and 0.188 ± 1.75 mm gingival (P >0.01) relative to the trifurcation of the molar roots. In the anteroposterior (y-axis) and the vertical (z-axis) planes, the C_Res showed significant association with root divergence (P <0.01).

CONCLUSIONS

The C_Res of the maxillary first molar was located apical and distal to the trifurcation area. It showed significant variation in its location. The 3D location of and also varied with the force direction. In some samples, this deviation was large. For accurate and predictable movement, tooth-specific C_Res need to be calculated.

Comparative study of effect of different lever arm positions and lengths on transverse and vertical bowing in lingual orthodontics - An FEM study

OBJECTIVE

To assess the influence of changing the size and position of lever arm on transverse and vertical bowing effects during retraction in lingual biomechanics.

METHODS

A three dimensional 3D finite element method was used to simulate en masse anterior teeth retraction using lingual appliance with sliding mechanics. Two groups were made, 1st group had lever arm mesial to canine while 2nd group had distal to canine. Each group had 4 subgroups with lever arm height varying from 0mm to 12mm. Displacements of the maxillary anterior teeth were noted in each group.

RESULTS

As the Lever Arm Height (LAH) increased in group I, the vertical bowing effect reduced while the transverse bowing increased with respect to canines. In group II, both vertical and transverse bowing effects increased but transverse bowing was less as compared to group I.

CONCLUSION

Strategic location of the lever arm is required in different clinical situations during en-mass retraction, keeping in mind the location of Centre of Resistance (CRes) as well as the vertical and the transverse bowing effects.

Simulation of orthodontic tooth movement during activation of an innovative design of closing loop using the finite element method

INTRODUCTION

Although many attempts have been made to study the mechanical behavior of closing loops, most have been limited to analyses of the magnitude of forces and moments acting on the end of the closing loop. The objectives of this study were to simulate orthodontic tooth movement during the activation of a newly designed closing loop combined with a gable bend and to investigate the optimal loop activation condition to achieve the desired tooth movement.

METHODS

We constructed a 3-dimensional model of maxillary dentition reproducing the state wherein a looped archwire combined with a gable bend was engaged in brackets and tubes. Orthodontic tooth movements were simulated for both anterior and posterior teeth while varying the degree of gable bend using the finite element method.

RESULTS

The incorporation of a 5° gable bend into the newly designed closing loop produced lingual crown tipping for the central incisor and bodily movement for the first molar. The incorporation of 10° and 15° gable bends produced bodily movement and root movement, respectively, for the central incisor and distal tipping for the first molar.

CONCLUSIONS

Torque control of the anterior teeth and anchorage control of the posterior teeth can be carried out effectively and simply by reducing by half the thickness of a teardrop loop with a height of 10 mm and a 0.019 × 0.025-in cross-section, to a distance of 3 mm from its apex, and by incorporating various degrees of gable bend into the loop corresponding to the treatment plan.

The ideal insertion angle after immediate loading in Jeil, Storm, and Thunder miniscrews: A 3D-FEM study

OBJECTIVE

The miniscrew is effectively used to provide additional anchorage for orthodontic purposes. The aim of this study was to identify an optimal insertion angle for Jeil, Storm, and Thunder miniscrews on stress distribution at the bone miniscrew interface.

MATERIALS AND METHODS

To perform 3-dimensional finite element model analysis, a 3-dimensional model with a bone block was constructed with type D2 of bone quality, and with miniscrews of Storm, Thunder, Jeil, with the diameter of 2, 1.5. 1.6mm and length 15.9, 12.4, 14.4mm respectively. The miniscrews were inserted at 15° 30°, 45°, 60°, 75° and 90° to the bone surface. A simulated horizontal orthodontic force of 200 gram was applied to the centre of the miniscrews head in all models, and stress distribution and its magnitude were evaluated with a 3-dimensional finite element analysis program.

RESULTS

In the cancellous bone, minimum stress was found at placement angles of 90° for Jeil and Storm, which was 0.37 and 0.39MPa respectively, and 15° for Thunder, which was 0.85MPa. The maximum von Mises stresses in the cancellous bone for Jeil was at 60°, which was 0.92MPa, and for Thunder at 90°, which was 1.3MPa.

CONCLUSION

Each miniscrew has an ideal insertion angle, optimal insertion positions were found within 90° for Jeil and for Storm but 15° for Thunder. Clinical significance 3-dimensional finite element analysis confirmed that each miniscrew has an ideal insertion angle according to its characteristics.

Effect of archwire stiffness and friction on maxillary posterior segment displacement during anterior segment retraction: A three-dimensional finite element analysis

Objective

Sliding mechanics using orthodontic miniscrews is widely used to stabilize the anchorage during extraction space closure. However, previous studies have reported that both posterior segment displacement and anterior segment displacement are possible, depending on the mechanical properties of the archwire. The present study aimed to investigate the effect of archwire stiffness and friction change on the displacement pattern of the maxillary posterior segment during anterior segment retraction with orthodontic miniscrews in sliding mechanics.

Methods

A three-dimensional finite element model was constructed. The retraction point was set at the archwire level between the lateral incisor and canine, and the orthodontic miniscrew was located at a height of 8 mm from the archwire between the second premolar and first molar. Archwire stiffness was simulated with rectangular stainless steel wires and a rigid body was used as a control. Various friction levels were set for the surface contact model. Displacement patterns for the posterior and anterior segments were compared between the conditions.

Results

Both the anterior and posterior segments exhibited backward rotation, regardless of archwire stiffness or friction. Among the conditions tested in this study, the least undesirable rotation was found with low archwire stiffness and low friction.

Conclusions

Posterior segment displacement may be unavoidable but reducing the stiffness and friction of the main archwire may minimize unwanted rotations during extraction space closure.

Evaluation of stress generation on the cortical bone and the palatal micro-implant complex during the implant-supported en masse retraction in lingual orthodontic technique using the FEM: Original research

Background. This study aimed to evaluate and analyze the distribution of stresses on the palatal micro-implants and the cortical bone at the micro-implant site with optimal orthodontic retraction force in lingual orthodontics. Methods. ANSYS 12.1 software was used to construct the finite element model of the maxillary bone, teeth and the periodontal ligament along with the lingual bracket set-up with wire and the micro-implant. Six- and 8-mm micro-implants were constructed. The final model consisted of 99190 nodes and 324364 elements. A 200-gram of retraction force was applied from the micro-implant to the anterior retraction hook. The micro-implant was embedded between the second premolar and the first molar. Hyper-view software was used to get the results in X-Y-Z dimensions. Results. The maximum von Mises stresses detected were 52.543 MPa for 6-mm micro-implant and 54.489 MPa for 8-mm micro-implant. Maximum stress was at the neck of the micro-implant. The 8-mm implant model showed 6×10-3 mm of lingual displacement. The least displacement of 1×10-3 mm was noticed for both the implant models in the apico-occlusal direction. The maximum von Mises stresses in the cortical bone at the micro-implant site was 18.875 MPa for 6-mm micro-implant and 21.551 MPa for 8-mm micro-implant. Conclusion. Six-mm micro-implant can be the choice for the implant-supported lingual orthodontic retraction as it produced minimal stresses on the cortical bone, and the initial stress displacements produced on the micro-implant were also minimal.

The effect of Alexander, Gianelly, Roth, and MBT bracket systems on anterior retraction: a 3-dimensional finite element study

OBJECTIVES

The aims of this study were to compare the effect of 4 bracket systems including Alexander, Roth, MBT, and Gianelly on upper anterior retraction and to quantify the amount of torque loss ratio in sliding mechanics by help of a 3-dimensional (3D) finite element method.

METHOD AND MATERIALS

3D FEM models were constructed in order to simulate anterior incisor retraction in first premolar extraction case. Displacement, stress, and strain on the incisal edge and apex of maxillary central incisor were calculated when 1-, 2-, and 3-N retraction forces were applied. Torque loss ratio was calculated by measuring the displacement of the teeth at crown tip and root apex in all 4 bracket systems on upper central incisor.

RESULTS

Uncontrolled lingual crown tipping of the incisor was observed in all bracket systems. The crown moved lingually by 9.5 μm and the root labially by 4.5 μm in MBT system with 3-N retraction force. The amount of crown movement was 8 μm and the root displacement was 4 μm in Gianelly system with the same retraction force. Torque loss ratio was 1.46 in Alexander and Gianelly with 3-N retraction force. However, the amount of torque loss ratio was 1.47 in MBT and Roth with the same retraction force.

CONCLUSIONS AND CLINICAL RELEVANCE

Uncontrolled tipping was the least in Gianelly and was the highest in MBT. The amount of torque loss ratio was the lowest in Gianelly and Alexander systems and the amount of torque loss ratio was the highest in MBT system.

Evaluation of initial displacement and stresses in the maxillary dentition following en masse retraction using fixed lingual appliance and micro-implants: A finite element analysis

OBJECTIVE

To evaluate and compare the stress and displacement pattern between conventional and micro-implant supported retraction in lingual orthodontic system.

MATERIALS AND METHODS

A finite element model of the maxilla, teeth, periodontal ligament, lingual-orthodontic appliance and a micro-implant complex was constructed using ANSYS 12.1 software. Two sizes of micro-implants, 6mm and 8mm, were constructed producing a simulated model of 99,190 nodes and 32,4364 elements. A retraction force of 200g was applied from an anterior retraction hook to the molar tube in the conventional model and from the micro-implants in the implant supported model. The initial displacement and stress patterns in the X-Y-Z axes were obtained using Hyper-view software.

RESULTS

The maximum von Mises stresses in the 6mm, 8mm and conventional model were 22.164 Megapascals (MPa), 28.603MPa and 16.491MPa respectively. The bucco-lingual displacement of the maxillary anteriors was greater in the 8mm implant model with 23×10^-3mm lingual displacement observed. The least lingual displacement of 11×10^-3mm was noted for the conventional model while a slightly higher moderate reading of 15×10^-3mm was seen in the 6mm micro-implant supported model. The maximum displacement of the periodontal ligament was noted in the 8mm micro-implant model.

CONCLUSION

Within the limitations of this research, the 8mm micro-implant model displayed high initial stresses and greater initial displacement of the anterior teeth in the X-Y-Z coordinates in comparison to conventional retraction method.

Force direction using miniscrews in sliding mechanics differentially affected maxillary central incisor retraction: Finite element simulation and typodont model

Background/purpose

En masse retraction was still controversy in orthodontics. The aim of this study was to investigate the effect of force directions created by different miniscrew positions and lever arm heights on maxillary central incisor movement using Finite Element (FE) simulation and a Typodont model.

Materials and methods

A typodont model and 3-dimensional FE were used to simulate en masse anterior teeth retraction in sliding mechanics. The lever arm and the miniscrew positions were varied to change the force direction. The maxillary central incisor displacement was recorded and analyzed.

Results

The typodont results revealed that miniscrew vertical position and lever arm height affected the type of tooth movement. The best control in the vertical plane was achieved by a 7 mm lever arm height and miniscrew 9 mm from the archwire. When the lever arm height and miniscrew were 7 mm from the archwire, the tooth extruded. When the lever arm height was 9 mm and the miniscrew was 7 or 9 mm from the archwire, the tooth intruded. The FE stimulation determined that near bodily movement of the maxillary central incisor was achieved when the lever arm height and miniscrew was 9 mm from the archwire. The highest strain distribution in the periodontal ligament was observed at the apical third of the lateral incisor.

Conclusion

In en masse retraction, the appropriate direction of force or the height of the miniscrew and the lever arm may enable orthodontists to maintain better control of the anterior teeth in sliding mechanics.

Can interfaces at bracket-wire and between teeth in multi-teeth finite element model be simplified?

OBJECTIVE

Finite element (FE) method's correctness depends heavily on modeling method. This study aimed at determining whether the interfaces at bracket-wire and between teeth can be simplified for multi-teeth FE analysis.

METHOD

A three-dimensional FE model of a mandible was created from cone-beam computed tomography scan. Due to symmetry, only a half of the mandible was modeled, which consisted of five teeth (first premolar extraction and only first molar), brackets and archwire, periodontal ligament (PDL), cortical bone, and cancellous bone. All the bone, teeth, and PDL were considered to be isotropic and linear. The En-masse retraction case was simulated. A detailed model, which has contact elements between the bracket and archwire and between teeth, was developed to allow relative motion at the interfaces. A model with simplified interfacial conditions, which does not allow the relative motion, was also created. The stresses and displacements as results of the treatment on these two models were calculated and compared.

RESULTS

The stress and displacement distributions from the detailed model were more close to reality based on the expected displacement pattern of the clinical case than from the simplified model. The maximum stresses from the two methods were also different. The highest stress from the detailed model is twice as high as from the simplified model.

CONCLUSIONS

The detailed model provides much more reasonable results than the simplified model. Thus, the simplified model should not be used to replace the detailed model if the stress magnitude and highest stress location are the expected outcomes.

Finite Element Analysis of Stress in Maxillary Dentition during En-masse Retraction with Implant Anchorage

The goal of this study was to investigate how the height of the archwire hook and implant anchor affect tooth movement, stress in the teeth and alveolar bone, and the center of resistance during retraction of the entire maxillary dentition using a multibracket system. Computed tomography was used to scan a dried adult human skull with normal occlusion. Three-dimensional models of the maxillary bone, teeth, brackets, archwire, hook, and implant anchor were created and used for finite element analysis. The heights of the hook and the implant anchor were set at 0, 5, or 10 mm from the archwire. Orthodontic force of 4.9 N was systematically applied between the hook and the implant anchor and differential stress distributions and tooth movements observed for each traction condition. With horizontal traction, the archwire showed deformation in the superior direction anterior to the hook and in the inferior direction posterior to the hook. Differences in traction height and direction resulted in different degrees of deformation, with biphasic movement clearly evident both in front of and behind the hook. With horizontal traction of the hook at a height of 0 mm, all the teeth moved distally, but not with any other type of traction. At a height of 5 mm or 10 mm, deformation showed an increase. The central incisor showed extrusion under all traction conditions, with the amount showing a reduction as the height of horizontal or posterosuperior traction increased. The center of resistance was located at the root of the 6 anterior teeth and entire maxillary dentition. The present results suggest that it is necessary to consider deformation of the wire and the center of resistance during en-masse retraction with implant anchorage.

[Orthodontic intrusion using mini-screws]

INTRODUCTION

Dental intrusion has long been considered one of the most difficult movements to induce in orthodontics. Using conventional mechanics, the main difficulty lies in the need to ensure anchorage control, which is highly complicated to achieve, so as to avoid parasitic movements. In this framework, mini-screws have proven to offer a very effective means of anchorage, allowing greater control over intrusion of the anterior and posterior teeth and a simpler biomechanical movement opening up new therapeutic perspectives for the orthodontist.

OBJECTIVE

The aim of this study was to describe the clinical and biomechanical application of mini-screws for dental intrusion.

Mandibular anterior intrusion using miniscrews for skeletal anchorage: A 3-dimensional finite element analysis

INTRODUCTION

Deepbites can be corrected by intrusion of mandibular anterior teeth. Direct anchorage with miniscrews simplifies complex tooth movements; however, few studies have reported their use for mandibular anterior intrusion. The purpose of this study was to evaluate, by means of the finite element method, initial tooth displacement and periodontal stress distribution using various mandibular anterior intrusion mechanics. Miniscrews were used as skeletal anchorage devices.

METHODS

Cone-beam computed tomography scans were used for 3-dimensional reconstruction of the mandible and the mandibular anterior dentition. Models included the 4 incisors with or without the canines. After all surrounding periodontal and bony structures were determined brackets, segmental archwires, and miniscrews were added. Finite element studies were performed to assess initial tooth displacement and periodontal stress distribution with multiple intrusion force vectors. Changes in the location of the miniscrews and loading points on the archwire created 14 scenarios.

RESULTS

Minimum buccolingual displacements, a uniform distribution of periodontal stress, and overall group intrusion for both 4-tooth and 6-tooth scenarios were best achieved when applying distointrusive vectors. The highest peaks of periodontal stress were observed when the force was directed at the corners of the segmental archwire. It was found that, in addition to distointrusive vectors, 4 loading points on the archwire were necessary for pure intrusion and uniform distribution of periodontal stress in the 6-tooth scenarios.

CONCLUSIONS

The simulations in this study suggest that group intrusion of all 6 mandibular anterior teeth might be achieved by applying distointrusive vectors. Inserting a pair of miniscrews distal to the canine roots, 1 screw per side, and directing 4 loading points on the archwire generates uniform periodontal stress distribution and minimum buccolingual displacements. Local conditions, such as narrow bone width and attached gingiva level, play significant roles in the clinical viability of the proposed virtual scenarios.

Determination of the centre of resistance during en masse retraction combined with corticotomy: finite element analysis

OBJECTIVE

To determine the effect of corticotomy on the change in the centre of resistance of the six maxillary anterior teeth Materials and methods: Three-dimensional finite element models of the maxillary anterior teeth with and without corticotomy were constructed. Brackets (size 0.022 inch × 0.028 inch) were placed passively on all anterior teeth that were set at the centre of the labial surface in the mesio-distal dimension and 3 mm from the incisal edge to the bracket slot in the vertical direction. The power arm was set mesial of the canine bracket. For the model with corticotomy, the bone density was decreased from initial value at 5% to 25%. The point of force application was varied in order to locate the centre of resistance. The centre of resistance was located by measurement of the difference of the displacement between the apical and incisal edges. The position of force was varied by moving apically parallel to the occlusal plane to simulate tooth movement.

RESULTS

As the alveolar bone density decreased from initial value to 25%, the location of the centre of resistance moved apically from the bracket slot from 10.8 mm to 11.2 mm, respectively.

CONCLUSIONS

The change of alveolar bone density due to corticotomy was associated with the location of the centre of resistance. The location of the centre of resistance moved apically as the alveolar bone density decreased but it was not clinically noticeable.

Quantification of intrusive/retraction force and moment generated during en-masse retraction of maxillary anterior teeth using mini-implants: A conceptual approach

OBJECTIVE

The aim of the present study was to clarify the biomechanics of en-masse retraction of the upper anterior teeth and attempt to quantify the different forces and moments generated using mini-implants and to calculate the amount of applied force optimal for en-masse intrusion and retraction using mini-implants.

METHODS

The optimum force required for en-masse intrusion and retraction can be calculated by using simple mathematical formulae. Depending on the position of the mini-implant and the relationship of the attachment to the center of resistance of the anterior segment, different clinical outcomes are encountered. Using certain mathematical formulae, accurate measurements of the magnitude of force and moment generated on the teeth can be calculated for each clinical outcome.

RESULTS

Optimum force for en-masse intrusion and retraction of maxillary anterior teeth is 212 grams per side. Force applied at an angle of 5o to 16o from the occlusal plane produce intrusive and retraction force components that are within the physiologic limit.

CONCLUSION

Different clinical outcomes are encountered depending on the position of the mini-implant and the length of the attachment. It is possible to calculate the forces and moments generated for any given magnitude of applied force. The orthodontist can apply the basic biomechanical principles mentioned in this study to calculate the forces and moments for different hypothetical clinical scenarios.

Torque differences according to tooth morphology and bracket placement: a finite element study

Introduction

Torque of the maxillary incisors is essential in esthetics and proper occlusion, while torque expression is influenced by many factors. The aim of this finite element study was to assess the relative effect of tooth morphology, bracket prescription, and bracket positioning on tooth displacement and developed stresses/strains after torque application.

Methods

A three-dimensional upper right central incisor with its periodontal ligament (PDL) and alveolus was modelled. The tooth varied in the crown-root angle (CRA) between 156°, 170°, and 184°. An 0.018-inch slot discovery® (Dentaurum, Ispringen, Germany) bracket with a rectangular 0.018 × 0.025-inch β-titanium wire was modelled. Bracket torque prescription varied between 0°, 12°, and 22°, with bracket placement at the centre of the middle, gingival or incisal third of the crown. A total of 27 models were generated and a buccal root torque of 30° was applied. Afterwards, crown and apex displacement, strains in the PDL, and stresses in the bracket were calculated and analysed statistically.

Results

The palatal crown displacement was significantly affected by bracket positioning (up to 94 per cent), while the buccal apex displacement was significantly affected by bracket prescription (up to 42 per cent) and bracket positioning (up to 23 per cent). Strains in the PDL were affected mainly by CRA (up to 54 per cent), followed by bracket positioning (up to 45 per cent). Finally, bracket prescription considerably affected the stresses in the bracket (up to 144 per cent).

Limitations

These in silico results need to be validated in vivo before they can be clinically extrapolated.

Conclusion

Tooth anatomy and the characteristics of the orthodontic appliance should be considered during torque application.

Torque differences due to the material variation of the orthodontic appliance: a finite element study

Background

Torque of the maxillary incisors is crucial to occlusal relationship and esthetics and can be influenced by many factors. The aim of this study was to assess the relative influence of the material of the orthodontic appliance (adhesive, bracket, ligature, and wire) on tooth displacements and developed stresses/strains after torque application.

Methods

A three-dimensional upper right central incisor with its periodontal ligament (PDL) and alveolus was modeled. A 0.018-in. slot discovery® (Dentaurum, Ispringen, Germany) bracket with a rectangular 0.018 x 0.025-in. wire was generated. The orthodontic appliance varied in the material of its components: adhesive (composite resin or resin-modified glass ionomer cement), bracket (titanium, steel, or ceramic), wire (beta-titanium or steel), and ligature (elastomeric or steel). A total of 24 models were generated, and a palatal root torque of 5° was applied. Afterwards, crown and apex displacement, strains in the PDL, and stresses in the bracket were calculated and analyzed.

Results

The labial crown displacement and the palatal root displacement of the tooth were mainly influenced by the material of the wire (up to 150% variation), followed by the material of the bracket (up to 19% variation). The magnitude of strains developed in the PDL was primarily influenced by the material of the wire (up to 127% variation), followed by the material of the bracket (up to 30% variation) and the ligature (up to 13% variation). Finally, stresses developed at the bracket were mainly influenced by the material of the wire (up to 118% variation) and the bracket (up to 59% variation).

Conclusions

The material properties of the orthodontic appliance and all its components should be considered during torque application. However, these in silico results need to be validated in vivo before they can be clinically extrapolated.

Electronic supplementary material

The online version of this article (doi:10.1186/s40510-017-0161-5) contains supplementary material, which is available to authorized users.

Types of tooth movement, bodily or tipping, do not affect the displacement of the tooth's center of resistance but do affect the alveolar bone resorption

OBJECTIVE

To investigate how types of tooth movement, bodily or tipping, influence the displacement of the center of resistance in teeth and alveolar bone resorption.

MATERIALS AND METHODS

Ten-week-old female Wistar rats were divided into eight groups of different factors, as follows: type of movement (bodily and tipping) and force magnitude (10, 25, 50, and 100 cN). The maxillary left first molars were moved mesially with nickel-titanium coil springs for 28 days. Micro-computed tomography (micro-CT) images were taken before and after tooth movement. The position of the center of resistance was determined by using finite element models constructed from the micro-CT image. The displacement of the center of resistance and the volume of alveolar bone resorption were measured.

RESULTS

The displacement of the center of resistance showed no significant difference between the bodily and tipping groups. The displacements of the center of resistance were increased with force magnitude at 10 and 25 cN, whereas they were not further increased at 50 and 100 cN. On the other hand, cervical alveolar bone resorption was significantly greater in the tipping group than in the bodily group.

CONCLUSIONS

Displacement of the center of resistance was not influenced by the types of tooth movement. However, volume of cervical alveolar bone resorption was greater in the tipping movement group than in the bodily movement group.

Finite-element analysis of the center of resistance of the mandibular dentition

Objective

The aim of this study was to investigate the three-dimensional (3D) position of the center of resistance of 4 mandibular anterior teeth, 6 mandibular anterior teeth, and the complete mandibular dentition by using 3D finite-element analysis.

Methods

Finite-element models included the complete mandibular dentition, periodontal ligament, and alveolar bone. The crowns of teeth in each group were fixed with buccal and lingual arch wires and lingual splint wires to minimize individual tooth movement and to evenly disperse the forces onto the teeth. Each group of teeth was subdivided into 0.5-mm intervals horizontally and vertically, and a force of 200 g was applied on each group. The center of resistance was defined as the point where the applied force induced parallel movement.

Results

The center of resistance of the 4 mandibular anterior teeth group was 13.0 mm apical and 6.0 mm posterior, that of the 6 mandibular anterior teeth group was 13.5 mm apical and 8.5 mm posterior, and that of the complete mandibular dentition group was 13.5 mm apical and 25.0 mm posterior to the incisal edge of the mandibular central incisors.

Conclusions

Finite-element analysis was useful in determining the 3D position of the center of resistance of the 4 mandibular anterior teeth group, 6 mandibular anterior teeth group, and complete mandibular dentition group.

The effects of alveolar bone loss and miniscrew position on initial tooth displacement during intrusion of the maxillary anterior teeth: Finite element analysis

Objective

The aim of this study was to determine the optimal loading conditions for pure intrusion of the six maxillary anterior teeth with miniscrews according to alveolar bone loss.

Methods

A three-dimensional finite element model was created for a segment of the six anterior teeth, and the positions of the miniscrews and hooks were varied after setting the alveolar bone loss to 0, 2, or 4 mm. Under 100 g of intrusive force, initial displacement of the individual teeth in three directions and the degree of labial tilting were measured.

Results

The degree of labial tilting increased with reduced alveolar bone height under the same load. When a miniscrew was inserted between the two central incisors, the amounts of medial-lateral and anterior-posterior displacement of the central incisor were significantly greater than in the other conditions. When the miniscrews were inserted distally to the canines and an intrusion force was applied distal to the lateral incisors, the degree of labial tilting and the amounts of displacement of the six anterior teeth were the lowest, and the maximum von Mises stress was distributed evenly across all the teeth, regardless of the bone loss.

Conclusions

Initial tooth displacement similar to pure intrusion of the six maxillary anterior teeth was induced when miniscrews were inserted distal to the maxillary canines and an intrusion force was applied distal to the lateral incisors. In this condition, the maximum von Mises stresses were relatively evenly distributed across all the teeth, regardless of the bone loss.

Stress distributions in peri-miniscrew areas from cylindrical and tapered miniscrews inserted at different angles

Objective

The purpose of this study was to analyze stress distributions in the roots, periodontal ligaments (PDLs), and bones around cylindrical and tapered miniscrews inserted at different angles using a finite element analysis.

Methods

We created a three-dimensional (3D) maxilla model of a dentition with extracted first premolars and used 2 types of miniscrews (tapered and cylindrical) with 1.45-mm diameters and 8-mm lengths. The miniscrews were inserted at 30°, 60°, and 90° angles with respect to the bone surface. A simulated horizontal orthodontic force of 2 N was applied to the miniscrew heads. Then, the stress distributions, magnitudes during miniscrew placement, and force applications were analyzed with a 3D finite element analysis.

Results

Stresses were primarily absorbed by cortical bone. Moreover, very little stress was transmitted to the roots, PDLs, and cancellous bone. During cylindrical miniscrew insertion, the maximum von Mises stress increased as insertion angle decreased. Tapered miniscrews exhibited greater maximum von Mises stress than cylindrical miniscrews. During force application, maximum von Mises stresses increased in both groups as insertion angles decreased.

Conclusions

For both cylindrical and tapered miniscrew designs, placement as perpendicular to the bone surface as possible is recommended to reduce stress in the surrounding bone.

Orthodontic intrusion of maxillary incisors: a 3D finite element method study

Objective:

In orthodontic treatment, intrusion movement of maxillary incisors is often necessary. Therefore, the objective of this investigation is to evaluate the initial distribution patterns and magnitude of compressive stress in the periodontal ligament (PDL) in a simulation of orthodontic intrusion of maxillary incisors, considering the points of force application.

Methods:

Anatomic 3D models reconstructed from cone-beam computed tomography scans were used to simulate maxillary incisors intrusion loading. The points of force application selected were: centered between central incisors brackets (LOAD 1); bilaterally between the brackets of central and lateral incisors (LOAD 2); bilaterally distal to the brackets of lateral incisors (LOAD 3); bilaterally 7 mm distal to the center of brackets of lateral incisors (LOAD 4).

Results and Conclusions:

Stress concentrated at the PDL apex region, irrespective of the point of orthodontic force application. The four load models showed distinct contour plots and compressive stress values over the midsagittal reference line. The contour plots of central and lateral incisors were not similar in the same load model. LOAD 3 resulted in more balanced compressive stress distribution.

Effect of material variation on the biomechanical behaviour of orthodontic fixed appliances: a finite element analysis

INTRODUCTION

Biomechanical analysis of orthodontic tooth movement is complex, as many different tissues and appliance components are involved. The aim of this finite element study was to assess the relative effect of material alteration of the various components of the orthodontic appliance on the biomechanical behaviour of tooth movement.

METHODS

A three-dimensional finite element solid model was constructed. The model consisted of a canine, a first, and a second premolar, including the surrounding tooth-supporting structures and fixed appliances. The materials of the orthodontic appliances were alternated between: (1) composite resin or resin-modified glass ionomer cement for the adhesive, (2) steel, titanium, ceramic, or plastic for the bracket, and (3) β-titanium or steel for the wire. After vertical activation of the first premolar by 0.5mm in occlusal direction, stress and strain calculations were performed at the periodontal ligament and the orthodontic appliance.

RESULTS

The finite element analysis indicated that strains developed at the periodontal ligament were mainly influenced by the orthodontic wire (up to +63 per cent), followed by the bracket (up to +44 per cent) and the adhesive (up to +4 per cent). As far as developed stresses at the orthodontic appliance are concerned, wire material had the greatest influence (up to +155 per cent), followed by bracket material (up to +148 per cent) and adhesive material (up to +8 per cent).

LIMITATIONS

The results of this in silico study need to be validated by in vivo studies before they can be extrapolated to clinical practice.

CONCLUSION

According to the results of this finite element study, all components of the orthodontic fixed appliance, including wire, bracket, and adhesive, seem to influence, to some extent, the biomechanics of tooth movement.

Distalization pattern of whole maxillary dentition according to force application points

Objective

The purpose of this study was to observe stress distribution and displacement patterns of the entire maxillary arch with regard to distalizing force vectors applied from interdental miniscrews.

Methods

A standard three-dimensional finite element model was constructed to simulate the maxillary teeth, periodontal ligament, and alveolar process. The displacement of each tooth was calculated on x, y, and z axes, and the von Mises stress distribution was visualized using color-coded scales.

Results

A single distalizing force at the archwire level induced lingual inclination of the anterior segment, and slight intrusive distal tipping of the posterior segment. In contrast, force at the high level of the retraction hook resulted in lingual root movement of the anterior segment, and extrusive distal translation of the posterior segment. As the force application point was located posteriorly along the archwire, the likelihood of extrusive lingual inclination of the anterior segment increased, and the vertical component of the force led to intrusion and buccal tipping of the posterior segment. Rotation of the occlusal plane was dependent on the relationship between the line of force and the possible center of resistance of the entire arch.

Conclusions

Displacement of the entire arch may be dictated by a direct relationship between the center of resistance of the whole arch and the line of action generated between the miniscrews and force application points at the archwire, which makes the total arch movement highly predictable.

Biomechanical aspects of segmented arch mechanics combined with power arm for controlled anterior tooth movement: A three-dimensional finite element study

The porpose of this study was to determine the optimal length of power arms for achieving controlled anterior tooth movement in segmented arch mechanics combined with power arm. A three-dimensional finite element method was applied for the simulation of en masse anterior tooth retraction in segmented power arm mechanics. The type of tooth movement, namely, the location of center of rotation of the maxillary central incisor in association with power arm length, was calculated after the retraction force was applied. When a 0.017 × 0.022-in archwire was inserted into the 0.018-in slot bracket, bodily movement was obtained at 9.1 mm length of power arm, namely, at the level of 1.8 mm above the center of resistance. In case a 0.018 × 0.025-in full-size archwire was used, bodily movement of the tooth was produced at the power arm length of 7.0 mm, namely, at the level of 0.3 mm below the center of resistance. Segmented arch mechanics required shorter length of power arms for achieving any type of controlled anterior tooth movement as compared to sliding mechanics. Therefore, this space closing mechanics could be widely applied even for the patients whose gingivobuccal fold is shallow. The segmented arch mechanics combined with power arm could provide higher amount of moment-to-force ratio sufficient for controlled anterior tooth movement without generating friction, and vertical forces when applying retraction force parallel to the occlusal plane. It is, therefore, considered that the segmented power arm mechanics has a simple appliance design and allows more efficient and controllable tooth movement.

Effects of mechanical stress and growth on the velocity of tooth movement

INTRODUCTION

In this study, we investigated the effects of the magnitudes of applied stress and growth status on the speed of tooth movement.

METHODS

Eighty-two maxillary canines in 41 subjects were retracted for 84 days by estimated stresses of 4, 13, 26, 52, or 78 kPa applied continuously via segmental mechanics. Dental impressions made at intervals of 1 to 14 days resulted in 9 or 10 dental casts per subject. Three-dimensional tooth movements were quantified using these casts, custom reference templates, and a measuring microscope. Serial height and cephalometric measurements determined growth status.

RESULTS

Distal tooth movement was linear with no lag phase in 96% of the teeth. Speeds averaged 0.028, 0.040, 0.050, 0.054, and 0.061 mm per day (standard errors, ± 0.004) for 4, 13, 26, 52, and 78 kPa, respectively. The maximum difference in speed between teeth was 9:1. Teeth moved significantly faster (P <0.0001) in growing compared with nongrowing subjects, on average by 1.6-fold. Stress and speed of tooth movement were logarithmically related in growing (R(2) = 0.47) and nongrowing (R(2) = 0.34) subjects. Other tooth movements were relatively small, except for the distopalatal rotation of teeth moved by 78 kPa that averaged more than 19°.

CONCLUSIONS

The speed of retraction was logarithmically related to the applied stress and was significantly faster in actively growing subjects compared with those who were not growing.

Finite element analysis of rapid canine retraction through reducing resistance and distraction

Objective

The aims of this study were to compare different surgical approaches to rapid canine retraction by designing and selecting the most effective method of reducing resistance by a three-dimensional finite element analysis.

Material and Methods

Three-dimensional finite element models of different approaches to rapid canine retraction by reducing resistance and distraction were established, including maxillary teeth, periodontal ligament, and alveolar. The models were designed to dissect the periodontal ligament, root, and alveolar separately. A 1.5 N force vector was loaded bilaterally to the center of the crown between first molar and canine, to retract the canine distally. The value of total deformation was used to assess the initial displacement of the canine and molar at the beginning of force loading. Stress intensity and force distribution were analyzed and evaluated by Ansys 13.0 through comparison of equivalent (von Mises) stress and maximum shear stress.

Results

The maximum value of total deformation with the three kinds of models occurred in the distal part of the canine crown and gradually reduced from the crown to the apex of the canine; compared with the canines in model 3 and model 1, the canine in model 2 had the maximum value of displacement, up to 1.9812 mm. The lowest equivalent (von Mises) stress and the lowest maximum shear stress were concentrated mainly on the distal side of the canine root in model 2. The distribution of equivalent (von Mises) stress and maximum shear stress on the PDL of the canine in the three models was highly concentrated on the distal edge of the canine cervix.

Conclusions

Removal of the bone in the pathway of canine retraction results in low stress intensity for canine movement. Periodontal distraction aided by surgical undermining of the interseptal bone would reduce resistance and effectively accelerate the speed of canine retraction.

Effect of play between bracket and archwire on anterior tooth movement in sliding mechanics: A three-dimensional finite element study

OBJECTIVES

The aim of this study was to clarify the effect of the play between the bracket and the archwire on anterior tooth movement subjected to the retraction force from various lengths of power arms in sliding mechanics.

MATERIALS AND METHODS

A three-dimensional finite element method was used to simulate en masse anterior tooth retraction in sliding mechanics. The displacements of the maxillary incisor and the archwire deformation were calculated when the retraction force was applied.

RESULTS

When a play did not exist, bodily movement was obtained at 5.0 mm length of power arm. In case a play existed, bodily movement was observed at the power arm length of 11.0 mm.

CONCLUSIONS

In the actual clinical situation, a bracket/archwire play and the torsion of the archwire within the bracket slot should be taken into consideration to prescribe an optimal power arm length and to achieve effective anterior tooth movement.

Analysis of stress in bone and microimplants during en-masse retraction of maxillary and mandibular anterior teeth with different insertion angulations: a 3-dimensional finite element analysis study

INTRODUCTION

The proper angle of microimplant insertion is important for cortical anchorage, patient safety, and biomechanical control. However, the actual impact of different insertion angulations on stability is unknown.

METHODS

To perform 3-dimensional finite element analysis, finite element models of a maxilla and a mandible with types D3 and D2 bone quality, and of microimplants with a diameter of 1.3 mm and lengths of 8 and 7 mm were generated. The microimplants were inserted at 30°, 45°, 60°, and 90° to the bone surface. A simulated horizontal orthodontic force of 200 g was applied to the center of the microimplant head, and stress distribution and its magnitude were analyzed with a 3-dimensional finite element analysis program.

RESULTS

The maximum von Mises stresses in the microimplant and the cortical bone decreased as the insertion angle increased. Analysis of the stress distribution in the cortical and cancellous bones showed that the stress was absorbed mostly in the cortical bone, and little was transmitted to the cancellous bone. The maximum von Mises stress was higher in type D3 bone quality than type D2 bone quality.

CONCLUSIONS

Placement of microimplants at a 90° angulation in the bone reduces the stress concentration, thereby increasing the likelihood of implant stabilization. Perpendicular insertion offers more stability to orthodontic loading.

Comparison of the intrusive effects of miniscrews and utility arches

INTRODUCTION

The aim of this prospective study was to compare the effects of incisor intrusion obtained with the aid of miniscrews and utility arches.

METHODS

Twenty-four patients (10 male, 14 female) with a deepbite of at least 4 mm were divided to 2 groups. In group 1, 13 patients (3 male, 10 female; mean age, 20.90 ± 7.12 years) in the postpubertal growth period were treated by using miniscrews; in group 2, 11 patients (7 male, 4 female; mean age, 15.25 ± 3.93 years) were treated with utility arches. Lateral cephalometric headfilms were taken at the beginning of treatment and after intrusion for the evaluation of the treatment changes. Statistical analyses of the data were performed with a significance level of P <0.05.

RESULTS

Intrusion lasted 6.61 ± 2.95 months for group 1 and 6.61 ± 2.46 months for group 2. The changes in the center of resistance of the incisors were 1.75 ± 0.4 mm (P <0.05) for group 1 and 0.86 ± 0.5 mm (P >0.05) for group 2; the difference between the groups was significant (P <0.05). In the miniscrew group, the incisors were protruded 0.79 ± 1.4 mm (P >0.05) relative to pterygoid vertical and 3.85° ± 2.4° (P >0.05) relative to the palatal plane. In group 2, the incisors showed 3.91 ± 0.7 mm (P <0.05) of protrusion relative to pterygoid vertical and 13.55° ± 2.4° (P <0.05) relative to the palatal plane. The maxillary first molars showed significant distal tipping in group 2 (P <0.05).

CONCLUSIONS

Unlike with utility arches, true maxillary incisor intrusion can be achieved by application of intrusive forces close to the center of resistance by using miniscrews with no counteractive movements in the molars.

Numeric simulations of en-masse space closure with sliding mechanics

INTRODUCTION

En-masse sliding mechanics have been typically used for space closure. Because of friction created at the bracket-wire interface, the force system during tooth movement has not been clarified.

METHODS

Long-term tooth movements in en-masse sliding mechanics were simulated with the finite element method.

RESULTS

Tipping of the anterior teeth occurred immediately after application of retraction forces. The force system then changed so that the teeth moved almost bodily, and friction occurred at the bracket-wire interface. Net force transferred to the anterior teeth was approximately one fourth of the applied force. The amount of the mesial force acting on the posterior teeth was the same as that acting on the anterior teeth. Irrespective of the amount of friction, the ratio of movement distances between the posterior and anterior teeth was almost the same. By increasing the applied force or decreasing the frictional coefficient, the teeth moved rapidly, but the tipping angle of the anterior teeth increased because of the elastic deflection of the archwire.

CONCLUSIONS

Finite element simulation clarified the tooth movement and the force system in en-masse sliding mechanics. Long-term tooth movement could not be predicted from the initial force system. The friction was not detrimental to the anchorage. Increasing the applied force or decreasing the friction for rapid tooth movement might result in tipping of the teeth.