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Comparison of zygoma plates and infrazygomatic crest miniscrews used open bite treatment: A 3-dimensional finite element study

Comparison of zygoma plates and infrazygomatic crest miniscrews used open bite treatment: A 3-dimensional finite element study



American Journal of Orthodontics and Dentofacial Orthopedics, 2022-05-01, Volume 161, Issue 5, Pages e466-e474, Copyright © 2022


Introduction

This study aimed to evaluate infrazygomatic crest (IZC) miniscrews, which represent a new approach to maxillary posterior tooth intrusion and extra-alveolar skeletal anchorage, using 3-dimensional finite element stress analysis to predict the clinical usability instead of zygoma plates.

Methods

Six different models were developed. The direction of the intrusion force was generated parallel to the maxillary first molar, and the posterior bite-block intrusion appliance was connected with 2 steel arches. A zygoma plate, stainless steel (SS)-IZC miniscrew, and a titanium alloy (TiA)-IZC miniscrew were used as anchorage units, and 200 g and 400 g of intrusion forces were applied. The cortical bone, spongious bone, stress values, and displacements in the anchorage unit were examined. The anchored materials were considered nonosseointegrated.

Results

Zygoma plates formed low-stress values under 200 g and 400 g of force. The greatest stress and displacement occurred in the first miniscrew that fixed the plate. SS-IZC miniscrews gave lower stress and displacement values than TiA-IZC miniscrews. The most stress was detected inferior to the screw-to-bone contact in IZC miniscrews.

Conclusions

Considering the initial value of resorption in cortical bone, it is predicted that SS-IZC and TiA-IZC miniscrews can be used in the clinic under 200 g of intrusion force, although there is not as little stress and displacement as for zygoma plates. Under 400 g of intrusion force, clinical use cannot be recommended because of the critical stress value they generate in the cortical bone.

Highlights

  • Zygoma plates formed low-stress values under 200 g and 400 g of force.

  • SS-IZC and TiA-IZC miniscrew formed low-stress values under 200 g force.

  • SS-IZC and Ti-IZC miniscrew formed critical stress values under 400 g of force.

  • SS-IZC miniscrews induced less stress and were displaced less than the TiA-IZC.

The anterior open bite is one of the malocclusions characterized by the absence of vertical contact between the edges of the mandibular and maxillary incisors, whereas the mandibular and maxillary posterior teeth close, making both treatment and posttreatment retention very difficult.

Treatment varies depending on the stage of growth and development and whether the problem is dental or skeletal. The most critical factor in the treatment of adults is the control of increased vertical dimension. The best way to solve this problem is the intrusion of the posterior teeth. With the widespread use of temporary anchorage devices, the procedure of posterior intrusion, which is difficult for orthodontists, has become more applicable.

They are miniplates with the best stability among skeletal anchors. The posterior segment can be successfully embedded with miniplates placed in the area of the zygomatic buttress. In addition to the successful use of miniplates in molar intrusion, it also has disadvantages, such as making an appointment with the surgical clinic for its use, the need for 2 surgical procedures, causing pain, swelling, and tenderness after the procedure.

Miniscrews are often used because they are more economical than plates, easier to apply, cause less discomfort after the procedure, and keep hygienically. Although various tooth movements can be successfully performed with the miniscrews placed between the roots, the limited range of movement, especially for intrusive movements, can lead to problems, such as the risk of injury to the root and cessation of movement. Therefore, the infrazygomatic crest (IZC) region is preferred for skeletal anchorage because it is far from the roots and has the best bone capacity in the maxilla. IZC miniscrews are longer than interradicular miniscrews and are made of stainless steel rather than a biocompatible titanium alloy to avoid breakage in practice.

As a new approach, IZC miniscrews have been used for various purposes in the clinic because they have a high success rate, are easier and more practical to use, the orthodontist can use them in the same session, they are more economical, cause less pain and cause less dissatisfaction during and after use.

In the literature, miniscrews in posterior intrusion have been investigated by finite element analysis and clinical studies. Distalization and en masse retraction movements have been studied with IZC miniscrews, but a study of posterior intrusion with IZC miniscrews could not be found. This study aims to compare the stresses and displacements in cortical and spongious bone, zygoma plate, and IZC miniscrews after applying 2 different intrusion forces to the zygoma plate, stainless steel IZC (SS-IZC) miniscrew and titanium alloy IZC (TiA-IZC) miniscrew using three-dimensional finite element analysis.

Material and methods

The study was conducted in Kırıkkale University Faculty of the Dentistry Department of Orthodontics and in collaboration with Tempa Engineering, using the three-dimensional finite element stress analysis method. Approval to conduct the study was obtained from the Kırıkkale University Noninterventional Research Ethics Committee (Date: September 30, 2020; Decision No: August 13, 2020). It was supported by the Kırıkkale University Scientific Research Projects Coordination Unit with the project number 2020/098.

The obtaining of the stereolithography model from the tomography data of Visible Human Project was carried out in the software 3DSlicer. The 3-dimensional model was transferred to the software Altair Evolve (Altair, Troy, Mich), in which 1.5 mm cortical bone, spongious bone, teeth, and 0.25 mm thick uniform periodontal ligament, acrylic bite-block, I-shaped titanium miniplate (Multipurpose Anchor MPI 1000; Tasarım Med, İstanbul, Türkiye), titanium miniscrews (2 × 5 mm) and IZC miniscrew (2 × 12 mm OrthoBoneScrews; Newton’s A, Hsinchu City, Taiwan) were modeled in the Altair Evolve software. The mesh structures were coordinated in the software Altair Hypermesh to ensure the transmission of forces between the models. The Nastran-based Altair Optistruct (Altair) implicit solver was used to solve the finite element models created.

The screws used in our study were considered nonosseointegrated. In the spongious and cortical bone contact regions of the IZC screw and the miniscrews of the zygoma plate, a nonlinear contact structure with a friction coefficient of 0.37 was defined. All materials were considered homogeneous and isotropic. Table I shows the number of elements and nodes used in the models. Table II shows the elastic modulus and Poisson’s ratio of the materials.

Table I
Number of elements and nodes
Name Model-1 Model-2
Miniplate IZC screw
N o. of Nodes 229810 206704
N o. of Elements 907427 820324

Table II
Characteristics of the materials used
Material Elastic Modulus [MPa] Poisson’s Ratio
Cortical Bone 14,700 0.3
Spongious Bone 1500 0.3
Teeth 20,700 0.3
Titanium 110,000 0.35
SS 193,000 0.3
TiA 114,000 0.34
Acrylic 15 0.35
Peridontal ligament 500 0.49

All models used an acrylic bite-block covering the maxillary first premolar, second premolar, first molar, and second molar. The acrylic blocks in the right and left areas were connected with 2 stainless steel palatal bars with a diameter of 1.4 mm. The bars were guided 5 mm away from the palate not to become embedded in the tissue. A bar of 0.9 mm round steel wire was attached to the buccal side of the acrylic plate ( Fig 1 ). The interface between the teeth and bite-block was assumed to be fully bonded.

Maxillary jaw model and acrylic appliance.
Fig 1
Maxillary jaw model and acrylic appliance.

Two types of loading conditions, 200 g and 400 g of force, were created for each model. The force applied to the zygoma plate and IZC miniscrews, placed in the zygoma and infrazygomatic area, respectively, were transferred to the acrylic block’s metal bar via a closed spring parallel to the axis of the first molar tooth ( Figs 2 , A and B ). A total of 6 analyses were carried out nonlinearly under 2 different loading conditions and with different material properties of the IZC miniscrew.

A, Zygoma plate model and force application; B, IZC miniscrew model, and force application.
Fig 2
A, Zygoma plate model and force application; B, IZC miniscrew model, and force application.

In the first model, 200 g of force was applied to the zygoma plate; in the second model, 400 g of force. In the third model, 200 g of force was applied to the SS-IZC miniscrew; in the fourth model, 400 g of force. In the fifth model, 200 g of force was applied to the TiA-IZC miniscrew; in the sixth model, 400 g of force.

Stress and displacement values obtained by finite element analysis are shown according to the color scale; stress values are given in Mega Pascals (Mpa = N/mm 2 ) displacement values in millimeters (mm). The stress distribution throughout the anchorage unit, cortical bone, and spongious bone was studied, and stress values were calculated using von Mises stresses. Displacements were evaluated in 3 different axes: transversal (x), sagittal (y), and vertical (z). The “+” values on the x-axis indicated lateral movement, “−” values indicated medial movement, “+” values on the y-axis indicated posterior movement, “−” values indicated anterior movement, “+” values on the z-axis indicated apical movement, and “−” values indicated coronal movement.

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