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 Table of Contents  
ORIGINAL RESEARCH
Year : 2021  |  Volume : 13  |  Issue : 5  |  Page : 462-469

Displacement patterns of the maxillary anterior teeth during total distalization and en masse anterior retraction using interradicular and infrazygomatic crest mini-implants with varying power arm heights: A finite element analysis


1 Department of Orthodontics, Saveetha Dental College, Saveetha Institute of Medical and Technical Sciences, Saveetha University, Chennai, Tamil Nadu, India
2 Department of Orthodontics, Saveetha Dental College, Saveetha Institute of Medical and Technical Sciences, Saveetha University, Chennai, Tamil Nadu, India; Department of Orthodontics, Wongwang Dental Research Institute, Iksan, Korea; Postgraduate Orthodontic Program, Arizona School of Dentistry and Oral Health, A. T. Still University, Mesa, AZ, USA
3 Department of Orthodontics, Saveetha Dental College, Saveetha Institute of Medical and Technical Sciences, Saveetha University, Chennai, Tamil Nadu, India; Department of Orthodontics, University of Puthisastra, Phnom Penh, Cambodia

Date of Submission17-Apr-2021
Date of Decision04-Jul-2021
Date of Acceptance16-Jul-2021
Date of Web Publication11-Oct-2021

Correspondence Address:
Dr. Aravind K Subramanian
Department of Orthodontics, Saveetha Dental College, Saveetha Institute of Medical and Technical Sciences, Saveetha University, Chennai, Tamil Nadu.
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/JIOH.JIOH_99_21

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  Abstract 

Aim: To evaluate the initial displacement of the maxillary anterior teeth during distalization of the whole maxillary dentition and en masse retraction of the maxillary anterior teeth using interradicular MIs (IRMIs) and infrazygomatic crest mini-implants (IZCMI) with varying power arm heights. Materials and Methods: The study is a finite element (FE) study. Two FE models were created for total distalization of the maxillary dentition and en masse retraction of the maxillary six anterior teeth. Mini-implants (MIs) were placed in the IZC and IR areas. The power arms were placed on the arch wire between the lateral incisors and canines at heights of 5, 8, and 12 mm. Total distalization (3N) and en masse anterior retraction (2N) forces were applied to the power arm. Results: The von Mises stress increased as power arm height increased with no significance, but the values were significantly greater in the IRMI than IZCMI in both models. Initial backward displacement of the maxillary anterior teeth increased as power arm height increased with no significance, but the values were significantly greater in IZCMI than IRMI in both models. Initial upward displacement of the maxillary anterior teeth decreased as power arm height increased with no significance, but the values were significantly greater in IRMI than IZCMI in both models. Conclusions: A careful consideration of the MI location and the power arm height should be preceded to obtain a desired tooth movement when planning to retract or distalize the maxillary dentition.

Keywords: Finite Element Analysis, Mini-implants, Power Arm Height, Retraction


How to cite this article:
Sreenivasagan S, Subramanian AK, Chae JM, Venugopal A, Marya A. Displacement patterns of the maxillary anterior teeth during total distalization and en masse anterior retraction using interradicular and infrazygomatic crest mini-implants with varying power arm heights: A finite element analysis. J Int Oral Health 2021;13:462-9

How to cite this URL:
Sreenivasagan S, Subramanian AK, Chae JM, Venugopal A, Marya A. Displacement patterns of the maxillary anterior teeth during total distalization and en masse anterior retraction using interradicular and infrazygomatic crest mini-implants with varying power arm heights: A finite element analysis. J Int Oral Health [serial online] 2021 [cited 2021 Dec 6];13:462-9. Available from: https://www.jioh.org/text.asp?2021/13/5/462/327876


  Introduction Top


In routine orthodontic practice, retraction of the anterior teeth is often indicated to establish a good lip seal and a pleasing aesthetic profile. Space closure in extraction cases is usually brought about by an en masse anterior retraction.[1] For orthodontic camouflage treatment in Class II malocclusion subjects, distalization of the dentition is accompanied by a change in the occlusal plane. This change is a result of intrusion of the dentition during distal movement.[2],[3],[4],[5]

The MIs are an excellent source of absolute anchorage for various orthodontic tooth movements, which include intrusion, retraction, distalization, and protraction of an individual tooth, a unit, or even the whole dentition at times. Their ability to distalize the entire dentition has widened the horizons of non-extraction treatment.[6] Achieving true intrusion of the dentition is a challenging procedure as the force applied on the anterior region will produce a counter moment in the posterior. However, the MIs will negate the effect and will allow for true intrusion.[7],[8],[9] The maxillary posterior MIs can be used for intrusion and retraction of the anterior teeth without anterior MIs, especially in the treatment of the bialveolar dental protrusion.[10],[11]

Finite element analysis (FEA) has been used for the creation of models based on the information of human anatomy and experimentation of the force models, type of reaction, and bone stress levels during retraction and distalization using various mechanics.[12],[13],[14] The FEA allows for the evaluation of an initial displacement in an irregular geometry, with different physical properties of materials.[13] The MIs were used as a stable anchorage while performing biomechanical tooth movements while varying the height of the power arm in the FE model.[14]

The center of resistance (CRes) should be considered when applying forces for retraction, intrusion, and distalization of the maxillary dentition. In the previous studies,[15],[16] the CRes of the maxillary six anterior teeth as well as the whole maxillary dentition was identified, but it showed a lot of variability based on different factors.

The location of the MI, height of the power arm, and CRes of the segment, being retracted or distalized, should be considered for desired biomechanical tooth movement when determining the direction of force. There have been previous FE studies[12],[13],[14],[15],[16] demonstrating retraction of the maxillary anterior teeth using IRMIs; however, very few finite element method (FEM) studies are available on IZCMIs. The length of the power arm plays a crucial role in the Cres in determining tooth movement.[17] The increase in the power arm height has been reported to decrease the tendency of tipping and increase in bodily movement.[18] The need of the hour is to have proper guidelines regarding the height and site of placement of the MI and the required length of the power arm depending on the type of tooth movement and its relation to the CRe of the anterior teeth.

The null hypothesis we propose is that the change in the height of the power arm during retraction or en masse anterior distalization using MIs as adjuvant anchorage will not cause any difference in the tooth movements or direction. Thus, the alternate hypothesis we need to take into consideration is that the shorter power arm will bring about intrusion of the maxillary anterior teeth along with retraction or distalization using MIs as an adjuvant anchorage, whereas a longer power arm will direct the force along the CRe and bring about bodily movement. Therefore, the aim of this study was to compare the differences between the initial tooth displacements when applying retractive forces using IRMIs and IZCMIs with varying power arm heights.


  Materials and Methods Top


The study is an FE analysis. The project was approved by the scientific review board of the university (SRB Ref No.: SRB/SDC/ORTHO-1804/21/102). Computed tomographic scans of a healthy human skull with full component of teeth and needing no orthodontic correction for his malocclusion were obtained. The scans were obtained in DICOM format. The format of the images was then converted to stereolithographic (STL) files using the software MIMICS 8.11 (Materialise, Ann Arbor, MI). The MI design was obtained from Fav Anchor (Skeletal Anchorage system, India). The IRMI (Titanium) and IZCMI (Stainless Steel) were 1.6 × 10 mm and 2 × 12 mm in diameter and length, respectively. Surface modeling of the FEA model was done using HYPERMESH software (2021 Altair Engineering Inc., version 10.0). ABAQUS software (Dassault Systems) was used for material designing. The hinge portion of the maxilla was fixed in all degrees of freedom. The properties of the materials are depicted in Young’s modulus, such as Poisson’s ratio, assumed isotropic and homogenous.

Once the maxillary model was generated, all teeth were set up with metal brackets of the MBT system and aligned with a 0.019 × 0.025″ stainless steel arch wire. For the model to be used for en masse anterior retraction, the first premolars were removed on both sides. After that, IZCMIs and IRMIs were placed in both models. The IRMIs were placed between the roots of the maxillary second premolar and first molar at a height of 7 mm above the cementoenamel junction (CEJ) angulated 40° to the occlusal plane. The IZCMIs were placed between the maxillary first and second molars, 13 mm above the CEJ at an angulation of 70° to the occlusal plane. The initial angle of insertion, for both implants, was perpendicular to the buccal bone, that is, 90°, and later angulated to 40° and 70° to obtain ideal insertion angles for IRMIs and IZCMIs, respectively [Figure 1].[13]
Figure 1: Finite element models according to location of mini-implant (IZCMI, infrazygomatic crest mini-implant; IRMI, interradicular mini-implant), height of power arm, and premolar extraction: (A) and (B), total distalization of the maxillary dentition; (C) and (D), en masse retraction of the maxillary anterior teeth

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Power arms of 5-, 8-, and 12-mm height were placed on the arch wire between the lateral incisors and canines in each model. These three variants were chosen while keeping in mind the factors such as the distance from the vestibule, the CRe, the understanding that a longer power arm would be in line with the CRes, and a shorter hook will apply a force below the CRes. For en masse anterior retraction and whole arch distalization models, the forces applied on either side were fixed at 2N and 3N per side using nickel-titanium (NiTi) closed coil springs, respectively. These models were fixed in all directions and initial displacements were determined in the X, Y, and Z axis [Figure 1].


  Results Top


The properties of the materials used in this FEM setup have been depicted in Young’s modulus and as Poisson’s ratio [Table 1], and they were assumed to be isotropic and homogenous. [Table 2] shows the von Mises stress in FE models according to the MI location and height of the power arm. The von Mises stress increased as the height of the power arm increased, but it showed no significant difference although there was a significant difference when the stresses with different MI locations were compared [Figure 2] and [Figure 3].
Table 1: Table of material properties

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Table 2: von Mises stress in finite element models according to the mini-implant location and power arm height (MPa)

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Figure 2: The von Mises stress comparison in finite element models using infrazygomatic crest mini-implant and interradicular mini-implant with varying power arm heights (MPa)

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Figure 3: Initial backward displacement in magnitude during total distalization of the maxillary dentition and en masse retraction of the maxillary anterior teeth using infrazygomatic crest mini-implant and inter-radicular mini-implant

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[Table 2] and [Figure 3] show the initial backward displacement. Backward displacement of the maxillary anterior teeth increased as the height of the power arm increased. The values were greater with IZCMI than IRMI models. The values were also greater for IZCMI during en masse anterior retraction than for total distalization. The maximum backward displacement was shown with the 12 mm of power arm using IZCMI for the en masse anterior retraction model. There were no significant differences between IRMIs for en masse anterior retraction and total distalization.

[Table 3] and [Figure 4] show the initial upward displacement. Upward displacement of the maxillary anterior teeth decreased as the power arm height increased. The values were greater in IRMI than IZCMI models. The values were greater in IRMI for total distalization compared with en masse anterior retraction. The maximum upward displacement was seen at 5 mm of power arm height with IRMI for the total distalization model. There were no significant differences in these values when IZCMIs for en masse anterior retraction and total distalization were compared.
Table 3: Displacement of the maxillary teeth

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Figure 4: Initial upward displacement in magnitude during total distalization of the maxillary dentition and en masse retraction of the maxillary anterior teeth using interradicular mini-implant

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  Discussion Top


Orthodontic tooth movement results from biological responses, such as periodontal ligament and alveolar bone remodeling. In this FEA study, we assumed that there was no deformation in the tooth or bracket by the external forces. Our study designated the properties to the teeth, bracket, and arch wire based on the research by Song et al.[19]

The occluso-gingival location of the MI or the use of a power arm/crimpable hook can be critical factors in determining the vertical component of the total force.[18] In this study, we observed that, when the same amount of force was applied keeping all the characteristics constant, except for the height of the anterior hooks, there were no significant differences in the bone stress levels in the anterior teeth area. The bone stress pattern varied with different types of tooth movement and depended on the CRes of the teeth.[20],[21]

Different opinions have been voiced in terms of the attributes of the power arm. Some authors have suggested that the height of the power arm was more critical, whereas others point out that the location of attachment is more important.[22],[23],[24],[25]

In our study, the maximum backward displacement was shown with the 12 mm power arm for en masse anterior retraction using IZCMI models. The most efficient total distalization or en masse anterior retraction was obtained using IZCMIs with power arm heights of 12 mm. These results were inconsistent with the results of Sia et al.,[26] which indicated that the power arm should lie on the same level as the CRes in order to achieve bodily displacement. In another study that was similar to ours conducted by Suzuki et al., there were variations of 0, 5, and 10 mm variations in the hook and they evaluated the stress patterns using FEA. In all their models, the central incisors had shown extrusion but this was decreased as there was an increase in the posterosuperior traction.[27] This indicates that the length of the power arm should be kept as long as possible to bring about efficient tooth movement that will be bodily and through the CRe. Previous studies have shown that a segmented power arm has been shown to produce bodily movement when the height of the power arm is almost at the level of that of the CRes, but there is an issue of soft tissue trauma in the vestibular aspect.[28] Our results were also consistent with that of Sung et al., where they observed that an 8 mm high anterior retraction hook was not efficient in bringing about a bodily retraction with high MI traction. In their study, a 2 mm hook brought about intrusion, which is also similar to our study findings where a short hook intruded the maxillary anterior teeth.[3]

Among all models, there was no anchorage loss observed in the molar region. The MIs as an anchorage will, thus, reduce the treatment time by minimizing the anchorage loss. The center of rotation is often insensitive to elastic properties of the periodontal ligament, and the instantaneous center of rotation on the application of force is independent on the load.[29] The CRes of the maxillary six anterior teeth can be estimated to be located halfway between the CRes of the four maxillary incisors and canines.[13] Deepening of the bite is often noted with retraction and when taking into consideration from a vertical plane, placing the e-chain or NiTi coil spring at the level of molar height will tend to exhibit the extrusion of the tooth and the use of MIs will bring about comparative intrusion.[30] When the force vector passes at, or above the CRes of the anterior teeth, there will be a clockwise or counter clockwise rotation and subsequently, uncontrolled tipping of the anterior teeth occurs.[31]

In this study, height of the power arm showed no significant influence on the type tooth movement, which was consistent with the study by Rokutanda et al.,[17] which evaluated the optimal height of the power arm to attain a controlled movement of the anterior teeth in segmented power arm mechanics at various heights. Segmented arch mechanics with power arms can provide a higher moment-to-force ratio that is sufficient for a controlled anterior tooth movement.[28] In another study by Tominaga et al.,[32] the authors estimated that a power arm height of 5.5 mm produced bodily movement as well as caused less arch wire deformation. When the power arm height was increased by more than 5.5 mm, the anterior segment of the arch wire was deformed, causing lingual root movement. In an FEM study conducted by Hedayati et al., they had assessed different anteroposterior positions and the height of the anterior hook at 0, 3, 6, and 9 mm; they had observed that a bodily tooth movement occurred when they had a hook height of 9 mm in both the implant positions that they assessed at mesial and distal to the second premolar.[33] In our study, we thus prove the alternate hypothesis that we had formulated that the shorter power arm will bring about intrusion of the maxillary anterior teeth along with retraction or distalization using MIs as an adjuvant anchorage, whereas a longer power arm will direct the force along the CRe and bring about bodily movement.

The FEA has some limitations in being only an engineering model with only the initial displacement calculated. The FEA is a momentary evaluation of the force application; therefore, further studies including creep strain and force application over time cycles would be beneficial in understanding the various biological responses. In future, a well-planned randomized controlled trial where implant-aided retraction can be done using varying power arm heights and evaluating the CRe of the maxillary anterior teeth and the type of tooth movement would be beneficial in creating a clear guideline in terms of biomechanical application of force using sliding mechanics when using temporary anchorage devices.

The backward displacement of the maxillary anterior teeth increases as the power arm height increases. The stress patterns were higher for IRMI. In the model setup of en masse distalization, IZCMI is better than IRMI. The power arm height of 12 mm followed by 8 mm had the maximum backward displacement. Upward displacement can be obtained using a shorter power arm height of 5 mm.


  Conclusions Top


Displacement pattern of the maxillary anterior teeth during distalization of the whole maxillary dentition and en masse retraction of the maxillary anterior teeth could be evaluated by an FEA. The following conclusions were achieved:

  1. The von Mises stress and backward displacement of the maxillary anterior teeth increased, and upward displacement of the maxillary anterior teeth decreased as power arm height increased, but they showed no significance.


  2. The von Mises stress values of the maxillary anterior teeth were significantly greater in the IRMI than IZCMI.


  3. Backward displacements of the maxillary anterior teeth were significantly greater in IZCMI of en masse anterior retraction than total distalization.


  4. Upward displacements of the maxillary anterior teeth were significantly greater in IRMI of total distalization than en masse anterior retraction.


  5. The maximum backward displacement was shown at 12 mm of the power arm height in IZCMI of an en masse anterior retraction.


  6. The maximum upward displacement was shown at 5 mm of the power arm height in IRMI of a total distalization.


  7. The authors recommend using MI in the infrazygomatic crest region when en masse distalization is needed, and either IRMIs or infrazygomatic crest mini-screw can be used successfully when retraction of maxillary anterior teeth is needed.


A careful consideration of the MI location and the power arm height should be preceded to obtain a desired tooth movement when planning to retract or distalize the maxillary dentition.

Acknowledgments

The author would like to acknowledge their CAE specialist Mr Ramkumar Murugan for helping with designing and running of this FEA.

Financial support and sponsorship

Nil.

Conflicts of interest

Not applicable.

Authors’ contributions

The study was designed and carried out, as well as the drafting of the article were done by the first author. The second author conceived the design, guided and designed the study, and aided in correction of the article. The third author helped in the designing of the study and with statistics. The fourth author helped with correction of the article and provided revisions to the scientific content of the article. All the authors had given consent for publication.

Ethical policy and Institutional Review board statement

Ethical approval was obtained from the university review board, and consent to orthodontic treatment was obtained.

Patient declaration of consent

Not applicable.

Data availability statement

All the supporting data for this article can be provided on request.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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