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 Table of Contents  
ORIGINAL ARTICLE
Year : 2018  |  Volume : 11  |  Issue : 2  |  Page : 130-135

Comparison of stress distribution on bone and mini-implants during en-masse retraction of maxillary anterior teeth in labial and lingual orthodontics: A three-dimensional finite element analysis


Department of Orthodontics and Dentofacial Orthopaedics, KLE Academy of Higher Education and Research, Belagavi, Karnataka, India

Date of Web Publication18-May-2018

Correspondence Address:
Dr. Ankita Anil Ringane
KLE Academy of Higher Education and Research, Belagavi, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/kleuhsj.kleuhsj_248_17

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  Abstract 


INTRODUCTION: In fixed orthodontic treatment, space closure is one of the most challenging aspects. The common method of en-masse retraction in sliding mechanics is the use of the elastomeric chain and power arm. Recently, titanium mini-implants are used as absolute sources of orthodontic anchorage. Lingual and labial bracket placement influences the pattern of tooth movement, but the stress that occurs around the teeth, the mini-implants, and the surrounding bone can be accurately mapped using a three-dimensional (3D) finite element method.
AIM: The aim of this study was to evaluate the stress distribution on bone and mini-implants during en-masse retraction of maxillary anterior teeth in labial and lingual orthodontics with the help of finite element analysis.
MATERIALS AND METHODS: Following the standard protocol for 3D finite element models, two models were created for en-masse retraction of the six anterior teeth: one model using the labial technique with the mini-implant placed at a height of 5 mm from the alveolar crest bucally and the second model using the lingual technique with the mini-implant placed at a height of 5 mm from the gingival margin palatally, and 200 g of retraction forces was given using a elastomeric chain.
RESULTS AND CONCLUSIONS: The Von Mises stresses in the bone and mini-implant were significantly higher in the lingual technique as compared to the labial technique. The variations in stress patterns in the bone and the mini-implant in the labial and lingual technique could be the result of difference in the inter-bracket distance, point of force application, and its location to the center of resistance of the dentition.

Keywords: En-masse retraction, finite element method, mini-implant, stress


How to cite this article:
Ringane AA, Hattarki R. Comparison of stress distribution on bone and mini-implants during en-masse retraction of maxillary anterior teeth in labial and lingual orthodontics: A three-dimensional finite element analysis. Indian J Health Sci Biomed Res 2018;11:130-5

How to cite this URL:
Ringane AA, Hattarki R. Comparison of stress distribution on bone and mini-implants during en-masse retraction of maxillary anterior teeth in labial and lingual orthodontics: A three-dimensional finite element analysis. Indian J Health Sci Biomed Res [serial online] 2018 [cited 2020 Feb 29];11:130-5. Available from: http://www.ijournalhs.org/text.asp?2018/11/2/130/232690




  Introduction Top


Lingual appliances have marked a great leap forward in terms of esthetics in orthodontics. Lingual appliances have their own peculiar biomechanics, distinct from that of conventional orthodontics, and special care must be taken in their application. In particular, for esthetic reasons, the six anterior teeth are retracted as a unit in the lingual technique so as not to create any space between canines and lateral incisors. The biomechanics of using various appliances differs in lingual appliance as compared to labial appliance.[1]

Among the three stages of comprehensive fixed orthodontic treatment, the second stage, i.e., space closure is one of the most challenging aspects. The biomechanics involved in the second phase of orthodontic treatment is either friction mechanics (en-masse retraction/sliding mechanics) or frictionless mechanics (loop mechanics). Sliding mechanics utilizes minimal archwire bending and is, therefore, quicker, and offers better sliding of the wire through the slots, and reactivation with these mechanics is simple.

In friction mechanics, there are several commonly used methods for en-masse retraction, out of which elastomeric chain and nickel titanium (NiTi) closed coil springs are commonly used. Elastomeric chain is relatively consistent in producing tooth movements but has a major drawback, i.e., rapid decay of forces. To compensate for this, the initial forces must often be greater than what is desirable.[2]

Anchorage control plays a pivotal role in the effective management of orthodontic patients for obtaining both structural balance and facial esthetics. In recent years, titanium mini-implants have gained enormous popularity in the orthodontic community and are being considered as absolute sources of orthodontic anchorage.[3]

The clinical success of a mini-implant is largely determined by the manner in which the mechanical stresses are transferred from the mini-implant to the surrounding bone without generating forces of a magnitude that would jeopardize the longevity of the mini-implant.[4]

The finite element method (FEM), a modern tool of numerical stress analytical technique, has the advantage of being applicable to solids of irregular geometry that contain heterogeneous material properties; it also provides an approximate solution for the response of the three-dimensional structures to the applied external loads under certain boundary conditions.[5] It is virtually impossible to measure stress accurately around mini-implants in vivo and the reactions and interactions of individual tissues.[6]

Very little data are available in literature depicting the stress distribution on mini-implant and bone during en-masse retraction; moreover, no study has been conducted comparing the stress distribution on bone and the mini-implant under en-masse retraction force in labial and lingual orthodontics in the maxilla, so this study was undertaken to compare the two techniques with the help of FEM.

Aim

The aim of this study was to evaluate the stress distribution on bone and mini-implants during en-masse retraction of maxillary anterior teeth in labial and lingual orthodontics.


  Materials and Methods Top


The FEM is a numerical procedure used for analyzing structures and consists of a computer model of a material or design that is stressed and analyzed for specific results. FEM uses a complex system of points (nodes) and elements, which make a grid called as mesh. This mesh is programmed to contain the material and structural properties (elastic modulus, Poisson's ratio, and yield strength), which define how the structure will react to certain loading conditions.[7]

Basic steps involved in carrying out FEM are as follows:[8]

  1. Preprocessing


    1. Construction of the geometric model
    2. Conversion of the geometric model into a finite element model
    3. Assembly/material property data representation
    4. Defining the boundary conditions
    5. Loading configuration


  2. Processing
  3. Postprocessing.


Separate models were constructed for mini-implants of 1.5 mm diameter and 8 mm length and 0.019” × 0.025” stainless steel wire along with 0.022” × 0.025” MBT brackets and molar tubes in the labial technique. Similarly, separate models were constructed for mini-implants of 1.5 mm diameter and 8 mm length and 0.016” × 0.022” stainless steel wire along with 0.018” × 0.025” STB brackets and molar tubes in the lingual technique. In the labial technique, bilaterally, a hook (power arm) of 3 mm length was placed between the lateral incisor and canine, and mini-implants were placed perpendicularly in the bone between the roots of maxillary second premolar and first molar 5 mm from the alveolar bone bucally. In the lingual technique, the mini-implants were placed perpendicular to the occlusal plane between the maxillary first and second molars 5 mm from the gingival margin palatally.

The different structures involved in this study include alveolar bone, periodontal ligament, teeth, NiTi coil spring, elastomeric chain, stainless steel archwire, and hook and titanium mini-implant. Each structure has a specific material property. Some of the material properties used in this study were derived from the study by Chang et al.,[9] and since the elastomeric chain is a polyurethane material, as reported by Eliades et al.,[10] elastomeric chain was given the properties of polyurethane [11] [Table 1].
Table 1: Material properties

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After preparation of the two finite element models, constant retraction forces were applied in each model, i.e., 200 g bilaterally from mini-implant to hook (power arm) of 3 mm length placed between the lateral incisor and canine [Figure 1], [Figure 2], [Figure 3], [Figure 4].
Figure 1: Complete finite element model - frontal view (labial technique)

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Figure 2: Occlusal view (lingual technique)

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Figure 3: Elastomeric chain with a buccal mini-implant (labial technique)

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Figure 4: Elastomeric chain with a palatal mini-implant in lingual technique

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  Results Top


Stresses (MPa) in the mini-implant and the alveolar bone during en-masse retraction and displacement (mm) of the anterior teeth were calculated and presented in colorful bands; eight different colors represented different stress levels and displacement patterns in the deformed state.

Determination of Von Mises stress in alveolar bone

Highest stress value of 51.81 MPa in the cortical bone and 4.45MPa in the cancellous bone was [Figure 5] and [Figure 6] seen at bone–mini-implant interface when the en-masse retraction was carried out with the help of elastomeric chain in the labial technique. Highest stress value of 107.79 MPa in the cortical bone and 3.09 MPa was [Figure 7] and [Figure 8] seen at the bone–mini-implant interface when en-masse retraction was carried out with the help of elastomeric chain in the lingual technique.
Figure 5: Stress patterns in cortical bone (lateral view) in labial technique

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Figure 6: Stress patterns in cancellous bone (lateral view) in labial technique

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Figure 7: Stress patterns in cortical bone (occlusal view) in lingual technique

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Figure 8: Stress patterns in cancellous bone (occlusal view) in lingual technique

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Determination of Von Mises stress in mini-implant

Highest stress value of 272.71 MPa was seen at the mini-implant head followed by lesser amount of stress value of 121.55 MPa at the mini-implant neck and very minimal or no stress value of 30.86 MPa at the mini-implant tip, when the en-masse retraction was carried out with the help of elastomeric chain in the labial technique [Figure 9]. Highest stress value of 285.67 MPa was seen at the mini-implant head followed by lesser amount of stress value of 158.72 MPa at the mini-implant neck and very minimal or no stress value of 31.77 MPa at the mini-implant tip, when en-masse retraction was carried out with the help of elastomeric chain in the lingual technique [Figure 10].
Figure 9: Stress patterns in mini-implant in labial technique

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Figure 10: Stress patterns in mini-implant in lingual technique

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  Discussion Top


Finite element analysis is a computerized numeric method for solving complex problems by dividing complex structures into many small interconnected simple structures. However, it has certain limitations; several assumptions are made in the development of the model in FEM. The structures in the model are all assumed to be homogenous and isotropic and to have linear elasticity.[6]

In the present study, FEM was used to evaluate the stress distribution on bone and mini-implants during en-masse retraction of the maxillary anterior teeth in labial and lingual technique.

In this study, the power arm for en-masse retraction was placed in between lateral incisor and canine as suggested by Kim et al.[12] According to them, the application of retraction force at this position indicates more stable movement of the anterior teeth than when the power arm is positioned between the canine and the first premolar.

The mini-implants used in the labial technique were 1.5 mm in diameter and 8 mm in length and were placed perpendicular to the inter-radicular bone in the region between the second premolar and first molar, similar to a study carried out by Issa Jasmine et al.[6] The 90° placement angulation in the bone reduces the stress concentration, thereby increasing the likelihood of implant stabilization and providing more stability to the orthodontic loading.[6],[4] As various dimensions of mini-implants are available and 1.5 mm diameter is widely used, a similar diameter mini-implant has been used in this study. According to Miyawaki et al.[13] and Machado,[4] the 1.5 mm diameter mini-implant can be successfully used with good stability in the buccal inter-radicular bone between the second premolar and first molar as an anchorage device. The importance of mini-implant length has been investigated in several studies, most of which have concluded that it does not significantly affect mini-implant stability.[4],[13],[14] The length of the mini-implant used should be decided based on the surrounding structures and location of placement.[4]

The mini-implants used in the lingual technique were 1.5 mm in diameter and 8 mm in length and placed inter-radicularly between the first and second molar, 5 mm–6 mm from gingival margin, and inserted perpendicular to the occlusal plane as given by Echarri et al.[15]

The forces exerted on the mini-implant were 200 g for both the techniques, which were within the physiologic limit.

This FEM study was carried out in an attempt to compare the stresses on the bone and mini-implant in the labial and lingual technique, which would make it possible to determine the underlying biomechanical mechanism of the mini-implants and thus provide reliable clinical usage guidelines.

Due to its mechanical nature, it is important to understand the mechanical rationale of mini-implant usage. It would be difficult to determine the underlying biomechanical mechanisms for mini-implant applications through an experimental approach because of the limited measureable mechanical index, imprecise parameter control, and large variations among samples. On the other hand, finite element analysis provides a more manageable and flexible approach for the evaluation of dental biomechanics than the experimental approach.[16]

Von Mises stresses in bone

Bone tissue is known to remodel its structure in response to mechanical stress.[17] The clinical success of a mini-implant is largely determined by the manner in which the mechanical stresses are transferred from the mini-implant to the surrounding bone without generating forces of a magnitude that would jeopardize the longevity of the mini-implant. Very low stress levels around a mini-implant may result in poor connection with bone or bone atrophy. On the other hand, abnormally, high stress concentrations in the supporting tissues can result in pressure necrosis and subsequently mini-implant failure.[4]

The results of this study showed that highest stress value of 51.81 MPa in the cortical bone and 4.45MPa in the cancellous bone was seen at bone–mini-implant interface when the en-masse retraction was carried out in the labial technique and a highest stress value of 107.79 MPa in the cortical bone and 3.09 MPa was seen at the bone–mini-implant interface when en-masse retraction was carried out in the lingual technique. The stress value in the cortical bone was higher than that of the cancellous bone in both the techniques. This is similar to the findings of Liu et al.,[16] who stated two reasons for this behavior. First, cortical bone with a higher Young's modulus resists more deformation and sustains higher loads than does cancellous bone. Second, the bending mode, as identified in the mini-implant stress, has more effect at the base support region, as justified by the concentrated high base stress in the entrance region of the cortex than the rest of the embedded region, a straighter and less bent region.

All the stress values in the bone are well below the yield stress of the bone (200 MPa),[4] indicating that the bone has sufficient strength to resist retraction forces by both the methods in clinically acceptable range.

Von Mises stresses in mini-implant

The stresses in mini-implant showed a variable pattern, i.e., stresses associated with the lingual technique were greater (maximum stress was 285.67 MPa at mini-implant head, 158.72 MPa at the mini-implant neck, and 31.77 at the mini-implant tip) than that in labial technique (maximum stress was 272.71 MPa at mini-implant head, 121.55MPa at the miniscrew neck, and 30.86 MPa at the mini-implant tip).

According to Singh et al.,[18] the finite element model neglects the stress produced by the insertion of the screw and considers only the stresses produced by horizontal and torsional loads. Hsieh et al.[19] claimed that stress investigated by finite element analyses could be considered to act on the bony tissue at a given time after screw insertion when the viscoelastic phenomena produced a relaxation of the stress field. Gracco et al.[20] followed the same methodology in their finite element study, supporting the argument that bony tissue behaves as a viscoelastic material, which results in relaxation in the stress fields generated by implant insertion. In spite of these limitations, the finite element predictions in our investigation are in good agreement with the results of Dalstra et al.[21] and Gallas et al.,[22] who reported that, when force is applied perpendicularly to the long axis of the implant, the maximum stresses were located around the neck of the implant at the bone–implant interface.

The increased stress and displacement values obtained at the eye of the mini-implant might be explained by the reduced bulk (quantity) of the material in this region. According to Melsen,[23] if an Allen wrench is used for insertion and removal, the hole at the center of the screw, which weakens the neck, might cause the screw to fracture and suggested the use of a slotted head instead of a hollow one to prevent the screw from breaking at this region.

Similar to the bone stresses, the stress values in the mini-implant were also below the yield stress of titanium (692 MPa),[4] indicating that the mini-implant has sufficient strength to resist retraction forces in both the methods.

According to Mathew et al.,[24] there are many clinical and biomechanical differences between the lingual and the labial orthodontic techniques. These can be attributed to many factors such as difference in the position of brackets, reduced arch perimeter, and variations in the lingual anatomy. These differences between the two techniques do influence the movement of the teeth. The stress exerted by the lingual bracket system is always greater than that generated by the labial appliance, except at the molars. This was due to the smaller inter-bracket distance in the anterior segment, which results in a greater load on the teeth even if an undersized archwire is used.[1]

In this study, the stresses on the mini-implant and the bone were higher in the lingual technique as compared to the labial technique.


  Conclusions Top


The Von Mises stresses in the bone and mini-implant were significantly higher in the lingual technique as compared to the labial technique, and the stresses were concentrated more in the cortical bone than the cancellous bone and were higher at the mini-implant head compared to the rest of the mini-implant body in both the techniques. The variations in stress patterns in the bone and the mini-implant in the labial and lingual technique could be the result of difference in the inter-bracket distance, point of force application, and its location to the center of resistance of the dentition.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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Lombardo L, Scuzzo G, Arreghini A, Gorgun O, Ortan YO, Siciliani G, et al. 3D FEM comparison of lingual and labial orthodontics in en masse retraction. Prog Orthod 2014;15:38.  Back to cited text no. 1
    
2.
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Gracco A, Cirignaco A, Cozzani M, Boccaccio A, Pappalettere C, Vitale G, et al. Numerical/experimental analysis of the stress field around miniscrews for orthodontic anchorage. Eur J Orthod 2009;31:12-20.  Back to cited text no. 20
    
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[PUBMED]  [Full text]  


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
 
 
    Tables

  [Table 1]


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