The Effect of Varied Positioning of Mini-screw,Anterior Retraction Hook,and Resultant Force Vector on Efficient En-Masse Retraction Using Finite Element Method: A Systematic Review

Abstract

Objective:

The objective of this systematic review was to assess the available evidence to evaluate the effectiveness of en-masse retraction design with mini-screw with respect to the retraction hook and mini-implant position and height.

Methods:

The following electronic databases were searched till July 31, 2020: Pro-Quest Dissertation Abstracts and Thesis database Cochrane Central Register of Controlled Trials (CENTRAL), PubMed, Google Scholar, US National Library of Medicine, and National Research Register. En-masse retractions with anterior retraction hooks assisted by mini-implant three-dimensional finite element method (3D FEM) models were included in the study. The selected studies were assessed for the risk of bias using the Cochrane Collaboration risk of bias tool. The “traffic plot” and “weighted plot” risk of bias distribution were designed using the ROBVIS tool. The authors extracted and analyzed the data.

Results:

Twelve studies fulfilled the inclusion criteria. The risks of biases were low for 9 studies and high for 3 studies. Data on mini-implant, retraction hook, and the center of resistance/force vectors were extracted. The outcomes of the included studies were heterogeneous.

Conclusions:

According to the currently available literature review for successful bodily en-masse tooth movement, the force vector should pass through the center of resistance, which can be achieved by the clinical judgment of placing a mini-screw and an anterior retraction hook. The force from an implant placed at a higher level from the anterior retraction hook will cause intrusion; an implant placed at the medium level shows bodily movement; and an implant placed at a lower level shows tipping forces in consolidated arches.

Keywords

Mini-screw anterior retraction hook FEM en-masse retraction

Introduction

Anchorage is a crucial step in planning for orthodontic treatment. It can be provided by both intraoral and extraoral techniques and which can act as primary or reinforced anchorage. Newton’s third law states, for every action, there is an equal and opposite reaction, which is applicable in orthodontics. With recent advances in the specialty of orthodontics, it is established that skeletal anchorage provides absolute stability to the segments by providing resistance toward undesirable reactionary tooth movements. Bone screws, Y-shaped plates, and onplants¹ are used as means of achieving absolute anchorage. Intraoral anchorage using appliances such as transpalatal arch and nance holding arch does not require patient cooperation, whereas extraoral anchorage using appliances such as headgear demands patient’s cooperation for successful treatment outcome. The first successful orthodontic implant was placed by Creekmore and Eklund² in 1983, but it was Kanomi³ in 1997 who described a mini-implant specifically designed for orthodontic use. In anterior en-masse retraction, anchorage planning is critical for successful orthodontic treatment. Conditions such as skeletal open bite⁴ and skeletal class III malocclusion may require absolute anchorage. Planning anchorage, especially in extraction cases, is critical in determining both the treatment outcome and the selection of appropriate mechanisms⁵.

The photoelasticity method, the strain gauge method, laser holography, and the finite element method (FEM) are some of the advanced technologies used to analyze the biological and biomechanical properties of orthodontic tooth movement. FEM is a tool used to gauge stress and strain by simulating a biological entity to a computer-generated model⁶. FEM evaluates the stress distributions and three-dimensional (3D) displacements in different organic/inorganic models with irregular geometry and nonhomogeneous physical properties⁷. In 1943, R. Courant⁸ first formulated mathematical techniques to analyze the biomechanical properties of the object. It was Weinstein⁹ in 1976, who first applied FEM to dentistry to measure the various occlusion loads on implants. Since then, the FEM method has been used in the field of orthodontics to decipher the physiologic responses of dentoalveolar complex to orthodontic forces by exhibiting quantitative data.

Objectives

The aim of this study was therefore to compare and evaluate the effectiveness of en-masse retraction design with mini-screw with respect to the retraction hook and mini-implant position and height.

Material and Methods

Protocol and Registrations

The systematic review was performed in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analysis¹⁰ guidelines and the main research question was formatted according to the patient, problem or population intervention comparison, control or comparator outcome (PICO). The protocol for a systematic review of mini-screw-enabled en-masse retraction was registered on the National Institute of Health Research Database (PROSPERO https://www.crd.york.ac.uk/prospero/registration ID-CRD42020194373).

Eligibility Criteria

The following selection criteria were applied for the review.

Study design: Only original research articles were included.

Participants: Either patient 3D CT scanned model or ideal dental cast 3D model was considered. Sliding mechanics with first premolar extraction was the treatment protocol to be simulated.

Interventions: En-masse retraction with anterior retraction hooks (ARH) and assisted anchorage from mini-screw.

Exclusion criteria: Studies related to retraction performed with frictionless mechanisms and studies without FEM simulation were omitted from the review.

Outcome measures: The primary outcome of this review is to find the optimum height and position of ARH and mini-implant during en-masse retraction.

Information Sources, Search Strategy, and Study Selection

A comprehensive electronic database search was conducted with no limit of date of publication. Search engines such as Cochrane Central Register of Controlled Trials (CENTRAL), PubMed, and Google Scholar were included (Table 1). In addition to this, unpublished research studies were electronically searched using electronic databases such as the US National Library of Medicine (https://clinicaltrials.gov/) and National Research Register (ISRCTN Registry http://www.isrctn.com/), using the terms “En-masse retraction” and “FEM.” The Pro-Quest Dissertation Abstracts and Thesis database were searched (https://about.proquest.com/libraries/academic/dissertations-theses/) using the terms “orthodontic*” and “En-masse retraction*.” The gray literature (http://www.opengrey.eu/) was also searched. The search strategy and terms used for the PubMed search engine are presented in Table 1. An e-mail alert was set for the PubMed search that allowed updates of results during the process of developing the review. Only studies included in English literature were selected for this systematic review. Hand searches were done on the list of references in the selected article.

Table 1.

List of Search Engines and the Result Obtained Using MeSH Terms With Sample of PubMed Search Terms.

S. No	Search Engine	Results
1	PubMed	64
2	Cochrane	32
3	Google Scholar	104
4	US National library of medicine(https://clinicaltrials.gov/)	10
MeSh terms
((((extraction) OR ((space clos) OR (retraction))) AND (((((((((maxilla) OR (mandible)) OR (upper jaw)) OR (lower jaw))) OR (dental)) AND ((((((((((3-dimensional modeling) OR (3 dimensional modelling)) OR (3-dimensional analysis)) OR (3 dimensional analysis)) OR (fea)) OR (fem)) OR (finite element analysis)) OR (finite element method)) OR (three-dimension)) OR (three dimension))) AND (orthodont*)) AND ((stress on the cortical bone) OR (stress on dentition)))) NOT (lingual)) NOT ((surgery) OR (orthognathic surgery))
No.	PubMed Search Term	Results
#1	((extraction) OR (space closure)) OR (retraction)	942,234
#2	((((dental) OR (lower jaw)) OR (upper jaw)) OR (mandible)) OR (maxilla)	644,846
#3	((((((((((3-dimensional modeling) OR (3 dimensional modelling)) OR (3-dimensional analysis)) OR (3 dimensional analysis)) OR (fea)) OR (fem)) OR (finite element analysis)) OR (finite element method)) OR (threedimension)) OR (three dimension)))	356,670
#4	(orthodont*))	75,229
#5	((stress on the cortical bone) OR (stress on dentition))))	15,689
#6	(lingual) OR (((surgery) OR (orthognathic surgery)))	4,847,949
#7	#1 AND #2 AND #3 AND #4 AND #5 NOT #6	64

The study selection was done in 2 phases. In the first phase, the title and abstract were screened independently by 2 reviewers (AA, PS). The short-listed studies are selected for full-text reading. Strict inclusion criteria scrutinizing was done. Any disagreements between the 2 reviewers were sorted by a discussion with third and fourth reviewers.

Data Items and Collection

The data extraction was performed by 2 reviewers (AA, PS) independently to record the general information such as author, year of publication, and study setting, study method, FEM software used, a technique used (En-masse retraction method, friction mechanics), the primary outcome, that is, position of ARH and mini-screw and secondary outcome, that is, force vector considerations, and inference/recommendations.

Risk of Bias/Quality Assessment in Individual Studies

The quality of the selected studies was evaluated by a pair of observers (AA, PS) on ROBVIS¹¹ (Covidence) using the Cochrane Collaboration risk of bias tool, as described in the Cochrane Handbook for Systematic Reviews of Interventions¹². A total of 6 domains of bias were assessed: (a) random sequence generation, (b) allocation concealment, (c) blinding of outcome assessment, (d) incomplete outcome data assessment, (e)selective reporting, and (f) another source of bias. The blinding of participants and operators was not included. The sequence generation was not explained, as the included studies did not report it. The use of sequence generation is questionable for in vitro studies or perhaps impossible, particularly for FEM study. Evaluation of allocation concealment of in vitro studies was not appropriate.

Results

Study Selection and Characteristics

A total of 210 articles were identified after electronic search (PubMed, CENTRAL, Google Scholar, and US National Library of Medicine) and 8 articles were included by hand search. After removing the duplicates, 104 articles were short-listed, out of which 40 articles were included for abstract reading for Phase I screening. Totally, 18 articles selected for Phase II full-text reading (Figure 1). Finally, 12 articles were selected for qualitative review. Six articles were excluded, and the reasons for their exclusion are presented in Table 2. All the 12 articles were original research articles, and basic characteristics and inference were tabulated separately.

Figure 1.

Source: Moher et al.¹⁰

Table 2.

List of the Excluded Studies and Their Respective Reasons for Exclusion.

Article	Reason for Rejection
Tominago et al	Outcome
Kim et al	Different technique
Kojima et al	Outcome
Ozaki et al	Comparison done on different technique
Yan et al	Outcome
Othaman et al	Outcome

Risk of Bias Within Studies

All the included studies were assessed for risk of bias through the Cochrane Collaboration risk of bias tool, as described in the Cochrane Handbook for Systematic Reviews of Interventions (Figures 2 and 3). The overall assessment of bias was done under high, low, unclear, and not applicable as per Cochrane Handbook for Systematic Review. Three articles were having a high risk of bias, whereas 9 articles were having a low risk of bias. Tests for the risk of bias across studies were not undertaken.

Figure 2.

Source: Risk-of-bias VISualization (ROBVIS).

Figure 3.

Source: Risk-of-bias VISualization (ROBVIS).

Synthesis of Results

Meta-analysis was not possible due to sizeable heterogeneity in various methodological, clinical, and statistical variations. Solid heterogeneity is present in the various methods of outcome measurement, the different landmarks used for positioning the mini-implant and ARH, and application of force via elastics versus coil. Hence, only a qualitative report was obtained.

Results of Individual Studies

The summary of the included variables is outlined in Tables 3 and 4.

Position of Mini-screw

The position of mini-screw was described in 10 studies (Table 5), where the height varied from 4 mm to 13.5 mm. In 8 studies, the position of mini-implant placement is determined from the level of the archwire. Zhang et al¹³ considered the height of placement from molar hook whereas. Jain et al²² measured it from the alveolar bone. Six studies have used 150 g of force for the retraction. Namburi et al²⁰ and Bohara et al²³ used intrusion force (60 g) from the anterior region in addition to the retraction force. Chetan et al¹⁷ showed retraction component of force reduces by around 1% and the intrusion component of force increases by around 0.3% for each millimeter apical placement of the screw. Ashekar et al¹⁶ proved that low mini-implant placement (6 mm) causes anterior teeth to tip, mid-implant placement (8 mm) causes bodily movement, and high implant placement (10 mm) shows intrusion in the FEM model. The ideal height for the bilateral implant position for the retraction of the anterior teeth is 10 mm²⁰. But on the contrary, Chetan et al¹⁷ suggested that the position of the implant in the vertical plane will have very little effect on the type of tooth movement. Bohara et al²³ showed force vector passing away from the center of resistance (CR) will show tipping movement independent of implant position.

Table 3.

Basic Characteristics of the Included Studies.

S.NO	Author	Model	Mesh Design	Technique Used	Number of nodes	Number of Element
1.	Zhang et al	3D FEM	Not mentioned	ANSYS 8.1	186322	130616
2.	Sung et al	3D FEM	4-node tetrahedron element	ANSYS (solid45)	Model 1-49039 Model 2-48429 Model 3-49039	Model 1-249796 Model 2-246646 Model 3-249796
3.	Kojima et al	3D FEM	Not mentioned	ANSYS software	Not mentioned	Not mentioned
4.	Ashekar et al	3D FEM	Not mentioned	ANSYS version 13	249796	49039
5.	Chetan S et al	3D FEM	Not mentioned	ANSYS 11	187937	41802
6.	Parashar et al	3D FEM	Not mentioned	MIMICS	Not mentioned	Not mentioned
7.	Hedayati et al	3D FEM		ANSYS Software	371100	Not mentioned
8.	Namburi et al	3D FEM	Not mentioned	ANSYS 12.1 version software	43887	209807
9.	Shreevats et al	3D FEM	Not mentioned	MIMICS and ANSYS	Not mentioned	Not mentioned
10.	Abhishek Jain et al	3D FEM	tetrahedron shape	Not mentioned	294124	66448
11.	Bohara et al	3D FEM	Not mentioned	software-MIMICS (version 8.11) ANSYS (version 12.1)	Model A 98972Model B 313944Model C 335781Model D 313944	Model A 335781Model B 93622Model C 98972Model D 93622
12.	Suzuki et al	3 D FEM	Not mentioned	ANSYS 11.0	222391	136459

Table 4.

Characteristics of the Included Studies.

Methods

Retraction Hook

Mini-screw

Conclusion

S. No.

Name of the Author

Year

Objectives

Model Design

Reference

Position

Reference

Position

Force

Consideration of CR (En-masse Retraction)

Retraction Hook

Mini-screw

Recommendations

Zhang et al

2008

To investigate the stress and displacement of maxillary anterior teeth during retraction by FEM.

Ideal modelNISSINB3-305

Molar hook

Case 1-2 mmCase 24 mm

Apical to molar tube

10 mm

150 gm

Mentioned about CR but not specified the position

Not specified

Retraction force passing near the center of resistance of 6 anterior teeth makes their displacement uniform.

Sung et al

2010

To examine effective en-masse retraction with mini-implant anchorage with varying height and position of mini-implant and retraction hooks by FEM.

Ideal modelNISSINi21D-400G

From arch wire

1. 0 mm2. 2 mm3. 5 mm4. 8 mm

From arch-wire level

1. 10 mm2. 12 mm

200 gm

Yes, and center of resistance was 9 mm superiorly and 13.5 mmposteriorly from the midpoint of the labial splinting wire

Not specified

As the height of the ARH increased, lingual tipping of the central and lateral incisors was reduced.

For bodily retraction of 6 anterior teeth, theline of action of the force passing through the CR is desirable.

Kojima et al.

2012

To evaluate the length and position of the anterior retraction hook and placement of mini-implant by FEM.

Patient CT-scan to 3D model—FEM model

From arch wire

1. 1 mm2. 4 mm4. 8 mm

From arch wire

1. 4 mm2.8 mm

1.5 N

No and the position of CR is not specified

The high-position (8 mm) mini-screw, bodily tooth movement was almost achieved

When the powerarm was lengthened, rotation of the entire dentitiondecreased.

Retraction force passing near the center of resistance of 6 anterior teeth makes their displacement uniform.

Ashekar et al

2013

To evaluate the proper position of anterior retraction hook and mini-implant position for retraction by FEM Study.

Model based on values of wheeler’s dental anatomy.

From arch wire

1. 0 mm2. 5 mm3. 8 mm

From arch wire

1. 6 mm2. 8 mm3. 10 mm

150 gm

Yes, and center of resistance was 9 mm superiorly and 13.5 mmposteriorly from the midpoint of the labial splinting wire

1. (6 mm) mini-implant shows tipping movements.2. Mid-implant (8 mm) shows bodily movement3. High implant condition (10 mm) shows intrusion and retraction.

Varies according to the position of mini-implant. 1. for 6 mm implant, 5 mm ARH is preferable2. 8-mm implant—8-mm ARH showed bodily movement 3. 10-mm implant position—0-mm intrusion +retraction seen

To produce more amount of retraction, the force vector must pass through or near the center of resistance of all anterior teeth.

Chetan et al

2014

To evaluate the proper position of mini-implant for en-masse retraction by FEM study.

Model based on values of wheeler’s dental anatomy.

From arch wire

2 mm

From arch wire

1. 13.5 mm2. 9 mm3. 4.5 mm4. molar hook

150 gm

Yes, and center of resistance was 9 mm superiorly and 13.5 mmposteriorly from the midpoint of the labial splinting wire

For every mm of apical displacement of implant, the retraction component of force reduces approximately by around 1% and intrusion component of force increases approximately by around 0.3%.

Not specified

The position of implant in vertical plane will have very little effect on the type of tooth movement.

Abhishek Parashar et al

2014

To evaluate anterior en-masse retraction with force vectors from 2 different levels of mini-implants by FEM.

CT-scan to 3D model—FEM model

From arch wire

1. 5 mm2. 8 mm

From arch wire

1. 13.5 mm2. 8 mm3. molar hook

200 gm

Yes, and center of resistance was 9 mm superiorly and 13.5 mmposteriorly from the midpoint of the labial splinting wire

Bodily movement with very minimal torque loss was observed in high implant position to low retraction hook.

Not specified

The force vector passing close to the center of resistance produced bodily movement.

Hedayati et al

2016

To evaluate the proper antero-posterior position of mini-implant and anterior retraction hook size for retraction by FEM Study.

Model based on values of wheeler’s dental anatomy.

From arch wire

1. 0 mm2. 3 mm3. 6 mm4. 9 mm

From arch wire

1. mesial2. distal of secondPremolar

150 gm

Yes, and center of resistance was 9 mm superiorly and 13.5 mmposteriorly from the midpoint of the labial splinting wire

the mini-screw was placed between the second premolar and first molar; the rotationof the anterior teeth segment increased while themovement in the vertical plane decreased.

Bodily movements occurred with the 9-mm height of the power arm in both mini-screw positions.

Rotation of anterior dentition was decreased with a longer anterior power arm and the mesial placementof the mini-screw.

Namburi et al

2017

To compare the 2 mini-screw vs 3 mini-screw with different heights of mini-implant for the en-masse retraction by FEM study.

Not mentioned

From arch wire

1)9 mm

From arch wire

Group 11. 7 mm2. 10 mm3. 13 mmGroup 21. 7 mm2. 10 mm3. 13 mm4. 12 mm anterior implant.

120 gm+60 gm

Yes,center of resistance for the present study model might be located at 10 mm

1. Low mini-implant—tipping movements.2. Mid-implant—bodily movement3. High implant condition shows intrusion and retraction.The ideal height for bilateral implant position for retraction of the anterior teeth is 10 mm.

Not specified.

In 3-implant system, more intrusion and less stress are seen in PDL than 2-implant system.

Ruchira Shreevats et al

2017

To evaluate the effective en-masse design with varying heights of mini-screw and alveolar bone by FEM method.

CT-scan to 3D model—FEM model

From arch wire

1. 1 mm2. 2 mm…10. 10 mm

From alveolar crest

8 mm

150 gm

Yes, and it varies according to the alveolar bone height. 100%—8 mm apically75% bone height—10 mm apically50% bone height—14 mm apically

Not specified

In cases where the bone loss is 50% or more, bodily movement is not feasible.

10.

Abhishek Jain et al

2017

Comparison of varying height of mini-implant with force application from NiTi coil vs E-chain for en-masse retraction by FEM method.

Not mention d

Not mentioned

From alveolar crest

1. 3 mm2. 5 mm

150 gm

Mentioned about CR but not specified the position

Increasing the height of mini-screw placement encouraged the translatory movement of anterior teeth

Not specified

The overall displacement of anterior teeth in both vertical and sagittal directions was found to be more with elastomeric chain as compared to NiTi coil spring.

11.

Bohara et al

2017

To understand the nature of stresses and displacement patterns of anterior teeth during en-masse intrusion and retraction on force application with different combinations of mini-implants and retraction hooks height by FEM method.

Not mentioned

From arch wire

1. 6 mm gingivally2. 2 mm incisally

From arch wire

1. 6 mmOr2. 6 mm +In the incisor region

150 gm

Mentioned about CR but not specified the position

Not specified

The retraction hook height could be the most easily modifiable clinical factor in determining and achieving the most desirable direction of anterior teethdisplacement during intrusion and retraction of anterior teeth. The teeth showed almost bodily movement and controlled lingual crown tipping when a 6-mm gingival retraction hook was used.

The force passing near the center of resistance exhibits bodily movement.

12.

Suzuki et al

2019

To evaluate the height of the retraction hook and implant anchorage effect on the tooth movement during en-masse retraction of anterior teeth by FEM method.

Dried adult skull CT-scan to 3D model—FEM model

From arch wire

1. 0 mm2. 5 mm3. 10 mm

From arch wire

1. 0 mm2. 5 mm3. 10 mm

150 g

Yes, and it is located 5 mm above the alveolar crest

Not specified

1. The traction at 0 mm height hook, all teeth move distally2. With traction of hook at 0 mm, with higher the implant position extrusion of central decreased.

Force should pass through or near the center of resistance to avoid tipping of the teeth.*

Note: * All the included articles studied via FEM and sliding mechanics.

Table 5.

Results of Individual Studies on Vertical Position of Mini-screw and Height of Anterior Retraction Hook.

Study	Height of the Mini-implant	Height Measured Level	Force
Sung et al	10 mm (low)12 mm (high)	Arch-wirelevel	200 g
Kojima et al	4 mm 8 mm	Arch-wirelevel	1.5 N (150 gf)
Ashekar et al	6 mm (low)8 mm (medium)10 mm (high)	Arch-wirelevel	150 g
Chetan et al	0 mm (molar hook)4.5 mm (low)9 mm (medium)13.5 mm (high)	Arch-wirelevel	150 g
Abhishek Parashar et al	8 mm (low)13.5 mm (high)	Arch-wirelevel	200 g
Namburi et al	7 mm 10 mm 13 mm	Arch-wirelevel	1. 120 g and 2. 60 g
Abhishek Jain et al	3 mm 5 mm	Alveolar bone	150 g
Bohara et al	6 mm	Arch-wirelevel	1. 150 g2. 60 g
Suzuki et al	0 mm 5 mm 10 mm	Arch-wirelevel	4.9 N (500 gf)
Zhang et al	10 mm	Molar hook	150 g
Study	Height of anterior retraction hook
Zhang et al	2 mm, 4 mm
Sung et al	0 mm, 2 mm, 5 mm, and8 mm
Kojima et al	1 mm, 4 mm, and 8 mm
Ashekar et al	0 mm, 5 mm, and 8 mm
Abhishek Parashar et al	5 mm, 8 mm
Hedayati et al	0 mm, 3 mm, 6 mm, and 8 mm
Ruchira Shreevats et al	1 mm, 2 mm, …, 10 mm
Bohara et al	6 mm (gingivally)2 mm (incisally)
Suzuki et al	0 mm, 5 mm, and 10 mm

Position of Anterior Retraction Hooks

A total of 9 articles have considered the varying height of the ARH for en-masse retraction (Table 5). Hedayati et al¹⁹ suggested that placing an ARH size of 9 mm exhibits bodily movement independent of mini-screw position, mesial or distal to premolar. The rotation of anterior dentition was also decreased with a longer anterior power arm. On the contrary, Zhang et al¹³ found that when an ARH of 2 mm is used, the lateral incisor showed controlled lingual crown tipping; on the other hand, when an ARH of 4 mm is used, the central and canine teeth showed lingual crown tipping and laterals shows bodily retraction and intrusion.

Tooth Movement

Seven studies had discussed tipping, torque, and translation of incisor while retraction. Parashar et al¹⁸ suggested positioning the mini-implant in a higher position (13.5 mm from the archwire plane) while retraction prevents torque loss and also positioning the mini-screws in a lower position (8 mm from the archwire plane) leads to maximum torque loss. Namburi et al²⁰ suggested placing mini-screw 10 mm from arch wire as a reference point. Chetan et al¹⁷ suggested the more apical the placement of the mini-screw, the more is the intrusion achieved. Sung et al quoted increasing the length of ARH (<8 mm) reduces unwanted tipping of the central and lateral incisors. Shreevats²¹ et al suggested that bodily movement during en-masse retraction is feasible only when 50% alveolar bone height is present in the anterior. In addition to the bilateral implants, an inter-radicular implant in the incisor region reduces the stresses in periodontal ligament (PDL) and achieves greater intrusion in comparison to the 2-implant system. Retraction with elastomeric chain produces lesser amount of von Mises stress on the bone as well as mini-implant and produces more displacement of anterior teeth in both vertical and sagittal directions as compared to that with nickel titanium wire (NiTi) coil spring.

Discussion

At the time of registration with PROSPERO, this systematic review was the first to specifically investigate the position of mini-screw, the height of ARH, and the CR of 6 anterior teeth for en-masse retraction. However, before writing this systematic review, several in-vitro FEM studies were published which have addressed various aspects of en-masse retraction as stated. Studies such as Zhang et al,¹³ Sung et al,¹⁴ Chetan et al,¹⁷ Parashar et al,¹⁸ Namburi et al,²⁰ Jain et al,²² Shreevats et al,²¹ and Suzuki et al²⁴ have not concluded either optimum position of mini-screw, size of ARH, reference plane to be considered, or the exact position of CR in their respective published articles. To elucidate the above aspects, it was imperative that a systematic review be conducted for discernment. A total of 218 articles were selected using medical subject headings (MeSH) terms, and few articles were handpicked. After regress scrutinization using inclusion and exclusion criteria, 12 articles were finally considered for a systematic review.

Summary of Evidence

The methodological heterogeneity was noted in all these studies. Parashar et al, Sreevats et al, and Bohara et al used MIMICS software for simulation. Zhang et al,¹³ Sung et al,¹⁴ Kojima et al,¹⁵ Ashekar et al,¹⁶ Chetan et al,¹⁷ Hedayati et al,¹⁹ Namburi et al,²⁰ Bohara et al,²³ and Suzuki et al²⁴ used ANSYS software for simulation, whereas Jain et al have not mentioned the software used for the study. Kojima et al,¹⁵ Suzuki et al,²⁴ and Parashar et al¹⁸ designed an FEM model based on the CT scan and converted it into a 3D model, while other studies such as Ashekar et al,¹⁶ Chetan et al,¹⁷ and Hedayati et al¹⁹ designed a model based on Wheeler’s dental anatomy textbook. Jain et al²² and Bohara et al²³ did not quote the model design.

Namburi et al²⁰ compared the stresses on the 2-implant systems with 3-implant systems for retraction. He stated that the stresses on the hard bone, PDL, and implant showed less stress in 3-implant systems. Zhang et al¹³found more displacement and stress in the cervix and root apex in teeth and also that lateral incisors experience more compressive stress on the labial and lingual surfaces with mini-screw assisted retraction. Jain et al²² used NiTi coil spring and elastomeric chain on mini-screws for retraction force. They inferred a higher level of Von Mises stresses in bone while using the NiTi coil spring and mini-screw. Sung et al¹⁴ found higher periodontal tensile stress (0.67 g mm⁻²) on the bucco-cervical third of canine when the force vector passed through CR during en-masse retraction. Bohara et al²³ stated that the nature of stresses changes from tensile to compressive from the cervical area to the apical area while retraction.

The method of retraction and force used for retraction varied in the studies. The force of 150 gm was used in studies conducted by Zhang et al,¹³ Ashekar et al,¹⁶ Chetan et al,¹⁷ Hedayati et al,¹⁹ Shreevats et al,²¹ Jain et al,²² Bohara et al,²³ and Suzuki et al.²⁴ Sung et al¹⁴ and Parashar et al¹⁸ used 200 gm of force and Namburi et al²⁰ used 120 g for retraction and 60 gm for the intrusion. Jain et al²² showed the method of en-masse retraction using NiTi spring in comparison with the power chain. Other included studies did not mention the method of retraction.

The positioning of the ARH differed inciso-gingivally and mesiodistally in included studies. Bohara et al²³ compared the position of ARH incisally (inverted) and gingivally with respect to the arch wire. The positioning of ARH in between lateral incisors and canine was common, and this was specifically mentioned in the 12 studies. Zhang et al¹³ considered a molar hook for deciding the dimensions of ARH, whereas an arch wire was reported as the reference plane in studies by Sung et al,¹⁴ Kojima et al,¹⁵ Ashekar et al,¹⁶ Chetan et al,¹⁷ Parashar et al,¹⁸ Hedayati et al,¹⁹ Namburi et al,²⁰ Shreevats et al,²¹ and Suzuki et al.²⁴ Biomechanical effects of different positions of mini-screw were compared with the fixed dimension of ARH by Chetan et al¹⁷ (2-mm ARH), Namburi et al²⁰ (9 mm), and Bohara et al²³ (6-mm ARH for gingival model and 2-mm ARH for incisal model), respectively, whereas other studies such as those by Zhang et al,¹³ Sung et al,¹⁴ Kojima et al,¹⁵ Ashekar et al,¹⁶ Parashar et al,¹⁸ Hedayati et al,¹⁹ and Suzuki et al²⁴ evaluated different dimensions of ARH with fixed position of mini-screw. Sung et al¹⁴ and Hedayati et al¹⁹ varied the ARH for four different heights. Kojima et al,¹⁵ Ashekar et al,¹⁶ and Suzuki et al,²⁴ varied the height of ARH thrice in their studies. Zhang et al¹³and Parashar et al¹⁸ changed the height of the ARH twice in their studies. Shreevats et al²¹ studied the effects by varying the height of ARH in increments of millimeter. Park et al²⁵ considered short ARH to increase the vertical component and decrease the horizontal component and vice versa.

In terms of mini-implant, the number of implants, height, and position were discussed. Namburi et al²⁰ and Bohara et al²³ compared 2-mini-implant models versus 3-mini-implant models for an anterior retraction and showed that the 3-implant system has better biomechanical advantages for better intrusion and retraction and minimum stress on the periodontium. Hedayati et al¹⁹ studied the mesiodistal positioning of the mini-screws by comparing the bio-mechanical effects by placing one implant placed at the mesial and other at the distal of the second premolar. Shreevats et al²¹ and Jain et al²² considered the alveolar crest as a reference point for the mini-screw placement. Zhang et al¹³ considered apical to Molar tubes. Park et al²⁵ took the bracket slot as a reference plane for placement of mini-screw. Studies such as Sung et al,¹⁴ Kojima et al,¹⁵ Ashekar et al,¹⁶ Chetan et al,¹⁷ Parashar et al,¹⁸ Hedayati et al,¹⁹ Namburi et al,²⁰ Bohara et al,²³ and Suzuki et al²⁴ had taken arch-wire level as reference. Zhang et al¹³and Shreevats et al²¹ placed the mini-implant at a fixed height from the respective reference planes and varied the ARH height. Ashekar et al,¹⁶ Chetan et al,¹⁷ Namburi et al,²⁰ and Suzuki et al²⁴ used 3 different implant positions for comparison. Sung et al¹⁴ (10 mm, 12 mm), Kojima et al¹⁵ (4 mm, 8 mm), Parashar et al¹⁸ (13.5 mm, 8 mm), and Jain et al²² (3 mm, 5 mm) used 2 different implant positions.

The CR is critical for assessing the resultant direction of force for en-masse retraction when applied from mini-screw to ARH. The line of force vector passing through CR will bring about the bodily retraction. Melsen et al²⁶ considered CR at a point 9 mm superiorly and 13.5 mm posteriorly from the center of the arch wire. The same point is considered in studies by Sung et al,¹⁴ Ashekar et al,¹⁶ Chetan et al,¹⁷ Parashar et al,¹⁸ and Hedayati et al.¹⁹ Namburi et al²⁰ estimated the location to be at 10 mm from the alveolar bone in between lateral and canine. Suzuki et al²⁴ considered CR to be at 5 mm above the alveolar crest apically above the central incisor. Zhang et al,¹³ Kojima et al,¹⁵ Bohara et al,²³ and Jain et al²² have mentioned the importance of CR but not specified its location. Shreevats et al²¹ discussed CR in relation to periodontally compromised patients. He stated that as the alveolar bone loss increased, the CR moved apically.

Upadhayay et al²⁷ suggested retraction force to be angulated when applied from mini-screw whereas in the conventional techniques force vector remains parallel to the occlusal plane. Antoszewska-Smith et al²⁸ stated retraction with mini-screw is more efficient with minimum undesirable effects. Koyama et al²⁹ and Rizk et al³⁰ laid stress on en-masse retraction using mini-screw, as it reduces the treatment time period and augments anchorage. Kim et al³¹ corroborate the above results by retracting 6 anterior teeth using mini-screws without bonding posterior segments.

Limitations

The FEM model is an emerging diagnostic and investigating method in the engineering and biomedical fields, but the simulation and original result may vary due to clinical and biological factors such as bone level, bone density, age/gender of the patient, and periodontal integrity. The studies included in this systematic review were in English, although few studies in different languages were published but not included, suggesting incomplete data collection and probably publication bias. The 12 articles discussed in the systematic review used FEM as an investigating tool however, the results of these studies were not corroborated with clinical studies under the same orientation of mini-screw and ARH.

Conclusions

According to the currently available literature review for successful bodily en-masse tooth movement, the force vector should pass through CR, which can be achieved by the clinical judgment of placing a mini-screw in the right position and assessing the height of ARH.

The retraction force should pass through the CR, to achieve the bodily movement and controlled lingual crown tipping.

The force from an implant placed at a higher level from the ARH will cause intrusion; an implant placed at a medium level shows bodily movement, and an implant placed at a lower level shows tipping forces in consolidated arches.

The use of an extra implant in the incisor region improves intrusion and reduces stress in the periodontal ligament.

Footnotes

Declaration of Conflicting Interests

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

P Subash

References

Costa

Raffainl

Melsen

Miniscrews as orthodontic anchorage: A preliminary report. Int J Adult Orthodon Orthognath Surg . 1998;13(3):201-209.

Creekmore

Eklund

MK.

The possibility of skeletal anchorage. J Clin Orthod . 1983; 17:266-269.

Kanomi

Mini-implant for orthodontic anchorage. J Clin Orthod . 1997; 31:763-767.

H-S

Park

Kwon

OW.

The treatment of open bite with micro screw implants anchorage. Am J Orthod Dentofacial Orthop . 2004; 126:627-636.

H-S

Park

Bae

Kyung

Sung

Micro-implant anchorage for treatment of skeletal Class I bialveolar protrusion. J Clin Orthod . 2001; 35:417-422.

Knop

Gandini

Jr. Shintcovsk

Gandini

MREAS.

Scientific use of the finite element method in Orthodontics. Dental Press J Orthod . 2015 Apr;20(2): 119-125. doi:10.1590/2176-9451.20.2.119-125.sa.

Kim

Suh

Kim

Lee

Optimum conditions for parallel translation of maxillary anterior teeth under retraction force determined with the finite element method. Am J Orthod Dentofacial Orthop . 2010;137(5):639-647. doi:10.1016/j.ajodo.2008.05.01.

Courant

Variational methods for the solution of problems of equilibrium and vibrations. Bull Am Math Soc . 1943; 49:1-23.

Weinstein

Klawitter

Anand

Schuessler

Stress analysis of porous rooted dental implants. J Dent Res . 1976;55(5):772-777. doi:10.1177/0022034576055005100.

10.

Moher

Liberati

Tetzlaff

AltmanDG

PRISMAGroup.

Preferred reporting items for systematic reviews and meta-analyses: The PRISMA statement. J Clin Epidemiol . 2009; 62:1006-1012.

11.

McGuinness

Higgins

JPT.

Risk-of-bias VISualization (robvis): An R package and Shiny web app for visualizing risk-of-bias assessments. Res Syn Meth . 2020;1-7. doi:10.1002/jrsm.141.

12.

Higgins

JPT

Green

Cochrane Handbook for Systematic Reviews of Interventions . Chichester: John Wiley & Sons; 2011.

13.

Zhang

Zhong

PP.

3D finite element study of en masse retraction of maxillary anterior teeth in two typical force directions. Chin J Dental Res . 2008; 11:101-107.

14.

Sung

Jang

Chun

Moon

YS.

Effective en-masse retraction design with orthodontic mini-implant anchorage: A finite element analysis. Am J Orthod Dentofacial Orthop . 2010;137(5):648-657. doi:10.1016/j.ajodo.2008.06.036.

15.

Kojima

Kawamura

Fukui

Finite element analysis of the effect of force directions on tooth movement in extraction space closure with miniscrew sliding mechanics. Am J Orthod Dentofacial Orthop . 2012;142(4):501-508. doi:10.1016/j.ajodo.2012.05.01.

16.

Ashekar

Deshpande

Shetty

Lele

Patil

SS.

Evaluation of optimal implant positions and height of retraction hook for intrusive and bodily movement of anterior teeth in sliding mechanics: A FEM study. J Indian Orthod Soc . 2013;47(4):479-482.

17.

Shankar

Keluskar

Vasisht

Revankar

En-masse retraction of the maxillary anterior teeth by applying force from four different levels—A finite element study. J Clin Diagn Res . 2014;8:ZC26-ZC30. doi:10.7860/JCDR/2014/8408.483.

18.

Parashar

Aileni

Rachala

Torque loss in En-Masse retraction of maxillary anterior teeth using Miniimplants with force vectors at different levels: 3D FEM study. J Clin Diagn Res . 2014 Dec;8(12):ZC77-ZC80.doi:10.7860/jcdr/2014/10099.5353

19.

Hedayati

Shomali

Maxillary anterior en masse retraction using different antero-posterior position of mini screw: A 3D finite element study. Prog Orthod . 2016;17(1):31. doi:10.1186/s40510-016-0143-.

20.

Namburi

Nagothu

Kumar

Evaluating the effects of consolidation on intrusion and retraction using temporary anchorage devices-A FEM study. Prog Orthod . 2017 Dec;18(1):2.doi: 10.1186/s40510-016-0155-8

21.

Shreevats

Prashanth

Amarnath

Shetty.

Akshai

An fem study on anterior tooth movement, in sliding mechanics with varying bracket slot and archwire dimension with and without alveolar bone loss. Int J Appl Dent Sci . 2018; 4:197-202.

22.

Jain

Hattarki

Priyanka

Vandana

Janhavi

Comparison of stress distribution in bone and miniscrew and displacement pattern of maxillary anterior teeth by two methods of en-masse retraction: A 3-D finite element analysis. Indian J Orthod Dentofacial Res . 2017;3(1):23-30.

23.

Bohara

Kumar

Sharma

Jayprakash

Misra

Savana

Stress distribution and displacement of maxillary anterior teeth during En-masse intrusion and retraction: A FEM study. J Indian Orthod Soc . 2017;51:152. doi:10.4103/jios.jios_199_1.

24.

Suzuki

Sueishi

Katada

Togo

Finite element analysis of stress in maxillary dentition during En-masse retraction with implant anchorage. Bull Tokyo Dent Coll . 2019;60(1):39-52. doi:10.2209/tdcpublication.2017-005.

25.

H-S

Park

Kwon

TG.

Sliding mechanics with microscrew implant anchorage. Angle Orthod . 2004;74(5):703-710. doi:10.1043/0003-3219(2004)074<0703:SMWMIA>2.0.CO;.

26.

Melsen

Fotis

Burstone

CJ.

Vertical force considerations in differential space closure. J Clin Orthod . 1990;24(11):678-683

27.

Upadhyay

Yadav

Nanda

Biomechanics of incisor retraction with mini-implant anchorage. J Orthod . 2014;41 Suppl 1:S15-S23. doi:10.1179/1465313314Y.000000011.

28.

Antoszewska-Smith

Sarul

Łyczek

Konopka

Kawala

Effectiveness of orthodontic miniscrew implants in anchorage reinforcement during en-masse retraction: A systematic review and meta-analysis. Am J Orthod Dentofacial Orthop . 2017;151(3):440-455. doi:10.1016/j.ajodo.2016.08.02.

29.

Koyama

Iino

Abe

Takano-Yamamoto

Miyawaki

Differences between sliding mechanics with implant anchorage and straight-pull headgear and intermaxillary elastics in adults with bimaxillary protrusion. Eur J Orthod . 2011;33(2):126-131. doi:10.1093/ejo/cjq04.

30.

Rizk

Mohammed

Ismael

Bearn

DR.

Effectiveness of en masse versus two-step retraction: A systematic review and meta-analysis. Prog Orthod . 2018;18(1):41. doi:10.1186/s40510-017-0196-.

31.

Kim

Hwang

Ferreira

Chung

KR.

Analysis of temporary skeletal anchorage devices used for en-masse retraction: A preliminary study. Am J Orthod Dentofacial Orthop . 2009;136(2):268-276. doi:10.1016/j.ajodo.2007.08.02.