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Table of Contents   
ORIGINAL RESEARCH  
Year : 2013  |  Volume : 24  |  Issue : 6  |  Page : 701-707
An in vitro comparison of resistance to second and third order archwire activations of three different varieties of esthetic brackets


1 Department of Periodontics, Sri Hasanamba Dental College, Hassan, Karnataka, India
2 Department of Orthodontics, Sri Hasanamba Dental College, Hassan, Karnataka, India
3 Department of Pedodontics, Vyas Dental College, Jodhpur, Rajasthan, India
4 Department of Orthodontics, Malbar Dental College and Research Centre, Manoor, Kerala, India
5 Department of Orthodontics, Vyas Dental College, Jodhpur, Rajasthan, India
6 Department of Prosthodontics, MIDSR Dental College, Latur, Maharashtra, India

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Date of Submission07-Dec-2012
Date of Decision07-Aug-2013
Date of Acceptance22-Aug-2013
Date of Web Publication20-Feb-2014
 

   Abstract 

Background: When ceramic brackets were introduced as an esthetic alternative to the stainless steel brackets, it was a step ahead in the use of esthetic appliances for orthodontic treatment. Although ceramic brackets had overcome the drawbacks of the initial polycarbonate brackets such as staining and bracket slot distortion, they posed an altogether different problem. This was on account of the physical properties of the ceramic material, namely brittleness.
Purpose of the Study: The purpose of this study is to investigate the physical properties of three different varieties of esthetic brackets, i.e., "MXI" ceramic bracket, "Spirit MB" bracket (Ormco), a composite bracket with metal slot reinforcement and the third one fiber bracket, with silicious copolymer, Natura (Leone Co.).
Aims and Objective of the Study : The objective of this study is to compare the torsional resistance of the brackets and resistance of brackets to second order archwire activations.
Conclusion: The strength of the selected brackets was more than sufficient to withstand orthodontic load without any fracture or deformation.

Keywords: Arch wire, composite bracket, MXI′ ceramic bracket, spirit MB′ bracket, torsional resistance

How to cite this article:
Ajith S, Gowda AR, Babaji P, Shivaprakash S, Dmello K, Kamble SS. An in vitro comparison of resistance to second and third order archwire activations of three different varieties of esthetic brackets. Indian J Dent Res 2013;24:701-7

How to cite this URL:
Ajith S, Gowda AR, Babaji P, Shivaprakash S, Dmello K, Kamble SS. An in vitro comparison of resistance to second and third order archwire activations of three different varieties of esthetic brackets. Indian J Dent Res [serial online] 2013 [cited 2019 Oct 21];24:701-7. Available from: http://www.ijdr.in/text.asp?2013/24/6/701/127615
Patients' desire for improved esthetic dental restorations has caused general dentistry to expand its horizons into the field of cosmetic dentistry, orthodontics being no exception. Today, many adult patients are demanding high quality orthodontic treatment that is esthetically minimally obtrusive. The introduction of direct bonding technique initiated the movement from an obvious to a more esthetic appliance.

During the late 1960s, plastic brackets were introduced as an esthetic alternative to metal brackets. Although these brackets received initial acceptance, they were soon abandoned because of slot dimension distortion [1] and staining.

In the late 1980s, ceramic brackets were introduced. Unlike plastic brackets, the ceramic brackets resist staining as also slot distortions and are chemically inert to fluids likely to be ingested. However, there are a few disadvantages associated with ceramic brackets, which are related to their strength and integrity. [2] There have been several attempts in the recent years to improve the physical properties of both the ceramic and the plastic brackets, so as to increase the efficiency of these brackets in clinical use.

Plastic brackets have been reinforced by the insertion of precision stamped stainless steel slots, ceramic powder or both. Similar improvements have been attempted in the ceramic brackets in terms of providing metal lining to the slots, polycarbonate base and employing injection molding procedures during manufacture to reduce surface irregularities. [3]

Previous studies have demonstrated that among polycarbonate brackets, only metal reinforced polycarbonate brackets provide torque control, which is comparable to that of metal brackets. [4] Furthermore injection molded ceramic brackets should possess better fracture toughness and torsional resistance on account of a smoother surface topography. However, there is no data to substantiate these assertions. Newly introduced fiber brackets are also due to be tested for their physical properties.


   Aims and Objectives Top


  • To determine the resistance of the brackets to third order archwire activation
  • To determine the resistance of the brackets to second order archwire activation.



   Materials and Methods Top


Armrntarium and materials

  • Universal Testing Machine: Instron (Model No. 4505)
  • Loctite 495 industrial adhesive
  • Bonding agent, composite material, light curing gun
  • brackets.


The brackets included in this study are [Figure 1].
Figure 1: Three brackets used for the study (L to R) i.e., MXI ceramic bracket, spirit MB and natura fiber bracket

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  • MXI ceramic brackets (Polycrystalline ceramic brackets having a polycarbonate base [T.P. Orthodontics, Inc.])
  • Spirit MB (Composite brackets with metal slot reinforcement [Ormco Corporation])
  • Natura (Fiber bracket [Leone Co]).


All brackets had slot dimensions of 0.022″ × 0.028″.

The experiments were carried out on a Universal Testing Machine: Instron (Model No. 4505) for each of the following tests.

  • Torsional resistance
  • Resistance to tipping activation.


The apparatus designed for this purpose was similar to the one designed by Rhodes et al. (1992) [7] for their study on the torsional resistance of ceramic brackets. The apparatus consists of following parts, i.e., [Figure 2]:
Figure 2: Schematic representation of the apparatus used to check torsional resistance of brackets

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  • A - Base
  • B - Support posts
  • C - Crossbar 1
  • D - Crossbar 2
  • E - Specimen holder
  • F - Pin
  • G - Load arm
  • H - Attachment to the load cell of Instron Machine
  • I - Hemi sectioned part of the crosshead
  • J - Acrylic mount.


The apparatus was designed to maintain the orientation of the bracket to the archwire in all the three planes of space during the testing procedure. Photograph in [Figure 3] shows the apparatus along with the materials used to determine torsional resistance.

Acrylic mounts in 2.5 mm thickness were prepared in self-cured acrylic resin. The brackets being tested were fixed on the acrylic mounts using Loctite 495 adhesive, before subjecting them to the torsional test [Figure 4]. Ten maxillary right central incisor brackets from each company were tested for this property.
Figure 3: Apparatus along with the materials used to determine torsional resistance

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Figure 4: A ceramic bracket being tested. The brackets were fixed on the acrylic mounts using Loctite 495 adhesive, before subjecting them to the torsional test

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Specimen preparation for ceramic brackets

Ceramic brackets were ligated on to the 0.021″ × 0.025″ stainless steel wire, which was held between the two crossbars using elastomeric rings (Ormco) such that the center of the bracket was at a distance of 6 mm from the point where the archwire gripped in the crossbar emerged out of it [Figure 4]. This distance was chosen because it was considered to be an average inter-bracket distance in a clinical situation. After ligating the bracket to wire, the bracket base developed a mild angulation to the mount below as a result of the in-built tip and torque in the bracket. The bracket base was then attached to the mount below maintaining the same angulation using a fluid industrial adhesive (Loctite 495). By this procedure, the tip and torque built in the bracket was negated. The archwire rested passively in the bracket slot, whose base was attached to the acrylic mount.

Testing procedure

At the time of testing, the apparatus was placed on the Instron crosshead so that the hole in the load arm was in line with the load cell attachment. A thick silk thread was tied between the hole in the load arm and the load cell above. The Instron crosshead was moved at a rate of 1 inch/min for each sample until the bracket fractured.

The force was recorded graphically on the X-Y Recorder. The highest point on the graph was taken as the point of failure of the ceramic bracket. The force obtained was multiplied by the distance from the center of the hole on the load arm of the apparatus to the center of the crosshead (61 mm), to obtain the torque in gram millimeters.

Specimen preparation for composite and fibre brackets

A 90° bend was given in the vertical direction in the 0.021″ × 0.025″ stainless steel wire, which was attached to the crossbar 1. The bracket to be tested was ligated to the horizontal section of the wire arising from the crossbar such that the center of the bracket was at a distance of 6 mm from the end of the crossbar as done earlier for the ceramic bracket [Figure 5]. A protractor was placed just adjacent to the vertical section of the wire to record the deformation in degrees.
Figure 5: The lateral view of the apparatus with the modification to test composite and fiber brackets with a 90° bend in the wire and a protractor

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In this set up, the movement of the vertical section of the wire indicated the amount of deformation of the bracket in degrees. Here again, the Instron crosshead was moved at a rate of 1 inch/min. The force from the Instron machine was released at regular intervals to record the permanent deformation of the brackets on the protractor. The force required to permanently deform the brackets to the extent of 4°, 8°, 12° and 16° was recorded. This force in grams was multiplied by the length of the load arm (61 mm) as done previously to obtain torque in gram-millimeters.

Description of the apparatus used to determine resistance of brackets to second order activation

This part of the study was designed to determine the second order moments at which ceramic bracket failure and distortion of composite and fiber brackets occurred [Figure 6]. The design of the apparatus was similar to the one used by Lindauer et al. [6] to determine the second order archwire activations. Ten maxillary right central incisor brackets from each company were tested.
Figure 6: The apparatus along with the materials used to determine resistance of the bracket to second order archwire activation

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The brackets were first ligated at the center of the 0.021″ × 0.025″ stainless steel wire using elastomeric rings (Ormco) such that the base of the bracket faced the acrylic cylinder. The position of the acrylic cylinder was adjusted, to closely approximate the base of the bracket. Then a high strength fluid industrial adhesive (Loctite 495) was applied to attach the bracket base to the acrylic cylinder. This procedure resulted in the 0.021″ × 0.025″ wire lying passively in the bracket slot before subjecting the brackets to the test.

An Instron Universal Testing Machine apssplied measured force to the load arm projecting from the rotating cylinder [Figure 7] and [Figure 8]. The distance from the position of force application to the center of rotation of the cylinder (the length of the load arm) was fixed and known to be 39 mm. When force was applied to the arm of the cylinder, the archwire tipped mesiodistally in the bracket slot. The cross head speed was maintained at one inch/min. The force magnitude at which breakage occurred for the ceramic brackets was recorded. Similarly, force values required to permanently deform the composite and the fiber brackets to extent of 4°, 8°, 12° and 16° were recorded. The angular deflections were measured on a protractor from a pin placed vertically on the rotating cylinder. The resultant force values in grams were multiplied by 39 mm to obtain the moments in gram-millimeters.
Figure 7: The same apparatus in the Instron Universal Testing Machine

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Figure 8: Close up of a bracket undergoing testing for second order archwire activations

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   Results Top


Interpretation of results

The values obtained from the tests were statistically evaluated. The mean and standard deviation of each of the tests were obtained. Each of the bracket varieties were then evaluated for every property using the analysis of variance (ANOVA) test. As the ANOVA test indicated a statistically significant difference when the three bracket varieties were evaluated, a Student t-test was performed to compare the brackets with each other.

Torsional resistance

Among the brackets compared, the fiber brackets started to deform earliest, with the mean minimum moment required to deform then permanently by 4° being 3,860 g/mm. The average moments, at which the fiber brackets deformed by 8° and the ceramic bracket fractured, were comparable, with no statistical difference between the two groups. All other groups showed a highly significant difference statistically (P < 0.01), when they were compared with each other.

The metal slot reinforced composite brackets showed higher resistance to deformation. The moment required to deform them beyond 8° was found to be greater than the average moment required to fracture the ceramic bracket.

Resistance to second order archwire activations

The moments involved in this test were generally much higher than what was observed in the previous test. The mean moment required to fracture the ceramic bracket was 33706 g/mm. This was comparable with the mean moment at which the fiber brackets got deformed by 12° (34823 g/mm). These two groups showed no statistically significant difference (P > 0.05). The mean moment at which the composite brackets got deformed by 8° was found to be 29587 g/mm, which was lower than the fracture moment of the ceramic brackets. To obtain 12° of plastic deformation of the composite bracket, a mean moment of 41940 g/mm was required, which was much higher than the fracture moment of the ceramic brackets. These groups showed highly statistically significant differences (P < 0.01). The mean moment required to permanently deform composite and fiber brackets to the extent of 4° and 16° showed a statistically significant difference at a lesser significance level (P < 0.05).


   Discussion Top


The introduction of esthetic brackets was a much-heralded development in the treatment of adult patients. Their acceptance by patients has been unprecedented in the practice of orthodontics. However, orthodontists have noticed certain problems related to esthetic brackets during the course of treatment. The present study was taken up to address these professional concerns.

The purpose of this study was to test the physical properties of esthetic brackets in vitro and to determine, from the results obtained, the efficacy of these brackets for clinical use. The various esthetic brackets currently available were analyzed and three different categories of brackets were chosen, after sufficient evidence was gathered to prove that the selected brackets had overcome the major drawbacks associated earlier with their categories. The brackets tested were:

  • MXI ceramic brackets (Polycrystalline ceramic brackets having a polycarbonate base [T.P. Orthodontics, Inc.])
  • Spirit MB (Composite brackets with metal slot reinforcement [Ormco Corporation])
  • Natura (Fiber bracket [Leone Co.]).


All brackets had the slot dimensions of 0.022″ × 0.028″.

The present study was undertaken in two parts, each part to determine one particular property. The properties studied were:

  • Resistance of the brackets to third order archwire (torsional) activation
  • Resistance of the brackets to second order archwire (tip) activation.


This required the fabrication of apparatus for the first three properties tested. The basic designs of the apparatus were the same as those developed by earlier authors for similar studies. [5],[6],[7] A few refinements were made in the apparatus design for obtaining a better comparison between the brackets.

The findings from this study indicated that ceramic brackets fractured at a mean torque of 9396 g/mm [Table 1]. The minimum moment required for plastic deformation of the composite/fiber bracket to the extent of 4° was 3806 g/mm. These values appear to be above the amount of force recommended for effectively torquing maxillary incisors.
Table 1: Torsional resistance: Ceramic bracket-moments required to fracture the bracket


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Reitan [8] suggested that a force of 130 g should be used at the root apex during torquing movements. The average distance from the bracket to the root apex on an upper central incisor is 18.25 mm. [9] Therefore, the applied torque would be 2373 g/mm. Nikolai [10] suggested that for an average sized maxillary incisor segment, the total torque requirement is 3000-3500 g/mm.

It can be concluded that the range of moments required for successful torquing displacements varies from about 2000 to 3500 g/mm. In this regard, the maximum force recommended is still less than the lowest mean torque required for plastic deformation of composite/fiber brackets (3806 g/mm) [Table 2] and [Table 3] used in this study. Hence, it can be concluded that the more evolved esthetic brackets currently available, can well withstand the torquing forces applied in the clinical situation, without undergoing fracture/plastic deformation.
Table 2: Torsional resistance: Composite bracket with metal slot reinforcement - moments required to permanently deform the bracket


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Table 3: Torsional resistance: Fiber bracket-moments required to permanently deform the bracket


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Another area of concern, related again to the strength of the bracket material, is the resistance of the brackets to second order archwire activations. Lindauer et al. [6] have measured the force, which resulted on stretching an orthodontic elastic or three links of an elastic chain module across a 22 mm molar to canine distance. The average force was 350 g. Since these forces were acting approximately 10 mm away from the center of resistance of these teeth, the second order moments exerted at the brackets were calculated to be about 3500 g/mm. The same authors also measured the moments produced at the brackets from 'V-bends' of 5° to 45° angulations placed across a 15 mm interbracket distance. These yielded average moments of 600 g/mm and 2800 g/mm respectively.

The second order loads determined in this study to fracture the ceramic brackets and induce plastic deformation of the composite/fiber brackets were substantial higher, with the minimum moment required to deform the fiber bracket to extent of 4° being 13072 g/mm [Table 4]. Hence, it is very unlikely that second order archwire forces generated in clinical situations may cause failure of the esthetic brackets used in the present study.
Table 4: Resistance to M-D tipping activation: Ceramic bracket-moments required to fracture the bracket


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The fiber bracket showed the least resistance to deformation when subjected to torsional forces. The minimum mean moment required to permanently deform the fiber bracket to extent of 4° being, 3806 g/mm [Table 5]. This was followed by the metal slot reinforced composite bracket, which deformed to the same extent at a mean moment of 4493 g/mm [Table 6]. The ceramic bracket resisted higher moments and fractured at a mean moment of 9369 g/mm [Table 1]. The moments required to deform the composite and fiber brackets beyond 4° were much higher than the mean moment required to fracture the ceramic bracket.
Table 6: Resistance to M-D tipping activation: Composite bracket with metal slot reinforcement - moments required to permanently deform the bracket


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The moments at which the brackets failed from second order archwire activation were much higher than in the previous test, but followed the same pattern. The earliest signs of deformation were seen with the fibre bracket at a mean moment of 13072 g/mm.

A study by Holt et al. [5] showed the ceramic brackets to be capable of withstanding torque values between 3706 and 6177 g/mm. The mean torque value in the present study to fracture ceramic brackets was higher, at 9369 g/mm. This increase in the torsional resistance could be attributed to the continued improvements in the ceramic brackets that have been taking place. The brackets selected for this study were injection molded during the manufacturing process, resulting in lesser or no scratches on the surface. Furthermore, care was taken not to induce any surface scratches during the testing procedure.

Other authors like Feldner et al. [4] and Dobrin et al. [1] have investigated the torque deformation characteristic of orthodontic polycarbonate brackets. The moments reported in these studies were much lower than the values obtained in the present study. The plastic deformation and not the initial (elastic) deformation of the brackets were evaluated in our study, which was considered to be more appropriate. This is probably the reason why the values obtained in the present study were in a higher range.

Authors like Rhodes et al. [5] have investigated this property of ceramic brackets. They mention the forces (in grams), which were required to fracture the ceramic brackets. The distance from the point of force application has not been considered. The moment expressed at the bracket will depend upon the distance factor also. Hence, from the results obtained out of these studies, meaningful comparisons cannot be made with those obtained from other studies.

To overcome this, Lindauer et al. [6] studied the moments required to fracture seven varieties of ceramic brackets from second order archwire activations. The mean moments for fracture of ceramic brackets ranged from 15905 g/mm to 35291 g/mm. These results were comparable with our study, where the mean moment for the fracture of ceramic brackets was 33706 g/mm. From the previous study, the resistance of Transend 2000 (3M Unitek) and Fasination (Dentaurum) ceramic brackets to second order archwire activations were comparable to the MXI ceramic brackets (T.P. Ortho) used in this study.

There was no literature available for comparison of the resistance of polycarbonate/fiber brackets to second order archwire activations.


   Conclusion Top


The results obtained indicated that all the three selected brackets were strong enough to sufficiently withstand torsional and tipping forces, which are applied in the clinical practice. The loads at which these brackets fractured/deformed were much higher than the calculated clinical loads. It is very unlikely that they would fail on account of these archwire activations.

To conclude, the strength of the selected brackets was more than sufficient to withstand orthodontic load without any fracture or deformation. With a variety of esthetic brackets available now, the Orthodontist should judge the most suitable appliance for any given patient.

 
   References Top

1.Dobrin RJ, Kamel IL, Musich DR. Load-deformation characteristics of polycarbonate orthodontic brackets. Am J Orthod 1975;67:24-33.  Back to cited text no. 1
[PUBMED]    
2.Winchester LJ. A comparison between the old Transcend and the new Transcend series 2000 bracket. Br J Orthod 1992;19:109-16.  Back to cited text no. 2
[PUBMED]    
3.Kusy RP. Morphology of polycrystalline alumina brackets and its relationship to fracture toughness and strength. Angle Orthod 1988;58:197-203.  Back to cited text no. 3
[PUBMED]    
4.Feldner JC, Sarkar NK, Sheridan JJ, Lancaster DM. In vitro torque-deformation characteristics of orthodontic polycarbonate brackets. Am J Orthod Dentofacial Orthop 1994;106:265-72.  Back to cited text no. 4
[PUBMED]    
5.Holt MH, Nanda RS, Duncanson MG Jr. Fracture resistance of ceramic brackets during arch wire torsion. Am J Orthod Dentofacial Orthop 1991;99:287-93.  Back to cited text no. 5
[PUBMED]    
6.Lindauer SJ, Macon CR, Browning H, Rubenstein LK, Isaacson RJ. Ceramic bracket fracture resistance to second order arch wire activations. Am J Orthod Dentofacial Orthop 1994;106:481-6.  Back to cited text no. 6
[PUBMED]    
7.Rhodes RK, Duncanson MG Jr, Nanda RS, Currier GF. Fracture strengths of ceramic brackets subjected to mesial-distal archwire tipping forces. Angle Orthod 1992;62:67-76.  Back to cited text no. 7
[PUBMED]    
8.Reitan K. Some factors determining the evaluation of forces in orthodontics. Am J Orthod 1957;43:32-45.  Back to cited text no. 8
    
9.Wheeler RC. A Text Book of Dental Anatomy and Physiology. 4 th ed. Philadelphia: W.B. Saunders 1965. p. 136.  Back to cited text no. 9
    
10.Nikolai RJ. Bioengineering Analysis of Orthodontic Mechanics. Philadelphia: Lea and Febiger; 1985. p. 299-305.  Back to cited text no. 10
    

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Correspondence Address:
Prashant Babaji
Department of Pedodontics, Vyas Dental College, Jodhpur, Rajasthan
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.127615

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