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Influence of the cross-section of orthodontic wires on the surface friction of self-ligating brackets

 

 

Roberta Buzzoni^I; Carlos N. Elias^II; Daniel J. Fernandes^III; José Augusto M. Miguel^IV

^IOrthodontist, State University of Rio de Janeiro (UERJ)
^IIAssociate Professor, Department of Mechanical Engineering and Materials Science, Military Institute of Engineering (IME)
^IIIDoctoral Student in Orthodontics and in Materials Science, State University of Rio de Janeiro and Military Institute of Engineering (IME)
^IVAdjunct Professor and Coordinator, Masters Degree Course in Orthodontics, State University of Rio de Janeiro

Contact Address

 

 

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ABSTRACT

OBJECTIVES: The purpose of this study was to assess the surface friction produced between self-ligating stainless steel brackets equipped with a resilient closure system and round and rectangular orthodontic wires made from the same material.
METHODS: Thirty maxillary canine brackets were divided into six groups comprising Smartclip and In-Ovation R self-ligating brackets, and conventional Gemini brackets tied with elastomeric ligatures. This investigation tested the hypothesis that self-ligating brackets are susceptible to increases in friction that are commensurate with increases and changes in the cross-section of orthodontic wires. Traction tests were performed with the aid of thirty segments of 0.020-in and 0.019 x 0.025-in stainless steel wires in an Emic DL 10000 testing machine with a 2N load cell. Each set of bracket/wire generated four samples, totaling 120 readings. Comparisons between means were performed using analysis of variance (one way ANOVA) corrected with the Bonferroni coefficient.
RESULTS AND CONCLUSION: The self-ligating brackets exhibited lower friction than conventional brackets tied with elastomeric ligatures. The Smartclip group was the most effective in controlling friction (p<0.01). The hypothesis under test was confirmed to the extent that the traction performed with rectangular 0.019 x 0.025-in cross-section wires resulted in higher friction forces than those observed in the 0.020-in round wire groups (p<0.01). The Smartclip system was more effective even when the traction produced by rectangular wires was compared with the In-Ovation R brackets combined with round wires (p<0.01).

Keywords: Brackets. Orthodontic wires. Stainless steel. Friction.

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INTRODUCTION

The increasingly frequent use of sliding mechanics underscores the importance of controlling friction when performing orthodontic movement.¹⁰ Friction can be defined as a force that opposes or slows down the movement of two bodies in contact.^5,8,10 Before orthodontic movement can be produced it is necessary that the force delivered exceed the frictional resultant present in the bracket/wire interface. However, when high friction rates are observed in this situation, force can be reduced to as low as 60%¹¹ of its original intensity, which may clinically cause a delay in biological response.^8,10

A wide range of variables can modulate the amount of friction generated, among which are type of material, size and shape of brackets^4,10 and orthodontic wires,^1,4,10 and types of ligation used at the bracket/wire interface.^1,15 Wire thickness has a direct relationship with friction forces while rectangular archwires feature higher friction than their round counterparts.⁹  Regarding ligation type, self-ligating brackets are reported in the literature as appliances that allow friction control of the orthodontic archwires engaged in their slots¹⁵ (Fig 1).

Self-ligating systems are divided into two categories according to how the mechanical ligation method operates. Action can be either active or passive.^4,15 Passive systems have a sliding clip that entraps the archwire inside the bracket slot without applying any pressure.¹⁵  Active models exert a continuous pressure on the archwire,⁴ enabling faster alignment with greater speed and torque control.¹⁵ In some models, pressure becomes more intense as archwire size is increased,⁹ which may also result in the incorporation of higher frictional forces. This investigation tested the hypothesis that self-ligating brackets are susceptible to increases in friction that are commensurate with increases and changes in the cross-section of orthodontic wires used in traction. The aim of this study was to assess the surface friction produced by self-ligating stainless steel brackets equipped with a resilient closure system and compare the friction generated during traction of round and rectangular orthodontic wires made from the same material using round and rectangular cross-section wires.

 

MATERIAL AND METHODS

For this study, 30 maxillary canine brackets were divided into six distinct groups each with five brackets. The groups were composed of SmartClip (3M/Unitek, CA, USA) and In-Ovation R (GAC, NY, USA) self-ligating brackets, and conventional Gemini (3M/Unitek, CA, USA) brackets tied with gray elastomeric ligatures (TP Orthodontics, IN, USA) (Table 1).  Five segments of round 0.020-in orthodontic wires and five segments of rectangular 0.019 x 0.025-in (TP Orthodontics, IN, USA) wires measuring 8.0 cm were used in each of the three types of brackets evaluated (Table 1). The surface friction generated by traction of the steel wires inside the bracket slots was evaluated according to four consecutive readings of each bracket/orthodontic wire pair. Twenty readings were conducted of each group in a total of 120 readings.

 

 

The bracket/wire models were tested for surface friction in an EMIC DL 10000 (EMIC, PR, Brazil) testing machine with a load cell of 2.0 kilograms (kg) of force (Fig 2). To perform the tests and compensation of different angulations (tips) built into the pre-adjusted brackets, metal cylinders were especially developed with compensatory tips so that all the wires were placed in parallel relationship with the bracket slots. These stainless steel cylinders were connected to the testing machine and the brackets bonded with the aid of Super Bonder instant adhesive (Loccite, SP, Brazil). The bracket bases were first filled with Transbond XT (3M/Unitek, CA, USA) light cure resin in order to determine the area for bonding (Fig 3).

 

 

 

Ligation of orthodontic wires to the Gemini brackets was performed using elastomeric ligatures (TP Orthodontics, IN, USA), which were replaced at each trial. The orthodontic wires were pulled at a speed of 5 mm/min at a distance of 3.5 mm (Fig 2). Maximum force (gF) values were recorded by computer program Tesc, version 3.04 (Emic, PR, Brazil).

The results were summarized as means and standard deviations. Comparisons between means were performed using analysis of variance (one way ANOVA) corrected with the Bonferroni coefficient.

 

RESULTS

Comparison between surface friction force means in grams-force (gF) under traction with 0.020-in and 0.019 x 0.025-in stainless steel wire is shown in Table 2. The values ​​found show lower mean friction in the self-ligating groups compared with conventional brackets tied with elastomeric ligatures. The most effective friction control was afforded by the groups with  ​​SmartClip self-ligating brackets, regardless of the wire cross-section under traction (p<0.01). Increases and changes in the cross-section of wires, from round to rectangular, caused friction to increase in all groups tested (p<0.01).

 

 

Distribution of values ​​was analyzed using the Shapiro-Wilk normality test (p<0.05) and presented in a box-plot graph (Fig 4). The resulting pattern showed normal distribution for all groups except for SmartClip brackets combined with 0.020-in archwires.

 

 

Analysis of variance (ANOVA) values revealed associations between frictional forces and self-ligating systems. The p value (p<0.01) found with the Bonferroni multiple comparisons test indicated statistical differences between groups (Table 2).

 

DISCUSSION

In this study, twenty specimens from each group were obtained through surface tensile tests. Each bracket provided four consecutive friction readings representative of the traction of each archwire type in the slot, totaling twenty measurements for each group. This methodology was employed with the purpose of simulating the clinical conditions involved in sliding repeatedly — be it the bracket sliding on the wire or vice versa — according to the time period established in the treatment goals. The results showed reduced standard deviations, which would make redundant repetitions with the same brackets. Although some authors argue that the bracket/wire surface is susceptible to wear,^6,7 the authors believe that, in this test, wear occurs significantly only in materials with a high coefficient of friction such as crystalline matrix or polycarbonate. This methodology is similar to research conducted by Voudoris¹⁵ in which only eight brackets in each group were subjected to sliding, totaling twenty four samples for each type of bracket.

Only maxillary canine brackets were evaluated. This choice is justified by the fact that, clinically, canine teeth are often involved in sliding mechanics, especially in cases that require premolar extraction. The same brackets were selected in studies conducted by Berger² and Brown et al.³ However, the bases of these brackets were found to be too concave, hindering the bonding procedure. To ensure proper bonding, the bracket bases had to be filled flush with light cure resin. Voudouris¹⁵ also used the same resin filling technique in his study and noted no variation in friction values.

Each manufacturer developed their own self-ligating model with individual angulations and torques. It has been argued that there is a direct relationship between angulation (tip), active torques and increased surface friction.⁹ Metal cylinders were fabricated with the purpose of standardizing the various prescriptions in bonding the brackets subjected to traction. A compensatory angulation was pre-adjusted in the bonding surface so as to neutralize the original bracket tip, ensuring parallelism between slot and wire. This standardization system is consistent with the methodology developed by Sims et al,¹¹ who also used a piece of metal support for compensatory bonding. Some authors have used different methods to standardize the pre-existing angulations.^3,15

An important factor influencing the magnitude of friction is how the bracket and orthodontic wire are ligated.¹¹ The conventional brackets were ligated with the aid of elastomeric ligatures in order to standardize the force delivered to entrap the orthodontic wire inside the bracket slot. Voudouris¹⁵ reported that the loss of elasticity experienced by the material is subjective and directly proportional to the length of time it remains stretched, and such loss could exert significant influence on the surface friction. For this reason, the ligatures were replaced at each test.

Piozzoni, Ravnholt and Melsen⁹ argued that thicker wires produce higher friction values and, in general, rectangular archwires generate more friction than round archwires due to a broader contact area with the brackets under traction. Others believe that the most important variable is the extent to which bracket slots are filled by the archwires.^3,11  In this research, after comparing different stainless alloy wires, it was found that increases in friction are directly proportional to increases in the cross-section of the archwires, regardless of the bracket model evaluated. This was shown to have a statistically significant correlation.

Self-ligating brackets with a resilient closure system feature a self-closing anterior cap that entraps — by pressure — the orthodontic wires inside the bracket slots.⁹ It is believed that the permanent contact between the self-ligating cap and the wire contribute to increasing the friction generated in sliding.¹⁴ This interaction was evidenced in this study, where the friction generated by rectangular wires was found to be higher than that of round wires. A possible explanation could be that rectangular wires contact the self-closing cap on its 0.025-in face, while round wires contact only the 0.020-in face. As the bracket slot is filled by the wire, the resilient cap exerts more pressure on the wires, consequently increasing the friction resultant on the brackets that experience traction.

The results of this study showed that In-Ovation R brackets yielded friction means above those observed in the SmartClip appliance when 0.020-in and 0.019 x 0.025-in cross-section wires were used. The group comprising SmartClip brackets was statistically more effective in controlling friction even when the traction of rectangular wires was compared with the test involving In-Ovation R brackets in combination with round wires.
The mean friction values produced by conventional Gemini brackets indicate that the self-ligating groups were more effective in controlling surface friction. This finding is in agreement with several previous studies.^1,2,10,11 The aim of this study was to assess the behavior of self-ligating brackets with resilient ligation systems in combination with orthodontic wires of different cross-sections. The mean values found for the conventional Gemini system served only as reference and were therefore not treated statistically.

 

CONCLUSIONS

The ​​SmartClip self-ligating bracket system allowed better control of friction forces regardless of the type of orthodontic wire used for traction. Rectangular 0.019 x 0.025-in wires produced a greater amount of friction than round 0.020-in wires made from the same stainless alloy. This finding supports the hypothesis of this investigation as regards the self-ligating brackets tested. The group comprised of SmartClip brackets was more effective even when the traction of rectangular wires was compared with the test involving In-Ovation R brackets in combination with round wires.

 

REFERENCES

1. Berger JL. The SPEED appliance: a 14-year update on this unique self-ligated orthodontic mechanism. Am J Orthod Dentofacial Orthop. 1994;105(3):217-23.         [ Links ]

2. Berger JL. The influence of the SPEED brackets self-ligating design on force levels in tooth movement: a comparative in vitro study. Am J Orthod Dentofacial Orthop. 1990;97:219-28.         [ Links ]

3. Braun S, Bluestein M, Moore K, Benson G. Friction in perspective. Am J Orthod Dentofacial Orthop. 1999;115(6):619-27.         [ Links ]

4. Closs LQ, Mundostock KS, Gandini LG Jr, Raveli DB. Os diferentes sistemas de bráquetes self-ligating: revisão de literatura. Rev Clín Ortod Dental Press. 2005;4(2):60-6.         [ Links ]

5. Harradine NWT, Birnie DJ. The clinical use of Activa self-ligating brackets. Am J Orthod Dentofacial Orthop. 1996;109(3):319-28.         [ Links ]

6. Kapur R, Sinha P, Nanda RS. Comparison of frictional resistance in titanium an stainless steel brackets. Am J Orthod Dentofacial Orthop. 1999;116(3):271-4.         [ Links ]

7. Lofttus BP, Årtun J, Nicholls JI, Alonzo TA, Stoner JA. Evaluation of friction during sliding tooth movement in various bracket/arch wire combinations. Am J Orthod Dentofacial Orthop. 1999;116(3):336-45.         [ Links ]

8. Moore MM, Harrington E, Rock P. Factors affecting friction in the pre-adjusted appliance. Eur J Orthod. 2004;26(6):579-83.         [ Links ]

9. Piozzoni L, Ravnholt G, Melsen B. Frictional forces related to self-ligating brackets. Eur J Orthod. 1998;20(3):283-91.         [ Links ]

10. Read-Ward GE, Jones SP, Daviest EH. A comparison of self-ligating and conventional orthodontic bracket systems. Br J Orthod. 1997;24:209-17.         [ Links ]

11. Sims APT, Waters NE, Birnie DJ, Pethybridget RJ. A comparison of the forces required to produce tooth movement in vitro using two self-ligating brackets and pre-adjusted bracket employing two types of ligation. Eur J Orthod. 1993;15(5):377-85.         [ Links ]

12. Stolzenberg J. The Russell attachment and its improved advantages. Int J Orthod Dent Child. 1935;21(9):837-40.         [ Links ]

13. Taloumis LJ, Smith TM, Hondrum ST, Lorton L. Force decay and deformation of orthodontic elastomeric ligatures. Am J Orthod Dentofacial Orthop. 1997;111(1):1-11.         [ Links ]

14. Thorstenson GA, Kusy RP. Resistance to sliding of self-ligating brackets versus conventional stainless steel twin brackets with second order angulation in the dry and wet (saliva) states. Am J Orthod Dentofacial Orthop. 2001;120(4):361-70.         [ Links ]

15. Voudouris JC. Interactive Edgewise mechanism: form and function — a comparison with conventional Edgewise brackets. Am J Orthod Dentofacial Orthop. 1997;111(2):199-40.         [ Links ]

 

 

Contact Address
Daniel J. Fernandes
Faculdade de Odontologia – UERJ
Blvd. 28 de Setembro 157, sala 230 – Vila Isabel
CEP: 20.551-030 – Rio de Janeiro/RJ, Brazil
E-mail: fernandes.dj@gmail.com

Submitted: April 23, 2009
Revised and accepted: April 14, 2010