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ORIGINAL ARTICLE
Year : 2012  |  Volume : 3  |  Issue : 3  |  Page : 186-192

Evaluation of shear bond strength of a polyacid modified composite resin used as orthodontic bonding material


1 Department of Orthodontics, SRM Dental College, Chennai, Tamil Nadu, India
2 Department of Orthodontics, Sree Balaji Dental College and Hospital, Chennai, Tamil Nadu, India
3 Department of Orthodontics, Annoor Dental College, Muvattu Puzha, Kerala, India
4 Department of Orthodontics, Government Dental College, Chennai, Tamil Nadu, India

Date of Web Publication19-Feb-2013

Correspondence Address:
Sangeetha Duraisamy
Department of Orthodontics, SRM Dental College, Bharathi Salai, Ramapuram, Chennai, Tamil Nadu
India
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DOI: 10.4103/0976-433X.107400

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  Abstract 

Background: Incorporating fluoride in the orthodontic bonding resin to prevent demineralization around orthodontic brackets reduces the bond strength, which is critical in the clinical performance of the resin. Aim and Objective: The aim of the study is to evaluate the shear bond strength of a polyacid-modified composite resin and compare it with a conventional composite resin and a resin-modified glass ionomer cement and to evaluate the site of bond failure and the amount of adhesive remaining on the tooth surface after debonding. Materials and Methods: Brackets were bonded using the three orthodontic bonding agents to 120 human premolars embedded in self-cure acrylic blocks, and shear bond strength was evaluated using the Instron universal testing machine. The debonded bracket surfaces were examined with a stereomicroscope to evaluate the site of bond failure and the presence of residual adhesive. Results: Composite resin bonding material showed the maximum initial and final bond strength followed by the polyacid-modified composite resin and resin-modified glass ionomer cement. The mode of bond failure for the polyacid-modified composite was adhesive failure at bracket adhesive interface or cohesive failure. Conclusion: The polyacid-modified composite resin produced shear bond strength in vitro within the range that is considered in the literature to be adequate for routine clinical use. Further clinical research is recommended to validate this finding.

Keywords: Bond strength, fluoride release, polyacid-modified composite resin


How to cite this article:
Duraisamy S, Manjula W S, Muthusamy L, Vimala G. Evaluation of shear bond strength of a polyacid modified composite resin used as orthodontic bonding material. SRM J Res Dent Sci 2012;3:186-92

How to cite this URL:
Duraisamy S, Manjula W S, Muthusamy L, Vimala G. Evaluation of shear bond strength of a polyacid modified composite resin used as orthodontic bonding material. SRM J Res Dent Sci [serial online] 2012 [cited 2020 May 28];3:186-92. Available from: http://www.srmjrds.in/text.asp?2012/3/3/186/107400


  Introduction Top


Fixed orthodontic appliances retain plaque and make oral hygiene maintenance difficult. Decalcification of enamel surface adjacent to orthodontic bracket is a common side-effect of orthodontic treatment. [1] Fluoride increases the resistance of enamel to demineralization. [2],[3] Various fluoride-releasing regimes were recommended for patients undergoing fixed orthodontic treatment. Most of them are dependent on patient compliance. [4],[5] Topical fluorides reduce the bond strength, and fluoridated elastomeric ligatures are not effective in reducing the decalcification. [6],[7] The advantage of a bracket bonding material with sustained release of fluoride is that a low-dose fluoride is continuously released at the bracket enamel interface the site which is at risk. [8],[9],[10] Low bond strength associated with glass ionomer cements limits their orthodontic use primarily to band cementation. [11],[12],[13],[14]

Incorporation of inorganic fluorides into composite resins creates problems of phase separation and loss of mechanical integrity because of the highly polar nature of the fluoride salts and low polarity of the resins. Organic fluoride incorporation has a plasticizing effect that also yields poor properties. [9]

Antonucci et al. introduced resin-modified glass ionomer cements (RMGICs) in 1988. [13] Addition of resin monomers to the poly alkeonic acid solution resulted in resin-modified glass ionomer cements. The bond strength of RMGIC is superior to that of glass ionomer cements but inferior to conventional composite resins when used in unetched enamel. [15],[16] Polyacid-modified composite resins (PMCR) or compomer are single component systems consisting of aluminosilicate glass in the presence of carboxyl-modified resin monomers and light-activated conventional resin monomers. After light-activation of the compomer and initial setting, water sorbs into the compomer from the oral environment producing a delayed acid-base reaction releasing fluoride from the aluminosilicate glass. [16] Physical properties are acquired quickly as the compomer polymerize, and their early setting strengths are superior to those of the resin-modified glass ionomer and are adequate for orthodontic purposes. [17],[18]

During debonding, the mode of failure may be adhesive failure at the enamel resin interface, making enamel clean up easy, or at bracket base resin interface, making enamel clean up difficult. In certain situations, it may be cohesive failure within the resin matrix. The site of bond failure and the amount of adhesive remaining on the tooth determines the type of clean up procedures and hence the collateral damages caused to the enamel during such procedures.

In spite of having relatively lesser bond strength, the bond reliability and clinical performance of a fluoride-releasing composite resin is comparable to the conventional composite resins [19],[20],[21] with favorable ARI scores. [22],[23]

The purpose of the study is to evaluate and compare the shear bond strength of a polyacid-modified composite resin, a conventional composite resin, and resin-modified glass ionomer cement and to evaluate the site of bond failure and the amount of adhesive remaining on the tooth surface.


  Materials and Methods Top


Hundred and twenty human premolars with intact buccal enamel without any hypoplastic spots, extracted for orthodontic treatment purpose, were collected, cleaned of debris, and stored in distilled water. The samples were randomly divided into six groups of 20 each [Table 1] and embedded in color-coded self-cure acrylic blocks of cuboid shape up to the level of the cementoenamel junction. The tooth was oriented in the block such that the buccal surface will be parallel to the force during the shear bond strength test [Figure 1]. Roth prescription 0.022" slot, stainless steel brackets (Gemini Series, 3M Unitek, Monrovia, Calif.), with an average bracket base surface area of 10.61 mm 2 , were bonded to each premolar tooth following the manufactures instructions.
Table 1: Six sample groups


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Figure 1: Sample groups

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The orthodontic bonding material evaluated in the study are a light-cured PMCR system-releasing fluoride that is recommended for use to bond brackets on an uncontaminated etched enamel surface (Python light cure adhesive, TP Orthodontics, Inc., LaPorte, Ind). A light-cured composite resin system that is recommended for use to bond brackets on an uncontaminated etched enamel surface (Transbond, 3M Unitek, Monrovia, Calif) and a RMGIC-releasing fluoride recommended for use to bond brackets on an unetched enamel surface. (Fuji Ortho LC, GC, Tokyo, Japan) were used as controls [Figure 2].
Figure 2: Adhesives evaluated

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The specimens were stored in an incubator at 100% relative humidity and 37°C [Figure 3]. At the end of 30 minutes, group I, III, and group V samples were subjected to shear bond strength (SBS) analysis using a Instron universal testing instrument, (Model 1195, Instron Corp., Canton, Mass.) to find the initial bond strength. Group II, IV, and group VI samples were subjected to shear bond strength analysis after 24 hours to find the final bond strength [Figure 4] and [Figure 5]. An occlusogingival load was applied to the bracket producing a shear force with a 50 KN load cell and a crosshead speed of 1 mm/minute until shearing of the bracket from the tooth occurred. The shearing force required for debonding was recorded in kilograms. The values are converted to newtons and then to megapascals using the surface area of the bracket base.
Figure 3: Incubator

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Figure 4: Instron testing machine

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Figure 5: Mounted sample

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After debonding, the enamel surface of the samples were examined with a stereomicroscope (stereozoom microscope, Carl zeiss, Germany) under 20X magnification to evaluate the site of bond failure and the presence of residual adhesive [Figure 6]. Any adhesive remaining after removal of the bracket was assessed according to the Adhesive Remnant Index (ARI) as given by Artun and Bergland, and scored on a scale of 0 to 3 [24] [Figure 7]. The results were subjected to statistical analysis. The bond strength data were subjected to a Weibull analysis to predict the probability of failure of the bracket bonding system at any level of shear stress. [24]
Figure 6: Steriomicroscope

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Figure 7: Adhesive remnant index, Score 0 - No adhesive left on the tooth, Score 1 - Less than half of the adhesive remaining on the tooth surface, Score 2 - More than half of the adhesive remaining on the tooth surface, Score 3 - All of the adhesive remaining on the tooth surface

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  Results Top


The descriptive statistics including mean shear bond strength, standard deviation, standard error, maximum and minimum value for all the six groups are shown in the [Table 2]. The mean shear bond strength of all the six groups is depicted in the [Graph 1 [Additional file 1]]. Transbond XT showed the maximum mean shear bond strength at 30 min (6.541 Mpa) and 24 hours (12.514 Mpa), followed by Python (5.139 Mpa and 10.361 Mpa) and Fuji Ortho LC (2.478 Mpa and 6.276 Mpa).

Statistical analysis of the mean shear bond strength of the three adhesives at 30 minutes using analysis of variance and Tuckey HSD test showed significant difference between the three groups tested [Table 3] and [Table 4]. Statistical analysis of the mean shear bond strength of the three adhesives at 24 hours using analysis of variance showed significant difference between the groups [Table 5]. Tuckey HSD test showed that the mean shear bond strength of composite resin and PMCR at 24 hours were not significantly different from each other [Table 6]. Student's independent - t test showed that the mean shear bond strength of all the three adhesives at 24 hours were significantly greater than the mean shear bond strength at 30 min [Table 7]. [Table 8] shows the weibull modulus for the three materials tested. The highest weibull modulus of 3.83 was recorded with Transbond XT indicating the greatest bond reliability, followed by Python light cure adhesive (3.593). The highest characteristic strength of 13.858 Mpa was recorded with Transbond XT indicating the greatest bond strength, followed by Python light cure adhesive (11.492 Mpa). The reliability of the material is a function of weibull modulus and the characteristic strength. The probability of bond failure to applied shear stress for the three light cure adhesive materials are graphically represented in the [Graphs 2 [Additional file 2]], [Graph 3 [Additional file 3]], [Graph 4 [Additional file 4]].
Table 2: Descriptive statistics for shear bond strength values


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Table 3: Analysis of variance results - Group I, III and V


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Table 4: Statistical comparison of mean shear bond strength using Tukey HSD - Group I, III and V


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Table 5: Analysis of variance results - Group II, IV and VI


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Table 6: Statistical comparison of mean shear bond strength of using Tukey - HSD - Group II, IV and VI


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Table 7: Student's independent t-test to compare mean shear bond strength of the materials at 30 min and 24 hours


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Table 8: Parameters of the weibull analysis


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The adhesive remnant index of each sample group in each group is shown in [Graph 5 [Additional file 5]]. The results of Chi-square test showed that there is a significant association between the ARI score and different groups [Table 9]. The Spearmen rank correction analysis between the bond strength and ARI scores shows a significant relationship between bond strength and ARI score in all the groups [Table 10]. The mode of bond failure for the PMCR was adhesive failure at bracket adhesive interface or cohesive failure. The predominant type of bond failure for the composite resin was adhesive bond failure at bracket adhesive interface. The predominant mode of bond failure of RMGIC was adhesive bond failure at bracket enamel.
Table 9: Analysis of adhesive remnant index score-results of Chi-square test


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Table 10: Spearmen rank correlation analysis of shear bond strength and adhesive remnant index scores


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  Discussion Top


Decalcification of enamel can occur within 4 weeks of initiating orthodontic treatment. [25],[26] Using bonding resin containing fluoride reduces the incidence of decalcification around the brackets. [11],[12] Incorporation of fluoride in the composite resin and continuous leaching of fluoride affects the bond strength and the clinical performance of the resin. [9]

The shear bond strength of all the three bonding materials were measured at 30 minutes (Initial bond strength) and at 24 hours (Final bond strength). This was designed to simulate a clinical situation, because arch wires are placed at the bonding appointment when the light cure materials might not have been completely polymerized. [27] The mean shear bond strength of all the three adhesive materials evaluated in this present study was significantly higher at 24 hours than at 30 minutes. This result agrees with the findings of Bishara et al. and Andrew summers et al. and is probably related to the incomplete polymerization of the light-cured materials at 30 minutes. [15],[27]

The initial bond strength of the polyacid-modified composite resin was significantly greater than that of the RMGIC but inferior to those of the conventional composite resin. The final bond strength of the polyacid-modified composite resin was significantly greater than that of the RMGIC and was comparable to the conventional composite resin. The reduced bond strength of the RMGIC may be due to the fact that the material was used on unetched enamel, and the chemical bond between the glass ionomer adhesive is weaker than the mechanical bond produced by the resin adhesives in etched enamel.

All the adhesives tested in this investigation produced shear bond strength greater and within the range of 5.9 to 7.8 Mpa considered by Reynolds to be adequate for routine clinical uses, except the initial bond strength of resin-modified glass ionomer cement. [28] Transbond XT had the highest characteristic bond strength and greater bond reliability as shown by the weibull modulus followed by the Python light cure adhesive. [12],[13],[22],[28],[29]

In this present study, the predominant mode of bracket failure for polyacid-modified composite resin was adhesive failure at bracket adhesive interface or cohesive in nature. The predominant mode of failure for conventional resin was adhesive in nature at the adhesive bracket interface leaving adhesive on the tooth surface. This is probably because of the incomplete polymerization of resin below the bracket base. [29] Air entrapment behind the mesh of bracket can also affect polymerization, because oxygen inhibits free radical polymerizing in light cure composite materials. [30] Hence, bonding the adhesive to the acid-etched enamel surface is greater than to the bracket base making enamel cleanup extensive after debonding. The predominant mode of failure for RMGIC was at the enamel adhesive interface. This suggest that the chemical bonding of glass ionomer cement to the bracket base is stronger than to the unetched enamel surface, even in the presence of resin component. This mode of failure in RMGIC makes enamel clean up less extensive after debonding.


  Summary and Conclusion Top


The final bond strength of all the three adhesives tested was significantly higher than the initial bond strength. The final bond strength of polyacid-modified composite resin was significantly greater than that of RMGIC and comparable to that of composite resin. The highest weibull modulus of 3.83 was recorded with conventional composite resin, indicating the greatest bond reliability, followed by PMCR and RMGIC. All the adhesives tested in this investigation produced shear bond strength greater and within the range to be adequate for routine clinical uses, except the initial bond strength of RMGIC. The mode of bond failure for polyacid-modified composite resin was adhesive failure at bracket adhesive interface or cohesive.

Polyacid-modified composite resin can be reserved as bracket adhesives for patients who have poor oral hygiene and high caries rate. Further in vitro studies simulating oral environment and clinical trials should be performed before putting this material into routine orthodontic use.

 
  References Top

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    Figures

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

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



 

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