DR. MAHESH JAIN (M.D.S.)Assistant ProfessorDepartment of Orthodontics & Dentofacial OrthopedicsManipal College of Dental sciencesManipal University, Mangalore (India)Email id: doctormaheshjain@gmail.com

I finished my Bachelors in Dental Surgery in 2007 and Masters in Orthodontics & Dentofacial Orthopedics in 2011 from Manipal University, India.

Academic Credentials

Received Student Plaque Award 2006 for outstanding achievement during the years of dental study from International College of Dentists (India & Srilanka section).

Presented paper on “Role of Dentistry in Bioterrorism” at Indian Dental Association student conference held at Bangalore, 2006.

Presented Poster on “Clinical manipulation of Herbst fixed functional appliance” at 13th Indian Orthodontic Society Post- Graduate Student’s Convention 2009 held at Davengere, India.

Presented Paper on “Treatment of Retrusive Midface by Rapid Palatal Expansion and Facemask Therapy: Case Report Series” at 14th Indian Orthodontic Society Post-Graduate Student’s Convention 2010 held at Chennai, India

Presented Paper on “Determination of optimum adhesive thickness using varying degree of force application and its effect on shear bond strength: an in-vitro study” at 46th Indian Orthodontic Conference 2011 held at Khajuraho, India.

Determination of Optimum adhesive thickness using differential force application with light cure adhesive &

its effects on shear bond strength: an in-vitro study

Introduction

The main concern with the orthodontists is to achieve stable bond strength which depends on many factors which includes:

Bracket base design Adhesive thickness Adhesive composition Type of enamel conditioner Length of etching time and so on

Many studies have evaluated the effect of these mentioned variables on shear bond strength, however the effect of adhesive thickness on bond strength largely remain unstudied.

Uniform adhesive thickness present between bracket base and tooth surface may prevent the need for compensatory bends that to be placed in finishing arch wires thereby, help in full utilization of PEA and to obtain perfect finish.

Aims & Objectives

Effect of varying thickness of adhesive on shear bond strength of orthodontic bracket.

To determine the optimum adhesive thickness achieved between tooth surface and bracket with light cured adhesive.

Materials and MethodsBased on amount of force applied while bonding

Materials &Methods

Bucco-lingual width of each premolar along with bracket was measured

Bucco-lingual width of each premolar was measured again after bonding

Bracket was subjected to force applied at centre of bracket using a fork end of Dontrix guage

Determining adhesive thickness

Hence, difference in bucco lingual width of each premolar with bracket prior to bonding and after bonding is measured ;thereby adhesive thickness is determined for specified force application.

The mean, standard deviation and confidence interval for mean were calculated for each group

Analysis of variance (ANOVA) was used to determine whether significant differences existed between the various group compared.

Significance of level was predetermined at p value <0.001.

The Bonferroni multiple comparison test was applied to find the group with significant difference.

Pearson Correlation test was performed to correlate the relationship between adhesive thicknesses and shear bond strength

Weibull analysis was used to calculate the probability of failure at given values of applied force.

STATISTICAL ANALYSIS

Results

20 .9950 .04968 .01111 .9717 1.0183 127.591 p<0.00120 .8365 .03951 .00883 .8180 .8550 HS20 .7245 .06825 .01526 .6926 .7564

.

Group AGroup BGroup C

adhesive thickness(mm)

N MeanStd.

DeviationStd.Error Lower Bound Upper Bound

95% Confidence Interval forMean ANOVA F

value p value

Table I: Descriptive statistics of the three groups and comparison of adhesive thickness by ANOVA tests.

Results

Multiple Comparisons

Bonferroni

.15850 .01702 p<0.001 HS

.27050 .01702 p<0.001 HS

.11200 .01702 p<0.001 HS

(J) GroupGroup B

Group C

Group C

(I) GroupGroup A

Group B

Dependent Variableadhesive thickness (mm)

MeanDifference

(I-J) Std. Error p

Table II: Multiple comparisons of adhesive thickness between three groups using Bonferroni test.

20 6.364 1.2727 .2846 5.768 6.960 23.956 p<0.00120 9.559 1.4383 .3216 8.885 10.232 HS20 8.577 1.7372 .3884 7.764 9.390

Group AGroup BGroup c

Shear bondstrength(MPa)

N MeanStd.

DeviationStd.Error Lower Bound Upper Bound

95% Confidence Interval for Mean ANOVA F

value p value

Table III: Descriptive statistics of the three groups and comparison of shear bond strength by ANOVA tests.

 Multiple Comparisons

Bonferroni

-3.1945 .4728 p<0.001 HS

-2.2130 .4728 p<0.001 HS

.9815 .4728 .127 NS

(J) GroupGroup B

Group C

Group C

(I) GroupGroup A

Group B

Dependent VariableShear bondstrength(MPa)

MeanDifference

(I-J) Std. Error p

Table IV: Multiple comparisons of shear bond strength between three groups using Bonferroni test

Correlations

-.528 .017 sig

.408 .074 NS

.122 .607 NS

Shear bondstrength(MPa)

Shear bondstrength(MPa)

Shear bondstrength(MPa)

adhesive thickness (mm)

adhesive thickness (mm)

adhesive thickness (mm)

GroupGroup A

Group B

Group C

Pearson Correlation p value

Table V: Pearson's correlation test

% failure Reliability Group A Group B Group C

99.9% 0.001 1.109 0.925 0.885

99% 0.01 1.089 0.909 0.856

95% 0.05 1.069 0.893 0.826

5% 0.95 0.892 0.756 0.590

1% 0.99 0.829 0.707 0.515

Beta ( shape parameter/weibull modulus) 22.49 24.31 12.08

Alpha (characteristic life) 1.018 0.8545 0.7547

Mean adhesive thickness(mm) 0.995 0.836 0.724

Table VI: Weibull analysis for adhesive thickness

% failure Reliability Group A Group B Group C

99.9% 0.001 9.62 13.18 13.16

99% 0.01 8.96 12.48 12.24

95% 0.05 8.31 11.78 11.32

5% 0.95 4.08 6.83 5.43

1% 0.99 3.07 5.49 4.04

Beta ( shape parameter/weibull modulus) 5.725 7.467 5.539

Alpha (characteristic life) 6.867 10.175 9.291

Mean shear bond strength (MPa) 6.36 9.55 8.57

Table VII: Weibull analysis for shear bond strength

Thus, the increase in the applied force while bonding results in the decreased thickness of the adhesive between bracket base and tooth surface. However, the amount of thickness may vary not only due to the pressure, but also to the viscosity of the adhesive used.

Discussion

Most of the studies done till now use firm pressure while placing bracket on the enamel surface with assumption that uniform thickness of adhesive is achieved, but as shown in the present study even a change of 1ounce force brings about significant change in adhesive thickness.

Mackay also noted that the stiffness of each material depends on size, type and amount of the filler content and monomer used. Therefore, adhesive thickness obtained in this study by varying the force of application is meant only for adhesive Transbond XT. Hence, different types of adhesive resins will produce different thickness while applying same amount of force.

However, Mackay (1992) reported that increasing the thickness of adhesives had no statistically significant effect on their mean shear bond strength, although the trend was to decreased. Evans, Powers (1985) noted that the tensile bond strength was decreased with increased adhesive thickness. These results are similar to that reported by Jost-Brinkmann , Schechter & others.

As adhesive thickness reduces, shear bond strength first increases from group A (6.36 MPa) to group B (9.55 MPa), but then it decreases in group C (8.57MPa); and it is highly statistically significant.

As suggested by Buonocore (1963) & Alster(1995), increasing the thickness of the adhesive layer results in a weaker joint due to the increased polymerization shrinkage seen in thicker layers, along with the imperfections which lead to increase in stress concentrations and hence decreasing the strength.

Interesting finding to be noted here is that shear bond strength reduces in group C (8.57 MPa), although its thickness is minimum (0.72mm). Although, these findings are quite similar to our findings, but this cannot be compared directly as material and methodology in these studies were different from the present study.

An explanation of this finding could be that strength of any material depends on its volume to a larger extent, and since the thickness of group C is minimum we can expect bond failure to occur early.

Li et al too reported that volume of filler had a greater effect on physical and mechanical properties than filler size. Arici et al noticed that mean shear bond strength increased as adhesive thickness increased; but they have tested shear bond strength at interval of 0.25mm thickness starting from 0mm to 0.5mm.

Hence, Weibull analysis is performed here to analyze the characteristic bond strength and to know the probability of failure for each of the material.

Higher weibull modulus (β) indicates a more predictable system and possibly, a more clinically reliable system. Group B produced higher modulus (β) values than other groups, hence the group B could be consider as more predictable and clinically reliable system.

Hence, we can infer that optimum adhesive thickness is required; because shear bond strength has a tendency to decrease with decreasing thickness. As indicated by Fox et al, mean and standard deviation values of bond strengths are not the best indicators of the performance of any bonding material.

Weibull analysis also showed that force needed for 99.9% chance of failure is maximum for group B (13.18 MPa) and minimum for group A(9.62 MPa). It means that group B can withstand higher forces when compared to all other groups.

Therefore, group B with mean shear bond strength of 9.55MPa having a mean adhesive thickness of 0.83mm could be considered as the group having optimum adhesive thickness required to achieve sufficient bond strength to prevent chances of bond failure.

Thus, application of controlled bonding pressure (2 oz as we obtained in this study) through a bracket holder that has a pressure gauge could be the correct way to gain a uniform optimum adhesive thickness (0.83mm) between the bracket and enamel.

Adhesive thickness between bracket base and tooth surface decreases with increasing the amount of force application from 1ounce to 3 ounces.

Mean shear bond strength increases when adhesive thickness decreases from group A to Group B and then it has a tendency to decrease as shown in group C.

0.83mm could be considered as optimum adhesive thickness which is required to achieve sufficient bond strength to prevent chances of bond failure.

Conclusion

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