1. DR. MAHESH JAIN (M.D.S.)
Assistant Professor
Department of Orthodontics & Dentofacial Orthopedics
Manipal College of Dental sciences
Manipal University, Mangalore (India)
Email id: doctormaheshjain@gmail.com

2.  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.

3. Determination of Optimum adhesive thickness using
differential force application with light cure adhesive &
its effects on shear bond strength: an in-vitro study

4. 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.

5. 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.

6. 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.

7. Materials and Methods
Based on amount of force applied while bonding
Materials &Methods

8. 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

9. 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.

10.  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

11. Results
20 .9950 .04968 .01111 .9717 1.0183 127.591 p<0.001
20 .8365 .03951 .00883 .8180 .8550 HS
20 .7245 .06825 .01526 .6926 .7564
.
Group A
Group B
Group C
adhesive thickness
(mm)
N Mean
Std.
Deviation
Std.
Error Lower Bound Upper Bound
95% Confidence Interval for
Mean ANOVA F
value p value
Table I: Descriptive statistics of the three groups and comparison of adhesive thickness by ANOVA tests.
Results

12. Multiple Comparisons
Bonferroni
.15850 .01702 p<0.001 HS
.27050 .01702 p<0.001 HS
.11200 .01702 p<0.001 HS
(J) Group
Group B
Group C
Group C
(I) Group
Group A
Group B
Dependent Variable
adhesive thickness (mm)
Mean
Difference
(I-J) Std. Error p
Table II: Multiple comparisons of adhesive thickness
between three groups using Bonferroni test.

13. 20 6.364 1.2727 .2846 5.768 6.960 23.956 p<0.001
20 9.559 1.4383 .3216 8.885 10.232 HS
20 8.577 1.7372 .3884 7.764 9.390
Group A
Group B
Group
c
Shear bond
strength(MPa)
N Mean
Std.
Deviation
Std.
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.

14.
Multiple Comparisons
Bonferroni
-3.1945 .4728 p<0.001 HS
-2.2130 .4728 p<0.001 HS
.9815 .4728 .127 NS
(J) Group
Group B
Group C
Group C
(I) Group
Group A
Group B
Dependent Variable
Shear bond
strength(MPa)
Mean
Difference
(I-J) Std. Error p
Table IV: Multiple comparisons of shear bond strength between three
groups using Bonferroni test

15. Correlations
-.528 .017 sig
.408 .074 NS
.122 .607 NS
Shear bond
strength(MPa)
Shear bond
strength(MPa)
Shear bond
strength(MPa)
adhesive thickness (mm)
adhesive thickness (mm)
adhesive thickness (mm)
Group
Group A
Group B
Group C
Pearson Correlation p value
Table V: Pearson's correlation test

16. % 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

17. % 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

18. 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.

19. 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.

20. 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.

21. 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.

22. 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.

23. 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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