DR. MAHESH JAIN (M.D.S.)Assistant ProfessorDepartment of Orthodontics & Dentofacial OrthopedicsManipal College of Dental sciencesManipal University, Mangalore (India)Email id: firstname.lastname@example.org
I finished my Bachelors in Dental Surgery in 2007 and Masters in Orthodontics & DentofacialOrthopedics in 2011 from Manipal University, India.Academic Credentials Received Student Plaque Award 2006 for outstanding achievement during the years of dentalstudy from International College of Dentists (India & Srilanka section). Presented paper on “Role of Dentistry in Bioterrorism” at Indian Dental Association studentconference held at Bangalore, 2006. Presented Poster on “Clinical manipulation of Herbst fixed functional appliance” at 13thIndianOrthodontic Society Post- Graduate Student’s Convention 2009 held at Davengere, India. Presented Paper on “Treatment of Retrusive Midface by Rapid Palatal Expansion andFacemask Therapy: Case Report Series” at 14thIndian Orthodontic Society Post-GraduateStudent’s Convention 2010 held at Chennai, India Presented Paper on “Determination of optimum adhesive thickness using varying degree offorce application and its effect on shear bond strength: an in-vitro study” at 46thIndianOrthodontic Conference 2011 held at Khajuraho, India.
Determination of Optimum adhesive thickness usingdifferential 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 strengthwhich depends on many factors which includes: Bracket base design Adhesive thickness Adhesive composition Type of enamel conditioner Length of etching time and so onMany studies have evaluated the effect of these mentioned variables onshear bond strength, however the effect of adhesive thickness on bondstrength largely remain unstudied.
Uniform adhesive thickness present between bracket base and toothsurface may prevent the need for compensatory bends that to beplaced in finishing arch wires thereby, help in full utilization of PEAand to obtain perfect finish.
Aims & Objectives Effect of varying thickness of adhesive on shear bond strength oforthodontic bracket. To determine the optimum adhesive thickness achieved betweentooth surface and bracket with light cured adhesive.
Materials and MethodsBased on amount of force applied while bondingMaterials &Methods
Bucco-lingual width of each premolar alongwith bracket was measuredBucco-lingual width of each premolar wasmeasured again after bondingBracket was subjected to force appliedat centre of bracket using a fork end ofDontrix guageDetermining adhesive thickness
Hence, difference in bucco lingual width of each premolarwith bracket prior to bonding and after bonding ismeasured ;thereby adhesive thickness is determined forspecified force application.
The mean, standard deviation and confidence interval for mean were calculated foreach group Analysis of variance (ANOVA) was used to determine whether significant differencesexisted 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 withsignificant difference. Pearson Correlation test was performed to correlate the relationship between adhesivethicknesses and shear bond strength Weibull analysis was used to calculate the probability of failure at given values ofapplied force.STATISTICAL ANALYSIS
Results20 .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 Cadhesive thickness(mm)N MeanStd.DeviationStd.Error Lower Bound Upper Bound95% Confidence Interval forMean ANOVA Fvalue p valueTable I: Descriptive statistics of the three groups and comparison of adhesive thickness by ANOVA tests.Results
Multiple ComparisonsBonferroni.15850 .01702 p<0.001 HS.27050 .01702 p<0.001 HS.11200 .01702 p<0.001 HS(J) GroupGroup BGroup CGroup C(I) GroupGroup AGroup BDependent Variableadhesive thickness (mm)MeanDifference(I-J) Std. Error pTable II: Multiple comparisons of adhesive thicknessbetween 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 9.390Group AGroup BGroupcShear bondstrength(MPa)N MeanStd.DeviationStd.Error Lower Bound Upper Bound95% Confidence Intervalfor Mean ANOVA Fvalue p valueTable III: Descriptive statistics of the three groups and comparison of shear bond strength by ANOVA tests.
Multiple ComparisonsBonferroni-3.1945 .4728 p<0.001 HS-2.2130 .4728 p<0.001 HS.9815 .4728 .127 NS(J) GroupGroup BGroup CGroup C(I) GroupGroup AGroup BDependent VariableShear bondstrength(MPa)MeanDifference(I-J) Std. Error pTable IV: Multiple comparisons of shear bond strength between threegroups using Bonferroni test
% failure Reliability Group A Group B Group C99.9% 0.001 1.109 0.925 0.88599% 0.01 1.089 0.909 0.85695% 0.05 1.069 0.893 0.8265% 0.95 0.892 0.756 0.5901% 0.99 0.829 0.707 0.515Beta ( shape parameter/weibull modulus) 22.49 24.31 12.08Alpha (characteristic life) 1.018 0.8545 0.7547Mean adhesive thickness(mm) 0.995 0.836 0.724Table VI: Weibull analysis for adhesive thickness
% failure Reliability Group A Group B Group C99.9% 0.001 9.62 13.18 13.1699% 0.01 8.96 12.48 12.2495% 0.05 8.31 11.78 11.325% 0.95 4.08 6.83 5.431% 0.99 3.07 5.49 4.04Beta ( shape parameter/weibull modulus) 5.725 7.467 5.539Alpha (characteristic life) 6.867 10.175 9.291Mean shear bond strength (MPa) 6.36 9.55 8.57Table VII: Weibull analysis for shear bond strength
Thus, the increase in the applied force while bonding results in the decreased thickness ofthe adhesive between bracket base and tooth surface. However, the amount of thicknessmay vary not only due to the pressure, but also to the viscosity of the adhesive used.DiscussionMost of the studies done till now use firm pressure while placing bracket on the enamelsurface with assumption that uniform thickness of adhesive is achieved, but as shown inthe present study even a change of 1ounce force brings about significant change inadhesive thickness.Mackay also noted that the stiffness of each material depends on size, type and amount ofthe filler content and monomer used. Therefore, adhesive thickness obtained in this studyby varying the force of application is meant only for adhesive Transbond XT. Hence,different types of adhesive resins will produce different thickness while applying sameamount of force.
However, Mackay (1992) reported that increasing the thickness of adhesives had nostatistically significant effect on their mean shear bond strength, although the trend wasto decreased. Evans, Powers (1985) noted that the tensile bond strength was decreasedwith 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 (6.36MPa) to group B (9.55 MPa), but then it decreases in group C (8.57MPa); and it ishighly statistically significant.As suggested by Buonocore (1963) & Alster(1995), increasing the thickness of theadhesive layer results in a weaker joint due to the increased polymerization shrinkageseen in thicker layers, along with the imperfections which lead to increase in stressconcentrations and hence decreasing the strength.
Interesting finding to be noted here is that shear bond strength reduces in group C (8.57MPa), although its thickness is minimum (0.72mm). Although, these findings are quitesimilar to our findings, but this cannot be compared directly as material and methodologyin these studies were different from the present study.An explanation of this finding could be that strength of any material depends on itsvolume to a larger extent, and since the thickness of group C is minimum we can expectbond failure to occur early.Li et al too reported that volume of filler had a greater effect on physical and mechanicalproperties than filler size. Arici et al noticed that mean shear bond strength increased asadhesive thickness increased; but they have tested shear bond strength at interval of0.25mm thickness starting from 0mm to 0.5mm.
Hence, Weibull analysis is performed here to analyze the characteristic bond strengthand to know the probability of failure for each of the material.Higher weibull modulus (β) indicates a more predictable system and possibly, a moreclinically reliable system. Group B produced higher modulus (β) values than othergroups, hence the group B could be consider as more predictable and clinically reliablesystem.Hence, we can infer that optimum adhesive thickness is required; because shear bondstrength has a tendency to decrease with decreasing thickness.As indicated by Fox et al, mean and standard deviation values of bond strengths are notthe best indicators of the performance of any bonding material.
Weibull analysis also showed that force needed for 99.9% chance of failure is maximumfor group B (13.18 MPa) and minimum for group A(9.62 MPa). It means that group B canwithstand higher forces when compared to all other groups.Therefore, group B with mean shear bond strength of 9.55MPa having a mean adhesivethickness of 0.83mm could be considered as the group having optimum adhesive thicknessrequired 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 auniform optimum adhesive thickness (0.83mm) between the bracket and enamel.
Adhesive thickness between bracket base and tooth surface decreases withincreasing the amount of force application from 1ounce to 3 ounces.Mean shear bond strength increases when adhesive thickness decreases fromgroup A to Group B and then it has a tendency to decrease as shown in groupC.0.83mm could be considered as optimum adhesive thickness which is required toachieve sufficient bond strength to prevent chances of bond failure.Conclusion
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