The independent sample t test showed a statistically significant difference (P <
.001) between flowable and nonflow-able groups in both the buccal and lin-gual cusps. This finding led to a rejection of the null hypothesis. Specimens with flowable composite resin had higher cuspal deflection values than speci-mens without flowable composite resin.
The results of the statistical analyses are shown in Tables 2 and 3. Figure 4 shows an example of the output curves.
The mean cuspal deflection values obtained from the buccal cusp for flowable composite resin groups were 41.91 μm/m for Tetric EvoCeram and 44.59 μm/m for Grandio. The mean cuspal deflection values of the non-flowable groups were 32.64 μm/m and 39.40 μm/m for Tetric EvoCeram and Grandio, respectively. This showed
Fig 3 Graphical user interface of the Strain Indicator Program (SIProg).
Title bar
Menu bar
Display area
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that groups without flowable composite resin exhibited less cuspal deflection than groups with flowable composite resin (Table 2).
The mean cuspal deflection val-ues obtained from the lingual cusp for flowable composite resin groups were 22.24 μm/m for Tetric EvoCeram and 21.73 μm/m for Grandio. The mean cus-pal deflection values of the nonflowable groups were 16.02 μm/m and 16.51 μm/m for Tetric EvoCeram and Grandio, re-spectively. This showed that groups without flowable composite resin exhib-ited less cuspal deflection than groups with flowable composite resin (Table 3).
This study used two commercial nanohybrid composite resin systems, each with its own flowable composite
resin. This was done to exclude the manufacturing factor and to avoid bias toward one brand. The width of the buc-colingual cavity was slightly larger than that of an ideal clinical situation. This was selected to decrease the remaining buccal and lingual walls and increase the sensitivity of the strain gauge. The selection of a nanofilled universal two-step etch-and-rinse adhesive system was based on previous studies showing that this system is the gold standard for bonding with dentin. The advantage of the new computerized measuring sys-tem used in this study was that it provid-ed accurate details about the process of deflection in a curve format consist-ing of peaks and valleys, which will be further explained in future studies. The Table 2 Comparison (t test) of cuspal deflection in specimens with
and without flowable composite resin (buccal cusp)
P SE SD Mean
SE = standard error; SD = standard deviation.
*Significantly different at P < .05.
Table 3 Comparison (t test) of cuspal deflection in specimens with and without flowable composite resin (lingual cusp)
P SE SD Mean
SE = standard error; SD = standard deviation.
*Significantly different at P < .05.
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accuracy of measurements even when based only on deflection range values (difference between the postcuring and precuring values) is more reliable than with conventional measuring systems because the data are automatically cal-culated by SIProg. This new measuring system also provides a large amount of data storage and easy recall.
The results showed that the highest levels of strain were produced during exposure of the restoration to the light source for polymerization. The high stresses associated with the applica-tion of flowable composite resin may be explained by the material’s low fill-er content and high resinous content, which increases the polymerization shrinkage.9,32–35 The strain levels did not decrease during placement of the restoration, and any stress relaxation resulting from the flow of the material was not sufficient to overcome the poly-merization shrinkage.
The results support the findings of previous studies. One study suggested
that flowable composite resins shrink more than conventional composite res-ins, creating more stress on the bond-ing agent durbond-ing curbond-ing and possibly allowing for premature deflection of the overlaying conventional composite resin. The authors attributed this to the difference in flexural modulus caused by the changes in filler content.33 An-other study disagreed with the concept that shrinkage stress generated by a subsequent layer of higher modulus composite resin could be absorbed by an elastic intermediary layer.17 A review article on polymerization shrink-age noted that flowable composite resins produced stress levels similar to those of nonflowable materials.36 In addition, an in vitro study concluded that the use of flowable materials does not lead to marked stress reduction and increases the risk of debonding at the adhesive interface as a result of polymerization shrinkage.37 Oliveira et al38 strongly confirmed that using flow-able composite resin as a liner or base
Fig 4 Sample of an output curve drawn using SIProg.
3630
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material under a composite resin resto-ration increases polymerization shrink-age stresses at the adhesive interface, potentially leading to adhesive failure.
Chuang et al39 concluded that the use of flowable composite resin lining may aggravate cusp flexure.
On the other hand, some studies did not support the current results. One study showed that the use of composite resin liners with a low elastic modulus was a satisfactory technique for partial absorption of the stress generated by polymerization shrinkage; however, this study was performed using composite resin blocks, which were not inserted in prepared cavities.40 Another study supported the inverse relationship be-tween filler percentage and shrinkage strain, which was explained by the cor-responding decrease in the volume fraction of monomers present to un-dergo polymerization. The conclusion of this study should not be generalized, however, because it showed bias to-ward two types of flowable composite resins and ultimately stated that these types still suffered from polymerization shrinkage.41 Finally, a study introduc-ing a new method of measurintroduc-ing cuspal deflection reported that polymerization shrinkage tended to decrease as filler content increased. This study used a noncontact cuspal deflection measur-ing method based on a laser displace-ment sensor; unfortunately, the validity of noncontact measurements is still un-der investigation.22
Based on the results of this study, the null hypothesis that application of flowable composite resin does not in-crease cuspal deflection of MOD
com-posite resin restorations was rejected.
Further, the validity of the new comput-erized modification of the strain gauge method was proven by the agreement of the results with those of other cuspal deflection studies.
CONCLUSIONS
Under the conditions of this study, it was concluded that the application of flowable composite resin at the inter-nal cavity line angles increased cus-pal deflection, possibly due to the material’s high volumetric shrinkage, which exerts more stress at the tooth-restoration interface. The limitation of the new measuring system used was the need to obtain two symmetric buc-cal and lingual channels to be able to judge whether the deflection occurs simultaneously in both cusps or in one cusp more than the other. This limita-tion can now be addressed by the en-gineering team of this study.
The use of flowable composite resin is not preferred in MOD cavities be-cause it increases internal polymeri-zation stress, which leads to greater cuspal deflection. Further investigation is needed to carefully study the SIProg output curves.
ACKNOWLEDGMENTS
The authors express their deepest gratitude and appre-ciation to Dr Geoff Smith, Center for Applied English Studies, The University of Hong Kong, for his work in revising the article’s language. They also thank VOCO for supplying the Grandio and Grandio Flow materials.
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REFERENCES
1. Jokstad A, Mjör IA. Analyses of long-term clinical behavior of class-II amalgam restorations.
Acta Odontol Scand 1991;49:
47–63.
2. Marchesi G, Breschi L, Antoniolli F, Di Lenarda R, Ferracane J, Cadenaro M.
Contraction stress of low-shrinkage composite materials assessed with different testing systems. Dent Mater 2010;26:
947–953.
3. Ozgünaltay G, Görücü J.
Fracture resistance of class II packable composite restora-tions with and without flowable liners. J Oral Rehabil 2005;32:
111–115.
4. Eick JD, Welch FH. Polymeri-zation shrinkage of posterior composite resins and its possible influence on postop-erative sensitivity. Quintes-sence Int 1986;17:103–111.
5. Li QS, Jepsen S, Albers HK, Eberhard J. Flowable materials as an intermediate layer could improve the marginal and internal adaptation of composite restorations in Class-V-cavities.
Dent Mater 2006;22:250–257.
6 McCullock AJ, Smith BG. In vitro studies of cusp reinforce-ment with adhesive restorative material. Br Dent J 1986;161:
450–452.
7. Suliman AH, Boyer DB, Lakes RS. Polymerization shrinkage of composite resins: Compari-son with tooth deformation. J Prosthet Dent 1994;71:7–12.
8. Van Ende A, De Munck J, Mine A, Lambrechts P, Van Meer-beek B. Does a low-shrinking composite induce less stress at the adhesive interface? Dent Mater 2010;26:215–222.
9. Labella R, Lambrechts P, Van Meerbeek B, Vanherle G.
Polymerization shrinkage and elasticity of flowable compos-ites and filled adhesives. Dent Mater 1999;15:128–137.
10. Lee IB, Min SH, Kim SY, Ferracane J. Slumping tendency and rheological prop-erties of flowable composites.
Dent Mater 2010;26:443–448.
11. Beun S, Bailly C, Dabin A, Vreven J, Devaux J, Leloup G.
Rheological properties of experimental Bis-GMA/
TEGDMA flowable resin composites with various macrofiller/microfiller ratio.
Dent Mater 2009;25:198–205.
12. Lindberg A, van Dijken JMV, Hörstedt P. In vivo interfacial adaptation of class II resin composite restorations with and without a flowable resin composite liner. Clin Oral Investig 2005;9:77–83.
13. Roberson TM, Heymann H, Swift EJ, Sturdevant CM.
Sturdevant’s Art and Science of Operative Dentistry, ed 5. St Louis: Elsevier Mosby, 2006.
14. Summitt JB. Fundamentals of Operative Dentistry: A Contem-porary Approach, ed 3.
Chicago: Quintessence, 2006.
15. Stefanski S, van Dijken JW.
Clinical performance of a nanofilled resin composite with and without an intermediary layer of flowable composite: A 2-year evaluation [epub ahead of print 23 Nov 2010]. Clin Oral Investig.
16. Gallo JR, Burgess JO, Ripps AH, et al. Three-year clinical evaluation of two flowable composites. Quintessence Int 2010;41:497–503.
17. Unterbrink GL, Liebenberg WH.
Flowable resin composites as
“filled adhesives”: Literature review and clinical recommen-dations. Quintessence Int 1999;30:249–257.
18. Lee MR, Cho BH, Son HH, Um CM, Lee IB. Influence of cavity dimension and restoration methods on the cusp deflec-tion of premolars in composite restoration. Dent Mater 2007;
23:288–295.
19. Segura A, Donly KJ. In vitro posterior composite polymeri-zation recovery following hygroscopic expansion. J Oral Rehabil 1993;20:495–499.
20. Alomari QD, Reinhardt JW, Boyer DB. Effect of liners on cusp deflection and gap formation in composite restorations. Oper Dent 2001;
26:406–411.
21. Suliman AA, Boyer DB, Lakes RS. Cusp movement in premolars resulting from composite polymerization shrinkage. Dent Mater 1993;
9:6–10.
22. Miyasaka T, Okamura H.
Dimensional change measure-ments of conventional and flowable composite resins using a laser displacement sensor. Dent Mater J 2009;
28:544–551.
23. Sun J, Lin-Gibson S. X-ray microcomputed tomography for measuring polymerization shrinkage of polymeric dental composites. Dent Mater 2008;
24:228–234.
24. Meredith N, Setchell DJ. In vitro measurement of cuspal strain and displacement in composite restored teeth.
J Dent 1997;25:331–337.
25. Suliman AA, Boyer DB, Lakes RS. Interferometric measure-ments of cusp deformation of teeth restored with compos-ites. J Dent Res 1993;72:
1532–1536.
26. Jantarat J, Panitvisai P, Palamara JE, Messer HH.
Comparison of methods for measuring cuspal deformation in teeth. J Dent 2001;29:75–82.
27. Li JY, Fok ASL, Satterthwaite J, Watts DC. Measurement of the full-field polymerization shrinkage and depth of cure of dental composites using digital image correlation. Dent Mater 2009;25:582–588.
28. Pearson GJ, Hegarty SM.
Cusp movement of molar teeth with composite filling materials in conventional and modified MOD cavities. Br Dent J 1989;
166:162–165.
29. Pearson GJ, Hegarty SM.
Cusp movement in molar teeth using dentine adhesives and composite filling materials.
Biomaterials 1987;8:473–476.
30. Dietschi D, Olsburgh S, Krejci I, Davidson C. In vitro evalua-tion of marginal and internal adaptation after occlusal stressing of indirect class II composite restorations with different resinous bases. Eur J Oral Sci 2003;111:73–80.
HAMAMA ET AL
investigation of the effects of glass inserts on the effective composite resin polymerization shrinkage. J Dent Res1989;68:1234–1237.
32. Alvarez-Gayosso C, Barceló-Santana F, Guerrero-Ibarra J, Sáez-Espínola G, Canseco-Martínez MA. Calculation of contraction rates due to shrinkage in light-cured composites. Dent Mater 2004;20:228–235.
33. Bayne SC, Thompson JY, Swift EJ Jr, Stamatiades P, Wilkerson M. A characterization of first-generation flowable composites. J Am Dent Assoc 1998;129:567–577.
34. Kleverlaan CJ, Feilzer AJ.
Polymerization shrinkage and contraction stress of dental resin composites. Dent Mater 2005;21:1150–1157.
35. Stansbury JW, Trujillo-Lemon M, Lu H, Ding X, Lin Y, Ge J.
Conversion-dependent shrinkage stress and strain in dental resins and composites.
Dent Mater 2005;21:56–67.
36. Braga RR, Ferracane JL.
Alternatives in polymerization contraction stress manage-ment. Crit Rev Oral Biol Med 2004;15:176–184.
37. Cadenaro M, Marchesi G, Antoniolli F, Davidson C, Dorigo ED, Breschi L. Flowabil-ity of composites is no guarantee for contraction stress reduction. Dent Mater 2009;25:649–654.
38. Oliveira LCA, Duarte S Jr, Araujo CA, Abrahão A. Effect of low-elastic modulus liner and base as stress-absorbing layer in composite resin restorations.
Dent Mater 2010;26:
e159–e169.
39. Chuang SF, Chang CH, Chen TY. Spatially resolved assess-ments of composite shrinkage in MOD restorations using a digital-image-correlation technique. Dent Mater 2011;
27:134–143.
40. Cunha LG, Alonso RC, Sobrinho LC, Sinhoreti MA.
Effect of resin liners and photoactivation methods on the shrinkage stress of a resin composite. J Esthet Restor Dent 2006;18:29–36.
41. Baroudi K, Saleh AM, Silikas N, Watts DC. Shrinkage behaviour of flowable resin-composites related to conversion and filler-fraction. J Dent 2007;35:
651-655.
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