Shrinkage assessment of low shrinkage composites using micro-computed tomography Ronaldo Hirata,1 Emanuele Clozza,2 Marcelo Giannini,3 Ehsan Farrokhmanesh,1 Malvin Janal,1 Nick Tovar,1 Estevam A. Bonfante,4 Paulo G. Coelho1,2 1
Department Department 3 Department 4 Department 2
of of of of
Biomaterials and Biomimetics, New York University College of Dentistry, New York Periodontology and Implant Dentistry, New York University College of Dentistry, New York Restorative Dentistry, State University of Campinas, Piracicaba Dental School, Piracicaba, SP, Brazil Prosthodontics, University of Sao Paulo - Bauru College of Dentistry, Bauru, SP, Brazil
Received 6 March 2014; revised 3 June 2014; accepted 29 June 2014 Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.33258 Abstract: Objectives: The aim of this study was to quantify the polymerization volumetric shrinkage of one regular and two low shrinkage bulk fill composites in class I cavities with or without an adhesive layer, using three-dimensional (3D) micro-computed tomography (lCT). Methods: Class I cavity preparations (2.5 mm depth 3 4 mm length 3 4 mm wide) were standardized in 36 extracted human third molars, which were randomly divided in six groups (n 5 6 each) as follows: Group VIT (regular composite without bonding agent); Group SDR (low shrinkage flowable composite without bonding agent); Group TET (low shrinkage composite without bonding agent); Group VIT/P (regular composite with bonding agent); Group SDR/X (low shrinkage flowable composite with bonding agent); TET/T (low shrinkage composite with bonding agent). Each tooth was scanned via mCT at cavity preparation, immediately after
cavity filling, and after light-curing. Acquired lCT data were imported into Amira software for analysis and volume values evaluated between steps from cavity preparation until light-curing. Results: Both low shrinkage composites showed a significantly less volumetric shrinkage than VIT. The use of dental adhesive significantly decreased the average volumetric contraction similarly for the three composites, by about 20%. Conclusion: Both low shrinkage composites showed less volumetric polymerization contraction than the regular composite. The use of dental adhesive decreased the total volumetric shrinkage for all evaluated C 2014 Wiley Periodicals, Inc. J Biomed Mater Res composites. V Part B: Appl Biomater 00B: 000–000, 2014.
Key Words: polymerization, x-ray micro-computed tomography, composite resins, volume shrinkage, 3D imaging
How to cite this article: Hirata R, Clozza E, Giannini M, Farrokhomanesh E, Janal M, Tovar N, Bonfante EA, Coelho PG. 2014. Shrinkage assessment of low shrinkage composites using micro-computed tomography. J Biomed Mater Res Part B 2014:00B:000–000.
INTRODUCTION
Restorative practice currently fosters an increase in the development and use of dental materials other than amalgam, such as resin composites, because of a legal restrain treaty on mercury-based products recently announced by the United Nations Environment Programme.1,2 Despite the excellent esthetic results achieved with composite resins, an inherent concern still persists because of light curing polymerization shrinkage stress that can be high enough to debond adhesive interfaces.3,4 Potential clinical implications are marginal infiltration, post operative sensitivity, dental microcracking, and a decrease in adhesive bond strength.5 The composite resins contain different monomers and fillers, which directly influence polymerization shrinkage. Contraction stress produced during the polymerization process is acknowledged as a multifaceted phenomenon which
is not only a result of the composite shrinkage process alone, but also is related to its formulation, degree of conversion, polymerization kinetics, confinement conditions, viscoelastic behavior, and elastic modulus.6,7 The elastic modulus is crucial to predict the contraction stress potential that such resin materials can generate at the bonded interface.8–10 The lower the Young’s modulus, more reduction on internal stresses during polymerization shrinkage can be expected compared to a higher modulus composite.10 In addition, patient related variables such as cavity configuration factor (C-factor), known as the ratio between bonded and unbonded surfaces of the composite restoration, play a major role in class I cavities, for instance, where five bonded cavity walls and only the occlusal unbonded wall for polymerization stress relief poses a taxing shrinkage scenario.4,11
Correspondence to: R. Hirata; e-mail: [email protected] Contract grant sponsor: CNPq; contract grant number: 305777-2010-6 Contract grant sponsor: Department of Biomaterials and Biomimetics at NYUCD
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TABLE I. Composition and Lot Number of the Tested Materials Type of Material Vitalescence (regular composite) SureFil SDR Flow (bulk fill composite)
Tetric N-ceram Bulk Fill Peak Universal Bond (adhesive) XP Bond (adhesive)
Tetric N-Bond Self-Etch (adhesive)
Composition
Batch Number
Bisphenol A glycidyl methacrylate, fillers (75 weight percent or 58 volume percent) with average size of 0.7 mm. Barium-alumino-fluoro-borosilicate glass, strontium alumino-fluoro-silicate glass, modified urethane dimethacrylate resin, ethoxylated bisphenol A dimethacrylate, triethyleneglycol dimethacrylate, camphorquinone, photo-accelerator, butylated hydroxyl toluene; UV stabilizer; titanium dioxide; iron oxide pigments, fluorescing agent. Dimethacrylates, Prepolymers, barium glass filler, ytterbium trifluoride, mixed oxide, additive, initiators, stabilizers, pigments 2-Hydroxyethyl methacrylate, methacrylic acid dehydrated alcohol, chlorhexidine diacetate. Carboxylic acid modified dimethacrylate, phosphoric acid modified acrylate resin, urethane dimethacrylate, triethylene glycol dimethacrylate, 2hydroxy ethyl methacrylate, butylated benzenediol, ethyl-4-dimethyl aminobenzoate, camphorquinone, functionalised amorphous silica, tbutanol. Bis-acrylamide derivative, Bis-methacrylamide dihydrogenphosphate, amino acid acrylamide, hydroxyalkyl methacrylamide, nano-fillers, water, initiators and stabilizers
BS96Z
Because polymers in general may have their composition modified to tailor specific demands, understanding the impact of such alterations on shrinkage by means of an accurate and reliable method able to quantify and image volume changes is of major interest. To date, different methods to measure shrinkage have been described. Historically, a variety of substrates and composite restorations interface have been evaluated by dye penetration tests,12 which in fields such as endodontics have been strongly discouraged because evidence questioning its scientific relevance has been raised.13 Besides, this method is destructive and in tandem with marginal gap evaluations, have been regarded to deliver inaccurate predictions of clinical performance.14 The use of resin replicas has presented as an alternative technique to identify marginal failures on the toothrestoration interface in vivo.15 Other conventional methods to measure the polymerization shrinkage of composite resins have been described, as the use of optical instruments to determine the linear polymerization,16–18 fiber-optic sensing method based on a Fizeau-type interferometric scheme,19 mercury dilatometer,20 electromagnetic balance with a force transducer with a photo detector,21 and videoimaging device (AccuVol; Bisco) in single-view mode.8,22 More recent non-destructive methods include the use of finite element analysis,23 and the use of micro-computed tomography (mCT) generating 2D and 3D images and data,24–28 providing the possibility to analyze the material behavior inside a given cavity configuration. Recently, companies have marketed some composite materials as low-shrinkage stress, where their formulation potentially allows them to be placed as a flowable base or in bulk-fill of up to 4 mm thickness.29,30 However, bulk-fill composites should not be generically claimed as low-shrinkage stress and it has been advised that clinicians should be cautious when placing them in high C-factor cavity preparations.30,31 Therefore, quantification of their volume changes relative to conventional composites is warranted.
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110527
R52450 J084 1104001218
R59912
The aim of this study was to quantify the polymerization volumetric shrinkage of one regular and two low shrinkage bulk fill composites in class I cavities with or without an adhesive layer, using three-dimensional (3D) microcomputed tomography (lCT). The research hypotheses were that: 1. The tested regular composite would present higher volumetric shrinkage than the low shrinkage bulk filled ones. 2. The mCT method is able to detect the effect of adhesive application on polymerization volume shrinkage.
MATERIALS AND METHODS
Specimen preparation Thirty-six recently extracted human third molars were collected under an Institutional Review Board protocol approved by the NYU Medical School. Standardized box-shaped class I cavities (2.5 mm depth 3 4 mm length 3 4 mm wide) were prepared in each teeth. The cavities were prepared using a diamond bur (AD20 Occlusal Reduction Bur, Strauss/Code 845-022), which presents a standardized active head size and a vertical stop to deliver consistent cavity preparation depth. The final cavity preparation was then checked with a digital caliper. All teeth were maintained in distilled water at room temperature (25 C) before and after preparation procedures. The teeth were allocated to six groups (n 5 6 each), as follows: Group VIT, (unbonded, regular composite/Vitalescence, Ultradent Product, South Jordan, UT); Group SDR (unbonded, low shrinkage flowable composite/SureFil SDR Flow, Dentsply Caulk, Milford, DE); Group TET (unbonded low shrinkage composite/Tetric Bulk fill, Ivoclar Vivadent, Schaan, Lichtenstein); Group VIT/P (regular composite with dental adhesive/Peak Universal Bond, Ultradent Product); Group SDR/X (low shrinkage flowable composite with dental adhesive/XP Bond, Dentsply Caulk, Milford, DE); and
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FIGURE 1. Flow chart describing shrinkage quantification. The single letters within parentheses indicate the outcome of the performed steps. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Group TET/T (low shrinkage composite with dental adhesive/Tetric N-bond Self-etch pen, Ivoclar Vivadent, Schaan, Lichtenstein). Bonding agents were used according to respective manufacturer’s protocols. Table I depicts the composition and lot number for materials tested. Filling procedures All teeth underwent mCT analysis following cavity preparation for a first scanning. In the bonded groups, with exception of TET/T, the cavity (enamel and dentin) was etched with phosphoric acid (Ultra etch 35%, Ultradent Product) for 15 s and subsequently rinsed for 20 s. Excess water was removed with thin absorbent paper. For the VIT/P group, a layer of the above mentioned one bottle primer/adhesive system was applied in the cavity with a microbrush and light-cured for
20 s, as recommended by the manufacturer. In the group SDR/X, the selected one-bottle primer/adhesive system was applied following the manufacturer’s instructions. For the group TET/T, no etching was performed because the Tetric N-bond Self-Etch pen (Ivoclar Vivadent, Schaan, Lichtenstein) is a self-etching system, and adhesive application also followed the manufacturer’s recommendations. In all groups, the cavities were filled in bulk using their assigned composites and air-dried for 5 s. Samples were immediately protected from any light sources in dark plastic vials and placed in the mCT imaging chamber for a scan and volume quantification before light curing. Then composites were light cured for 40 s with a LED unit (800 mW/cm2, Ultra Lume 5, Ultradent Product) and inserted into the holder for the third mCT scanning and measurement. The
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(ANOVA). Post-hoc comparisons were carried out using 95% confidence intervals based on the pooled estimate of residual variability. RESULTS
FIGURE 2. Volume of shrinkage (%) among all groups. Error bars reflects the 95% confidence limits. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
polymerization unit step-over distance was kept constant at approximately 5 mm. mCT analysis The flow chart in Figure 1 illustrates how the experiment was conducted. Each tooth was scanned using a mCT apparatus (mCT40, Scanco Medical, AG, Basserdorf, Switzerland) after initial cavity preparation, before light-curing polymerization, and after light-curing polymerization.32 The mCT was calibrated using a phantom standard at 70 Kvp/BH 200 mgHA/ccm. The operating condition for the mCT device was: energy (70 Kvp–114 microamperes) with a medium resolution (16 mm/slice) using a sample holder of 16.5 mm. The average of the total number of slices was 250, with an average scan time of 28 min. Acquired mCT data were imported into a workstation and evaluated with Amira software (version 5.5.2, VSG, Burlington, MA). The measurement of the cavity is required to superimpose the two subsequent scans (before and after light-curing) with the Amira software tool called “superimposition”, providing a perfect arrangement of the three images, and exempting the presence of a reference mark in the stub during mCT scanning. This procedure avoids the scattering and possible noise between the restorative material and the tooth structure due its similar radiondensity. To minimize possible artifacts in the threshold segmentation, the following steps were undertaken: lCT data at cavity preparation were registered with lCT data obtained respectively before and after lightcure polymerization. Next, the two registered images were subjected to Boolean operation (registered lCT data minus lCT data at cavity preparation) to subtract the tooth from the composite restoration. This step sequence enabled isolation and quantification of the composite restoration’s volume. The cropped volume was automatically labeled with the “segmentation editor” command and subsequently reconstructed in a 3D manner. The “Material Statistics” command computed the volume changes. From these measurements, the volumetric loss following polymerization shrinkage was calculated as percentages. Statistical analysis Data were analyzed using a two-way (three levels of composite and two levels of adhesive) analysis of variance
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Figure 2 shows the mean (95% CL) level of volumetric shrinkage in each of the six treatment groups. Inspection suggests more shrinkage in the VITA composite then either of the two low-shrinkage composites, and that each composite shrank less when used with adhesive. ANOVA supported these observations, showing significant differences (p < 0.001) among composites and between adhesive treatments (p 5 0.02) (Table II). Post-hoc testing then showed more shrinkage in VITA [M(SD)5 3.5% (0.8)] than either SDR [M(SD)5 1.7% (0.7)] or TET [M(SD)5 2.0% (0.3)], both p < 0.05; and more shrinkage without adhesive [M(SD)5 2.7% (1.0)] than with adhesive [M(SD) 5 2.2% (1.0)]. Although there was less of an adhesive effect for TET than the other composites (25.0% for SDR, 18.4% for VIT, and 13.6% for the TET groups), analysis failed to support an interaction (p 5 0.60). Thus, use of the lower shrinkage composites reduced shrinkage by about 50%, and adhesive reduced shrinkage by about 20%; both effects were statistically significant. Qualitative 2D analysis of polymerization shrinkage for the VIT group resulted in gaps along the pulpal floor and parts of the axial walls. For the SDR as for the TET groups, smaller gaps along the cavity floor were found. Analysis of the bonded groups showed the same pattern described for the unbonded samples, however with fewer gaps (Figures 3 and 4). The 3D reconstructions (Figures 5 and 6), showed volume shrinkage mainly at the occlusal surface and some parts of the cavity floor for VIT group. The SDR group presented most of the volume rendering at the cavity floor, but also at the occlusal surface. The TET group showed the least amount of volumetric changes and these were mainly detected at the occlusal surface. Amongst the bonded groups, the VIT/P presented a decrease in volume shrinkage at the pulpal floor, and such a decrease was even more pronounced in the SDR/ X group. Images and shrinkage amount rendered for the TET/T group was very similar when compared to the TET. DISCUSSION
It is acknowledged that polymerization stress and shrinkage strongly depend, among other factors, on the C-factor of the cavity.33 In our study, the rationale for selecting a class I cavity configuration was to provide one of the most challenging TABLE II. Polymerization Shrinkage (n 5 6) Determined for the Composite Resins With or Without Adhesive Reported as % (SE) Using mCT Composite Resin Vitalescence SureFil SDR Tetric bulk fill
Bonded Groups
Unbonded Groups
3.1 (0.33) A a 1.5 (0.36) B b 1.9 (0.1) B b
3.8 (0.25) B a 2.0 (0.19) B b 2.2 (0.1) B b
Uppercase letters compare values (with or without adhesive) within the same row (composite resin). Lowercase letters compare values (between composite resins) within the same column.
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FIGURE 3. Figure representing the 2D sections of groups without adhesive (A) VIT before curing; (B) VIT after curing; (C) SDR before curing; (D) SDR after curing; (E) TET before curing; (F) TET after curing. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
shrinkage scenarios among cavity designs.34 The diamond burs used for cavity preparation presented cutting tips that served as a guiding reference during preparation for standardized cavity depth. Because of natural teeth variability and differences in occlusal anatomy, some areas of the cavity floors still presented enamel after preparation. Because samples were randomly assigned to different groups, bias originating from substrate variation was minimized. All composite materials used in this study were placed in bulk (one increment) in an attempt to evaluate the composite volumetric shrinkage and the effect of bonding procedures on polymerization shrinkage. Both of our postulated research hypotheses that stated that regular composite would present higher volumetric shrinkage than low shrinkage bulk fill restorative materials, and that the mCT method would be able to detect the effect of adhesive application on polymerization volume shrinkage were accepted. Potential composite dimensional alterations during scanning were not considered and warrant further investigation. It has been previously suggested that the lower polymerization shrinkage observed for the TET and SDR composite relative to the VIT composite is related to TET and
SDR chemical formulation.29,30,35 VIT is a regular composite, Bis-GMA-based material and contains 75% weight percent or 58% volume of fillers. The volumetric shrinkage of VIT/P group was 3.1% and for the VIT it was 3.8%. This volumetric shrinkage is a common value for methacrylate and dimethacrylate-based composites.17,21,22,24–26 On the other hand, the polymerization shrinkage for the SDR group ranged from 1.5 to 2.0%, with or without adhesive application, respectively. The results obtained in the present study are lower than previously reported where a Kaman Linometer was used for shrinkage evaluation (determined to be at 3.5%).18 The discrepancy between results likely arose from the fact that in the present study the composite was placed in situ in a class I cavity preparation, suggesting that general comparisons on shrinkage even between the same brand of composites, not accounting for methodological differences, should be made with caution,17 even when using different softwares with lCT.24 Volumetric shrinkage evaluation of several composites has shown, for instance, a 1–3% range for packable and up to 6% for flowable composites, which is in contrast to our findings.9 The SureFil SDR low shrinkage composite is composed by a
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FIGURE 4. Figure representing the 2D sections of groups with adhesive (A) VIT/P before curing; (B) VIT/P after curing; (C) SDR/X before curing; (D) SDR/X after curing; (E) TET/T before curing; (F) TET/T after curing. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
modified urethane dimethacrylate resin and ethoxylated bisphenol A dimethacrylate as main monomers. Lower volumetric shrinkage is due in part to the larger molecular weight of initial monomers compared to conventional methacrylated-based resin systems.35,36 The molecular weight of modified urethane dimethacrylate resin is 849 g/ mol, whereas Bis-GMA presents 513 g/mol. In addition, previous studies suggest that photoinitiators incorporated into urethane-based methacrylate resins facilitate free-radical polymerization decreasing the overall curing stress,37 thereby decreasing volumetric shrinkage. The observed volumetric shrinkage for TET and TET/T groups ranged from 1.9 to 2.2%, respectively. To the best of our knowledge, no volumetric shrinkage measurements are yet available in the literature even though the manufacturer’s materials specification sheet reports values at 2%,29 which is in close agreement with our results. Different than the modified UEDMA approach above mentioned to reduce polymerization shrinkage, Tetric N-ceram bulk composite uses a new germanium-based photoinitiator to tailor polymerization kinetics and a pre-polymer stress reliever that supposedly decrease polymerization stress and shrinkage.29,38 The evaluation of the shrinkage without adhesive application, although not clinically relevant, allowed the observation
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of the immediate role of adhesives in counteracting composite polymerization shrinkage in standardized cavities.39 Such an approach has been previously used for even more challenging C-factor scenarios, such as in bonding posts to root canals, where it was shown that resin cement shrinkage stresses are so high that push-out bond strength measurements have resulted in no differences when an adhesive system is or is not used, suggesting that friction seems to be more relevant than adhesion as a retention mechanism in this particular scenario.40,41 On the other hand, the clinically relevant measurement, where an adhesive was applied to the tooth substrate before restoration fabrication, did allow for a quantitative observation of how bonding between composite and tooth structure decreased the overall volumetric shrinkage of all composites.32 For bonded restorations, the presence of gaps decreased for VIT and SDR groups, whereas the volumetric shrinkage decreased for all bonded groups. The mCT has been previously regarded as a key tool to three-dimensionally quantify and characterize the polymerization shrinkage of composite in cavities.24,25,27,32 The accuracy of the method allows the measurement of current failures on the composite insertion like bubbles,27 whereas traditional volumetric shrinkage methods do not account for. One current limitation of the method is that materials without sufficient radiopacity, and without or with low filler
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FIGURE 5. Figure representing the 3D renderings of groups without adhesive (A) VIT shrinkage and composite filling; (B) VIT shrinkage; (C) SDR shrinkage and composite filling; (D) SDR shrinkage; (E) TET shrinkage and composite filling; (F) TET shrinkage. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
content, such as dental adhesives, are difficult to resolve through software reconstruction.32,42 In the present study, the 2D location of marginal gaps partially represent the overall shrinkage scenarios presented in 3D rendering.25 Gaps were generated by the stress and dynamics of the volumetric shrinkage inside the cavity. It reinforces the importance of 3D analysis instead of 2D methods.13,14 In the 2D analysis, polymerization shrinkage of the regular methacrylated-based material either bonded or unbonded resulted in gaps along the pulpal floor and parts of the axial walls. Although gaps were also noted for bonded samples, they were reduced along the cavity floor. The absence of opening occlusal margins even in the non bonded samples was found in other studies, and was explained by the wetting capacity of the composite before curing promoting reduced wetting on the enamel margins.39,43 The 3D images of the VIT and VIT/P showed shrinkage located at the occlusal surface and some parts of the pulpal floor; even with the bonding procedure the shrinkage mainly in the occlusal surface
was sustained.44 Previous studies have depicted gap formations in areas similar to the ones observed in the present study, supporting shrinkage vectors pointing towards the restoration occlusal surface resulting in restoration debonding from the pulpal walls.25,32,44–46 Another potential explanation for this deformation is that the occlusal outer area of the cavity is closer to the light tip and consequently polymerization may take place faster than at the cavity floor.42,47,48 Furthermore, the viscous flow characteristic of composite resins prior the pre-gel phase cause a slight concavity on the occlusal surface.32,44 The SDR groups presented the same contraction pattern observed for the VIT group. However, the use of the adhesive XP Bond significantly decreased shrinkage along the pulpal floor as observed in the 3D reconstruction. We speculate that because these materials are flowable composites presenting higher viscoelasticity relative to conventional composites, they may better dissipate polymerization shrinkage stress when bonded to the cavity walls reducing
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FIGURE 6. Figure representing the 3D renderings of groups with adhesive (A) VIT/P shrinkage and composite filling; (B) VIT/P shrinkage; (C) SDR/X shrinkage and composite filling; (D) SDR/X shrinkage; (E) TET/T shrinkage and composite filling; (F) TET/T shrinkage. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
the gaps around the restoration surfaces. According to Versluis et al.,45 for a well-bonded composite restoration, the shrinkage vectors are oriented down and toward the bonded margins, which may explain the misfit of the flowable composite (SDR/X group) and its volume reduced at the occlusal surface24 observed in the 3D reconstruction.43 Another explanation of the influence of the adhesive in this specific material is the delay of the gel point resulting in a slow polymerization rate of the flowable composite compared to a paste, thereby allowing deformation without overstressing areas near the cavity walls.33,49 Such theory may be supported by a study that showed that SDR low shrinkage composite presented a delayed gelation point compared to regular composite and conventional flowable composites.35 In the 2D analysis the gaps were mainly at the pulpal floor. Finally, the Tetric N-ceram low shrinkage bulk fill material presented shrinkage deformation, more restricted more to the occlusal surface, with and without the bonding agent. The 2D slices showed that gaps formed at the pulpal floor.
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Further studies considering cavity size and design are warranted for appropriate mapping and quantification of different resin composite based restorative materials. Further quantitative analysis and correlation of the 2D gaps and 3D shrinkage reconstruction are fundamental to predict the relation between volumetric shrinkage and gap formation.
CONCLUSION
Based on the results of this research, it can be concluded that: 1. The low shrinkage composites showed less volumetric polymerization shrinkage than regular resin composite. 2. The use of dental adhesive decreased the overall volumetric shrinkage of all evaluated composites. 3. Micro-computed tomography 3D reconstruction allowed quantification and realistic imaging of volumetric shrinkage changes within a restoration
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REFERENCES 1. Kessler R. The Minamata Convention on Mercury: A first step toward protecting future generations. Environ Health Perspect 2013;121:A304–A309. 2. Rekow ED, Fox CH, Petersen PE, Watson T. Innovations in materials for direct restorations: Why do we need innovations? Why is it so hard to capitalize on them? J Dent Res 2013;92:945–947. 3. Carvalho RM, Pereira JC, Yoshiyama M, Pashley DH. A review of polymerization contraction: The influence of stress development versus stress relief. Oper Dent 1996;21:17–24. 4. Feilzer AJ, De Gee AJ, Davidson CL. Setting stress in composite resin in relation to configuration of the restoration. J Dent Res 1987;66:1636–1639. 5. Hirata R. TIPS: Dicas em Odontologia Estetica. Sao Paulo-Brasil: Artes Medicas; 2010. 6. Chen HY, Manhart J, Hickel R, Kunzelmann KH. Polymerization contraction stress in light-cured packable composite resins. Dent Mater 2001;17:253–259. 7. Bouschlicher MR, Vargas MA, Boyer DB. Effect of composite type, light intensity, configuration factor and laser polymerization on polymerization contraction forces. Am J Dent 1997;10:88–96. 8. Labella R, Lambrechts P, Van Meerbeek B, Vanherle G. Polymerization shrinkage and elasticity of flowable composites and filled adhesives. Dent Mater 1999;15:128–137. 9. Kleverlaan CJ, Feilzer AJ. Polymerization shrinkage and contraction stress of dental resin composites. Dent Mater 2005;21:1150–1157. 10. Braga RR, Ballester RY, Ferracane JL. Factors involved in the development of polymerization shrinkage stress in resin-composites: A systematic review. Dent Mater 2005;21:962–970. 11. Feilzer AJ, De Gee AJ, Davidson CL. Quantitative determination of stress reduction by flow in composite restorations. Dent Mater 1990;6:167–171. 12. Loguercio AD, de Oliveira Bauer JR, Reis A, Grande RH. In vitro microleakage of packable composites in Class II restorations. Quintessence Int 2004;35:29–34. 13. Wu MK, Wesselink PR. Endodontic leakage studies reconsidered. Part I. Methodology, application and relevance. Int Endod J 1993;26:37–43. 14. Heintze SD. Systematic reviews: I. The correlation between laboratory tests on marginal quality and bond strength. II. The correlation between marginal quality and clinical outcome. J Adhes Dent 2007;9:546. 15. Roulet JF, Salchow B, Wald M. Margin analysis of posterior composites in vivo. Dent Mater 1991;7:44–49. 16. Lee IB MS, Seo DG. A new method to measure the polymerization shrinkage kinetics of composites using a particle tracking method with computer vision. Dent Mater 2012;28:212–218. 17. de Melo Monteiro GQ MM, Rolim TV, de Oliveira Mota CC, de Barros Correia Kyotoku B, Gomes AS, de Freitas AZ. Alternative methods for determining shrinkage in restorative resin composites. Dent Mater 2011;27:e176–e185. 18. Garcia D, Yaman P, Dennison J, Neiva G. Polymerization shrinkage and depth of cure of bulk fill flowable composite resins. Oper Dent 2014;39:441–448. 19. Mucci V, Duchowicz R, Cook WD, Vallo C. Influence of thermal expansion on shrinkage during photopolymerization of dental resins based on bis-GMA/TEGDMA. Dent Mater 2009;25:103–114. 20. Eick JD, Kotha SP, Chappelow CC, Kilway KV, Giese GJ, Glaros AG, Pinzino CS. Properties of silorane-based dental resins and composites containing a stress-reducing monomer. Dent Mater 2007;23:1011–1017. 21. Lee IB, Cho BH, Son HH, Um CM. A new method to measure the polymerization shrinkage kinetics of light cured composites. J Oral Rehabil 2005;32:301–314. 22. Arrais CA, Oliveira MT, Mettenburg D, Rueggeberg FA, Giannini M. Silorane- and high filled-based"low-shrinkage" resin composites: Shrinkage, flexural strength and modulus. Braz Oral Res 2013;27:97–102. 23. Li J, Li H, Fok AS, Watts DC. Numerical evaluation of bulk material properties of dental composite using two-phase finite element models. Dent Mater 2012;28:996–1003. 24. Zeiger DN, Sun J, Schumacher GE, Lin-Gibson S. Evaluation of dental composite shrinkage and leakage in extracted teeth using X-ray microcomputed tomography. Dent Mater 2009;25:1213–1220.
25. Sun J, Eidelman N, Lin-Gibson S. 3D mapping of polymerization shrinkage using X-ray micro-computed tomography to predict microleakage. Dent Mater 2009;25:314–320. 26. Sun J, Fang R, Lin N, Eidelman N, Lin-Gibson S. Nondestructive quantification of leakage at the tooth-composite interface and its correlation with material performance parameters. Biomaterials 2009;30:4457–4462. 27. Sun J, Lin-Gibson S. X-ray microcomputed tomography for measuring polymerization shrinkage of polymeric dental composites. Dent Mater 2008;24:228–234. 28. Kim HJ, Park SH. Measurement of the internal adaptation of resin composites using micro-CT and its correlation with polymerization shrinkage. Oper Dent 2014;39:E57–E70. 29. .Tetric N-ceram Bulk Fill: Scientific Documentation. Volume 2012: Ivoclar Vivadent. 30. El-Damanhoury H, Platt J. Polymerization shrinkage stress kinetics and related properties of bulk-fill resin composites. Oper Dent 2014;39:374–382. 31. Van Ende A, Mine A, De Munck J, Poitevin A, Van Meerbeek B. Bonding of low-shrinking composites in high C-factor cavities. J Dent 2012;40:295–303. 32. Chiang YC, Rosch P, Dabanoglu A, Lin CP, Hickel R, Kunzelmann KH. Polymerization composite shrinkage evaluation with 3D deformation analysis from microCT images. Dent Mater 2010;26: 223–231. 33. Feilzer AJ, De Gee AJ, Davidson CL. Quantitative determination of stress reduction by flow in composite restorations. Dent Mater 1990;6:167–171. 34. Van Ende A, De Munck J, Van Landuyt KL, Poitevin A, Peumans M, Van Meerbeek B. Bulk-filling of high C-factor posterior cavities: Effect on adhesion to cavity-bottom dentin. Dent Mater 2013;29:269–277. 35. Ilie N, Hickel R. Investigations on a methacrylate-based flowable composite based on the SDR technology. Dent Mater 2011;27:348–355. es-Salgado NR, Boaro LC, Braga RR. Monomers 36. Gajewski VE1 PC, Fro used in resin composites: Degree of conversion, mechanical properties and water sorption/solubility. Braz Dent J 2012;23:508–514. 37. Jin X, Bertrand S, Hammesfahr PD. New radically polymerizable resins with remarkably low curing stress. J Dent Res 2009; 88(Spec Oss A):1651. 38. Moszner N, Fischer UK, Ganster B, Liska R, Rheinberger V. Benzoyl germanium derivatives as novel visible light photoinitiators for dental materials. Dent Mater 2008;24:901–917. 39. Kakaboura A, Rahiotis C, Watts D, Silikas N, Eliades G. 3D-marginal adaptation versus setting shrinkage in light-cured microhybrid resin composites. Dent Mater 2007;23:272–278. 40. Goracci C, Fabianelli A, Sadek FT, Papacchini F, Tay FR, Ferrari M. The contribution of friction to the dislocation resistance of bonded fiber posts. J Endod 2005;31:608–612. 41. Tay FR, Pashley DH. Monoblocks in root canals: A hypothetical or a tangible goal. J Endod 2007;33:391–398. 42. Papadogiannis D, Kakaboura A, Palaghias G, Eliades G. Setting characteristics and cavity adaptation of low-shrinking resin composites. Dent Mater 2009;25:1509–1516. 43. Chiang YC, Hickel R, Lin CP, Kunzelmann KH. Shrinkage vector determination of dental composite by mu CT images. Compos Sci Technol 2010;70:989–994. 44. Cho E, Sadr A, Inai N, Tagami J. Evaluation of resin composite polymerization by three dimensional micro-CT imaging and nanoindentation. Dent Mater 2011;27:1070–1078. 45. Versluis A, Tantbirojn D, Douglas WH. Do dental composites always shrink toward the light? J Dent Res 1998;77:1435–1445. 46. Souza-Junior EJ, de Souza-Regis MR, Alonso RC, de Freitas AP, Sinhoreti MA, Cunha LG. Effect of the curing method and composite volume on marginal and internal adaptation of composite restoratives. Oper Dent 2011;36:231–238. 47. Sato T, Miyazaki M, Rikuta A. Real-time dimensional change in light-cured composites at various depths using laser speckle contrast analysis. Eur J Oral Sci 2004;112:538–544. 48. Sato T, Miyazaki M, Rikuta A, Kobayashi K. Application of the laser speckle-correlation method for determining the shrinkage vector of a light-cured resin. Dent Mater J 2004;23:284–290. 49. Ilie N HR. Investigations on a methacrylate-based flowable composite based on the SDRTM technology. Dent Mater 2010;27:348–355.
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Department Department 3 Department 4 Department 2
of of of of
Biomaterials and Biomimetics, New York University College of Dentistry, New York Periodontology and Implant Dentistry, New York University College of Dentistry, New York Restorative Dentistry, State University of Campinas, Piracicaba Dental School, Piracicaba, SP, Brazil Prosthodontics, University of Sao Paulo - Bauru College of Dentistry, Bauru, SP, Brazil
Received 6 March 2014; revised 3 June 2014; accepted 29 June 2014 Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.33258 Abstract: Objectives: The aim of this study was to quantify the polymerization volumetric shrinkage of one regular and two low shrinkage bulk fill composites in class I cavities with or without an adhesive layer, using three-dimensional (3D) micro-computed tomography (lCT). Methods: Class I cavity preparations (2.5 mm depth 3 4 mm length 3 4 mm wide) were standardized in 36 extracted human third molars, which were randomly divided in six groups (n 5 6 each) as follows: Group VIT (regular composite without bonding agent); Group SDR (low shrinkage flowable composite without bonding agent); Group TET (low shrinkage composite without bonding agent); Group VIT/P (regular composite with bonding agent); Group SDR/X (low shrinkage flowable composite with bonding agent); TET/T (low shrinkage composite with bonding agent). Each tooth was scanned via mCT at cavity preparation, immediately after
cavity filling, and after light-curing. Acquired lCT data were imported into Amira software for analysis and volume values evaluated between steps from cavity preparation until light-curing. Results: Both low shrinkage composites showed a significantly less volumetric shrinkage than VIT. The use of dental adhesive significantly decreased the average volumetric contraction similarly for the three composites, by about 20%. Conclusion: Both low shrinkage composites showed less volumetric polymerization contraction than the regular composite. The use of dental adhesive decreased the total volumetric shrinkage for all evaluated C 2014 Wiley Periodicals, Inc. J Biomed Mater Res composites. V Part B: Appl Biomater 00B: 000–000, 2014.
Key Words: polymerization, x-ray micro-computed tomography, composite resins, volume shrinkage, 3D imaging
How to cite this article: Hirata R, Clozza E, Giannini M, Farrokhomanesh E, Janal M, Tovar N, Bonfante EA, Coelho PG. 2014. Shrinkage assessment of low shrinkage composites using micro-computed tomography. J Biomed Mater Res Part B 2014:00B:000–000.
INTRODUCTION
Restorative practice currently fosters an increase in the development and use of dental materials other than amalgam, such as resin composites, because of a legal restrain treaty on mercury-based products recently announced by the United Nations Environment Programme.1,2 Despite the excellent esthetic results achieved with composite resins, an inherent concern still persists because of light curing polymerization shrinkage stress that can be high enough to debond adhesive interfaces.3,4 Potential clinical implications are marginal infiltration, post operative sensitivity, dental microcracking, and a decrease in adhesive bond strength.5 The composite resins contain different monomers and fillers, which directly influence polymerization shrinkage. Contraction stress produced during the polymerization process is acknowledged as a multifaceted phenomenon which
is not only a result of the composite shrinkage process alone, but also is related to its formulation, degree of conversion, polymerization kinetics, confinement conditions, viscoelastic behavior, and elastic modulus.6,7 The elastic modulus is crucial to predict the contraction stress potential that such resin materials can generate at the bonded interface.8–10 The lower the Young’s modulus, more reduction on internal stresses during polymerization shrinkage can be expected compared to a higher modulus composite.10 In addition, patient related variables such as cavity configuration factor (C-factor), known as the ratio between bonded and unbonded surfaces of the composite restoration, play a major role in class I cavities, for instance, where five bonded cavity walls and only the occlusal unbonded wall for polymerization stress relief poses a taxing shrinkage scenario.4,11
Correspondence to: R. Hirata; e-mail: [email protected] Contract grant sponsor: CNPq; contract grant number: 305777-2010-6 Contract grant sponsor: Department of Biomaterials and Biomimetics at NYUCD
C 2014 WILEY PERIODICALS, INC. V
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TABLE I. Composition and Lot Number of the Tested Materials Type of Material Vitalescence (regular composite) SureFil SDR Flow (bulk fill composite)
Tetric N-ceram Bulk Fill Peak Universal Bond (adhesive) XP Bond (adhesive)
Tetric N-Bond Self-Etch (adhesive)
Composition
Batch Number
Bisphenol A glycidyl methacrylate, fillers (75 weight percent or 58 volume percent) with average size of 0.7 mm. Barium-alumino-fluoro-borosilicate glass, strontium alumino-fluoro-silicate glass, modified urethane dimethacrylate resin, ethoxylated bisphenol A dimethacrylate, triethyleneglycol dimethacrylate, camphorquinone, photo-accelerator, butylated hydroxyl toluene; UV stabilizer; titanium dioxide; iron oxide pigments, fluorescing agent. Dimethacrylates, Prepolymers, barium glass filler, ytterbium trifluoride, mixed oxide, additive, initiators, stabilizers, pigments 2-Hydroxyethyl methacrylate, methacrylic acid dehydrated alcohol, chlorhexidine diacetate. Carboxylic acid modified dimethacrylate, phosphoric acid modified acrylate resin, urethane dimethacrylate, triethylene glycol dimethacrylate, 2hydroxy ethyl methacrylate, butylated benzenediol, ethyl-4-dimethyl aminobenzoate, camphorquinone, functionalised amorphous silica, tbutanol. Bis-acrylamide derivative, Bis-methacrylamide dihydrogenphosphate, amino acid acrylamide, hydroxyalkyl methacrylamide, nano-fillers, water, initiators and stabilizers
BS96Z
Because polymers in general may have their composition modified to tailor specific demands, understanding the impact of such alterations on shrinkage by means of an accurate and reliable method able to quantify and image volume changes is of major interest. To date, different methods to measure shrinkage have been described. Historically, a variety of substrates and composite restorations interface have been evaluated by dye penetration tests,12 which in fields such as endodontics have been strongly discouraged because evidence questioning its scientific relevance has been raised.13 Besides, this method is destructive and in tandem with marginal gap evaluations, have been regarded to deliver inaccurate predictions of clinical performance.14 The use of resin replicas has presented as an alternative technique to identify marginal failures on the toothrestoration interface in vivo.15 Other conventional methods to measure the polymerization shrinkage of composite resins have been described, as the use of optical instruments to determine the linear polymerization,16–18 fiber-optic sensing method based on a Fizeau-type interferometric scheme,19 mercury dilatometer,20 electromagnetic balance with a force transducer with a photo detector,21 and videoimaging device (AccuVol; Bisco) in single-view mode.8,22 More recent non-destructive methods include the use of finite element analysis,23 and the use of micro-computed tomography (mCT) generating 2D and 3D images and data,24–28 providing the possibility to analyze the material behavior inside a given cavity configuration. Recently, companies have marketed some composite materials as low-shrinkage stress, where their formulation potentially allows them to be placed as a flowable base or in bulk-fill of up to 4 mm thickness.29,30 However, bulk-fill composites should not be generically claimed as low-shrinkage stress and it has been advised that clinicians should be cautious when placing them in high C-factor cavity preparations.30,31 Therefore, quantification of their volume changes relative to conventional composites is warranted.
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110527
R52450 J084 1104001218
R59912
The aim of this study was to quantify the polymerization volumetric shrinkage of one regular and two low shrinkage bulk fill composites in class I cavities with or without an adhesive layer, using three-dimensional (3D) microcomputed tomography (lCT). The research hypotheses were that: 1. The tested regular composite would present higher volumetric shrinkage than the low shrinkage bulk filled ones. 2. The mCT method is able to detect the effect of adhesive application on polymerization volume shrinkage.
MATERIALS AND METHODS
Specimen preparation Thirty-six recently extracted human third molars were collected under an Institutional Review Board protocol approved by the NYU Medical School. Standardized box-shaped class I cavities (2.5 mm depth 3 4 mm length 3 4 mm wide) were prepared in each teeth. The cavities were prepared using a diamond bur (AD20 Occlusal Reduction Bur, Strauss/Code 845-022), which presents a standardized active head size and a vertical stop to deliver consistent cavity preparation depth. The final cavity preparation was then checked with a digital caliper. All teeth were maintained in distilled water at room temperature (25 C) before and after preparation procedures. The teeth were allocated to six groups (n 5 6 each), as follows: Group VIT, (unbonded, regular composite/Vitalescence, Ultradent Product, South Jordan, UT); Group SDR (unbonded, low shrinkage flowable composite/SureFil SDR Flow, Dentsply Caulk, Milford, DE); Group TET (unbonded low shrinkage composite/Tetric Bulk fill, Ivoclar Vivadent, Schaan, Lichtenstein); Group VIT/P (regular composite with dental adhesive/Peak Universal Bond, Ultradent Product); Group SDR/X (low shrinkage flowable composite with dental adhesive/XP Bond, Dentsply Caulk, Milford, DE); and
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FIGURE 1. Flow chart describing shrinkage quantification. The single letters within parentheses indicate the outcome of the performed steps. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Group TET/T (low shrinkage composite with dental adhesive/Tetric N-bond Self-etch pen, Ivoclar Vivadent, Schaan, Lichtenstein). Bonding agents were used according to respective manufacturer’s protocols. Table I depicts the composition and lot number for materials tested. Filling procedures All teeth underwent mCT analysis following cavity preparation for a first scanning. In the bonded groups, with exception of TET/T, the cavity (enamel and dentin) was etched with phosphoric acid (Ultra etch 35%, Ultradent Product) for 15 s and subsequently rinsed for 20 s. Excess water was removed with thin absorbent paper. For the VIT/P group, a layer of the above mentioned one bottle primer/adhesive system was applied in the cavity with a microbrush and light-cured for
20 s, as recommended by the manufacturer. In the group SDR/X, the selected one-bottle primer/adhesive system was applied following the manufacturer’s instructions. For the group TET/T, no etching was performed because the Tetric N-bond Self-Etch pen (Ivoclar Vivadent, Schaan, Lichtenstein) is a self-etching system, and adhesive application also followed the manufacturer’s recommendations. In all groups, the cavities were filled in bulk using their assigned composites and air-dried for 5 s. Samples were immediately protected from any light sources in dark plastic vials and placed in the mCT imaging chamber for a scan and volume quantification before light curing. Then composites were light cured for 40 s with a LED unit (800 mW/cm2, Ultra Lume 5, Ultradent Product) and inserted into the holder for the third mCT scanning and measurement. The
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(ANOVA). Post-hoc comparisons were carried out using 95% confidence intervals based on the pooled estimate of residual variability. RESULTS
FIGURE 2. Volume of shrinkage (%) among all groups. Error bars reflects the 95% confidence limits. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
polymerization unit step-over distance was kept constant at approximately 5 mm. mCT analysis The flow chart in Figure 1 illustrates how the experiment was conducted. Each tooth was scanned using a mCT apparatus (mCT40, Scanco Medical, AG, Basserdorf, Switzerland) after initial cavity preparation, before light-curing polymerization, and after light-curing polymerization.32 The mCT was calibrated using a phantom standard at 70 Kvp/BH 200 mgHA/ccm. The operating condition for the mCT device was: energy (70 Kvp–114 microamperes) with a medium resolution (16 mm/slice) using a sample holder of 16.5 mm. The average of the total number of slices was 250, with an average scan time of 28 min. Acquired mCT data were imported into a workstation and evaluated with Amira software (version 5.5.2, VSG, Burlington, MA). The measurement of the cavity is required to superimpose the two subsequent scans (before and after light-curing) with the Amira software tool called “superimposition”, providing a perfect arrangement of the three images, and exempting the presence of a reference mark in the stub during mCT scanning. This procedure avoids the scattering and possible noise between the restorative material and the tooth structure due its similar radiondensity. To minimize possible artifacts in the threshold segmentation, the following steps were undertaken: lCT data at cavity preparation were registered with lCT data obtained respectively before and after lightcure polymerization. Next, the two registered images were subjected to Boolean operation (registered lCT data minus lCT data at cavity preparation) to subtract the tooth from the composite restoration. This step sequence enabled isolation and quantification of the composite restoration’s volume. The cropped volume was automatically labeled with the “segmentation editor” command and subsequently reconstructed in a 3D manner. The “Material Statistics” command computed the volume changes. From these measurements, the volumetric loss following polymerization shrinkage was calculated as percentages. Statistical analysis Data were analyzed using a two-way (three levels of composite and two levels of adhesive) analysis of variance
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Figure 2 shows the mean (95% CL) level of volumetric shrinkage in each of the six treatment groups. Inspection suggests more shrinkage in the VITA composite then either of the two low-shrinkage composites, and that each composite shrank less when used with adhesive. ANOVA supported these observations, showing significant differences (p < 0.001) among composites and between adhesive treatments (p 5 0.02) (Table II). Post-hoc testing then showed more shrinkage in VITA [M(SD)5 3.5% (0.8)] than either SDR [M(SD)5 1.7% (0.7)] or TET [M(SD)5 2.0% (0.3)], both p < 0.05; and more shrinkage without adhesive [M(SD)5 2.7% (1.0)] than with adhesive [M(SD) 5 2.2% (1.0)]. Although there was less of an adhesive effect for TET than the other composites (25.0% for SDR, 18.4% for VIT, and 13.6% for the TET groups), analysis failed to support an interaction (p 5 0.60). Thus, use of the lower shrinkage composites reduced shrinkage by about 50%, and adhesive reduced shrinkage by about 20%; both effects were statistically significant. Qualitative 2D analysis of polymerization shrinkage for the VIT group resulted in gaps along the pulpal floor and parts of the axial walls. For the SDR as for the TET groups, smaller gaps along the cavity floor were found. Analysis of the bonded groups showed the same pattern described for the unbonded samples, however with fewer gaps (Figures 3 and 4). The 3D reconstructions (Figures 5 and 6), showed volume shrinkage mainly at the occlusal surface and some parts of the cavity floor for VIT group. The SDR group presented most of the volume rendering at the cavity floor, but also at the occlusal surface. The TET group showed the least amount of volumetric changes and these were mainly detected at the occlusal surface. Amongst the bonded groups, the VIT/P presented a decrease in volume shrinkage at the pulpal floor, and such a decrease was even more pronounced in the SDR/ X group. Images and shrinkage amount rendered for the TET/T group was very similar when compared to the TET. DISCUSSION
It is acknowledged that polymerization stress and shrinkage strongly depend, among other factors, on the C-factor of the cavity.33 In our study, the rationale for selecting a class I cavity configuration was to provide one of the most challenging TABLE II. Polymerization Shrinkage (n 5 6) Determined for the Composite Resins With or Without Adhesive Reported as % (SE) Using mCT Composite Resin Vitalescence SureFil SDR Tetric bulk fill
Bonded Groups
Unbonded Groups
3.1 (0.33) A a 1.5 (0.36) B b 1.9 (0.1) B b
3.8 (0.25) B a 2.0 (0.19) B b 2.2 (0.1) B b
Uppercase letters compare values (with or without adhesive) within the same row (composite resin). Lowercase letters compare values (between composite resins) within the same column.
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FIGURE 3. Figure representing the 2D sections of groups without adhesive (A) VIT before curing; (B) VIT after curing; (C) SDR before curing; (D) SDR after curing; (E) TET before curing; (F) TET after curing. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
shrinkage scenarios among cavity designs.34 The diamond burs used for cavity preparation presented cutting tips that served as a guiding reference during preparation for standardized cavity depth. Because of natural teeth variability and differences in occlusal anatomy, some areas of the cavity floors still presented enamel after preparation. Because samples were randomly assigned to different groups, bias originating from substrate variation was minimized. All composite materials used in this study were placed in bulk (one increment) in an attempt to evaluate the composite volumetric shrinkage and the effect of bonding procedures on polymerization shrinkage. Both of our postulated research hypotheses that stated that regular composite would present higher volumetric shrinkage than low shrinkage bulk fill restorative materials, and that the mCT method would be able to detect the effect of adhesive application on polymerization volume shrinkage were accepted. Potential composite dimensional alterations during scanning were not considered and warrant further investigation. It has been previously suggested that the lower polymerization shrinkage observed for the TET and SDR composite relative to the VIT composite is related to TET and
SDR chemical formulation.29,30,35 VIT is a regular composite, Bis-GMA-based material and contains 75% weight percent or 58% volume of fillers. The volumetric shrinkage of VIT/P group was 3.1% and for the VIT it was 3.8%. This volumetric shrinkage is a common value for methacrylate and dimethacrylate-based composites.17,21,22,24–26 On the other hand, the polymerization shrinkage for the SDR group ranged from 1.5 to 2.0%, with or without adhesive application, respectively. The results obtained in the present study are lower than previously reported where a Kaman Linometer was used for shrinkage evaluation (determined to be at 3.5%).18 The discrepancy between results likely arose from the fact that in the present study the composite was placed in situ in a class I cavity preparation, suggesting that general comparisons on shrinkage even between the same brand of composites, not accounting for methodological differences, should be made with caution,17 even when using different softwares with lCT.24 Volumetric shrinkage evaluation of several composites has shown, for instance, a 1–3% range for packable and up to 6% for flowable composites, which is in contrast to our findings.9 The SureFil SDR low shrinkage composite is composed by a
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FIGURE 4. Figure representing the 2D sections of groups with adhesive (A) VIT/P before curing; (B) VIT/P after curing; (C) SDR/X before curing; (D) SDR/X after curing; (E) TET/T before curing; (F) TET/T after curing. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
modified urethane dimethacrylate resin and ethoxylated bisphenol A dimethacrylate as main monomers. Lower volumetric shrinkage is due in part to the larger molecular weight of initial monomers compared to conventional methacrylated-based resin systems.35,36 The molecular weight of modified urethane dimethacrylate resin is 849 g/ mol, whereas Bis-GMA presents 513 g/mol. In addition, previous studies suggest that photoinitiators incorporated into urethane-based methacrylate resins facilitate free-radical polymerization decreasing the overall curing stress,37 thereby decreasing volumetric shrinkage. The observed volumetric shrinkage for TET and TET/T groups ranged from 1.9 to 2.2%, respectively. To the best of our knowledge, no volumetric shrinkage measurements are yet available in the literature even though the manufacturer’s materials specification sheet reports values at 2%,29 which is in close agreement with our results. Different than the modified UEDMA approach above mentioned to reduce polymerization shrinkage, Tetric N-ceram bulk composite uses a new germanium-based photoinitiator to tailor polymerization kinetics and a pre-polymer stress reliever that supposedly decrease polymerization stress and shrinkage.29,38 The evaluation of the shrinkage without adhesive application, although not clinically relevant, allowed the observation
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of the immediate role of adhesives in counteracting composite polymerization shrinkage in standardized cavities.39 Such an approach has been previously used for even more challenging C-factor scenarios, such as in bonding posts to root canals, where it was shown that resin cement shrinkage stresses are so high that push-out bond strength measurements have resulted in no differences when an adhesive system is or is not used, suggesting that friction seems to be more relevant than adhesion as a retention mechanism in this particular scenario.40,41 On the other hand, the clinically relevant measurement, where an adhesive was applied to the tooth substrate before restoration fabrication, did allow for a quantitative observation of how bonding between composite and tooth structure decreased the overall volumetric shrinkage of all composites.32 For bonded restorations, the presence of gaps decreased for VIT and SDR groups, whereas the volumetric shrinkage decreased for all bonded groups. The mCT has been previously regarded as a key tool to three-dimensionally quantify and characterize the polymerization shrinkage of composite in cavities.24,25,27,32 The accuracy of the method allows the measurement of current failures on the composite insertion like bubbles,27 whereas traditional volumetric shrinkage methods do not account for. One current limitation of the method is that materials without sufficient radiopacity, and without or with low filler
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FIGURE 5. Figure representing the 3D renderings of groups without adhesive (A) VIT shrinkage and composite filling; (B) VIT shrinkage; (C) SDR shrinkage and composite filling; (D) SDR shrinkage; (E) TET shrinkage and composite filling; (F) TET shrinkage. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
content, such as dental adhesives, are difficult to resolve through software reconstruction.32,42 In the present study, the 2D location of marginal gaps partially represent the overall shrinkage scenarios presented in 3D rendering.25 Gaps were generated by the stress and dynamics of the volumetric shrinkage inside the cavity. It reinforces the importance of 3D analysis instead of 2D methods.13,14 In the 2D analysis, polymerization shrinkage of the regular methacrylated-based material either bonded or unbonded resulted in gaps along the pulpal floor and parts of the axial walls. Although gaps were also noted for bonded samples, they were reduced along the cavity floor. The absence of opening occlusal margins even in the non bonded samples was found in other studies, and was explained by the wetting capacity of the composite before curing promoting reduced wetting on the enamel margins.39,43 The 3D images of the VIT and VIT/P showed shrinkage located at the occlusal surface and some parts of the pulpal floor; even with the bonding procedure the shrinkage mainly in the occlusal surface
was sustained.44 Previous studies have depicted gap formations in areas similar to the ones observed in the present study, supporting shrinkage vectors pointing towards the restoration occlusal surface resulting in restoration debonding from the pulpal walls.25,32,44–46 Another potential explanation for this deformation is that the occlusal outer area of the cavity is closer to the light tip and consequently polymerization may take place faster than at the cavity floor.42,47,48 Furthermore, the viscous flow characteristic of composite resins prior the pre-gel phase cause a slight concavity on the occlusal surface.32,44 The SDR groups presented the same contraction pattern observed for the VIT group. However, the use of the adhesive XP Bond significantly decreased shrinkage along the pulpal floor as observed in the 3D reconstruction. We speculate that because these materials are flowable composites presenting higher viscoelasticity relative to conventional composites, they may better dissipate polymerization shrinkage stress when bonded to the cavity walls reducing
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FIGURE 6. Figure representing the 3D renderings of groups with adhesive (A) VIT/P shrinkage and composite filling; (B) VIT/P shrinkage; (C) SDR/X shrinkage and composite filling; (D) SDR/X shrinkage; (E) TET/T shrinkage and composite filling; (F) TET/T shrinkage. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
the gaps around the restoration surfaces. According to Versluis et al.,45 for a well-bonded composite restoration, the shrinkage vectors are oriented down and toward the bonded margins, which may explain the misfit of the flowable composite (SDR/X group) and its volume reduced at the occlusal surface24 observed in the 3D reconstruction.43 Another explanation of the influence of the adhesive in this specific material is the delay of the gel point resulting in a slow polymerization rate of the flowable composite compared to a paste, thereby allowing deformation without overstressing areas near the cavity walls.33,49 Such theory may be supported by a study that showed that SDR low shrinkage composite presented a delayed gelation point compared to regular composite and conventional flowable composites.35 In the 2D analysis the gaps were mainly at the pulpal floor. Finally, the Tetric N-ceram low shrinkage bulk fill material presented shrinkage deformation, more restricted more to the occlusal surface, with and without the bonding agent. The 2D slices showed that gaps formed at the pulpal floor.
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HIRATA ET AL.
Further studies considering cavity size and design are warranted for appropriate mapping and quantification of different resin composite based restorative materials. Further quantitative analysis and correlation of the 2D gaps and 3D shrinkage reconstruction are fundamental to predict the relation between volumetric shrinkage and gap formation.
CONCLUSION
Based on the results of this research, it can be concluded that: 1. The low shrinkage composites showed less volumetric polymerization shrinkage than regular resin composite. 2. The use of dental adhesive decreased the overall volumetric shrinkage of all evaluated composites. 3. Micro-computed tomography 3D reconstruction allowed quantification and realistic imaging of volumetric shrinkage changes within a restoration
ASSESSMENT OF LOW SHRINKAGE COMPOSITES USING MCT
ORIGINAL RESEARCH REPORT
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