INTRODUCTION

Several improvements have been made regarding direct restorative materials, leading to the interest of developing more complex and better properties. One of the major difficulties related to composite resin restorations has been to overcome their intrinsic volumetric polymerization shrinkage and polymerization stress, which can lead to marginal seal loss and, subsequently, bacterial infiltration and recurrent caries1). Cuspal deflection is a clinical manifestation caused by volumetric shrinkage and polymerization stress that affects the dental structure due to the adhesion of composite to the tooth2).

Free radical polymerization of resin-based materials is associated to a post gel contraction3), in which its stress is reflected in the adhesive interface due to the bonding of resin-based material to cavity walls, promoted by adhesive systems4). In a microscopic level, monomers establish covalent bonds, reducing the distance within each other and, decreasing the free volume4). According to Hooke’s Law, the applied stress is a factor of the modulus and resultant strain of a composite in its elastic, or solid state.

Resin composite composition is paramount; filler content and organic matrix have significant effect on reaction kinetics, and as such, the elastic modulus,

shrinkage and stress generation. Nowadays, filler loading usually exceeds 60% filler volume fraction5,6) and, together with the composition of the resin matrix, determine the magnitude of volumetric shrinkage experienced4).

Some other factors have been described to affect shrinkage stress and volumetric shrinkage phenomena, such as the cavity C-factor7), the depth and volume of the restorative material8), the use of different light irradiation methods9), among others. Also, it has been purposed in the literature the incremental layer technique, which came to be known as the “elastic cavity wall” concept, described clinically and thought to have significant benefit decreasing shrinkage stress and cuspal strain2).

In order to overpass those issues of resin composites, low volumetric shrinkage and low shrinkage stress composite resins with the advantage to be placed and polymerized in bulk were introduced. These materials allow increments of 4–5 mm thick, thus skipping the layering technique and reducing clinician chair time10). Some of these bulk-fill resin composites have shown lower elastic modulus than the nanohybrid and microhybrid composites10), thus affecting their performance under stress and increasing their deformability, which is directly related to their filler content5,10).

In order to evaluate volumetric polymerization shrinkage, novel contemporary methodology utilizing 2D and 3D non-destructive imaging methods such as

Assessment of cuspal deflection and volumetric shrinkage of different bulk fill composites using non-contact phase microscopy and micro-computed tomographyMartin PRAGER1, Mark PIERCE2, Pablo J. ATRIA3,4, Camila SAMPAIO3,5, Eduardo CÁCERES4,6, Mark WOLFF1, Marcelo GIANNINI5 and Ronaldo HIRATA4

1 Department of Cariology and Comprehensive Care, New York University College of Dentistry, New York, USA2 Department of Biomedical Engineering Rutgers, The State University of New Jersey, Piscataway, USA3 Department of Biomaterials, College of Dentistry, Universidad de Los Andes, Santiago, Chile4 Department of Biomaterials and Biomimetics, New York University College of Dentistry, New York, USA5 Department of Restorative Dentistry, State University of Campinas, Piracicaba Dental School, Piracicaba, Brazil6 Department of Biomaterials, Universidad Andres Bello, Faculty of Dentistry Viña del Mar, Viña del Mar, ChileCorresponding author, Pablo J. ATRIA; E-mail: Atria.pablo@gmail.com

The understanding of cuspal deflection and volumetric shrinkage of resin composites is necessary to assess and improve the placement techniques of resin-based materials. The aim of this study was to investigate the cuspal deflection and its relationship with volumetric polymerization shrinkage of different bulk-fill resin composites. The investigation was conducted using non-contact phase microscopy and micro-computed tomography. Thirty custom-milled aluminum blocks were fabricated for microscopy analysis and thirty-six tooth models with standardized Class I cavities were used for micro-computed tomography analysis. Results showed that high-viscosity composites present higher cuspal deflection compared to bulk-fill composites. The filler loading of resin composites seems to have an effect on cusp deflection, since the higher the filler content percentage, the higher the cusp deflection. On the other hand, it seems to have an opposite effect on volumetric shrinkage, since higher filler loadings produced lower volumetric shrinkage percentages.

Keywords: Shrinkage, Composite, Cuspal, Light-curing, Computed-tomography

Color figures can be viewed in the online issue, which is avail-able at J-STAGE.Received Apr 24, 2017: Accepted Aug 15, 2017doi:10.4012/dmj.2017-136 JOI JST.JSTAGE/dmj/2017-136

Dental Materials Journal 2018; 37(3): 393–399

Fig. 1 Milled aluminum block.

Table 1 Materials, filler loading, manufacturers and batch numbers of the materials

Group Products Manufacturer Batch number

AMPFiller Loading: 76 wt%

Amelogen Plus+Peak Universal Bond

Ultradent Products, South Jordan, UT, USA

N553090L114

FBFFiller Loading: 64.5 wt%

Filtek Bulk Fill Flowable+Scotchbond Universal

3M ESPE, St. Paul, MN, USA

N458529508936

VBFFiller Loading: 65 wt%

Venus Bulk Fill Flowable+i Bond Total Etch

Heraeus Kulzer, South Bend, IN, USA

010032S010043

SDRFiller Loading: 68 wt%

SureFil SDR Flow+Prime & Bond NT

Dentsply Caulk, Milford, DE, USA

1106071121206

TBFFiller Loading: 71 wt%

Tetric EvoCeramBulk Fill+Excite F

Ivoclar Vivadent, Schaan, Liechtenstein

S09217R77474

SBFFiller Loading: 83.5 wt%

SonicFill+OptiBond FLKerr, Orange, CA, USA

50072774808062

micro-computed tomography (µCT) have been used with promising results11-13). On the other hand, cuspal deflection is a measure of shrinkage stress, and can be evaluated using methods with devices such as linear variable differential transformers or direct current differential transformers14) and strain gauges15). As well as the volumetric shrinkage measuring methods, a non-destructive measuring technique is desirable in order to facilitate reproducibility and sequential measurements of cuspal deflection16).

The understanding of cuspal deflection and volumetric shrinkage of bulk-fill resin composites is necessary in order to assess and improve the placement techniques of resin-based composites. Thus, the aim of this study was to evaluate and measure cuspal deflection and volumetric shrinkage of different bulk-fill resin composites using non-contact phase microscopy and micro-computed tomography. The two null hypotheses tested were that different bulk fill composites would not generate1) equal cuspal deflection and2) equal volumetric shrinkage values.

MATERIALS AND METHODS

Spectral-domain phase microscopyThirty custom milled aluminum blocks [10 mm (W)×8 mm (L)×30 mm (D)] with a MOD cavity [6 mm (W)×8 mm (L)×4 mm (D)] were fabricated using a milling machine, creating two remaining cusps [2 mm (W)×8 mm (L)×4 mm (D)] (Fig. 1)17). The inner walls of the cavity were air-abraded with 50 µm Al2O3 powder to simulate acid-etched enamel surface, then steam cleaned and air-dried.

One regular hybrid composite, three bulk-fill flowable and two high-viscosity bulk fill composites with their respective bonding agents were assigned into six groups and tested (Table 1): Amelogen Plus/Peak Universal Bond (Group AMP, Ultradent Product); Filtek Bulk Fill Flowable Restorative/Scotchbond Universal

(Group FBF, 3M ESPE); Venus Bulk Fill Flowable/i Bond Total Etch (Group VBF, Heraeus Kulzer); SureFil SDR Flow/Prime & Bond NT (Group SDR, Dentsply); Tetric EvoCeram Bulk Fill/Excite F (Group TBF, Ivoclar Vivadent); SonicFill Bulk Fill/Optibond FL (Group SBF, Kerr).

The displacement of one cusp wall was measured before, during, and after light curing. Measurements were acquired utilizing spectral-domain phase microscopy (SDPM), an interferometric technique used previously for dynamic nanoscale measurement in biological and non-biological materials18,19). Blocks were secured at their bases in a 3-axis positioning stage (Newport 460A series) with one cusp wall facing the SDPM sample beam. The open (“proximal”) sides of the block were closed with aluminum plates that were

394 Dent Mater J 2018; 37(3): 393–399

Fig. 2 Aluminum block positioned on stage in front of cover glass, facing SDPM.

Fig. 3 Diagram of SDPM set-up.

Fig. 4 Light-activation of composite with UltraLume LED 5.

lubricated with petroleum jelly, simulating a matrix band (Fig. 2). A reference signal was obtained from the reflection at a cover glass, positioned 0.75 mm from the cusp surface in a “common-path” configuration (Fig. 3). The signal formed by interference of reference and sample beams was recorded as a function of optical wavelength at a custom-designed spectrometer. Fourier transformation of the signal yielded the magnitude and phase of the signal returning from the cusp surface. The measured phase ∆φ was unwrapped and converted to physical cusp displacement ∆z using the relationship ∆z=(λ0/4π)∆φ.

SDPM measured single cusp displacement at 5 s intervals at the block midpoint, 0.5 mm from top of the cusp, beginning 30 s prior to the onset of illumination, then every 1 s from the onset of irradiance for 30 s, then every 5 s for a total duration of 10 min.

Real-time monitoring of cuspal deflection was conducted, in addition to recording data as described above for post-processing. As with all interferometry-based methods, phase shifts greater than ±π result in a calculated phase, which is “wrapped”, but this was corrected in the post-processing. The data showed a level baseline during the first 30 s (prior to illumination), followed by a brief positive displacement (expansion), coincident to the onset of illumination. The sample then underwent rapid, large contraction over the remainder of the experiment, with multiple cycles of phase wrapping

occurring within the initial photo polymerization phase. Data were analyzed by one-way ANOVA at the 5% level of significance using IBM SPSS software (IBM, Armonk, NY, USA) followed by a post-hoc Tukey-Kramer test.

Volumetric polymerization shrinkageThirty-six models with standardized class I cavity (4 mm depth×4 mm length×4 mm wide) were fabricated with methyl methacrylate resin (Ortho Jet, Lang Dental Manufacturing, Wheeling, IL, USA), simulating a cavity designed in a natural tooth20). The replicas were obtained by silicone impressions made with Express Light Body (3M ESPE). PMMA (polymethyl metacrylate). Class-I cavities were randomly divided in 6 groups (n=6).

All resin composites were placed in a single 4 mm “bulk” increment and cured with Ultralume LED 5 (Ultradent) at 545 mW/cm2 irradiance for 20 s (Fig. 4). The radiometer used to measure the LED output was a Newport model 818-ST2/DB (Newport, Irvine, CA, USA) detector, with responsivity set for the LED wavelength.

Each sample was scanned twice using µCT40 (Scanco Medical, Basserdorf, Switzerland) before and after light-activation11). The µCT was calibrated with a phantom standard at 70 Kvp/BH 200 mgHA/ccm. The operating condition for the µCT device was: energy (70 Kvp–114 µA) with a resolution of 16 µm/slice, using a sample holder of 16.5 mm. Acquired µCT data were imported into a workstation and evaluated with Amira software (version 5.5.2, VSG) The total volumetric shrinkage of the resin composite was evaluated. The uncured and cured scans were superimposed with the Amira software tool called “superimposition”, providing an arrangement of both images, and exempting the presence of a reference mark in the stub during µCT scanning. After that, the two registered images were subjected to Boolean operations (registered µCT uncured data minus µCT cured data) to subtract the cured from the uncured composite restoration (after volumetric shrinkage). This sequence enabled isolation and visualization of the composite restoration’s shrinkage volume. The cropped volume was automatically labeled

395Dent Mater J 2018; 37(3): 393–399

Table 2 Means (SD) of single cusp deflection (nm) and volumetric polymerization shrinkage (%)

Composite resin Cusp deflection Volumetric shrinkage

AMP 7,472 (1,675) A 2.10 (0.74) CD

FBF 4,454 (691) B 3.02 (0.68) BC

VBF 3,677 (962) B 4.07 (0.57) B

SDR 3,676 (988) B 6.55 (1.59) A

TBF 3,847 (1,016) B 1.26 (0.18) D

SBF 8,016 (1,574) A 1.02 (0.15) D

(%). Same superscript letters within a column indicate no significant difference between the means.

Fig. 5 Graph comparing the average rate and amount of displacement (nm) of single aluminum cusp facing SDPM of all groups.

Fig. 6 MicroCT images of volumetric polymerization shrinkage for each group.

Note that polymerization shrinkage was mostly observed in occlusal free surface, followed by pulpal floor for all material tested.

with the “segmentation editor” command and reconstructed in a 3D manner. The “Material Statistics” command computed the volumetric changes. From these measurements, the volumetric loss following polymerization shrinkage was calculated as percentages12,13). Data was analyzed by one-way analysis of variance (ANOVA).

RESULTS

Cusp deflection (nm) and volumetric shrinkage (%) means from different groups are shown in Table 2. Data showed significant difference between groups for the two tests evaluated (p<0.05) Total displacement of both “cusps” is assumed to be twice the reported (single cusp) value.

Concerning cusp deflection, the high viscosity, regular resin composite group AMP and the SBF bulk-fill high viscosity resin composite presented the highest values (7,472 and 8,016 nm, respectively), significant different than the other bulk fill flowable resin composites groups FBF, VBF and SDR (p<0.05). The regular viscosity bulk fill resin composite TBF did not

present statistical difference when compared to the bulk-fill flowable resin composites (p>0.05), but was significant different than the regular viscosity AMP and SBF ones (p>0.05) (Table 2). A plot of displacement vs. time revealed that the rate of displacement of the aluminum “cusp” for all composites tested was extremely fast from the initiation of light-activation and for 60 s thereafter (Fig. 5). After 60 s, the rate for all composites was decreased dramatically until 600 s, but kept deflecting weakly.

When volumetric polymerization shrinkage was

396 Dent Mater J 2018; 37(3): 393–399

evaluated, SDR presented the highest percentages (6.55%), significant different than all other groups (p<0.05). After this group, the second higher values of shrinkage was observed for VBF (4.07%), not significantly different than FBF (3.02%) (p=0.249), which by itself was not significant different than AMP (2.10%) (p=0.389). The lowest values of shrinkage were seen for groups TBF (1.26%) and SBF (1.02%), although not significant different than AMP (2.10%) (p>0.05). Figure 6 shows the volumetric polymerization shrinkage (%) for different resin composites.

Qualitative analysis from µCT images shows volumetric shrinkage mostly seen in the free occlusal surface for all products. In some samples of all groups, shrinkage was also observed at the pulpal floor surface. In proximal walls, almost no shrinkage was observed.

DISCUSSION

Numerous approaches have been made over the years to reduce polymerization shrinkage, and therefore, clinical implications of this phenomenon, such as cuspal deflection and secondary caries formation21). The selection of an appropriate resin composite material would influence the clinical performance of the restoration4). This study quantified the degree of deflection of aluminum “cusps” and related it to the volumetric polymerization of several bulk-fill resin composites and one hybrid resin composite.

Based on the results of this study, both null hypotheses were rejected since significant differences in cusp deflection and volumetric polymerization shrinkage were observed among products. In terms of cusp displacement, the three flowable bulk fill resin composites (FBF, VBF and SDR) were not significantly different within each another, but presented significantly lower values than that of the regular viscosity resin composites, either conventional or bulk fill (AMP and SBF), which were not significantly different between themselves. Although TBF resin composite is shown in a regular consistency, it presented statistically similar cusp deflection as of the flowable composites, despite the fact of being statistically different when compared to the regular viscosity ones. On the other hand, volumetric polymerization shrinkage was higher for the flowable composites SDR and VBF, reaching values of approximately 6 and 4% respectively. In general, flowable composites promoted lower values of cusp deflection, even presenting higher values of volumetric shrinkage. These high volumetric shrinkage results are related to higher concentration of organic matrix and lower percentage of filler particle, which increase the volumetric polymerization shrinkage. However this low-viscosity material is able to absorb the polymerization stress due to its low modulus, inducing lower effects on cusps deflection22).

In this study a SDPM setup and a microCT technique were used to measure dimensional changes during polymerization. SDPM is an optical technique based on white light interferometry, which determines

the relative distance between reflective surfaces of an object and a stationary mirror18,19). SDPM was employed for dynamic measurement of cuspal deflection due to polymerization stress of composite resin before, during, and after polymerization, allowing for real time observations of the rate of deflection of the aluminum cusps17). The microCT technique allowed the visualization of volume differences before and after light polymerization, allowing comparing the behavior of the resin composite itself and how it relates to the cuspal deflection observed by SDPM method12).

The election of the material (aluminum) was done based on a study that stated that elastic modulus of aluminum (68.5 GPa) might simulate the entire tooth (dentin 18.3 GPa and enamel 84.1 GPa)17). In the current study, each aluminum block showed deflection, which indicates that adhesion between composite/aluminum was established. This setting was used in order to standardize samples, avoiding dimensional differences and its influence on the results. Class I cavities were selected for analyzing volumetric shrinkage due to their common clinical use; its high C-factor value influences polymerization shrinkage stress, making them a suitable model measurement23).

It is known that a higher percentage of inorganic fillers present in the resin composites tend to increase the elastic modulus of this material, thus increasing the polymerization shrinkage stress22). As a result from this stress, greater cusp deflection can be observed7). Stresses build up at the bonded interface when composite polymerization shrinkage is restricted by the adhesion to the cavity walls8). This goes in accordance to the results to our study, when the resin composites with the highest amounts of inorganic fillers AMP (76 wt%) and SBF (83.5 wt%) showed the highest values of cusp deflections deformations, significantly different than the other groups (p<0.05). It was shown that resin composites with higher elastic modulus show a higher number of gap-free enamel margins, but an increased incidence of marginal enamel fractures24). This results also correlates to the results from our study, once qualitative analysis from µCT images showed volumetric shrinkage mostly seen in the free occlusal surfaces, and proximal bonded enamel walls demonstrated gap-free margins. One study found different patterns of cusps displacement, showing that the addition of filler content does not efficiently reduce post-gel shrinkage and polymerization stress25). Nevertheless, some other authors state that this can lead to a reduction in cuspal deflection measurements6).

There is an inverse correlation between the percentage of inorganic filler particles in a composite resin and the degree of unrestrained shrinkage when cured26). This would necessarily imply that the higher the elastic modulus of a composite, the less it would shrink. However, when restrained, such as it is when bonded between the walls of a cavity preparation, the more highly filled resins may exert uncompensated stress on the adhesive bond27), which can have an impact to the cusp deflection, as seen in this study.

Essentially, the degree to which a composite resin

397Dent Mater J 2018; 37(3): 393–399

restoration exerts stress on the tooth is a function of its chemical composition, viscoelastic properties, reaction kinetics, elastic modulus, and volumetric shrinkage, as well as the geometry of the cavity preparation (C-factor)26,27). As a result of these very complex interactions and factors, the extent to which one or more may contribute to the ultimate stress remains controversial.

Contrary to the results found for cusp deflection, volumetric shrinkage showed that the highest the amount of inorganic filler, the less the volumetric polymerization shrinkage is observed. The extent of polymerization shrinkage is dependent on the mobility, the molecular weight, and the functionality of the monomers28). The percentage of inorganic filler content has been shown to influence the viscosity and flowability of unpolymerized material7), and a higher percentage is expected to cause less volumetric polymerization shrinkage29), which goes in accordance to our study. Our results showed higher polymerization shrinkage for groups SDR (6.55%), VBF (4.07%) and FBF (3.02%), which happen to have the lowest filler percentages, 68, 65 and 64.5 wt%, respectively. On the contrary, high-filled resin composites showed the lowest values of volumetric shrinkage, such as SBF (1.02%), TBF (1.26%) and AMP (2.10%), that have respectively 83.5, 71 and 76 wt% filler content.

The regular viscosity bulk-fill TBF resin composite showed low values of cusp deflection and also low values of volumetric shrinkage. Besides the high filler content (71 wt%) that explains the low values of volumetric shrinkage, the low values of cusp deflection can be explained by a stress reliever as a special patented filler functionalized by silanes, which is present in the materials composition and features a lower modulus of elasticity, generating an attenuation of forces during polymerization30). Presence of pre-polymerized fillers might also contribute to the lower elastic modulus10).

For years it has been suggested incremental filling the cavity in order to decrease polymerization stress; however, this is a time demanding procedure. Bulk-fill materials are being deeply investigated6,10,25,30). Those materials lessen the time demanded to build a restoration, since it can be introduced in a cavity until 4 mm deep. This ensures that, clinically, the use of flowable bulk-fill composites could be feasible in class-I cavities as a “core” following recommendation of manufactures. Further in vivo and in vitro studies most focus on gap and sensibility on this new class of dental materials.

Within the limitations of this study, it can be concluded that different restorative systems affect volumetric shrinkage and cusp deflection. The filler loading of resin composites seems to have a direct effect on cusp deflection, since in general higher the filler content percentage, the higher the cusp deflection. On the other hand, it seems to have an opposite effect on volumetric shrinkage, since higher filler loadings produced lower volumetric shrinkage percentages. Bulk filling class-I and class-II cavities with bulk-fill resin composites represents a viable alternative to the

traditional incremental filling technique. It is important to attempt that polymerization shrinkage stress depends not only on volumetric shrinkage, but also on filler loadings, monomeric composition and the tooth/restoration interface characteristic, being either enamel or dentin.

CONFLICTS OF INTEREST

Authors declare that they have no conflicts of interest.

REFERENCES

1) Carvalho RMd, Pereira JC, Yoshiyama M, Pashley DH. A review of polymerization contraction: The influence of stress development versus stress relief. Oper Dent 1995; 21: 17-24.

2) Cara R, Fleming G, Palin W, Walmsley A, Burke F. Cuspal deflection and microleakage in premolar teeth restored with resin-based composites with and without an intermediary flowable layer. J Dent 2007; 35: 482-489.

3) Davidson C, De Gee A, Feilzer A. The competition between the composite-dentin bond strength and the polymerization contraction stress. J Dent Res 1984; 63: 1396-1399.

4) 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.

5) Ilie N, Hickel R. Investigations on mechanical behaviour of dental composites. Clin Oral Investig 2009; 13: 427-438.

6) Moorthy A, Hogg C, Dowling A, Grufferty B, Benetti AR, Fleming G. Cuspal deflection and microleakage in premolar teeth restored with bulk-fill flowable resin-based composite base materials. J Dent 2012; 40: 500-505.

7) Lee MR, Cho BH, Son HH, Um CM, Lee IB. Influence of cavity dimension and restoration methods on the cusp deflection of premolars in composite restoration. Dent Mater 2007; 23: 288-295.

8) Braga RR, Boaro LC, Kuroe T, Azevedo CL, Singer JM. Influence of cavity dimensions and their derivatives (volume and ‘C’factor) on shrinkage stress development and microleakage of composite restorations. Dent Mater 2006; 22: 818-823.

9) Dennison JB, Yaman P, Seir R, Hamilton JC. Effect of variable light intensity on composite shrinkage. J Prosthet Dent 2000; 84: 499-505.

10) Ilie N, Bucuta S, Draenert M. Bulk-fill resin-based composites: an in vitro assessment of their mechanical performance. Oper Dent 2013; 38: 618-625.

11) Chiang YC, Rösch P, Dabanoglu A, Lin CP, Hickel R, Kunzelmann KH. Polymerization composite shrinkage evaluation with 3D deformation analysis from µCT images. Dent Mater 2010; 26: 223-231.

12) Hirata R, Clozza E, Giannini M, Farrokhmanesh E, Janal M, Tovar N, Bonfante EA, Coelho PG. Shrinkage assessment of low shrinkage composites using micro-computed tomography. J Biomed Mater Res 2015; 103: 798-806.

13) Sampaio CS, Chiu KJ, Farrokhmanesh E, Janal M, Puppin-Rontani RM, Giannini M, Bonfante EA, Coelho PG, Hirata R. Microcomputed tomography evaluation of polymerization shrinkage of class I flowable resin composite restorations. Oper Dent 2017; 42: E16-E23.

14) Grimaldi J, Hood J. Lateral deformation of tooth crown under axial cuspal loading. J Dent Res 1973; 52: 584.

15) Palin WM, Fleming GJ, Nathwani H, Burke FT, Randall RC. In vitro cuspal deflection and microleakage of maxillary premolars restored with novel low-shrink dental composites. Dent Mater 2005; 21: 324-335.

398 Dent Mater J 2018; 37(3): 393–399

16) Jantarat J, Panitvisai P, Palamara JEA, Messer HH. Comparison of methods for measuring cuspal deformation in teeth. J Dent 2001; 29: 75-82.

17) Kwon Y, Ferracane J, Lee IB. Effect of layering methods, composite type, and flowable liner on the polymerization shrinkage stress of light cured composites. Dent Mater 2012; 28: 801-809.

18) Joo C, Akkin T, Cense B, Park BH, de Boer JF. Spectral-domain optical coherence phase microscopy for quantitative phase-contrast imaging. Opt Lett 2005; 30: 2131-2133.

19) Choma MA, Ellerbee AK, Yang C, Creazzo TL, Izatt JA. Spectral-domain phase microscopy. Opt Lett 2005; 30: 1162-1164.

20) Milosevic M, Mitrovic N, Stankovic TS. Analysis of local shrinkage patterns of self-adhering and flowable composites using 3D digital image correlation. Quintessence Int 2011; 42: 797-804.

21) Deliperi S, Bardwell DN. An alternative method to reduce polymerization shrinkage in direct posterior composite restorations. J Am Dent Assoc 2002; 133: 1387-1398.

22) Condon JR, Ferracane JL. Assessing the effect of composite formulation on polymerization stress. J Am Dent Assoc 2000; 131: 497-503.

23) De Munck J, Van Landuyt K, Coutinho E, Poitevin A,

Peumans M, Lambrechts P, Van Meerbeek B. Micro-tensile bond strength of adhesives bonded to Class-I cavity-bottom dentin after thermo-cycling. Dent Mater 2005; 21: 999-1007.

24) Benetti AR, Peutzfeldt A, Lussi A, Flury S. Resin composites: Modulus of elasticity and marginal quality. J Dent 2014; 42: 1185-1192.

25) El-Damanhoury H, Platt J. Polymerization shrinkage stress kinetics and related properties of bulk-fill resin composites. Oper Dent 2014; 39: 374-382.

26) Gonçalves F, Azevedo CLN, Ferracane JL, Braga RR. BisGMA/TEGDMA ratio and filler content effects on shrinkage stress. Dent Mater 2011; 27: 520-526.

27) Gonçalves F, Kawano Y, Braga R. Contraction stress related to composite inorganic content. Dent Mater 2010; 26: 704-709.

28) Ellakwa A, Cho N, Lee IB. The effect of resin matrix composition on the polymerization shrinkage and rheological properties of experimental dental composites. Dent Mater 2007; 23:1229-1235.

29) Peutzfeldt A. Resin composites in dentistry: the monomer systems. Eur J Oral Sci 1997; 105: 97-116.

30) Jang J, Park S, Hwang I. Polymerization shrinkage and depth of cure of bulk-fill resin composites and highly filled flowable resin. Oper Dent 2015; 40: 172-180.

399Dent Mater J 2018; 37(3): 393–399