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Theeffectofbulk-fillflowablecompositesonthefractureresistanceandcuspaldeflectionofendodonticallytreatedpremolars

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The effect of bulk-fill flowablecomposites on the fracture resistanceand cuspal deflection of endodonticallytreated premolarsTuğba Toza, Safa Tuncerb, Funda Öztürk Bozkurta, Aysun Kara

Tuncerc & Harika Gözükara Bağd

a Department of Restorative Dentistry, School of Dentistry,Istanbul Medipol University, Istanbul, Turkeyb Department of Restorative Dentistry, School of Dentistry,Istanbul University, Istanbul, Turkeyc Department of Endodontics, School of Dentistry, BezmialemUniversity, Istanbul, Turkeyd Faculty of Medicine, Department of Biostatistics, InonuUniversity, Malatya, TurkeyPublished online: 27 Apr 2015.

To cite this article: Tuğba Toz, Safa Tuncer, Funda Öztürk Bozkurt, Aysun Kara Tuncer & HarikaGözükara Bağ (2015): The effect of bulk-fill flowable composites on the fracture resistance andcuspal deflection of endodontically treated premolars, Journal of Adhesion Science and Technology,DOI: 10.1080/01694243.2015.1037381

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The effect of bulk-fill flowable composites on the fracture resistanceand cuspal deflection of endodontically treated premolars

Tuğba Toza, Safa Tuncerb, Funda Öztürk Bozkurta*, Aysun Kara Tuncerc and HarikaGözükara Bağd

aDepartment of Restorative Dentistry, School of Dentistry, Istanbul Medipol University, Istanbul,Turkey; bDepartment of Restorative Dentistry, School of Dentistry, Istanbul University, Istanbul,

Turkey; cDepartment of Endodontics, School of Dentistry, Bezmialem University, Istanbul,Turkey; dFaculty of Medicine, Department of Biostatistics, Inonu University, Malatya, Turkey

(Received 24 December 2014; final version received 28 March 2015; accepted 31 March 2015)

The aim of this study was to evaluate the effect of bulk-fill flowable composites oncuspal deflection and fracture resistance of endodontically treated teeth. Forty-twomaxillary premolars were subjected to endodontic treatment followed by thepreparation of mesioocclusodistal cavities. Teeth were divided into six groupsaccording to restorative materials as follows: Group 1: Clearfil Majesty Flow andClearfil Majesty Posterior; Group 2: Venus Bulk Fill and Clearfil Majesty Posterior;Group 3: Clearfil Majesty Posterior; Group 4: Vertise Flow and Clearfil MajestyPosterior; Group 5: SDR and Clearfil Majesty Posterior; and Group 6: x-tra baseand Clearfil Majesty Posterior. A single-step self-etch adhesive (OptiBondAll-in-One) was applied to all groups, except Group 4. The cavities were restoredwith a centripetal incremental insertion technique and flowable composites using a2-mm-thick base material, except for Group 3. The distance between cusp tips wasmeasured before and after the cavity preparations, after the restorations, and afterthermal cyclus with a digital micrometer. After measuring, each tooth was subjectedto compressive loading with a stainless steel ball (4 mm diameter) perpendicular tothe occlusal surface with a crosshead speed of 1 mm/min, and mean loads necessaryto fracture were recorded in Newtons. The data were statistically analyzed byKruskal–Wallis test. No statistically significant differences were found betweengroups in fracture strength or cuspal deflections (p > 0.05). Bulk-fill flowablecomposite bases did not change the cuspal deflection or fracture resistance ofendodontically treated teeth, compared with that of a conventional flowable baseand conventional resin composite.

Keywords: bulk-fill composite; fracture resistance; cuspal deflection; endodonticallytreated teeth

1. Introduction

The extraction of restored endodontically treated teeth generally depends on crownfractures, secondary caries, and failed restorations because they are more susceptible totooth fractures due to the coronal destruction observed after removing of deep caries orprevious restorations, endodontic treatments [1,2] that lead moisture loss and increasedbrittleness.[1] The restorative techniques for endodontically treated teeth have varying

*Corresponding author. Email: fbozkurt@medipol.edu.tr

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drawbacks such as the amount of remaining tooth structure that have resulted inimportant criticisms.[3] Mesioocclusodistal preparations generally caused the formationof extended cavity size that are subjected to cuspal fracture. It was revealed in theliterature that adhesive restorations might have the potential to restore sound tooth stiff-ness and has to mimic the missing tooth structure and also increase fracture resis-tance.[4,5] Current adhesive technology offers direct adhesive restorations forendodontically treated teeth as an alternative, low-cost esthetic technique. Surely, thedecision of restorative technique would depend on the missing tooth structure and it isalso definitely that in addition to the esthetic expectations, an acceptable restorationmust assure function and maintain the tooth structure against fracture.[3]

One important disadvantage of resin composite restorations is polymerizationshrinkage that may cause shrinkage stress on the tooth and tooth–restoration interfaces.Cuspal deflection is result of the interactions between this shrinkage stress and theadaptation of restorative material to cavity. This may breakdown the adhesion at thetooth–restoration interface, generally leading to nanoleakage and recurrent caries.[6,7]Cuspal deflection influenced from two different category of factors. The factors ofmaterial itself (polymerization shrinkage, flow, marginal adaptation) and geometricproperties (cavity size and remaining tooth structure) [6,8–10] could be accepted as thefirst category. The other category includes clinical factors for the use of cavity liner[6,10] restorative [11,12] and polymerization [13] technique. Cuspal deflection increasesparallel with cavity size. With larger restorations, the stress is occurred higher on thetooth, while it is observed lower on the restoration and tooth–restoration interface. Therisk of tooth fracture is also influenced by this stress distribution.[6,7]

Flowable composites can be a different alternative material for increasing fractureresistance. One of the advantages of these composites is their close adaptation withcavity walls. It has also been reported that the use of flowable composites as cavity lin-ers reduces cuspal flexure.[14] Elastic cavity wall concept that is explained with theapplication of flowable composites generally to the cervical part of the tooth as anintermediate layer and suggested for direct restorative techniques.[15] In this concept,the shrinkage stress of a resin composite which shows higher elastic modulus can becompensated by this elastic flowable increment.[10] It was also known that the cuspaldisplacement was affected by the elastic modulus of the composites.[16]

To simplify the restoration procedure, nowadays contribution to adhesive dentistryis bulk-fill flowable composites that can be placed up to 4 mm thick.[17,18] The poly-merization kinetics of these materials are claimed to be better due to the enhancedtranslucency and enable them to be cured up to a 4 mm depth. The polymerizationstress of bulk-fill flowable composites was lower than conventional flowable compos-ites,[19,20] and the marginal integrity of these materials are observed like convention-ally layered composite restorations.[18,19,21]

The aim of the current study was to evaluate the fracture strength, failure modes,and cuspal deflection of endodontically treated maxillary premolars restored with differ-ent types of bulk-fill flowable and flowable resin-based composites. The null hypothe-ses were that bulk-fill flowable composites did not have an effect on the cuspaldeflection of direct restorations of endodontically treated teeth and there was not sta-tistically significant difference on the fracture resistance of conventional flowable base,conventional resin composite and bulk-fill flowables.

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2. Materials and methods

The tooth samples were collected from a pool of extracted teeth of Dental Clinics ofİstanbul Medipol University. The teeth were examined under 10× stereomicroscope forcracks, developmental defects, caries, and forty-two maxillary single-rooted premolarswere selected for the study. Teeth of similar sizes (width varied between 6.38 and7.08 mm) were chosen by measuring the buccolingual width in millimeters with usinga digital micrometer gauge (Series 480-505, resolution 1 μm; SHAN TM PrecisionMeasuring Instruments, Guilin, China) and allowing a maximum deviation of 10%from the determined mean. The teeth were washed in tap water and stored in distilledwater at 37 °C, which was changed every 5 days during the study.

2.1. Cuspal deflection measurement

The buccal and palatal cusp tips of each specimen were acid-etched (Prime-Dent, PrimeDental, USA) for 30 s, washed for 30 s, and dried. A two-step etch and rinse adhesive(Single Bond 2, 3M ESPE, USA) was applied according to the manufacturer’s recom-mendations and polymerized for 20 s. Conventional flowable composite (ClearfilMajesty Flow, Kuraray, USA) was applied to the buccal and palatal cusps as referenceballs to measure the intercuspal distance followed by polymerization (20 s). The dis-tance between the reference balls was measured with a digital micrometer and recordedas the ‘initial distance’ and measurements were done after all restorative procedures(before the endodontic and restorative procedure, after restorations and the data wererecorded.[22]

2.2. Preparation procedure

Before the preparations, the outlines of the cavities were drawn with pencil, and paral-lel-sided MOD standardized cavities were prepared with a 1 mm gingival cavosurfacemargin coronal to the cementoenamel junction (CEJ). The bur (835-012-4, Diatech,Switzerland) was replaced after every fourth cavity preparation in order to ensure high-cutting efficacy. The cavities have a flat floor, with a width that was one-third of theintercuspal distance for the occlusal portions of the preparations and one-third of thetotal faciolingual dimension for the proximal boxes. The cavosurface margins wereprepared at 90°, and all internal angles were rounded.

2.3. Endodontic treatment

After the preparations, a conservative, endodontic access was performed on the pulpchamber wall. After that, all canals were prepared with Pro-Taper Ni-Ti Rotary System(Dentsply Malleiffer) and obturated with an AH 26 sealer (Dentsply; DeTrey, Konstanz,Germany) and gutta-percha using a lateral compaction technique. Gutta-percha wasremoved, and excess sealer was removed by cotton pellet with alcohol. Following theendodontic treatments, the teeth were randomly divided into six groups (n = 7) accord-ing to the restorative materials. The materials for the restorative procedures are listed inTable 1.

2.4. Restorative procedure

Group 1: A single-step self-etch adhesive (OptiBond All-in-One, Kerr, Italy) wasapplied to the enamel and dentin according to the manufacturer’s instructions. The

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surfaces were dried with gentle air blowing and then with a medium for at least 5 sand light cured for 10 s (Elipar Free Light, 3M ESPE, AG, Germany, 1007 mW/cm2).After adhesive polymerization, a metal matrix band (Adapt SuperCap Matrices, Kerr,Switzerland) was placed around the tooth. A thin layer of a composite (Clearfil MajestyEsthetic, Kuraray, Japan) was applied toward the metallic matrix contacting the cavo-surface of the proximal box up to half of the occlusal-cervical extension. The secondlayer was applied over the previous increment contacting the cavosurface margin of theproximal box and forming the marginal ridge. This procedure was applied for both themesial and distal margins and applied resin composites were polymerized for 40 saccording to the manufacturer’s instruction. Thus, the cavity turned into a class I asmentioned in centripetal technique. The flowable composite, Clearfil Majesty Flow, wasapplied at a 2 mm thickness horizontally to the resulting class I cavity and cured for20 s. After the flowable composite application, two increments of composite (ClearfilMajesty Esthetic, Kuraray, Japan) were applied to the buccal and palatal sides of theocclusal cavity obliquely and cured for 40 s.

Table 1. Materials tested in the study.

Materials Material type Composition ManufacturerLot

number

OptiBondAll-in-One

Single-stepself-etchadhesive

Acetone, hydroxyethylmethacrylate(HEMA), ethyl alcohol, disodiumhexafluorosilicate

KerrCorporation,USA

3514858

ClearfilMajestyFlow

Flowablecomposite

Triethylene glycol dimethacrylate(TEGDMA), hydrophobic aromaticdimethacrylate, silanated barium glassfiller, silanated colloidal silica, dl-Camphorquinone, accelerators, pigments,others

KurarayDental,Japan

00310B

ClearfilMajestyEsthetic

Nanohybridcomposite

Bisphenol A, diglycidylmethacrylate,hydrophobic aromatic, dimethacrylatehydrophobic, aliphatic methacrylate,silanated barium glass filler, pre-polymerized organic filler, dl-camphorquinone Initiators, accelerators,pigments others

KurarayDental,Japan

00041B

VenusBulkFill

Bulk-fillflowablecomposite

Methacrylate monomers (UDMA,EBADMA), Ba–Al–F silicate glass,YbF3, SiO2

HeraeusKulzer,Germany

66046795

VertiseFlow

Bulk-fillflowablecomposite

hydroxyethylmethacrylate (HEMA), 4methoxyphenol (MEHQ), zinc oxide(ZnO), pigments

KerrCorporation,USA

3500809

SDR Posteriorbulk-fillflowable base

Polymerizable dimethacrylate resins,polymerizable urethane dimethacrylate,barium boron fluoro alumino silicateglass, silicon dioxide – amorphous,strontium aluminosilicate glass, titaniumdioxide, synthetic inorganic iron oxides(colorants)

DENTSPLYCaulk, USA

1208224

x-trabase Bulk-fillflowablecomposite

Aliphaticdimethacrylate, catalyst Voco,Germany

1216598

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Group 2: OptiBond All-in-One was applied to the enamel and dentin according tothe manufacturer’s instructions. After the matrix application, Clearfil Majesty Estheticwas applied toward the metallic matrix on both mesial and distal margins and cured for40 s as described above. The bulk-fill flowable composite (Venus Bulk Fill, HeraeusKulzer, Germany) was applied at a 2 mm thickness horizontally to the resulting class Icavity and cured for 20 s. After the flowable composite application, the remaining cav-ity was restored with Clearfil Majesty Esthetic, as mentioned above.

Group 3 (control): After the application of OptiBond All-in-One, the cavities wererestored with Clearfil Majesty Esthetic by using a centripetal technique. In this tech-nique after polymerization of mesial and distal margins, Clearfil Majesty Esthetic wasinserted to the created class I cavity as four oblique layers.

Group 4: Self-adhering flowable composite (Vertise Flow, Kerr Corporation,Orange, CA, USA) was dispensed in the cavity, brushed onto all cavity walls and thebeveled area with moderate pressure for 15–20 s, and light cured for 20 s. After therestorations of the marginal ridges with Clearfil Majesty Esthetic using a centripetaltechnique, Vertise Flow was applied at a 2 mm thickness horizontally and cured for20 s. After the flowable composite application, the remaining cavity was restored withClearfil Majesty Esthetic, as mentioned for Groups 1 and 2.

Group 5: After the restorations of the mesial and distal marginal ridges with Clear-fil Majesty Esthetic, a bulk-fill flowable composite (SDR, Dentsply, Germany) wasapplied at a 2 mm thickness horizontally to the resulting class I cavity and cured for20 s. The remaining cavity was restored as described in Groups 1, 2, and 4.

Group 6: After the restorations of the mesial and distal marginal ridges with Clear-fil Majesty Esthetic, bulk-fill flowable composite (x-trabase, Voco, Germany) wasapplied at a 2 mm thickness horizontally, and the remaining cavity was restored asdescribed in all groups except Group 3.

2.5. Fracture testing

One dental operator made all the preparations and restorations. After the cuspal deflec-tion measurements, the teeth were then subjected to 500 thermal cycles, each with adwell time of 20 s at 5 and 55 °C and handled in moist gauze to prevent dehydration.A thin coat of wax (0.3 mm) was applied on the external root surface of all teeth. Theteeth were vertically mounted to a level of 1.0 mm apical to the CEJ, with the longaxis of the tooth parallel to that of the acrylic resin molds. The wax on the root sur-faces was purified with boiling water, and this space was filled with silicone paste(Oranwash vl, Zhermark, Germany) 1 mm apical to the CEJ to simulate periodontalligament. The specimens were stored in 100% humidity for 24 h before fracture testing.The specimens were placed in a universal testing machine (Autograph AG-IS,Shimadzu Co, Kyoto, Japan) with the long axis of the roots parallel to the load direc-tion after storing. A stainless steel ball (4 mm diameter) was used to load the speci-mens until fracture, with a crosshead speed of 1 mm/min. The loading site was thecentral fissure and the ball was contacted to occlusal surface of the restorations and thebuccal and palatinal cusps of the teeth, while the mean loads were recorded inNewtons. The failure modes (adhesive, cohesive, or mixed) were evaluated andassessed using standard criteria [23] at a magnification of 20×. Fractures were charac-terized as ‘restorable’ if they were limited to the coronal portion and included retentionfailures and ‘not restorable’ if the fractures reached the root.

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2.6. Statistical analysis

The fracture load and cuspal deflection data were expressed as median, minimum, andmaximum values. Since the data had a non-normal distribution, the difference amonggroups was statistically analyzed using a non-parametric one-way ANOVA (Kruskal–Wallis) test at a 5% significance level. All data were analyzed using SPSS 11.5 forWindows software (SPSS Inc., Chicago, IL, USA).

3. Results

The mean buccal–palatal width of the maxillary premolar teeth did not vary signifi-cantly between the specimen groups (p > 0.05). The forces (N) required to fracture theteeth in each group are displayed in Figure 1 and Table 2. The Kruskal–Wallis testdemonstrated no significant differences (p = 0.18) with respect to fracture resistanceamong all groups. The influence of using bulk-fill flowables on the cuspal movementwas not statistically significant (p = 0.63) for all groups. The cuspal deflection valuesfor the experimental groups are shown in Figure 2 and Table 3. Table 4 shows the fail-ure mode results of all the experimental groups in the study. It was observed that thefractures were generally not restorable.

Figure 1. Fracture resistance values.

Table 2. The minimum, maximum, and median forces of experimental groups (N).

Groups Median Minimum Maximum

Group 1 (Clearfil Majesty Flow) 653.59 486.72 886.09Group 2 (Venus Bulk Fill) 642.81 523.75 919.84Group 3 (Clearfil Majesty Posterior) 782.97 534.06 998.13Group 4 (Vertise Flow) 830.00 623.59 948.13Group 5 (SDR) 692.97 426.09 806.09Group 6 (x-trabase) 691.17 503.13 749.06

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4. Discussion

Maxillary premolar restorations may undergo palatal and buccal strains, as a result ofocclusal chewing loads which is related with high stress inside the tooth–restorationstructure. However, if the strain is higher from the maximum resistance of the tooth,gap formation, nanoleakage, crack, or fracture may be occurred and also the restorationmay have failed.[24] Thus, in this study, maxillary premolars were used to compare thefracture resistance of different restorative materials regard to mechanical occlusal load-ing.

Numerous investigations have been carried out to decide out the ideal way torestore endodontically treated teeth. In a literature, it was mentioned that tooth fracturedecreased when MOD cavities were restored with direct composite restoratives.[25]According to Reeh and Messer,[26] the access of endodontic treatment within intactteeth reduces fracture resistance and when it was combined with an MOD cavitypreparation, the resistance is reduced more. Mondelli et al. [27] reported that maxillarypremolars (no endodontic treatment) with MOD cavity preparations close to 2.5 mmdeep presented only 36% of the fracture resistance recorded for that of sound premo-lars. They related the difference to the smaller remaining tooth structure followingMOD cavity preparations, similar to the study of Burke.[28] Adhesive restorations havealso been suggested for larger cavities of endodontically treated teeth.[29] Large MODcavities were prepared in maxillary premolars with endodontic access in our studyaccording to the knowledge of these restorations with low fracture toughness.

Figure 2. Cuspal deflection values.

Table 3. The minimum, maximum and median values of cuspal deflection (μm).

Groups Median Minimum Maximum

Group 1 (Clearfil Majesty Flow) 11.30 8.77 13.00Group 2 (Venus Bulk Fill) 11.77 5.81 15.20Group 3 (Clearfil Majesty Posterior) 11.30 7.40 13.97Group 4 (Vertise Flow) 9.90 9.40 14.03Group 5 (SDR) 10.00 8.60 12.30Group 6 (x-trabase) 8.27 7.60 13.03

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It is believed that the clinical performance of endodontically treated teeth dependson both root canal treatment and the restorative technique. Deciding the best techniqueand material according to functional demands and lost tooth structure are the key fac-tors for clinical success.[30,31] Direct adhesive restorations are able to resist functionalstresses and reinforce the weakened tooth structure.[32,33] The low elastic modulus ofresin composites may explain the energy transmitted to the tooth structure.[32,34]Today’s nano technology made small modifications on resin composites to have supe-rior mechanical properties like increased elastic modulus and easy adaptation tech-niques.[35] Here in this study, we decided to use direct adhesive restorative materials,which were developed with nanotechnology to present superior properties.

Although it was known that flowable composites shrink more than traditional resincomposites,[36] bulk-fill flowables have lower shrinkage stress. The stress-relievingflowability is potentially claimed by the manufacturer (Dentsply) as “SDR” resin mono-mer [20]; SDR’s enhanced translucency promotes light transmittance and better poly-merization kinetic up to 4 mm. Several new materials are also claimed bulk-fillpolymerization up to 4 mm (Tetric EvoCeram Bulk Fill, Ivoclar-Vivadent, Schaan,Liechtenstein) and 5 mm (SonicFILL, Kerr, Orange, CA, USA). Bulk-fill restorativeswith enhanced polymerization and controlled shrinkage [35–37] reduce chairside timeof restorative treatments.[21]

Van Meerbeek et al. [15] observed that the use of a low modulus flowable compos-ite may increase the flexibility by allowing it to act as a stress breaker. Alshali et al.[38] reported that the bulk-fill composites’ (SDR and Venus Bulk Fill) shrinkage weregenerally comparable to those of the conventional composites that they tested. However,in other studies,[20] it was reported that SDR showed 60–70% less polymerizationshrinkage stress than that of the conventional resin composites. In our study, the frac-ture resistance of four different bulk-fill flowable composites was not statistically dif-ferent. The use of flowable composite resin material is believed to relieve stress,promote adaptation, and work against fractures; however, it did not support the findings(the differences were not statistically significant) of our study. The median fractureforces were observed as 653.59 in Group 1, 642.85 in Group 2, 782.97 in Group 3, 830in Group 4, 692.97 in Group 5, and 691.17 in Group 6. This similarity would be relatedto the bulk application technique limited with 2 mm thickness. The manufacturers also

Table 4. The failure mode results of experimental groups.

Groups Restorable Unrestorable Adhesive Cohesive Mix

Group 1 (Clearfil Majesty Flow) 1(14.28%)

6 (85.72%) 1(14.28%)

0 (0%) 6(85.72%)

Group 2 (Venus Bulk Fill) 2(28.57%)

5 (71.43%) 3(42.85%)

0 (0%) 4(57.14%)

Group 3 (Clearfil MajestyPosterior)

1(14.28%)

6 (85.72%) 7 (100%) 0 (0%) 0 (0%)

Group 4 (Vertise Flow) 4(57.14%)

3 (42.86%) 5(71.43%)

0 (0%) 2(28.57%)

Group 5 (SDR) 3(42.85%)

4 (57.14%) 4(57.14%)

1(14.28%)

2(28.57%)

Group 6 (x-trabase) 2(28.57%)

5 (71.43%) 3(42.86%)

0 (0%) 4(57.14%)

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mentioned that bulk-fill flowables could be applied in bulk with a single layer of up to4 mm thickness; perhaps this thickness would change the polymerization shrinkagestress as well as fracture resistance.

Regarding fracture mode, majority of the specimens in the groups exhibited non-restorable failures, except in the group restored with Venus fill. However, the differencewas not statistically different. Failures typically occurred by adhesive breakdown at thebuccal or palatal interface, with a cuspal fracture obliquely extending from the lowerpart of the buccal or palatal line angle. Fracture patterns generally varied related toloading conditions for example with vertical loading, the fracture was occurred verti-cally through the restoration extending into the root and cause marginal failure wherethe cuspal fracture was also seen.[2,23]

The cuspal deflection occurred during restorative treatment is related to many fac-tors, such as size and configuration of preparations or the adhesive materials used.[7]Although the incremental technique is preferred, it is not clear that this technique canreduce the deflection compared to the bulk-fill restorations. Some studies also reportedthat cuspal deflection was reduced with incremental technique,[6,39] whereas othersmentioned that this technique was not better than bulk-fill restorations.[40,41] It wasalso reported that the cuspal deflection with bulk-fill application technique wasobserved significantly lower compared with incremental cure due to the incompletepolymerization of resin composite applied as bulk.[40] However, this fact cannot beexpected for bulk-fill flowables applied as up to 4–5 mm thickness due to the bettercontrolled polymerization kinetics of these materials. To standardize the materials cur-ing depths, the cavities were restored with centripetal technique, and the thickness ofall flowables including bulk fills applied in bulk and 2 mm thick in our study. It wassubjected that flowable composites shrink more[20]; thus, we could except that cuspaldeflection values must be higher for Groups 1 and 4; and through the knowledge of theeffect of incremental technique on shrinkage stress decrease,[6,39] it could be exceptedthat cuspal deflection values must be lower for Group 3. However, the cuspal deflectionvalues did not vary among the groups. This finding could be related to the lower poly-merization shrinkage of bulk-fill flowable composites.

Taha et al. [7] observed the mean cuspal deflections as 6 μm for low shrink com-posites and 10–40 μm reported for high shrink composites similar to some other litera-ture.[10,42,43] In our study, the cuspal deflection values we observed were 11.30(Group 1), 11.17 (Group 2), 11.30 (Group 3), 9.9 (Group 4), 10.00 (Group 5), and8.27 (Group 6), respectively, like high shrink composites. It is important to determinewhether the lower cuspal deflection in bulk cure is because of an incomplete cure oranother reason, as Versluis et al. [44] indicated. If it is occurred due to an incompletepolymerization, it would have different results when curing time or density wasextended.

On fracture mode, adhesive failure was observed on all specimens of Group 3. Thissituation could be explained with better mechanical properties of traditional resin com-posites when compared to flowable ones. However, it can be thought that due to thehigher elastic modulus of these conventional resins, 85.72% of the fractures observedin Group 3 specimens were unrestorable failures. In addition to this, more restorablefractures were observed in the Groups 4 and 5 but these differences were not statisti-cally significant. Further studies with larger specimen numbers and with differentrestorative groups are needed to find out the differences between the flowable andconventional resin composites.

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Consequently, the null hypothesis tested, which stated that bulk-fill flowablecomposites would not have an effect on the cuspal deflection and fracture resistance ofdirect restorations of endodontically treated teeth, was accepted.

5. Conclusion

Within the limitations of this study, it can be concluded that using bulk-fill flowablesdid not change the cuspal deflection or fracture resistance of endodontically treatedpremolars. The findings would be different if the restorations were done only in bulkand not by using a centripetal technique or if the thickness of bulk-fill composite were4 mm or more.

Disclosure statementNo potential conflict of interest was reported by the authors.

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