Mechanical properties, shrinkage stress, cuspal strain and fracture resistance of molars restored with bulk-fill composites and incremental filling technique

Accepted Manuscript Title: Mechanical properties, shrinkage stress, cuspal strain and fracture resistance of molars restored with bulk-fill composites and incremental filling technique Author: C.M.P. Rosatto A.A. Bicalho C. Ver´ıssimo G.F. Braganc¸a Rodrigues M.P. D. Tantbirojn A. Versluis C.J. Soares PII: DOI: Reference:

S0300-5712(15)30047-6 http://dx.doi.org/doi:10.1016/j.jdent.2015.09.007 JJOD 2527

To appear in:

Journal of Dentistry

Received date: Revised date: Accepted date:

1-7-2015 25-9-2015 28-9-2015

Please cite this article as: Rosatto CMP, Bicalho AA, Ver´issimo C, Braganc¸a GF, Rodrigues MP, Tantbirojn D, Versluis A, Soares C.J.Mechanical properties, shrinkage stress, cuspal strain and fracture resistance of molars restored with bulk-fill composites and incremental filling technique.Journal of Dentistry http://dx.doi.org/10.1016/j.jdent.2015.09.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mechanical properties, shrinkage stress, cuspal strain and fracture

resistance

of

molars

restored

with

bulk-fill

composites and incremental filling technique

Short title: Biomechanical behavior of bulk-fill composites

C.M.P. Rosattoa; A.A. Bicalhoa; C. Veríssimoa; G.F. Bragançaa; Rodrigues M.P.a; D. Tantbirojnb; A. Versluisc; C.J. Soaresa,*

a

Department of Operative Dentistry and Dental Materials, Dental School, Federal

University of Uberlândia, Uberlândia, Minas Gerais, Brazil. b

Department of Restorative Dentistry, College of Dentistry, University of Tennessee

Health Science Center, Memphis, Tennessee, USA. c

Department of Bioscience Research, College of Dentistry, University of Tennessee

Health Science Center, Memphis, Tennessee, USA.

*Corresponding author: Prof. Dr. Carlos José Soares Av. Pará, 1720, Bloco 4L, Anexo A, Campos Umuarama, 38400-902 Uberlândia, Minas Gerais, Brasil Phone: +55-34-3225-8106 E-mail: [email protected]

1

ABSTRACT Objectives: To compare bulk-fill with incremental filling techniques for restoring large mesio-occlusal-distal (MOD) restorations. Methods: Seventy-five molars with MOD preparations were divided into five groups: Z350XT, incrementally filled with Filtek Z350XT and four bulk-fills - FBF/Z350XT, Filtek Bulk Fill/Filtek Z350XT; VBF/CHA, Venus Bulk Fill/Charisma Diamond; SDR/EST-X, SDR/Esthet-X HD; TEC, TetricEvoCeram Bulk Fill. Cuspal strains were measured using strain-gauges (n=10): CSt-Re, during restorative procedure; CSt-100N, during 100N occlusal loading; CSt-Fr, at fracture load. Before fracture load, teeth were load-cycled. Fracture resistance, fracture mode, and enamel cracks were recorded. The other five teeth were used for Elastic modulus (E) and Vickers hardness (VH). Post-gel shrinkage (Shr), diametral tensile strength (DTS) and compressive strength (CS) were determined (n=10). Shrinkage stresses were analyzed using finite element analysis. Results: SDR had similar CS values as TEC, lower than all other composites. CHA had similar DTS values as Z350XT, higher than all other composites. Z350XT had the highest mean Shr and SDR the lowest Shr. New enamel cracks and propagation was observed after the restoration, regardless of filling technique. Z350XT had lower fracture resistance than bulk-fill composite techniques. No significant differences in failure modes were found. E and VH were constant through the depth for all techniques. Bulk-filling techniques had lower stresses compared to Z350XT. Conclusions: Flowable bulk-fill composites had lower mechanical properties than paste bulk-fill and conventional composites. All bulk-fill composites had lower post-gel shrinkage than conventional composite. Bulk-fill filling techniques resulted in lower cusp strain, shrinkage stress and higher fracture resistance.

Clinical significance. Using bulk-fill composites cause lower CSt wich indicates lower stress in restored tooth. Furthermore, bulk-fill composites have a higher fracture resistance. Therefore, clinicians may choose the bulk-fill composite to decrease undesirable effects of restoration while simplifying filling procedure.

Keywords: composite resin, bulk-fill, cuspal strain, shrinkage stress, mechanical properties, finite element analysis.

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1. Introduction Resin composites have been extensively employed in restorative dentistry for several decades [1]. More than five hundred millions direct dental restorations are placed every year around the world which representes one of the most prevalent medical interventions in the human body [2]. Incremental filling technique has been used for placement of resin composite restoration [3-5]. However, the post-operative sensivity is frenquently observed, which is commonly associated with polymerization shrinkage stresses [5]. Different filling techniques and composite resins have been developed in order to minimize polymerization shrinkage and their clinical effects [6]. The latest trend in composite technology was the development of the so-called “bulk-fill” composites [7]. These new materials were created in order to cure up to 4 mm deep [8]. Bulk-fill composites are gaining popularity among the clinicians because they simplify the restorative procedure by reducing the number of composite layers and thus the curing time [2]. The deeper cure in bulk-fill composites is made possible by adjustements in translucency and photoinitiators. An additional initiator system has been introduced in Tetric EvoCeram Bulk Fill (Ivocerin), which is described as a germanium based initiator system with a higher photocuring activity than camphoroquinone [9]. Meanwhile, their opacity is an advantage over other bulk-fill composites in terms of acceptable esthetics for placement in the visible zone, e.g., mesial class II restorations [10]. Composite resin composition and filling techniques are among the primary approaches to reduce volumetric contraction and shrinkage stress development [11, 12]. Restoration placement in increments or bulk is also widely considered a factor in the modification of shrinkage stresses [3, 4, 13]. Although Incremental filling techniques have often been assumed to decrease the shrinkage stresses, finite element analysis has shown that bulk placement may produce lower residual shrinkage stresses [5,14]. The concept of bulk-filling is not a novel idea [15], and has been evaluated numerous times in the literature [13, 16-21]. However, how much stress is generated by polymerization shrinkage depends on more factors than mechanical properties of resin composites and restorative filling techniques, such as curing light intensity, photoactivation time, mechanical properties of the tooth structure, and geometry and extent of the cavity [1422]. 3

The success of composite resin restorations is associated with their mechanical properties [4]. The first-generation flowable composites were not suitable for full-depth posterior fillings because of their inferior mechanical properties and increased volumetric shrinkage compared to conventional paste-like composites, primarily due to the lower filler content [23-26]. Bulk-fill seems very similar in chemical composition as regular nanohybrid and microhybrid resin composites [27]. Some bulk-fill composites require a final 2 mm increment of a conventional composite material while other bulkfill composites can be placed without this final layer. This different application of the same material class may confuse some practitioners [28]. Few studies have examined the complete biomechanical performance of the new bulk-fill materials. Therefore, the aim of this study was to evaluate the mechanical properties expressed by hardness, elastic modulus, post-gel shrinkage, compressive and tensile strength and the biomechanical performance expressed by cuspal strain, enamel crack detection, fracture resistance and stress distributions of new bulk-fill composites in molars with large class II mesio-occlusal-distal (MOD) restorations. The null hypothesis tested was that the biomechanical behavior would not be affected by the restorative material and filling technique (bulk-fill or conventional composites).

2. Materials and Methods 2.1. Study design Four bulk-fill and one incrementally placed conventional composites were tested in this study. All composite resins were tested for compressive strength (CS), diametral tensile strength (DTS), which were used to determine the Modified von Mises stresses, and post-gel shrinkage (Shr). Human molars with MOD cavities were restored according to manufacturer’s instructions. Teeth were tested for cuspal strain using strain-gauges during filling (CSt- Re), during 100N occlusal loading (CSt- 100N), and at fracture (CSt-Fr). Enamel crack were detected and tracked (Ect) using transillumination. Fracture strength (Fs) and fracture mode (Fm) were tested in axial occlusal compressive loading. Vickers hardness (VH) and Elastic modulus (E) of the composites were tested at different depths using dynamic indentation. Finally, shrinkage stresses and the stresses during compressive loading were evaluated by finite element analysis (FEA) using Modified von Mises (mvm) and Critical Modified von Misses (Crmvm). 4

2.2. Composite resins Seven commercial materials including four bulk-fill and three conventional composites were tested. The composites and their filler percentage and matrix information are listed in Table 1. The MOD restorations were perfomed using an incremental technique (Filtek Z350XT, 3M ESPE) or a bulk fill technique (Filtek Bulk Fill/Filtek Z350XT, 3M ESPE; Venus Bulk Fill/Charisma Diamond, Heraeus Kulzer; SDR/Esthet-X HD, Dentsply; TetricEvoCeram Bulk Fill, Ivoclar Vivadent), according to manufacturer’s instructions.

2.3. Tooth selection and cavity preparation Seventy-five extracted intact caries-free human molars were used (Ethics Committee in Human Research approval #721985). The teeth were selected to have an intercuspal width within a maximum deviation of 10% from the determined mean (range between 4.8 mm to 6.0 mm). The roots were covered with a 0.3 mm layer of a polyether impression material (Impregum; 3M ESPE, St Paul, Minn) to simulate periodontal ligament, and embedded in a polystyrene resin (Cristal, Piracicaba, SP, Brazil) up to 2 mm below the cemento-enamel junction to simulate the alveolar bone [29]. The teeth were cleaned using a rubber cup and fine pumice water slurry. Class II mesio-occlusal-distal (MOD) cavities were prepared in all specimens with 4/5 of the intercuspal width, 4 mm depth in occlusal box and 1 mm in proximal box with a diamond bur (#3146 diamond bur, Microdont) with copious air-water spray using a cavity preparation machine [30]. The teeth were randomly divided into five groups (n = 15) following the materials and filling techniques. Ten restored teeth per group were used for measuring cuspal strain and afterwards for transillumination, mechanical fatigue, marginal adaptation and compressive fracture resistance tests. The other five restored teeth were used for Vickers Hardness (VH) and Elastic modulus (E) measurements.

2.4. Cuspal strain during restorative procedure (CSt-Re) Cuspal strain was measured with strain gauges (PA-06-060CC-350L, Excel Sensores, Embú, SP, Brazil), which had an internal electrical resistance of 350 Ω, a gauge factor of 2.07, and a grid size of 21.02 mm2. The strain gauges were fixed in the region where the finite element model indicated the presence of the highest 5

polymerization strains and stresses [5]. Two strain gauges were placed on the external surface of the buccal and lingual cusps, next to the class II MOD cavity base. In addition, two strain gauges were fixed to another tooth with the same cavity preparation to compensate for temperature effects. The strain gauges were bonded with cyanoacrylate-based adhesive (Super Bonder; Loctite, Itapeví, SP, Brazil), and connected to a data acquisition device (ADS2000; Lynx, São Paulo, SP, Brazil). Following the cavity preparation, the teeth were restored using a bulk-fill material in the dentin area (4.0mm in thickness), which was subsequently covered with the conventional resin composite in the enamel area (1.0mm in thickness). The composites were light-activated in 2 increments using a quartz-tungsten-halogen unit (800 mW/cm2; Optilux 501, Kerr Mfg. Co., Orange, CA, USA) according to manufacture’s instructions: 40 s for Filtek Bulk Fill and 20 s for all other composites. The Tetric EvoCeram Bulk Fill was used for restoring both dentin and enamel in bulk increment. Cavities in the control group were filled incrementally in six increments with the conventional resin composite. A Teflon matrix was made to standardize each composite resin increment before the insertion into the cavity [4]. In addition, a device was created to simulate adjacent premolar and molar to allow interproximal contact during restoration. The cuspal strains were acquired at 4 Hz during the restoration procedures and continued for 5 minutes after curing the last increment (AqDados 7.02 and AqAnalisys data acquisition software; Lynx).

2.5. Enamel crack detection and tracking The samples were evaluated three instances during the experiment to detect enamel crack presence and propagation in buccal and lingual cusps (29): A) intact tooth before preparation; B) after cavity preparation; and C) 24h after restoration. The images of the sample were captured at ×1.5 magnification under standardized conditions (Nikon D60 and Nikkor 105 mm macro lens, Chiyoda, Tokyo, Japan) using transillumination LED light (Photonita, P1050, Florianópolis, SC, Brazil), with the optic fiber illuminator positioned on the occlusal surface of the tooth. Three examiners, blinded to the group identities, evaluated the images. Preexisting cracks were differentiated from thos created during composite polymerization.

Cracks were

6

categorized as described by Batalha-Silva et al., 2013 [31]: (I) no cracks visible, (II) visible cracks smaller than 3 mm, and (III) visible cracks larger than 3 mm (Figure 1).

2.6. Mechanical cycling test After cuspal strain measurements, chewing cycles were simulated to induce mechanical fatigue (Biocycle, Biopdi, São Paulo, SP, Brazil). The specimens were submerged in water maintained at about 37oC and cycled 1,200,000 times from 0 to 50 N axial compressive loading with a 8-mm diameter stainless steel sphere on the occlusal cusps with a 2 Hz frequency.

2.7. Cuspal strain during fracture procedure (CSt-Fr), fracture resistance and fracture mode Axial compressive loading with a stainless steel sphere 8 mm in diameter at a crosshead speed of 0.5 mm/min in a universal testing machine (DL2000; EMIC, São Jose dos Pinhais, PR, Brazil) with a 5000 N load cell. Strains were recorded under 100 N loading (CSt-100N) with strain gauges. The load required (N) to cause fracture of specimens was recorded on a computer with control and data acquisition software (TESC; EMIC). Strains were also recorded at failure load (CSt-Fr). The fracture modes of each specimen were evaluated by three operators and then assigned to one of four categories [32]: (I) fractures involving a small portion of the coronal tooth structure; (II) fractures involving a small portion of the coronal tooth structure and cohesive failure of the restoration; (III) fractures involving the tooth structure, cohesive and/or adhesive failure of the restoration, with root involvement that can be restored in association with periodontal surgery; and (IV) severe root and crown fracture, which require extraction of the tooth.

2.8. Vickers hardness (VH) and Elastic modulus (E) The remaining five specimens from each group were used for analysis of the mechanical properties (VH and E) of the composite resins at 5 depths. Each restored tooth was sectioned in the mesial-distal direction into two halves using a precision saw (Isomet 1000, Buehler, Lake Bluff, IL, USA). One section per tooth was randomly selected for assessment of the mechanical properties. The specimens were embedded in methacrylate resin (Instrumental Instrumentos de Medição Ltda, São Paulo, SP, Brazil). 7

Prior to testing, the surfaces were finished with silicon-carbide papers (#600, 800, 1200, and 2000 grit; Norton, Campinas, SP, Brazil) and polished with metallographic diamond pastes (6-, 3-, 1-, and 0.25-µm; Arotec, São Paulo, SP, Brazil). After polishing the specimens were cleaned using ultrasound for 10 minutes in distilled water. Using a Vickers

indenter

(CSM

Micro-Hardness

Tester;

CSM

Instruments,

Peseux,

Switzerland), indentations were made every 1.0 mm from 4.5 mm to 0.5 mm, starting from the restoration pulp walls. The indentations were carried out with controlled force, whereby the test load was increased or decreased at a constant speed ranging between 0 and 500 mN in 20 s intervals. The maximum force of 500 mN was held for five seconds. The load and the penetration depth of the indenter were continuously measured during the load-unload-cycle. The universal hardness was defined as the applied force divided by the apparent area of the indentation at the maximum force. The measurements were expressed in VH units by applying the conversion factor supplied by the manufacturer. The indentation modulus, comparable to the material’s elastic modulus (E), was calculated from the slope of the tangent of the indentation depth curve at the maximum force [33].

2.9. Post-gel shrinkage Post-gel linear shrinkage was determined using the strain gauge method [34]. Ten specimens were tested for each restorative material. The materials were shaped into a hemisphere on top of a biaxial strain gauge (CEA-06-032WT-120, Measurements Group, Raleigh, NC, USA) that measured shrinkage strains in two perpendicular directions. A strain conditioner (ADS0500IP, Lynx Tecnologia Eletrônica) converted electrical resistance changes in the strain gauge to voltage changes through a quarterbridge circuit with an internal reference resistance. The strain values measured along the two axes were averaged since the material properties were homogeneous and isotropic on a macro scale. All materials were light-cured using a quartz-tungsten-halogen unit (800 mW cm2, Optilux 501, Kerr Mfg. Co.) with the light tip held at 1 mm distance from the surface of the composite and the strain values were collected for 5 minutes. The mean shrinkage strain was used as linear post-gel shrinkage input for the finite element analysis, and could be converted to volumetric percentage by multiplying by 3 and 100%.

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2.10. Compressive and diametral tensile strength Compressive and diametral tensile strength (n = 10) of each composite resin were tested. The composite resin was placed into a cylindrical Teflon mold for the compressive strength test (6 mm height, 3 mm diameter) or the diametral tensile strength test (2 mm height, 4 mm diameter). The specimens for the compressive test made with bulk-fill composites were polymerized with 4.0 mm for the first increment and 2.0 mm for second increment. For conventional composite the specimens were polymerized in three 2.0 mm increments. The light source used (Optilux 501, Kerr Mfg. Co)had 800 mW/cm2 light intensity and was applied for the recommended curing times. Afterwards, the specimens were stored in distilled water for 24 hours at 37oC. The specimens were submitted to compressive strength and diametral tensile testing in a universal testing machine (DL2000, EMIC) at a crosshead speed of 0.5 mm/min until failure occurred. Compressive strength values (kgf/cm2) were calculated by dividing the fracture load (F) by the cross-sectional area and converted into MPa. Diametral tensile strength values (kgf/cm2) were calculated using the equation: DTS = 2F/πdt, where d is the specimen diameter, and t is the height of the specimen. DTS values were converted into MPa.

2.11. Residual Stress Calculation - Finite element analysis In order to calculate corresponding residual stress in the tooth, a twodimensional (2D) finite element simulation was carried out for the occlusal portion of a mesial-occlusal-distal restoration with the cavity floor in dentin with same dimensions as the experimental test. The geometric model was based on a digitized buccolingual cross section of a molar embedded in an acrylic resin block with similar dimensions and conditions (PDL simulation) as the teeth in the laboratory tests. Coordinates were obtained using ImageJ software (public domain, Java-based image processing and analysis software developed at The National Institutes of Health, Bethesda, MD, USA). Material properties used were: enamel elastic modulus 84 GPa and Poisson’s ratio 0.30; dentin elastic modulus 18 GPa and Poisson’s ratio 0.23, respectively [35]. The elastic moduli of the restorative materials were experimentally determined in this study and are shown in Table 6. The Poisson’s ratio was chosen to be the same for all composites at 0.24 [36].

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The finite element analysis was performed using MSC.Mentat (preprocessor and postprocessor) and MSC.Marc (solver) software (MSC Software Corporation, Santa Ana, CA, USA). The total number of FEA models was five for the different restorative materials used. A plane strain condition was assumed for the tooth cross-sections (enamel, dentin) and plane stress elements for the composites. Due to the 2D conditions, no distinction was made between the mesial and distal increments. Nodal displacements were constraint in both directions at the bottom and lateral surfaces of the support cylinder. Polymerization shrinkage was simulated by thermal analogy. Temperature was reduced by 1oC, while the linear shrinkage value (post-gel shrinkage) was entered as the coefficient of linear thermal expansion. After polymerization shrinkage simulation the models were submitted to a 100 N axial compressive load with a simulated sphere as was used in laboratory tests. Modified von Mises equivalent stress was used to express the stress conditions, using the ratio between compressive-tensile strength ratios (Stress Differential Effect SDE). The compressive-tensile strength ratios (SDE) for the restorative materials used are shown in the Table 2 and were calculated based on the experimental tests (compression strength/diametral tensile strength). The compressive and tensile strengths of enamel were 384.0 and 10.3 MPa and for dentin 297.0 and 98.7 MPa, respectively [37]. The Critical modified von Mises stress takes the difference between compressive and tensile strengths into account and scales the equivalent values relative to the enamel tensile strength. This was used to demonstrate the relative significance of the Modified von Mises stresses of all materials within one distribution plot since the significance of a stress in a material depends on its strength. In this study the relative value was the tensile strength of the enamel (10.3 MPa). Furthermore, microstrain values in the Ydirection (vertical) were obtained during the analyses at nodes of the buccal and lingual external surface corresponding to the same position where the strain gauge was fixed in laboratory tests.

2.12. Statistical Analysis The cuspal strain, fracture resistance, post-gel shrinkage, elastic modulus and hardness data were tested for normal distribution (Shapiro-Wilk, p>0.05) and equality of variances (Levene’s test), followed by parametric statistical tests. One-way analysis of variance (ANOVA) was performed for CS, DTS, CSt at each cusp, fracture 10

resistance, and Shr values. One-way ANOVA was performed in a split-plot arrangement for E and VH values, with the plot represented by restorative protocol and the subplot represented by depth of the cavity. Multiple comparisons were made using Tukey’s test. The failure mode and enamel cracks data were subjected to Chi-square test. All tests employed α=.05 significance level and all analyses were carried out with the statistical package Sigma Plot version 13.1 (Systat Software Inc, San Jose, CA, USA).

3. Results 3.1. Compressive strength (CS) and diametral tensile strength (DTS) The mean CS and (standard deviation) for the seven composite resins are shown in Table 2. One-way ANOVA showed significant difference among composite resins tested (P <0.001). The SDR had similar CS values as TEC, and lower values than all other composite resins. No difference was found among all other composite resins. The mean DTS and (standard deviation) for the seven composite resins are shown on Table 2. One-way ANOVA showed significant difference among composite resins tested (P <0.001). CHA had similar DTS values as Z350XT, and higher values than all other composites. Z350XT had significant higher DTS values than FBF and TEC.

3.2. Volumetric post-gel shrinkage (Shr) The Shr mean values and (standard deviations) of seven composites are presented in Figure 2 and Table 2. One-way ANOVA revealed statistical difference among the composites (P <0.001). Z350XT had had the highest mean volumetric shrinkage value and SDR had the lowest value.

3.3. Cuspal strain (CSt) The values of cuspal strain for the filling techniques during restorative procedure, during the simulation of 100 N occlusal loading and at the maximum fracture loading are shown in Table 3. The lingual cusp had higher cusp deformation than buccal cusp irrespective of filling technique or time of measurement. For CS measured during the restorative procedure the Z350XT group had significantly higher cuspal strain values than other groups. For CS measured during the 100 N occlusal loading no difference was found among the filling techniques. For CS measured at 11

fracture, the Filtek Z350XT group had significantly higher cuspal strain values than other groups.

3.4. Enamel crack detection and tracking The results of the crack analysis are shown in Table 4. The majority of the teeth showed no visible cracks. The occurrence of new cracks and crack propagation (larger than 3 mm) was higher after restoration regardless of restorative material or cusp. Z350XT/incremental technique showed this behavior only for the lingual cusp. Lingual cusps exhibited more cracks after restoration than the buccal cusp for the bulk-fill composites.

3.5. Vickers Hardness (VH) Vickers Hardness was constant in the mesial, center and distal regions. Therefore data were averaged for each cavity depth. VH of the five filling techniques at various depths are shown in Figure 3A and Table 6. The VH of the Z350XT and TEC were constant through the entire restoration. For the FBF/Z350XT, SDR/EST-X and VBF/CHA the VH was constant in enamel restoration (conventional composite resin portion) and decreased significantly in dentin restoration (bulk-fill composite resin). No difference was found between the VH in the enamel restoration for all filling techniques (Table 5). The VH measured in the dentin restoration was significantly lower for the groups that used flowable bulk-fill composite (FBF/Z350XT, SDR/EST-X and VBF/CHA) than the filling techniques that used the Tetric EvoCeram bulk-fill composite resin (TEC) or incremental filling technique (FZ350XT) (Table 5).

3.6.

Elastic modulus (E) Elastic Modulus was constant in the mesial, center and distal regions. Therefore

data were averaged for each depth cavity. E of the five filling techniques at various depths of the restorations are shown in Figure 3B and Table 6. The E of the Z350XT and TEC filling technique were constant through the entire restoration. For the FBF/Z350XT, SDR/EST-X and VBF/CHA filling techniques the E was constant in enamel restoration (conventional composite resin portion) and decreased significantly in dentin restoration (bulk-fill composite resin). The E of the VBF/CHA in the enamel restoration (Charisma Diamond) was significantly higher than all other groups (Table 12

5). The E measured in the dentin restoration for Z350XT (Filtek Z350XT) and TEC (Tetric EvoCeram Bulk Fill) had higher E values, whereas VBF/CHA (Venus Bulk Fill) had the lowest E values (Table 5).

3.7. Fracture resistance and failure mode The mean fracture resistance and (standard deviation) for the three restorative techniques are shown in Table 6. One-way ANOVA showed significant difference among groups (P < 0.001). The FBF/Z350XT group had significantly lower fracture resistance than the other filling techniques that used bulk-fill composite resins. No difference was found among all bulk-fill composite resin techniques. No significant differences in failure modes (Table 6) were found among the groups (P = 0.386). The ratio between the maximum resistance and cusp deformation at fracture moment is shown in Table 6. No difference was found among the groups (P = 0.631).

3.8. Finite Element Analysis Shrinkage stress distributions during restoration (modified von Mises stress) and at 100 N occlusal loading (critical modified von misses stress) are shown in Figures 4 and 5, respectively. The bulk-filling technique resulted in lower stresses compared to the Z350XT incremental technique, irrespective of bulk-fill composite used (Figure 4). The Z350XT had higher critical von Mises stresses in the dentin and restoration, mainly on the lingual cusp, than all other filling techniques (Figure 5). TEC showed higher critical von Misses stresses in the composite resin restoration than other filling techniques that used bulk-fill composites. Figures 6 shows cuspal strains obtained by FEA at the buccal and lingual surfaces. Cuspal strain behavior from the laboratory study (Table 3) during filling technique and 100 N loading were very similar to the deformation values calculated by FEA (Figure 6). The strain in the enamel reduced drastically, increasing significantly in the dentin, irrespective of the filling technique during the load application.

4. Discussion The results of the present study confirmed that the use of bulk-fill composite resins in posterior restorations reduces the cusp deformation, post-gel shrinkage, and shrinkage stress, and increases the fracture resistance. Therefore the null hypothesis was 13

rejected. The mechanical characterization of restorative materials is very important to understand the biomechanical behavior during oral function. Although a tooth is subjected to a compressive occlusal load, tensile stresses are also generated in the tooth structure. Not all changes in stress conditions have equal weight because the tooth structure is able to withstand compressive stresses better than tensile stresses. (34) This study showed that SDR composite had the lowest compressive-tensile strength ratio (SDE), while FBF had the highest ratio (Table 2). This means that tensile stress components are relatively more critical in FBF restorations than for the other composites. This behavior was expressed in the Modified von Mises stress values, which took those compressive-tensile stress ratios into account. To calculate the shrinkage stresses, polymerization shrinkage behavior must be modeled. Since not all shrinkage generates stresses, a ‘‘post-gel’’ shrinkage value was calculated in this study. Post-gel shrinkage was defined as the portion of the total polymerization shrinkage that causes stresses, and was measured using the strain gauge technique. The results of this study showed that the post-gel shrinkage of the conventional composite (Z350XT) was higher than those of the bulk-fill composites tested. It is widely accepted that volumetric contraction and solidification during the polymerization process of restorative composites in combination with bonding to hard tissues result in stress generation and inward deformation of the cavity walls of restored teeth [14]. Shrinkage stresses cannot be measured directly; however the resulting deformation and cuspal strain can be easily measured [22]. The results of CSt showed that both buccaland lingual cusps experienced inward cuspal strains, where lingual cusps showed more cuspal deformation than buccal cusps. This behavior was also demonstrated in previous studies and is mainly attributed to the smaller lingual cusp being most weakened during tooth preparation [4, 5]. Z350XT and VBF/CHA showed the highest CSt values, which may be explained by the higher E values of the composites used to restore enamel restoration. During the simulation of 100 N occlusal loading no difference was found among filling techniques. But during restoration different techniques may change the optimal coronal stress distribution. As a result, the same coronal tissues may be able to withstand the same masticatory forces equally well [36]. The resulting coronal deformation may result in post-operative sensitivity and may propagate pre-existing enamel microcracks. Shrinkage induced enamel 14

microfracture reportedly occurs immediately after polymerization [31], as was seen in this study. In this study we observed between 80 and 100% of previous cracks propagated after restoration (type III). Another study reported a fracture line (enamel crack) in the cusp after a large MOD restoration was placed [22]. The fracture line corresponds with the area of relatively high cuspal strain. It is possible that the crack propagated from an already existing enamel crack. These observations suggest that Shr and the resulting CSt are associated with crack propagation. However this results should be analysed with caution, because several teeth included in this study had small cracks before the cavity preparation, which are clinically common. . Crack propagation usually increases with increasing cavity dimensions, increasing the risk of tooth fracture [38-40]. In this study fracture resistance was statistically similar for all bulk-fill techniques. The failure mode also showed that fractures in restorations with bulk-fill composites are less catastrophic than Z350XT. This behavior was also observed in the finite element analysis using the critical modified von Mises stress. While intact teeth seldom fracture under normal functional loading, for intracoronally restored teeth fracture is a major concern [32]. It is important to note that because fracture is caused by local stresses exceeding the tissue strengths, any shift in stress concentrations due to changes in loading conditions or composite properties, may affect the fracture resistance of a tooth. The degree of conversion as well as depth of cure of a composite influences the development of stresses [41]. An inadequate cure tends to compromise the mechanical properties of a composite restoration and its adhesion and, consequently, its long-term clinical success [42]. For the flowable bulk-fill composite resins, the manufacturers recommend an occlusal coverage with a conventional resin composite to offer improved esthetics and better mechanical performance [15]. Higher depth of cure has been reported earlier for bulk-fill resin composites due to improvements in their initiator system and increased translucency [35]. The VH was significantly lower for the flowable bulk-fill composite resins compared with the conventional composite and paste bulk-fill composite resins. Figure 3 indicates that all composite resins were well cured, without significant differences in depth of cure. A previous study confirmed that the mechanical properties of SDR composite are maintained at 4 mm depth after curing in bulk [7]. There is a difference between inherently low hardness as a material property, and reduced hardness due to incomplete polymerization. In this study, all 15

composites appeared very well cured (Figure 3). The inferior hardness of SDR and Venus Bulk Fill can be explained by their lower filler contents used for obtaining reduced viscosity [33]. Another important physical property that influences the stress development is the elastic modulus, which is also associated with the composition of a material. Elastic modulus has been shown to increase with filler content [3, 43]. During polymerization, the composite changes from a predominantly viscous to a predominantly solid substance, which can be characterized by the development of the elastic modulus (E) [14]. Residual shrinkage stresses can be created when volumetric polymerization contraction is accompanied by this E development and the surrounding tooth structure restricts the volumetric changes [12, 44]. Composite resins with high elastic modulus produce more rigid restorations, which increase the effect of polymerization contraction on residual shrinkage stresses [35]. Restorative materials with low elastic modulus may reduce shrinkage stresses, but may not sufficiently recover the structural integrity of the original tooth to support masticatory loads [3]. Lower filler content was clearly reflected in lower mechanical properties (Filtek Bulk Fill and Venus Bulk Fill). This confirms previous reports of the correlation between filler content and elastic modulus [7]. Cuspal strain measured by strain gauges validated the finite element analysis. Also the stress distributions caused by post-gel shrinkage showed that the model restored with Z350XT using an incremental technique had higher stresses. This higher stress levels in the enamel correspond with the higher strain values measured for the Z350XT group. Bulk-fill composites were associated with lower stress levels, regardless of the materials used. The validation and correlation of experimental and computational methods is an important part in a comprehensive approach and is essential to justify conclusions drawn from in vitro analyses [5]. This finding helps to gain better understanding and insight into the nature and development of stress and strain distributions. Stress depends on multiple factors, such as how the tooth is loaded and where it is fixed, and the tooth anatomy comprises the shape and the combination of different hard tissues. Assessment of the stresses in a tooth is thus a complex task, which was achieved with the help of finite element analysis [45].

16

5. Conclusion Flowable bulk-fill composites had lower mechanical properties than paste bulkfill composite and conventional composites. However all bulk-fill composites had lower post-gel shrinkage than the conventional composites. Teeth should be carefully examined for enamel cracks after large restorations. The use of bulk-fill filling techiniques resulted in lower cuspal strains and shrinkage stresses. Furthermore, teeth restored with bulk-fill composites hadhigher fracture resistance. Therefore, the clinicians may choose bulk-fill composites for decreasing undesirable polymerization shrinkage effects while simplifying the filling technique.

Acknowledgements This project was funded by grants from the CNPq - National Council for Scientific and Technological Development and FAPEMIG. This research was performed at CPbio-Biomechanics, Biomaterials and Cell Biology Research Center, UFU.

17

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25. Chuang SF, Liu JK, Chao CC, Liao FP, Chen YH. Effects of flowable composite lining and operator experience on microleakage and internal voids in class II composite restorations. The Journal of prosthetic dentistry 2001;85(2):177-83. 26. Labella R, Lambrechts P, Van Meerbeek B, Vanherle G. Polymerization shrinkage and elasticity of flowable composites and filled adhesives. Dental materials 1999;15(2):128-37. 27. Ilie N, Rencz A, Hickel R. Investigations towards nano-hybrid resin-based composites. Clinical oral investigations 2013;17(1):185-93. 28. Ilie N, Bucuta S, Draenert M. Bulk-fill resin-based composites: an in vitro assessment of their mechanical performance. Operative dentistry 2013;38(6):618-25. 29. Soares CJ, Pizi EC, Fonseca RB, Martins LR. Influence of root embedment material and periodontal ligament simulation on fracture resistance tests. Brazilian oral research 2005;19(1):11-6. 30. Soares CJ, Fonseca RB, Gomide HA, Correr-Sobrinho L. Cavity preparation machine for the standardization of in vitro preparations. Brazilian Oral Research 2008;22(3):281-7. 31. Batalha-Silva S, de Andrada MA, Maia HP, Magne P. Fatigue resistance and crack propensity of large MOD composite resin restorations: direct versus CAD/CAM inlays. Dental materials 2013;29(3):324-31. 32. Burke FJ, Wilson NH, Watts DC. The effect of cavity wall taper on fracture resistance of teeth restored with resin composite inlays. Operative dentistry 1993;18(6):230-6. 33. Flury S, Hayoz S, Peutzfeldt A, Husler J, Lussi A. Depth of cure of resin composites: is the ISO 4049 method suitable for bulk fill materials? Dental materials 2012;28(5):521-8. 34. Sakaguchi RL, Versluis A, Douglas WH. Analysis of strain gage method for measurement

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37. Craig RG. Evaluation of an automatic mixing system for an addition silicone impression material. Journal of America Dental Association 1985;110(2): 213-5. 38. He Z, Shimada Y, Sadr A, Ikeda M, Tagami J. The effects of cavity size and filling method on the bonding to Class I cavities. The journal of adhesive dentistry 2008;10(6):447-53. 39. 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. Dental materials 2007;23(3):288-95. 40. Moorthy A, Hogg CH, Dowling AH, Grufferty BF, Benetti AR, Fleming GJ. Cuspal deflection and microleakage in premolar teeth restored with bulk-fill flowable resinbased composite base materials. Journal of dentistry 2012;40(6):500-5. 41. Boaro LC, Goncalves F, Guimaraes TC, Ferracane JL, Versluis A, Braga RR. Polymerization stress, shrinkage and elastic modulus of current low-shrinkage restorative composites. Dental materials 2010;26(12):1144-50. 42. Jadhav S, Hegde V, Aher G, Fajandar N. Influence of light curing units on failure of directcomposite restorations. Journal of conservative dentistry : JCD 2011;14(3):225-7. 43. Masouras K, Silikas N, Watts DC. Correlation of filler content and elastic properties of resin-composites. Dental materials 2008;24(7):932-9. 44. Clifford SS, Roman-Alicea K, Tantbirojn D, Versluis A. Shrinkage and hardness of dental composites acquired with different curing light sources. Quintessence international 2009;40(3):203-14. 45. Korioth TWP, Versluis A. Modeling the mechanical behavior of the jaws and their related structures by finite element (FE) analysis. Crit Rev Oral Biol M 1997;8(1):90104.

21

Figures and legends

Figure 1. Crack detection examined for buccal and lingual cusps: A) intact tooth before preparation; B) after tooth cavity preparation; and C) 24 hours after composite resin restoration. Red arrows indicate crack type II on buccal cusp and type III in lingual cusp.

22

Figure 2. Post-gel shrinkage curves measured for 5 minutes after start of light activation

23

Figure 3. Vickers Hardness (A) and elastic modulus at various restoration depths for five filling techniques.

24

Figure 4. Modified von Mises stress distributions for the different restorative filling materials and techniques 5 min after polymerization.

25

Figure 5. Critical Modified von Mises stress distributions for the different restorative filling materials and techniques during 100 N occlusal loading.

26

Figure 6. Strain values calculated with finite element analysis at the lingual and buccal cusp during polymerization (4 increments – Z350XT and 2 increments for all other composite resins) and occlusal loading.

27

Tables Table 1. Composite resins composition.

Material

CODE

Shade

Composite Type

Increment size and light activation time

Filler Organic matrix

Filler

EvoCeram

IVA

bulk fill

UDMA, BISGMA

composite

VBF

A2

4.0 mm – 20s

FBF

A2

UDMA, TEGDMA

SDR

Esthet X HD

SDR

EXT-X

A2

A2

4.0 mm – 40s

UDMA, BISGMA, EBPADMA,

composite

Procrylat resin 4.0 mm – 20s

Modified

composite

difunctional diluents

d composite

64/42.5

UDMA, Barium and strontium

dimethacrylate 2.0 mm – 20s

treated

ceramic and YbF3

flowable

Microhybri

glass,

ytterbium trifluoride, 65/38

Silane

Bis-GMA EBPADMA,

and alumino-fluoro-

silica

Schaan,

Heraeus-Kuzer, (Hanau, Germany)

3M-ESPE (St. Paul, MN, USA) Dentsply,

68/44

silicate glasses

adduct, Ba-F-Al-B-Si-glass,

Vivadent,

Liechtenstein)

silicon dioxide

flowable

Bulk-fill

79/61

mixed

Barium

composite

Filtek bulk fill

(Ivoclar

oxide
prepolymer

flowable

Bulk-fill

glass,

ytterbium trifluoride,

paste

Bulk-fill Venus bulk fill

Barium

Bulk-fill TEC

Manufacturer

wt/Vol 4.0 mm – 20s

Tetric

%

(Konstanz, Germany)

76/60

Dentsply, (Konstanz,

TEGDMA Charisma Diamond

CHA

A2

Nanohybrid

2.0 mm – 20s

composite

TCD-DI-HEA,

Bariumm, aluminium,

UDMA

fluoride glass

2.0 mm – 20s

Filtek Z350XT

Z350X T

A2

Nanofilled composite

Germany)

Silica Bis-GMA, EMA, TEGDMA

and

81/64

zirconia-silica

(Hanau, Germany)

zirconia

Bis- nanofillers, UDMA, agglomerated

Heraeus-Kuzer,

82/60

3M-ESPE (St. Paul, MN, USA)

nanoclusters

29

Table 2. Mean (SD) of Compressive Strength, Diametral Tensile Strength, Volumetric Post-gel Shrinkage*, and Stress differential effect (SDE) ‡ Stress

Compressive

Diametral Tensile Post-gel

Strength (MPa)

Strength (MPa)

shrinkage %

Z350 XT

257.0 (36.1)A

47.3 (7.5)AB

0.74 (0.07)D

EST-X

252.9 (30.6)A

43.6 (5.9)BC

0.46 (0.01)BC

CHA

250.8 (36.0)A

54.2 (3.7)A

0.44 (0.04)BC

5.43

FBF

245.1 (37.4)A

38.6 (5.9)C

0.50 (0.05)C

6.35

VBF

229.1 (44.8)A

43.8 (8.2)BC

0.41 (0.03)AB

5.23

TEC

213.3 (37.4)AB

37.8 (7.7)C

0.42 (0.04)AB

5.64

SDR

182.3 (14.6)B

43.5 (3.7)BC

0.34 (0.03)A

4.18

Groups

differential effect (SDE) 5.43 5.80

* Different uppercase letters indicate significant difference between the composites (p<0.05). † Compressive strength/Tensile strength?

Table 3. Cuspal strain (µS) measured by strain gauges (n = 10). Cusp Groups

strain Cusp strain at 100N loading (μS)

Filling technique Buccal Cusp

Lingual Cusp

Cusp strain

Mean cusps

at fracture load(μS)

Buccal

Lingual

Mean

Cusp

Cusp

cusps

TEC

184.8 (65.1)

233.3 (84.3)

209.1 (55.3)A

16.6 (4.3)

33.3 (9.2)

SDR/EST-X

167.6 (53.1)

277.2 (73.9)

222.4 (40.7)A

21.5 (6.9)

25.2 (6.3)

VBF/CHA

264.6 (66.6)

343.8 (77.0)

304.2 (55.4)AB

20.8 (6.2)

23.1 (7.0)

FBF/Z350XT

318.2 (111.0)

526.5 (172.7)

422.3 (138.2)B

16.5 (3.6)

22.8 (7.2)

Z350XT

527.7 (110.4)

591.8 (88.4)

559.7 (75.4)B

17.3 (5.2)

29.7 (9.3)

24.9

Buccal Cusp

Lingual Cusp

Mean cusps

481.4 (149.5)

699.0 (128.9)

590.2 (131.5B

23.3 (5.0)A 487.9 (171.7)

744.8 (240.8)

616.4 (112.5)B

427.9 (135.7)

700.6 (192.2)

564.3 (116.1B

585.3 (144.8)

768.1 (173.7)

676.7 (115.8)B

301.7 (97.0)

419.3 (89.8)

360.5 (66.5) A

(5.0)A

21.9 (3.7)A 19.7 (5.1)A 23.5 (5.3)A

Different letters indicate significant difference between the restorative techniques (p<0.05).

31

Table 4 – Crack incidences in intact teeth, after cavity preparation and after the filling technique. Intact teeth

Prepared teeth

24 h after restoration

Buccal Cusp Lingual Cusp Buccal Cusp Lingual

Buccal Cusp Lingual

Cusp

Cusp

I

II

III I

II

III I

II

III I

II

III I

II

III I

II

III

TEC

8

1

1

8

1

1

8

1

1

8

1

1

3

-

7

1

-

9

SDR/EST-X

7

1

2

6

1

3

7

1

2

6

1

3

3

-

7

-

-

10

VBF/CHA

4

2

4

4

2

4

2

3

5

3

1

6

1

1

8

-

-

10

FBF/Z350XT

6

2

2

8

-

2

2

2

6

6

2

2

2

2

6

2

1

7

Z350XT

7

3

-

6

3

1

7

2

1

6

1

3

6

2

2

3

-

9

(I) no cracks visible, (II) visible cracks smaller than 3 mm, and (III) visible cracks larger than 3 mm.

32

Table 5 – Elastic modulus (GPa) and Vickers Hardness (N/mm2) averaged from measurement points on the composite restoration placed to substitute dentin or enamel. Elastic Modulus (GPa) Filling technique

Vickers Hardness (N/mm2)

Composite of Composite of Composite of Composite of Enamel

Dentin

Enamel

Dentin

substitute

substitute

substitute

substitute

Z350XT

14.9 (0.4) Ba

14.4 (0.6) Aa

114.4 (7.1) Aa

115.6 (7.9) Aa

FBF/Z350XT

15.2 (0.6) Ba

10.1 (0.4) Cb

114,4 (9.0) Aa

62.7 (2.6) Bb

TEC

14.5 (0.3) Ba

14.4 (0.5) Aa

106.7 (4.2) Aa

105.4 (6.1) Aa

SDR/EST-X

14.9 (0.4) Ba

12.7 (3.7) Bb

103.9 (1.7) Aa

61.1 (3.7) Bb

VBF/CHA

20.3 (9.4) Aa

9.4 (0.3) Db

117.3 (7.2)Aa

52.8 (1.7) Bb

Different uppercase letters in columns compare restorative technique for each mechanical property. Lowercase letters in rows compare restoration location for each mechanical property (p < 0.05).

33

Table 6. Fracture resistance (N), mode of fracture and the ratio between maximum cusp deformation/fracture resistance measured by axial compression test (n = 10).

Groups

n

Fracture resistance (N)

Ratio between

Fracture mode

Strain/Fracture I

II

III

IV

resistance

FBF/Z350XT

10

2373.8 (332.5)A

0

0

5

5

0.29

SDR/EST-X

10

2164.1 (469.1)A

0

2

2

6

0.30

TEC

10

2029.0 (449.0)A

0

6

2

2

0.31

VBF/CHA

10

1971.2 (381.5)A

0

2

5

3

0.29

Z350XT

10

1380.9 (285.6)B

0

2

3

5

0.28

Different letters indicate significant difference between the restorative techniques for fracture resistance (p<0.05)

34