Polymerization shrinkage and contraction stress of dental resin composites

Impact of refrigeration of different Resin composite restorative materials on the marginal adaptation in class II restorations

BACKGROUND

The pre-polymerization temperature of resin composite restorative materials could influence their adaptation to cavity details. As a current debate is existing about the refrigeration of resin composite restorative materials, this study was designed to assess the effect of refrigeration of 3 types of resin composite restorative materials with different matrix systems on their marginal adaptation in Class II restorations.

METHODS

Forty-two sound maxillary molars, each with two separated Class II cavities, were used in this study. The teeth were assigned into 3 main groups (n = 14) according to the restorative /adhesive system used; an Ormocer-based composite (Admira Fusion/Futurabond M+, Voco GmbH, Cuxhaven, Germany), a methacrylate modified Ormocer-based (Ceram.X SphereTEC One/Prime&Bond Universal, Dentsply Sirona GmbH Konstanz, Germany), and a methacrylate-based (Tetric N-Ceram/Tetric N-Bond Universal, Ivoclar Vivadent AG, Schaan, Liechtenstein). Each group was then divided into 2 subgroups (n = 14) according to the gingival margin location; 1 mm above and 1 mm below the cemento-enamel junction (C.E.J). Each subgroup was further divided into 2 categories (n = 7) according to the storage temperature; stored at room temperature or stored in refrigerator at 4°- 5° C. Epoxy resin replicas were observed under scanning electron microscope (SEM) to examine the marginal gaps. A gab scoring system was used to assess the marginal adaptation of each restoration by giving scores on the basis of measurements of the maximum marginal gaps. The data obtained were statistically analyzed using the Chi-square test at a significance level of p < 0.05.

RESULTS

None of the tested groups exhibited 100% gap-free margins irrespective of margin location or storage temperature. For both storage temperatures, no statistically significant difference was observed among all tested groups either with margins located above or below C.E.J (p > 0.05). As well, there was no statistically significant difference when comparing both marginal locations for each material (p > 0.05). Regarding the effect of storage temperature, statistically significant difference was only observed between the room-temperature stored groups with margins located above C.E.J and their corresponding groups stored in refrigerator (p < 0.05).

CONCLUSION

The refrigeration of resin composite restorative materials prior to the restorative procedures revealed a deleterious effect on marginal adaptation of the restorations with margins located in enamel regardless the type of material used.

Release of contraction stress of dental resin composites by water sorption

Polymerization shrinkage of bonded resin composite restorations will result in the development of curing contraction stresses during setting and can cause debonding of the restoration or failure of the surrounding tooth structure. However, the hygroscopic expansion that occurs after exposure of the restorative to the wet oral environment can compensate for this shrinkage.

OBJECTIVES

The purpose of this study was to determine the hygroscopic expansion of six commercial resin composites and relate it to their composition, mechanical properties, shrinkage, and contraction stress development.

METHODS

Short-term volumetric shrinkage and contraction stress of the different composites were measured by mercury dilatometry and a universal testing machine. The long-term contraction stress was measured by the deflection of a bilayer strip of metal and a resin composite, which were stored dry as well as wet to determine the effect of hygroscopic expansion. The curvature of the strip was measured by profilometry over a period of 3 months.

RESULTS

The curvature of the strip correlated well (r2 =0.74) with the initial contraction stress, showing that the contraction stress is an important factor in initial deformation. The water sorption in all specimens showed that the initial deformation, within 2-4 weeks after curing, was completely counteracted. A high correlation (r2 =0.90) between deflection and relative water sorption was found, where the relative water sorption is defined as the absolute water sorption corrected for the inorganic filler volume of the composite.

SIGNIFICANCE

Within a period of 2-4 weeks after curing most of the curing contraction stresses of resin composite restoratives will be released by hygroscopic expansion.

Effect of ceramic thickness on the polymerization quality and film thickness of dual-polymerizing versus heated light-polymerizing adhesive cement

STATEMENT OF PROBLEM

The clinical success of ceramic veneers cemented with preheated composite resin has been reported to be acceptable. Although the cementing technique requires sufficient light energy to activate its polymerization, the ability of light to penetrate through ceramic restorations of different thicknesses is unclear.

PURPOSE

The purpose of this in vitro study was to evaluate the polymerization quality and bond joint thickness of a dual-component adhesive and a heated composite resin when bonding ceramic onlays of varying thicknesses.

MATERIAL AND METHODS

Sixty noncarious maxillary premolars, extracted for orthodontic or periodontal reasons, were divided into 6 groups (n=10) and sectioned apically to the marginal ridge. Computer-aided design and computer-aided manufacturing (CAD-CAM) was used to create onlays of varying thicknesses. Following the manufacturers ' instructions, the onlays were cemented using a dual-polymerizing resin for group DP1, DP2, and DP3 groups and preheated light-polymerizing resin for groups LP1, LP2, and LP3. The numbers indicate onlay thickness in each group. Vickers testing was performed 24 hours after polymerization, with a 0.49-N static load applied for 10 seconds. Film thickness was analyzed under an optical microscope, and temperature changes were measured using a thermal imaging camera. Statistical analysis was done with the 2-factor mixed ANOVA tests (α=.05).

RESULTS

The Vickers hardness of a dual-polymerizing cement was lower compared with heated light-polymerizing cement (P<.05). Moreover, the thickness of the resin-ceramic restoration did not significantly affect the polymerization of a dual-polymerizing cement. However, the polymerization of the preheated light-polymerizing composite resin in group LP3 was significantly lower compared with other thicknesses (P<.05). In group LP3, the Vickers hardness of the tooth adjacent to the surface of the cement (DX2) was lower compared with indentations closer to the restoration surface (DX3, DX6), demonstrating a reduction in polymerization in the internal part. In addition, the mean film thickness of the dual-polymerizing groups (96 ±18 µm) was significantly lower than of the heated light-polymerizing groups (294 ±64 µm) (P<.05). A temperature reduction of 15 °C in the heated composite resin was also observed after 8 seconds CONCLUSIONS: The thickness of the ceramic restoration did not significantly affect the polymerization of a dual-polymerizing cement. However, the polymerization of the preheated light-polymerizing composite resin under the same ceramic restoration with a thickness of 3 mm was significantly lower. In addition, the mean film thickness for the dual-polymerizing composite resin groups was significantly lower than for the heated light-polymerizing composite resin groups.

Marginal integrity in minimally invasive molar resin composite restorations: Impact of polymerization shrinkage

OBJECTIVES

This study utilized non-linear finite element (FE) models to explore polymerization shrinkage and its impact on marginal integrity in molars following both selective caries removal (SCR) and conventional treatment. Specifically, we performed 2D in silico simulations to study residual stresses post-resin polymerization shrinkage and their influence on the marginal integrity of various restoration types.

METHODS

Initially, FE models were developed based on a cohesive zone framework to simulate crack propagation along the bonded interfaces between restoration and tooth structure in SCR-treated molars with class I and class II restorations. The modeled resin composite restorations first underwent polymerization shrinkage and were then subjected to various occlusal loading conditions. Stress magnitudes and distributions were identified to evaluate the margin integrity and predict the mechanism and location of interfacial failure.

RESULTS AND DISCUSSION

The FE models computed polymerization shrinkage stresses of less than 1 MPa, exerting a minor influence on the composite/tooth interface. Occlusal loading, however, significantly impacted the load-bearing capacity of the composite/tooth (c/t) interface, potentially jeopardizing the restoration integrity. Especially under bi-axial occlusal loading, interfacial debonding occurred in the vertical cavity walls of the class I restorations, increasing the risk of failure. Notably, SCR-treated teeth exhibited better margin integrity than restored teeth after complete caries removal (NCR). These findings provide valuable insights into the mechanical behavior of SCR-treated teeth under different loading conditions and highlight the importance of considering the load scenarios that may lead to failure at the c/t interface. By investigating the factors influencing crack initiation and delamination, this novel research contributes to the optimization of restorative treatments and aids in the design of more resilient dental restorations.

Assessments of polymerization shrinkage by optical coherence tomography-based digital image correlation analysis-Part II: Effects of restorative composites

OBJECTIVES

To examine the polymerization shrinkage of different resin-based composite (RBC) restorations using optical coherence tomography (OCT) image-based digital image correlation (DIC) analysis.

METHODS

The refractive index (RI) of three RBCs, Filtek Z350XT (Z350), Z350Flowable (Z350F), and BulkFill Posterior (Bulkfill), was measured before and after polymerization to calibrate their axial dimensions under OCT. Class I cavities were prepared in bovine incisors and individually filled with these RBCs under nonbonded and bonded conditions. A series of OCT images of these restorations were captured during 20-s light polymerization and then input into DIC software to analyze their shrinkage behaviors. The interfacial adaptation was also examined using these OCT images.

RESULTS

The RI of the three composites ranged from 1.52 to 1.53, and photopolymerization caused neglectable increases in the RI values. For nonbonded restorations, Z350F showed maximal vertical displacements on the top surfaces (-16.75 µm), followed by Bulkfill (-8.81 µm) and Z350 (-5.97 µm). In their bonded conditions, all showed increased displacements. High variations were observed in displacement measurements on the bottom surfaces. In the temporal analysis, the shrinkage of nonbonded Z350F and Bulkfill decelerated after 6-10 s. However, Z350 showed a rebounding upward displacement after 8.2 s. Significant interfacial gaps were found in nonbonded Z350 and Z350F restorations.

SIGNIFICANCE

The novel OCT image-based DIC analysis provided a comprehensive examination of the shrinkage behaviors and debonding of the composite restorations throughout the polymerization process. The flowable composite showed the highest shrinkage displacements. Changes in the shrinkage direction may occur in nonbonded conventional composite restorations.

Polymerization shrinkage stress of contemporary dental composites: Comparison of two measurement methods

The aim of this study was to compare two testing methodologies employed for assessing the polymerization shrinkage stress of dental resin composites. Ten commercial resin composites were investigated (EverX Posterior & Flow; G-ænial Anterior, Posterior, A'CHORD & Universal Injectable; Filtek One Bulk Fill & Universal Restorative; SDR flow+ and Aura Bulk Fill). Photoelastic and contraction forces measurement methods were performed. The slope of the linear trendline and C-factor of specimens were calculated. The shrinkage stress values (range between: 6.4-13.4 MPa) obtained by the photoelastic method were higher for all resin composites than the values obtained by contraction forces measurements (range between 1.2-4.8 MPa). However, there was a strong linear correlation between these methods (r=0.8). The use of both investigated methods revealed important information about the shrinkage behavior of the restorative resin composites.

Evaluation of Color Stability and Translucency of Different Composite Resins Exposed to Alcohol

BACKGROUND

As there is evidence showing the effects of alcohol on the surface properties of composite resins, it is of great importance to understand the effects of commonly consumed beverages on the discoloration of composite restorations in people who consume alcohol.

AIM

The aim of this study was to investigate the changes in the color and translucency of composite resins immersed in different beverages after exposure to alcohol.

METHODS

Disk-shaped samples from each composite resin (Filtek Z250 (Z250), Clearfil Majesty Esthetic (CME), and Esthelite Bulk Fill Flow (EBF), were prepared (n = 60) and randomly divided into two groups (exposed or unexposed to alcohol). The samples were immersed in distilled water, black tea, and coffee. The color parameter (∆E00) and translucency parameter (∆TP) were calculated after 24 h, 1 week, and 3 weeks. Four-way ANOVA and Tukey's post-hoc analysis were used for the statistical analysis (α = 0.05).

RESULTS

Alcohol significantly increased the discoloration of Z250 and EBF in black tea. The discoloration of CME was not affected by alcohol exposure. Z250 and EBF showed clinically unacceptable discoloration (ΔE00 > 2.25) after immersion in black tea and coffee, whereas CME showed clinically unacceptable discoloration after immersion in black tea. Black tea caused the greatest color change in all composite materials regardless of alcohol exposure. EBF showed the lowest ΔTP values (∆TP = -1.82) at 3 weeks of immersion in black tea.

CONCLUSIONS

The effect of alcohol on discoloration depended on the composite type used. The translucency of bulk-fill flowable composite decreased with alcohol exposure.

Assessment of Degree of Conversion and Volumetric Shrinkage of Novel Self-Adhesive Cement

The degree of monomer conversion and polymerization shrinkage are two of the main reasons for potential adhesion failure between the tooth structure and the restoration substrate. To evaluate the degree of conversion and polymerization shrinkage of a newly developed self-adhesive resin cement, the degree of conversion (DC) was measured using FTIR under different activation modes, temperatures, and times. Volumetric shrinkage was tested using the AcuVol video imaging method. The experimental cement showed a higher DC than other cements under self-curing. The DC of the experimental cement was higher than that of other cements, except SpeedCem Plus under light curing. The experimental cement had a higher DC than other cements, except SpeedCem Plus in some conditions under dual curing. All self-adhesive cements had a higher DC at 37 °C than at 23 °C under self-curing, and there was no statistical difference between 23 °C and 37 °C under light curing. All self-adhesive cements showed a significantly higher DC at 10 min than at 5 min under self-curing. There was no statistical difference between 5 min and 10 min for most cements under dual curing. All self-adhesive cements statistically had the same volumetric shrinkage under light curing and self-curing. The newly developed self-adhesive resin cement exhibited a higher degree of conversion and similar volumetric shrinkage compared to these commercial self-adhesive resin cements.

Does the universal adhesive's film thickness affect dentin-bonding effectiveness?

OBJECTIVES

To investigate the influence of adhesive resin application modalities on the film thickness of the adhesive resin and the effectiveness of a two-step universal adhesive (UA) bonded in self-etch (SE) bonding mode to high C-factor class-I cavity-bottom dentin.

MATERIALS AND METHODS

After application of the primer of G2-Bond Universal (G2B, GC), the adhesive resin was applied into standard class-I cavities (human molars) following four application modalities: (1) one layer, strongly air-blown; (2) one layer, gently air-blown; (3) two layers, each gently air-blown; (4) one layer, not air-blown. After being restored with composite, each tooth was sectioned to obtain one micro-specimen (n = 10), of which the adhesive resin film thickness was measured using optical microscopy. The micro-tensile bond strength (μTBS) was tested immediately or upon 100,000 thermocycles. Statistical analyses involved Kruskal-Wallis and Mann-Whitney U testing (p < 0.05).

RESULTS

G2B's μTBS was significantly affected by the adhesive resin application modality and aging. Gently air-blowing the adhesive resin resulted in significantly higher immediate μTBS than strong air-blowing or no air-blowing. No significant difference in μTBS was found between single or double gently air-blown adhesive resin applications. The adhesive resin film thickness significantly varied with the application modalities.

CONCLUSIONS

A too thin or too thick adhesive resin film thickness adversely affected bond strength of the two-step UA applied in SE mode and high C-factor condition.

CLINICAL RELEVANCE

The adhesive resin layer thickness can affect the bonding performance of two-step UAs in high C-factor cavities. Dental clinicians remain advised to avoid improper air-blowing of UAs and strictly follow the application instructions.

Marginal adaptation of bulk-fill resin composites with different viscosities in class II restorations: a micro-CT evaluation

BACKGROUND

The purpose of this study is to evaluate the marginal adaptation of bulk-fill resin composites with different viscosities (paste-like and flowable) in Class II restorations using micro-CT imaging.

METHODS

Forty extracted human molars were used. Mesial and distal Class II box cavities (approximately 3 mm x 3 mm x 4 mm) were prepared for each tooth, with cavity floors located 1 mm below the enamel-cementum junction. Following adhesive application, teeth were restored using eight different groups: Group XB: X-tra Base Bulk-fill Flowable (VOCO), Group XF: X-tra Fill Bulk-fill (VOCO), Group FB: Filtek Bulk-fill Posterior (3 M ESPE), Group FF: Filtek Bulk-fill Flowable (3 M ESPE), Group BB: Beautifil-Bulk (SHOFU), Group BF: Beautifil-Bulk Flowable (SHOFU), and Group CO: "as a control group", Clearfil Majesty Posterior (KURARAY) and Group CF: "as a control group", Clearfil Majesty Flow + Clearfil Majesty Posterior (KURARAY). The restored teeth underwent an aging protocol involving 1000 cycles in a water bath fluctuating between 5 ± 1.0 °C and 55 ± 1.0 °C. Post-aging, teeth were immersed in 50% silver nitrate solution for 24 h and then in a film developer solution for 8 h. Microleakage analysis was performed using micro-CT, evaluated with 3D Slicer software. A two-way ANOVA was employed for statistical analysis.

RESULTS

Two-way ANOVA results indicated significant effects of both viscosity (p < 0.0001) and composite type (p < 0.0001) on marginal adaptation. Viscosity analysis (comparing flowable and paste-like) revealed no significant differences in the FB-FF, XB-XF and BB-BF groups but significant differences in the and CO-CF group, with flowable type exhibiting less microleakage than paste-like type.

CONCLUSIONS

The study suggests that while the viscosity of bulk-fill composites did not significantly affect marginal adaptation, the brand of bulk-fill composite did influence it.

Contemporary flowable bulk-fill resin-based composites: a systematic review

Flowable bulk-fill resin-based composites (BF-RBCs) represent a new and interesting alternative for the bulk-fill restorative techniques in the posterior region. However, they comprise a heterogeneous group of materials, with important differences in composition and design. Therefore, the aim of the present systematic review was to compare the main properties of flowable BF-RBCs, including their composition, degree of monomer conversion (DC), polymerization shrinkage and shrinkage stress, as well as flexural strength. The search was conducted following PRISMA guidelines in the Medline (PubMed), Scopus and Web of Science databases. In vitro articles reporting on the DC, polymerization shrinkage/shrinkage stress, and flexural strength of flowable BF-RBCs strength were included. The QUIN risk-of-bias (RoB) tool was used for assessing the study quality. From initially 684 found articles, 53 were included. Values for DC ranged between 19.41 and 93.71%, whereas polymerization shrinkage varied between 1.26 and 10.45%. Polymerization shrinkage stresses reported by most studies ranged between 2 and 3 MPa. Flexural strength was above 80 MPa for most materials. A moderate RoB was observed in most studies. Flowable BF-RBCs meet the requirements to be indicated for bulk fill restoration technique in the posterior region. However, important variations among composition and properties hinder extrapolation of the results to materials different from those reported here. Clinical studies are urgently required to assess their performance under a real working scenario.

Enhancing the mechanical stability of restored teeth with interfacial cracks: Finite element analysis

OBJECTIVES

This study aims to enhance the mechanical stability of restored molar teeth with class II occlusal-distal (OD) cavities. We seek to achieve this goal through a comprehensive investigation of three primary factors: (1) the choice of restoration material properties, (2) internal cavity geometries, and (3) the impact of double-layered restoration configurations.

METHODS

To achieve our objectives, we initiated by creating two-dimensional (2D) models of restored teeth featuring class II OD cavities, utilizing scanned and segmented images of maxillary molar teeth. We drew 2D profiles of dentine and enamel, which were then imported into finite element analysis (FEA) software. To explore various cavity geometries, we implemented a total of thirteen different designs, encompassing straight, oblique, grooved, curved, and double-layered configurations. We utilized a semi-circular stone to simulate the application of contact load on the restored tooth. We applied identical boundary conditions and contact loading across all models. To assign material properties, we developed a Python code, enabling the automatic assignment of seven elastic moduli ranging from 2 GPa to 26 GPa to the restoration materials. Meanwhile, constant material properties were assigned to the enamel and dentine. In total, we conducted 133 FEA simulations to comprehensively analyse the effects of the aforementioned factors on the strength and performance of restored molar teeth.

RESULTS

Our analysis revealed two key factors significantly influencing the mechanical resistance of treated teeth, particularly in the presence of a crack or debonding: (1) the marginal geometry of the OD cavity and (2) the elastic modulus of the restorative material. However, altering the internal cavity angle and implementing a double-layered restoration did not significantly influence the restored tooth's overall strength and performance in the face of crack or debonding situations.

SIGNIFICANCE

The findings of this study have substantial implications for designing and restoring class II OD cavities to enhance resistance to cracks or debonding. The use of curved marginal geometries in restoration design can significantly improve fracture resistance, with double-curved geometries reducing stress concentrations by approximately 43% compared to straight cavities. These results offer valuable guidance for strengthening the structural integrity of restored teeth, calling for further experimental investigations to explore practical applications and benefits.

5-year clinical performance of posterior bulk-filled resin composite restorations: A double-blind randomized controlled trial

OBJECTIVES

The aim of this double-blind, and randomized controlled clinical trial was to evaluate the 5-year clinical performance of posterior resin composite restorations placed with the incremental filling technique [IF] or the bulk-fill technique [BF]. Two different adhesive systems were used: etch-&-rinse (ER) or self-etch (SE).

METHODS

Posterior dental teeth of 72 participants (n = 236), with a cavity depth of at least 3 mm, were randomly divided into four groups. Restorations were applied with either Tetric N-Bond or Tetric N-Bond SE. The composite resin Tetric N-Ceram Bulk-Fill was placed either with IF or BF. Restorations were evaluated using FDI criteria at baseline and after 1, 2, 3, 4, and 5 years. Statistical analysis was performed using the Wilcoxon Signed rank test (a=0.05).

RESULTS

Two hundred and four restorations were evaluated after 5 years. Eleven restorations were considered 'failed', ten due to fracture (4 IF and 6 BF) and one due to secondary caries (IF). The annual failure rate was 1.2% for BF and 1% for IF (p = 0.35). When comparing BF and IF, no significant differences were found for any of the parameters evaluated (p > 0.05). Regarding the adhesive systems, 44 and 51 restorations showed minor problems in terms of marginal adaptation and staining, with significantly more marginal discoloration when the self-etch adhesive was used (p = 0.002).

SIGNIFICANCE

The bulk-fill restorative technique showed good clinical behavior compared to the incremental filling technique, especially when using an etch-&-rinse adhesive, after 5 years of clinical evaluation.

Application and limitations of configuration factor (C-factor) in stress analysis of dental restorations

OBJECTIVE

The configuration factor (C-factor) is an index used to evaluate the relationship between cavity configuration and the development of polymerization shrinkage stress in dental restorations. Although C-factor has been widely researched, its correlation with stress analysis in dental restorations remains controversial. This review aims to discuss the application and limitations of C-factor and define the restricted conditions under which the C-factor "rule of thumb" is applicable.

METHODS

A thorough literature review was conducted on the application and limitations of C-factor in stress analysis of dental restorations. This was principally based on MEDLINE/PubMed and Web of Science databases and a review of the relevant studies and publications in scientific papers in international peer-reviewed journals for the specific topic of C-factor and polymerization shrinkage.

RESULTS

The C-factor alone cannot provide an accurate prediction of the shrinkage stress of restorations and the mechanical behavior of material-tooth interfaces. C-factor is only applicable under one condition not typically seen in clinical practice: low, near-rigid compliance.

SIGNIFICANCE

Conditions for the application of C-factor have been explicitly defined. A more accurate and precise understanding and utilization of the C-factor is of benefit as it contributes to better understanding of polymerization shrinkage behavior of restorations.

Influence of the Type of Nanofillers on the Properties of Composites Used in Dentistry and 3D Printing

Photopolymerization is a growing field with an extensive range of applications and is environmentally friendly owing to its energy-efficient nature. Such light-assisted curing methods were initially used to cure the coatings. However, it has become common to use photopolymerization to produce 3D objects, such as bridges or dental crowns, as well as to cure dental fillings. In this study, polymer nanocomposites containing inorganic nanofillers (such as zinc nano-oxide and zinc nano-oxide doped with two wt.% aluminum, titanium nano-oxide, kaolin nanoclay, zirconium nano-oxide, aluminum nano-oxide, and silicon nano-oxide) were fabricated and studied using Real Time FT-IR to investigate the effects of these nanoadditives on the final conversion rates of the obtained nanocomposites. The effects of the fillers on the viscosity of the produced nanocomposites were also investigated, and 3D prints of the selected nanocomposites were presented.

Microleakage and Marginal Integrity of Ormocer/Methacrylate-Based Bulk-Fill Resin Restorations in MOD Cavities: SEM and Stereomicroscopic Evaluation

This in vitro study aimed to compare the microleakage and marginal integrity of methacrylate/ormocer-based bulk-fill composite (BFC) restorations used in cervical marginal relocation with two different layering thicknesses in mesio-occlusal-distal (MOD) cavities exposed to thermo-mechanical loading. Standard MOD cavities were prepared in 60 mandibular molars and assigned into three groups: x-tra fil/AF + x-tra base/XB, Tetric N-Ceram Bulk Fill/TNB + Tetric N-Flow Bulk Fill/TFB, and Admira Fusion x-tra/AFX + Admira Fusion x-base/AFB. Each group was further divided into two subgroups (2 mm and 4 mm) based on the thickness of flowable BFCs (n = 10). The specimens were subjected to thermo-mechanical loading (240,000 cycles) and immersed in 0.2% methylene blue. Following mesiodistal sectioning, the specimens were examined under stereomicroscope (×25) and scored (0-3) for microleakage. Marginal integrity was examined using a scanning electron microscope (SEM). Descriptive statistical methods and the chi-square test were used to evaluate the data (p < 0.05). While there was no statistically significant difference in gingival cement microleakage in the XB and AFB specimens with a 4 mm thickness, microleakage was significantly increased in the TFB specimen (p = 0.604, 0.481, 0.018 respectively). A significantly higher amount of score 0 coronal microleakage was detected in the AFX2 mm + AFB4 mm compared to the TNB2 mm + TFB4 mm (p = 0.039). The SEM examination demonstrated better marginal integrity in groups with 2 mm thick flowable BFCs. Ormocer and methacrylate-based materials can be used in marginal relocation with thin layers.

Cytotoxic, Elastic-Plastic and Viscoelastic Behavior of Aged, Modern Resin-Based Dental Composites

The development of resin-based composites (RBCs) is a delicate balance of antagonistic properties with direct clinical implications. The clear trend toward reducing filler size in modern RBCs solves esthetic deficiencies but reduces mechanical properties due to lower filler content and increases susceptibility to degradation due to larger filler-matrix interface. We evaluated a range of nano- and nano-hybrid RBCs, along with materials attempting to address shrinkage stress issues by implementing an Ormocer matrix or pre-polymerized fillers, and materials aiming to provide caries-protective benefit by incorporating bioactive fillers. The cytotoxic response of human gingival fibroblast (HGF) cells after exposure to the RBC eluates, which were collected for up to six months, was analyzed using a WST-1 assay. The microstructural features were characterized using a scanning electron microscopy and were related to the macroscopic and microscopic mechanical behaviors. The elastic-plastic and viscoelastic material behaviors were evaluated at the macroscopic and microscopic levels. The data were supplemented with fractography, Weibull analysis, and aging behavioral analysis. The results indicate that all RBCs are non-cytotoxic at adequate exposure. The amount of inorganic filler affects the elastic modulus, while only to a limited extent the flexural strength, and is well below the theoretical estimates. The nanoparticles and the agglomeration of nanoparticles in the RBCs help generate good mechanical properties and excellent reliability, but they are more prone to deterioration with aging. The pre-polymerized fillers lower the initial mechanical properties but are less sensitive to aging. Only the Ormocer retains its damping ability after aging. The strength and modulus of elasticity on the one hand and the damping capacity on the other are mutually exclusive and indicate the direction in which the RBCs should be further developed.

Microleakage Evaluation of Two Methacrylate-Based Composites (GC Kalore and Luna SDI) in Class II Restorations: A Laboratory Study

Objective

In recent years, dental composite resins such as tooth-colored restoration are frequently used to restore dental cavities, coronal fractures, and congenital defects. This study aimed to evaluate the microleakage of two methacrylate-based composites (GC Kalore and Luna SDI) in class II restorations.

Materials and Methods

In this experimental study, a total of 18 intact human premolars previously extracted for periodontal and orthodontic reasons were randomly divided into two groups. Similar class II cavities (box only) were prepared on all teeth and restored with two different composites. In group 1, a bonding agent (Single Bond 2-SB2; 3M ESPE) and Luna SDI composite in mesial cavities and GC Kalore composite in distal cavities were used. In group 2, Single Bond 2 and GC Kalore composite in mesial cavities and Luna SDI composite in distal cavities were applied. They were then subjected to 2000 thermal cycles in a water bath between 5-55°C (dwell time: 30 seconds in every bath and transfer time: 10 seconds). Then, they were immersed in a 2% basic fuchsin dye solution for 24 hours. After rinsing with water, they were sectioned mesiodistally and evaluated for microleakage using a stereomicroscope.

Results

Independent t-test (Mann-Whitney test) showed no statistically significant difference for microleakage in mesial and distal class II restorations between GC Kalore composite and Luna SDI composite (p = 1.000) (p= 0.852). A total of 83.4% of the Luna SDI composite samples and 66.6% of the GC Kalore composite had a microleakage score of ≤3 in class II cavities.

Conclusion

In the present study, marginal microleakage was found mainly at the gingival floor extending to 1/3 of the axial wall for the Luna SDI composite and GC Kalore composite. Furthermore, no statistically significant difference was found between the microleakage of the Class II cavities restored with Luna SDI composite and GC Kalore composite.

Noninvasive Adaptation Appraisal of Antimicrobial Nano-Filled Composite

OBJECTIVE

The aim of this research was to assess the effect of incorporating zein-coated magnesium oxide (zMgO) nanofillers to resin-based composite on the internal adaptation of the restorations using cross-polarisation optical coherence tomography (CP-OCT).

METHODS

Thirty noncarious human molar teeth were used. Class V cavities (3 × 5 mm) were prepared on the buccal and lingual surfaces of each tooth. Clearfil SE Bond 2 was applied to all the cavities and then the teeth were divided into 3 groups (n = 10) as follows: group 1-restored with N-Flow composite; group 2 and group 3-restored with N-Flow composite mixed with different zMgO nanoparticle concentrations (0.3% and 0.5% by weight, respectively) and then light cured using an LED curing device. Specimens were examined for interfacial adaptation examination under CP-OCT. Characterisation of the dental composite incorporating zMgO was done by Fourier-transform infrared spectroscopy (FTIR), x-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM). Results were analysed with Kruskall-Wallis test followed by Mann-Whitney U test, at a significance level of P < .05.

RESULTS

XRD spectra exhibited the sharp peaks of zMgO in the composite enhanced with zMgO nanoparticles. FESEM analysis showed a uniform distribution of the zMgO nanoparticles in the composite and FTIR illustrated no change in the spectra. The gap percentage along the cavity floor was significantly lower in groups 2 and 3 in comparison to group 1 (P < .05). Also there was a significant difference in gap percentages between groups 2 and 3 (P < .05), with group 3 showing the lowest gap percentage.

CONCLUSIONS

The incorporation of 0.3% and 0.5% zMgO nanoparticles in flowable composite assists in improving the internal adaptation of the composite to the tooth surface.

Polymerization and shrinkage kinetics and fracture toughness of bulk-fill resin-composites

OBJECTIVES

To determine degree of conversion (DC), maximum polymerization rate (RPmax), polymerization shrinkage (PS), maximum shrinkage rate (PS Rmax) and fracture toughness (KIC) of different types of bulk-fill (BF) composites plus the effect of viscosity reduction techniques.

METHODS

BF specimens were created in 2 mm deep molds: SonicFill 3 (SF3), Viscalor (VC), One Bulk Fill (OBF) and Beautifil Bulk (BBR). SF3 was applied via sonic insertion using a SonicFill handpiece (Kerr Corp. USA). Viscalor was pre-heated in a Caps Warmer in T3 mode (at 68 °C) for 30 s (T3-30 s) and 3 min (T3-3 min), respectively. Specimens were irradiated at zero distance from the upper surface with an Elipar S10 LED unit (3 M ESPE, USA) of mean irradiance 1200 mW/cm2 for 40 s. Real-time polymerization kinetics and DC at 5 min and 24 h post-irradiation (DC5 min and DC24 h) were measured using ATR-FTIR (n = 3). PS was measured up to 1 h on 1 mm thick discs via the bonded-disk technique (n = 3) and PS Rmax obtained by numerical differentiation (n = 3). For fracture toughness, single-edge-notched specimens (32 × 6 ×3 mm) of each BF composite were prepared and measured by three-point bending after 7 d water storage (n = 5). Data were analysed using One-way ANOVA, independent T-tests and Tukey post-hoc tests (p < 0.05).

RESULTS

SF3 showed the significantly highest DC5 min, DC24 h and RPmax (p < 0.05), followed by OBF (p < 0.05). Regardless of pre-heating, VC showed comparable conversion kinetics to BBR (p > 0.05). There was no significant difference in PS of these BF composites, except OBF had the highest PS (p > 0.05). However, PS Rmax significantly varied among materials (p = 0.047) and SF3 had the highest PS Rmax. Regarding fracture toughness, BBR had the lowest KIC (p < 0.05), whereas other composites showed similar KIC (p > 0.05). Strong correlations of filler content (wt%)-PS/KIC were found. Different pre-heating times had no significant influences on DC %, RPmax, PS, PS Rmax and KIC of VC (p > 0.05).

SIGNIFICANCE

Different types of bulk-fill composites showed comparable shrinkage. A highly filled BF giomer composite (BBR) had the lowest fracture toughness, whereas others had similar KIC. Pre-heating had no adverse effects on Viscalor properties. Sonication and pre-heating are beneficial techniques to enhance composite flowability without either increasing shrinkage or reducing fracture toughness.

Posterior ceramic versus metal restorations: A systematic review and meta-analysis

OBJECTIVES

The goal of this systemic review and meta-analysis was to evaluate the longevity of indirect adhesively-luted ceramic compared to conventionally cemented metal single tooth restorations.

DATA

Randomized controlled trials (RCT) investigating indirect adhesively-luted ceramic restorations compared to metal or metal-based cemented restorations in permanent posterior teeth.

SOURCES

Three electronic databases (PubMed, CENTRAL (Cochrane) and Embase) were screened. No language or time restrictions were applied. Study selection, data extraction and quality assessment were done in duplicate. Risk of Bias and level of evidence was graded using Risk of Bias 2.0 tool and Grade Profiler 3.6.

RESULTS

A total of 3056 articles were found by electronic databases. Finally, four RCTs were selected. Overall, 443 restorations of which 212 were adhesively-luted ceramic restorations and 231 conventionally cemented metal restorations have been placed in 314 patients (age: 22-72 years). The highest annual failure rates were found for ceramic restorations ranging from 2.1% to 5.6%. Lower annual failure rates were found for metal (gold) restorations ranging from 0% to 2.1%. Meta-analysis could be performed for adhesively-luted ceramic vs. conventionally cemented metal restorations. Conventionally cemented metal restoration showed a significantly lower failure rate than adhesively-luted ceramic ones (visual-tactile assessment: Risk Ratio (RR)[95%CI]=0.31[0.16,0.57], low level of evidence). Furthermore, all studies showed a high risk of bias.

CONCLUSION

Conventionally cemented metal restorations revealed significantly lower failure rates compared to adhesively-luted ceramic ones, although the selected sample was small and with medium follow-up periods with high risks of bias.

Comparison of polymerization shrinkage of a new bulk-fill flowable composite with other composites: An in vitro study

OBJECTIVE

Since composites still face a critical problem called polymerization shrinkage and bulk-fill composites have reported acceptable results for this issue, this study aims to assess the polymerization shrinkage of a new bulk-fill flowable composite (G-aenial bulk injectable [GBI]) and compare it to other bulk-fill and conventional composites.

MATERIALS AND METHODS

In this in vitro study, 25 composite discs were fabricated using three bulk-fill and two conventional composites. They were bonded to a microscopic slide and were covered by a coverslip. This assembly was transferred to a linear variable differential transformer and composite samples were cured from underneath the slides. Dimensional changes formed in composite samples were recorded. Data were analyzed using analysis of variance followed by post hoc Tukey's and Dunnett's tests.

RESULTS

The groups were significantly different regarding polymerization shrinkage. G-aenial bulk injectable and G-aenial universal flo showed significantly higher polymerization shrinkage than other composites at 30, 60, and 1800 s after light irradiation, while X-tra fil and Filtek Z250 showed the lowest polymerization shrinkage at the aforementioned time points.

CONCLUSION

According to the results, the new composite had polymerization shrinkage similar to the conventional one. Bulk-fill composites reported similar or lower shrinkage to conventional composites.

Utility of Dental Operating Microscopes in Assessing Microleakage of Nanohybrid Resin Restorations Using Different Placement Techniques

Background

Composite resin has become a material of choice due to its aesthetic potential and durability. It is at a lower cost compared to ceramic restorations. But it has a tendency to shrink during polymerization, leading to the formation of gaps at the margins. Placement techniques of restoration can be modified to reduce stress shrinkage. A dental operating microscope will help in the precise and thin layering of composite against the cavity wall and the matrix band.

Methodology

Class I cavities were prepared in 30 human permanent premolars. All cavity surfaces were dried and acid-etched. A bonding agent was applied and they were light-cured. Specimens were divided into three groups: I, II, and III, where restorations were performed using horizontal layering, oblique layering, and split-increment horizontal layering techniques respectively. Samples were then submerged in silver nitrate solution and were examined under a stereomicroscope after a longitudinal section and graded for dye penetration.

Results

The dye penetration scores were subjected to a statistical analysis using Analysis of variance (ANOVA) with post hoc Tukey’s test with the statistical software Statistical Package for Social Sciences (SPSS version 12). The level of significance was set at 0.05 for all statistical inferences.

Conclusion

The split-increment horizontal layering technique under the dental operating microscope showed less microleakage when compared to other methods and is the preferred method for composite restorations.

Investigation of Thermal-Induced Changes in Molecular Order on Photopolymerization and Performance Properties of a Nematic Liquid-Crystal Diacrylate

Polymerization shrinkage and associated stresses are the main reasons for dental restorative failure. We developed a series of liquid crystal diacrylates and dimethacrylates which have markedly low polymerization shrinkage. In order to fully understand the effects of temperature-induced changes of molecular order on the photopolymerization process and performance properties of the generated polymers, the photopolymerization of a difunctional acrylate, 2-t-butyl-1,4-phenylene bis (4-(6-(acryloyloxy)hexyloxy)benzoate), which exists in the nematic liquid crystalline phase at room temperature, was investigated as a function of photopolymerization temperature over the nematic to isotropic range. Morphological studies suggested that a mesogenic phase was immediately formed in the polymer even if polymerization in thin films occurred above the nematic-to-isotropic (N→I) transition temperature of the monomer (Tn-i = 45.8 °C). Dynamic mechanical analysis of 2 × 2 mm cross-section bar samples polymerized at 60 °C showed reduced elastic moduli, increased glass transition temperature and formation of a more crosslinked network, in comparison to polymers formed at lower polymerization temperatures. Fractography analysis showed that polymers generated from the nematic liquid crystalline phase underwent a different fracture pattern in comparison to those generated from the isotropic phase. Volumetric shrinkage (2.2%) found in polymer polymerized from the nematic liquid crystalline phase at room temperature was substantially less than the 6.0% observed in polymer polymerized from an initial isotropic phase at 60 °C, indicating that an organized monomer can greatly contribute to reducing cure shrinkage.

Low-Shrinkage Resin Matrices in Restorative Dentistry-Narrative Review

Dimethacrylate-based resin composites restorations have become widely-used intraoral materials in daily dental practice. The increasing use of composites has greatly enhanced modern preventive and conservative dentistry. They have many superior features, especially esthetic properties, bondability, and elimination of mercury and galvanic currents. However, polymeric materials are highly susceptible to polymerization shrinkage and stresses that lead to microleakage, biofilm formation, secondary caries, and restoration loss. Several techniques have been investigated to minimize the side effects of these shrinkage stresses. The primary approach is through fabrications and modification of the resin matrices. Therefore, this review article focuses on the methods for testing the shrinkage, as well as formulations of resinous matrices available to reduce polymerization shrinkage and its associated stress. Furthermore, this article reviews recent cutting-edge developments on bioactive low-shrinkage-stress nanocomposites to effectively inhibit the growth and activities of cariogenic pathogens and enhance the remineralization process.

Influence of thermal and thermomechanical stimuli on a molar tooth treated with resin-based restorative dental composites

OBJECTIVES

In-vivo experimental techniques to understand the biomechanical behavior of a restored tooth, under varying oral conditions, is very limited because of the invasive nature of the study and complex tooth geometry structure. Therefore, 3D-Finite element analyses are used to understand the behavior of a restored tooth under varying oral conditions. In this study, the distribution of maximum principal stress (MaxPS) and the location of MaxPS on a restored tooth using six different commercially available dental resin composites under the influence of thermal and thermomechanical stimuli are performed.

METHODS

An intact tooth was scanned using µ-CT and segmented to obtain separate geometric models of the tooth, including enamel and dentine. Then, a class II mesial-occlusal-distal (MOD) cavity was constructed for the tooth model. The restored tooth model was further meshed and imported to the commercial Finite Element (FE) software ANSYS. Thermal hot and cold stimuli at 50 °C and 2 °C, respectively, were applied on the occlusal and lingual surface of the tooth model with the tooth's ambient temperature set at 37 °C. A uniform loading of 400 N was applied on the occlusal surface of the tooth to imitate the masticatory forces during the cyclic thermal stimuli.

RESULTS

The results of this study showed that the restorative materials with higher thermal conductivity showed a lower temperature gradient between the restoration and enamel, during the application of thermal stimuli, leading to a higher value of MaxPS on the restoration. Moreover, on applying thermal stimuli, the location of MaxPS at the restoration-enamel junction (REJ) changes based on the value of the coefficient of thermal expansion (CTE). The MaxPS distribution on the application of simultaneous thermal and mechanical stimuli was not only dependent on the elastic modulus of restorative materials but also their thermal properties such as the CTE and thermal conductivity. The weakest part of the restoration was at the REJ, as it experienced the peak stress level during the application of thermomechanical stimuli.

SIGNIFICANCE

The findings from this study suggest that restorative materials with lower values of elastic modulus, lower coefficient of thermal expansion and higher values of thermal conductivity result in lower stresses on the restoration. The outcomes from this study also suggest that the thermal and mechanical properties of a restorative material can have a considerable effect on the selection of restorative materials by dental clinicians over conventional restorative materials.

Effect of Preheating and Vibration on Microhardness and Microleakage of Microhybrid Resin Composite (In Vitro Study)

BACKGROUND: Preheating and sonic vibration are two methods for the treatment of microhybrid resin composites that may effect on their mechanical and physical properties. AIM: This study was conducted to assess the effect of using preheating and sonic vibration on microhardness and microleakage of microhybrid resin composite METHODS AND MATERIALS: For microhardness test, a total of 30 samples of resin composite discs were prepared. Samples were divided into three groups according to the method of treatment of resin composite, controlled group (T0), preheated group (T1), and sonic vibration group (T2). Surface microhardness values were evaluated at baseline and after thermocycling. For microleakage test, a total of 30 Class-V cavities were prepared on the labial surfaces of extracted human anterior teeth. The cavities were then divided into three groups according to the method of resin composite treatment as mentioned before in the microhardness test. All samples were sectioned; then two-dimensional cross-sectional images from each sample. Each cross-sectional image was analyzed using Image J software to quantify interfacial microleakage at the cavity floor. STATISTICAL ANALYSIS USED: Two-way ANOVA analysis was used to test the effects of thermocycling on three groups of each test. One-way ANOVA was used to compare between three different methods of resin composite treatment. RESULTS: For microhardness at baseline revealed that the highest mean value was recorded for the control group, followed by the sonic vibration group meanwhile, the lowest mean value was recorded for the sonic vibration group, followed by preheated group at microleakage test. CONCLUSION: Preheating and sonic vibration of microhybrid resin composite does not improve its microhardness; however, sonic vibration provides better marginal adaptation than the preheating and the conventional methods.

A Novel Low-Shrinkage Resin for 3D Printing

OBJECTIVE

To evaluate the shrinkage, accuracy, and mechanical properties of a newly developed photo-polymerizable resin material for 3D printing dental applications and compare it with three commercially available resins.

METHODS

An experimental novel proprietary resin material for 3D printing was formulated. This new resin, Die and Model Tan (SprintRay), Formlabs Grey (FormLabs), and LCD Grey (Roxel 3D) were evaluated and tested for volumetric shrinkage, accuracy, and flexural and tensile properties. Volumetric shrinkage was measured using the AcuVol video imaging method. The accuracy of the 3D printed objects through the DLP (digital light processing) printing system was determined by the 3D deviation between the scanned model and the original CAD digital model. Parallelepiped specimens (2 × 2 × 25 mm, n= 5) were printed for flexural strength, flexural modulus, and modulus of resilience and measured in accordance with ISO-4049. Dumbbell-shaped specimens (Type V, n = 5) were printed to investigate the tensile strength and tested according to ASTM-D638. Data were analyzed using one-way ANOVA and post-hoc Tukey tests (p≤0.05).

RESULTS

The experimental resin exhibited significantly lower volumetric shrinkage and significantly higher accuracy than the other commercially available resins (p < 0.001). The experimental resin showed flexural strength, flexural modulus, and tensile strength similar to Die and Model Tan resin (SprintRay) (p > 0.05), and these parameters were significantly higher than those of Formlabs Grey resin (Formlabs) and lower than those of LCD Gray resin (Roxel3D)(p <0.001).

CONCLUSION

The newly formulated 3D printing resin demonstrated lower volumetric shrinkage, high accuracy, and adequate mechanical properties compared to the commercially available resin materials.

CLINICAL SIGNIFICANCE

The new 3D printed objects exhibited higher accuracy, good stability over time, and adequate mechanical properties. The new 3D printing resin is a good candidate for modeling applications for restoration, orthodontics and implants.

Affordable Biocidal Ultraviolet Cured Cuprous Oxide Filled Vat Photopolymerization Resin Nanocomposites with Enhanced Mechanical Properties

In this study, Cuprous Oxide (Cu2O), known for its mechanism against bacteria, was used as filler to induce biocidal properties on a common commercial resin stereolithography (SLA) 3D printing resin. The aim was to develop nanocomposites suitable for the SLA process with a low-cost process that mimic host defense peptides (HDPs). Such materials have a huge economic and societal influence on the global technological war on illness and exploiting 3D printing characteristics is an additional asset for these materials. Their mechanical performance was also investigated with tensile, flexural, Charpy's impact, and Vickers microhardness tests. Morphological analysis was performed through scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy-dispersive X-ray spectroscopy (EDS) analysis, while the thermal behavior was studied through Thermogravimetric Analysis (TGA). The antibacterial activity of the fabricated nanocomposites was investigated using a screening agar well diffusion method, for a gram-negative and a gram-positive bacterium. Three-dimensional printed nanocomposites exhibited antibacterial performance in all loadings studied, while their mechanical enhancement was approximately 20% even at low filler loadings, revealing a multi-functional performance and a potential of Cuprous Oxide implementation in SLA resin matrices for engineering and medical applications.

Preparation and evaluation of novel bio-based Bis-GMA-free dental composites with low estrogenic activity

OBJECTIVE

Although bisphenol Aglycidyl methacrylate (Bis-GMA) are widely used in the dental composite, its raw materials include the petroleum-based product bisphenol A (BPA) with high estrogenic activity (EA). In this study, two new BPA-free dimethacrylate monomers from bio-based material creosol were synthesized and evaluated.

METHODS

The renewable bisphenol monomer 5, 5'-methylenedicreosol (BCF) was prepared from bio-based material creosol. By the human breast cancer cells (MCF-7 cells) proliferation assay, a risk assessment of BCF was performed to determine if BCF possessed reduced EA in comparison to BPA. Then, the novel monomers 5, 5'-methylenedicreosol diglycidyl ether diacrylate (BCF-EA) and 5, 5'-methylenedicreosol diglycidyl ether dimethacrylate (BCF-GMA) were synthesized from BCF with epichlorohydrin and (meth)acrylate. All products were investigated by ¹H NMR and FT-IR spectra. The control resin was a mixture based on Bis-GMA and tri(ethyleneglycol) dimethacrylate (TEGDMA) with a weight ratio of 5:5 (5B5T). Similarly, experimental resin matrix was a mixture based on BCF-EA/TEGDMA (5E5T) and BCF-GMA/TEGDMA (5G5T). And their corresponding composites were then prepared with corresponding resin matrices and hybrid SiO₂ (5E5TC, 5G5TC and 5B5TC). The properties of these composites were investigated according to the standard or referenced methods. Each sample was evaluated for double bond conversion (DC), shrinkage stress (SS) and volumetric polymerization shrinkage (VS). Water sorption (WS), water solubility (SL), mechanical properties and cytotoxicity were also measured.

RESULTS

¹H NMR and FT-IR spectra confirmed the chemical structure of each monomer. EA test revealed that bio-based bisphenol monomer BCF as the precursor of BCF-EA and BCF-GMA showed lower EA than BPA. Cured resin matrix: Both 5E5T and 5G5T had nearly the same DC (p < 0.05), which was higher than 5B5T (p < 0.05); 5E5T and 5G5T had lower VS, SL and cytotoxicity than 5B5T (p < 0.05); mechanical properties of 5E5T and 5G5T were all better than those of 5B5T (p < 0.05). Cured composite: There was no significant difference in conversion (p < 0.05); 5E5TC and 5G5TC had significantly lower VS (p < 0.05); WS of 5E5TC and 5G5TC were similar (p < 0.05), but higher compared to 5B5TC (p < 0.05); 5E5TC and 5G5TC had the deeper depth of cure (p > 0.05); before water immersion, there was no significant difference in flexural strength between 5E5TC and 5G5TC (p > 0.05), and higher than 5B5TC (p < 0.05); 5E5TC and 5G5TC showed less cytotoxicity than 5B5TC (p < 0.05).

SIGNIFICANCE

The new BPA-free di(meth)acrylates are promising photocurable dental monomers owning to bio-based raw material, high degree of conversion coupled with low curing shrinkage and good mechanical properties. Therefore, BCF-EA and BCF-GMA has a potential to be used as the substitution for Bis-GMA to prepare Bis-GMA-free dental composite.

Effect of Conventional Adhesive Application or Co-Curing Technique on Dentin Bond Strength

The aim of this in vitro study was to assess the effect of two different adhesive application methods on shear dentin bond strength (ISO 29022) using three various adhesive systems. A mid-coronal section of 77 intact third human molars with fully developed apices was made to create flat bonding substrates. The materials used in the study were Excite F (Ivoclar Vivadent), Prime&Bond Universal (Dentsply Sirona) and G-Premio Bond (GC). The application of each adhesion system was performed in two different ways. In the first group, the bonding agent was light cured immediately after the application (conventional method), while in the second group the adhesive and composite were cured concurrently (“co-curing” method). A total of 180 specimens were prepared (3 adhesives × 2 method of application × 30 specimens per experimental group), stored at 37 °C in distilled water and fractured in shear mode after 1 week. Statistical analysis was performed using ANOVA and Weibull statistics. The highest bond strength was obtained for Prime&Bond conventional (21.7 MPa), whilst the lowest bond strength was observed when co-curing was used (particularly, Excite F 12.2 MPa). The results showed a significant difference between conventional and co-curing methods in all materials. According to reliability analysis, the co-curing method diminished bond reliability. Different application techniques exhibit different bond strengths to dentin.

Effect of thickness on shrinkage stress and bottom-to-top hardness ratio of conventional and bulk-fill composites

This study evaluated the effect of the material thickness on shrinkage stress and bottom-to-top hardness ratio of conventional and bulk-fill composites. Six commercial composites were selected based on their different technologies: Two conventional (C1, C2), two high-viscosity bulk-fill (HVB1, HVB2), and two low-viscosity bulk-fill (LVB1, LVB2). Shrinkage stress was analyzed for five specimens with 2 mm thickness (C-factor 0.75 and volume 24 mm³ ) and five specimens with 4 mm thickness (C-factor 0.375 and volume 48 mm³ ) for 300 s in a universal testing machine. Bottom-to-top hardness ratio values were obtained from Knoop microhardness measurements in specimens with 2- and 4-mm thickness (n = 5). Thickness increase resulted in significantly higher shrinkage stress for all materials with the exception of HVB2 and LVB1. C1, C2, HVB2, and LVB1 showed lower bottom-to-top hardness ratios at 4 mm than at 2 mm. Only LVB2 presented a bottom-to-top hardness ratio lower than 80% at 2 mm, while HVB1 surpassed this threshold at 4 mm of depth. The results suggest that the increase of composite thickness affected the shrinkage stress values. Also, thickness increase resulted in lower bottom-to-top hardness ratio. HVB1 showed better behavior than other bulk-fill materials, with low stress and adequate bottom-to-top hardness ratio at 4 mm thickness.

A Comparative Study of the Mechanical Properties of Selected Dental Composites with a Dual-Curing System with Light-Curing Composites

Dual-curing composites have a wide spectrum of use in practice (rebuilding, reconstruction, and luting). The characterization of this type of material and comparative study of selected mechanical properties with light-cured materials were carried out for this paper. In this study, we used six materials with a dual-cure system—Bulk EZ, Fill-Up!, StarFill 2B, Rebilda DC, MultiCore Flow, Activa Bioactive-Restorative—and three light-cured materials—Filtek Bulk Fill Posterior, Charisma Classic, and G-aenial Universal Flo. The materials were conditioned for 24 h in water at 37 °C before testing. Selected material properties were determined: three-point bending flexural strength, diametral tensile strength, hardness, microhardness, and shrinkage stress. The highest three-point bending flexural strength (TPB) was 137.0 MPa (G-aenial Universal Flo), while the lowest amounted to 86.5 MPa (Activa Bioactive). The diametral tensile strength (DTS) values were in a range from 39.2 MPa (Rebilda DC) to 54.1 MPa (Charisma Classic). The lowest hardness (HV) value of 26 was obtained by the Activa Bioactive material, while the highest values were recorded for Filtek Bulk Fill Posterior and Charisma Classic-53. The shrinkage stress of the tested materials ranged from 6.3 MPa (Charisma Classic) to 13.2 MPa (G-aenial Universal Flo). Dual-curing composites were found to have similar properties to light-cured composites.

Efficient prediction of the packing density of inorganic fillers in dental resin composites for excellent properties

OBJECTIVE

The purpose of this study is to develop a mathematical model for efficient prediction of the packing density of different filler formulations in dental resin composites (DRCs), and to study properties of DRCs at the maximum filler loading (MFL), thereby providing an effective guidance for the design of filler formulations in DRCs to obtain excellent properties.

METHODS

The packing density data generated by discrete element model (DEM) simulation were used to re-derive the parameters of 3-parameter model. The modifier effect was also induced to modify the 3-parameter model. DRCs with 10 filler formulations were selected to test properties at the MFL. The packing densities of binary and ternary mixes in DRCs were calculated by 3-parameter model to explore the regularity of composite packing.

RESULTS

The predicted packing density was validated by simulation and experimental results, and the prediction error is within 1.40 vol%. The optimization of filler compositions to obtain a higher packing density is beneficial to enhancing the mechanical properties and reducing the polymerization shrinkage of DRCs. In binary mixes, the maximum packing density occurs when the volume fraction of small fillers is 0.35-0.45, and becomes higher with the reduction of particle size ratio. In ternary mixes, the packing density can reach the maximum value when the volume fractions of large and small fillers are in the 0.5-0.75 and 0.15-0.4 ranges, respectively.

SIGNIFICANCE

The modified 3-parameter model can provide an effective method to design the multi-level filler formulations of DRCs, thereby improving the performance of the materials.

Comparing marginal microleakage in Class V cavities restored with flowable composite and Cention-N using confocal microscope-an in-vitro study

Context

Class V cavity presents a clinical challenge in the field of restorative dentistry as the margin placement is partially in enamel and partly in cementum, and the trouble associated with this design is the microleakage at the tooth restoration interface.

Aims

To evaluate and compare marginal microleakage in Class V cavities when restored with flowable composite and Cention-N restorative material.

Methods and Material

This in-vitro study was performed on 30 human maxillary premolars, which were extracted for orthodontic reasons. Class V cavities were prepared on the buccal surfaces of each tooth. After application of seventh-generation bonding agent (3M ESPE, Single Bond Universal Adhesive), the teeth were divided into two groups of 15 each and restored with flowable composite (3M ESPE Filtek Z350 XT) and Cention-N (Ivoclar Vivadent) and were subjected to thermocycling. The teeth were then immersed in 0.1% Rhodamine B dye for 48 h. They were sectioned longitudinally into mesial and distal parts with the help of an isomet diamond saw. The sections were observed under a confocal laser scanning microscope (CLSM).

Statistical Analysis Used

The results were subjected to statistical analysis using the Mann-Whitney U test and Pearson's Chi-square test.

Results

The analysis showed statistically significant results among the groups. Cention-N showed lesser microleakage, which was statistically significant when compared to flowable composite (P = 0.005).

Conclusions

Cention-N showed significantly lesser leakage and better adaptation than flowable composite.

Improved performance of Bis-GMA dental composites reinforced with surface-modified PAN nanofibers

In the present work, polyacrylonitrile (PAN) nanofibers reinforced dental composites were investigated to achieve the improved interfacial adhesion between the PAN nanofiber and resin matrix using surface modification of nanofibers. PAN nanofibers mat were prepared by electrospinning and then, surface treated with the activated bisphenol A glycidyl methacrylate (Bis-GMA)/triethyleneglycol dimethacrylate (TEGDMA) (50/50 mass ratio) dental resin followed by photo-curing. Also, the treated nanofibers mat was milled into a powder to achieve the uniform distribution of nanofibers in the matrix resin. The reinforced dental composite were prepared by mixing the various mass fraction of the powder (0.5-15 wt%) with the Bis-GMA/TEGDMA dental monomers. The effect of weight ratio of surface-modified nanofibers to blend resin on the chemical structure, morphology, compression and flexural properties, color and polymerization shrinkage of dental composites was evaluated. The results showed that using surface-treated nanofibers with content of 5 wt% enhanced the compression strength, flexural strength, flexural modulus and work of rupture of the resultant dental composite by factors of 23%, 7%, 80%, and 145%, respectively, comparing to the unreinforced neat resin. Also, the polymerization shrinkage reduces by 37%. These significant improved properties of the dental composite could be due to the semi-interpenetration network formation between surface-modified nanofibers and resin matrix and well distribution of nanofibers in the dental resin. Further increasing the nanofiber content led to poor mechanical properties of obtained dental composites. The results also, revealed that the color of resin composite could be whiter using modified PAN nanofibers as the filler.

24 hour polymerization shrinkage of resin composite core materials

PURPOSE

The study's purpose was to evaluate the 24-hour polymerization shrinkage of resin composite core materials.

MATERIAL AND METHODS

Eleven resin composite core material samples (n = 12) were evaluated using a non-contact imaging device with measurements obtained over 24 h. Shrinkage values were determined corresponding to proposed times involved with CAD/CAM same-day treatment and at 24 h. Shrinkage data was statistically compared using Friedman/Dunn's test for intragroup analysis and Kruskal Wallis/Dunn's test for intergroup analysis, all at a 95% level of confidence (α = 0.05).

RESULTS

Mean results identified a wide volumetric shrinkage range with considerable similarity overlap. Inconsistent shrinkage behavior was evident and all materials reached maximum values before 24 h. No significant difference was observed during proposed digital same day all ceramic crown procedures, but some differences were noted at 24 h.

CONCLUSIONS

Under this study's conditions results were material specific, at times inconsistent, with wide variation. Shrinkage consistently increased for all products and it is not known if the continued shrinkage magnitude may compromise the stability and fit of all-ceramic crowns at 24 h.

Probing the mineralized tissue-adhesive interface for tensile nature and bond strength

The mechanical performance of the dentin-adhesive interface contributes significantly to the failure of dental composite restorations. Rational material design can lead to enhanced mechanical performance, but this requires accurate characterization of the mechanical behavior at the dentin-adhesive interface. The mechanical performance of the interface is typically characterized using bond strength tests, such as the micro-tensile test. These tests are plagued by multiple limitations including large variations in the test results. The challenges associated with conventional tensile tests limit our ability to unravel the complex relationships that affect mechanical behavior at the dentin-adhesive interface. This study used the diametral compression test to overcome the challenges inherent in conventional bond strength tests. The bovine femur cortical bone tissue was considered as a surrogate material (the mineralized tissue) for human dentin. Two different adhesive formulations, which differed by means of their self-strengthening properties, were studied. The tensile behavior of the mineralized tissue, the adhesive polymer, and the bond strength of the mineralized tissue - adhesive interface was determined using the diametral compression test. The diametral compression test improved the repeatability for both the tensile and bond strength tests. The rate dependent mechanical behavior was observed for both single material and interfacial material systems. The tensile strength and bond strength of the mineralized tissue-adhesive interface was greater for the self-strengthening formulation as compared to the control.

Physicochemical and Mechanical Properties of Bis-GMA/TEGDMA Dental Composite Resins Enriched with Quaternary Ammonium Polyethylenimine Nanoparticles

Modification of dental monomer compositions with antimicrobial agents must not cause deterioration of the structure, physicochemical, or mechanical properties of the resulting polymers. In this study, 0.5, 1, and 2 wt.% quaternary ammonium polyethylenimine nanoparticles (QA-PEI-NPs) were obtained and admixed with a Bis-GMA/TEGDMA (60:40) composition. Formulations were then photocured and tested for their degree of conversion (DC), polymerization shrinkage (S), glass transition temperature (T_g), water sorption (WS), solubility (SL), water contact angle (WCA), flexural modulus (E), flexural strength (σ), hardness (HB), and impact resistance (a_n). We found that the DC, S, Tg, WS, E, and HB were not negatively affected by the addition of QA-PEI-NPs. Changes in these values rarely reached statistical significance. On the other hand, the SL increased upon increasing the QA-PEI-NPs concentration, whereas σ and a_n decreased. These results were usually statistically significant. The WCA values increased slightly, but they remained within the range corresponding to hydrophilic surfaces. To conclude, the addition of 1 wt.% QA-PEI-NPs is suitable for applications in dental materials, as it ensures sufficient physicochemical and mechanical properties.

Improving the Physical-Mechanical Property of Dental Composites by Grafting Methacrylate-Polyhedral Oligomeric Silsesquioxane onto a Filler Surface

Endowing dental composites with excellent interfacial bonding through filler surface modification is pivotal to improve the physical-mechanical property and prolong the life of composite fillings. In this study, methacrylate-polyhedral oligomeric silsesquioxane (MA-POSS) acts as a "molecular bridge" between the commonly used SiO₂ particles and the methacrylate-based resin matrix via a thiol-ene click reaction to construct MA-POSS/SiO₂ (p-SiO₂) hybrid particles. Synthesized p-SiO₂ exhibited the roughest surface morphology and had more polymerizable groups, in comparison with SiO₂ and silanized SiO₂. Furthermore, the p-SiO₂ particles were used as a reinforcement to fabricate bisphenol A glycerolate dimethacrylate/tri(ethyleneglycol) dimethacrylate-based dental composites, where the SiO₂- and silanized SiO₂-filled composites served as the control groups, and the filler loading was fixed at 65 wt %. Results of the mechanical properties indicated that the hybrid p-SiO₂ particles significantly improved the flexural strength, flexural modulus, compressive strength, and work of fracture of dental composites, giving improvements of 251.2, 17.89, 122.3, and 1094%, respectively, over the SiO₂-filled composites due to the strong interfacial interaction between the resin matrix and p-SiO₂. Additionally, this optimal p-SiO₂-loaded composite also presented better polymerization shrinkage, acceptable degree of conversion, curing depth, and cell viability. Grafting of MA-POSS onto a filler surface is a promising filler surface modification to improve the resin matrix/filler interfacial interaction, leading to the enhanced overall performance of composites.

In vitro comparison of the surface roughness of polymethyl methacrylate and bis-acrylic resins for interim restorations before and after polishing

STATEMENT OF PROBLEM

Polymethyl methacrylate and bis-acrylic based resins are widely used for interim restorations. Their initial surface roughness is important because it determines their aesthetic properties and the potential for biofilm adhesion.

PURPOSE

The purpose of this in vitro study was to assess the surface roughness and morphology of 6 bis-acrylic and 2 polymethyl methacrylate resins widely used for interim dental restorations, both before and after polishing.

MATERIAL AND METHODS

Specimens made of different bis-acrylic resins (Protemp 4, Luxatemp Star, Systemp, Telio, Structur Premium, Structur 3) or of polymethyl methacrylate (Unifast Trad, Unifast 3) were polished using a 2-step polishing system (Diatech). The average surface roughness before and after polishing (10 seconds at low speed in dry conditions) was measured by optical profilometry. Atomic force microscopy and scanning electron microscopy were used to analyze surface morphology. The Mann-Whitney and Kruskal-Wallis tests were used to evaluate the differences in roughness among specimens (α=.05), and the Pearson r correlation was computed to assess the relationship between fillers and average surface roughness.

RESULTS

In the 8 groups evaluated, the roughness significantly increased (P<.001) for Protemp 4 (from 0.12 to 0.50 μm), Luxatemp Star (0.17 to 1.19 μm), Unifast 3 (0.40 to 1.00 μm), Systemp (0.46 to 1.51 μm), Structur 3 (0.85 to 1.06 μm), Structur Premium (1.00 to 1.74 μm), and Telio (1.13 to 1.21 μm), except for Unifast Trad (9.20 to 2.59 μm). Pairwise multiple comparisons identified Protemp 4 as having the smoothest surface before and after polishing, while Unifast Trad was the roughest in both. Atomic force microscopy and scanning electron microscopy observations showed that the surface roughness of bis-acrylic resins was related to their surface morphology and average filler sizes. A positive relation between fillers and roughness was assessed (r=0.345, P<.001).

CONCLUSIONS

For the bis-acrylic interim resins, the surface roughness after polishing was correlated to the material used and its filler sizes. Nanofiller-based resins showed the smoothest surfaces. For the polymethyl methacrylate-based resins, the recently marketed Unifast 3 had the lowest overall roughness values.

Dental Restorative Materials for Elderly Populations

The incidence of dental caries, especially root caries, has risen in elderly populations in recent years. Specialized restorative materials are needed due to the specific site of root caries and the age-related changes in general and oral health in the elderly. Unfortunately, the restorative materials commonly used clinically cannot fully meet the requirements in this population. Specifically, the antibacterial, adhesive, remineralization, mechanical, and anti-aging properties of the materials need to be significantly improved for dental caries in the elderly. This review mainly discusses the strengths and weaknesses of currently available materials, including amalgam, glass ionomer cement, and light-cured composite resin, for root caries. It also reviews the studies on novel anti-caries materials divided into three groups, antimicrobial, remineralization, and self-healing materials, and explores their potential in the clinical use for caries in the elderly. Therefore, specific restorative materials for caries in the elderly, especially for root caries, need to be further developed and applied in clinical practice.

The Benefits of Smart Nanoparticles in Dental Applications

Dentistry, as a branch of medicine, has undergone continuous evolution over time. The scientific world has focused its attention on the development of new methods and materials with improved properties that meet the needs of patients. For this purpose, the replacement of so-called "passive" dental materials that do not interact with the oral environment with "smart/intelligent" materials that have the capability to change their shape, color, or size in response to an externally stimulus, such as the temperature, pH, light, moisture, stress, electric or magnetic fields, and chemical compounds, has received much attention in recent years. A strong trend in dental applications is to apply nanotechnology and smart nanomaterials such as nanoclays, nanofibers, nanocomposites, nanobubbles, nanocapsules, solid-lipid nanoparticles, nanospheres, metallic nanoparticles, nanotubes, and nanocrystals. Among the nanomaterials, the smart nanoparticles present several advantages compared to other materials, creating the possibility to use them in various dental applications, including preventive dentistry, endodontics, restoration, and periodontal diseases. This review is focused on the recent developments and dental applications (drug delivery systems and restoration materials) of smart nanoparticles.

Dimensional stability of short fibre reinforced flowable dental composites

Fibre-reinforced dental composites are proven to have superior mechanical properties in comparison with micro/nano/hybrid filled composites. However, the addition of small quantities of short glass fibres could affect the dimensional stability of the restoration both during initial stages as well as through the life of the restoration. This in-vitro study aims at evaluating the physical properties of short S-Glass reinforced flowable dental composites. Two S-Glass short fibre-particulate reinforced (5 wt% of aspect ratios 50 and 70) and one particulate only reinforced flowable dental composites were prepared with UDMA-TEGDMA based dental monomer systems. Samples were photopolymersied for 60 s and stored in distilled water at 37 °C for 24 h before testing. Depth of cure (through-thickness microhardness), volumetric shrinkage (Archimedes technique), polymerisation stress (cantilever based tensometer), curing exotherm (thermocouple), water sorption and solubility (ISO 4049) and thermal expansion coefficient (dilatometer) were determined. The test results were statistically analysed using one-way ANOVA (p < 0.05). Depth of cure increased by 41%, volumetric shrinkage increased by 8.3%, shrinkage stress increased by 37.6%, exotherm increased by 20.2%, and thermal expansion reduced by 6.4% while water sorption and solubility had a negligible effect with the inclusion of short glass fibres. The study demonstrates that within the same organic resin system and quantity, a small replacement of fillers with short fibres could significantly affect the dimensional stability of the composite system. In conjunction with mechanical properties, this study could help clinicians to gain confidence in fibre reinforced dental composite restorative system.

The Influence of Cement Layer Thickness on the Stress State of Metal Inlay Restorations-Photoelastic Analysis

The successful restoration of teeth requires a good connection between the inlay and natural tissue. A strong bond may improve retention and reinforce tooth structure. The purpose of this study was to evaluate the influence of cement layer thickness on contraction stress generated during photopolymerization, and to determine the changes in stress state of the cement occurring during aging in water (over 84 days). Two cements were used: resin composite cement (NX3) and self-adhesive resin cement (Maxcem Elite Chroma). A cylindrical sample made of CuZn alloy was used to imitate the inlay. The stress state was measured by photoelastic analysis. The contraction stress of the inlay restoration was calculated for cement layer thicknesses of 25 µm, 100 µm, 200 µm, and 400 µm. For both tested materials, the lowest contraction stress was observed for the thinnest layer (25 µm), and this increased with thickness. Following water immersion, a significant reduction in contraction stress was observed due to hygroscopic expansion. Applying a thin layer (approximately 25 µm) of composite and self-adhesive resin cements resulted in high levels of expansion stresses (over −6 MPa) after water aging.

Optical comparison between micro-CT and OCT in imaging of marginal composite adaptation: Observational study

Dental composite is the most used aesthetic restorative biomaterial worldwide. However, it undergoes polymerisation shrinkage that could lead to loss of the interfacial seal between tooth and resin in some circumstances. This demands high-resolution imaging technologies to detect these defects. This study carried out a comparison between microcomputed tomography (micro-CT; Shimadzu, Japan) and swept-source optical coherence tomography (SS-OCT; Santec, Japan) in the detection of marginal adaptation defects at the tooth-resin interface. Unlike in micro-CT, it was possible to outline interfacial gaps along with tooth-resin interfaces with SS-OCT, which was attributed to the Fresnel diffraction of light. This in vitro comparison demonstrates SS-OCT has great potential in dental imaging to effectively assess dental composite adaptation and marginal defects when high resolution is desired in real time. LAY DESCRIPTION: Detection of tooth-colored restoration defects had been assessed by different radiographic methods. However, most of these methods are either invasive or suffer from low-resolution. In this study, a comparison has been carried out between two different high-resolution imaging systems; microcomputed tomography and optical coherence tomography, to explore their potentials in detecting restorations defects. The results showed optical coherence tomography has a great accuracy in locating the underlying defects when the obtained images were validated against confocal laser scanning microscopy images.