Delayed Photo-activation Effects on Mechanical Properties of Dual Cured Resin Cements and Finite Element Analysis of Shrinkage Stresses in Teeth Restored With Ceramic Inlays

Mechanical properties of dual-polymerizing resin-core material, dual-polymerizing resin cement, and bulk-fill composite resin used for restoring endodontically treated teeth

STATEMENT OF PROBLEM

How resin-based material and a light-activation protocol influence the mechanical properties of materials used to cement glass fiber post-and-cores in endodontically treated teeth is unclear.

PURPOSE

The purpose of this in vitro study was to evaluate the influence of immediate or 5-minute delayed light activation on the mechanical properties of dual-polymerizing resin cements and dual-polymerizing resin-core materials compared with bulk-fill composite resins.

MATERIAL AND METHODS

Nine resin-based materials were tested: 4 dual-polymerizing resin-core materials, (Allcem Core; FGM, LuxaCore Z; DMG, Rebilda DC; VOCO, and (Clearfil DC Core Plus; KURARAY), 3 dual-polymerizing resin cements, (RelyX Universal; 3M ESPE, RelyX U200; 3M ESPE, and Allcem Dual; FGM), and 2 bulk-fill composite resins, (Opus Bulk Fill APS; FGM, and Filtek One Bulk Fill; 3M ESPE). The dual-polymerizing materials were light activated using both protocols. The postgel shrinkage (Shr), flexural strength (FS), elastic modulus (E), Knoop hardness (KH), degree of conversion (DC), and depth of polymerization (DoP) were measured (n=10). The data for Shr, FS, E, and DoP were analyzed using 2-way ANOVA, and for KH and DC using 2-way repeated measurement ANOVA and the Tukey HSD test (α=.05).

RESULTS

A 5-minute delay before light-activation significantly reduced Shr for all materials (P<.001). Increasing the depth significantly reduced the KH for all materials (P<.001). Bulk-fill composite resins and dual-polymerizing resin-core had higher KH values than dual-polymerizing resin cements (P<.001).

CONCLUSIONS

Delayed 5-minute light-activation reduced postgel shrinkage and had no negative effect on mechanical properties. Dual-polymerizing resin-core materials exhibited higher KH values than dual-polymerizing resin cement and mechanical properties similar to those of bulk-fill composite resin.

Bacterial Adhesion and In Situ Biodegradation of Preheated Resin Composite Used as a Luting Agent for Indirect Restorations

OBJECTIVE

To evaluate surface roughness and bacterial adhesion after in situ biodegradation of the cementation interface of indirect restorations cemented with preheated resin composite.

METHODS AND MATERIALS

Resin composite blocks (Z250XT/3M ESPE) were cemented to bovine enamel (7 × 2.5 × 2 mm) using preheated microhybrid resin composites: (1) Filtek Z100 (3M ESPE) (Z100); (2) Gradia Direct X (GC America) (GDX); and (3) Light-cured resin cement RelyX Veneer (3M ESPE) (RXV) (n=21). The resin composites were preheated on a heating device (HotSet, Technolife) at 69°C for 30 minutes. Disk-shaped specimens (7 × 1.5 mm) were made for biodegradation analysis with the luting agents (n=25). The in situ phase consisted of 20 volunteers' using an intraoral palatal device for 7 days. Each device had six cylindrical wells for the blocks and the disk-shaped specimens. Biodegradation was evaluated through surface roughness (Ra), scanning electron microscopy (SEM) micromorphological analysis, and colony-forming unit (CFU) count. The film thickness of the luting agents was also measured under stereomicroscopy.

RESULTS

Increased surface roughness was observed after the cariogenic challenge without differences between the luting agents. Higher variation and surface flaws suggestive of particulate detachment were observed for Z100. No differences were observed in CFU counts.

CONCLUSIONS

All materials underwent surface biodegradation, and the surface roughness of the resin cements was similar to or lower than that of the preheated resin composites. The resin composites' film thickness was thicker than that of the resin cement. Clinicians should be aware of these factors when choosing the use of preheated resin composite since it can lead to reduced longevity of the cementation interface and, therefore, restorations.

Physicochemical properties of flowable composites using isobornyl methacrylate as diluent monomer

OBJECTIVE

this study sought to evaluate the effect of isobornyl methacrylate (IBOMA) as a diluent monomer on the physicochemical properties of experimental flowable resin composites.

METHODOLOGY

the organic resin matrix of a modal flowable resin composite was formulated with 50 wt.% of bisphenol-A-glycidyl methacrylate (Bis-GMA) and 50 wt.% of a diluent monomer, in which IBOMA was used as a combining or substituent diluent monomer to triethylene glycol dimethacrylate (TEGDMA). The resin matrices were filled with 55 wt.% particles, of which 10 wt.% was 0.05-μm fumed silica, and 45 wt.% was 0.7-μm BaBSiO2 glass. Polymerization shrinkage stress (PSS; n=10), degree of conversion (DC; n=3), maximum rate of polymerization (Rpmax; n=3), film thickness (FT; n=10), sorption (Wsp; n=10), solubility (Wsl; n=10), flexural strength (FS; n=10), flexural modulus (FM; n=10), Knoop microhardness (KH; n=10), and microhardness reduction after chemical softening (HR; n=10) were evaluated. Data were analyzed using one-way ANOVA, followed by Tukey's test (α=0.05; β=0.2).

RESULTS

the results showed that the substitution or addition of IBOMA reduced FT (p=0.001), PSS (p=0.013), Rpmax (p=0.001), DC (p=0.001), FM (p=0.006) Wsp (p=0.032), and Wsl (p=0.021). However, when used as a complete substituent, IBOMA demonstrated significantly lower FS (p=0.017) and KH (p=0.008), while TEGDMA demonstrated significantly lower HR (p=0.022).

CONCLUSION

the flowable composite containing IBOMA combined with TEGDMA showed no effect in KH and FS and effectively reduced the PSS, RP, FT, Wsp, and Wsl. However, it showed a reduction in DC, FS, and an increase in HR.

Influence of Battery Levels in Cordless LED Light-curing Units on Properties of Resin Cement and Glass Fiber Post Retention

PURPOSE

This study aimed to assess the impact of battery levels in single-peak and multi-peak light-curing units (LCUs) on irradiance, and their effects on glass fiber post push-out bond strength to root dentin and the degree of conversion of dual-cure universal resin cement.

METHODS AND MATERIALS

Sixty bovine roots underwent endodontic treatment and were randomly distributed into 6 groups (n=10), formed by combining two LCUs (single-peak and multipeak) and three battery levels (100%, 50%, and 10%). A spectrophotometer measured irradiance (mW/ cm2) and spectral irradiance (mW/cm2/nm). Push-out bond strength (PBS) tests occurred at three root regions (cervical, middle, and apical), with optical and scanning electron microscopy for failure mode analysis. Degree of conversion (DC) was evaluated across the root regions. Data were analyzed using two-way repeated measures ANOVA and the Tukey HSD test. The Fisher exact test verified failure modes (α=0.05).

RESULTS

As multipeak LCU battery levels decreased, emitted irradiance also diminished, with notable PBS reductions in the apical thirds. Failure modes were influenced by different conditions, primarily exhibiting mixed modes. Battery levels significantly impacted DC in the multipeak LCU, particularly in the cervical region, while the single-peak LCU exhibited DC reduction at the 10% battery level in the cervical region.

CONCLUSIONS

Emitted irradiance, resin cement DC, and glass fiber post bond strength to root dentin may be influenced by varying cordless LCUs and battery levels.

Novel Remineralizing and Antibiofilm Low-Shrinkage-Stress Nanocomposites to Inhibit Salivary Biofilms and Protect Tooth Structures

Recurrent caries remain a persistent concern, often linked to microleakage and a lack of bioactivity in contemporary dental composites. Our study aims to address this issue by developing a low-shrinkage-stress nanocomposite with antibiofilm and remineralization capabilities, thus countering the progression of recurrent caries. In the present study, we formulated low-shrinkage-stress nanocomposites by combining triethylene glycol divinylbenzyl ether and urethane dimethacrylate, incorporating dimethylaminododecyl methacrylate (DMADDM), along with nanoparticles of calcium fluoride (nCaF2) and nanoparticles of amorphous calcium phosphate (NACP). The biofilm viability, biofilm metabolic activity, lactic acid production, and ion release were evaluated. The novel formulations containing 3% DMADDM exhibited a potent antibiofilm activity, exhibiting a 4-log reduction in the human salivary biofilm CFUs compared to controls (p < 0.001). Additionally, significant reductions were observed in biofilm biomass and lactic acid (p < 0.05). By integrating both 10% NACP and 10% nCaF2 into one formulation, efficient ion release was achieved, yielding concentrations of 3.02 ± 0.21 mmol/L for Ca, 0.5 ± 0.05 mmol/L for P, and 0.37 ± 0.01 mmol/L for F ions. The innovative mixture of DMADDM, NACP, and nCaF2 displayed strong antibiofilm effects on salivary biofilm while concomitantly releasing a significant amount of remineralizing ions. This nanocomposite is a promising dental material with antibiofilm and remineralization capacities, with the potential to reduce polymerization-related microleakage and recurrent caries.

Novel Dental Low-Shrinkage-Stress Composite with Antibacterial Dimethylaminododecyl Methacrylate Monomer

OBJECTIVES

Current dental resins exhibit polymerization shrinkage causing microleakage, which has the potential to cause recurrent caries. Our objectives were to create and characterize low-shrinkage-stress (LSS) composites with dimethylaminododecyl methacrylate (DMADDM) as an antibacterial agent to combat recurrent caries.

METHODS

Triethylene glycol divinylbenzyl ether and urethane dimethacrylate were used to reduce shrinkage stress. DMADDM was incorporated at different mass fractions (0%, 1.5%, 3%, and 5%). Flexural strength, elastic modulus, degree of conversion, polymerization stress, and antimicrobial activity were assessed.

RESULTS

The composite with 5% DMADDM demonstrated higher flexural strength than the commercial group (p < 0.05). The addition of DMADDM in BisGMA-TEGDMA resin and LSS resin achieved clinically acceptable degrees of conversion. However, LSS composites exhibited much lower polymerization shrinkage stress than BisGMA-TEGDMA composite groups (p < 0.05). The addition of 3% and 5% DMADDM showed a 6-log reduction in Streptococcus mutans (S. mutans) biofilm CFUs compared to commercial control (p < 0.001). Biofilm biomass and lactic acid were also substantially decreased via DMADDM (p < 0.05).

CONCLUSIONS

The novel LSS dental composite containing 3% DMADDM demonstrated potent antibacterial action against S. mutans biofilms and much lower polymerization shrinkage-stress, while maintaining excellent mechanical characteristics. The new composite is promising for dental applications to prevent secondary caries and increase restoration longevity.

Experimental Investigations of the Dental Filling Materials: Establishing Elastic Moduli and Poisson's Ratios

The mechanical properties of the dental filling material (DFMs) strongly influence the lifetime and durability of the tooth reparation performed. Among the most significant mechanical characteristics, one has to mention the Poisson's ratio and the elastic modulus (Young's modulus). They, during the cyclic mastication load, can prevent or aid in the prevention of secondary dental decays by provoking micro-cracks, the de-bonding of the filling material from the natural dental tissue, as well as fatigue at the level of their interface. The authors performed a scoping analysis of the nowadays-involved experimental methods, together with a critical review, putting in evidence of their advantages and limits. Based on the developments, they propose a new approach in this sense by involving the electronic speckle pattern interferometry (ESPI)/shearography high-accuracy optical method. They illustrate the advantages of this method in establishment of the elastic modulus, but they also propose a high-accuracy methodology in the estimation of Poisson's ratio. Based on the briefly-illustrated experimental results, one can conclude that ESPI/shearography can become a very useful tool for research, even though it is not a common (nowadays widely applied) method, such as three-point bending or strain gauge methods.

Impact of Immediate and Delayed Photo-activation of Self-adhesive Resin Cements on Bonding Efficacy and Water Uptake Under Simulated Pulpal Pressure

This study investigated the effect of immediate versus delayed photo-activation on the bonding performance and water uptake of self-adhesive (SA) resin cements under simulated pulpal pressure (SPP). The occlusal dentin surface was exposed in 66 extracted third molars. Resin composite cylinders were cemented to dentin under SPP, with either RelyX Unicem 2 (RU) (3M Oral Care, St Paul, MN, USA) or Maxcem Elite (MC) (Kerr, Orange, CA, USA). Each cement group was equally divided into three groups (n=8 each) according to the time elapsed between placement and photo-activation: immediate activation (IM), 30-second delayed activation (D30), or 120-second delayed activation (D120). Shear bond strength (SBS) was measured, and the type of failure was determined using a stereomicroscope. Three additional samples from each experimental subgroup were used for confocal laser scanning microscopy (CLSM) analysis. A fluorescent dye solution was added to the pulpal fluid reservoir, then a CLSM was used to detect the dye distribution within the tooth-restoration interface. Two-way analysis of variance (ANOVA) and the Tukey post-hoc test were used to analyze the SBS results (α=0.05). D30 resulted in a significantly higher mean SBS in the two cement groups than IM and D120 (p<0.05). RU showed significantly higher SBS values than MC regardless of the time of light activation (p<0.05). RU showed less dye uptake confined to the cement-dentin interface compared to the MC groups, which showed dye uptake throughout the entire thickness of the cement layer and gap formation at the interface, especially in the D120 group. The 30-second photo-activation delay group significantly improved the bond strength of SA cements. Delaying the photo-activation to 120 seconds increased pulpal fluid uptake by SA cements and compromised the integrity of the bonded interfaces.

Shrinkage Stress and Temperature Variation in Resin Composites Cured via Different Photoactivation Methods: Insights for Standardisation of the Photopolymerisation

The literature has shown that there is no consensus regarding the best resin composite photoactivation protocol. This study evaluated the efficiency of the conventional, soft-start, pulse-delay and exponential protocols for photoactivation of resin composites in reducing the shrinkage stress and temperature variation during the photopolymerisation. The photoactivation processes were performed using a photocuring unit and a smartphone app developed to control the irradiance according each photoactivation protocol. These photoactivation methods were evaluated applying photoactivation energies recommended by the resins manufactures. Three brands of resin composites were analysed: Z-250, Charisma and Ultrafill. The cure effectiveness was evaluated through depth of cure experiments. All results were statistically evaluated using one-way and multi-factor analysis of variance (ANOVA). The use of exponential and pulse-delay methods resulted in a significant reduction of the shrinkage stress for all evaluated resins; however, the pulse-delay method required too long a photoactivation time. The increases on the temperature were lower when the exponential photoactivation was applied; however, the temperature variation for all photoactivation protocols was not enough to cause damage in the restoration area. The evaluation of the depth of cure showed that all photoactivation protocols resulted in cured resins with equivalent hardness, indicating that the choice of an alternative photoactivation protocol did not harm the polymerisation. In this way, the results showed the exponential protocol as the best photoactivation technique for practical applications.

Development of dual-cured resin cements with long working time, high conversion in absence of light and reduced polymerization stress

OBJECTIVE

This study evaluated the properties of experimental dual-cured cements containing thiourethane (TU) and low concentrations of p-Tolyldiethanolamnie (DHEPT) and benzoyl peroxide (BPO) as chemical initiators.

METHODS

BisGMA/TEGDMA-based dual-cured cement was formulated with 1.0 wt% DHEPT and 0.75 wt% BPO as initiators and used as control. The concentration of BPO was adjusted to 0.1 wt% in catalyst paste of experimental cements, and two base pastes containing TU and 0.5 wt% or 0.25 wt% of DHEPT were formulated. The rheological behavior and kinetics of polymerization of cements were assessed in the absence of light activation. The kinetics of polymerization was also evaluated for cements light-activated immediately or 5 min after the start of mixing. Polymerization stress, flexural strength and elastic modulus (n = 5) were also evaluated under these conditions.

RESULTS

Cements with TU presented lower viscosity than the control, improved working time (0.25% DHEPT > 0.5% DHEPT) and higher conversion in the absence of light-activation. Delaying the light-activation reduced the maximum rate of polymerization (Rpmax) but did not affect the conversion or stress. The addition of TU increased the Rpmax and conversion, and reduced the stress when compared to the control, without affecting the flexural strength. Except for the control with delayed light-activation (highest values), the other experimental conditions yielded similar modulus.

SIGNIFICANCE

Adding TU and using a low concentration of DHEPT/BPO resulted in dual-cured cements with longer working time, reduced polymerization stress and increased conversion even in the absence of light, with no significant effect on the mechanical properties.

Effect of Tack Cure on Polymerization Shrinkage of Resin-based Luting Cements

CLINICAL RELEVANCE

Self-cure after tack cure could result in a lower polymerization shrinkage in some resin-based luting cements, which is closely related to lower degree of cure.

SUMMARY

Effect of luting materials, presence of tooth preparation, and functional loading on stress distribution on ceramic laminate veneers: A finite element analysis

STATEMENT OF PROBLEM

How the polymerization shrinkage, loading, and mechanical properties of luting materials affect the shrinkage and functional stresses in ceramic laminate veneers (CLVs) with and without tooth preparation is unclear.

PURPOSE

The purpose of this finite element analysis (FEA) study was to evaluate the effect of the polymerization shrinkage, functional loading, and mechanical properties of different luting materials on the stresses in ultrathin 0.3-mm CLVs with and without tooth preparation.

MATERIAL AND METHODS

Three resin cements, RelyX Veneer (RV), Allcem Veneer APS (AV), Variolink Esthetic LC (VE), and 1 flowable composite resin, Tetric N-Flow (TF), were tested for post-gel shrinkage (Shr), Knoop hardness (KHN), elastic modulus (E), compressive strength (CS), and diametral tensile strength (DTS). IPS e.max CAD disks of 0.3-mm thickness were made for simulating the effects of light attenuation. Eight 2-dimensional finite element models (Marc-Mentat) of a maxillary central incisor were generated to evaluate the polymerization shrinkage stress of different materials for luting 0.3-mm CLVs with or without tooth preparation and the stress during functional loading by using a modified von Mises criterion (mvm). Collected data from Shr, KHN, and E were submitted to 2-way ANOVA and the Tukey HSD test (α=.05).

RESULTS

Light attenuation by the 0.3-mm ceramic disk did not significantly affect the E values, but Shr was significantly lower in VE (26%) and TF (35%). TF had lower volumetric Shr (%) when interposing a ceramic disk (0.31%). Both tested tooth preparation options showed similar stress distributions from polymerization shrinkage or functional loading, with higher stress concentration on the incisal edge and also on the cervical surface. The model featuring tooth preparation and RV resin cement had the highest and VE the lowest stress levels.

CONCLUSIONS

The flowable composite resin had similar mechanical properties as the resin cements. The stress distribution from shrinkage and functional loading was similar for both techniques with or without tooth preparation.

Improved mechanical performance of self-adhesive resin cement filled with hybrid nanofibers-embedded with niobium pentoxide

OBJECTIVES

In this study hybrid nanofibers embedded with niobium pentoxide (Nb₂O₅) were synthesized, incorporated in self-adhesive resin cement, and their influence on physical-properties was evaluated.

METHODS

Poly(D,L-lactide), PDLLA cotton-wool-like nanofibers with and without silica-based sol-gel precursors were formulated and spun into submicron fibers via solution blow spinning, a rapid fiber forming technology. The morphology, chemical composition and thermal properties of the spun fibers were characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and Fourier-transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC), respectively. Produced fibers were combined with a self-adhesive resin cement (RelyX U200, 3M ESPE) in four formulations: (1) U200 resin cement (control); (2) U200+1wt.% PDLLA fibers; (3) U200+1wt.% Nb₂O₅-filled PDLLA composite fibers and (4) U200+1wt.% Nb₂O₅/SiO₂-filled PDLLA inorganic-organic hybrid fibers. Physical properties were assessed in flexure by 3-point bending (n=10), Knoop microhardness (n=5) and degree of conversion (n=3). Data were analyzed with One-way ANOVA and Tukey's HSD (α=5%).

RESULTS

Composite fibers formed of PDLLA-Nb₂O₅ exhibited an average diameter of ∼250nm, and hybrid PDLLA+Nb₂O₅/SiO₂ fibers were slightly larger, ∼300nm in diameter. There were significant differences among formulations for hardness and flexural strength (p<0.05). Degree of conversion of resin cement was not affected for all groups, except for Group 4 (p<0.05).

SIGNIFICANCE

Hybrid reinforcement nanofibers are promising as fillers for dental materials. The self-adhesive resin cement with PDLLA+Nb₂O₅ and PDLLA+Nb₂O₅/SiO₂ presented superior mechanical performance than the control group.

Biomechanical behavior of cavity design on teeth restored using ceramic inlays: An approach based on three-dimensional finite element analysis and ultrahigh-speed camera

Ceramic fracture and debonding are the primary failures that follow ceramic inlay and can lead to stress and tooth fracture. In this study, we examined two designs-concave and flat-of the gingival cavity bottom for tooth cavities restored using ceramic inlays. We investigated the biomechanical behavior of ceramic inlay-restored teeth (concave and flat) through three-dimensional finite element analysis (FEA) and experimentally validated the results using an ultrahigh-speed camera. We conducted in vitro real-time recording of the deformation of a restored tooth during loading using an ultrahigh-speed camera. This technique enables further image registration to observe deformation variation and vector fields. The deformation vector fields revealed that the concave design moved the deformation toward the buccal side of the cavity bottom, whereas the flat design moved it toward the palatal side. These findings correlated with the FEA results, which indicated that the concave design constrained stress in the dentin cavity and relieved palatal stress. Our results suggest that incorporating a concave design in cavity preparation can improve the fracture resistance of ceramic inlay-restored teeth, preventing unrestorable fractures. The current study is the first to utilize an ultrahigh-speed camera in dental biomechanics, and such cameras are useful for nondestructive and dynamic analysis. STATEMENT OF SIGNIFICANCE: First utilize ultrahigh-speed cameras in dental biomechanics analysis. Tooth fracture videos captured by ultrahigh-speed camera helps us learn fracture mechanics in between tooth cavity design and ceramic inlay. Concave design leads to stress in safer areas that causes a less damaging fracture. Minimal invasive preparation by concave design strengthens tooth fracture resistance. Non-destructive data from ultrahigh-speed cameras combined with FEA can get more insight into how the stress and strain derived in biomaterials.

Delayed Photoactivation of Dual-cure Composites: Effect on Cuspal Flexure, Depth-of-cure, and Mechanical Properties

Objectives:

This study tested whether delayed photoactivation could reduce shrinkage stresses in dual-cure composites and how it affected the depth-of-cure and mechanical properties.

Methods and Materials:

Two dual-cure composites (ACTIVA and Bulk EZ) were subjected to two polymerization protocols: photoactivation at 45 seconds (immediate) or 165 seconds (2 minutes delayed) after extrusion. Typodont premolars with standardized preparations were restored with the composites, and cuspal flexure caused by polymerization shrinkage was determined with three-dimensional scanning of the external tooth surfaces before restoration (baseline) and at 10 minutes and one hour after photoactivation. Bond integrity (intact interface) was verified with dye penetration. Depth-of-cure was determined by measuring Vickers hardness through the depth at 1-mm increments. Elastic modulus and maximum stress were determined by four-point bending tests (n=10). Results were analyzed with two- or three-way analysis of variance and pairwise comparisons (Bonferroni; α=0.05).

Results:

Delayed photoactivation significantly reduced cuspal flexure for both composites at 10 minutes and one hour (p≤0.003). Interface was &gt;99% intact in every group. Depth-of-cure, elastic modulus, and flexural strength were not significantly different between the immediate and delayed photoactivation (p&gt;0.05). The hardness of ACTIVA reduced significantly with depth (p&lt;0.001), whereas the hardness of Bulk EZ was constant throughout the depth (p=0.942).

Conclusions:

Delayed photoactivation of dual-cure restorative composites can reduce shrinkage stresses without negatively affecting the degree-of-cure or mechanical properties (elastic modulus and flexural strength).

In vitro antibacterial activity of self-etch bio-active dental adhesives after artificial aging

Purpose:

The aims to evaluate the antibacterial effect of different bioactive component containing dental adhesives before and after artificial aging.

Materials and methods:

Two bio-active adhesives; Clearfil Protect Bond and FL Bond II, two non-bioactive adhesives, Clearfil SE Bond and Clearfil S3 Bond were used for this study. Antibacterial activities of the fresh and aged samples against Streptococcus mutans were investigated with Direct Contact Test. Data were analyzed with Kruskal Wallis and Mann Whitney U multiple comparison tests.

Results:

For fresh samples FL Bond II and Clearfil Protect Bond exhibit similar antibacterial effect but Clearfil Protect Bond showed significantly higher antibacterial effect after aging the samples (p<0.05).

Conclusion:

The incorporation of bio-active antibacterial components into adhesive systems may be considered as a fundamental component in inhibiting residual Streptococcus mutans when considering the antibacterial effect of fresh samples of bio-active adhesives.

Effect of cavity preparation design and ceramic type on the stress distribution, strain and fracture resistance of CAD/CAM onlays in molars

Objective

This study aimed to evaluate the effect of the cavity preparation and ceramic type on the stress distribution, tooth strain, fracture resistance and fracture mode of human molar teeth restored with onlays.

Material and Methods

Forty-eight molars were divided into four groups (n=12) with assorted combinations of two study factors: BL- conventional onlay preparation with boxes made from leucite ceramic (IPS-Empress CAD, Ivoclar Vivadent); NBL- conservative onlay preparation without boxes made from leucite ceramic; BD- conventional onlay preparation with boxes made from lithium disilicate glass ceramic (IPS e.max CAD, Ivoclar Vivadent); NBL- conservative onlay preparation with boxes made from lithium disilicate glass ceramic cuspal deformation (µS) was measured at 100 N and at maximum fracture load using strain gauge. Fracture resistance (N) was measured using a compression test, and the fracture mode was recorded. Finite element analysis was used to evaluate the stress distribution by modified von Mises stress criteria. The tooth strain and fracture resistance data were analyzed using the Tukey test and two-way ANOVA, and the fracture mode was analyzed by the chi-square test (α=0.05).

Results

The leucite ceramic resulted in higher tooth deformation at 100 N and lower tooth deformation at the maximum fracture load than the lithium disilicate ceramic (P<0.001). The lithium disilicate ceramic exhibited higher fracture resistance than the leucite ceramic (P<0.001). The conservative onlay resulted in higher fracture strength for lithium disilicate ceramic. Finite element analysis results showed the conventional cavity preparation resulted in higher stress concentration in the ceramic restoration and remaining tooth than the conservative onlay preparation. The conservative onlays exhibited increased fracture resistance, reduced stress concentration and more favorable fracture modes.

Conclusion

Molars restored with lithium disilicate CAD-CAM ceramic onlays exhibited higher fracture resistance than molars restored with leucite CAD-CAM ceramic onlays.

Polymerization shrinkage stress of composite resins and resin cements - What do we need to know?

Polymerization shrinkage stress of resin-based materials have been related to several unwanted clinical consequences, such as enamel crack propagation, cusp deflection, marginal and internal gaps, and decreased bond strength. Despite the absence of strong evidence relating polymerization shrinkage to secondary caries or fracture of posterior teeth, shrinkage stress has been associated with post-operative sensitivity and marginal stain. The latter is often erroneously used as a criterion for replacement of composite restorations. Therefore, an indirect correlation can emerge between shrinkage stress and the longevity of composite restorations or resin-bonded ceramic restorations. The relationship between shrinkage and stress can be best studied in laboratory experiments and a combination of various methodologies. The objective of this review article is to discuss the concept and consequences of polymerization shrinkage and shrinkage stress of composite resins and resin cements. Literature relating to polymerization shrinkage and shrinkage stress generation, research methodologies, and contributing factors are selected and reviewed. Clinical techniques that could reduce shrinkage stress and new developments on low-shrink dental materials are also discussed.

Titanium dioxide nanotubes addition to self-adhesive resin cement: Effect on physical and biological properties

OBJECTIVES

This study has investigated the influence of Titanium dioxide nanotubes (TiO₂-nt) addition to self-adhesive resin cement on the degree of conversion, water sorption, and water solubility, mechanical and biological properties.

METHODS

A commercially available auto-adhesive resin cement (RelyX U200™, 3M ESPE) was reinforced with varying amounts of nanotubes (0.3, 0.6, 0.9wt%) and evaluated at different curing modes (self- and dual cure). The DC in different times (3, 6, 9, 12 and 15min), water sorption (Ws) and solubility (Sl), 3-point flexural strength (σf), elastic modulus (E), Knoop microhardness (H) and viability of NIH/3T3 fibroblasts were performed to characterize the resin cement.

RESULTS

Reinforced self-adhesive resin cement, regardless of concentration, increased the DC for the self- and dual-curing modes at all times studied. The concentration of the TiO₂-nt and the curing mode did not influence the Ws and Sl. Regarding σf, concentrations of both 0.3 and 0.9wt% for self-curing mode resulted in data similar to that of dual-curing unreinforced cement. The E increased with the addition of 0.9wt% for self-cure mode and H increased with 0.6 and 0.9wt% for both curing modes. Cytotoxicity assays revealed that reinforced cements were biocompatible.

SIGNIFICANCE

TiO₂-nt reinforced self-adhesive resin cement are promising materials for use in indirect dental restorations. Taken together, self-adhesive resin cement reinforced with TiO₂-nt exhibited physicochemical and mechanical properties superior to those of unreinforced cements, without compromising their cellular viability.

Degree of conversion and bond strength of resin-cements to feldspathic ceramic using different curing modes

Resin cements have led to great advances in dental ceramic restoration techniques because of their ability to bond to both dental structures and restorative materials.

Comparison of immediate and delayed light-curing on nano-indentation creep and contraction stress of dual-cured resin cements

STATEMENT OF PROBLEM

Polymerization protocol of dual-cured resin cements plays a great role in the success of restorations. Knowledge about the effect of light-curing protocol on the contraction stress value and the mechanical properties would assist in the development, selection and handling of the resin cement material.

PURPOSE

This study was undertaken to assess the effects of two different light-curing protocols on the polymerization shrinkage stress and nano-creep of two current dual-cured resin cements.

MATERIALS AND METHODS

Twenty-four soda-lime glass disks, measuring 1mm in thickness, with a perforation in their center that measured 3mm in diameter, received four Vickers indentations at a distance of 500µ from the margin. The indent cracks were measured before and at 15-, 30- and 60-min intervals after restoration of the cavity with either CLEARFIL™ SA or RelyX™ Ultimate cement. Twelve specimens were prepared from each resin cement and divided into two groups according to the curing protocol used: immediate and delayed light-curing. Stresses at the indent impression were calculated by considering glass fracture toughness and the increase in crack length. Stress at the bonded interface was calculated using the Lame equation for thick-walled cylinder under an internal pressure. Nano-indentation hardness, modulus of elasticity and creep of each cement were measured under a 10-mN load for 20s holding time. The specimens were stored in distilled water at 37°C for 24h before the nano-indentation tests. Data were analyzed with two-way and repeated-measures ANOVA (α≤0.05).

RESULTS

Curing protocol had a significant effect on both resin cements; higher stress values were recorded with the immediate curing mode (P=0.033). Curing mode did not exert a significant effect on the modulus of elasticity (P=0.595) and hardness (P=0.454) of resin cements. However, in relation to creep, it exhibited significant effects (P<0.0001).

CONCLUSIONS

Immediate light-curing resulted in higher polymerization stress and lower nano-creep values compared to the delayed protocol with the resin cements assessed.

Dynamic measurement of local displacements within curing resin-based dental composite using optical coherence elastography

This study aimed to determine the feasibility of using optical coherence elastography to measure internal displacements during the curing phase of a light-activated, resin-based composite material. Displacement vectors were spatially mapped over time within a commercial dental composite. Measurements revealed that the orientation of cure-induced displacement vectors varied spatially in a complex manner; however, each vector showed a systematic evolution with time. Precision of individual displacements was estimated to be ∼1 ∼1 to 2  μm 2  μm , enabling submicrometer time-varying displacements to be detected.