System compliance dictates the effect of composite filler content on polymerization shrinkage stress

Polymerization shrinkage of contemporary dental resin composites: Comparison of three measurement methods with correlation analysis

The aim of this study was to evaluate a novel approach for measuring the polymerization shrinkage of dental resin composites - measurement of sample depth variation. This new method was compared with two testing methodologies used for assessing the polymerization shrinkage (buoyancy and strain gauge methods). Eleven commercial resin composites were investigated (EverX Posterior; EverX Flow Bulk & Dentin; G-aenial Anterior, Posterior, A'chord & Universal Injectable; Filtek One Bulk Fill & Universal Restorative; SDR + Flow and Aura Bulk Fill). In addition, filler content (wt. %), flexural modulus, and the degree of conversion were evaluated. Shrinkage values, obtained by the buoyancy method, are greater than shrinkage evaluated by the strain gauge. There are significant differences in polymerization shrinkage among the tested resin composite materials. There is a strong correlation between the newly proposed method of shrinkage measurement and the buoyancy method (r2 = 0.8; p < 0.01). There is no correlation between volumetric shrinkage measurement (depth changes and buoyancy method) and linear strain measurement. Volumetric filler amount correlates with shrinkage values evaluated by all three methods. The degree of conversion for the tested resin composites ranges from 36 % to 52 %. There are some differences (around 10 %) between the filler content (wt. %) measured by the ashing-in-air method and the data given by the manufacturers. The highest flexural modulus is 14.8 GPa and the lowest 6.6 GPa. New formulations may introduce unknown relationships between the fundamental properties of dental resin composites.

Do nanofillers provide better physicomechanical properties to resin-based pit and fissure sealants? A systematic review

The purpose of this study was to systematically review the impact of nanofillers on the physicomechanical properties of resin-based pit and fissure sealants (RBS). This review included in vitro studies with full-length English-language articles reporting on the physicomechanical properties of nanofilled RBS until February 2023. PubMed, Web of Sciences, Scopus, and LILACS databases were accessed for literature searches. The review was formulated based on the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines and used the Consolidated Standards of Reporting Trials (CONSORT) guidelines and risk of bias Cochrane tool for quality assessment. The search resulted in 539 papers, of which 22 were eligible to be included in the review. Inorganic, polymeric, core-shell, and composite nanomaterials were used to reinforce the studied RBS. The inherent nature of the nanomaterial used, its morphology, concentration, and volume used were the primary parameters that determined the nanomaterial's success as a filler in RBS. These parameters also influenced their interaction with the resin matrix, which influenced the final physicomechanical properties of RBS. The use of nanofillers that were non-agglomerated and well dispersed in the resin matrix enhanced the physicomechanical properties of RBS.

Effect of elastomeric urethane monomer on physicochemical properties and shrinkage stress of resin composites

This study aimed to evaluate the effect of an elastomeric urethane monomer (Exothane-24) in different concentrations on physicochemical properties, gap formation, and polymerization shrinkage stress of experimental resin composites. All experimental composites were prepared with 50 wt.% of Bis-GMA and 50 wt.% of TEGDMA, to which 0 wt.% (control), 10 wt.%, 20 wt.%, 30 wt.%, and 40 wt.% of Exothane-24 were added. Filler particles (65 wt.%) were then added to these resin matrixes. Ultimate tensile strength (UTS: n = 10), flexural strength (FS: n = 10), flexural modulus (FM: n = 10), hardness (H: n = 10), hardness reduction (HR: n = 10), degree of conversion (DC: n = 5), gap width (GW: n = 10), and polymerization shrinkage stress in Class I (SS-I: n = 10) and Class II (SS-II: n = 10) simulated configuration. All test data were analyzed using one-way ANOVA and Tukey's test (α = 0.05;  = 0.2). Exothane-24 in all concentrations decreased the H, HR, DC, GW, SS-I, and SS-II (p < 0.05) without affecting the UTS, and FS (p > 0.05). Reduction in FM was observed only in the Exothane 40% and 30% groups compared to the control (p < 0.05). Exothane-24 at concentrations 20% and 30% seems suitable since it reduced GW and polymerization SS without affecting the properties of the composite resins tested, except for H.

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.

Optimal resin monomer ratios for light-cured dental resins

Monomer ratios play a crucial role on the performances of dental resins, the optimal monomer ratios for dental resins are determined by combining the degree of conversion (DC), rate of polymerization (Rp), and mechanical properties, based on commonly-used Bis-GMA (bisphenol A-glycidyl methacrylate), UDMA (urethane dimethacrylate), and TEGDMA (triethyleneglycol dimethacrylate) resins. The DC and mechanical properties of the dental resins are examined by NIR (Near Infrared Ray) spectroscopy and nanoindentation tests, respectively. The results indicate that the Rp increases while the DC decreases with the loading content of Bis-GMA or UDMA in dental resins (i.e., Bis-GMA/TEGDMA and UDMA/TEGDMA). Meanwhile, both elastic modulus and hardness also present a tendency to increase. Various different monomers maybe create a strong polymer matrix in proper proportions, comprehensively comparing the performances of dental resins in different monomer ratios, the cured resins containing Bis-GMA (15–35 wt%), UDMA (37–60 wt%) and TEGDMA (20–35 wt%) show better material properties. The present study offers a quantitative analysis for Bis-GMA/UDMA/TEGDMA dental resins as well as provides guidance for the research of dental resins.

Polymerization Kinetics and Development of Polymerization Shrinkage Stress in Rapid High-Intensity Light-Curing

This study investigated polymerization kinetics, linear shrinkage, and shrinkage stress development for six contemporary composite materials of different viscosities cured using radiant exitances of 1100–2850 mW/cm². Real-time measurements of degree of conversion, linear shrinkage, and shrinkage stress were performed over 5 min using Fourier-transform infrared spectrometry, a custom-made linometer, and a custom-made stress analyzer, respectively. For most tested variables, the factor “material” had a higher effect size than the factor “curing protocol”. Maximum polymerization rate and maximum shrinkage stress rate were the most affected by changes in curing conditions. In contrast, no significant effects of curing conditions were identified within each material for shrinkage stress values measured at the end of the 5 min observation period. Linear shrinkage and shrinkage stress values measured after 5 min were closely correlated (R = 0.905–0.982). The analysis of polymerization kinetics suggested that the two composites specifically designed for rapid light-curing responded to higher radiant exitances differently than other composites. Polymerization kinetics and shrinkage stress behavior of contemporary restorative composite materials of different viscosities were overall more affected by material type than differences in curing conditions. Subtle differences in polymerization kinetics behavior shown by the two composites specifically designed for rapid high-intensity light-curing did not translate into significant differences in the development of polymerization shrinkage stress.

Full mapping tensile bond strength of luting in search for differences due to centripetal curing shrinkage

OBJECTIVES

testing if hypothetical transverse centripetal strains due to polymerization contraction of luting materials produce differential alterations in its bonding to luted structures, depending on distances to the center of the luting mass, and if this effect is C-factor related. Two hypotheses were tested: (1) there is a statistically significant decreasing relationship between the bonding strength and the transverse distances to the center of the luting material, and (2) there is a statistically significant difference between bonding strengths among luting spaces with different configuration factors.

METHODS

10 PMMA (15 mm Ø) pairs of cylinders were cemented (Scotchbond Universal adhesive & Relyx Universal, both chemically cured) in a compliant setup under two (20 and 70 N) luting forces forming 2 groups (5 samples each), resulting in different C-factors. Whole samples were sectioned in x and y directions obtaining non-trimmed beams from all along the luting surfaces. Their relative positions in each sample were assessed before separating and categorized (10 categories) according to their distances to the center of the sample. All beams were tested in tension and, because of their uneven bonding areas and to balance its influence, UTS results were transformed into UTS_res. First hypothesis was tested trough a linear relationship between UTS_res and distances to vertical centers per samples. Second hypothesis was tested using Mann-Whitney U tests to compare UTS_res between groups, along all categories. Further Weibull analysis was applied.

RESULTS

ANOVA's p of the regression UTS_res - categories were statistically significant for all samples in group 70 N and for all except one in group 20 N: first hypothesis is partially maintained. Although Mann-Whitney tests p comparing UTS_res of both groups for all categories but the first were statistically significant this hypothesis was maintained relying in Weibull analysis.

SIGNIFICANCE

bonded attachment of cemented materials decreases from centers to outbounds in plane, extensive surfaces, and this decrease is C-factor related.

The use of an elastomeric methacrylate monomer (Exothane 24) to reduce the polymerization shrinkage stress and improve the two-body wear resistance of bulk fill composites

OBJECTIVES

Characterize the chemical structure of an elastomeric monomer (Exothane 24) and evaluate the degree of conversion (DC), polymerization shrinkage stress (PSS), rate of polymerization (Rp), flexural strength (F_Strenght), flexural modulus (F_Modulus), Vickers hardness (V_Hardness) and two-body wear resistance of dental bulk fill composites (BFCs) containing Exothane 24.

METHODS

The Exothane 24 was characterized using mass spectroscopy, elemental analysis, ¹³C- and ¹H NMR. BFCs were formulated containing Exothane 24 (E10, E25, and E50). Similar BFCs containing regular UDMA (U10, U25, and U50), commercial conventional, and BFCs were used as control groups. ATR-FTIR spectroscopy was used to measure DC and the Rp of the composites. The PSS was measured using the universal testing machine method. Specimen bars were used to assess the F_Strenght, F_Modulus, and V_Hardness. RBCs were submitted to a two-body wear test using a chewing simulator machine; the rate and volumetric wear loss were evaluated using an optical scanner. Data were analyzed statistically with α = 0.05 and β = 0.2.

RESULTS

Exothane 24 is a urethane isophorone tetramethyl methacrylate monomer with polymerization stress-relieving properties. No differences were found in the DC up to 4 mm in depth for E25. All BFCs had similar F_Strenght, except for E50. E25 had the lowest volumetric wear loss and wear rate. E25 had approximately 30% lower PSS and slower Rp than commercial BFCs with similar wear resistance to conventional commercial composites.

SIGNIFICANCE

The Exothane 24 reduced the PSS and increased the wear resistance of BFCs; however, the formulation is important to optimize the properties of the BFCs.

Polymerization kinetics of experimental resin composites functionalized with conventional (45S5) and a customized low-sodium fluoride-containing bioactive glass

This study aimed to investigate polymerization kinetics and curing light transmittance of two series of experimental dental resin composites filled with 0–40 wt% of either 45S5 bioactive glass (BG) or a customized low-Na F-containing BG. Polymerization kinetics in 0.1-mm and 2-mm thick layers were investigated through real-time degree of conversion measurements using a Fourier transform infrared (FTIR) spectrometer. FTIR spectra were continuously collected at a rate of 2 s⁻¹ during light-curing (1340 mW/cm²). Light transmittance through 2-mm thick composite specimens was measured using a UV–Vis spectrometer at a rate of 20 s⁻¹. Unlike BG 45S5, which led to a dose-dependent reduction in the rate and extent of polymerization, the customized low-Na F-containing BG showed a negligible influence on polymerization. The reduction in light transmittance of experimental composites due to the addition of the low-Na F-containing BG did not translate into impaired polymerization kinetics. Additionally, the comparison of polymerization kinetics between 0.1-mm and 2-mm thick layers revealed that polymerization inhibition identified for BG 45S5 was not mediated by an impaired light transmittance, indicating a direct effect of BG 45S5 on polymerization reaction. A customized low-Na F-containing BG showed favourable behaviour for being used as a functional filler in light-curing dental resin composites.

Low-shrinkage-stress nanocomposite: An insight into shrinkage stress, antibacterial, and ion release properties

The aims are: (a) To develop the first low-shrinkage-stress nanocomposite with antibacterial and remineralization capabilities through the incorporation of dimethylaminododecyl methacrylate (DMAHDM) and nanoparticles of amorphous calcium phosphate (NACP); (b) to investigate the effects of the new composite on biofilm inhibition, mechanical properties, shrinkage stress, and calcium (Ca) and phosphate (P) ion releases. The low-shrinkage-stress resin consisted of urethane dimethacrylate and triethylene glycol divinylbenzyl ether. Composite was formulated with 3% DMAHDM and 20% NACP. Mechanical properties, shrinkage stress, and degree of conversion were evaluated. Streptococcus mutans biofilm growth on composites was assessed. Ca and P ion releases were measured. The shrinkage stress of the low-shrinkage-stress composite containing 3% DMAHDM and 20% NACP was 36% lower than that of traditional composite control (p < 0.05), with similar degrees of conversion of 73.9%. The new composite decreased the biofilm colony-forming unit by 4 log orders and substantially reduced biofilm lactic acid production compared to control composite (p < 0.05). Incorporating DMAHDM to the low-shrinkage-stress composite did not adversely affect the Ca and P ion release. A novel bioactive nanocomposite was developed with low shrinkage stress, strong antibiofilm activity, and high levels of ion release for remineralization, without undermining the mechanical properties and degree of conversion.

Polymerization shrinkage behaviour of resin composites functionalized with unsilanized bioactive glass fillers

Previous work has shown that partial replacement of reinforcing fillers with unsilanized silica particles can diminish polymerization shrinkage stress of dental resin composites. The aim of the present study was to investigate whether such an effect can be attained by using unsilanized bioactive glass (BG). Incorporating BG fillers into resin composites is interesting due to their potential for exerting caries-preventive effects. Experimental light-curable composites with a total filler load of 77 wt% were prepared. Reinforcing fillers were partially replaced with 0-60 wt% of BG 45S5 and an experimental low-sodium fluoride-containing BG. The following properties were investigated: linear shrinkage, degree of conversion, shrinkage stress, maximum shrinkage stress rate, and time to achieve maximum shrinkage stress rate. The diminishing effect of BG 45S5 on shrinkage stress was mediated by a decrease in degree of conversion caused by this BG type. In contrast, as the degree of conversion remained unaffected by the experimental BG, the resulting shrinkage behaviour was governed by the effect of varying amounts of silanized and unsilanized fillers on material's viscoelastic properties. The replacement of silanized reinforcing fillers with unsilanized BG did not reduce polymerization shrinkage stress unless the reduction was attained indirectly through a diminished degree of conversion.

Novel low-shrinkage-stress nanocomposite with remineralization and antibacterial abilities to protect marginal enamel under biofilm

OBJECTIVES

Polymerization shrinkage stress may lead to marginal damage, microleakage and failure of composite restorations. The objectives of this study were to : (1) develop a novel nanocomposite with low-shrinkage-stress, antibacterial and remineralization properties to reduce marginal enamel demineralization under biofilms; (2) evaluate the mechanical properties of the composite and calcium (Ca) and phosphate (P) ion release; and (3) investigate the cytotoxicity of the new low-shrinkage-stress monomer in vitro.

METHODS

The low-shrinkage-stress resin consisted of urethane dimethacrylate (UDMA) and triethylene glycol divinylbenzyl ether (TEG-DVBE), and 3 % dimethylaminohexadecyl methacrylate (DMAHDM) and 20 % calcium phosphate nanoparticles (NACP) were added. Mechanical properties, polymerization shrinkage stress, and degree of conversion were evaluated. The growth of Streptococcus mutans (S. mutans) on enamel slabs with different composites was assessed. Ca and P ion releases and monomer cytotoxicity were measured.

RESULTS

Composite with DMAHDM and NACP had flexural strength of 84.9 ± 10.3 MPa (n = 6), matching that of a commercial control composite. Adding 3 % DMAHDM did not negatively affect the composite ion release. Under S. mutans biofilm, the marginal enamel hardness was 1.2 ± 0.1 GPa for the remineralizing and antibacterial group, more than 2-fold the 0.5 ± 0.07 GPa for control (p < 0.05). The polymerization shrinkage stress of the new composite was 40 % lower than that of traditional composite control (p < 0.05). The new monomers had fibroblast viability similar to that of traditional monomer control (p > 0.1).

CONCLUSION

A novel low-shrinkage-stress nanocomposite was developed with remineralizing and antibacterial properties. This new composite is promising to inhibit recurrent caries at the restoration margins by reducing polymerization stress and protecting enamel hardness.

Effect of Dentin Bonding Agents, Various Resin Composites and Curing Modes on Bond Strength to Human Dentin

This study investigated the influence of several dentin bonding agents, resin composites and curing modes on push-out bond strength to human dentin. 360 extracted caries-free third molars were prepared, cut into slices, embedded in epoxy resin and perforated centrally. One half of the specimens (180) were treated by using one-step adhesive systems and the other half (180) with multi-step adhesive systems. Subsequently, the cavities were filled with either universal, flowable or bulk-fill resin composite according to the manufactures’ product line and cured with either turbo or soft start program. After storage the push-out test was performed. The data was analyzed using Kolmogorov-Smirnov, three- and one-way ANOVA followed by the Scheffé post-hoc test, unpaired two-sample t-test (p < 0.05). The strongest influence on push-out bond strength was exerted by the resin composite type (partial eta squared η_P² = 0.505, p < 0.001), followed by the adhesive system (η_P² = 0.138, p < 0.001), while the choice of the curing intensity was not significant (p = 0.465). The effect of the binary or ternary combinations of the three parameters was significant for the combinations resin composite type coupled adhesive system (η_P² = 0.054, p < 0.001), only. The flowable resin composites showed predominantly mixed, while the universal and bulk-fill resin composite showed adhesive failure types. Cohesive failure types were not observed in any group. Multi-step adhesive systems are preferable to one-step adhesive systems due to their higher bond strength to dentin. Flowable resin composites showed the highest bond strength and should become more important as restoration material especially in cavity lining. The use of a soft start modus for polymerization of resin composites does not enhance the bond strength to dentin.

Spatial and temporal tunability of magnetically-actuated gradient nanocomposites

Natural biological materials usually adopt functional gradient designs within interfacial regions to fulfil unusual mechanically-challenging demands. Manufacturing analogous gradients to alleviate premature failures for synthetic interfaces has remained challenging until recently, where magnetically-actuated gradient nanocomposites (MA-G-NCs) have emerged as a promising processing technique. The essence of this technique lies in controlling the spatial distribution of nanoreinforcements (usually particles) inside a polymer matrix through a magnetophoresis process. Herein, we present a theory-experiment-combined study on the evolution kinetics and equilibrium distribution of the nanoparticles during the magnetophoresis process and consequently to explore the spatial and temporal tunability of the MA-G-NCs. Using a simplified drift-diffusion theory as the guide, we determine two critical processing parameters for the MA-G-NCs: the applied magnetic field and the actuation duration. By systematically varying these two parameters independently, we experimentally demonstrate that the profile of the nanoparticle distribution inside the MA-G-NCs can be finely tuned both spatially and temporally. In order to quantify the volume fraction of the nanoparticles along the cross section of the MA-G-NCs, we propose a mechanics-based method by site-specifically measuring the local elastic modulus and converting back to the volume fractions based on an established modulus-fraction correlation. The nanoparticle concentration profiles obtained thereby are validated by morphological characterizations and also agree well with theoretical predictions based on the drift-diffusion theory. Our combined results indicate that the magnetophoresis-induced evolution of the nanoparticles follows approximately the drift-diffusion transport process and the gradient profile of the MA-G-NCs is highly controllable and programmable. The presented study not only advances the fundamental understanding of the evolution kinetics of the nanoparticles under the effect of magnetophoresis, but also establishes the critical processing-structure-property relationships for the MA-G-NCs that should guide future development of customized interfaces with desired mechanical and physical property gradients.

Development of mechanical properties in dental resin composite: Effect of filler size and filler aggregation state

The aim of this work was to study the effect of filler size and filler aggregation state on the mechanical properties of dental resin composites evaluated at filler loadings between 20 wt% and up to 76.5 wt%. Non-aggregated silica nanoparticles (SiNP_MPS) (80 nm), doughnut-shaped silica nanoclusters obtained by spray drying (SDSiNP_MPS) (3.5 μm) and amorphous barium-alumina borosilicate microparticles (BaAlBoSi_MPS) (1.0 μm), functionalized by 3-methacryloxypropyl trimethoxysilane (MPS), were the fillers incorporated into resin matrix dental composites composed of triethylene glycoldimethacrylate (TEGDMA), urethane dimethylacrylate (UDMA), bisphenol A polyethylene glycol diether dimethacrylate (Bis EMA), and bisphenol A glycidyl methacrylate (BisGMA) (0.3:0.7:1:1 weight ratio, respectively). The mechanical properties developed in the resin composites were correlated with the formation of percolated-like particle networks, as observed by scanning electron microscopy (SEM), and volume fraction percolation thresholds (ϕ_c) calculated from a percolation model. Resin composites with non-aggregated SiNP_MPS showed an apparent percolation threshold ϕ_c = 0.15 (i.e. 27 wt%); above this filler concentration and up to a volume fraction of particles (ϕ_P) of 0.24 (i.e. 40 wt%) there was an increase in the flexural modulus and the compressive strength of the resin composite. However, a further increase in filler concentration diminished all its mechanical properties due to a decrease in the particle-matrix adhesion strength, demonstrated by the increase in surface roughness and fracture steps as observed by SEM images. On the other hand, a resin composite filled with doughnut-shaped silica nanoclusters (SDSiNP_MPS) showed an apparent percolation threshold ϕ_c = 0.41 (i.e. 60 wt%); increasing filler loading over this concentration generated an improvement in its mechanical properties, except the flexural strength also due to a decrease in the particle-matrix adhesion strength. The resin composites obtained with amorphous individual BaAlBoSi_MPS microparticles (1.0 μm) and BaAlBoSi_MPS microparticle aggregates (ca. 40.0 μm) showed an apparent percolation threshold ϕ_c = 0.41 (i.e. 64 wt%) that promoted an improvement in all their mechanical properties. SEM image of BaAlBoSi_MPS resin composite at high filler loading (≥ 60 wt%) showed a decrease in fracture steps and no presence of voids, indicating a better adhesion between amorphous BaAlBoSi_MPS particles and the polymeric matrix, which explains the improvement of mechanical properties. Resin composites filled exclusively with silica doughnut-shape nanoclusters or amorphous BaAlBoSi_MPS microparticles could develop mechanical properties similar to or even better than those obtained by mixing nanofillers with spherical nanoclusters, which are commonly used in commercial resin composites.

Polymerization shrinkage and shrinkage force kinetics of high- and low-viscosity dimethacrylate- and ormocer-based bulk-fill resin composites

The aim of the present study was to investigate polymerization shrinkage, shrinkage force development, and degree of monomer conversion of high- and low-viscosity dimethacrylate- and ormocer-based bulk-fill resin composites. Two flowable bulk-fill composites (SDR, x-tra base), two high-viscosity bulk-fill composites (Bulk Ormocer, SonicFill), and two conventional composite materials (Esthet X flow, Esthet X HD) were photoactivated for 20 s at 1275 mW/cm². Linear polymerization shrinkage and shrinkage force were recorded in real time using custom-made devices, and the force rate and time to achieve maximum force rate were determined. Degree of conversion was measured using Fourier-transform infrared spectroscopy. Data were analyzed with one-way ANOVA and Tukey's HSD post-hoc test, and bivariate correlations were computed (α = 0.05). The category of high-viscosity bulk-fill resin composites showed the significantly lowest polymerization shrinkage and force development. Within the tested flowable composite materials, SDR bulk-fill generated the significantly lowest shrinkage forces during polymerization and attained the significantly highest degree of conversion. Strong positive correlations were revealed between shrinkage force and both linear polymerization shrinkage (r = 0.902) and maximum force rate (r = 0.701). Linear shrinkage and shrinkage force both showed a negative correlation with filler volume content (r = - 0.832 and r = - 0.704, respectively). Bulk-fill resin composites develop lower shrinkage forces than their conventional flowable and high-viscosity counterparts, respectively, which supports their use for restoring high C-factor posterior cavities. Overall, bulk-fill composites with high filler amount and low force rate showed the most favorable shrinkage force characteristics.

Real-time synchronous measurement of curing characteristics and polymerization stress in bone cements with a cantilever-beam based instrument

An instrumentation capable of simultaneously determining degree of conversion (DC), polymerization stress (PS), and polymerization exotherm (PE) in real time was introduced to self-curing bone cements. This comprises the combination of an in situ high-speed near-infrared spectrometer, a cantilever-beam instrument with compliance-variable feature, and a microprobe thermocouple. Two polymethylmethacrylate-based commercial bone cements, containing essentially the same raw materials but differ in their viscosity for orthopedic applications, were used to demonstrate the applicability of the instrumentation. The results show that for both the cements studied the final DC was marginally different, the final PS was different at the low compliance, the peak of the PE was similar, and their polymerization rates were significantly different. Systematic variation of instrumental compliance for testing reveals differences in the characteristics of PS profiles of both the cements. This emphasizes the importance of instrumental compliance in obtaining an accurate understanding of PS evaluation. Finally, the key advantage for the simultaneous measurements is that these polymerization properties can be correlated directly, thus providing higher measurement confidence and enables a more in-depth understanding of the network formation process.

Antimicrobial Monomers for Polymeric Dental Restoratives: Cytotoxicity and Physicochemical Properties

A trend for the next generation of polymeric dental restoratives is to incorporate multifunctional capabilities to regulate microbial growth and remineralize tooth surfaces. Polymerizable 2-(methacryloyloxy)-N-(2-(methacryloyloxy)ethyl)-N,N-dimethylethan-1-aminium bromide (IDMA1) and N,N′-([1,1′-biphenyl]-2,2′-diylbis(methylene))bis(2-(methacryloyloxy)-N,N-dimethylethan-1-aminium) bromide (IDMA2), intended for utilization in bi-functional antimicrobial and remineralizing composites, were synthesized, purified with an ethanol-diethyl ether-hexane solvent system, and validated by nuclear magnetic resonance (¹H and ¹³C NMR) spectroscopy, mass spectrometry, and Fourier-transform infrared spectroscopy. When incorporated into light-curable urethane dimethacrylate (UDMA)/polyethylene glycol-extended UDMA (PEG-U)/ethyl 2-(hydroxymethyl)acrylate (EHMA) (assigned UPE) resins, IDMAs did not affect the overall resins’ hydrophilicity/hydrophobicity balance (water contact angle: 60.8–65.5°). The attained degrees of vinyl conversion (DVC) were consistently higher in both IDMA-containing copolymers and their amorphous calcium phosphate (ACP) composites (up to 5% and 20%, respectively) reaching 92.5% in IDMA2 formulations. Notably, these high DVCs values were attained without an excessive increase in polymerization stress. The observed reduction in biaxial flexure strength of UPE-IDMA ACP composites should not prevent further evaluation of these materials as multifunctional Class V restoratives. In direct contact with human gingival fibroblasts, at biologically relevant concentrations, IDMAs did not adversely affect cell viability or their metabolic activity. Ion release from the composites was indicative of their strong remineralization potential. The above, early-phase biocompatibility and physicochemical tests justify further evaluation of these experimental materials to identify formulation(s) suitable for clinical testing. Successful completion is expected to yield a new class of restoratives with well-controlled bio-function, which will physicochemically, mechanically, and biologically outperform the conventional Class V restoratives.

Polymer-Based Direct Filling Materials

After a brief review of current restorative materials and classifications, this article discusses the latest developments in polymer-based direct filling materials, with emphasis on products and studies available in the last 10 years. This will include the more recent bulk fill composites and self-adhesive materials, for which clinical evidence of success, albeit somewhat limited, is already available. The article also introduces the latest cutting edge research topics on new materials for composite restorations, and an outlook for the future of how those may help to improve the service life of dental composite restorations.

Polymerization stress evolution of a bulk-fill flowable composite under different compliances

OBJECTIVE

To use a compliance-variable instrument to simultaneously measure and compare the polymerization stress (PS) evolution, degree of conversion (DC), and exotherm of a bulk-fill flowable composite to a packable composite.

METHODS

A bulk-fill flowable composite (Filtek Bulk-fill, FBF) and a conventional packable composite (Filtek Z250, Z250) purchased from 3M ESPE were investigated. The composites were studied using a cantilever-beam based instrument equipped with an in situ near infrared (NIR) spectrometer and a microprobe thermocouple. The measurements were carried out under various instrumental compliances (ranging from 0.3327μm/N to 12.3215μm/N) that are comparable to the compliances of clinically prepared tooth cavities. Correlations between the PS and temperature change as well as the DC were interpreted.

RESULTS

The maximum PS of both composites at 10min after irradiation decreased with the increase in the compliance of the cantilever beam. The FBF composite generated a lower final stress than the Z250 sample under instrumental compliances less than ca. 4μm/N; however, both materials generated statistically similar PS values at higher compliances. The reaction exotherm and the DC of both materials were found to be independent of compliance. The DC of the FBF sample was slightly higher than that of the packable Z250 composite while the peak exotherm of FBF was almost double that of the Z250 composite. For FBF, a characteristic drop in the PS was observed during the early stage of polymerization for all compliances studied which was not observed in the Z250 sample. This drop was shown to relate to the greater exotherm of the less-filled FBF sample relative to the Z250 composite.

SIGNIFICANCE

While the composites with lower filler content (low viscosity) are generally considered to have lower PS than the conventional packable composites, a bulk-fill flowable composite was shown to produce lower PS under a lower compliance of constraint as would be experienced if the composite was used as the base material in clinical procedures.