Impact of curing protocol on conversion and shrinkage stress

Impact of optical fiber-based photo-activation on dental composite polymerization

OBJECTIVES

The study aimed to introduce a novel two-step optical fiber-based photo-activation of dental resin-based composites (RBCs) for reducing polymerization shrinkage stress (PSS).

METHODS

Proposed protocol design - in the first step, two flexible plastic optical fibers connected to a dental light curing unit (LCU), were used as light guides inserted into the filling to initiate low-irradiance polymerization from within; in the second step, fibers were extracted and remaining voids were filled with RBC, followed by conventional high-irradiance curing to finalize polymerization. Three bulk-fill RBCs were tested (Beautifil-Bulk Restorative, Filtek Bulk-fill Posterior, Tetric PowerFill) using tooth cavity models. Three non-invasive examination techniques were employed: Digital Holographic Interferometry, Infrared Thermography, and Raman spectroscopy for monitoring model deformation, RBC temperature change, and degree of conversion (DC), respectively. A control group (for each examined RBC) underwent conventional photo-activation.

RESULTS

The experimental protocol significantly reduced model deformation by 15 - 35 %, accompanied by an 18 - 54 % reduction in RBC temperature change, emphasizing the impact of thermal shrinkage on PSS. Real-time measurements of deformation and temperature provided indirect insights into reaction dynamics and illuminated potential mechanisms underlying PSS reduction. After a 24-hour dark-storage period, DC outcomes comparable to conventional curing were observed, affirming the clinical applicability of the method.

CONCLUSIONS

Protocol involving the use of two 1.5 mm fibers in the first step (300 mW/cm2 x 10 s), followed by a second conventional curing step (1000 mW/cm2 x 10 s), is recommended to achieve the desired PSS reduction, while maintaining adequate DC and ensuring efficient clinical application.

CLINICAL SIGNIFICANCE

Obtained PSS reduction offers promise in potentially improving the performance of composite restorations. Additionally, leveraging the flexibility of optical fibers improves light guide approach for restorations on posterior teeth. Meanwhile, implementation in clinical practice is easily achievable by coupling the fibers with commercial dental LCUs using the provided plastic adapter.

Upconversion particle-assisted NIR polymerization enables microdomain gradient photopolymerization at inter-particulate length scale

High crosslinking and low shrinkage stress are difficult to reconcile in the preparation of performance-enhancing photopolymer materials. Here we report the unique mechanism of upconversion particles-assisted NIR polymerization (UCAP) in reducing shrinkage stress and enhancing mechanical properties of cured materials. The excited upconversion particle emit UV-vis light with gradient intensity to the surroundings, forming a domain-limited gradient photopolymerization centered on the particle, and the photopolymer grows within this domain. The curing system remains fluid until the percolated photopolymer network is formed and starts gelation at high functional group conversion, with most of the shrinkage stresses generated by the crosslinking reaction having been released prior to gelation. Longer exposures after gelation contribute to a homogeneous solidification of cured material, and polymer materials cured by UCAP exhibit high gel point conversion, low shrinkage stress and strong mechanical properties than those cured by conventional UV polymerization techniques.

Composite Pre-heating: A Novel Approach in Restorative Dentistry

Resin composite pre-heating is a novel approach that might improve handling and marginal adaptation of the unset material paste in clinical application. The goal of this review article is to compile all laboratory experiments on resin composite preheating and see how it impacts the mechanical properties of the material. Results have shown that preheating composite resins improves the degree of conversion, stiffness, marginal adaptability, and microhardness. While flexural strength is unbothered, polymerization shrinkage is hindered, and the microleakage results are unknown.

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.

Autopolymerizing acrylic repair resin containing low concentration of dimethylaminohexadecyl methacrylate to combat saliva-derived bacteria

Biofilm accumulation on the polymethyl methacrylate (PMMA) restorations negatively affect the prognosis of the provisional restorations or the following treatment. This study developed a novel antibacterial PMMA resin containing low concentration of dimethylaminohexadecyl methacrylate (DMAHDM). Four resins were tested: (1) PMMA resin (Control), (2) 1.25% DMAHDM, (3) 2.5% DMAHDM, (4) 5% DMAHDM. Adding 1.25% DMAHDM into the PMMA resin did not influence the mechanical properties, degree of conversion, monomer releasing, and color stability of the specimens (p > 0.05). The incorporation of DMAHDM into PMMA resin could greatly prevent saliva-derived biofilms adhesion compared with the control group (p < 0.05). The metabolism level of saliva-derived biofilms on the 1.25%, 2.5%, and 5% DMAHDM resins were reduced by 20%, 54%, and 62%, respectively. And the mechanism of DMAHDM disturbing the integrity of bacterial cell walls was confirmed by flow cytometric analysis. Adding 1.25% and 2.5% DMAHDM did not compromise cytocompatibility of the modified resin (p > 0.05). Therefore, novel PMMA resin containing low concentration DMAHDM is promising as a future antimicrobial provisional restoration material for preventing microbial-induced complications in clinical settings. Graphical abstract.

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.

Properties of A Model Self-Healing Microcapsule-Based Dental Composite Reinforced with Silica Nanoparticles

Aim: The purpose of this study was to evaluate the mechanical properties of an experimental self-healing dental composite model (SHDC) composed of SiO₂ nanoparticles with varying percentages of triethylene glycol dimethacrylate (TEGDMA) monomer and N,N-dihydroxyethyl-p-toluidine (DHEPT) amine microcapsules. Materials and methods: Microcapsules were prepared by in-situ polymerisation of PUF shells, as explained in our previous work. The model SHDC included bisphenol A glycidyl dimethacrylate (Bis-GMA:TEGDMA) (1:1), 1 wt% phenyl bis(2,4,6-trimethylbenzoyl) phosphine oxide (BAPO), 0.5 wt% benzoyl peroxide (BPO) catalyst, 20 wt% silanised silica dioxide (SiO₂) (15 nm) and (0, 2.5, 5, 7.5, 10 wt%) of microcapsules (120 ± 45 μm). Light transmission, hardness, degree of conversion (DC), flexural strength and elastic modulus of the SHDC model were measured. Results: The degree of conversion of the SHDC ranged from 73 to 76% 24 h after polymerisation. Hardness measurements ranged from 22 to 26 VHN (p > 0.05); however, the flexural strength was adversely affected from 80 to 55 MPa with increasing microcapsules of up to 10 wt% in the composites (p < 0.05). Conclusion: Only flexural strength decreased drastically ~30% with increasing microcapsules (>10 wt%) in the composites. All other measured properties were not significantly affected. Accordingly, we recommend a stronger composite material that could be created by increasing the filler content distribution in order to achieve a hybrid self-healing composite with enhanced mechanical properties.

Thermal shrinkage reveals the feasibility of pulse-delay photocuring technique

OBJECTIVES

To resolve the feasibility of the pulse-delay photocuring technique as a clinical strategy for reducing the detrimental polymerization stress induced in dental composites during the photocuring process.

METHODS

Model dental composites with high and low-filler contents were cured with the pulse-delay photocuring technique using different combinations of photocuring variables (irradiance, exposure time, and delay time). Irradiance used ranged from 0.1W/cm² to 4W/cm². The exposure time of the first pulse varied from 0.2s to 27.2s and the delay times ranged from 10s to 120s. The radiant exposure was varied from 4J/cm² to 20J/cm². A cantilever-beam based instrument (NIST Standards Reference Instrument 6005) was used to implement the photocuring technique for the measurement of the polymerization properties (the degree of monomer conversion, polymerization stress induced due to shrinkage, and temperature change due to the reaction exotherm and curing light absorbance) simultaneously in real-time. These properties were compared with those obtained using the conventional photocuring technique (i.e., using a constant irradiance for a fixed exposure time, a uniform exposure).

RESULTS

There exists a minimum radiant exposure, such that a reduction in the polymerization stress can be achieved without sacrificing the degree of monomer conversion by using the pulse-delay over the conventional photocuring technique. More specifically, stress reductions of up to 19% and 32% was observed with the pulse-delay when compared with the conventional photocuring technique at an irradiance of 0.5W/cm² and 4W/cm², respectively. The reduction occurred when the exposure time of the first pulse was greater than, but closer to, the gelation time (i.e., lower than the vitrification time) of the composite, regardless of the delay time used. Lower thermal shrinkage (contraction) during the post-curing time, rather than the stress relaxation during the delay time or lower degree of monomer conversion as claimed in the literature, is the cause of the reduction in the polymerization stress.

SIGNIFICANCE

The study clarifies a long-standing confusion and controversy on the applicability of the pulse-delay photocuring technique for reducing the polymerization stress and promotes its potential clinical success for dental restorative composites.

Visible-Light-Sensitive Triazine-Coated Silica Nanoparticles: A Dual Role Approach to Polymer Nanocomposite Materials with Enhanced Properties

Nanocomposite materials are of great interest because of their superior properties. Besides the traditional synthesis methods that require high temperatures or toxic solvents, photopolymerization technology provides a simple, low-cost, and environmentally friendly route in preparing nanocomposites. In this research, the preparation of blue-light-sensitive triazine derivative-coated silica nanoparticles is presented. The resulting triazine-coated silica nanoparticles can play a dual role, i.e., acting as both photoinitiators to trigger photopolymerization reactions under the irradiation of LED@410 nm and fillers to endow the produced photopolymer nanocomposite materials with enhanced properties. Specifically, the triazine-coated silica nanoparticles can successfully induce free radical polymerization of trimethylolpropane triacrylate efficiently under the irradiation of LED@410 nm and demonstrate comparable photoinitiation ability to the triazine derivative-based photoinitiator. The effects of different loading amounts of triazine-coated silica nanoparticles toward the photopolymerization kinetics are also evaluated. By coating with the triazine derivative, the nanoparticles show good dispersion in the polymer matrix and significantly reduce the shrinkage of the samples during the photopolymerization. Moreover, the photocured nanocomposites exhibit enhanced migration stability and mechanical properties when an optimal amount of triazine-coated silica nanoparticles is added in the formulation.

Light-curing units used in dentistry: Effect of their characteristics on temperature development in teeth

This study aimed to investigate pulp chamber and surface temperature development using different LED light curing units (LCUs). Eight brands of LED-LCUs were tested in a laboratory bench model. The pulp chamber and surface temperature were recorded with a type T thermocouple and infrared cameras, respectively. The highest pulp chamber and surface temperature increase was 6.1±0.3°C and 20.1±1.7°C, respectively. Wide-spectrum LED-LCUs produced higher pulp chamber temperature increase at 0 mm and 2 mm but lower at 4 mm. Narrow-spectrum LED-LCUs produced higher surface temperature increase. LED-LCU featuring modulated output mode resulted in lower increase in pulp chamber temperature but higher on surface temperature. LED-LCU with light guide tip delivering an inhomogeneous beam caused higher increase in temperature on the surface and in the pulp chamber. LED-LCUs with different spectral emission, output mode and light guide tip design contributed to different temperature development in the pulp chamber and at the surface of teeth.

Effect of co-precipitation plus spray-drying of nano-CaF2 on mechanical and fluoride properties of nanocomposite

OBJECTIVE

Fluoride (F)-releasing restoratives typically are either weak mechanically or release only low levels of F ions. The objectives of this study were to: (1) develop a novel photo-cured nanocomposite with strong mechanical properties and high levels of sustained F ion release via a two-step "co-precipitation + spray-drying" technique to synthesize CaF₂ nanoparticles (nCaF₂); and (2) investigate the effect of spray-drying treatment after co-precipitation of nCaF₂ on mechanical properties and F ion release of composite.

METHODS

Two types of CaF₂ particles were synthesized: A co-precipitation method yielded CaF₂cp; "co-precipitation + spray-drying" yielded nCaF₂cpsd. Composites were fabricated with fillers of: (1) 0% CaF₂ + 70% glass; (2) 10% CaF₂cp + 60% glass; (3) 15% CaF₂cp + 55% glass; (4) 20% CaF₂cp + 50% glass; (5) 10% nCaF₂cpsd + 60% glass; (6) 15% nCaF₂cpsd + 55% glass; and (7) 20% nCaF₂cpsd + 50% glass. A commercial F-releasing nanocomposite served as control.

RESULTS

The nCaF₂cpsd had much smaller particle size (median = 32 nm) and narrower distribution (22-57 nm) than CaF₂cp (median = 5.25 μm, 162 nm-67 μm). The composite containing nCaF₂cpsd had greater flowability, flexural strength, elastic modulus and hardness than CaF₂cp composite and commercial control composite. At 84-day immersion in water, the nanocomposites containing 20% nCaF₂cpsd had 65 times higher cumulative F release, and 77 times greater long-term F-release rate, than commercial control.

CONCLUSIONS

A novel two-step "co-precipitation + spray-drying" technique of synthesizing nCaF₂ was developed. The photo-cured nanocomposite containing 20% nCaF₂cpsd possessed strong mechanical properties and excellent long-term F-release ability, and hence is promising for dental restoration applications to inhibit secondary caries.

Interfacial Interactions during Demolding in Nanoimprint Lithography

Nanoimprint lithography (NIL) is a useful technique for the fabrication of nano/micro-structured materials. This article reviews NIL in the field of demolding processes and is divided into four parts. The first part introduces the NIL technologies for pattern replication with polymer resists (e.g., thermal and UV-NIL). The second part reviews the process simulation during resist filling and demolding. The third and fourth parts discuss in detail the difficulties in demolding, particularly interfacial forces between mold (template) and resist, during NIL which limit its capability for practical commercial applications. The origins of large demolding forces (adhesion and friction forces), such as differences in the thermal expansion coefficients (CTEs) between the template and the imprinted resist, or volumetric shrinkage of the UV-curable polymer during curing, are also illustrated accordingly. The plausible solutions for easing interfacial interactions and optimizing demolding procedures, including exploring new resist materials, employing imprint mold surface modifications (e.g., ALD-assisted conformal layer covering imprint mold), and finetuning NIL process conditions, are presented. These approaches effectively reduce the interfacial demolding forces and thus lead to a lower defect rate of pattern transfer. The objective of this review is to provide insights to alleviate difficulties in demolding and to meet the stringent requirements regarding defect control for industrial manufacturing while at the same time maximizing the throughput of the nanoimprint technique.

Effects of Post-Curing Time on the Mechanical and Color Properties of Three-Dimensional Printed Crown and Bridge Materials

Three-dimensional (3D) printing is increasingly being utilized in the dental field. After fabricating a prosthesis using a 3D printed resin, a post-curing process is required to improve its mechanical properties, but there has been insufficient research on the optimal post-curing conditions. We used various 3D printed crown and bridge materials in this study, and evaluated the changes in their properties according to post-curing time by evaluating the flexural strength, Weibull modulus, Vickers hardness, color change, degree of conversion, and biocompatibility. The obtained results confirmed that the strength of the 3D printed resin increased when it was post-cured for 60–90 min. The Vickers hardness, the degree of conversion, and biocompatibility of the 3D printed resins increased significantly around the beginning of the post-curing time, and then increased more gradually as the post-curing time increased further. It was observed that the color tone also changed as the post-curing time increased, with some groups showing a ΔE₀₀ value of ≥ 2.25, which can be recognized clinically. This study has confirmed that, after the printing process of a 3D printed resin was completed, a sufficient post-curing time of at least 60 min is required to improve the overall clinical performance of the produced material.

Influence of an Alkoxylation Grade of Acrylates on Shrinkage of UV-Curable Compositions

Commercially available UV curable restorative materials are composed of inorganic filler hydroxyapatite, multifunctional methacrylate, photoinitiator and alkoxylated acrylate. Especially, the application of alkoxylated monomers with different alkoxylation grade allows the reduction of polymerization shrinkage which plays the major role by application of low shrinkage composites as high quality restorative dental materials or other adhesive materials in the form of UV-polymerized self-adhesive acrylics layers (films). There are several ways to reduce polymerization shrinkage of restorative compositions, for example, by adjusting different alkoxylated acrylic monomers, which are integral part of investigated UV curable restorative composites. This article is focused on the studies of contraction-stress measured as shrinkage during UV-initiated curing of restorative composites containing various commercially available alkoxylated acrylates. Moreover, studies with experimental restorative materials and recent developments typical for UV curing technology using special photoreactive monomers are described.

Polymerization Stress and Gap Formation of Self-adhesive, Bulk-fill and Flowable Composite Resins

CLINICAL RELEVANCE

Bulk-fill materials show a similar or better performance than control flowable materials regarding interfacial integrity. However, some self-adhesive composites need improvements to achieve competitive performance.

SUMMARY

Objective: This laboratory study compared the polymerization stress and gap formation of self-adhesive, bulk-fill and control flowable composites. The degree of conversion (DC) and post-gel shrinkage were also assessed.Methods: Two self-adhesive (Vertise Flow and Fusio Liquid Dentin), two bulk-fill (Tetric N-Flow Bulk-Fill and Filtek Bulk-Fill Flowable Restorative), and two control flowable (Z350 XT Flowable Restorative and Tetric N-Flow) composites were evaluated. Polymerization stress (PS) was determined in a universal testing machine (n=5). Gap formation was evaluated by scanning electron microscopy in class I restorations (n=6). DC was measured by Fourier transform infrared spectroscopy (n=3). Post-gel volumetric shrinkage (VS) was measured using the strain gauge method (n=5). Data were submitted to one-way analysis of variance or a Kruskal-Wallis test (α=0.05).Results: Vertise Flow and Fusio Liquid Dentin presented the highest interfacial gap (27%±5% and 21%±6%, respectively), which was associated with their highest PS (4.1±0.8 MPa and 3.5±0.6 MPa, respectively) and DC (63%±2% and 60%±2%, respectively) in spite of the lowest VS (1.0%±0.2% and 1.0%±0.3%, respectively). Tetric N-Flow Bulk-Fill and Filtek Bulk-Fill Flowable Restorative presented similar PS (2.9± 0.3 MPa and 2.4±0.2 MPa, respectively) to both control materials. However, the Tetric N-Flow Bulk-Fill showed the lowest gap (7%±2%) and the highest DC (64.3%±0.4%), and the Filtek Bulk-fill presented a marginal gap (17.8%±3.4%) and a DC (54.5%±2.7%) similar to the control materials. The VS values of both bulk-fill materials were similar to those of Tetric N-Flow and lower than that of Z350 XT Flowable Restorative.Conclusions: Bulk-fill composites showed either similar or significantly lower interfacial gaps and PS than the control flowable composites. The self-adhesive composites showed a significantly higher gap percentage and PS than the control and bulk-fill materials.

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.

Novel Nanocomposite Inhibiting Caries at the Enamel Restoration Margins in an In Vitro Saliva-Derived Biofilm Secondary Caries Model

Secondary caries often occurs at the tooth-composite margins. This study developed a novel bioactive composite containing DMAHDM (dimethylaminohexadecyl methacrylate) and NACP (nanoparticles of amorphous calcium phosphate), inhibiting caries at the enamel restoration margins in an in vitro saliva-derived biofilm secondary caries model for the first time. Four composites were tested: (1) Heliomolar nanocomposite, (2) 0% DMAHDM + 0% NACP, (3) 3% DMAHDM + 0% NACP, (D) 3% DMAHDM + 30% NACP. Saliva-derived biofilms were tested for antibacterial effects of the composites. Bovine enamel restorations were cultured with biofilms, Ca and P ion release of nanocomposite and enamel hardness at the enamel restoration margins was measured. Incorporation of DMAHDM and NACP into composite did not affect the mechanical properties (p > 0.05). The biofilms’ CFU (colony-forming units) were reduced by 2 logs via DMAHDM (p < 0.05). Ca and P ion release of the nanocomposite was increased at cariogenic low pH. Enamel hardness at the margins for DMAHDM group was 25% higher than control (p < 0.05). With DMAHDM + NACP, the enamel hardness was the greatest and about 50% higher than control (p < 0.05). Therefore, the novel composite containing DMAHDM and NACP was strongly antibacterial and inhibited enamel demineralization, resulting in enamel hardness at the margins under biofilms that approached the hardness of healthy enamel.

In vitro evaluation of composite containing DMAHDM and calcium phosphate nanoparticles on recurrent caries inhibition at bovine enamel-restoration margins

OBJECTIVE

Recurrent caries is a primary reason for restoration failure caused by biofilm acids. The objectives of this study were to: (1) develop a novel multifunctional composite with antibacterial function and calcium (Ca) and phosphate (P) ion release, and (2) investigate the effects on enamel demineralization and hardness at the margins under biofilms.

METHODS

Dimethylaminohexadecyl methacrylate (DMAHDM) and nanoparticles of amorphous calcium phosphate (NACP) were incorporated into composite. Four groups were tested: (1) Commercial control (Heliomolar), (2) Experimental control (0% DMAHDM + 0% NACP), (3) antibacterial group (3% DMAHDM + 0% NACP), (D) antibacterial and remineralizing group (3% DMAHDM + 30% NACP). Mechanical properties and Ca and P ion release were measured. Colony-forming units (CFU), lactic acid and polysaccharide of Streptococcus mutans (S. mutans) biofilms were evaluated. Demineralization of bovine enamel with restorations was induced via S. mutans, and enamel hardness was measured. Data were analyzed via one-way and two-way analyses of variance and Tukey's multiple comparison tests.

RESULTS

Adding DMAHDM and NACP into composite did not compromise the mechanical properties (P > 0.05). Ca and P ion release of 3% DMAHDM + 30% NACP was increased at cariogenic low pH. Biofilm lactic acid and polysaccharides were greatly decreased via DMAHDM, and CFU was reduced by 4 logs (P < 0.05). Under biofilm acids, enamel hardness at the margins was decreased to about 0.5 GPa for control; it was about 1 GPa for antibacterial group, and 1.3 GPa for antibacterial and remineralizing group (P < 0.05).

CONCLUSIONS

The novel 3% DMAHDM + 30% NACP composite had strong antibacterial effects. It substantially reduced enamel demineralization adjacent to restorations under biofilm acid attacks, yielding enamel hardness that was 2-fold greater than that of control composites. The novel multifunctional composite is promising to inhibit recurrent caries.

Effect of different composite modulation protocols on the conversion and polymerization stress profile of bulk-filled resin restorations

OBJECTIVE

The aim of this in vitro study was to test the effect of different composite modulation protocols (pre-heating, light-curing time and oligomer addition) for bulk filling techniques on resin polymerization stress, intra-pulpal temperature change and degree of conversion.

METHODS

Class I cavities (4mm depth×5mm diameter) were prepared in 48 extracted third molars and divided in 6 groups. Restorations were completed with a single increment, according to the following groups: (1) Filtek Z250XT (room temperature - activated for 20s); (2) Filtek Z250XT (at room temperature - activated for 40s); (3) Filtek Z250XT (pre-heated at 68°C - activated for 20s); (4) Filtek Z250XT (pre-heated at 68°C - activated for 40s); (5) Filtek BulkFill (at room temperature - activated for 20s); (6) Filtek Z250XT (modified by the addition of a thio-urethane oligomer at room temperature - activated for 40s). Acoustic emission test was used as a real-time polymerization stress (PS) assessment. The intra-pulpal temperature change was recorded with a thermocouple and bottom/top degree of conversion (DC) measured by Raman spectroscopy. Data were analyzed with one-way ANOVA/Tukey's test (α=5%).

RESULTS

Pre-heating the resin composite did not influence the intra-pulpal temperature (p=0.077). The thio-urethane-containing composite exhibited significantly less PS, due to a lower number of acoustic events. Groups with pre-heated composites did not result in significantly different PS. Filtek BulkFill and the thio-urethane experimental composite presented significantly higher DC.

SIGNIFICANCE

Resin composite pre-heating was not able to reduce polymerization stress in direct restorations. However, thio-urethane addition to a resin composite could reduce the polymerization stress while improving the DC.

The effect of rapid high-intensity light-curing on micromechanical properties of bulk-fill and conventional resin composites

Rapid high-intensity light-curing of dental resin composites is attractive from a clinical standpoint due to the prospect of time-savings. This study compared the effect of high-intensity (3 s with 3,440 mW/cm²) and conventional (10 s with 1,340 mW/cm²) light-curing on micromechanical properties of conventional and bulk-fill resin composites, including two composites specifically designed for high-intensity curing. Composite specimens were prepared in clinically realistic layer thicknesses. Microhardness (MH) was measured on the top and bottom surfaces of composite specimens 24 h after light-curing (initial MH), and after subsequent immersion for 24 h in absolute ethanol (ethanol MH). Bottom/top ratio for initial MH was calculated as a measure of depth-dependent curing effectiveness, whereas ethanol/initial MH ratio was calculated as a measure of crosslinking density. High-intensity light-curing showed a complex material-dependent effect on micromechanical properties. Most of the sculptable composites showed no effect of the curing protocol on initial MH, whereas flowable composites showed 11–48% lower initial MH for high-intensity curing. Ethanol/initial MH ratios were improved by high-intensity curing in flowable composites (up to 30%) but diminished in sculptable composites (up to 15%). Due to its mixed effect on MH and crosslinking density in flowable composites, high-intensity curing should be used with caution in clinical work.

A new safranin based three-component photoinitiating system for high resolution and low shrinkage printed parts via digital light processing

Additive manufacturing or 3D printing has attracted the interest of researchers in industry and academia because of its outstanding features. In this study, a new three-component photoinitiating system (PIS) consisting of safranin O (SFH⁺), thiol derivatives and diphenyl iodonium salt was used for the free radical photopolymerization of a diacrylate monomer (SR349) in DLP 3D printing. The photoinitiating characteristics of this PIS were evaluated and advantageously compared to those of a conventional PI (TPO) by using RT-FTIR. It is shown that the proposed PIS could be used as an efficient PIS for free radical photopolymerization. In addition, the resolution and shrinkage of printed parts in the presence of this three-component PIS were measured and compared to those printed using TPO as a photoinitiator. The resolution of printed parts was determined by using SEM and profilometry techniques. In addition, photorheometry was used to evaluate the linear shrinkage of samples. Moreover, the initiating mechanism of the three-component PIS was studied by using laser flash photolysis (LFP). A photocyclic mechanism was outlined for the three-component PIS which demonstrated this mechanism would be very beneficial for DLP 3D printing.

The resolution and shrinkage of DLP 3D printed parts improve remarkably when SFH⁺/RSH/IOD⁺ is used as a photoinitiating system.

[Effect of preheating on the properties of resin composite]

Resin composite, which is commonly used as a dental filling material, has some problems, such as poor wear resistance, polymerization shrinkage, and poor dentin marginal adaptability. Preheating of resin composite improves its pro-perties. This paper reviewed the effects of resin composite preheating on its monomer conversion, marginal microleakage, mechanical properties, and irritation on dental pulp.

Influence of Modulated Photo-Activation on Shrinkage Stress and Degree of Conversion of Bulk-Fill Composites

Abstract This study aimed to evaluate the polymerization properties of bulk-fill materials (low and high-viscosity) by using high-intensity continuous light and intermittent photo-activation in terms of polymerization shrinkage stress and degree of conversion (DC). The following Bulk-fill and Conventional nanofilled resin composites were evaluated: Filtek Z350XT Flow (3M/ESPE), SureFil SDR Flow (Dentsply), Filtek Bulk Fill Flow (3M/ESPE), Filtek Z350XT (3M/ESPE) and Filtek Bulk Fill Posterior (3M/ESPE). A LED device (DB 685, Dabi Atlante) was used for both protocols: continuous uniform and intermittent photo-activation (light-on and light-off cycles) with identical radiant exposure (14 J/cm2). The polymerization shrinkage stress (n=6) was evaluated by inserting a single increment of 12 mm3 between two stainless steel plates (6×2 mm) adapted to a Universal Testing Machine (UTM), at different times. Measurements were recorded after photo-activation. The degree of conversion was evaluated by Fourier transformed infrared spectroscope (FTIR) with an attenuated total reflectance (ATR) accessory (n=5). Data were analyzed by three-way ANOVA and Tukey’s HSD (α=0.05) tests. Bulk Fill Posterior presented higher shrinkage stress values when photo-activated with the intermittent technique (p<0.05). The intermittent photo-activation increased the degree of conversion for the low-viscosity bulk-fills (p<0.05). Therefore, the use of modulated photo-activation (intermittent) must be indicated with caution since its use can influence the shrinkage stress and degree of conversion of composites, which varies according to the resin formulations.

Preparation of Biodegradable Polyethylene Glycol Dimethacrylate Hydrogels via Thiol-ene Chemistry

Through the control of the molecular weight, water content and monomer concentration, polyethylene glycol dimethacrylate (PEGDMA) based hydrogels have been adapted for numerous applications, including as structural scaffolds, drug delivery vehicles and cell carriers. However, due to the low biodegradability rates, the use of PEGDMA in tissue engineering has been limited. Thiol-based monomers have been shown to improve the degradation rates of several PEG-based hydrogels, though their impact on several material properties has not been as well defined. In this work, several mercaptopropianoates, as well as mercaptoacetates, were mixed with PEGDMA and copolymerized. Following an initial polymerization check, it was determined that mercaptoacetate-based thiol monomers did not polymerize in the presence of PEGDMA, whereas mercaptopropionates were more successful. The wettability, and the compressive and tensile strength, in addition to the thermal properties, were determined for successfully copolymerized samples via a combination of differential scanning calorimetry, dynamic mechanical analysis, unconfined compression, and goniometry. Further study determined that dipentaerythritol hexa(3-mercaptopropionate) (DiPETMP) successfully enhanced the biodegradability of PEGDMA.

Effects of S. mutans gene-modification and antibacterial calcium phosphate nanocomposite on secondary caries and marginal enamel hardness

Secondary caries at the restoration-tooth margins is a main reason for dental restoration failures. Gene-modification for Streptococcus mutans (S. mutans) and composites containing dimethylaminohexadecyl methacrylate (DMAHDMA) and nanoparticles of amorphous calcium phosphate (NACP) all have the potential to suppress bacterial acids and promote remineralization. However, there has been no report of their effects on marginal caries-inhibition and enamel hardness. The objective of this study was to investigate the effects of gene-modification and DMAHDM–NACP composite restoration on enamel demineralization and hardness at the margins under biofilm acids for the first time. Parent S. mutans and rnc gene-deleted S. mutans were tested side by side. The bioactive composite contained 3% DMAHDM and 30% NACP. Mechanical properties and calcium (Ca) and phosphate (P) ion releases were measured. Colony-forming units (CFU), MTT, lactic acid and polysaccharide of biofilms were evaluated. Demineralization of bovine enamel with composite restorations was induced via biofilms, then enamel hardness was measured. The dual strategy of combining rnc-deletion with DMAHDM+30NACP: (1) achieved the strongest biofilm-inhibition, with the greatest reduction in biofilm CFU by 6 logs; (2) decreased biofilm lactic acid and polysaccharide production by more than 80%; (3) achieved enamel hardness that was 140% higher than that of a commercial fluoride-releasing composite under 30 days of biofilm acids. Therefore, the novel dual approach of rnc gene-deletion and DMAHDM+NACP nanocomposite is promising to inhibit secondary caries at the margins and increase the longevity of tooth restorations.

Secondary caries at the restoration-tooth margins is a main reason for dental restoration failures.

Evaluation of chromatic changes of a nanocomposite resin using the new whitness index

Background and aims

To evaluate the staining effects of two brands of coffee and the bleaching efficiency of two in-office bleaching methods, upon different opacities of a commercial nanocomposite.

Methods

Twenty four specimens of each opacity, A3 Dentin, A3 Body and A3 Enamel, were fabricated from Filtek Supreme (3MEspe). The specimens were further divided into two groups (n=12) and were immersed in two coffee solutions (Bio Organic Coffee Bellarom, 100% Arabica, and Iulius Meinl Coffee), for 24 hours. Between the staining sessions, the specimens were stored in sterile water, at 37°C. Each group was further divided into three (n=4), in order to be bleached, as follows: Group 1 - Beyond 35% in office, for 4 applications of 15 minutes each, Group 2 – Zoom Day White 6% in office, for 4 applications of 15 minutes each, Group 3 – Control Group, stored in sterile water. Color values were measured with a dental spectrophotometer Vita EasyShade 4.0 and five measurements were recorded for each sample at a time. Lightness L*, color coordinates a* and b* were recorded, at baseline, after staining in coffee and after bleaching. Whiteness index (WID) of the three composite resins (A3D, A3B, A3E) in the three moments were calculated, as well as the color difference Delta E* correspondent to the staining and bleaching process. Data were analyzed using one-way repeated measures ANOVA and the WID index was calculated WID (p<0.05). Univariate analysis of variance was performed for assessing the influence of staining solution upon composite resins, as well as for testing the effect of bleaching agents. The significance level was set at α=0.05 and pairwise comparisons were adjusted by the Least Significant Difference method.

Results

The pairwise comparisons showed no significant difference between the effects of the two bleaching agents upon the WID, meaning that they induce almost similar color changes. The results of the univariate ANOVA test indicated a significant effect of the composite resin and the staining solution upon the WID (p<0.05). However, no significant interaction effect was found between the composite resin and the staining solution (p=0.095). There was a significant difference in the staining effect of the two coffee solutions only for A3B and A3E composite resins (p<0.05).

Conclusions

The chromatic changes of the nanocomposite resin could be evaluated by the variation of the whiteness index. The staining effect induced by the two types of coffee was similar. The most effective protocol was the in-office bleaching method based on Beyond 35%.

Nanofilled Resin Composite Properties and Clinical Performance: A Review

The aim of this review was to compile recent evidence related to nanofilled resin composite materials regarding the properties and clinical performance. Special attention was given to mechanical properties, such as strength, hardness, abrasive wear, water sorption, and solubility. The clinical performance of nanocomposite materials compared with hybrid resin composites was also addressed in terms of retention and success rates, marginal adaptation, color match, and surface roughness. A search of English peer-reviewed dental literature (2003-2017) from PubMed and MEDLINE databases was conducted using the terms "nanocomposites" or "nanofilled resin composite" and "clinical evaluation." The list was screened, and 82 papers that were relevant to the objectives of this work were included in the review. Mechanical properties of nanocomposites are generally comparable to those of hybrid composites but higher than microfilled composites. Nanocomposites presented lower abrasive wear than hybrids but higher sorption values. Their clinical performance was comparable to that of hybrid composites.

Polymerization shrinkage stress of resin-based dental materials: A systematic review and meta-analyses of technique protocol and photo-activation strategies

PURPOSE

A systematic review was conducted to determine whether there were any alternative technique or additional step strategies available to reduce and control polymerization shrinkage stress development in dental resin-based restorative materials.

DATA SOURCES

This report followed the PRISMA Statement. A total of 36 studies were included in this review. Two reviewers performed a literature search up to December 2016, without restriction of the year of publication, in seven databases: PubMed, Web of Science, Scopus, SciELO, LILACS, IBECS, and BBO.

STUDY SELECTION

Only in vitro studies that evaluated polymerization shrinkage stress by direct testing were included. Pilot studies, reviews and in vitro studies that evaluated polymerization shrinkage stress by indirect methods (e.g., microleakage or cuspal deflection measurements), finite elemental analysis or mathematical models were excluded. Of the 6.113 eligible articles, 36 studies were included in the qualitative analysis, and the meta-analysis was performed with 25 studies. A global comparison was performed with random-effects models (α = 0.05). The strategies were subdivided as follows: the use of an alternative technique protocol of placing the material inside the tooth cavity; the modification of the irradiation intensity or total energy delivered to the material; the use of an alternative light-curing source; or the use of an alternative photo-activation mode. All alternative strategies showed statistically significant differences when compared with their respective controls (p < 0.05).

CONCLUSION

The use of alternative light-curing sources contributed more to minimizing stress development than placing the material by means of an alternative technique protocol or by modifying the irradiant intensity or total energy delivered to the material during photo-activation. Moreover, the use of an alternative photo-activation mode (intermittent light, exponential, soft-start or pulse delay modes) was shown to be an effective strategy for reducing and controlling stress development in resin-based dental materials.

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.

Visible Light Photoinitiator for 3D-Printing of Tough Methacrylate Resins

Lithography-based additive manufacturing was introduced in the 1980s, and is still the method of choice for printing accurate plastic parts with high surface quality. Recent progress in this field has made tough photopolymer resins and cheap LED light engines available. This study presents the influence of photoinitiator selection and post-processing on the thermomechanical properties of various tough photopolymers. The influence of three photoinitiators (Ivocerin, BAPO, and TPO-L) on the double-bond conversion and mechanical properties was investigated by mid infrared spectroscopy, dynamic mechanical analysis and tensile tests. It was found that 1.18 wt % TPO-L would provide the best overall results in terms of double-bond conversion and mechanical properties. A correlation between double-bond conversion, yield strength, and glass transition temperature was found. Elongation at break remained high after post-curing at about 80-100%, and was not influenced by higher photoinitiator concentration. Finally, functional parts with 41 MPa tensile strength, 82% elongation at break, and 112 °C glass transition temperature were printed on a 405 nm DLP (digital light processing) printer.

Light Curing in Dentistry

The ability to light cure resins 'on demand' in the mouth has revolutionized dentistry. However, there is a widespread lack of understanding of what is required for successful light curing in the mouth. Most instructions simply tell the user to 'light cure for xx seconds' without describing any of the nuances of how to successfully light cure a resin. This article provides a brief description of light curing. At the end, some recommendations are made to help when purchasing a curing light and how to improve the use of the curing light.

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.

Quantifying the sensitivity of the network structure and properties from simultaneous measurements during photopolymerization

We present a method that combines experimental and computational approaches to assess a comprehensive set of structural and functional evolution during a network formation process via photopolymerization. Our work uses the simultaneous measurement of the degree of conversion, polymerization stress, the change in reaction temperature, and shrinkage strain in situ. These measurements are combined with the theory of viscoelastic materials to deduce the relaxation time and frequency-dependent modulus of the polymerizing network. The relaxation time and degree of conversion are used to demonstrate the effect of processing parameters (e.g. curing protocol adjusted by the light intensity) in creating different network structures for the same initial resin. We describe experimental trends using effective medium calculations on a cross-linked polymer network model. In particular, we show that the effect of curing conditions on the spatial heterogeneity in crosslink density can be quantified using multiparametric measurements and modeling. Collectively, the present method is a way to examine holistically the complex structural and functional evolution of the network formation process.

Do Dental Resin Composites Accumulate More Oral Biofilms and Plaque than Amalgam and Glass Ionomer Materials?

A long-time drawback of dental composites is that they accumulate more biofilms and plaques than amalgam and glass ionomer restorative materials. It would be highly desirable to develop a new composite with reduced biofilm growth, while avoiding the non-esthetics of amalgam and low strength of glass ionomer. The objectives of this study were to: (1) develop a protein-repellent composite with reduced biofilms matching amalgam and glass ionomer for the first time; and (2) investigate their protein adsorption, biofilms, and mechanical properties. Five materials were tested: A new composite containing 3% of protein-repellent 2-methacryloyloxyethyl phosphorylcholine (MPC); the composite with 0% MPC as control; commercial composite control; dental amalgam; resin-modified glass ionomer (RMGI). A dental plaque microcosm biofilm model with human saliva as inoculum was used to investigate metabolic activity, colony-forming units (CFU), and lactic acid production. Composite with 3% MPC had flexural strength similar to those with 0% MPC and commercial composite control (p > 0.1), and much greater than RMGI (p < 0.05). Composite with 3% MPC had protein adsorption that was only 1/10 that of control composites (p < 0.05). Composite with 3% MPC had biofilm CFU and lactic acid much lower than control composites (p < 0.05). Biofilm growth, metabolic activity and lactic acid on the new composite with 3% MPC were reduced to the low level of amalgam and RMGI (p > 0.1). In conclusion, a new protein-repellent dental resin composite reduced oral biofilm growth and acid production to the low levels of non-esthetic amalgam and RMGI for the first time. The long-held conclusion that dental composites accumulate more biofilms than amalgam and glass ionomer is no longer true. The novel composite is promising to finally overcome the major biofilm-accumulation drawback of dental composites in order to reduce biofilm acids and secondary caries.

Composite resin reinforced with silver nanoparticles-laden hydroxyapatite nanowires for dental application

OBJECTIVE

The object is to find a functional one-dimensional nanofibrous filler for composite resin, which is able to provide both efficient reinforcement and high antibacterial activity.

METHODS

Hydroxyapatite (HA) nanowires were synthesized via hydrothermal technique using calcium oleate as the precursor. Polydopamine (PDA)-coated HA (HA-PDA) nanowires were prepared by soaking HA nanowires in dopamine (DA) aqueous solution. Silver nanoparticles (AgNPs)-laden HA (HA-PDA-Ag) nanowires were prepared via reduction reaction by adding silver nitrate and glucose into HA-PDA suspensions in DI water. The resulted HA-PDA-Ag nanowires were then mixed into Bis-GMA/TEGDMA (50/50, w/w) at 4-10wt.%, thermal-cured, and submitted to characterizations including mechanical properties, interfacial adhesion between filler and resin matrix, distribution of HA nanowires and AgNPs, as well as silver ion release, cytotoxicity and antibacterial activity.

RESULTS

HA-PDA-Ag nanowires were readily obtained and the loading amounts of AgNPs could be controlled by adjusting the feeding doses of silver nitrate and HA-PDA nanowires. Benefiting from the PDA surface layer, HA-PDA-Ag nanowires could disperse well in composite resin and form good interfacial adhesion with the resin matrix. In comparison with neat resin, significant increases in flexural strength and modulus of cured composites were achieved at the addition amounts of HA-PDA-Ag nanowires being 6-8wt.%. The distribution of AgNPs was homogeneous throughout the resin matrix in all designs, which endowed the composites with high antibacterial activity against streptococcus mutans. Continuous silver ion release from composites was detected, however, it was determined the composites would have insignificant cytotoxicity based on the proliferation of L929 fibroblasts in extracts of HA-PDA-Ag nanowires.

SIGNIFICANCE

The finding proved that HA-PDA-Ag nanowires could serve as functional nanofillers for composite resins, which should help much in developing materials for satisfactory long-term clinical restorations.

One-year water-ageing of calcium phosphate composite containing nano-silver and quaternary ammonium to inhibit biofilms

Dental composites are commonly used restorative materials; however, secondary caries due to biofilm acids remains a major problem. The objectives of this study were (1) to develop a composite containing quaternary ammonium dimethacrylate (QADM), nanoparticles of silver (NAg), and nanoparticles of amorphous calcium phosphate (NACP), and (2) to conduct the first investigation of the mechanical properties, biofilm response and acid production vs water-ageing time from 1 day to 12 months. A 4 × 5 design was utilized, with four composites (NACP-QADM composite, NACP-NAg composite, NACP-QADM-NAg composite, and a commercial control composite), and five water-ageing time periods (1 day, and 3, 6, 9, and 12 months). After each water-ageing period, the mechanical properties of the resins were measured in a three-point flexure, and antibacterial properties were tested via a dental plaque biofilm model using human saliva as an inoculum. After 12 months of water-ageing, NACP-QADM-NAg had a flexural strength and elastic modulus matching those of the commercial control (P>0.1). Incorporation of QADM or NAg into the NACP composite greatly reduced biofilm viability, metabolic activity and acid production. A composite containing both QADM and NAg possessed a stronger antibacterial capability than one with QADM or NAg alone (P<0.05). The anti-biofilm activity was maintained after 12 months of water-ageing and showed no significant decrease with increasing time (P>0.1). In conclusion, the NACP-QADM-NAg composite decreased biofilm viability and lactic acid production, while matching the load-bearing capability of a commercial composite. There was no decrease in its antibacterial properties after 1 year of water-ageing. The durable antibacterial and mechanical properties indicate that NACP-QADM-NAg composites may be useful in dental restorations to combat caries.

Novel protein-repellent dental adhesive containing 2-methacryloyloxyethyl phosphorylcholine

OBJECTIVES

Biofilms at tooth-restoration margins can produce acids and cause secondary caries. A protein-repellent adhesive resin can potentially inhibit bacteria attachment and biofilm growth. However, there has been no report on protein-repellent dental resins. The objectives of this study were to develop a protein-repellent bonding agent incorporating 2-methacryloyloxyethyl phosphorylcholine (MPC), and to investigate its resistance to protein adsorption and biofilm growth for the first time.

METHODS

MPC was incorporated into Scotchbond Multi-Purpose (SBMP) at 0%, 3.75%, 7.5%, 11.25%, and 15% by mass. Extracted human teeth were used to measure dentine shear bond strengths. Protein adsorption onto resins was determined by a micro bicinchoninic acid (BCA) method. A dental plaque microcosm biofilm model with human saliva as inoculum was used to measure biofilm metabolic activity and colony-forming unit (CFU) counts.

RESULTS

Adding 7.5% MPC into primer and adhesive did not decrease the dentine bond strength, compared to control (p>0.1). Incorporation of 7.5% of MPC achieved the lowest protein adsorption, which was 20-fold less than that of control. Incorporation of 7.5% of MPC greatly reduced bacterial adhesion, yielding biofilm total microorganism, total streptococci, and mutans streptococci CFU that were an order of magnitude less than control.

CONCLUSIONS

A protein-repellent dental adhesive resin was developed for the first time. Incorporation of MPC into primer and adhesive at 7.5% by mass greatly reduced the protein adsorption and bacterial adhesion, without compromising the dentine bond strength.

CLINICAL SIGNIFICANCE

The novel protein-repellent primer and adhesive are promising to inhibit biofilm formation and acid production, to protect the tooth-restoration margins and prevent secondary caries.

Five second photoactivation? A microhardness and marginal adaptation in vitro study in composite resin restorations

INTRODUCTION

Studies defining the characteristics of light curing units and photoactivation methods are necessary to allow the correct choices to be made in daily practice. This study aimed to determine whether different photoactivation protocols for composite resins [periodic level shifting (PLS) - 5 second and soft-start] are able to maintain or enhance the mechanical properties and marginal adaptation of restorations.

METHODS

Restorations were placed in bovine teeth using the following photoactivation methods: continuous light for 20 seconds (control group); PLS technology (PLS - 5 second group); and continuous light and a light guide tip distance of 6 mm after which the tip was placed at the surface of the restoration (soft-start group). The teeth were transversely sectioned in the incisal-cervical direction. Thirty halves were randomly selected for Knoop microhardness testing (n = 10). The other 30 halves were subjected to scanning electron microscopy analysis. The images obtained were measured to identify the highest marginal gap, and statistical tests for variance analysis were conducted.

RESULTS

Microhardness tests showed no statistically significant difference between the photoactivation methods analysed (P ≥ 0.01). The tests showed a difference among depths (P < 0.01), with the deeper layers being the hardest. In analysing marginal adaptation, no significant difference was identified between the higher marginal gap values in the continuous (mean = 10.36) and PLS - 5 second (mean = 10.62) groups, and the soft-start group (mean = 5.83) presented the lowest values (P < 0.01).

CONCLUSIONS

The PLS - 5 second and soft-start protocols did not alter the hardness of the restorations. Moreover, the PLS - 5 second protocol did not alter the marginal adaptation, whereas the soft-start protocol improved marginal adaptation.

The Effect of Resin-modified Glass-ionomer Cement Base and Bulk-fill Resin Composite on Cuspal Deformation

Objectives: This study investigated cuspal deformation in teeth restored with different types of adhesive materials with and without a base.

Methods: Mesio-occluso-distal slot cavities of moderately large dimension were prepared on extracted maxillary premolars (n=24). Teeth were assigned to one of four groups and restored with either a sonic-activated bulk-fill resin composite (RC) (SonicFill), or a conventional nanohybrid RC (Herculite Ultra). The base materials used were a flowable nanofilled RC (Premise Flowable) and a high-viscosity resin-modified glass-ionomer cement (RMGIC) (Riva Light-Cure HV). Cuspal deflection was measured with two direct current differential transformers, each contacting a buccal and palatal cusp. Cuspal movements were recorded during and after restoration placement. Data for the buccal and palatal cusp deflections were combined to give the net cuspal deflection.

Results: Data varied widely. All teeth experienced net inward cuspal movement. No statistically significant differences in cuspal deflection were found among the four test groups.

Conclusions: The use of a flowable RC or an RMGIC in closed-laminate restorations produced the same degree of cuspal movement as restorations filled with only a conventional nanohybrid or bulk-fill RC.

Effects of water-aging on self-healing dental composite containing microcapsules

OBJECTIVES

The objectives of this study were to develop a self-healing dental composite containing poly(urea-formaldehyde) (PUF) shells with triethylene glycol dimethacrylate (TEGDMA) and N,N-dihydroxyethyl-p-toluidine (DHEPT) as healing liquid, and to investigate the mechanical properties of the composite and its self-healing efficacy after water-aging for 6 months.

METHODS

PUF microspheres were synthesized encapsulating a TEGDMA-DHEPT healing liquid. Composite containing 30% of a resin matrix and 70% of glass fillers by mass was incorporated with 0%, 2.5%, 5%, 7.5% and 10% of microcapsules. A flexural test was used to measure flexural strength and elastic modulus. A single edge V-notched beam method was used to measure fracture toughness (KIC) and self-healing efficacy. Specimens were water-aged at 37 °C for 1 day to 6 months and then tested for self-healing. Fractured specimens were healed while being immersed in water to examine self-healing efficacy, in comparison with that in air.

RESULTS

Incorporation of up to 7.5% of microcapsules into the resin composite achieved effective self-healing, without adverse effects on the virgin mechanical properties of the composite (p>0.1). An excellent self-healing efficacy of 64-77% recovery was obtained (mean±sd; n=6). Six months of water-aging did not decrease the self-healing efficacy compared to 1 day (p>0.1). Exposure to water did not decrease the healing efficacy, compared to that healed in air (p>0.1).

CONCLUSIONS

A composite was developed with excellent self-healing efficacy even while being immersed in water. The self-healing efficacy did not decrease with increasing water-aging time for 6 months.

CLINICAL SIGNIFICANCE

The novel self-healing composite may be promising for dental applications to heal cracks, resist fracture, and increase the durability and longevity.

Effect of dimethylaminohexadecyl methacrylate mass fraction on fracture toughness and antibacterial properties of CaP nanocomposite

OBJECTIVES

Biofilm acids contribute to secondary caries which is a reason for restoration failure. Previous studies synthesized nanoparticles of amorphous calcium phosphate (NACP) and dimethylaminohexadecyl methacrylate (DMAHDM). The objectives of this study were to develop DMAHMD-NACP nanocomposite for double benefits of antibacterial and remineralization capabilities, and investigate the DMAHMD mass fraction effects on fracture toughness and biofilm response of NACP nanocomposite for the first time.

METHODS

DMAHDM was incorporated into NACP nanocomposite at mass fractions of 0% (control), 0.75%, 1.5%, 2.25% and 3%. A single edge V-notched beam method was used to measure fracture toughness K(IC). A dental plaque microcosm biofilm model using human saliva as inoculum was used to measure the antibacterial properties of composites.

RESULTS

K(IC) was about 1 MPa×m(1/2) for all composite (mean±sd; n=6). Adding DMAHDM from 0% to 3% did not affect K(IC) (p>0.1). Lactic acid production by biofilms on composite containing 3% DMAHDM was reduced to less than 1% of that on composite control. Metabolic activity of adherent biofilms on composite containing 3% DMAHDM was reduced to 4% of that on composite control. Biofilm colony-forming unit (CFU) counts were reduced by three orders of magnitude on NACP nanocomposite containing 3% DMAHDM.

CONCLUSIONS

DMAHDM-NACP nanocomposite had good fracture resistance, strong antibacterial potency, and NACP for remineralization (shown in previous studies). The DMAHDM-NACP nanocomposite may be promising for caries-inhibiting dental restorations, and the method of using double agents (DMAHDM and NACP) may have a wide applicability to other dental materials including bonding agents and cements.

Rechargeable dental adhesive with calcium phosphate nanoparticles for long-term ion release

OBJECTIVES

The tooth-resin bond is the weak link of restoration, with secondary caries as a main reason for failure. Calcium phosphate-containing resins are promising for remineralization; however, calcium (Ca) and phosphate (P) ion releases last only a couple of months. The objectives of this study were to develop the first rechargeable CaP bonding agent and investigate the key factors that determine CaP ion recharge and re-release.

METHODS

Nanoparticles of amorphous calcium phosphate (NACP) were synthesized. Pyromellitic glycerol dimethacrylate (PMGDM), ethoxylated bisphenol-A dimethacrylate (EBPADMA), 2-hydroxyethyl methacrylate (HEMA), and bisphenol-A glycidyl dimethacrylate (BisGMA) were used to synthesize three adhesives (denoted PE, PEH and PEHB). NACP were mixed into adhesive at 0-30% by mass. Dentin shear bond strengths were measured. Adhesive specimens were tested for Ca and P initial ion release. Then the ion-exhausted specimens were immersed in Ca and P solution to recharge the specimens, and the recharged specimens were then used to measure ion re-release for 7 days as one cycle. Then these specimens were again recharged and the re-release was measured for 7 days as the second cycle. Three recharge/re-release cycles were tested.

RESULTS

PEHB had the highest dentin bond strength (p<0.05). Increasing NACP content from 0 to 30% did not affect dentin bond strength (p>0.1), but increased CaP release and re-release (p<0.05). PEHB-NACP had the greatest recharge/re-release, and PE-NACP had the least (p<0.05). Ion release remained high and did not decrease with increasing the number of recharge/re-release cycles (p>0.1). After the third cycle, specimens without further recharge had continuous CaP ion release for 2-3 weeks.

SIGNIFICANCE

Rechargeable CaP bonding agents were developed for the first time to provide long-term Ca and P ions to promote remineralization and reduce caries. Incorporation of NACP into adhesive had no negative effect on dentin bond strength. Increasing NACP filler level increased the ion recharge and re-release capability. The new CaP recharge method and PMGDM-EBPADMA-NACP composition may have wide application in adhesives, composites and cements, to combat caries and remineralize lesions.

A novel protein-repellent dental composite containing 2-methacryloyloxyethyl phosphorylcholine

Secondary caries due to biofilm acids is a primary cause of dental composite restoration failure. To date, there have been no reports of dental composites that can repel protein adsorption and inhibit bacteria attachment. The objectives of this study were to develop a protein-repellent dental composite by incorporating 2-methacryloyloxyethyl phosphorylcholine (MPC) and to investigate for the first time the effects of MPC mass fraction on protein adsorption, bacteria attachment, biofilm growth, and mechanical properties. Composites were synthesized with 0 (control), 0.75%, 1.5%, 2.25%, 3%, 4.5% and 6% of MPC by mass. A commercial composite was also tested as a control. Mechanical properties were measured in three-point flexure. Protein adsorption onto the composite was determined by the microbicinchoninic acid method. A human saliva microcosm biofilm model was used. Early attachment at 4 h, biofilm at 2 days, live/dead staining and colony-forming units (CFUs) of biofilms grown on the composites were investigated. Composites with MPC of up to 3% had mechanical properties similar to those without MPC and those of the commercial control, whereas 4.5% and 6% MPC decreased the mechanical properties (P<0.05). Increasing MPC from 0 to 3% reduced the protein adsorption on composites (P<0.05). The composite with 3% MPC had protein adsorption that was 1/12 that of the control (P<0.05). Oral bacteria early attachment and biofilm growth were also greatly reduced on the composite with 3% MPC, compared to the control (P<0.05). In conclusion, incorporation of MPC into composites at 3% greatly reduced protein adsorption, bacteria attachment and biofilm CFUs, without compromising mechanical properties. Protein-repellent composites could help to repel bacteria attachment and plaque build-up to reduce secondary caries. The protein-repellent method might be applicable to other dental materials.

The Use of Quaternary Ammonium to Combat Dental Caries

Resin composites and adhesives are increasingly popular in dental restorations, but secondary caries is one of the main reasons for restoration failure. Quaternary ammonium monomers (QAMs) have an anti-microbial effect and are widely used in many fields. Since the concept of the immobilized antibacterial effect was put forward, dental restorations containing QAMs have been studied to reduce secondary caries. Previous studies have been struggling to develop novel anti-caries materials which might have triple benefits: good mechanical properties, antibacterial effects and remineralization potentials. Different kinds of QAMs have been proven to be effective in inhibiting the growth and metabolism of biofilms. Combination of QAMs and other nanoparticles in resin composites and adhesives could enhance their anti-caries capability. Therefore, QAMs are promising to show significant impact on the future of restorative and preventive dentistry.

Effect of modulated photo-activation on polymerization shrinkage behavior of dental restorative resin composites

This study investigated the influence of modulated photo-activation on axial polymerization shrinkage, shrinkage force, and hardening of light- and dual-curing resin-based composites. Three light-curing resin composites (SDR bulk-fill, Esthet X flow, and Esthet X HD) and one dual-curing material (Rebilda DC) were subjected to different irradiation protocols with identical energy density (27 J cm(-2) ): high-intensity continuous light (HIC), low-intensity continuous light (LIC), soft-start (SS), and pulse-delay curing (PD). Axial shrinkage and shrinkage force of 1.5-mm-thick specimens were recorded in real time for 15 min using custom-made devices. Knoop hardness was determined at the end of the observation period. Statistical analysis revealed no significant differences among the curing protocols for both Knoop hardness and axial shrinkage, irrespective of the composite material. Pulse-delay curing generated the significantly lowest shrinkage forces within the three light-curing materials SDR bulk-fill, Esthet X flow, and Esthet X HD. High-intensity continuous light created the significantly highest shrinkage forces within Esthet X HD and Rebilda DC, and caused significantly higher forces than LIC within Esthet X flow. In conclusion, both the composite material and the applied curing protocol control shrinkage force formation. Pulse-delay curing decreases shrinkage forces compared with high-intensity continuous irradiation without affecting hardening and axial polymerization shrinkage.

Development of novel dental adhesive with double benefits of protein-repellent and antibacterial capabilities

OBJECTIVES

Secondary caries at the tooth-restoration margins remains a main reason for restoration failure. The objectives of this study were to: (1) combine protein-repellent 2-methacryloyloxyethyl phosphorylcholine (MPC) with quaternary ammonium dimethylaminohexadecyl methacrylate (DMAHDM) to develop a new dental adhesive with double benefits of protein-repellent and antibacterial capabilities for the first time; and (2) investigate the effects on protein adsorption, anti-biofilm activity, and dentin bond strength.

METHODS

MPC and DMAHDM were incorporated into Scotchbond Multi-Purpose (SBMP) primer and adhesive. Dentin shear bond strengths were measured using extracted human molars. Protein adsorption onto the adhesive resin surfaces was determined by the micro bicinchoninic acid (BCA) method. A dental plaque microcosm biofilm model with human saliva as inoculum was used to investigate biofilm metabolic activity, colony-forming unit (CFU) counts, lactic acid production and live/dead staining of biofilms on resins.

RESULTS

Incorporation of 7.5% MPC and 5% DMAHDM into primer and adhesive did not adversely affect the dentin shear bond strength (p>0.1). The resin with 7.5% MPC+5% DMAHDM had protein adsorption that was nearly 20-fold less than SBMP control (p<0.05). The resin with 7.5% MPC+5% DMAHDM had much stronger antibacterial effects than using MPC or DMAHDM alone (p<0.05). Biofilm CFU counts on the resin with 7.5% MPC+5% DMAHDM were reduced by more than 4 orders of magnitude, compared to SBMP control.

SIGNIFICANCE

The use of double agents (protein-repellent MPC+antibacterial DMAHDM) in dental adhesive achieved much stronger inhibition of biofilms than using each agent alone. The novel protein-repellent and antibacterial bonding agent is promising to reduce biofilm/plaque buildup and reduce recurrent caries at the tooth-restoration margins.