Application of digital image correlation to full-field measurement of shrinkage strain of dental composites

Objectives： Polymerization shrinkage of dental composites remains a major concern in restorative dentistry because it can lead to micro-cracking of the tooth and debonding at the tooth-restoration interface. The aim of this study was to measure the full-field polymerization shrinkage of dental composites using the optical digital image correlation （DIC） method and to evaluate how the measurement is influenced by the factors in experiment setup and image analysis. Methods： Four commercial dental composites, Premise Dentine, Z 100, Z250 and Tetric EvoCeram, were tested. Composite was first placed into a slot mould to form a bar specimen with rectangular-section of 4 mmx2 mm, followed by the surface painting to create irregular speckles. Curing was then applied at one end of the specimen while the other part were covered against curing light for simulating the clinical curing condition of composite in dental cavity. The painted surface was recorded by a charge-coupled device （CCD） camera before and after curing. Subsequently, the volumetric shrinkage of the specimen was calculated with specialist DIC software based on image cross correlation. In addition, a few factors that may influence the measuring accuracy, including the subset window size, speckle size, illumination light and specimen length, were also evaluated. Results： The volumetric shrinkage of the specimen generally decreases with increasing distance from the irradiated surface with a conspicuous exception being the composite Premise Dentine as its maximum shrinkage occurred at a subsurface distance of about 1 mm instead of the irradiated surface. Z100 had the greatest maximum shrinkage strain, followed by Z250, Tetric EvoCeram and then Premise Dentine. Larger subset window size made the shrinkage strain contour smoother. But the cost was that some details in the heterogeneity of the material were lost. Very small subset window size resulted in a lot of noise in the data, making it difficult to discern the general pattern in the strain distribution. Speckle size did not seem to have obvious effect on the volumetric shrinkage strain along specimen length; however, larger speckles resulted in higher level of noise or heterogeneity in the shrinkage distribution. Compared with bright illumination, dimmer lighting produced larger standard deviations in the measured shrinkage~ indicating a higher level of noise. The longer the specimen, the greater was the rate of reduction with distance from the irradiated surface, especially for the longitudinal strain. Significance： The image correlation method is capable of producing full-field polymerization shrinkage of dental composites. The accuracy of the measurements relies on selection of optimal parameters in experimental setup and DIC analysis.

The effect of temperature on the wear behaviour of dental composites

Despite numerous published studies on the wear of dental composites,few have considered the influence of temperature on the two-body wear process.Additionally,no previous work has considered the influence of temperature on dominant wear mecha-nisms during the consumption of hot substances,hence the focus of this study.Recip-rocating wear tests were carried out at varying artificial saliva lubricant temperatures(37 and 57℃)and material loss was quantified using profilometry.The wear tracks were analysed using FIB/SEM/TEM.Results reveal that the wear rate of a dental composite can significantly increase with temperature,with fatigue/delamination and ploughing acting as dominant mechanisms.

Antibacterial Dental Resin Composites: A Narrative Review

Lack of antibacterial properties in resin-based composites (RBCs) is one of the flaws that cause the failure of filling clinically. Several agents have been incorporated to endow RBCs with antibacterial properties. In this review, we summarize the recent antibacterial agents between 2015 and 2020 using keywords of antibacterial or antimicrobial dental resin composites by PubMed databases. The most effective strategies are concerned with polymerizable monomers (50%), followed by filler particles (39%) and leachable agents (11%). A recent modification of the antibacterial agent is either by combining two agents from the same category or mixing agents from different categories in one. More than two methods were used in one study to assess antibacterial efficacy. The most common method was biofilm colony-forming units (CFUs) counting method (40%), followed by live/dead bacteria staining assay of biofilms (25%), metabolic activity assay of biofilms using MTT assay (16%), lactic acid production assay of biofilms (8%), agar diffusion test (8%), and other methods (3%) such as minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC).

Urchin-like multiscale structured fluorinated hydroxyapatite as versatile filler for caries restoration dental resin composites

Caries is one of the most prevalent human diseases,resulting from demineralization of tooth hard tissue caused by acids produced from bacteria,and can progress to pulpal inflammation.Filling restoration with dental resin composites(DRCs)is currently the most common treatment for caries.However,existing DRCs suffer from low fracture strength and lack comprehensive anti-caries bioactivity including remineralization,pulp protection,and anti-cariogenic bacteria effects.In this study,inspired by plant roots’ability to stabilize and improve soil,fluorinated urchin-like hydroxyapatite(FUHA)with a three-dimensional whisker structure and bioactive components of calcium,phosphorus,and fluorine was designed and synthesized by a dynamic self-assembly method.Furthermore,versatile FUHA particles with different loading fractions were used as functional fillers to fabricate methacrylate-based DRCs,where the urchin-like hydroxyapatite(UHA)filled DRCs and commercial DRCs(Z350XT and BEAUTIFIL II)served as the control groups.The results demonstrated that FUHA with 50 wt%loading in resin matrix endowed DRC(F5)with excellent physicochemical properties,dentin remineralization property,cell viability,promotion of dental pulp stem cells mineralization,and antibacterial properties.Meanwhile,F5 also presented good clinical handling and aesthetic characteristics.Therefore,structure/functional-integrated FUHA filled DRCs have potential as a promising strategy for tooth restoration and anti-caries bioactivity.

3D FINITE ELEMENT ANALYSIS OF THE DAMAGE EFFECTS ON THE DENTAL COMPOSITE SUBJECT TO WATER SORPTION

The damage effects of water sorption on the mechanical properties of the hydroxyapatite particle reinforced Bis-GMA/TEGDMA copolymer （HA/Bis-GMA/TEGDMA） h-ave been predicted using 3D finite cell models. The plasticizer effect on the polymer matrix was considered as a variation of its Young＇s modulus. Three different cell models were used to determine the influence of varying particle contents, interphase strength and moisture concentration on the debonding damage. The stress distribution pattern has been examined and the stress transfer mode clarified. The Young＇s modulus and fracture strength of the Bis-GMA/TEGDMA composite were also predicted using the model with and without consideration of the damage. ine Iormer results with consideration of the debonding damage are in good agreement with existing literature experimental data. The shielding effect of our proposed model and an alternative approach were discussed. The FCC cell model has also been extended to predict the critical load for the damaged and the undamaged composite subject to the 3-point flexural test.

Mechanical properties of dental composite resins containing nanodiamond of different diameters

In order to investigate the effect of nanodiamond （ND） on the mechanical properties of dental composite resins, bisphenol-A-glycidyl dimethacrylate （ Bis-GMA ）, triethylene glycol dime- thacrylate （ TEGDMA）, barium glass （BG） powder, DL-camphoroquinone （CQ） and dimethylamin- oethylmethacrylate （DMAEMA） were mixed as the matrix for the experiments. NDs of three diame- ters （ 100 nm, 220 nm, 500 nm） modified with 3-（trimethoxysilyl） propyl methacrylate （γ-MPS） were added to the mixture separately in 0. 2% to get test samples. The fiexural strength and elastic modulus of these samples and the mixture without ND （as the reference group, RG） were characterized with standard specification tests. Results reveal that the mechanical properties and elastic modulus of resins containing 220 nm ND （ R-220 ） and 500 nm ND （ R-500 ） were significantly different from those of RG. R-500 had the highest flexural strength and elastic modulus （ by average data）, and all of its five specimens were found to have mean flexural strength values greater than the minimum value （80MPa） prescribed by ISO 4049： 2000. Different sizes of diamonds have different effects on mechanical properties of dental composite resins, and the performances of 220 nm and 500 nm diamonds are better than that of 100 nm diamond.

A General Strategy for Efficiently Constructing Multifunctional Cluster Fillers Using a Three-Fluid Nozzle Spray Drying Technique for Dental Restoration

Multifunctional fillers are greatly required for dental resin composites(DRCs).In this work,a spray dryer with a three-fluid nozzle was applied for the first time to construct high-performance complex nanoparticle clusters(CNCs)consisting of different functional nanofillers for dental restoration.The application of a three-fluid nozzle can effectively avoid the aggregation of different nanoparticles with opposite zeta potentials before the spray drying process in order to construct regularly shaped CNCs.For a SiO_(2)–ZrO_(2) binary system,the SiO_(2)–ZrO_(2) CNCs constructed using a three-fluid nozzle maintained their excellent mechanical properties((133.3±4.7)MPa,(8.8±0.5)GPa,(371.1±13.3)MPa,and(64.5±0.7)HV for flexural strength,flexural modulus,compressive strength,and hardness of DRCs,respectively),despite the introduction of ZrO_(2) nanoparticles,whereas their counterparts constructed using a two-fluid nozzle showed significantly decreased mechanical properties.Furthermore,heat treatment of the SiO_(2)-ZrO_(2) CNCs significantly improved the mechanical properties and radiopacity of the DRCs.The DRCs containing over 10%mass fraction ZrO_(2) nanoparticles can meet the requirement for radiopaque fillers.More importantly,this method can be expanded to ternary or quaternary systems.DRCs filled with SiO_(2)-ZrO_(2)-ZnO CNCs with a ratio of 56:10:4 displayed high antibacterial activity(antibacterial ratio>99%)in addition to excellent mechanical properties and radiopacity.Thus,the three-fluid nozzle spray drying technique holds great potential for the efficient construction of multifunctional cluster fillers for DRCs.

Micromechanical interlocking-inspired dendritic porous silicabased multimodal resin composites for the tooth restoration

Porous silica particles have shown great potential application as reinforcing fillers in the field of dentistry due to their ability to construct the micromechanical interlocking effect at filler-matrix interface.However,how to accurately regulate the pore structure,especially the pore size,to increase the degree of the micromechanical interlocking and the performance of materials remains a challenge.Herein,we have proposed a facile self-assembly process to synthesize dendritic porous silica with tunable pore sizes(DPS-x)by adjusting the chain-length of the alcohols in the microemulsion.The mechanism of nucleation-growth is further put forward.The results indicate that the pore size of DPS-x indeed affects the mechanical property of composites,where the DPSpen particles with intermediate pore size are chosen as the optimal reinforcing fillers.The bimodal and multimodal filler formulations are further established to address the loading limitation of unimodal DPS-pen(46 wt.%).In virtue of the closepacked structure of identical spheres,the particle sizes of secondary silica embedded into the maximally loaded bimodal D3S7 composite(DPS-pen:Si430=30:70,w/w)are theoretically calculated without trials.Among all formulations,the developed multimodal D3S7+Si178+Si90 filler exhibits superior mechanical properties,the lowest shrinkage,and high polymerization conversion for dental composites,along with satisfied waster sorption and solubility,and good biocompatibility in vitro and in vivo,which are comparable to commercial composite Z350 XT(3M,USA).These DPS-x particles and their multimodal fillers can also be applied to other polymer-based biomaterials.

THE INFLUENCE OF SHRINKAGE AND MOISTURE DIFFUSION ON IDEALIZED TOOTH STRUCTURE INVOLVING DEBONDING DAMAGE

This study highlights the joint effect of early polymerization shrinkage and long-term moisture diffusion on the behavior of the restoration-tooth structure. The interphase debonding between particle and polymer resin in dental composite is taken into account by introducing the damage variable. The idealized model is designed and constructed for representing the restorationtooth structure, which consists of enamel, dentin, composite and interphase, each considered as homogenous material. The simulation is carried out using the general-purpose finite element software package, ABAQUS incorporated with a user subroutine for definition of damaged material behavior. The influence of Young's moduli of composite and interphase on stress and displacement is discussed. The compensating effect of water sorption on the polymerization shrinkage is examined with and without involving damage evolution. A comparison is made between the influence of hyper-, equi- and hypo-water sorption. Interfacial failure in the specific regions as well as cuspal movement has been predicated. The damage evolving in dental composite reduces the rigidity of composite, thus in turn reducing consequent stress and increasing consequent displacement. The development of stresses at the restoration-tooth interface can have a detrimental effect on the longevity of a restoration.

Influence of surface roughness on the color of dental-resin composites

This study deals with the influence of surface roughness on the color of resin composites.Ten resin composites (microfilled,hybrid,and microhybrid) were each polished with 500-grit,1 200-grit,2 000-grit,and 4 000-grit SiC papers.The roughness parameter (Ra) was measured using a Plμ confocal microscope,and field-emission scanning electron microscope (Fe-SEM) images were used to investigate filler morphology.Color was measured using a spectroradiometer and a D65 standard illuminant (geometry diffuse/0° specular component excluded (SCE) mode).Surface roughness decreased with grit number and was not influenced by filler size or size distribution.A significant influence of Ra on lightness (L*) was found.Lightness increased with decreases in roughness,except for specimens that underwent polishing procedure 4 (PP4;500-grit,1 200-grit,2 000-grit,and 4 000-grit SiC papers consecutively).Generally,it was found that surface roughness influenced the color of resin composites.The composites that underwent PP1 (500-grit SiC paper) exhibited significant differences in chroma (C*),hue (h°),and lightness (L*) compared to composites that underwent PP3 (500-grit,1 200-grit,and 2 000-grit SiC papers consecutively) and PP4.Color difference (E*) between the polishing procedures was within acceptability thresholds in dentistry.

Mechanical Properties, Biocompatibility and Anti-Bacterial Adhesion Property Evaluation of Silicone-Containing Resin Composite with Different Formulae

Novel branched silicone methacrylate was developed.The mechanical and biological properties of the resin system were investigated to select the formula proportion with the best overall performance.The novel silicone-containing monomers were combined with an incremental sequence of glass filler concentrations in commonly used Bis-GMA/TEGDMA(50/50,wt./wt.)dental resin systems.Physicochemical properties,surface properties,antibacterial adhesion effect,anti-biofilm effect,protein adsorption,and cytotoxicity were evaluated.The results showed that BSMs did not affect the double bond conversion of dental resin,but could reduce volumetric shrinkage(p<0.05).The BSM containing resins can resist protein and bacteria adhesion(S.mutans)because it has increased hydrophobicity and a lower free energy surface(p<0.05).However,there were no statistically significant differences in cytotoxicity,surface roughness,and double bond conversion rate.Overall,the results indicate that changes in a material’s properties are not strictly proportional to its composition.Synthetic silicone resin methacrylate can reduce the polymerization shrinkage,have low surface energy and anti-adhesion properties.Silicone composite resin containing 70%matrix has the best comprehensive properties.The silicone methacrylate composite represents an innovative method to improve the properties and reducing secondary caries.

Mechanical properties and biocompatibility of polymer infiltrated sodium aluminum silicate restorative composites

A new type of polymer-infiltrated-ceramic-network composites (PICNs) was fabricated by infiltrating methacrylate-based monomers into partially sintered porous ceramics.The mechanical properties (flexural strength,flexural modulus,elastic modulus,Vickers hardness,fracture toughness) were investigated and compared with that of the natural tooth and common commercial CAD/CAM blocks.Our results indicated that sintering temperature and corresponding density of porous ceramics have an obvious influence on the mechanical properties,and PICNs could highly mimic the natural tooth in mechanical properties.The biocompatibility experiments evaluated through in vitro cell attachment and proliferation of BMSCs showed good biocompatibility.The mechanical properties and biocompatibility confirmed that PICN could be a promising candidate for CAD/CAM blocks for dental restoration.