Changes in polymerization stress and elastic modulus of bulk-fill resin composites for 24 hours after irradiation

Effect of peroxide-free and peroxide-based in-office bleaching on the surface and mechanical properties of CAD/CAM esthetic restorative materials

The study aimed to investigate the influence of H2O2-based and H2O2-free in-office bleaching on the surface and mechanical attributes of CAD/CAM composite blocks. CAD/CAM composite blocks from five different composite materials (CC1, CC2, CC3, CC4, and CC5) were randomly divided into two groups according to bleaching application (H2O2-based and H2O2-free). The surface topography, morphology, nanohardness, elastic modulus, flexural strength, and fracture toughness were measured. A paired and unpaired sample t-tests gauged the effect of pre- and post-bleaching on the substrates. The estimated mean differences (before-after bleaching) suggested an increase in surface roughness for two materials CC2 and CC4, and a significant decrease in nanohardness for material CC4 and in elastic modulus for materials CC2 and CC4 with H2O2-based bleaching, whereas H2O2-free bleaching resulted in changes compatible with no change in these properties. Flexural strength and fracture toughness showed no evidence of changes, irrespective of the bleaching gel used. Scanning electron microscopic analysis revealed erosive effects and micropore formation due to H2O2-based bleaching. H2O2-based bleaching deteriorates the surface of CAD/CAM composite materials while H2O2-free bleaching gel had an insignificant effect on both surface and bulk properties. The clinician should carefully evaluate the potential effects of H2O2-based bleaching on the surface properties of CAD/CAM composites.

Residual stresses in glass crowns generated by polymerization and water sorption of resin cements

This study investigated residual stresses in glass crowns cemented with resin cements. Glass caps were cemented to cylindrical cores using a conventional resin composite cement, a self-adhesive resin cement, or a methyl methacrylate (MMA)-based cement in dual-cure or self-cure mode. The cemented caps were stored in 37°C water for 28 days, and stresses on the cap surface were repeatedly measured. The water sorption, water solubility, and elastic modulus of the cements were also measured. Polymerization of the cements initially generated compressive stresses on the surfaces. Dual-curing or a greater modulus yielded greater initial stress. The stresses gradually decreased over time and lingered on the surfaces at 28 days with all the cements. Greater sorption tended to lead to greater stress reduction; however, the MMA-based cement exhibited less stress reduction despite exhibiting the greatest sorption. The use of a resin composite cement or dual-curing is recommended to reinforce crown restorations.

Assessment of Micro-Hardness, Degree of Conversion, and Flexural Strength for Single-Shade Universal Resin Composites

Single-shade universal resin composites (SsURC) are preferred in clinical practice to reduce time for shade selection and obtain good esthetic results. In this study, the static mechanical properties of seven new SsURCs were investigated, their spectral analyzes were performed and scanning electron microscopy (SEM) evaluations were presented. Charisma Diamond One/DO, Admira Fusion x-tra/AFX, Omnichroma/OC, OptiShade/OS, Essentia Universal/EU, Zenchroma/ZC, Vittra APS Unique/VU were used in a three-point bending test to determine flexural strength (FS) and elastic modulus (EM); Vickers micro-hardness (VHN) and hardness-ratio (HR) were performed with a micro-hardness tester from top/bottom after 24-h/15-days of storage in distilled water at 37 °C (±1 °C). The degree of conversion (DC) was assessed by using Fourier transform infrared spectroscopy (FTIR). The structure of the resin matrix and filler content were assessed by SEM. Data were analyzed using IBM SPSS V23 and the R program and the significance level was taken as p < 0.05. The main effect of the tested SsURCs was found to be statistically significant on FS, EM, VHN, and DC values (p < 0.001). Bis-GMA free SsURCs (AFX, DO, VU) showed better DC and HR except for OC. All seven tested SsURCs conform to the requirements of ISO standards for dental resin composites for all tested categories.

Stress Point Monitoring Algorithm for Structure of Steel Cylinder Concrete Pipes in Large Buildings

The internal structure of steel cylinder concrete pipe in large buildings is complex. Traditionally, the safety monitoring method is unable to accurately monitor the situation of every stress point in the structure. Therefore, the wireless sensor network with practical value in the field of building safety monitoring was introduced. A monitoring algorithm for stress points in pipeline structure was put forward. The distribution law of circumferential prestress produced by prestress steel wire on the pipe core concrete was analyzed. According to the influencing factors, the mechanical performance of tube concrete pipe structure was discussed, and the method of calculating prestress of pipe structure was constructed. Combined with sensor network nodes, a series of basic hypothesis information was set. Moreover, the force between node and stress point was analyzed by virtual potential field. Based on the force analysis for the centroid of the sensing area, the monitoring of the stress point in pipeline structures was completed. After that, a rectangular area to be monitored was selected and the force points were established randomly. According to the relationship between network coverage rate and monitoring efficiency, we found that the proposed algorithm had good network immunity. According to the different number of nodes, sensing radii and perception angles, the influence of index on the monitoring accuracy was discussed. Experimental results show that the accuracy of the proposed algorithm is sensitive to the change of node parameter. When the number of nodes, sensing radius and sensing angle change, the maximum fluctuation range of monitoring accuracy is 0.08–0.99. From the application effect of the algorithm, we can see that the detection effect of the algorithm has obvious advantage.

Reinforcing the cervical dentin with bonded materials to improve fracture resistance of endodontically treated roots

Introduction:

Endodontic procedure leads to the loss of tooth structure resulting in fractures. Intraorifice barriers of bonded restorative materials placed in the cervical third of tooth may help in increasing fracture resistance.

Materials and Methods:

Human mandibular premolars (n = 75) underwent decoronation to adjust working length at 14 mm and prepared up to F3. They were obturated using gutta-percha and resin sealer AH-Plus and randomly divided into five groups (n = 15), Group 1: Control obturated with gutta-percha only. Groups 2, 3, 4, and 5 had placement of intraorifice barriers after the removal of 3 mm coronal gutta-percha such that Group 2: RMGI, Group 3: Self-adhering flowable composite, Group 4: Bulkfill Flowable Composite, and Group 5: mineral trioxide aggregate (MTA). Mounting of specimens was done in acrylic resin to expose coronal 3 mm and tested using the universal testing machine.

Results:

Group 1 (control) showed least fracture strength among all groups. Among those with intraorifice barriers, Group 2 Resin-modified glass ionomer cement showed maximum fracture resistance followed by Group 4 (Bulkfill composite) and Group 5 (self-adhering flowable composite) and least by Group 5 (MTA).

Conclusion:

The type of intraorifice barrier had a significant impact on root fracture resistance.

Effect of post-irradiation polymerization on selected mechanical properties of six direct resins

This study evaluated the post-irradiation mechanical property development of six resin composite-based restorative materials from the same manufacturer starting at 1 h post irradiation, followed by 24 h, 1 week, and 1 month after fabrication. Samples were stored in 0.2M phosphate buffered saline until testing. Flexural strength, flexural modulus, flexural toughness, modulus of resiliency, fracture toughness, and surface microhardness were performed at each time interval. Mean data was analyzed by Kruskal Wallis and Dunn's post hoc testing at a 95% level of confidence (α=0.05). Results were material specific but overall, all resin composite material mechanical properties were found to be immature at 1 h after polymerization as compared to that observed at 24 h. It may be prudent that clinicians advise patients, especially those receiving complex posterior composite restorations, to guard against overly stressing these restorations during the first 24 h.

Comparison of polymerization stresses of dental resin composites evaluated by two indentation fracture methods

BACKGROUND

Polymerization stress is a major problem in dental resin composite restorations. Two indentation fracture methods can be applied to evaluate the stress, however, they often calculate different values.

OBJECTIVE

To compare polymerization stresses of dental composites determined by the two methods.

METHODS

Glass disks with a central hole were used. Two indentation fracture methods (Methods 1 and 2) were employed to determine the polymerization stresses of low-shrinkage and bulk-fill composites. Method 1: Cracks were made in the glass surface at 300 μm from the hole. The hole was filled with the composite. Polymerization stresses at 30 min after filling were calculated from the lengths of crack extension. Method 2: The hole was filled with the composite. Cracks were introduced in the glass at 1,000 μm from the hole at 30 min after the polymerization and the stresses were calculated from the crack lengths. Stresses at composite-glass bonded interface were calculated from the stress values obtained by the two methods.

RESULTS

The bulk-fill composite generated the smallest interfacial stress, and Method 1 revealed lower values than Method 2.

CONCLUSIONS

The composites yielded relatively small stresses. Method 1 calculated smaller stress values, possibly affected by the lower threshold stress intensity factor.

Exposure of Biomimetic Composite Materials to Acidic Challenges: Influence on Flexural Resistance and Elastic Modulus

Acidic conditions of the oral cavity, including soft drinks and cariogenic bacteria, represent a damage for restorative biomimetic composite materials. The aim of this study is to assess the influence of two different acidic challenges on the flexural strength and elastic modulus of five composites: x-tra fil (Group 1, XTF), GrandioSO x-tra (Group 2, GXT), Admira Fusion x-tra (Group 3, AFX), VisCalor bulk (Group 4, VCB), and Enamel Plus HRi (Group 5, EPH). Thirty samples for each group were randomly divided and assigned to three different treatments: storage in distilled water as the controls (subgroups 1a-5a), 3 weeks distilled water + 1 week Coca-Cola (subgroups 1b-5b), and 4 weeks Coca-Cola (subgroups 1c-5c). For each subgroup, the flexural strength and elastic modulus were measured using an Instron universal testing machine, and data were submitted to statistical analysis. Considering subgroups B, no material showed a significant difference in the flexural strength with the controls (p > 0.05), whereas for subgroups C, only GXT and VCB showed significantly lower values (p < 0.05). AFX reported the lowest flexural strength among the materials tested. As regards the elastic modulus, no material showed a significant variation after acidic storages when compared with the respective control (p > 0.05). AFX and EPH reported the lowest elastic modulus compared to the other materials. All composites tested showed adequate flexural properties according to the standards, except for AFX. This biomimetic material, along with EPH, might be indicated for V class (cervical) restorations considering the lowest values of elasticity reported.

Residual polymerization stresses in human premolars generated with Class II composite restorations

The objective of this study was to assess the influence of filling techniques on residual polymerization stresses in resin composite restorations of the tooth. Flat planes were ground in buccal enamel surfaces of extracted human premolars, followed by preparing Class II cavities. Indentation cracks were introduced in the planes and crack lengths were measured mesio-distally (x-direction) and cervico-incisally (y-direction). Cavities were filled with a resin composite and an adhesive using three methods; one with bulk filling and two with differing incremental filling techniques. The x- and y-tensile stresses were calculated from crack lengths measured repeatedly over 360 min after filling. Elastic modulus and polymerization shrinkage of the composite were also measured. Filling technique and time after fillings were statistically significant only for the y-stress. The incremental techniques generated smaller stresses than the bulk filling. The stresses developed for 60 min after filling, while the modulus and the shrinkage stopped developing within 10 min and 2 min after irradiation, respectively. The incremental technique, in which the proximal portion of the cavity was filled first, was effective in decreasing the residual tensile stress generated by the polymerization of resin composite.

Flowable Bulk-Fill Materials Compared to Nano Ceramic Composites for Class I Cavities Restorations in Primary Molars: A Two-Year Prospective Case-Control Study

Background: The aim of this split-mouth study is to compare the results of 24 months’ clinical performance of primary molar Class I restorations with a nano-ceramic composite, Ceram•X mono (Dentsply) with a flowable bulk-fill material regular viscosity, SDR (Dentsply). Methods: Following the ethical approval, 27 patients with at least two class I cavities in primary molars were included in the study. A total number of 54 restorations were conducted (n = 27 for Ceram X and n = 27 for SDR). Restorations were evaluated at baseline, 6, 18, and 24 months, according to the modified Ryge criteria. The cavosurface marginal discoloration and color match were evaluated visually after air-drying the tooth and after removing the plaque (if necessary). Results: At 24 months’ follow-up, 54 restorations showed similar clinical performance. The statistical analysis did not reveal any statistical significance in the values between the groups in 7 out of 7 modified Ryge criteria. However, two restorations in both groups received Bravo ratings in the cavosurface marginal discoloration scoring. No side effects were reported by the participants of the study. Conclusion: Restorations with both materials (Ceram•X mono and SDR) have provided almost identical results.

Fracture resistance, gap and void formation in root-filled mandibular molars restored with bulk-fill resin composites and glass-ionomer cement base

AIM

To evaluate fracture resistance and gap/void presence of root-filled mandibular molars restored with 2 bulk-fill and 1 conventional resin composites, with or without a glass-ionomer cement (GIC) base.

METHODS

Coronal access and mesio-occlusal (MO) cavities were prepared, then root canal treatment was performed on 30 mol/L. The teeth were randomly divided, according to the cavity volume, into 6 experimental groups (N = 5) and restored with conventional/light-cured (Ceram-X), bulk-fill/light-cured (SureFil SDR) or bulk-fill/dual-cured (Core-X Flow) with/without a 2-mm thick GIC base. Gaps and voids (%) were determined using microcomputed tomography. Intact teeth and unrestored teeth were used as negative and positive controls. Fracture load (N) was determined using a universal testing machine.

RESULTS

No significant difference in fracture resistance or gap/void formation was found among the 3 resin composites. GIC-base groups revealed significantly lower fracture strength than intact teeth, while fracture strengths of no GIC-base groups were not significantly different from intact teeth. GIC-base groups revealed significantly more gaps and voids in the area of the GIC than the resin composite.

CONCLUSION

Conventional and bulk-fill resin composites provided similar fracture resistance and gaps/voids in root-filled molars with MO cavities. Placing a GIC base decreased fracture resistance and increased gap/void formation.

Flexural Properties and Elastic Modulus of Different Esthetic Restorative Materials: Evaluation after Exposure to Acidic Drink

BACKGROUND

Acidic beverages, such as soft drinks, can produce erosion of resin composites. The purpose of the present study was to investigate mechanical properties of different esthetic restorative materials after exposure to acidic drink.

METHODS

Nine different composites were tested: nanofilled (Filtek Supreme XTE, 3M ESPE), microfilled hybrid (G-ænial, GC Corporation), nanohybrid Ormocer (Admira Fusion, Voco), microfilled (Gradia Direct, GC Corporation), microfilled hybrid (Essentia, GC Corporation), nanoceramic (Ceram.X Universal, Dentsply De Trey), supranano spherical hybrid (Estelite Asteria, Tokuyama Dental Corporation), flowable microfilled hybrid (Gradia Direct Flo, GC Corporation), and bulk fill flowable (SureFil SDR flow, Dentsply De Trey). Thirty specimens of each esthetic restorative material were divided into 3 subgroups (n=10): specimens of subgroup 1 were used as control, specimens of subgroup 2 were immersed in 50 ml of Coca Cola for 1 week, and specimens of subgroup 3 were immersed in 50 ml of Coca Cola for 1 month. Flexural strength and elastic modulus were measured for each material with an Instron Universal Testing Machine. Data were submitted to statistical analysis.

RESULTS

After distilled water immersion, nanofilled composite showed the highest value of both flexural strength and elastic modulus, but its flexural values decreased after acidic drink immersion. No significant differences were reported between distilled water and acidic drink immersion for all other materials tested both for flexural and for elastic modulus values.

CONCLUSIONS

Even if nanofilled composite showed highest results, acidic drink immersion significantly reduced flexural values.

Properties of Experimental Dental Composites Containing Antibacterial Silver-Releasing Filler

Secondary caries is one of the important issues related to using dental composite restorations. Effective prevention of cariogenic bacteria survival may reduce this problem. The aim of this study was to evaluate the antibacterial activity and physical properties of composite materials with silver sodium hydrogen zirconium phosphate (SSHZP). The antibacterial filler was introduced at concentrations of 1%, 4%, 7%, 10%, 13%, and 16% (w/w) into model composite material consisting of methacrylate monomers and silanized glass and silica fillers. The in vitro reduction in the number of viable cariogenic bacteria Streptococcus mutans ATCC 33535 colonies, Vickers microhardness, compressive strength, diametral tensile strength, flexural strength, flexural modulus, sorption, solubility, degree of conversion, and color stability were investigated. An increase in antimicrobial filler concentration resulted in a statistically significant reduction in bacteria. There were no statistically significant differences caused by the introduction of the filler in compressive strength, diametral tensile strength, flexural modulus, and solubility. Statistically significant changes in degree of conversion, flexural strength, hardness (decrease), solubility (increase), and in color were registered. A favorable combination of antibacterial properties and other properties was achieved at SSHZP concentrations from 4% to 13%. These composites exhibited properties similar to the control material and enhanced in vitro antimicrobial efficiency.