Fracture toughness of seven resin composites evaluated by three methods of mode I fracture toughness (KIc)

Comparative Study on Interface Fracture of 4th Generation 3-Steps Adhesive and 7th Generation Universal Adhesive

The purpose of this paper is to compare the fracture behavior of interfaces obtained using fourth-generation and universal dental adhesives. The study relies on optic and SEM to evaluate the dentin-adhesive-restoration material interface of the samples and also on FEA simulation of fracture behavior. Specimen fabrication relied on 20 extracted teeth, in which class I cavities were created according to a protocol established based on the rules of minimally invasive therapy. For the direct adhesive technique, the adhesives used were: three-step All Bond, three-batch A and one-step Clearfil Universal Bond Quick-batch B. The restoration was performed with the same composite for both adhesives: Gradia direct posterior. The simulation used a 3D reconstructed molar on which geometric operations were performed to obtain an assembly that replicated a physical specimen. Material properties were applied to each component based on the information found in the literature. A simplified model for crack propagation was constructed, and using the fracture mechanics tool in Ansys 2019, the stress intensity factors that act at the crack tip of the adhesive interface were obtained. Mechanical simulation and microscopic investigation showed us how the interface of the dentine-adhesive-filling material performed in cases of both dental adhesives and for a certain loading condition. Important differences were identified among the adhesives, the fourth generation being superior to the fourth generation especially due to the separate steps in which the tooth surface was prepared for adhesion.

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.

A comprehensive review of the antibacterial activity of dimethylaminohexadecyl methacrylate (DMAHDM) and its influence on mechanical properties of resin-based dental materials

The repetitive restorative cycle should be avoided, aiming at the smallest number of restorations' replacements to ensure greater tooth longevity. Antibacterial materials associated with the control of caries etiological factors can help improve restoration's durability. This review aimed to analyze the results of in vitro studies that added Dimethylaminohexadecyl methacrylate (DMAHDM), an antibacterial monomer, to restorative materials. The PubMed, SCOPUS, Web of Science, and Biblioteca Virtual em Saúde databases were screened for studies published between 2015 and 2020. After full-text reading, 24 articles were included in the final sample. DMAHDM has demonstrated antibacterial efficacy against several bacteria related to dental caries and periodontal diseases, causing a transition in the biofilm balance without inducing resistance. When DMAHDM was included in acrylic resin, the material cytotoxicity increased, and changes in mechanical properties were observed. In contrast, resin composites had their mechanical properties maintained in most studies; however, toxicity was not examined. The association between DMAHDM and 2-methacryloyloxyethyl phosphorylcholine or silver nanoparticles improved the antibacterial effect. Besides, the association with nanoparticles of amorphous calcium phosphate or nanoparticles of calcium fluoride can provide remineralization capacity. There is a lack of information on the cytotoxicity and bacteria resistance induction, and further studies are needed to address this.

Thiourethane filler functionalization for dental resin composites: Concentration-dependent effects on toughening, stress reduction and depth of cure

OBJECTIVES

The aim of this study was to modify the surface of fillers used in dental composites by the synthesis of two novel thiourethane oligomeric silanes, used to functionalize the silica-containing inorganic particles. Several thiourethane silane concentrations were tested during the silanization process to systematically assess the effect of silane coverage on experimental composite conversion, polymerization stress and fracture toughness.

MATERIALS AND METHODS

Two different thiourethane silanes were synthesized based either on 1,6-hexanediol-diissocynate (HDDI), or 1,3-bis(1-isocyanato-1-methylethyl) benzene (BDI). Conventional 3-(Trimethoxysilyl)propyl methacrylate was used as the control. Glass fillers were silanized with 1, 2 or 4 wt% of each thiourethane silane, then evaluated by thermogravimetrical analysis. Photopolymerizable resin composites were prepared with Bis-GMA/UDMA/TEGDMA and 50 wt% silanized glass filler. Polymerization kinetics and degree of conversion were tested using Near-IR. Bioman was used to test polymerization stress. Data were analyzed with two-way ANOVA/Tukey's test (α = 5%).

RESULTS

The mass of silane coupled to the filler increased with the concentrations of thiourethane in the silanizing solution, as expected. Thiourethane-containing groups exhibited significantly higher degree of conversion compared to control groups, except for BDI 4%. HDDI 4%, BDI 2% and BDI 4% showed significantly lower polymerization stress than control groups. HDDI 4% exhibited significantly higher fracture toughness.

CONCLUSIONS AND CLINICAL SIGNIFICANCE

Novel filler functionalization with thiourethane silanes may be a promising alternative for improving dental composites properties by significantly increasing the degree of conversion, fracture toughness and reducing the polymerization stress.

Quantitative fractography as a novel approach to measure fracture toughness of direct resin composites

Dental resin composites generally fail from small cracks. Large crack techniques are not representative of in vivo failures. Quantitative fractography relies on the observation of small "natural flaws". This study investigates the effect of flaws that occur from fabrication and handling on the measurement of fracture toughness for four different direct resin composites. Microtensile dog-bone shaped specimens for each of four composites were fractured to measure the strength. In addition, we measured crack sizes of the fracture initiating cracks and determined the fracture toughness from these measurements using quantitative fractographic analysis on selected specimens. The characteristic strengths (28-51 MPa) and Weibull moduli (4.9-7.8) were also determined and related to the toughness values (0.5-0.9 MPa m^1/2) obtained. The elastic moduli (5-10 GPa) were measured using an indentation technique that has not been used before for direct resin composites. The indentation technique offers an alternative method for small specimens. Quantitative fractographic analysis offers a useful technique to assess fracture toughness of resin composites from cracks of the same size as observed in practice. The toughness of the direct resin composites results from a balance between the size, number and shape of filler particles and the viscosity of the resin matrix.

Fracture mechanics of circular discs with a V-notch subjected to wedging

OBJECTIVE

A method proposed for determining the fracture toughness (FT) of dental materials involves a 'roller' wedging open a V-notch in a cylindrical specimen. There are a number of problems with the design of this test and its mechanical analysis, and thus with the validity of the results obtained, were it to be used. Firstly, friction is ignored in calculating the horizontal wedging force. Secondly, the test specimen does not make use of a pre-crack at the notch tip. The aim of this study was to evaluate the effects of these factors on the FT calculated.

METHODS

An analytical solution for the mode-I stress intensity factor (K_I) of the compact tension specimen, which bears some similarities, is taken to be applicable. The mechanics of the specimen has been reanalysed, with a finite-element study of the resultant stresses, and compared with the compact-tension test.

RESULTS

The assumed analytical solution can provide accurate estimates for K_I for the V notched specimen. However, the apparent agreement is due to the fortuitous combination of an overestimated horizontal wedging force and an underestimated stress singularity at the crack tip. In any case, ignoring friction will lead to an overestimate of FT.

SIGNIFICANCE

It is concluded that the test as presented is invalid.

Influence of different curing modes on flexural properties, fracture toughness, and wear behavior of dual-cure provisional resin-based composites

We investigated the influence of different curing modes on the mechanical properties and wear behavior of dual-cure provisional resin-based composites (DCPRs). Three DCPRs and a self-curing bis-acryl provisional resin-based composite were used. Flexural strength (σ_F), elastic modulus (E), resilience (R), and fracture toughness (K_IC) were measured. The specimens were fabricated with and without light irradiation, stored in distilled water at 37°C for 24 h, and subjected to 5,000 or 10,000 thermal cycles. For sliding impact wear testing, 12 specimens were prepared with and without light irradiation. The maximum facet depth and volume loss were determined using a noncontact profilometer. Some of the mechanical properties and wear behavior of DCPRs are affected by light irradiation. This study indicated that proper light irradiation is important in polymerization process of the DCPRs to enhance the wear resistance and some mechanical properties.

Fracture Toughness Analysis of Ceramic and Resin Composite CAD/CAM Material

Objectives:

To evaluate and compare the fracture toughness of dental CAD/CAM materials of different material classes intended for in-office milling (glass ceramics, hybrid, resin composites) and the influence of aging on this property.

Methods and Materials:

The fracture toughness (critical intensity factor, KIc) values of 9 CAD/CAM restorative materials (Ambarino High-Class, Brilliant Crios, Cerasmart, exp. CAD/CAM composite, Katana Avencia, Lava Ultimate, VITA Enamic, IPS Empress CAD, and IPS e.max CAD) were determined using the SEVNB method in a four-point bending setup. Twenty bending bars of each material with a 4 × 3 cross and a minimum length of 12 mm were cut out of CAD/CAM milling blocks. Notching was done starting with a pre-cut and consecutive polishing and v-shaping with a razor blade, resulting in a final depth of v-shaped notches of between 0.8 and 1.2 mm. Half of the specimens were selected for initial fracture toughness measurements. The others were thermocycled in distilled water for 30,000× (5/55°C; 30-second dwell time) before testing. Specimen fracture surfaces were analyzed using confocal laser scanning microscopy.

Results:

All specimens for each material fractured into two fragments and showed the typical compression curl and brittle failure markings. Comparing initial KIc values, lithium disilicate ceramic IPS e.max CAD showed significantly the highest and leucite-reinforced ceramic IPS Empress CAD significantly the lowest KIc values (p<0.001). All tested CAD/CAM materials with a resin component ranged in the same KIc value group (p>0.999-0.060). After thermal cycling, the highest KIc values were measured for lithium disilicate ceramic IPS e.max CAD, followed by resin composite materials Ambarino High-Class (p<0.001-0.006) and hybrid material VITA Enamic (p<0.001-0.016), while the significantly lowest values were reflected for the resin composite materials Cerasmart, LAVA Ultimate (p<0.001-0.006), and Katana Avencia (p<0.001-0.009). The roughness of the fracture surfaces varied depending on the microstructure of the respective material. The ceramic surfaces showed the smoothest surfaces. The fracture surface of VITA Enamic revealed microstructural inhomogeneities and microcracks. For CAD/CAM resin composite materials, crack paths through the matrix and interfaces of matrix and fillers could be observed at the microstructure level.

Conclusions:

The materials tested show differences in fracture toughness typical for the class they belong to. With one exception (Ambarino High-Class), thermocycling affected the fracture toughness of materials with a resin component negatively, whereas the leucite and lithium disilicate ceramic showed stability.

The effect of aging methods on the fracture toughness and physical stability of an oxirane/acrylate, ormocer, and Bis-GMA-based resin composites

PURPOSE

To determine the effect of aging methods on the fracture toughness of a conventional Bis-GMA-based resin composite (Filtek Supreme), an ormocer-based resin composite (Admira), and an experimental hydrophobic oxirane/acrylate interpenetrating network resin system (OASys)-based composite.

METHODS

A 25 × 5 × 2.8-mm stainless-steel mold with 2.5 mm single-edge center notch, following ASTM standards [E399-90], was used to fabricate 135 specimens (n = 15) of the composite materials and randomly distributed into groups. For the baseline group, specimens were fabricated and then tested after 24-h storage in water. For the biofilm challenge, specimens were randomly placed in a six-well tissue culture plate and kept at 37 °C with bacterial growth media (Brain Heart Infusion (BHI); Streptococcus mutans) changed daily for 15 days. For the water storage challenge, specimens were kept in 5 ml of deionized distilled autoclaved water for 30 days at 37 °C. μCT evaluation by scanning the specimens was performed before and after the proposed challenge. Fracture toughness (K_Ic) testing was carried out following the challenges.

RESULTS

μCT surface area and volume analyses showed no significant changes regardless of the materials tested or the challenge. Filtek and Admira fracture toughness was significantly lower after the biofilm and water storage challenges. OASys mean fracture toughness values after water aging were significantly higher than that of baseline. Toughness values for OASys composites after biofilm aging were not statistically different when compared to either water or baseline values.

CONCLUSION

The fracture toughness of Bis-GMA and ormocer-based dental resin composites significantly decreased under water and bacterial biofilm assault. However, such degradation in fracture toughness was not visible in OASys-based composites.

CLINICAL SIGNIFICANCE

Current commercial dental composites are affected by the oral environment, which might contribute to the long-term performance of these materials.