Edge chipping resistance of ceramics bonded to a dentine analogue

Edge Chipping Resistance and Flexural Strength of CAD-CAM Ceramics Before and After Thermomechanical Aging

OBJECTIVES

To evaluate the complementary mechanical properties of dental ceramics using edge chipping resistance (Rea) and flexural strength before and after thermomechanical aging.

MATERIAL AND METHODS

Computer-aided design and computer-aided manufacturing of ceramic materials, including zirconia (ZR), lithium disilicate (LS2), and resin nanoceramics (RNC), were evaluated. Specimens for flexural strength testing were fabricated with dimensions of 3 × 4 × 25 mm, with 30 specimens per group. For the edge chipping test (ECT), specimens were fabricated with dimensions of 3 × 4 × 12 mm, with 48 specimens per group. Half of the specimens for both tests were subjected to thermomechanical aging for 200,000 cycles at 50 N. A Weibull analysis was performed to determine flexural strength. Fractographic analysis was performed before and after thermomechanical aging during the ECT. X-ray diffractometry (XRD) was performed on the ZR specimens before and after thermomechanical aging. Flexural strength was analyzed using a two-way repeated analysis of variance (ANOVA) with a t-test, and Rea was analyzed using Pearson correlation and a two-way repeated ANOVA.

RESULTS

The flexural strength and Rea differed according to the material (p = 0.001), whereas they were similar before and after thermomechanical aging (p > 0.05). The Weibull modulus of the flexural strength decreased after thermomechanical aging. In the fractographic analysis of the ECT, more than two fracture origins were identified after thermomechanical aging. The XRD analysis of ZR showed an increased monoclinic phase after thermomechanical aging.

CONCLUSION

All the materials met the ISO 6872 standards for bending test. However, LS2 and RNC show low edge chipping resistance within the range of the occlusal force. The thermomechanical aging did not significantly alter the mechanical properties of the materials. However, the Weibull coefficients of the flexural strength decreased, and an additional origin of fracture appeared after thermomechanical aging.

SIGNIFICANCE

Flexural strength and Rea differed depending on the material used. The probability of the material fracture increased after thermomechanical aging. To understand ceramic fractures, two mechanical tests and thermomechanical aging must be performed together.

Cyclic fatigue tests on non-anatomic specimens of dental ceramic materials: A scoping review

The aim of the present scoping review was to identify and discuss the methods, testing parameters, and characteristics used to induce cyclic fatigue on non-anatomic dental ceramic specimens. In vitro studies written in English which evaluated commercially-available non-anatomic dental ceramic specimens subjected to mechanical cyclic fatigue were selected. The search was performed in the PubMed, Scopus and Web of Science databases. The initial search yielded 1,636 articles, of which 81 were included. Based on the collected data, most of the included studies evaluated dental ceramic specimens cemented to supporting substrate (n= 42; 51.9%); used step-stress (n= 35; 42.2%) accelerated fatigue test, loading frequencies above 10 Hz (n= 31, 35.6%), stainless steel (n = 28, 32.6%) load applicator with spherical shaped tip 40 mm diameter (n= 25, 30.9%); applied only axial loads (n= 77, 95.1%); and considered a wet testing environment (n= 65, 78.3%). The definition of test geometry, method, and testing parameters must be cautiously considered according to the study objective and the scenario that is simulated. Accelerated fatigue tests, load frequencies up to 20 Hz, a 40 mm stainless steel spherical load applicator and a wet testing environment are the major common defined parameters presented in the existing literature. More studies exploring the influence of such factors on fatigue mechanism are necessary.

Effect of processing methods on the chipping resistance of veneered zirconia

OBJECTIVES

To evaluate the edge chipping resistance (R_eA) and the fracture toughness (K_C) of 3Y-TZP bilayers produced with the following materials/processing combinations: fluorapatite glass-ceramic applied on zirconia using the traditional layering and hot-pressing (press-on) techniques; feldspathic porcelain using rapid layer technology (RLT); and lithium disilicate glass-ceramic using CAD-on method. The influence of the cooling rate (slow and fast) was analyzed for layering and hot-pressing.

METHODS

Bilayer bars (25x4x2 mm) were made following manufacturers' instructions. The edge chipping test was performed in an universal testing machine, using a coupled Vickers indenter. R_eA was calculated dividing the critical load at fracture by the edge distance. Fracture toughness was calculated by a regression fit with a fixed slope of 1.5 correlating the critical chipping load regarding edge distance and also with indentation fracture (IF) method. Data were statistically analyzed using ANOVA and Tukey's test (α = 5%).

RESULTS

R_eA and K_C was significantly higher for the CAD-on bilayers. RLT showed intermediate R_eA means, and layering and hot-pressing techniques showed the lowest R_eA values. For both processing methods there was no effect of the cooling protocol on the R_eA and fracture toughness.

CONCLUSIONS

There is a significant effect of the material/processing association on the edge chipping resistance and fracture toughness of the bilayers. There was no effect of the cooling protocol on the edge chipping resistance and fracture toughness for the specimens processed by both the layering and hot-pressing techniques.

Non-silicate nanoparticles for improved nanohybrid resin composites

OBJECTIVE

Zirconia and alumina nanoparticles were coated with a silica-rich layer (ALSI and ZRSI) and used to prepare experimental nanohybrid resin composites, which were characterized and compared to a control commercial resin composite (Filtek Z350 XT).

METHODS

Silica nanoparticles with sizes compatible to ALSI (Aerosil 150) and ZRSI (Aerosil OX 50) were tested as references. The volume of nanoparticles was equivalent across the composites, which also had consistent content of glass microparticles. CC conversion, viscosity, depth of cure, surface topography, hardness, opacity, radio-opacity, and edge chipping resistance (ReA) were tested after 24 h. Flexural strength (σ_f) and fracture toughness (K_IC) were also tested after 15 K thermal cycles. Data were analyzed using one-way or two-way ANOVA and Tukey's test (α = 0.05).

RESULTS

ALSI and ZRSI yielded resin composites with lower viscosity and more irregular nanoagglomerates compared to nanosilica-based composites. CC conversion and depth of cure were lower for ZRSI composite, which had higher opacity, radio-opacity, and hardness. ReA was higher for ALSI composite. Composites with ALSI and ZRSI showed stable σ_f after aging, whereas the control and Aerosil 150 resin composites showed significant degradation. The commercial and nanosilica-based composites showed up to 42% reduction in K_IC after aging, whereas resin composites with ZRSI and ALSI showed a more stable K_IC.

SIGNIFICANCE

ALSI and ZRSI generated nanohybrid resin composites with improved and/or more stable physical properties compared with nanosilica-based and commercial composites. This study suggests that changing the composition of nanofillers is a simple method to enhance the performance of nanohybrid composites.

Edge chipping test in dentistry: A comprehensive review

OBJECTIVE

Literature on edge chipping test applied to dental materials and structures has been systematically reviewed with regard to the evaluation methods and parameters used.

DATA

A systematic search of the literature retrieved 3484 relevant studies. After removing duplicates, 1848 records were screened by titles and abstracts and 1797 were excluded; 51 papers were assessed full text for eligibility. Twenty papers were included in this study and they were organized according to the dental materials and structures tested as follows: 2 studies on human tooth, 9 on dental ceramics, 5 on polymer-based composites, and 4 studies evaluated both ceramic and polymer-based materials.

SOURCES

MEDLINE/PubMed, Scopus and Web of Science databases were searched up to June 2019 without restriction on date and language.

STUDY SELECTION

In vitro studies using edge chipping test on dental materials and structures were included.

CONCLUSIONS

Different methods have been used for edge chipping test, regardless of reported parameters. There is significant evidence that edge chipping test is a relevant approach to predict chipping behavior of dental materials and tooth tissues because chips produced from most edge chipping studies are similar to clinically reported chipping failures.

The critical bond strength of orthodontic brackets bonded to dental glass-ceramics

OBJECTIVES

To evaluate the critical bond strength (σ) of ceramic and metal brackets to a lithium disilicate-based glass-ceramic.

MATERIALS AND METHODS

Two hundred and forty ceramic specimens (IPS e-max CAD) were randomly distributed in 12 experimental groups (n = 20). Two ceramic brackets (monocrystalline, BCm; and polycrystalline, BCp) and a metal bracket (BM) were bonded to glass-ceramic specimens after one of the following surface treatments: HF-hydrofluoric acid applied for 60 s; S-silane applied for 3 min; HFS-HF followed by S; and MDP-application of an adhesive containing a phosphate monomer (MDP). All brackets were bonded to the treated glass-ceramic using a resin cement, stored in 37 °C water for 48 h before shear bond strength testing. Optical (OM) and scanning electron (SEM) microscopies were used for fractographic analysis. Data was statistically analyzed using Kruskal-Wallis and Student-Newman-Keuls (α = 0.05).

RESULTS

BCm bonded to glass-ceramic treated with either HFS or HF showed the highest median σ values, respectively, 10.5 MPa and 8.5 MPa. In contrast, the BCp bonded to glass-ceramic treated with MDP showed the lowest median σ value (0.8 MPa), which was not statistically different from other MDP-treated groups.

CONCLUSIONS

The failure mode was governed by the glass-ceramic surface treatment, not by the bracket type. Quantitative (σ values) and qualitative (fracture mode) data suggested a minimum of 5 MPa for brackets bonded to glass-ceramic, which is the lower critical limit bond strength for a comprehensive orthodontic treatment.

CLINICAL RELEVANCE

Bonding brackets to glass-ceramic requires micromechanical retention.