Influence of the foundation substrate on the fatigue behavior of bonded glass, zirconia polycrystals, and polymer infiltrated ceramic simplified CAD-CAM restorations

This study evaluated the influence of distinct substrates on the mechanical fatigue behavior of adhesively cemented simplified restorations made of glass, polycrystalline or polymer infiltrated-ceramics. CAD/CAM ceramic blocks (feldspathic - FEL; lithium disilicate - LD; yttria-stabilized zirconia - YZ; and polymer-infiltrated ceramic network - PICN) were shaped into discs (n = 15, Ø = 10 mm; thickness = 1.0 mm), mimicking a simplified monolithic restoration. After, they were adhesively cemented onto different foundation substrates (epoxy resin - ER; or Ni-Cr metal alloy - MA) of the same shape (Ø = 10 mm; thickness = 2.0 mm). The assemblies were subjected to fatigue testing using a step-stress approach (200N-2800 N; step-size of 200 N; 10,000 cycles per step; 20 Hz) upon the occurrence of a radial crack or fracture. The data was submitted to two-way ANOVA (α = 0.05) to analyze differences considering 'ceramic material' and 'type of substrate' as factors. In addition, a survival analysis (Kaplan Meier with Mantel-Cox log-rank post-hoc tests; α = 0.05) was conducted to obtain the survival probability during the steps in the fatigue test. Fractographic and finite element (FEA) analyzes were also conducted. The factors 'ceramic material', 'type of substrate' and the interaction between both were verified to be statistically significant (p < .001). All evaluated ceramics presented higher fatigue failure load (FFL), cycles for failure (CFF) and survival probabilities when cemented to the metallic alloy substrate. Among the restorative materials, YZ and LD restorations presented the best fatigue behavior when adhesively cemented onto the metallic alloy substrate, while FEL obtained the lowest FFL and CFF for both substrates. The LD, PICN and YZ restorations showed similar fatigue performance considering the epoxy resin substrate. A more rigid foundation substrate improves the fatigue performance of adhesively cemented glass, polycrystalline and polymer infiltrated-ceramic simplified restorations.

Trade-off between fracture resistance and translucency of zirconia and lithium-disilicate glass ceramics for monolithic restorations

High strength and translucency are generally not coincident in one restorative material and there is still a continuous development for a better balance between these two properties. Zirconia and lithium-disilicate glass-ceramics are currently the most popular alternatives for monolithic restorations. In this work, the mechanical properties and more important, the slow crack growth (SCG) resistance, which rules long-term durability, were thoroughly studied for three zirconia ceramics stabilized by 3, 4 and 5 mol% yttria in comparison to lithium-disilicate glass-ceramic. Translucency versus strength maps revealed that the more translucent zirconia compositions (i.e. with higher yttria contents) fill the gap between the standard 3 mol% yttria stabilized zirconia (3Y-TZP) and lithium-disilicate. Moreover, increasing yttria content did not always result in lower strength, as values for 3 mol% and 4 mol% yttria were the same. Independent on the yttria contents, all zirconia showed similar relative susceptibility to SCG under static and cyclic conditions and were significantly more SCG-resistant than lithium-disilicate glass ceramic. A concern with higher yttria contents (5 and 4 mol%) however could lie in the higher sensitivity to defects, resulting in a larger scatter in strength. STATEMENT OF SIGNIFICANCE: In addition to the common investigations on the generally reported strength, toughness and translucency, V-K_I diagrams (crack velocity versus stress-intensity factor) from fast fracture to threshold for three newly developed zirconia were directly measured by double torsion methods under static and cyclic loading conditions. The crack-growth mechanisms were analyzed in depth. Results were compared with another popular dental ceramic, namely lithium-disilicate glass-ceramic, revealing the pros and cons of polycrystalline and glass-ceramics in terms of long-term durability. This is the first time that V-K_I curves are compared for the major ceramic and glass-ceramic used for dental restorations. Strength versus translucency maps for different CAD/CAM dental restorative materials were described, showing the current indication range for zirconia ceramics.

Fatigue failure load of an adhesively-cemented lithium disilicate glass-ceramic: Conventional ceramic etching vs etch & prime one-step primer

OBJECTIVES

To evaluate the effect of different glass-ceramic surface treatments and aging on the fatigue failure load of a lithium disilicate glass-ceramic adhesively cemented to a dentin analogue material.

METHODS

One hundred and twenty (120) disc-shaped lithium disilicate specimens (Ø=10mm, thickness=1.5mm) were produced and randomly allocated (n=20) into 6 groups, considering 2 study factors: "surface treatment" in 3 levels (SIL-silane application only; HF5+SIL-5% hydrofluoric acid etching and silane application; ME&P-etching with an one-step ceramic primer), and "storage" in 2 levels (baseline-storage for 7 days; aging-storage for 90 days+12,000 thermal cycles). Ceramic discs were adhesively cemented to discs of a dentin analogue material (Ø=10mm, thickness=2.0mm) following the manufacturers' instructions. The fatigue failure load was determined by the staircase approach (250,000 cycles; 20Hz; initial load=1050N [∼70% of mean load-to-failure]; step size=52.5N [5% of initial load]). Micro-morphologic, fractographic, and atomic force microscope analysis were also performed. Fatigue failure load data were evaluated by one-way ANOVA, Bonferroni and t-tests for independent samples.

RESULTS

HF5+SIL presented higher fatigue failure load in both conditions (baseline and aging); ME&P presented intermediary mean values, while the SIL group presented the worst performance. All groups had a statistically significant decrease in the fatigue performance after aging.

SIGNIFICANCE

Hydrofluoric acid followed by silane application showed the best fatigue performance for an adhesively-cemented lithium disilicate ceramic. Aging negatively influenced the fatigue performance for all tested groups.

Edge chipping and flexural resistance of monolithic ceramics

OBJECTIVE

Test the hypothesis that monolithic ceramics can be developed with combined esthetics and superior fracture resistance to circumvent processing and performance drawbacks of traditional all-ceramic crowns and fixed-dental-prostheses consisting of a hard and strong core with an esthetic porcelain veneer. Specifically, to demonstrate that monolithic prostheses can be produced with a much reduced susceptibility to fracture.

METHODS

Protocols were applied for quantifying resistance to chipping as well as resistance to flexural failure in two classes of dental ceramic, microstructurally-modified zirconias and lithium disilicate glass-ceramics. A sharp indenter was used to induce chips near the edges of flat-layer specimens, and the results compared with predictions from a critical load equation. The critical loads required to produce cementation surface failure in monolithic specimens bonded to dentin were computed from established flexural strength relations and the predictions validated with experimental data.

RESULTS

Monolithic zirconias have superior chipping and flexural fracture resistance relative to their veneered counterparts. While they have superior esthetics, glass-ceramics exhibit lower strength but higher chip fracture resistance relative to porcelain-veneered zirconias.

SIGNIFICANCE

The study suggests a promising future for new and improved monolithic ceramic restorations, with combined durability and acceptable esthetics.