Optical Stability of High-translucency Resin-based Composites.

This study investigated the stability of the optical properties of high-translucent shades of dental resin-based composites. Four commercial materials (Filtek Z350 XT, Opallis, Amelogen Plus, and IPS Empress Direct) and 14 non-Vita shades were tested. Disc-shaped specimens for each resin-based composite-shade combination (n=6) were evaluated at T0 (baseline), T1 (after 30 days of storage in water), and T2 (after 30 days of storage each in water and a coffee solution). Color measurements were performed according to the L'C'h' color system. Translucency Parameter (TP) and CIEDE2000 color difference (ΔE00) were calculated. Data were statistically analyzed at α = 0.05. Baseline TP values varied from 43 ± 1 to 55 ± 1. Changes in TP at T1 varied from -3.0% (Opallis T-Neutral) to 4.2% (Amelogen Plus Trans Orange), with no major differences from T0. At T2, most resin-based composites showed significantly increased opacity, with changes varying between -15.0% (Empress Direct Trans 20) and -2.7% (Z350 XT Blue). However, the TP values were ≥40 throughout the study. Storage in water caused negligible color differences, with ΔE00 values at T1 ≤ 0.9 ± 0.6. At T2, all materials tested showed significant color difference, and ΔE00 ≥ 3.2 ± 0.2. The orange shades from Opallis and Amelogen Plus showed lower color variation than did the other shades. The most significant optical changes upon storage were detected in the hue and particularly the chroma color coordinate. In conclusion, the high-translucent resin-based composites showed large variability in the stability of their optical properties among the tested brands and different shades of the same material. Regardless of the storage condition, the tested resin-based composites retained their high-translucency character over time.

Masking Colored Substrates Using Monolithic and Bilayer CAD-CAM Ceramic Structures.

OBJECTIVE: To evaluate the masking ability and translucency of monolithic and bilayer CAD-CAM ceramic structures. METHODS: Discs of high translucency (HT) and low translucency (LT) lithium disilicate-based ceramic (IPS e.max CAD) with different thicknesses (0.7, 1, 1.5, and 2 mm) were evaluated as a monolithic structure or combined (bilayer) with a 0.5-mm-thick zirconia framework (IPS e.max ZirCAD). The masking ability and translucency were calculated based on CIE L*a*b* color coordinates measured with a spectrophotometer (SP60, X-Rite). The translucency parameter (TP) was calculated using color coordinates measured over standard white-and-black backgrounds. The masking ability was calculated by CIEDE2000 color difference metric (ΔE00) for each specimen measured over a tooth-colored substrate (shade A2) compared to three darker backgrounds (shade C4 and two metal substrates). Confidence intervals (CI) for the means (95% CI) were calculated for TP and ΔE00. The Pearson correlation between ΔE00 and TP was investigated for monolithic and bilayer structures over all backgrounds. RESULTS: The thinner the lithium disilicate layer, the greater the translucency and the higher the ΔE00 values. The effect of ceramic thickness on both translucency and masking ability was more pronounced for the monolithic structures. In addition, monolayers always presented a greater color variation than their bilayer counterparts. The metallic background produced greater ΔE00 than the C4-shaded substrate. CONCLUSION: Monolithic veneers were able to mask C4-shaded background but did not mask metallic backgrounds. Bilayer structures showed greater shade masking ability than monolithic structures.

Reliability and failure behavior of CAD-on fixed partial dentures.

OBJECTIVES: To estimate the reliability and failure behavior of fixed partial dentures (FPDs) fabricated using the CAD-on technique. METHODS: FPDs (n=25) were fabricated using a CAD/CAM system: IPS e.max ZirCAD - Crystall./Connect and IPS e.max CAD (Ivoclar). The restoration type ("three-unit bridge") and design method ("multilayer") based on Biogenerics were used. Framework and porcelain structures were united using a fusion ceramic (Crystall./Connect, Ivoclar). Mechanical fatigue was tested in a servohydraulic load frame machine at a cyclic loading (frequency: 2Hz; load ratio: 0.1). Based on previous data from specimens tested in fast fracture, three different stress profiles were used. The lifetime data were analyzed using an inverse power law-Weibull cumulative damage model (ALTA PRO, Reliasoft). All failed specimens were examined under a field emission scanning electron microscope. RESULTS: Porcelain chipping was the predominant (60%) mode of failure for FPDs tested in fast fracture and connector failure was predominant (67%) under fatigue. For fast fracture data, the Weibull modulus (ß) of FPDs was 7.8 combining the two failure modes. When chipping and connector fracture data were analyzed separately, ß values were 7.9 and 2.9. For the step stress fatigue test, ß values were lower than estimated using fast fracture, being 1.6 for connector fracture and 1.3 for porcelain chipping. SIGNIFICANCE: The test method (fast fracture or fatigue) significantly influenced the reliability of FPDs fabricated using the CAD-on technique, but it did not influence their failure behavior.

Flexural strength and reliability of monolithic and trilayer ceramic structures obtained by the CAD-on technique.

OBJECTIVE: To evaluate the flexural strength, Weibull modulus, fracture toughness, and failure behavior of ceramic structures obtained by the CAD-on technique, testing the null hypothesis that trilayer structures show similar properties to monolithic structures. METHODS: Bar-shaped (1.8mm×4mm×16mm) monolithic specimens of zirconia (IPS e.max ZirCAD - Ivoclar Vivadent) and trilayer specimens of zirconia/fusion ceramic/lithium dissilicate (IPS e.max ZirCAD/IPS e.max CAD Crystall./Connect/IPS e.max CAD, Ivoclar Vivadent) were fabricated (n=30). Specimens were tested in flexure in 37°C deionized water using a universal testing machine at a crosshead speed of 0.5mm/min. Failure loads were recorded, and the flexural strength values were calculated. Fractography principles were used to examine the fracture surfaces under optical and scanning electron microscopy. Data were statistically analyzed using Student's t-test and Weibull statistics (α=0.05). RESULTS: Monolithic and trilayer specimens showed similar mean flexural strengths, characteristic strengths, and Weibull moduli. Trilayer structures showed greater mean critical flaw and fracture toughness values than monolithic specimens (p<0.001). Most critical flaws in the trilayer groups were located on the Y-TZP surface subjected to tension and propagated catastrophically. Trilayer structures showed no flaw deflection at the interface. SIGNIFICANCE: Considering the CAD-on technique, the trilayer structures showed greater fracture toughness than the monolithic zirconia specimens.