Effect of the amount of thickness reduction on color and translucency of dental monolithic zirconia ceramics

PURPOSE

This study investigated the effect of amount of thickness reduction on color and translucency of dental monolithic zirconia ceramics.

MATERIALS AND METHODS

One-hundred sixty-five monolithic zirconia specimens (16.3 mm × 16.3 mm × 2.0 mm) were divided into 5 groups (Group I to V) according to the number of A2-coloring liquid applications. Each group was then divided into 11 subgroups by reducing the thickness up to 1.0 mm in 0.1-mm increments (Subgroup 0 to 10, n=3). Colors and spectral distributions were measured according to CIELAB on a reflection spectrophotometer. All measurements were performed on five different areas of each specimen. Color difference (ΔE^*_ab) and translucency parameter (TP) were calculated. Data were analyzed using one-way ANOVA and multiple comparison Scheffé test (α=.05).

RESULTS

There were significant differences in CIE L^* between Subgroup 0 and other subgroups in all groups. CIE a^* increased (0.52<R²<0.73), while CIE b^* decreased (0.00<R²<0.74) in all groups with increasing thickness reduction. Perceptible color differences (ΔE^* _ab>3.7) were obtained between Subgroup 0 and other subgroups. TP values generally increased as the thickness reduction increased in all groups (R²>0.89, P<.001).

CONCLUSION

Increasing thickness reduction reduces lightness and increases a reddish, bluish appearance, and translucency of monolithic zirconia ceramics.     

牙科氧化锆陶瓷的低温时效性能研究

目的 研究时效对5种牙科氧化锆陶瓷结构稳定性的影响.方法 5种氧化锆预烧结块(A组:TZ-3YS;B组:Vita In-Ceram YZ;C组:Ivoclar;D组:Cercon Smart;E组:Kavo)各切取15 mm×15 mm×1.5 mm氧化锆试件18片,常压致密烧结.时效处理如下:置于高温高压消毒炉中进行循环热处理,条件是134℃,0.2 MPa压力,维持时间为0h、1h、2h、3h、4 h、5h.X射线衍射分析(X-ray diffraction,XRD)试件的晶相结构,计算试件表面单斜相(m相)的相对含量.原子力显微镜观察A组和D组时效1、4 h的抛光试件.A组氧化锆再制备25 mm×4 mm×1.2 mm的试件30根,均分为未退火组(时效前)、退火组(时效前)和时效组(时效后),测试试件的三点弯曲强度.结果 除D组试件外,A、B、C、E组试件的m相含量均随时效时间的延长而增加,A组试件的m相含量最高,E组次之,B组和C组相近.XRD未检测到D组试件表面有m相,但使用原子力显微镜可观察到D组试件有m相晶核的成核和生长.时效前后A组试件的三点弯曲强度未衰减,未退火组试件三点弯曲强度[(1301±169)MPa]显著高于退火组[(1120±185)MPa]和时效组[(1158±111)MPa],差异有统计学意义(P＜0.05);而退火组与时效组间差异无统计学意义(P＞0.05).结论 牙科氧化锆陶瓷的低温时效效应具有时间依赖性,但时效效应并未使TZ-3YS的三点弯曲强度降低.     

牙科树脂渗透氧化锆陶瓷材料的制备与性能研究

目的 制备一种牙科树脂渗透氧化锆陶瓷（polymer infiltrated zirconia ceramic network,PICN）材料,检测其机械性能。方法 制备纯氧化锆陶瓷、纯树脂以及4种不同孔隙率的陶瓷支架,实验组的陶瓷支架用树脂进行渗透得到陶瓷树脂复合材料（PICN）。通过三点弯曲强度实验检测材料的弯曲强度和弹性模量,采用单边切口梁法检测断裂韧性,通过纳米压痕系统测得材料的硬度以及用扫描电镜观测材料的显微结构。结果 树脂渗透氧化锆陶瓷（PICN）材料的弯曲强度值为135~266 MPa,弹性模量值为41.3~99.3GPa,断裂韧性值为2.20~4.04 MPam1/2,硬度值为1.93~10.83GPa。扫描电镜显示PICN材料中树脂在陶瓷孔隙内渗透完全。结论 这类树脂渗透氧化锆陶瓷材料的机械性能与人类天然牙釉质和牙本质相似,在口腔修复领域具有良好的应用前景。     

In vitro fatigue tests and in silico finite element analysis of dental implants with different fixture/abutment joint types using computer-aided design models

Purpose

The aim of this study was to evaluate fatigue resistance of dental fixtures with two different fixture-abutment connections by in vitro fatigue testing and in silico three-dimensional finite element analysis (3D FEA) using original computer-aided design (CAD) models.

Wear behavior and microstructural characterization of translucent multilayer zirconia

ObjectiveTo characterize the composition, microstructure and wear properties of a multilayer translucent zirconia relative to the conventional 3Y-TZP.MethodsTwo types of ceramics were evaluated: a multilayer zirconia (MULTI, IPS e.max ZirCAD Multi, Ivoclar Vivadent) and a control 3Y-TZP (IPS e.max ZirCAD LT, Ivoclar Vivadent). Pre-sintered CAD-CAM blocks were cut, ground, sintered and polished to 1 μm finish. The phase fraction and grain size were measured using XRD and FE-SEM. For wear testing (n = 12), square-shaped specimens (16 × 16 × 1 mm) were adhesively bonded to a dentin analog. Sliding wear tests were performed using a spherical zirconia antagonist (r = 3.15 mm), with 30 N load at 1.5 Hz for 500,000 cycles in water. Optical and scanning electron microscopes and 3D laser scanner were used for quantitative wear analyses. Data were analyzed using Student’s t-test (α = 0.05).ResultsFor MULTI, the enamel layer had the highest cubic content and the largest grain size, followed by the two transition layers, and the dentin layer. 3Y-TZP showed the smallest grain size and cubic content. A significant amount of wear was observed in both materials up to 50,000 cycles until it reached a plateau. MULTI showed higher volume loss and greater wear depth than 3Y-TZP (p < 0.01). The higher volume loss was associated with extensive lateral fracture, leading to material spalling from the surface of cubic-containing zirconias.SignificanceThe wear pattern in multi-layered zirconia was more severe than 3Y-TZP. Additionally, the different layers of the multi-layered zirconia had similar wear behavior.     

Microshear bond strength of dual-cure resin cement in zirconia after different cleaning techniques: an in vitro study

PURPOSEThis study aimed to compare the microshear bond strength (µSBS) of dual-cure resin cement in CAD-CAM zirconia after different cleaning techniques.MATERIALS AND METHODSFifty discs of zirconia-based ceramic from Ivoclar Vivadent were embedded in acrylic resin. The discs were divided into five groups according to the cleaning methods used: Group 1: drying with spraying + sandblasting with Al₂O₃; Group 2: washed with water and dried with spraying + sandblasting with Al₂O₃; Group 3: washed with distilled water and dried with spraying + sandblasting with Al₂O₃ + zirconium oxide (Ivoclean); Group 4: washed with distilled water and dried with spraying + sandblasting with Al₂O₃ + potassium hydroxide (Zirclean); and Group 5: washed with distilled water and dried with spraying + sandblasting with Al₂O₃ + 1% NaClO. All of the groups were contaminated with artificial saliva for 1 minute and then cleaned. Statistical analyses were performed using ANOVA and Tukey''s tests. RESULTSThere were statistically significant differences among all groups for µSBS (P < .05). The group treated with zirconium oxide (Group 3) showed the highest µSBS (18.75 ± 0.23 MPa).CONCLUSIONWhen applied to zirconia, the cleaning methods affected the bonding with resin cement differently.     

Influence of thermal expansion mismatch and fatigue loading on phase changes in porcelain veneered Y-TZP zirconia discs

There is a rapidly growing interest for the use of Y-TZP zirconia as core material in veneered all-ceramic prostheses. It was hypothesized that a mismatch in coefficient of thermal expansion between the veneering porcelain and the Y-TZP zirconia core of these prostheses causes transformation of the tetragonal to the monoclinic structure in Y-TZP zirconia at the interface boundary when exposed to fatigue loading, resulting in fracture at the interface boundary. Y-TZP zirconia discs were veneered with three porcelains differing in coefficient of thermal expansion. Finite element analysis was used to investigate the stress distribution in the bi-layered discs because of the mismatch in thermal expansion. Two of these three groups were fatigued with the veneering ceramic in tension. X-ray diffraction was used to measure the intensity of monoclinic and tetragonal zirconia phase present at the zirconia core surface after sintering, airborne abrasion, veneering, and fatigue loading. It was found that the sintered tetragonal structure was converted to monoclinic up to a depth of 27 mum after airborne abrasion, and reversed back to tetragonal after porcelain veneering with porcelain. Fatigue loading of veneered discs did not, even with the highest possible thermal mismatch stress, cause any conversion from tetragonal to monoclinic phase.     

The fracture resistance of a CAD/CAM Resin Nano Ceramic (RNC) and a CAD ceramic at different thicknesses

Objectives

This study aimed to investigate the influence of restoration thickness to the fracture resistance of adhesively bonded Lava™ Ultimate CAD/CAM, a Resin Nano Ceramic (RNC), and IPS e.max CAD ceramic.

Methods

Polished Lava™ Ultimate CAD/CAM (Group L), sandblasted Lava™ Ultimate CAD/CAM (Group LS), and sandblasted IPS e.max CAD (Group ES) discs (n = 8, Ø = 10 mm) with a thickness of respectively 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, and 3.0 mm were cemented to corresponding epoxy supporting discs, achieving a final thickness of 3.5 mm. All the 120 specimens were loaded with a universal testing machine at a crosshead speed of 1 mm/min. The load (N) at failure was recorded as fracture resistance. The stress distribution for 0.5 mm restorative discs of each group was analyzed by Finite Element Analysis (FEA). The results of facture resistances were analyzed by one-way ANOVA and regression.

Results

For the same thickness of testing discs, the fracture resistance of Group L was always significantly lower than the other two groups. The 0.5 mm discs in Group L resulted in the lowest value of 1028 (112) N. There was no significant difference between Group LS and Group ES when the restoration thickness ranged between 1.0 mm and 2.0 mm. There was a linear relation between fracture resistance and restoration thickness in Group L (R = 0.621, P < 0.001) and in Group ES (R = 0.854, P < 0.001). FEA showed a compressive permanent damage in all groups.

Significance

The materials tested in this in vitro study with the thickness above 0.5 mm could afford the normal bite force. When Lava Ultimate CAD/CAM is used, sandblasting is suggested to get a better bonding.     

Effect of high-speed sintering on the flexural strength of hydrothermal and thermo-mechanically aged zirconia materials

ObjectiveTo investigate the influence of high-speed and conventional sintering on the flexural strength (FS) of three zirconia materials initial and after artificial aging.MethodsMilled zirconia specimens (3Y-TZP: ZI and Zolid; 4Y-TZP: Zolid HT+; Amann Girrbach AG; N = 288, n = 96/group) were sintered in a high-speed sintering protocol (final temperature 1580 °C, n = 48/subgroup) or a conventional sintering protocol (control group, final temperature 1450 °C, n = 48/subgroup). FS was tested initially and after artificial aging (10 h in an autoclave or 1,200,000 chewing cycles; n = 16/subgroup). Univariate ANOVAs, post-hoc Scheffé, partial eta-squared, Kolmogorov–Smirnov-, Kruskal–Wallis- and Mann–Whitney-U-test were performed (p < 0.05).ResultsZI showed the highest and HT+ the lowest FS, regardless of the sintering protocols and aging regimens (p < 0.001). High-speed sintered HT+ showed higher initial FS than the control group (p < 0.001). ZI (p < 0.001–0.004) and Zolid (p < 0.001–0.007) showed higher FS after thermo-mechanical aging. High-speed sintered HT+ showed higher FS in the initial stage (p < 0.001). The Weibull modulus of the three thermo-mechanically aged materials was negatively influenced by high-speed sintering.SignificanceAs shorter sintering times represent a cost and time efficient alternative, high-speed sintering is a valid alternative to conventional sintering protocols.     

Curing efficiency of various resin-based materials polymerized through different ceramic thicknesses and curing time

PURPOSE

The aim of this in vitro study was to examine the curing efficiency of various resin-based materials polymerized through ceramic restorations with 3 different thicknesses. Curing efficiency was evaluated by determining the surface microhardness (VHN) of the resin specimens.

MATERIALS AND METHODS

Four kinds of resin materials were used. Z350 (3M ESPE Filtek™ Z350: A2 Shade), Z250 (3M ESPE Filtek™ Z250: A2 Shade) and Variolink® II (VL: Ivoclar vivadent, base: transparent) either with or without a self-curing catalyst (VLC: Ivoclar vivadent, catalyst: low viscosity/transparent) were filled into the silicone mold (10 mm diameter, 1 mm thick). They were cured through ceramic discs (IPS e.max Press MO-0 ingot ivoclar vivadent, 10 mm diameter, 0.5, 1 and 2 mm thicknesses) by LED light-curing units for 20 and 40 seconds. Vicker''s microhardness numbers (VHNs) were measured on the bottom surfaces by a microhardness tester. Data were analyzed using a 3- way analysis of variance (ANOVA) at a significance level of 0.05.

RESULTS

The thickness of ceramic disc increased, the VHNs of all four resin types were decreased (P<.05). The mean VHN values of the resins light cured for 40 seconds were significantly higher than that of LED for 20 seconds in all four resin materials (P<.05). VLC showed significantly higher VHN values than VL regardless of other conditions (P<.05). Z350 and Z250 showed higher values than VL or VLC (P<.01).

CONCLUSION

Thinner ceramic disc with increased curing time resulted higher VHN values of all resin materials. The use of a catalyst produced a greater hardness with all polymerization methods. Restorative resin materials (Z350, Z250) showed higher VHN values than resin cement materials (VL, VLC).     

陶瓷厚度与基牙底色对4种常用CAD/CAM瓷修复体颜色的影响

目的 研究4种不同厚度和半透明度CAD/CAM陶瓷在不同基牙底色上的遮色效果，为临床的陶瓷材料选择提供参考。方法 4种CAD/CAM陶瓷分别是：IPS e.max CAD（硅酸锂玻璃）, VITA SUPRINITYPC（氧化锆增强玻璃）, VITABLOCS MarkⅡ（长石基玻璃）和Vita Enamic（聚合物渗透陶瓷），制作成不同厚度(1 mm、1.5 mm、2 mm)的瓷块试样，在7种基牙底色(A1、A2、ND7色调复合树脂、白色(White)、黑色(Black)、钴铬合金(CC)、贵金属合金(PMA)下使用分光光度计检测Lab值，同一种瓷块2 mm厚度在A2色上的表现作为对照组，横向比较各种瓷块在不同基牙底色上的色差值以及半透明度。结果 ΔL、Δa、Δb值随着陶瓷厚度的增加逐渐减小，且在4种陶瓷间存在明显差异；半透明性参数均值按半透明度递减顺序依次为长石基玻璃>硅酸锂玻璃>氧化锆增强玻璃>聚合物渗透陶瓷；其中半透明度越大，其陶瓷的遮色效果反而越差；当厚度达到2 mm时，4种陶瓷在各种基牙底色下的ΔE（临床可接受颜色阈值）均<3.7;4种陶瓷中ΔE与厚...     

Evaluation of surface treatments of monolithic zirconia in different sintering stages

Purpose

This study evaluated the effect of surface treatment of zirconia in pre-sintered and post-sintered stages on its surface roughness, phase transformation, and biaxial flexural strength (BFS).

Influence of surface treatments on the surface properties of different zirconia cores and adhesion of zirconia-veneering ceramic systems

Objective

The aim of this study was to assess the influence of surface treatments on the surface characteristics of different zirconia cores and the adhesion of the zirconia-veneering ceramic systems by means of strain energy release rate (G-value, J/m²).

Methods

Three types of zirconia cores (NANOZR (NZ), Vita In-Ceram YZ (VZ), and IPS e.max ZirCAD (IZ)) were used. The specimens were divided into four groups in each test according to the surface treatment used; Gr 1 (control; no treatment), Gr 2 (sandblasted), Gr 3 (CH₂Cl₂ for 60 min), and Gr 4 (experimental hot etching solution for 60 min). AFM, SEM, EDS, and XRD were carried out. Two types of veneering ceramics (Vita VM9 (V9) and IPS e.max Ceram (IC)) were used for testing the adhesion. The G-value (J/m²) was measured with a four-point bending configuration. Following fracture testing specimens were examined with SEM. Data were analyzed using ANOVA and Tukey's test.

Results

NZ treated with the experimental hot etching solution showed the highest R_a values (206.06 ± 9.98 nm) compared with the other groups (P < 0.05). The greatest amount of monoclinic phase is measured after sandblasting (19.59%) for NZ, followed by VZ (9.3%) and IZ (6.6%). The NZ/V9 (etching for 60 min) group showed the highest G-value (36.02 ± 2.80 J/m²) among the groups. Mode of failure was mostly cohesive failure within all the bonded veneering ceramic systems.

Significance

The experimental hot etching solution could be considered as alternative treatment modality to sandblasting for zirconia cores to avoid phase transition at the surface from tetragonal to monoclinic that may be detrimental for the longevity of the zirconia-veneering ceramic restoration.     

Biomechanical behavior of CAD/CAM cobalt-chromium and zirconia full-arch fixed prostheses

PURPOSETo verify the influence of computer-aided design/computer-aided manufacturing (CAD/CAM) implant-supported prostheses manufactured with cobalt-chromium (Co-Cr) and zirconia (Zr), and whether ceramic application, spark erosion, and simulation of masticatory cycles modify biomechanical parameters (marginal fit, screw-loosening torque, and strain) on the implant-supported system.MATERIALS AND METHODSTen full-arch fixed frameworks were manufactured by a CAD/CAM milling system with Co-Cr and Zr (n=5/group). The marginal fit between the abutment and frameworks was measured as stated by single-screw test. Screw-loosening torque evaluated screw stability, and strain analysis was explored on the implant-supported system. All analyses were performed at 3 distinct times: after framework manufacturing; after ceramic application in both materials'' frameworks; and after the spark erosion in Co-Cr frameworks. Afterward, stability analysis was re-evaluated after 10⁶ mechanical cycles (2 Hz/150-N) for both materials. Statistical analyses were performed by Kruskal-Wallis and Dunn tests (α=.05).RESULTSNo difference between the two materials was found for marginal fit, screw-loosening torque, and strain after framework manufacturing (P>.05). Ceramic application did not affect the variables (P>.05). Spark erosion optimized marginal fit and strain medians for Co-Cr frameworks (P<.05). Screw-loosening torque was significantly reduced by masticatory simulation (P<.05) regardless of the framework materials.CONCLUSIONCo-Cr and Zr frameworks presented similar biomechanical behavior. Ceramic application had no effect on the biomechanical behavior of either material. Spark erosion was an effective technique to improve Co-Cr biomechanical behavior on the implant-supported system. Screw-loosening torque was reduced for both materials after masticatory simulation.     

Effects of core characters and veneering technique on biaxial flexural strength in porcelain fused to metal and porcelain veneered zirconia

PURPOSE

The purpose of this study was to assess the impact of the core materials, thickness and fabrication methods of veneering porcelain on prosthesis fracture in the porcelain fused to metal and the porcelain veneered zirconia.

MATERIALS AND METHODS

Forty nickel-chrome alloy cores and 40 zirconia cores were made. Half of each core group was 0.5 mm-in thickness and the other half was 1.0 mm-in thickness. Thus, there were four groups with 20 cores/group. Each group was divided into two subgroups with two different veneering methods (conventional powder/liquid layering technique and the heat-pressing technique). Tensile strength was measured using the biaxial flexural strength test based on the ISO standard 6872:2008 and Weibull analysis was conducted. Factors influencing fracture strength were analyzed through three-way ANOVA (α≤.05) and the influence of core thickness and veneering method in each core materials was assessed using two-way ANOVA (α≤.05).

RESULTS

The biaxial flexural strength test showed that the fabrication method of veneering porcelain has the largest impact on the fracture strength followed by the core thickness and the core material. In the metal groups, both the core thickness and the fabrication method of the veneering porcelain significantly influenced on the fracture strength, while only the fabrication method affected the fracture strength in the zirconia groups.

CONCLUSION

The fabrication method is more influential to the strength of a prosthesis compared to the core character determined by material and thickness of the core.     

Reliability and aging behavior of three different zirconia grades used for monolithic four-unit fixed dental prostheses

ObjectiveTo investigate the fracture resistance and phase composition of tooth supported four-unit fixed dental prostheses (FDPs) made from three different zirconia grades after loading and aging.MethodsSeventy-two FDPs were fabricated from 3Y-TZP, 4Y-PSZ and 5Y-PSZ. This resulted in 24 FDPs per grade, subdivided into three groups (n = 8): a control group (C), a hydrothermally aged (H₂O, 85 °C, 90 days) group (A) and a group subjected to loading (2.5M cycles, 98N) with simultaneous thermal cycling (H₂O, 5–55 °C) subsequent to treatment A (AL). Subsequently, FDPs were statically loaded to fracture. Phase composition was quantified by X-ray diffraction (XRD) and μ-Raman spectroscopy. Focused ion beam (FIB) - Scanning electron microscopy (SEM) was used for visualization in-depth.ResultsCompared to 3Y-C FDPs (1233 ± 165N), reduced fracture load was found for 5Y-C FDPs (889 ± 80 N; p < .001). This did not apply for 4Y-C samples (1065 ± 111N). Treatments (A, AL) did not negatively affect the fracture load for the three grades of zirconia (p > .645). Both A and AL increased monoclinic phase content for 3Y and 4Y FDPs, whereas FIB-SEM suggests no transformability of 5Y-PSZ. 5Y-AL FDPs showed cracks and fractures at the abutment walls and restoration margins after dynamic loading. Reduced fracture load of 5Y samples as compared to 3Y and 4Y was associated with deficient transformability in the fracture zone.SignificanceAging and loading did not negatively affect the fracture resistance of monolithic four-unit FDPs made from three grades of zirconia. Due to cracks after dynamic loading, 5Y-PSZ cannot be recommended for the clinical application of four-unit FDPs.     

Contributions of stress corrosion and cyclic fatigue to subcritical crack growth in a dental glass-ceramic

ObjectiveThe objective of this study was to test the following hypotheses: (1) both cyclic degradation and stress-corrosion mechanisms result in subcritical crack growth (SCG) in a fluorapatite glass-ceramic (IPS e.max ZirPress, Ivoclar-Vivadent) and (2) there is an interactive effect of stress corrosion and cyclic fatigue to accelerate subcritical crack growth.MethodsRectangular beam specimens were fabricated using the lost-wax process. Two groups of specimens (N = 30/group) with polished (15 μm) or air-abraded surface were tested under rapid monotonic loading. Additional polished specimens were subjected to cyclic loading at two frequencies, 2 Hz (N = 44) and 10 Hz (N = 36), and at various stress amplitudes. All tests were performed using a fully articulated four-point flexure fixture in deionized water at 37 °C. The SCG parameters were determined using the ratio of inert strength Weibull modulus to lifetime Weibull modulus. A general log-linear model was fit to the fatigue lifetime data including time to failure, frequency, peak stress, and the product of frequency and logarithm of stress in ALTA PRO software.ResultsSCG parameters determined were n = 21.7 and A = 4.99 × 10⁻⁵ for 2 Hz, and n = 19.1 and A = 7.39 × 10⁻⁶ for 10 Hz. After fitting the general log-linear model to cyclic fatigue data, the coefficients of the frequency term (α₁), the stress term (α₂), and the interaction term (α₃) had estimates and 95% confidence intervals of α₁ = −3.16 (−15.1, 6.30), α₂ = −21.2 (−34.9, −9.73), and α₃ = 0.820 (−1.59, 4.02). Only α₂ was significantly different from zero.Significance(1) Cyclic fatigue does not have a significant effect on SCG in the fluorapatite glass-ceramic evaluated and (2) there was no interactive effect between cyclic degradation and stress corrosion for this material.     

Mechanical behavior and reinforcement mechanism of nanoparticle cluster fillers in dental resin composites: Simulation and experimental study

ObjectivesIn dental resin composites (DRCs), the structure of fillers has a great impact on the mechanical behavior. The purpose of this study is to gain an in-depth understanding of the reinforcement mechanism and mechanical behavior of DRCs with nanoparticle clusters (NCs) fillers, thereby providing a guidance for the optimal design of filler structures for DRCs.MethodsThis work pioneers the use of discrete element method (DEM) simulations combined with experiments to study the mechanical behavior and reinforcement mechanism of DRCs with NCs fillers.ResultsThe uniaxial compressive strength (UCS) of NCs-reinforced DRCs have an improvement of 9.58 % and 15.02 % in comparison with nanoparticles (NPs) and microparticles (MPs), respectively, because of the ability of NCs to deflect cracks and absorb stress through gradual fracturing. By using NCs and NPs as co-fillers, the internal defects of DRCs can be reduced, resulting in a further improvement of UCS of DRCs by 6.21 %. Furthermore, the mechanical properties of DRCs can be effectively improved by increasing the strength of NCs or reducing the size of NCs.SignificanceThis study deepens the understanding of relationship between filler structure and mechanical behavior in DRCs at the mesoscale and provides an avenue for the application of DEM simulations in composite materials.     

Residual stresses explaining clinical fractures of bilayer zirconia and lithium disilicate crowns: A VFEM study

ObjectiveTo understand the stress development in porcelain-veneered zirconia (PVZ) and porcelain-veneered lithium disilicate (PVLD) crowns with different veneer/core thickness ratios and cooling rates. To provide design guidelines for better performing bilayer restorations with the aid of Viscoelastic Finite Element Method (VFEM).MethodsThe VFEM was validated by comparing the predicted residual stresses with experimental measurements. Then, the model was used to predict transient and residual stresses in the two bilayer systems. Models with two different veneer/core thickness ratios were prepared (2:1 and 1:1) and two cooling protocols were simulated (Fast: ∼300 °C/min, Slow: ∼30 °C/min) using the heat transfer module, followed by stress analysis in ABAQUS. The physical properties of zirconia, lithium disilicate, and the porcelains used for the simulations were determined as a function of temperature.ResultsPVLD showed lower residual stresses than PVZ. The maximum tensile stresses in PVZ were observed in the cusp area, whereas those in PVLD were located in the central fossa. The 1:1 thickness ratio decreased stresses in both layers of PVZ. Slow cooling slightly decreased residual stresses in both systems. However, the cooling rate effect was more evident in transient stresses.SignificanceSlow cooling is preferable for both systems. A thinner porcelain layer over zirconia lowers stresses throughout the restoration. The different stress distributions between PVZ and PVLD may affect their failure modes. Smaller mismatches in modulus, CTE, and specific heat between the constituents, and the use of low T_g porcelains can effectively reduce the deleterious transient and residual tensile stresses in bilayer restorations.