Flexural strength of a layered zirconia and porcelain dental all-ceramic system

Color- and strength-graded zirconia: Strength, light transmission, and composition

STATEMENT OF PROBLEM

Natural teeth are graded in terms of translucency and strength. Graded zirconia materials are now available with a higher yttria content on the top or in the enamel zone to increase surface translucency and a lower yttria content on the bottom or in the dentin zone to increase strength. Such materials could provide multiple advantages over uniform materials and reduce the need for porcelain veneering in anterior artificial crowns; however, studies on the properties of these materials are lacking.

PURPOSE

The purpose of this in vitro study was to measure and compare the biaxial flexural strength, percentage light transmission, elemental content, and phase content of zones within and among color graded zirconia blocks and color- and strength-graded zirconia blocks.

MATERIAL AND METHODS

Disks of a color graded material (Katana STML) and a strength- and color-graded material (IPS e.max ZirCAD Prime) were fabricated (Zircom Furnace). Biaxial flexural strength was measured using a piston-on-3-ball test in a universal testing machine (n=10). Absolute light transmission was measured with a spectrophotometer. Elemental compositions were quantified for 3 zones in each of the 2 zirconia materials using X-ray fluorescence analysis. Zirconia phase fractions were quantified for 3 zones using X-ray diffraction analysis (XRD). Where appropriate, 2-way ANOVA, 1-way ANOVA, and Tukey multiple pairwise comparison testing were used to determine which of the 6 zones differed from one another (α=.05).

RESULTS

The color-graded zirconia exhibited gradients in light transmission and differences in phase content in the 3 zones measured. The color- and strength-graded zirconia exhibited gradients in light transmission, biaxial flexural strength, elemental composition, and phase content in the 3 zones measured. The bottom, dentin, or intaglio layer of a strength graded zirconia material was substantially stronger than all other zones of either material (P<.05). The top, enamel, zones of both materials possessed high light transmissibility (P<.05). The 2 materials differed with respect to biaxial strength (P<.001), light transmission (P<.02), elemental composition, and phase composition overall, as well as in most zone-by zone comparisons (P<.05). The performance and composition of the color graded material was consistent with it being a 5Y material throughout. The performance and composition of the color and strength graded material was consistent with it having a 3Y bottom zone and a 5Y top zone.

CONCLUSIONS

A strength-graded and color-graded zirconia material offers potential advantages in both strength and translucency.

Development of Y-TZP/MWCNT-SiO2 nanocomposite for dental protheses

Y-TZP/MWCNT-SiO2 nanocomposite was synthesized by co-precipitation and hydrothermal treatment methods. After the characterization of the MWCNT-SiO2 powder, specimens were obtained from the synthesized material Y-TZP/MWCNT-SiO2 by uniaxial pressing for a second characterization and later comparison of its optical and mechanical properties with the conventional Y-TZP. The MWCNT-SiO2 was presented in bundles of carbon nanotubes coated by silica (mean length: 5.10 ± 1.34 μm /D90: 6.9 μm). The composite manufactured was opaque (contrast ratio: 0.9929 ± 0.0012) and had a white color with a slightly difference from the conventional Y-TZP (ΔE00: 4.4 ± 2.2) color. The mechanical properties of Y-TZP/MWCNT-SiO2: vickers hardness (10.14 ± 1.27 GPa; p = 0.25) and fracture toughness (4.98 ± 0.30 MPa m1/2; p = 0.39), showed no significant difference from the conventional Y-TZP (hardness: 8.87 ± 0.89; fracture toughness: 4.98 ± 0.30 MPa m1/2). However, for flexural strength (p = 0.003), a lower value was obtained for Y-TZP/MWCNT-SiO2 (299.4 ± 30.5 MPa) when compared to the control Y-TZP (623.7 ± 108.8 MPa). The manufactured Y-TZP/MWCNT-SiO2 composite presented satisfactory optical properties, however the co-precipitation and hydrothermal treatment methods need to be optimized to avoid the formation of porosities and strong agglomerates, both from Y-TZP particles and MWCNT-SiO2 bundles, which lead to a significant decrease in the material flexural strength.

Effect of veneering material type and thickness ratio on flexural strength of bi-layered PEEK restorations before and after thermal cycling

OBJECTIVE

The purpose of this study was evaluating the biaxial strength of bi-layered PEEK restorations before and after aging using different veneering materials in different thickness ratios.

MATERIAL AND METHODS

Ninety specimens of thickness 1.5 mm were divided into three groups according to their veneering material. Group (CAD LD): BioHPP discs veneered with CAD milled lithium disilicate (n=30), group (CAD C): BioHPP discs veneered with CAD milled composite (n=30), and group (LC): BioHPP discs veneered with conventionally layered composite (n=30). Each group was subdivided into 3 subgroups (n=10) according to the different thickness ratios between the core and the veneering material (TC:TV). Subgroup 1: TC:TV=1:0.5, subgroup 2: TC:TV=0.7:0.8, and subgroup 3: TC:TV=0.5:1. Half of the specimens of each subgroup were subjected to thermocycling, and the bi-axial flexural strength of all specimens was tested before and after aging. Three-way ANOVA followed by Bonferroni's post hoc test were used for data analysis. The significance level was set at P ≤ 0.05.

RESULTS

Material, thickness ratio, and aging all had a significant effect on biaxial flexural strength. (LC) group had the highest biaxial flexural strength. TC:TV=0.5:1 showed the lowest biaxial flexural strength. All groups showed significant decrease in biaxial flexural strength after aging.

CONCLUSIONS

Veneering material for PEEK together with the thickness ratio between the core and veneering material greatly affect the flexural strength of bi-layered restorations. Thermocycling negatively impacts the flexural strength of PEEK bi-layered restorations.

CLINICAL SIGNIFICANCE

According to the results of that study, PEEK cores are best veneered with conventionally layered composite with core to veneering thickness ratio being 1:0.5.

Clinical evaluation of marginal fit of uncemented CAD-CAM monolithic zirconia three-unit restorations in anterior areas, using scannable and conventional polyvinyl siloxane impression materials

BACKGROUND

The accuracy of impression techniques determines the marginal fit of fixed prostheses. Marginal accuracy plays a main role in the success and failure of treatments. This in-vivo study evaluated the marginal fit of anterior three-unit monolithic zirconia fixed partial dentures (FPDs) using conventional and scannable polyvinyl siloxane impression materials.

METHODS

Ten patients were selected to replace the lateral teeth with a three-unit monolithic zirconia bridge. For each patient, in the first group, an impression was made with a two-step putty-wash technique using scannable polyvinyl siloxane material (BONASCAN; DMP, Greece). In the identical session, as the second group, an impression of conventional putty-wash polyvinyl siloxane was taken (BONASIL A⁺ Putty; DMP, Greece). The marginal discrepancy was measured through the replicas, which were cut perpendicularly within the buccolingual and mesiodistal directions. An Independent t-test was employed for data analyses (P < 0.05).

RESULTS

The marginal discrepancy in a conventional method for central abutment in mid-buccal, mid-lingual, mid-mesial, and mid-distal was higher than in the scannable method but it was not significant (P > 0.05). Also, the marginal discrepancy for canine abutment in the conventional method was higher than in the scannable method, but it was not significant, either (P > 0.05).

CONCLUSIONS

FPDs fabricated from both scannable and conventional impression materials were not superior to each other in marginal fit for both central and canine abutments by evaluation using the replica technique.

Finite Element Analysis of an Implant-Supported FDP with Different Connector Heights

All-ceramic fixed dental prostheses (FDPs) tend to fracture in the connector areas, due to the concentration of tensile stresses. This study aimed to evaluate the role of connector height on the stress distribution of a posterior three-unit implant-supported all-ceramic FDP using finite element analysis (FEA). Two titanium dental implants, their abutments, screws, and a three-unit all-ceramic FDP were scanned using a micro-CT scanner. Three 3D models with altered distal connector heights (3, 4, and 5 mm) were generated and analyzed on ABAQUS FEA software. The maximum principal stress values in MPa observed for each model with different connector heights and their respective locations (MA = mesial abutment; DA = distal abutment; F = framework; V = veneer) were: 3 mm—219 (MA), 88 (DA), 11 (F), 16 (V); 4 mm—194 (MA), 82 (DA), 8 (F), 18 (V); 5 mm—194 (MA), 80 (DA), 8 (F), and 18 (V). All the assembled models demonstrated the peak stresses at the neck area on the mesial abutments. The connector height had a significant influence on the stress distribution of the prosthesis. The models with higher distal connectors (4 and 5 mm) had a lower and more uniform distribution of maximum principal stresses (except for the veneer layer) when compared with the model with the smallest distal connector.

In-lab simulation of CAD/CAM grinding and intaglio surface treatments of 4YSZ monolithic restorations: Effect on its load-bearing capacity under fatigue

OBJECTIVES

To evaluate the effect of in-lab simulation of CAD/CAM grinding and intaglio surface treatments on the surface characteristics (topography and roughness) and fatigue behavior of adhesively luted 4YSZ simplified restorations.

METHODS

Ceramic discs (Ø = 10 mm, thickness = 1 mm) were randomly allocated into 6 groups considering: "In-lab simulation of CAD/CAM grinding" (ground or polished) and "intaglio surface treatments": Ctrl (without surface treatment), AlOx (aluminum oxide air abrasion) or GLZ (glaze spray application). The surface roughness of all samples was measured, the treated discs received a ceramic primer, were luted with resin cement onto a dentin analogue material (woven glass-reinforced epoxy resin) and tested under a cyclic fatigue test (step-stress approach, n = 15; 1.4 Hz, 10,000 cycles/step, step-size of 100N starting at 200N until failure). A complementary analysis was performed to corroborate the findings in the fatigue test that the glaze fill defects increase the mechanical properties of the ceramic. To do so, bars (n= 10; 1.0 × 1.0 × 12 mm; considering the groups: N-ID: non-indented; ID: indented; ID-GLZ: indented plus glaze spray application) were indented in a vickers hardness tester to produce a crack pattern, treated with glaze or not, and then submitted to flexural strength tests (FS). Fractographic and topographic analysis were performed.

RESULTS

In-lab simulation of CAD/CAM grinding decreased the fatigue failure load of the 4YSZ ceramic when comparing polished and ground groups, regardless of surface treatment. GLZ induced better fatigue performance compared to the air abrasion, regardless of the grinding condition (ground or polished surface). The results of the flexural strength test corroborated the findings in the fatigue test, as the ID-GLZ group presented superior FS than the ID group, however both had inferior FS than N-ID. There is an inverse association between roughness and fatigue failure load, as the higher the surface roughness, the lower the fatigue failure load. Failures in the fatigue and flexural strength tests started from the face subjected to tensile stresses.

CONCLUSION

In-lab simulation of CAD/CAM grinding had a detrimental effect on the fatigue behavior of 4YSZ and glaze spray induced better 4YSZ performance compared to the air abrasion. The intaglio surface treatments differently influenced the 4YSZ fatigue performance, however, only glaze spray can reverse the damage caused by the grinding.

Effect of pigmentation techniques on the fatigue mechanical behavior of a translucent zirconia for monolithic restorations

This study aims to characterize the effect of pre- and/or post-sintering pigmentation techniques on the fatigue behavior of a 4YSZ ceramic (4 mol% yttrium stabilized zirconia). First, 4YSZ ceramic discs (IPS e.max ZirCAD, 15 mm diameter, and 1.2 mm thickness) were obtained and allocated according to the 'pigmentation technique' factor into 5 groups: Ctrl - unshaded ceramic disc (IPS e.max ZirCAD BL); Manuf - ceramic discs shaded by the manufacturer at block formation (IPS e.max ZirCAD MT A2); Brush - unshaded ceramic disc pigmented in the pre-sintered stage using a brush; Stain - unshaded ceramic disc pigmented in the post-sintering stage using a stain glaze (shade A2); Brush + Stain - combination of Brush and Stain techniques. The specimens were sintered and analyzed by a spectrophotometer (VITA Easyshade) for color differences (ΔE₀₀ calculated according to CIEDE, 2000) to ensure that they present the same perceived color (ΔE₀₀ ≤ 1.77, acceptability threshold). The specimens (n = 15) were submitted to biaxial flexural fatigue testing using a cyclic fatigue method (frequency of 20 Hz; 10,000 cycles per step; initial stress of 200 MPa; and step-size of 25 MPa) until specimen fracture. Fatigue strength (FS) and number of cycles until failure (CFF) were recorded with statistical purposes. Fractographic, complementary topography, elemental and roughness analyses were performed. The Ctrl shown the highest FS, CFF, and survival rates compared to the other conditions (p < 0.05), being only statistically similar to Brush. Stain showed the lowest FS, CFF and survival rates (p < 0.05). The Manuf and Brush + Stain shown intermediary performance. In regards of Weibull moduli, Brush shown the lowest values (lowest structural reliability), for both FS and CFF, being only statistically similar to Manuf. Topography analysis shown a uniform size of zirconia crystals in Ctrl, Manuf and Brush. In addition, a smooth glassy surface with some spots of impregnated pigment was observed in the groups where glaze was applied (Stain and Brush + Stain). This finding was corroborated by elemental analysis. Despite that, Stain and Brush + Stain presented the roughest surface (p < 0.05). In summary, the pigmentation techniques used to provide a Vita classical A2 color shade have a detrimental effect on the mechanical fatigue properties of a 4YSZ ceramic, except for the technique performed with the brush in the block pre-sintering stage. Despite this, the brush technique presents great variability in mechanical performance, resulting in lower structural reliability.

Comparative analysis on intaglio surface trueness, wear volume loss of antagonist, and fracture resistance of full-contour monolithic zirconia crown for single-visit dentistry under simulated mastication

PURPOSE

This analysis aimed to evaluate the intaglio surface trueness, antagonist’s wear volume loss, and fracture resistance of full-contour crowns of (Y, Nb)-stabilized fully-sintered zirconia (FSZ), 4 mol% or 5 mol% yttria-stabilized partially sintered zirconia (4YZ or 5YZ) with high-speed sintering.

MATERIALS AND METHODS

A total of 42 zirconia crowns were separated into three groups: FSZ, 4YZ, and 5YZ (n = 14). The intaglio surface trueness of the crowns was evaluated at the inner surface, occlusal, margin, and axial areas and reported as root-mean-square, positive and negative average deviation. Half of the specimens were aged for 120,000 cycles in the chewing simulator, and the wear volume loss of antagonist was measured. Before and after chewing, the fracture load was measured for each group. The trueness values were analyzed with Welch's ANOVA, and the wear volume loss with the Kruskal-Wallis tests. Effect of the zirconia type and aging on fracture resistance of crowns was tested using two-way ANOVA.

RESULTS

The intaglio surface trueness measured at four different areas of the crown was less than 50 µm, regardless of the type of zirconia. No significant P in wear volume loss of antagonists were detected among the groups (P > .05). Both the type of zirconia and aging showed statistically significant effects on fracture resistance (P < .05).

CONCLUSION

The full-contour crowns of FSZ as well as 4YZ or 5YZ with high-speed sintering were clinically acceptable, in terms of intaglio surface trueness, antagonist’s wear volume loss, and fracture resistance after simulated mastication.

On the behaviour of zirconia-based dental materials: A review

Zirconia-based dental materials are extensively used in clinical practice due to their tooth-like appearance, biofunctionality, biocompatibility, and affordability. However, premature clinical failures of veneering porcelains raise a concern about their integrity. Extensive studies have been performed over a decade to resolve this issue, but it is challenging to reference all information effectively. A single source identifying the significance of potential parameters on material performance has not previously been available. An evidence-based meta-narrative review technique was used to review the characteristic parameters that can affect the overall behaviour of zirconia-based materials. Keywords were chosen to assess manuscripts based on scientific coherence with this paper's research objective. Online keyword searches were carried out on ScienceDirect, PubMed, and SAGE databases for relevant published manuscripts from year 1985-2020.261 out of 3170 identified manuscripts were included. A total of 10 parameters were identified and classified into the material, manufacturing, and geometric aspects. The effect of every parameter was reviewed on the performance of the material. A discrepancy in findings was observed and is attributed to the fact that there is no standard methodology. This review acts as a single source that summarizes various parameters' contribution to zirconia-based dental materials' performance. This review facilitates manufacturing improvements by accounting for every parameter's effect on overall performance.

Biomechanical evaluation of 3-unit fixed partial dentures on monotype and two-piece zirconia dental implants

This study aimed to evaluate the biomechanical behavior, stress distributions and bone microstrain of fixed partial dentures (FPD) with ceramic abutments supported on monotype zirconia implants, titanium implants and two-piece zirconia implants, using finite element analysis. A three-dimensional model of the jaw was simulated containing 1.0 mm thick cortical bone and cancellous bone tissue. A FPD and implant models (4.1 x 10 mm) were modeled containing a cement-retained implant abutment. These models were replicated in three groups with similar geometries: Titanium Implant and Zirconia Abutment (Ti-Zr); Zirconia Implant and Zirconia Abutment (Zr-Zr) and Monotype Zirconia Implant (Zr-S). An axial load of 300 N was applied to the center of the first premolar. The microstrain (με) and the Von-Mises stress (MPa) were assumed as failures criteria. For the three groups, a higher stress concentration was observed in the region of FPD connectors. The Ti-Zr group showed a higher stress concentration in the prosthesis and implant when compared to the other groups. However, the smaller elastic modulus of the titanium implant, in relation to the zirconia, provided a lower stress in the abutment and in the prosthetic screw. The monotype implant system allowed a more homogeneous stress distribution and its strain were predominantly located in the cervical region of the peri-implant bone tissue. Monotype or two-piece zirconia implants can be used for rehabilitation with FPD. However, the absence of separation between implant and abutment in the monotype system avoids the stress concentration in the prosthetic screw and reduced the peri-implant bone strain.

3D-Printed Polymer-Infiltrated Ceramic Network with Biocompatible Adhesive to Potentiate Dental Implant Applications

The aim of this work was to prepare and characterize polymer-ceramic composite material for dental applications, which must resist fracture and wear under extreme forces. It must also be compatible with the hostile environment of the oral cavity. The most common restorative and biocompatible copolymer, 2,2-bis(p-(2'-2-hydroxy-3'-methacryloxypropoxy)phenyl)propane and triethyleneglycol dimethacrylate, was combined with 3D-printed yttria-stabilized tetragonal zirconia scaffolds with a 50% infill. The proper scaffold deposition and morphology of samples with 50% zirconia infill were studied by means of X-ray computed microtomography and scanning electron microscopy. Samples that were infiltrated with copolymer were observed under compression stress, and the structure's failure was recorded using an Infrared Vic 2DTM camera, in comparison with empty scaffolds. The biocompatibility of the composite material was ascertained with an MG-63 cell viability assay. The microtomography proves the homogeneous distribution of pores throughout the whole sample, whereas the presence of the biocompatible copolymer among the ceramic filaments, referred to as a polymer-infiltrated ceramic network (PICN), results in a safety "damper", preventing crack propagation and securing the desired material flexibility, as observed by an infrared camera in real time. The study represents a challenge for future dental implant applications, demonstrating that it is possible to combine the fast robocasting of ceramic paste and covalent bonding of polymer adhesive for hybrid material stabilization.

Influence of yttria content and surface treatment on the strength of translucent zirconia materials

STATEMENT OF PROBLEM

Newly developed translucent zirconia materials have been used for anterior monolithic complete coverage restorations. Surface treatments can improve adhesion, as well as decrease or increase the strength of ceramics. However, information on the influence of surface treatments on the strength of translucent zirconias is sparse.

PURPOSE

The purpose of this in vitro study was to measure and characterize the effects of different surface treatments, including airborne-particle abrasion, on the strength of different translucent 4 mol% and 5 mol% yttria-stabilized zirconia materials.

MATERIALS AND METHODS

Disks (N=160) made from 4 types of translucent yttria-stabilized zirconia materials were surface-treated in 4 ways: Control groups were hand-polished with 2000-grit silicon carbide abrasive paper; as-machined; glass bead airborne-particle abraded; and alumina airborne-particle abraded. The biaxial flexural strength was measured by using a piston-on-3-ball test in a universal testing machine. The simple main effects of material type and surface treatment and their interaction on biaxial flexural strength were evaluated with 2-way ANOVA (α=.05). A priori, 1-way ANOVA and the Tukey multiple comparisons tests were used within material and treatment types (α=.05). Surface morphology was assessed by using scanning electron microscopy. Translucency, absolute transmittance, was measured by using a spectrophotometer.

RESULTS

Two-way ANOVA revealed that the effects of zirconia type, surface treatment, and their interaction all significantly affected biaxial flexural strength (P<.001). One-way ANOVA revealed that the 4Y material was stronger than all 5Y materials, regardless of surface treatment; all 5Y materials were ranked from strongest to weakest as polished; as-machined, or glass bead abraded; and alumina abraded. The 4Y material was stronger when alumina abraded than when glass bead abraded. Scanning electron microscopy showed that as-polished surfaces were smoother than all others; as-machined and glass bead abraded surfaces displayed little difference; alumina abraded was the roughest; and differences among materials were not discerned. The 1-way ANOVA and multiple comparisons testing showed that the 4Y material had less absolute transmittance, approximately 5% less, than all the 5Y materials.

CONCLUSIONS

Zirconia material type and surface treatment influenced the strength of translucent zirconia materials; a 4 mol% zirconia material was stronger than 5 mol% zirconia materials for all surface treatments tested; airborne-particle abrasion using alumina had a slight strengthening effect on a 4 mol% zirconia but had a weakening effect on 5 mol% materials; airborne-particle abrasion by using alumina produced the roughest surfaces on all materials; and the 4 mol% material was slightly less translucent than the 5 mol% materials.

Establishment of optimal variable elastic modulus distribution in the design of full-crown restorations by finite element analysis

To establish optimal elastic modulus distribution throughout the entire all-ceramic crown, aiming at improvement of the mechanical properties of the restoration as well as the adhesive interface, seven 3D models of mandibular first premolars of zirconia monolithic and bilayer crowns and lithium disilicate monolithic and bilayer crowns were constructed. The elastic modulus distribution of 8-layer crown A referred to human enamel, B was calculated by a genetic algorithm (GA) to minimize the principle stresses on the crown, and C minimized the shear stresses at the cementing lines. After applying a static load of 600 N, the maximum principle stresses were calculated and analyzed by finite element analysis (FEA). Group C were found to have the lowest peak shear stress at the cementing line and moderate peak tensile stress in the crown. Introduction of the modified elastic modulus distribution from human enamel into the entire all-ceramic crown reinforces the mechanical properties of the whole restoration as well as the adhesive interface against chipping and debonding.

[Study of the effect of the occlusal surface of a natural tooth and fixed partial dentures supported by dental implants on the stress distribution by finite element analysis]

OBJECTIVE

To study the effect of the occlusal surface of a natural tooth, cement-retained and screw-retained fixed partial dentures supported by dental implants and the coefficient of friction on the stress distribution in the peri-implant bone at maximum and minimum principal stresses.

MATERIAL AND METHODS

Study of maximum and minimum principal stresses in models with natural teeth and artificial crowns supported by dental implants, taking into account the coefficient of friction using the finite element analysis.

RESULTS

In models represented by fixed partial dentures supported by dental implants and a natural tooth, the maximum tensile stresses arise in the cortical bone in the cervical region of the artificial crown, and the maximum compressive stresses occur both in the cortical layer in the cervical region of the artificial crown and in the cervical region of the tooth. In models with two fixed partial dentures supported by dental implants or two natural teeth, the stress distributions in the cortical layers in the upper and lower jaw are almost identical.

CONCLUSION

Modeling the antagonist and adding to the FEA model is important in order to determine the precise and realistic direction of the resulting force vector. Amplification of the number of contact areas should be considered when modeling the occlusal surface of artificial crowns supported by dental implants.

Adhesion to Zirconia: A Systematic Review of Surface Pretreatments and Resin Cements

This systematic review aims to evaluate the different pretreatments of the zirconia surface and resin cement in order to determine a valid operative protocol for adhesive cementation. Methodologies conducted for this study followed the Prisma (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) guidelines. An electronic search was performed in four databases. The established focus question was: “What type of surface conditioning method is the one that obtains the best adhesion values to zirconia over time by applying a resin cement?” Forty-five relevant papers were found to qualify for final inclusion. In total, 260 different surface pretreatment methods, mainly combinations of air-abrasion protocols and adhesive promoters, were investigated. Altogether, the use of two artificial aging methods, three types of cement and four testing methods was reported. The results showed that mechanicochemical surface pretreatments offered the best adhesive results. Self-adhesive cement and those containing 10-MDP obtained the best results in adhesion to zirconia. Artificial aging reduced adhesion, so storage in water for 30 days or thermocycling for 5000 cycles is recommended. A standardized adhesive protocol has not been established due to a lack of evidence

Evaluation of fracture resistance of zirconia modification/polishing around implant abutments

Subcrestal placement of implants may have interproximal bone proximity issues that interfere with submucosal contour of the implant-supported Zirconia restorations during delivery of these restorations.  Modification of the mesial distal submucosal areas may be necessary to fully seat the restoration without impingement of the interproximal bone. Our aim was to determine if modification of submucosal cervical contour of implant supported zirconia titanium base (Zi-Ti base) restorations result in a significant change in fracture strength compared to Zi-Ti base restorations without any modification near the cervical submucosal area. Implant Zi-Ti base restorations, designed in the form of a maxillary premolar was made for the Straumann implant lab analog. Zirconia samples were cemented onto the Ti-base and the test group (N=20) underwent recontouring and polishing at the junction of the Zi-Ti-base cervical areas. The control group (N=20) did not undergo any modifications. All 40 samples underwent fracture testing with an Instron machine. We assessed differences between modified and unmodified implant restorations using a two-tailed t-test for independent samples.   Fracture strength values (N) ranged from 4,354.68 to 6,412.49 in the test group (N=20) and from 5,400.31 to 6,953.22 in the control group (N=20). The average fracture strength in the control group (6,154.84 ± 320.50) was higher than in the modified group (5,593.13 ± 486.51; p<.001)). Modification of submucosal contour significantly decreased the fracture strength. However, the average fracture strength exceeded the masticatory forces of humans.

Surface treatments and its effects on the fatigue behavior of a 5% mol yttria partially stabilized zirconia material

This study evaluated the effect of distinct surface treatments on the fatigue behavior (biaxial flexural fatigue testing) and surface characteristics (topography and roughness) of a 5% mol yttria partially stabilized zirconia ceramic (5Y-PSZ). Disc-shaped specimens of 5Y-PSZ (IPS e.max ZirCAD MT Multi) were manufactured (ISO 6872-2015) and allocated into six groups (n = 15) considering the following surface treatments: Ctrl - no-treatment; GLZ - low-fusing porcelain glaze application; SNF - 5 nm SiO₂ nanofilm; AlOx - aluminum oxide particle air-abrasion; SiC - silica-coated aluminum oxide particles (silica-coating); and 7%Si - 7% silica-coated aluminum oxide particles (silica-coating). The biaxial flexural fatigue tests were performed by the step-stress method (20Hz for 10,000 cycles) with a step increment of 50N starting at 100N and proceeding until failure detection. The samples were tested with the treated surface facing down (tensile stress side). Topography, fractography, roughness, and phase content assessments of treated specimens were performed. GLZ group presented the highest fatigue behavior, while AlOx presented the lowest performance, and was only similar to SiC and 7%Si. Ctrl and SNF presented intermediary fatigue behavior, and were also similar to SiC and 7%Si. GLZ promoted a rougher surface, Ctrl and SNF had the lowest roughness, while the air-abrasion groups presented intermediary roughness. No m-phase content was detected (only t and c phases were detected). In conclusion, the application of a thin-layer of low-fusing porcelain glaze, the deposition of silica nanofilms and the air-abrasion with silica-coated alumina particles had no detrimental effect on the fatigue behavior of the 5Y-PSZ, while the air-abrasion with alumina particles damaged the fatigue outcomes.

Evaluation of flexural strength of Zirconia using three different connector designs: An in vitro study

Aim:

The aim of this study is to evaluate the flexural strength of zirconia using three different connector designs under vertical and oblique loads.

Setting and Design:

Invitro - analytical study.

Materials and Methods:

For simulating zirconia fixed partial prosthesis, a specimen with three octagonal cylinders connected with each other was designed. Each face of the octagon was 3.75 mm ± 0.1 mm, and the total width was 9 mm ± 0.1 mm with a standard connector area of 10 mm² at cross-section. Three different connector designs, i.e., round, oval, and triangular were milled. Universal testing machine was used to test flexural strength with vertical and oblique forces.

Statistical Analysis Used:

Intergroup comparison of flexural strength was made using Descriptive statistics (1) one-way ANOVA, Bonferroni's post hoc test (2) Kruskal–Wallis test. The confidence interval was set at 95%, P < 0.05 was considered statistically significant for both the tests.

Results:

The highest flexural strength was observed in the triangle connector with vertical forces and lowest with oblique forces.

Conclusions:

Triangle connector design proved to be better than round and oval connectors on the application of vertical loads. Round connector design proved to be better than triangle and oval connector on application of oblique loads.

Biomechanical evaluation of anterior implants associated with titanium and zirconia abutments and monotype zirconia implants

PURPOSE

The present in silico study evaluated the behavior of titanium dental implants associated with abutments in zirconia and monotype zirconia implant using finite element analysis (FEA).

METHODS

A partial image of the anterior region of the maxilla was obtained by computed tomography. Three models of finite element were made using 3D modeling software (SolidWorks): Ti-Ti (control): implant morse cone (3.75 x 11mm; NobelActive) and titanium abutment (Esthetic Abutment); Ti-Zr: cone morse implant in titanium (3.75 x 11mm; NobelActive) and zirconia abutment (Procera Esthetic Abutment #9); Zr: monotype zirconia implant (4.1 x 12mm; Straumann Pure Ceramic). Computerized crowns of element 11 in lithium disilicate (IPS e.max Press, Ivoclar Vivadent) cemented in all groups were created. A load of 100N (45º) was applied simulating the excursion movement of the incisal guide. The von Mises, modified von Mises, maximum (tensile) and minimum (compression) principal stresses were obtained, compared and used for the quantitative and qualitative evaluation of the groups.

RESULTS

The Zr presented the lowest values of maximum, minimum, and von Mises tensions than the two pieces systems (Ti-Ti and Ti-Zr). Ti-Zr group had the highest values of tensions evaluated in this study.

CONCLUSIONS

The type of material as well as the geometry of implant influenc ed the tension values evaluated.

A State-of-the-Art Review on the Wear of the Occlusal Surfaces of Natural Teeth and Prosthetic Crowns

This review focuses on the wear mechanisms of natural and restorative dental materials, presenting a comprehensive description and analysis of the works published in the last two decades on the wear at the interface of occlusal surfaces. Different groups of tribological pairs were considered: tooth-tooth, tooth-restorative material (tooth-ceramic, tooth-resin-based-materials, and tooth-metal), and restorative-restorative materials. The lack of standardization of the wear tests impairs the direct comparison of the obtained results. However, it was possible to infer about the main wear mechanisms observed on the different classes of dental materials. Concerning ceramics, their toughness and surface finishing determines the wear of antagonist tooth. Abrasion revealed to be the main wear mechanisms at occlusal interface. In the case of resin-based composites, the cohesion of the organic matrix and the nature, shape, and amount of filler particles greatly influences the dental wear. The protruding and detachment of the filler particles are the main causes of abrasion of antagonist enamel. Metallic materials induce lower wear on antagonist enamel than the other classes of materials, because of their low hardness and high ductility. Most of the studies revealed plastic deformation and adhesive wear as the main wear mechanisms. Overall, more research in this area is needed for a better understanding of the mechanisms involved at the occlusal surfaces wear. This would be essential for the development of more suitable restoration materials.

Intraoral repair of a chipped porcelain-zirconia restoration

OBJECTIVE

Ceramic fracture is an undesirable outcome of the rehabilitation with fixed partial dentures (FPD), mainly because it may involve additional cost and clinical time for intraoral repair or replacement of the restoration. This clinical report describes a 5 years survival intraoral repair of a chipped porcelain veneered zirconia framework restoration using a resin-based composite.

CLINICAL CONSIDERATIONS

A FPD of porcelain veneered zirconia was made. After 18 months, the FPD presented a porcelain chip (porcelain fracture without exposure to the zirconia structure) on the buccal side of the pontic. An epoxy resin replica of the fractured surface was obtained and was examined under scanning electron microscopy. Fracture origin was found at the cervical area of the pontic. Intraoral repair by bonding the chipped fragment back in place was performed. After 15 days, the porcelain fragment debonded without patient knowledge and the fragment was lost. Then, intraoral repair using composite resin to restore the fractured area was performed and is still in function to date.

CONCLUSIONS

Based on the 5-years survival of the performed intraoral repair, the composite resin reconstruction technique has shown to be an adequate alternative treatment for fractured FPD.

CLINICAL SIGNIFICANCE

A resin composite repair of the fracture site can be performed in one clinical session, using much less time and cost than for the replacement of FPD. This clinical case survived 5 years to date.

Effect of coloring liquids on color of zirconia frameworks and bond strength of zirconia/veneering ceramic

STATEMENT OF PROBLEM

Zirconia substructures must be colored to fabricate esthetic restorations. However, the coloring procedure may affect the color match and bond strength of the veneering ceramic to the zirconia.

PURPOSE

The purpose of this in vitro study was to evaluate the bond strength of veneering ceramic to zirconia after dipping in coloring liquid for different durations and to compare the color of dipped blocks with precolored blocks.

MATERIAL AND METHODS

Sixty-four cube-shaped specimens were fabricated from uncolored zirconia blocks by using computer-aided design and computer-aided manufacturing and divided into 4 groups (n=16). The G30 group was subjected to coloring liquids for 30 seconds, G60 for 60 seconds, and G120 for 120 seconds. After the coloring procedures, the color of the specimens was evaluated by using a spectrophotometer. Precolored zirconia (PCZ) was used as the control. Once the color measurements were completed, the same specimens were prepared for the bond strength test, with 2 groups as the control: PCZ and the uncolored control group (control bond-strength [CBS]). Veneering ceramic was applied to the zirconia specimens and sintered. The shear bond test was conducted using universal testing equipment with a crosshead speed of 1 mm/min. Statistical analysis was carried out by using 1-way ANOVA followed by the Tukey HSD test (α=.05). A Weibull analysis was performed, and the Weibull modulus was calculated for each group.

RESULTS

The mean E value for the PCZ group (77.34 ±0.22) was statistically significantly different from that of all groups (PCZ-G30; P=.018, PCZ-G60; P= <.001, PCZ-G120; P<.001). The mean E value for G60 group was significantly different from mean E values of the G30 and G120 groups (G60-G30; P<.001, G60-G120; P<.001). The PCZ group had the highest mean shear bond strength value (31.5 ±7.67 MPa) and was significantly different from the mean bond strength values of the G120 and CBS groups (PCZ-G120; P=.008, PCZ-CBS; P<.001). The CBS had the lowest mean bond strength value (16.62 ±7.11 MPa) and was statistically significantly lower than that of the G60 group (CBS-G60; P=.001). The Weibull modulus was 64.78 for the PCZ group and 0.1 for the G120 group.

CONCLUSIONS

Immersing zirconia in coloring liquid altered the color and negatively affected the bond strength between the veneering ceramic and zirconia.

A simple 3-point flexural method for measuring fracture toughness of the dental porcelain to zirconia bond and other brittle bimaterial interfaces

PURPOSE

Porcelain fused to zirconia prostheses are widely used, but porcelain chipping, fracture, spalling and delamination are common clinical problems. Conventional bond strength testing is inherently unsuited for studying interfacial failure by cracking in brittle materials. Instead, fracture toughness is a more meaningful parameter because it can assess the robustness of the interface when subjected to loading, but fracture mechanics approaches have only rarely been used. Our purpose was to develop a novel, simple, 3-point flexural methodology and mathematical analysis to measure the fracture toughness of the porcelain to zirconia interface.

METHODS

Equations were derived to estimate the fracture toughness of the bond by computing the interfacial energy release rate for a novel simple 3-point flexural test model. The test was validated using two different configurations of layered zirconia/porcelain beams (n = 10), approximating the dimensions of a fixed dental prosthesis, fabricated from a tetragonal polycrystalline zirconium dioxide partially stabilized with yttria and a feldspathic dental porcelain.

RESULTS

Cracking along the bimaterial interface was produced and measured as a discrete event. Fracture toughness means (standard deviations) computed from the measured energy release rate, for the porcelain to zirconia interface in two different specimen configurations were 7.9 (1.3) and 5.3 (1.6) J/m².

CONCLUSIONS

Equations were derived to measure interfacial fracture toughness of brittle materials using a novel simple 3-point flexural test method. The test was then validated; estimates for the fracture toughness for the porcelain to zirconia bond, overlapped with previously published data derived from more complex 4-point notched tests.

The effect of abrading and cutting instruments on machinability of dental ceramics

The aim was to investigate the effect of machining instruments on machinability of dental ceramics. Four dental ceramics, including two zirconia ceramics were machined by three types (SiC, diamond vitrified, and diamond sintered) of wheels with a hand-piece engine and two types (diamond and carbide) of burs with a high-speed air turbine. The machining conditions used were abrading speeds of 10,000 and 15,000 r.p.m. with abrading force of 100 gf for the hand-piece engine, and a pressure of 200 kPa and a cutting force of 80 gf for the air-turbine hand-piece. The machinability efficiency was evaluated by volume losses after machining the ceramics. A high-abrading speed had high-abrading efficiency (high-volume loss) compared to low-abrading speed in all abrading instruments used. The diamond vitrified wheels demonstrated higher volume loss for two zirconia ceramics than those of SiC and diamond sintered wheels. When the high-speed air-turbine instruments were used, the diamond points showed higher volume losses compared to the carbide burs for one ceramic and two zirconia ceramics with high-mechanical properties. The results of this study indicated that the machinability of dental ceramics depends on the mechanical and physical properties of dental ceramics and machining instruments. The abrading wheels show autogenous action of abrasive grains, in which ground abrasive grains drop out from the binder during abrasion, then the binder follow to wear out, subsequently new abrasive grains come out onto the instrument surface (autogenous action) and increase the grinding amount (volume loss) of grinding materials.

Layered Manufacturing of Dental Ceramics: Fracture Mechanics, Microstructure, and Elemental Composition of Lithography-Sintered Ceramic

PURPOSE

To compare the fracture mechanics, microstructure, and elemental composition of lithography-based ceramic manufacturing with pressing and CAD/CAM.

MATERIALS AND METHODS

Disc-shaped specimens (16 mm diameter, 1.2 mm thick) were used for mechanical testing (n = 10/group). Biaxial flexural strength of three groups (In-Ceram alumina [ICA], lithography-based alumina, ZirkonZahn) were determined using the "piston on 3-ball" technique as suggested in test Standard ISO-6872. Vickers hardness test was performed. Fracture toughness was calculated using fractography. Results were statistically analyzed using Kruskal-Wallis test followed by Dunnett T3 (α = 0.05). Weibull analysis was conducted. Polished and fracture surface characterization was made using scanning electron microscope (SEM). Energy dispersive spectroscopy (EDS) was used for elemental analysis.

RESULTS

Biaxial flexural strength of ICA, LCM alumina (LCMA), and ZirkonZahn were 147 ± 43 MPa, 490 ± 44 MPa, and 709 ± 94 MPa, respectively, and were statistically different (P ≤ 0.05). The Vickers hardness number of ICA was 850 ± 41, whereas hardness values for LCMA and ZirkonZahn were 1581 ± 144 and 1249 ± 57, respectively, and were statistically different (P ≤ 0.05). A statistically significant difference was found between fracture toughness of ICA (2 ± 0.4 MPa⋅m^1/2 ), LCMA (6.5 ± 1.5 MPa⋅m^1/2 ), and ZirkonZahn (7.7 ± 1 MPa⋅m^1/2 ) (P ≤ 0.05). Weibull modulus was highest for LCMA (m = 11.43) followed by ZirkonZahn (m = 8.16) and ICA (m = 5.21). Unlike LCMA and ZirkonZahn groups, a homogeneous microstructure was not observed for ICA. EDS results supported the SEM images.

CONCLUSIONS

Within the limitations of this in vitro study, it can be concluded that LCM seems to be a promising technique for final ceramic object manufacturing in dental applications. Both the manufacturing method and the material used should be improved.

Using glass-graded zirconia to increase delamination growth resistance in porcelain/zirconia dental structures

OBJECTIVE

Porcelain fused to zirconia (PFZ) restorations are widely used in prosthetic dentistry. However, their tendency to delaminate along the P/Z interface remains a practical problem so that assessing and improving the interfacial strength are important design aspects. This work examines the effect of modifying the zirconia veneering surface with an in-house felspathic glass on the interfacial fracture resistance of fused P/Z.

METHODS

Three material systems are studied: porcelain fused to zirconia (control) and porcelain fused to glass-graded zirconia with and without the presence of a glass interlayer. The specimens were loaded in a four-point-bend fixture with the porcelain veneer in tension. The evolution of damage is followed with the aid of a video camera. The interfacial fracture energy G_C was determined with the aid of a FEA, taking into account the stress shielding effects due to the presence of adjacent channel cracks.

RESULTS

Similarly to a previous study on PFZ specimens, the fracture sequence consisted of unstable growth of channel cracks in the veneer followed by stable cracking along the P/Z interface. However, the value of GC for the graded zirconia was approximately 3 times that of the control zirconia, which is due to the good adhesion between porcelain and the glass network structure on the zirconia surface.

SIGNIFICANCE

Combined with its improved bonding to resin-based cements, increased resistance to surface damage and good esthetic quality, graded zirconia emerges as a viable material concept for dental restorations.

Effects of porcelain thickness on the flexural strength and crack propagation in a bilayered zirconia system

OBJECTIVE

This study evaluated the influence of porcelain (VM9, VITA Zahnfabrik, Germany) thickness on the flexural strength and crack propagation in bilayered zirconia systems (YZ, VITA Zahnfabrik, Germany).

MATERIAL AND METHODS

Thirty zirconia bars (20.0x4.0x1.0 mm) and six zirconia blocks (12.0x7.5x1.2 mm) were prepared and veneered with porcelain with different thickness: 1 mm, 2 mm, or 3 mm. The bars of each experimental group (n=10) were subjected to four-point flexural strength testing. In each ceramic block, a Vickers indentation was created under a load of 10 kgf for 10 seconds, for the propagation of cracks.

RESULTS

The results of flexural strength were evaluated by One-way ANOVA and Tukey's test, with a significance level of 5%. The factor "thickness of the porcelain" was statistically significant (p=0.001) and the l-mm group presented the highest values of flexural strength. The cracks were predominant among the bending specimens with 1 and 2 mm of porcelain, and catastrophic failures were found in 50% of 3-mm-thick porcelain. After the indentation of blocks, the most severe defects were observed in blocks with 3-mm-thick porcelain.

CONCLUSION

The smallest (1 mm) thickness of porcelain on the zirconia infrastructure presented higher values of flexural strength. Better resistance to defect propagation was observed near the porcelain/ zirconia interface for all groups. Higher flexural strength was found for a thinner porcelain layer in a bilayered zirconia system. The damage caused by a Vickers indentation near and far the interface with the zirconia shows that the stress profiles are different.

Evaluation of Fracture Resistance of Varying Thicknesses of Zirconia Around Implant Abutment Cylinders

Zirconia is becoming increasingly used as a restorative material for implant-supported restorations; however, information is lacking with respect to the minimum thickness of zirconia surrounding the implant components. The purpose of this study is to evaluate the resistance to fracture of different thicknesses of zirconia luted to implant components. Thirty cylinders of zirconia (Prettau, Zirkonzahn) with 13-mm height, designed with indented occlusal surface for loading, and varying wall thicknesses (0.5 mm, 1 mm, 1.5 mm; n = 10/group) were milled using a computer-aided design/computer-aided manufacturing system (Modellier, Zirkonzahn), after which they were sintered. Titanium temporary cylinders (ITCS41, Biomet3i) were attached to 30 implant analogs (ILA20, Biomet3i) that were embedded into polymethylmethacrylate blocks (Palapress Vario, Heraeus Kulzer) with dimensions of 4.5 × 1.8 × 2 cm. Zirconia specimens were cemented to the titanium cylinders using a self-adhesive, dual-cure resin cement (Panavia SA, Kuraray). Load to failure test was performed under compression until fracture using a universal testing machine (Instron5965, Instron) at a crosshead speed of 0.5 mm/min and measured in N (Newton). Statistical analysis was performed using 1-way analysis of variance and Tukey B test at α = .05 (SPSS19, IBM). Mean load to failure was 1059.94 N, 2019.46 N, and 4074.79 N for groups 0.5 mm, 1 mm, and 1.5 mm, respectively. Values were significantly different between the groups (P &lt; .05). Study limitations are that it is in vitro, specimens do not replicate tooth dimensions, and forces are static and directed toward the occlusal portion of each specimen. Within these limitations and considering the average human bite force, a thickness of 0.5 mm to 1 mm of this particular type of zirconia around this type of implant component can avoid fracture with these dimensions.

Dental ceramics: a review of new materials and processing methods

The evolution of computerized systems for the production of dental restorations associated to the development of novel microstructures for ceramic materials has caused an important change in the clinical workflow for dentists and technicians, as well as in the treatment options offered to patients. New microstructures have also been developed by the industry in order to offer ceramic and composite materials with optimized properties, i.e., good mechanical properties, appropriate wear behavior and acceptable aesthetic characteristics. The objective of this literature review is to discuss the main advantages and disadvantages of the new ceramic systems and processing methods. The manuscript is divided in five parts: I) monolithic zirconia restorations; II) multilayered dental prostheses; III) new glass-ceramics; IV) polymer infiltrated ceramics; and V) novel processing technologies. Dental ceramics and processing technologies have evolved significantly in the past ten years, with most of the evolution being related to new microstructures and CAD-CAM methods. In addition, a trend towards the use of monolithic restorations has changed the way clinicians produce all-ceramic dental prostheses, since the more aesthetic multilayered restorations unfortunately are more prone to chipping or delamination. Composite materials processed via CAD-CAM have become an interesting option, as they have intermediate properties between ceramics and polymers and are more easily milled and polished.

Effect of grinding and polishing on roughness and strength of zirconia

STATEMENT OF PROBLEM

The clinical applications of high-translucency monolithic zirconia restorations have increased. Chairside and laboratory adjustments of these restorations are inevitable, which may lead to increased roughness and reduced strength. The influence of grinding and polishing on high-translucency zirconia has not been investigated.

PURPOSE

The purpose of this in vitro study was to compare the roughness averages (Ra) of ground and polished zirconia and investigate whether roughness influenced strength after aging.

MATERIAL AND METHODS

High-translucency zirconia disks were milled, sintered, and glazed according to the manufacturer's recommendations. Specimens were randomized to 4 equal groups. Group G received only grinding; groups GPB and GPK received grinding and polishing with different polishing systems; and group C was the (unground) control group. All specimens were subjected to hydrothermal aging in an autoclave at 134°C at 200 kPa for 3 hours. Roughness average was measured using a 3-dimensional (3D) optical interferometer at baseline (Ra1), after grinding and polishing (Ra2), and after aging (Ra3). A biaxial flexural strength test was performed at a rate of 0.5 mm/min. Statistical analyses were performed using commercial software (α=.05).

RESULTS

Group G showed a significantly higher mean value of Ra3 (1.96 ±0.32 μm) than polished and glazed groups (P<.001), which showed no statistically significant difference among them (GPB, 1.12 ±0.31 µm; GPK, 0.88 ±0.31 µm; C: 0.87 ±0.25 μm) (P>.05). Compared with baseline, the roughness of groups G and GPB increased significantly after surface treatments and after aging, whereas aging did not significantly influence the roughness of groups GPK or C. Group G showed the lowest mean value of biaxial flexural strength (879.01 ±157.99 MPa), and the highest value was achieved by group C (962.40 ±113.84 MPa); no statistically significant differences were found among groups (P>.05). Additionally, no significant correlation was detected between the Ra and flexural strength of zirconia.

CONCLUSIONS

Grinding increased the roughness of zirconia restorations, whereas proper polishing resulted in smoothness comparable with glazed surfaces. The results provide no evidence that grinding and polishing affect the flexural strength of zirconia after aging.

Adhesive Bonding to Computer-aided Design/ Computer-aided Manufacturing Esthetic Dental Materials: An Overview

AIM

To review the adhesive bonding to different computer-aided design/computer-aided manufacturing (CAD/CAM) esthetic restorative materials.

BACKGROUND

The use of CAD/CAM esthetic restorative materials has gained popularity in recent years. Several CAD/ CAM esthetic restorative materials are commercially available. Adhesive bonding is a major determinant of success of CAD/ CAM restorations. Review result: An account of the currently available bonding strategies are discussed with their rationale in various CAD/ CAM materials.

CONCLUSION

Different surface treatment methods as well as adhesion promoters can be used to achieve reliable bonding of CAD/CAM restorative materials. Selection of bonding strategy to such material is determined based on its composition. Further evidence is required to evaluate the effect of new surface treatment methods, such as nonthermal atmospheric plasma and self-etching ceramic primer on bonding to different dental ceramics.

CLINICAL SIGNIFICANCE

An understanding of the currently available bonding strategies to CA/CAM materials can help the clinician to select the most indicated system for each category of materials.

The effect of direct and indirect force transmission on peri-implant bone stress - a contact finite element analysis

In almost all finite element (FE) studies in dentistry, virtual forces are applied directly to dentures. The purpose of this study was to develop a FE model with non-linear contact simulation using an antagonist as force transmitter and to compare this with a similar model that uses direct force transmission. Furthermore, five contact situations were created in order to examine their influence on the peri-implant bone stresses, which are relevant to the survival rate of implants. It was found that the peri-implant bone stresses were strongly influenced by the kind of force transmission and contact number.

Mechanical behavior of yttria-stabilized tetragonal zirconia polycrystalline ceramic after different zirconia surface treatments

This study investigated the effects of different zirconia surface conditioning methods on the biaxial flexure strength, surface characteristics and fractographic analysis of a Y-TZP ceramic. Disc-shaped specimens were manufactured according to ISO 6872-2008 for biaxial flexure strength testing, and then randomly assigned into seven groups (n=30). Control (CTRL): without treatment; Tribochemical silica coating (TBS): specimens were sandblasted with silica-coated aluminum oxide particles (CoJet-Sand) for 10s; Silica nanofilm (SNF): specimens were silica coated with a 5nm SiO₂ nanofilm; and four protocols of low-fusing porcelain glaze (GLZ): etching with 10% hydrofluoric acid gel (HF) for 1min (GLZ₁), 5min (GLZ₅), 10min (GLZ₁₀) and 15min (GLZ₁₅). Phase transformation, roughness, micro-morphological, flexural analysis tests, and fractographic analyses were performed. X-ray diffraction (XRD) analysis showed that the TBS promoted the highest m-phase content (20.35). However, for the GLZ groups, XRD analysis was not sensitive enough to obtain an accurate reading for phase transformation. The GLZ group had the highest roughness values. The TBS group had the highest characteristic strength (1291.38MPa), followed by SNF (999.26MPa). These results suggest that (TBS) and (SNF) treatments did not reduce the mechanical properties, while (GLZ) led to a degradation in the mechanical properties.

The effect of coloring liquid dipping time on the fracture load and color of zirconia ceramics

PURPOSE

The aims of the study were to evaluate the fracture load of zirconia core material after dipping in coloring liquid at different time intervals and to compare the color of dipped blocks with that of prefabricated shaded blocks.

MATERIALS AND METHODS

3-unit bridge frameworks were designed digitally. Sixty frameworks were fabricated using uncolored zirconia blocks by CAD/CAM and divided into 4 groups randomly (n = 15). Group 2 (G2) was subjected to coloring liquids for 2 minutes, Group 4 (G4) for 4 minutes, and Group 6 (G6) for 6 minutes. CFS group was not subjected to any coloring procedure. After coloring, color differences between the test groups and a prefabricated shaded zirconia group (CPZ, n = 15) were evaluated by using a spectrophotometer. Fracture test was conducted immediately after shade evaluation with a Testometric test device at a cross-head speed of 1 mm/sec. Statistical analysis for evaluating color and fracture load was performed by using one way ANOVA followed by Tukey HSD test (P ≤ .05). Weibull analysis was conducted for distribution of fracture load.

RESULTS

There was no difference in terms of fracture load and color between CFS (1176.681 N) and G2 (985.638 N) group and between CPZ (81.340) and G2 (81.140) group, respectively. Fracture load values of G4 (779.340 N) and G6 (935.491 N) groups were statistically significantly lower than that of CFS group (P ≤ .005). The color values of G4 (79.340) and G6 (79.673) groups were statistically different than that of CPZ group (P ≤ .005).

CONCLUSION

Prolonged immersion of zirconia in coloring liquid not only negatively affected the fracture load of the zirconia being tested in the current study but also deteriorated the desired shade of the restoration.

Mechanical performance of polymer-infiltrated zirconia ceramics

OBJECTIVE

The aim of this study was to evaluate the microstructure and mechanical behavior of polymer-infiltrated zirconia ceramics as a function of pre-sintering temperature (1000-1150°C).

METHODS

Polymer-infiltrated zirconia ceramics were prepared by combining the porous zirconia networks and polymer through infiltration and polymerization. XRD was employed to determine phase structure. The microstructure and fracture mechanism were observed by SEM. Flexural strength and fracture toughness were measured by three-point bending method and single-edge-notched beam method, respectively. A nanoindentation system was employed to determine elastic modulus and hardness.

RESULTS

Different porosities and polymer contents can be obtained by tuning the pre-sintered temperature of zirconia ceramic precursors. Zirconia network porosity varies from 46.3% to 34.7% and the relevant polymer content ranges from 18.4wt.% to 12.3wt.% when the pre-sintered temperature is set from 1000°C to 1150°C. The flexural strength, fracture toughness, hardness, and elastic modulus values of the specimen pre-sintered at 1150°C are 240.9MPa, 3.69MPam^1/2, 3.1GPa, and 58.8GPa, respectively.

CONCLUSION

The pre-sintering temperature has a significant effect on the microstructure and mechanical properties of polymer-infiltrated zirconia ceramics and the optimal pre-sintering temperature is 1150°C.

CLINICAL SIGNIFICANCE

Specimen pre-sintered at 1150°C shows tooth-like mechanical properties, suggesting a promising restorative material in dental clinic. Moreover, the synthesis process is simple and can be easily performed in a prosthesis laboratory.

Effect of Different Firing Temperatures on Structural Changes in Porcelain

PURPOSE

To study the structural changes occurring in the dental porcelain mass fired at various firing temperatures using scanning electron microscopy (SEM) and X-ray diffraction (XRD) spectroscopy. Also, additional tests, namely compressive strength, abrasion resistance analysis, and the amount of oxides released, were conducted at different firing temperatures.

MATERIALS AND METHODS

Six groups (40 specimens in each group) of porcelain mass were prepared. The dimensions and weight of all the specimens were kept constant. The specimens were then heat treated at different firing temperatures (660°C, 760°C, 860°C, 900°C, 960°C, 990°C). Half of the specimens of each group were subjected to a compressive strength test on a universal testing machine and then finely ground using an electrochemical grinder to prepare for XRD analysis. The other half of the specimens was weighed to analyze the amount of oxides released after each firing cycle. Following this, the specimens underwent an abrasion resistance test on a Nanovea Tribometer. The unaltered surface was scanned using SEM. The data (numerical and graphical) for all the tests were recorded and analyzed using one-way ANOVA and post hoc Tukey test.

RESULTS

The specimens fired at 900°C exhibited superior compressive strength and abrasion resistance. The quantity of oxides released by the specimens fired at 900°C was the least compared to specimens heat treated at the other firing temperatures. XRD analysis proved that the oxide released by the porcelain mass was calcium aluminum chromium oxide. Also, the fewer peaks obtained in the XRD graphs of specimens fired at 900°C signified lesser porosities in the porcelain specimens. SEM analysis depicted a homogeneous mass of porcelain at 900°C.

CONCLUSION

All the above findings validate the objective of studying the physical and internal structural changes of dental porcelain when subjected to an increasing firing temperature gradient. The specimens fired at 900°C exhibited superior strength and abrasion resistance. SEM analysis depicted a homogeneous mass of dental porcelain, implying that firing was complete at 900°C.

Wear of primary teeth caused by opposed all-ceramic or stainless steel crowns

PURPOSE

This study was conducted to evaluate the effects of full-coverage all-ceramic zirconia, lithium disilicate glass-ceramic, leucite glass-ceramic, or stainless steel crowns on antagonistic primary tooth wear.

MATERIALS AND METHODS

There were four study groups: the stainless steel (Steel) group, the leucite glass-ceramic (Leucite) group, the lithium disilicate glass-ceramic (Lithium) group, and the monolithic zirconia (Zirconia) group. Ten flat crown specimens were prepared per group; opposing teeth were prepared using primary canines. A wear test was conducted over 100,000 chewing cycles using a dual-axis chewing simulator and a 50 N masticating force, and wear losses of antagonistic teeth and restorative materials were calculated using a three-dimensional profiling system and an electronic scale, respectively. Statistical significance was determined using One-way ANOVA and Tukey's test (P<.05).

RESULTS

The Leucite group (2.670±1.471 mm³) showed the greatest amount of antagonist tooth wear, followed by in decreasing order by the Lithium (2.042±0.696 mm³), Zirconia (1.426±0.477 mm³), and Steel groups (0.397±0.192 mm³). Mean volume losses in the Leucite and Lithium groups were significantly greater than in the Steel group (P<.05). No significant difference was observed between mean volume losses in the Zirconia and Steel groups (P>.05).

CONCLUSION

Leucite glass-ceramic and lithium disilicate glass-ceramic cause more primary tooth wear than stainless steel or zirconia.

Influence of Veneering Fabrication Techniques and Gas-Phase Fluorination on Bond Strength between Zirconia and Veneering Ceramics

PURPOSE

Porcelain chipping has been one of the main problems of porcelain-fused-to-zirconia restorations. This study evaluates the bond strengths of layered, pressed, and adhesively bonded porcelain to yttria-stabilized zirconia substrates that have undergone traditional preparation or gas-phase fluorination.

MATERIALS AND METHODS

A three-point bending test was used to evaluate the bond strength of the porcelain and zirconia interface. Sixty-six specimens were prepared (n = 11) following ISO 9693 and loaded until failure using an Instron testing machine. One-half of the zirconia substrates received gas phase fluorination treatment before veneering application. Three porcelain veneering methods were evaluated: layered, pressed, and adhesively bonded porcelain. Bond strength results were interpreted using a two-way ANOVA and a Bonferroni multiple comparisons test. Statistical significance was set at α = 0.05.

RESULTS

ANOVA revealed a statistically significant effect of the veneering fabrication methods. No main effect was observed regarding the surface treatment to the zirconia. There was a significant effect related to the veneering method used to apply porcelain to zirconia. For untreated zirconia, layered porcelain had a significantly higher flexural strength compared to pressed or bonded, while pressed and bonded porcelains were not significantly different from one another. For zirconia specimens receiving fluorination treatment, both layered and pressed porcelains had significantly higher bond strengths than adhesively bonded porcelain. In addition, fluorinated pressed porcelain was not statistically different from the control layered or fluorinated layered porcelain.

CONCLUSION

The choice of veneering fabrication technique was critical when evaluating the zirconia to porcelain interfacial bond strength. Bonded porcelain to zirconia had a lower flexural strength than layered or pressed porcelain, regardless of zirconia surface treatment. In addition, fluorination had an effect on the bond strength of pressed porcelain.

Biaxial flexural strength of bilayered zirconia using various veneering ceramics

PURPOSE

The aim of this study was to evaluate the biaxial flexural strength (BFS) of one zirconia-based ceramic used with various veneering ceramics.

MATERIALS AND METHODS

Zirconia core material (Katana) and five veneering ceramics (Cerabien ZR; CZR, Lava Ceram; LV, Cercon Ceram Kiss; CC, IPS e.max Ceram; EM and VITA VM9; VT) were selected. Using the powder/liquid layering technique, bilayered disk specimens (diameter: 12.50 mm, thickness: 1.50 mm) were prepared to follow ISO standard 6872:2008 into five groups according to veneering ceramics as follows; Katana zirconia veneering with CZR (K/CZR), Katana zirconia veneering with LV (K/LV), Katana zirconia veneering with CC (K/CC), Katana zirconia veneering with EM (K/EM) and Katana zirconia veneering with VT (K/VT). After 20,000 thermocycling, load tests were conducted using a universal testing machine (Instron). The BFS were calculated and analyzed with one-way ANOVA and Tukey HSD (α=0.05). The Weibull analysis was performed for reliability of strength. The mode of fracture and fractured surface were observed by SEM.

RESULTS

It showed that K/CC had significantly the highest BFS, followed by K/LV. BFS of K/CZR, K/EM and K/VT were not significantly different from each other, but were significantly lower than the other two groups. Weibull distribution reported the same trend of reliability as the BFS results.

CONCLUSION

From the result of this study, the BFS of the bilayered zirconia/veneer composite did not only depend on the Young's modulus value of the materials. Further studies regarding interfacial strength and sintering factors are necessary to achieve the optimal strength.