Fatigue and damage accumulation of veneer porcelain pressed on Y-TZP

CAD-CAM resin composites: Effective components for further development

OBJECTIVE

This paper summarizes the effective components of computer-aided design and computer-aided manufacturing (CAD-CAM) resin composites that contribute to achieving greater mechanical properties and further development.

METHODS

In silico multi-scale analysis, in silico nonlinear dynamic finite element analysis (FEA), and artificial intelligence (AI) were used to explore the effective components of CAD-CAM resin composites. The effects of the filler diameter and silane coupling ratio on the mechanical properties of CAD-CAM resin composites have been clarified through multi-scale analysis. The effects of the filler contents, and filler and monomer compositions have been investigated by AI algorithms. The fracture behavior of CAD-CAM composite crown was analyzed using in silico non-linear dynamic FEA. The longevity of CAD-CAM composite crown was assessed through step-stress accelerating life testing (SSALT).

RESULTS

As the filler diameter decreases, there is an increase in elastic moduli and compressive strengths at the macroscale. At the nanoscale, a decrease in the filler diameter results in a decrease in the maximum value of the maximum principal strain. When the silane coupling ratio decreases, there is a decrease in the elastic modulus and compressive strength. According to the exhaustive search and feature importance analysis based on the AI algorithm, the combination of certain components was narrowed down to achieve a flexural strength of 269.5 MPa. The in silico non-linear FEA successfully detected the sign of the initial crack of the CAD-CAM composite molar crown. The SSALT revealed that CAD-CAM resin composite molar crowns containing nanofillers with a high fraction of resin matrix demonstrated great longevity.

SIGNIFICANCE

This paper summarized the effective components of CAD-CAM resin composites for their further development. The integration of in vitro and in silico approaches will expedite the advancement of CAD-CAM resin composites, offering benefits such as time efficiency and reduction of material waste for researchers and manufacturers.

Biomechanical performance of three fiberglass post cementation techniques: Imaging, in vitro, and in silico analysis

PURPOSE

The structural integrity of the resin cement layer, the bond strength, and the biomechanical behavior of different fiberglass post cementation techniques were evaluated.

METHODS

Thirty-three bovine incisors were divided into three groups (n = 11): conventional fiberglass post (CFP), conventional fiberglass post in flared root canals (CFL), and relined fiberglass post (RFP). Six specimens from each group were submitted for high-resolution microcomputed tomography (μCT) to evaluate the integrity and presence/volume of voids at the resin cement layer. Finite element analysis (FEA) of two three-dimensional (3D) models of each group were conducted, one considered ideal (without interface defects) and another containing the conditions identified in the μCT analysis. Push-out bond strength tests were conducted for all specimens.

RESULTS

The CFL group had the greatest mean values of void (Thirds cervical: 73.67; middle: 95.67; apical: 47.33) and gap concentration (Thirds cervical: 14.67; middle: 15.83; apical: 8.33) compared with CFP and RFP. A significant difference in bond strength was observed between the cervical (1.33 MPa) and middle thirds (1.85 MPa) compared with the apical third (4.85 MPa) of the CFL. A significant difference was observed in the bond strength in the CFL (1.33 MPa) and RFP (3.29 MPa) in the cervical third, which were statistically similar to the bond strength of the CFP. The tensile stress distributions were similar in most structures, localized in the cervical region on the lingual surface.

CONCLUSIONS

Structural defects in the interface layer might influence the bond strength and biomechanical behavior under the different fiberglass post cementations.

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

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

Monolithic zirconia crowns: effect of thickness reduction on fatigue behavior and failure load

PURPOSE

The objective of this study was to evaluate the effect of thickness reduction and fatigue on the failure load of monolithic zirconia crowns.

MATERIALS AND METHODS

140 CAD-CAM fabricated crowns (3Y-TZP, inCorisTZI, Dentsply-Sirona) with different ceramic thicknesses (2.0, 1.5, 1.0, 0.8, 0.5 mm, respectively, named G2, G1.5, G1, G0.8, and G0.5) were investigated. Dies of a mandibular first molar were made of composite resin. The zirconia crowns were luted with a resin composite cement (RelyX Unicem 2 Automix, 3M ESPE). Half of the specimens (n = 14 per group) were mouth-motion-fatigued (1.2 million cycles, 1.6 Hz, 200 N/ 5 – 55℃, groups named G2-F, G1.5-F, G1-F, G0.8-F, and G0.5-F). Single-load to failure was performed using a universal testing-machine. Fracture modes were analyzed. Data were statistically analyzed using a Weibull 2-parameter distribution (90% CI) to determine the characteristic strength and Weibull modulus differences among the groups.

RESULTS

Three crowns (21%) of G0.8 and five crowns (36%) of G0.5 showed cracks after fatigue. Characteristic strength was the highest for G2, followed by G1.5. Intermediate values were observed for G1 and G1-F, followed by significantly lower values for G0.8, G0.8-F, and G0.5, and the lowest for G0.5-F. Weibull modulus was the lowest for G0.8, intermediate for G0.8-F and G0.5, and significantly higher for the remaining groups. Fatigue only affected G0.5-F.

CONCLUSION

Reduced crown thickness lead to reduced characteristic strength, even under failure loads that exceed physiological chewing forces. Fatigue significantly reduced the failure load of 0.5 mm monolithic 3Y-TZP crowns.

Long-term fracture load of all-ceramic crowns: Effects of veneering ceramic thickness, application techniques, and cooling protocol

Background

To evaluate, in vitro, the effects of the cooling protocol, application technique, and veneering ceramic thickness on the fracture resistance of ceramic crowns with Y-TZP frameworks.

Material and Methods

80 frameworks were made from zirconia by the CAD/CAM technique and divided into 8 groups (n = 10) according to the factors: “application technique” (stratified-L and pressed -P), “thickness” (1 mm and 2 mm), and “cooling protocol” (slow-S and fast-F) of the feldspathic veneering ceramic. After, all crowns were cemented over G10 preparations with resin cement (Panavia F, Kuraray), mechanically cycled (2x106 cycles, 200 N, 3Hz), and subjected to the axial compression resistance test (0.5 mm/min, 10 kN). The data (N) underwent descriptive statistical analysis by 3-way ANOVA and Tukey’s test (5%). Fracture analysis was performed to determine the possible origin of failure.

Results

The factors “cooling protocol” (P=0.0058) and “application” technique (P=0.0001) influenced the fracture resistance of the crowns. For pressed veneer technique, the P2S (4608.9±464.5). A presented significantly higher results than that P2F(3621.1±523.0)BCD (Tukey’s test). For the stratified technique, this difference was not observed (P>0.05). The thickness of the veneering ceramic was not significant regardless of the cooling protocol and technique (P>0.05). The predominant failure mode was chipping of the ceramic veneer originating in the subsurface.

Conclusions

The pressed technique, used with a slow-cooling protocol, leads to the best outcome for the veneering of all-ceramic crowns.

Key words:Zirconia, ceramics, cooling protocol, thickness, application technique.

Minimal tooth preparation for posterior monolithic ceramic crowns: Effect on the mechanical behavior, reliability and translucency

OBJECTIVE

Despite the increased use of monolithic crowns, their performance has yet to be determined when the minimal tooth preparation (MTP) principle is considered. The goal of this study was to evaluate the effect of MTP on the mechanical behavior, reliability and translucency of posterior monolithic ceramic crowns.

METHODS

Dentin analogues were machined using two preparation designs (0.5 or 1 mm reduction) to receive first molar crowns in order to evaluate the monolithic crown performance. Next, 126 crowns were divided (21/g) according to the material (High translucent zirconia - YZHT, Zirconia reinforced lithium silicate - ZLS and Hybrid ceramic - HC) and thickness (0.5 or 1 mm). Tensile stress concentration was calculated using the finite element method. The crowns were adhesivelly cemented and step stress fatigued to calculate reliability for missions at 600 and 1000 N. Translucency was analyzed in 10 discs of each material and thickness.

RESULTS

Higher stress concentration was found in thinner crowns and those with higher elastic modulus. YZHT crowns were suspended when fatigue reached 1500 N load, thus 1-parameter Weibull was used to analyze the data. Reliability was only affected by thickness at 1000 N. ZLS.5 showed lower survival than HC.5, which was similar to the groups that presented 100% survival. YZHT showed the highest strength and data scattering. ZLS1 (22.3 ± 1.4) presented higher translucency than HC1 (19.2 ± 0.6) and YZHT1 (12.0 ± 2.9), whereas ZLS.5 and HC.5 were similar to each other (26.5 ± 2.3, 26.7 ± 2.2) and superior to YZHT.5 (12.7 ± 1.2).

SIGNIFICANCE

HC.5 combined high reliability and translucency with low stress concentration, yielding better crown performance and tooth preservation.

Static and Fatigue Loading of Veneered Implant-Supported Fixed Dental Prostheses

PURPOSE

This study aimed to compare the load to failure and the probability of survival of porcelain fused to zirconia (PFZ) three-unit, implant-supported, fixed dental prostheses (FDPs) to those of indirect composites veneered to either zirconia (CVZ) or milled fiber-reinforced composite (FRC) frameworks under static and fatigue loading.

MATERIALS AND METHODS

One-hundred and twenty posterior three-unit FDP (second premolar pontic) frameworks were fabricated via milling from a single Standard Tessellation Language (STL) file. The FDPs were divided into three groups. Each group (n = 40) was subjected to static (n = 20) and fatigue (n = 20) loading tests, as follows: (1) PFZ: zirconia framework layered with porcelain veneer; (2) CVZ: zirconia framework veneered with indirect composite resin; and (3) FRC: FRC framework veneered with indirect composite resin. After porcelain veneering onto sintered zirconia frameworks, or resin composite veneering onto zirconia or FRC frameworks, FDPs were cemented on their abutments using self-adhesive resin cement. After thermal cycling, half of the FDPs were subjected to an accelerated fatigue test. The other half of the FDPs were subjected to single load-to-failure (SLF) testing at a crosshead speed (1 mm/min). Lifetime analysis was conducted to determine the probability of survival, and fractographic analysis was performed.

RESULTS

Significant differences were observed among the studied groups for SLF with the highest characteristic strength values observed for PFZ (2154 N), followed by 1905.47 N for CVZ and 1679.56 N for FRC. The probability of survival for 100,000 cycles at 500 N was the highest for FRC (98%) and CVZ (100%) and was significantly lower for PFZ (88%). Different fracture patterns were observed in the fractography.

CONCLUSIONS

In fatigue testing, which simulates masticatory function better than static tests, a higher probability of survival was observed for FRC and CVZ than for PFZ. Framework fractures were not observed only for the FRC group, indicating that chairside repair with the addition of indirect composite could be performed for continued function.

A study on stress distribution to cement layer and root dentin for post and cores made of CAD/CAM materials with different elasticity modulus in the absence of ferrule

Background

To evaluate the stress distribution in a maxillary central incisor with different post and cores made of six CAD/CAM materials with different elastic modulus in the absence of ferrule using the finite element analysis.

Material and Methods

A three-dimensional endodontically treated maxillary central incisor restored with an all-ceramic crown was modelled in Rhinoceros (5.0 SR8, McNeel). The geometries were analyzed in ANSYS 17.2 (ANSYS Inc.) considering isotropic, homogeneous, linearly elastic materials with perfectly bonded contacts. The elastic moduli (E) of the post-and-cores defined the groups to be compared: nanoceramic resin (E=12.8GPa); composite resin (E=16GPa); hybrid ceramic (E=34.7GPa); lithium disilicate (E=95GPa); titanium (Ti-Al6-V4) (E=112GPa); and Y-TZP material (E=209.3GPa). The set was constrained in the cortical bone and loaded (45°/100 N) on the incisor palatine face. Stress distribution was analyzed by Maximum Principal Stress criteria for the crown-core cement line, Post-and-core's cement line, Post-and-core system and Dentin.

Results

The stress distribution at the crown-core cement line (11.4 - 13.2 MPa) was inversely proportional to the increase of the elastic modulus of the post-core approaches, while it was direct proportional on the post-and-core (4.7 - 40 MPa) and cement line (4.1 - 6.2 MPa). Stress distribution on the dentin was similar for all groups (24.7 - 25.3).

Conclusions

Post-and-core made by CAD/CAM seems to be an efficient treatment alternative, since it is a conservative approach, promotes better aesthetic quality and it allows the control of the cement line thickness. Key words:Endodontically treated teeth, Post-and-core technique, Ceramic crown, Finite element analysis, Biomimetics.

Fatigue resistance of all-ceramic fixed partial dentures - Fatigue tests and finite element analysis

OBJECTIVE

To estimate the fatigue resistance of a new translucent zirconia material in comparison to lithium disilicate for 3-unit fixed partial dentures (FPDs).

METHODS

Eighteen 3-unit FPDs (replacement of first upper molar) with a connector size of 4mm×4mm were dry milled with a five-axis milling machine (Zenotec Select, Wieland, Germany) using discs made of a new translucent zirconia material (IPS e.max ZirCAD MT, Ivoclar Vivadent). Another 9 FPDs with a reduced connector size (3mm×4mm) were milled. The zirconia FPDs were sintered at 1500°C. For a comparison, 9 FPDs were made of IPS e.max Press, using the same dimensions. These IPS e.max Press FPDs were ground from a wax disc (Wieland), invested and pressed at 920°C. All FPDs were glazed twice. The FPDs were adhesively luted to PMMA dies with Multilink Automix. Dynamic cyclic loading was carried out on the molar pontic using Dyna-Mess testing machines (Stolberg, Germany) with 2×10⁶ cycles at 2Hz in water (37°C). Two specimens per group and load were subjected to decreasing load levels (at least 4) until the two specimens no longer showed any failures. Another third specimen was subjected to this load to confirm the result. All the specimens were evaluated under a stereo microscope (20× magnification). The number of cycles reached before observing a failure, and their dependence on the load and on the material, were modeled, using a Weibull model. This made it possible to estimate the fatigue resistance as the maximum load for which one would observe less than 1% failure after 2×10⁶ cycles. In addition to the experimental study, Finite Element Modeling (FEM) simulations were conducted to predict the force to failure for IPS e.max ZirCAD MT and IPS e.max Press with a reduced cross-section of the connectors.

RESULTS

The failure mode of the zirconia FPDs was mostly the fracture of the distal connector, whereas the failure mode of the lithium disilicate FPDs observed to be the fracture of the connectors or multiple cracks of the pontic. The fatigue resistance with 1% fracture probability was estimated to be 488N for the IPS e.max ZirCAD MT FPDs (453N for repeated test), 365N for IPS e.max ZirCAD MT FPDs with reduced connector size and 286N for the e.max Press FPDs. All three IPS e.max ZirCAD groups statistically performed significantly better than IPS e.max Press (p<0.001). On the other hand, no significant difference could be established between the two IPS e.max ZirCAD MT3 groups with a 4mm×4mm connector size (p>0.05). The allowable maximum principal stress (σ_max) which did not lead to failure during fatigue testing for IPS e.max ZirCAD MT3 was calculated between 208MPa and 223MPa for FPDs with 4mm×4mm connectors for 2×10⁶ cycles. This value could also be verified for the FPDs of the same material with 3mm×4mm connectors. On the other hand fatigue strength in terms of σ_max at 2×10⁶ cycles of IPS e.max Press was calculated to be between 78 and 90MPa.

SIGNIFICANCE

The fatigue resistance of the translucent zirconia 3-unit FPDs was about 60-70% higher than that of the lithium disilicate 3-unit FPDs, which may justify their use for molar replacements, provided that a minimal connector size of 4mm×4mm is observed. Even with a limited number of specimens (n=9) per group it was possible to statistically differentiate between the tested groups.

Surface and Mechanical Characterization of Dental Yttria-Stabilized Tetragonal Zirconia Polycrystals (3Y-TZP) After Different Aging Processes

Yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) is a ceramic material used in indirect dental restorations. However, phase transformation at body temperature may compromise the material's mechanical properties, affecting the clinical performance of the restoration. The effect of mastication on 3Y-TZP aging has not been investigated. 3Y-TZP specimens (IPS E-max ZirCAD and Z5) were aged in three different modes (n=13): no aging (control), hydrothermal aging (HA), or chewing simulation (CS). Mechanical properties and surface topography were analyzed. Analysis of variance showed that neither aging protocol (p=0.692) nor material (p=0.283) or the interaction between them (p=0.216) had a significant effect on flexural strength, values ranged from 928.8 MPa (IPSHA) to 1,080.6 MPa (Z5HA). Nanoindentation analysis showed that material, aging protocol, and the interaction between them had a significant effect (p<0.001) on surface hardness and reduced Young's modulus. The compositional analysis revealed similar yttrium content for all the experimental conditions (aging: p=0.997; material: p=0.248; interaction material×aging: p=0.720). Atomic force microscopy showed an effect of aging protocols on phase transformation, with samples submitted to CS exhibiting features compatible with maximized phase transformation, such as increased volume of the material microstructure at the surface leading to an increase in surface roughness.

Effect of accelerated aging on dental zirconia-based materials

OBJECTIVE

To investigate the effect of aging on phase transformation and mechanical properties of yttria-tetragonal zirconia polycrystal (Y-TZP).

MATERIALS AND METHODS

Fully-sintered Y-TZP slabs, IPS E-max ZirCAD (ZC - Ivoclar) and Z-5 ceramic (Z5 - C5 Medical Werks), were artificially aged in autoclave for: 0, 30, 60 or 90min. Flexural strength (FS), crystalline changes (X-ray diffraction analysis - XRD) and surface topography were analyzed. 0 and 90min-aged samples were evaluated by nanoindentation to measure hardness and modulus, and results were compared using Wilcoxan Mann Whitney rank sum test (p≤0.05). FS results were compared using two-way ANOVA and Tukey HSD (α=0.05).

RESULTS

Material factor had significant effect (p=0.001) on flexural strength (Z5=966.95MPa; ZC=847.82MPa), but aging did not. Nanoindentation showed incidence of typical load/depth curves combined with some exhibiting features compatible with cracking. When typical curves were considered, aging had no effect on the modulus and hardness, but hardness was dependent on material type. A steady increase in the m phase related to aging time was observed for ZC samples. The maximum incidence of m phase was 6.56% for Z5/60min.

SIGNIFICANCE

Flexural strength is not affected by surface transformation in dental Y-TZP. Hydrothermal aging has an effect on m content and surface topography of different zirconia brands, but mechanical tests that can precisely characterize surface changes in aged Y-TZP are still missing.

Effect of different veneering techniques on the fracture strength of metal and zirconia frameworks

PURPOSE

To determine whether the fracture strengths and failure types differed between metal and zirconia frameworks veneered with pressable or layering ceramics.

MATERIALS AND METHODS

A phantom molar tooth was prepared and duplicated in 40 cobalt-chromium abutments. Twenty metal (IPS d.SIGN 15, Ivoclar, Vivadent, Schaan, Liechtenstein) and 20 zirconia (IPS e.max ZirCAD, Ivoclar) frameworks were fabricated on the abutments. Each framework group was randomly divided into 2 subgroups according to the veneering material: pressable and layering ceramics (n=10). Forty molar crowns were fabricated, cemented onto the corresponding abutments and then thermocycled (5-55℃, 10,000 cycles). A load was applied in a universal testing machine until a fracture occurred on the crowns. In addition, failure types were examined using a stereomicroscope. Fracture load data were analyzed using one-way ANOVA and Tukey HSD post-hoc tests at a significance level of 0.05.

RESULTS

The highest strength value was seen in metal-pressable (MP) group, whereas zirconia-pressable (ZP) group exhibited the lowest one. Moreover, group MP showed significantly higher fracture loads than group ZP (P=.015) and zirconia-layering (ZL) (P=.038) group. No significant difference in fracture strength was detected between groups MP and ML, and groups ZP and ZL (P>.05). Predominant fracture types were cohesive for metal groups and adhesive for zirconia groups.

CONCLUSION

Fracture strength of a restoration with a metal or a zirconia framework was independent of the veneering techniques. However, the pressing technique over metal frameworks resisted significantly higher fracture loads than zirconia frameworks.

Fatigue Resistance of Y-TZP/Porcelain Crowns is Not Influenced by the Conditioning of the Intaglio Surface

OBJECTIVES

The objective of this study was to investigate the effects of treatments of the intaglio surface of Y-TZP frameworks and luting agents on the fatigue resistance of all-ceramic crowns.

METHODS

A research design was chosen that attempted to reduce the likelihood of Hertzian cracking and to increase the probability of fracture initiation at the intaglio surface of the framework. Ninety identical preparations were machined in a dentin-like epoxy composite. Each preparation was restored with a Y-TZP framework made by a CAD/CAM system and veneered using feldspathic ceramic. Prior to cementation the intaglio surface of the ceramic was treated using one of four treatments: 1) cleaning with isopropyl alcohol; 2) application of an overglaze; 3) sandblasting with 125 μm aluminum oxide powder; and 4) sandblasting with 30 μm silica powder (CJ). One of three luting cements were used: 1) zinc phosphate; 2) glass ionomer; and 3) adhesive resin cement (PN). All three cements were tested against frameworks that were alcohol cleaned. Only the PN cements were tested against frameworks that had been sandblasted or glazed. Altogether, six groups of 15 specimens each were tested. Fatigue resistance was evaluated using stepwise loads at 1.4 Hz until failure: 5000 cycles at maximum load of 200 N, followed by 10,000 cycles at maximum loads of 800, 1000, 1200, and 1400 N. The cement thickness and failure modes were analyzed using a stereomicroscope and scanning electron microscopy. The results were analyzed using the Kaplan-Meier and Mantel-Cox log rank tests (5%), a one-way analysis of variance, Tukey multiple comparison test, and Weibull nonparametric test.

RESULTS

The predominant failure mode was chipping of the veneer. The crowns cemented with the adhesive resin cement exhibited chipping failure at higher mean loads than did crowns cemented with cements that usually do not bond strongly with dentin. When the adhesive cement was used, glazing and sandblasting intaglio framework surface treatments exhibited lower mean loads at chipping than did crowns whose intaglio surface was only cleaned with alcohol. Weibull analysis indicated that all specimens had a high ratio of late-to-early failures.

CONCLUSIONS

The fatigue experiment produced a pattern of failures that is very similar to that observed in clinical trials of Y-TZP crowns that are veneered with feldspathic porcelain. Crowns cemented with an adhesive resin cement exhibited chipping at a significantly higher mean load than those cemented with luting cements that do not usually form strong bonds with dentin. When cemented with adhesive resin cement, glazing or sandblasting the intaglio surface of the framework significantly reduced the mean fatigue loads at which chipping of veneers occurred, as compared to crowns whose intaglio surface had only been cleaned with alcohol. For this cement glazing or sandblasting the intaglio surface of the crown is not recommended.

Evaluation of zirconia bonding to veneering porcelain

Zirconium dioxide as core ceramic material for dental crowns and bridges, possess high strength, chemical stability and superior aesthetics after veneering. Veneering ceramic is considered to be the weakest part of all-ceramic restorations. The adhesion between the core and veneering porcelain is based on the manner in which the connection occurs in metal-ceramic structures. Standard procedures for connecting zirconia to hard dental tissues and veneering materials do not achieve the required strength of bonding. The aim of the paper is to investigate different surface treatments of the zirconium dioxide ceramic core and find the best, for achieving highest adhesive bonding values to veneering porcelain. The study was primarily designed to investigate the bonding strength of the veneering porcelain to zirconia with in vitro Macro shear bond strength test. The specimens with different surface treatment of the zirconia were divided in five groups of twelve according to the treatment of zirconium surface and results showed highest bonding values for specimens treated with Rocatec system.

Evidence of yttrium silicate inclusions in YSZ-porcelain veneers

This report introduces the discovery of crystalline defects that can form in the porcelain veneering layer when in contact with yttria-stabilized zirconia (YSZ). The focus was on dental prostheses and understanding the defects that form in the YSZ/porcelain system; however the data reported herein may have broader implications toward the use and stability of YSZ-based ceramics in general. Specimens were cut from fully sintered YSZ plates and veneering porcelain was applied (<1 mm thick) to one surface and fired under manufacturer's recommended protocol. Scanning electron microscopy (SEM) with integrated electron dispersive X-ray (EDAX) was used for microstructural and elemental analysis. EDAX, for chemical analysis and transmission electron diffraction (TED) for structural analysis were both performed in the transmission electron microscope (TEM). Additionally, in order to spatially resolve Y-rich precipitates, micro-CT scans were conducted at varying depths within the porcelain veneer. Local EDAX (SEM) was performed in the regions of visible inclusions and showed significant increases in yttrium concentration. TEM specimens also showed apparent inclusions in the porcelain and selected area electron diffraction was performed on these regions and found the inclusions to be crystalline and identified as either yttrium-silicate (Y2 SiO5 ) or yttrium-disilicate (Y2 Si2 O7 ). Micro-CT data showed that yttrium-silicate precipitates were distributed throughout the thickness of the porcelain veneer. Future studies are needed to determine whether many of the premature failures associated with this materials system may be the result of crystalline flaws that form as a result of high temperature yttrium diffusion near the surfaces of YSZ.

Randomized clinical trial on single retainer all-ceramic resin-bonded fixed partial dentures: Influence of the bonding system after up to 55 months

OBJECTIVES

This study evaluated the clinical outcome of all-ceramic resin-bonded fixed partial dentures (RBFPDs) with a cantilevered single-retainer design.

METHODS

Thirty anterior zirconia ceramic RBFPDs were inserted using either a phosphate monomer containing resin (Panavia 21 TC; N=16) or an adhesive bonding system with a phosphoric acid acrylate primer (Multilink-Automix with Metal/Zirconia primer; N=14).

RESULTS

During a mean observation time of 41.7 months one debonding occurred in each group. Both RBFPDs could be rebonded successfully resulting in a three-year survival rate of 100%.

CONCLUSION

Independent of the bonding system cantilevered zirconia ceramic RBFPDs showed promising results during the first three years.

CLINICAL SIGNIFICANCE

Single-retainer zirconia ceramic RBFPD present an alternative treatment option offering good aesthetics, a minimal invasive preparation, a high biocompatibility and can even be used to treat juvenile patients who do not yet come into consideration for implant placement. No significant influence of the bonding system used was detected so far.

Internal adaptation, marginal accuracy and microleakage of a pressable versus a machinable ceramic laminate veneers

OBJECTIVES

The aim of this study was to evaluate the internal adaptation and marginal properties of ceramic laminate veneers fabricated using pressable and machinable CAD/CAM techniques.

MATERIALS AND METHODS

40 ceramic laminate veneers were fabricated by either milling ceramic blocks using a CAD/CAM system (group 1 n=20) or press-on veneering using lost wax technique (group 2 n=20). The veneers were acid etched using hydrofluoric acid, silanated, and cemented on their corresponding prepared teeth. All specimens were stored under water (37 °C) for 60 days, then received thermocycling (15,000 cycles between 5 and 55 °C and dwell time of 90 s) followed by cyclic loading (100,000 cycles between 50 and 100 N) before immersion in basic fuchsine dye for 24 h. Half of the specimens in each group were sectioned in labio-lingual direction and the rest were horizontally sectioned using precision cutting machine (n=10). Dye penetration, internal cement film thickness, and vertical and horizontal marginal gaps at the incisal and cervical regions were measured (α=0.05).

RESULTS

Pressable ceramic veneers demonstrated significantly lower (F=8.916, P<0.005) vertical and horizontal marginal gaps at the cervical and incisal margins and lower cement film thickness (F=50.921, P<0.001) compared to machinable ceramic veneers. The inferior marginal properties of machinable ceramic veneers were associated with significantly higher microleakage values.

CONCLUSIONS

Pressable ceramic laminate veneers produced higher marginal adaptation, homogenous and thinner cement film thickness, and improved resistance to microleakage compared to machinable ceramic veneers.

CLINICAL SIGNIFICANCE

The manufacturing process influences internal and marginal fit of ceramic veneers. Therefore, dentist and laboratory technicians should choose a manufacturing process with careful consideration.

Influence of surface modification techniques on shear bond strength between different zirconia cores and veneering ceramics

PURPOSE

Veneering porcelain might be delaminated from underlying zirconia-based ceramics. The aim of this study was the evaluation of the effect of different surface treatments and type of zirconia (white or colored) on shear bond strength (SBS) of zirconia core and its veneering porcelain.

MATERIALS AND METHODS

Eighty zirconia disks (40 white and 40 colored; 10 mm in diameter and 4 mm thick) were treated with three different mechanical surface conditioning methods (Sandblasting with 110 µm Al₂O₃ particle, grinding, sandblasting and liner application). One group had received no treatment. These disks were veneered with 3 mm thick and 5 mm diameter Cercon Ceram Kiss porcelain and SBS test was conducted (cross-head speed = 1 mm/min). Two and one way ANOVA, Tukey's HSD Past hoc, and T-test were selected to analyzed the data (α=0.05).

RESULTS

In this study, the factor of different types of zirconia ceramics (P=.462) had no significant effect on SBS, but the factors of different surface modification techniques (P=.005) and interaction effect (P=.018) had a significant effect on SBS. Within colored zirconia group, there were no significant differences in mean SBS among the four surface treatment subgroups (P=0.183). Within white zirconia group, "Ground group" exhibited a significantly lower SBS value than "as milled" or control (P=0.001) and liner (P=.05) groups.

CONCLUSION

Type of zirconia did not have any effect on bond strength between zirconia core and veneer ceramic. Surface treatment had different effects on the SBS of the different zirconia types and grinding dramatically decreased the SBS of white zirconia-porcelain.

Reliability and failure modes of implant-supported zirconium-oxide fixed dental prostheses related to veneering techniques

OBJECTIVES

To compare fatigue failure modes and reliability of hand-veneered and over-pressed implant-supported three-unit zirconium-oxide fixed-dental-prostheses(FDPs).

METHODS

Sixty-four custom-made zirconium-oxide abutments (n=32/group) and thirty-two zirconium-oxide FDP-frameworks were CAD/CAM manufactured. Frameworks were veneered with hand-built up or over-pressed porcelain (n=16/group). Step-stress-accelerated-life-testing (SSALT) was performed in water applying a distributed contact load at the buccal cusp-pontic-area. Post failure examinations were carried out using optical (polarized-reflected-light) and scanning electron microscopy (SEM) to visualize crack propagation and failure modes. Reliability was compared using cumulative-damage step-stress analysis (Alta-7-Pro, Reliasoft).

RESULTS

Crack propagation was observed in the veneering porcelain during fatigue. The majority of zirconium-oxide FDPs demonstrated porcelain chipping as the dominant failure mode. Nevertheless, fracture of the zirconium-oxide frameworks was also observed. Over-pressed FDPs failed earlier at a mean failure load of 696 ± 149N relative to hand-veneered at 882 ± 61N (profile I). Weibull-stress-number of cycles-unreliability-curves were generated. The reliability (2-sided at 90% confidence bounds) for a 400N load at 100K cycles indicated values of 0.84 (0.98-0.24) for the hand-veneered FDPs and 0.50 (0.82-0.09) for their over-pressed counterparts.

CONCLUSIONS

Both zirconium-oxide FDP systems were resistant under accelerated-life-time-testing. Over-pressed specimens were more susceptible to fatigue loading with earlier veneer chipping.

A new method to test the fracture probability of all-ceramic crowns with a dual-axis chewing simulator

OBJECTIVES

The purpose of this study was to validate a new laboratory method to test all-ceramic systems with regard to the proportion of failures.

METHODS

Sixteen standardized mandibular molar crowns consisting of two different materials (IPS Empress, IPS e.max Press) were adhesively luted on CAD/CAM milled PMMA abutments (first lower molar, circular chamfer). All crowns were loaded applying an eccentric force in a Willytec chewing simulator (steel stylus, Ø 2.4 mm, 2 mm lateral movement from fossa to cuspal tip) with stepwise increase of the load (3, 5, 9 kg, 100,000 cycles each, 0.8 Hz) and simultaneous thermocycling (5°C/55°C×417 per phase). Another four crowns of each material were subjected to force measurements with a 3D force sensor during dynamic loading of each loading phase using two different lateral movements (from fossa to cusp and vice versa).

RESULTS

The cumulative forces for the three directions in space were much higher compared to the static load of the chewing simulator (maximal force at 3 kg 60 N, 5 kg 160 N, 9 kg 240 N). There was no statistically significant difference in the mean or maximal force between the two materials or two different lateral movements. During dynamic loading, no fractures occurred in the molar crowns made of IPS e.max Press, whereas 50% of the IPS Empress crowns showed failures (75% fractures and 25% chippings) (log-rank test p=0.002). Most of the Empress crowns fractured during the third loading phase (9 kg).

CONCLUSIONS

The forces that the dead weights exerted during dynamic loading were 2-3 times higher than those during static loading. None of the lithium disilicate ceramic molar crowns fractured, whereas half of the leucite reinforced molar crowns failed during dynamic loading.