Thermal-induced residual stresses affect the fractographic patterns of zirconia-veneer dental prostheses

Effect of different surface treatments and adhesive cementation on the surface topography and flexural strength of translucent and ultra-translucent monolithic zirconia

PURPOSE

To evaluate the effect of different surface treatments and adhesive cementation on the miniflexural strength (MFS) of monolithic zirconia.

MATERIALS AND METHODS

Two-hundred and forty (240) sintered bars of translucent zirconia (ZT) and ultra-translucent zirconia (ZUT) were obtained (8 mm ×2 mm ×1 mm). The bars were divided into 16 groups (n = 15) according to the factors "Zirconia" (ZT and ZUT), "Cementation" (Cem) and "surface treatment" (Ctrl:Control, Al:Aluminum oxide/Al2O3 50 µm, Si:Silica/SiO2 coated alumina particles oxide 30 µm, Gl:Glazing+hydrofluoric acid). Half of the bars received an adhesive layer application, followed by application of resin cement and light curing. The surface roughness was measured in non-cemented groups. All the bars were subjected to the MFS test (1.0 mm/min; 100 kgf). Scanning electron microscopy was used for qualitative analyses. MFS data (MPa) and roughness (µm) were statistically evaluated by three-way and two-way ANOVA respectively and Tukey's test (5%).

RESULTS

The surface treatment and the interaction were significant for roughness. Glazing promoted less roughness compared to silicatization. Regarding MFS, only the zirconia and surface treatment factors were significant. For ZT, the sandblasted groups had an increase in MFS and glazing reduced it. There was no difference between the groups without cementation for the ZUT; however, ZUT.Si/Cem, and ZUT.Al/Cem obtained superior MFS among the cemented groups.

CONCLUSIONS

Sandblasting increases the flexural strength for ZT, while glaze application tends to reduce it. Applying resin cement increases the flexural strength of ZUT when associated with sandblasting. Sandblasting protocols promote greater surface roughness.

3D Printing of Bioinert Oxide Ceramics for Medical Applications

Three-dimensionally printed metals and polymers have been widely used and studied in medical applications, yet ceramics also require attention. Ceramics are versatile materials thanks to their excellent properties including high mechanical properties and hardness, good thermal and chemical behavior, and appropriate, electrical, and magnetic properties, as well as good biocompatibility. Manufacturing complex ceramic structures employing conventional methods, such as ceramic injection molding, die pressing or machining is extremely challenging. Thus, 3D printing breaks in as an appropriate solution for complex shapes. Amongst the different ceramics, bioinert ceramics appear to be promising because of their physical properties, which, for example, are similar to those of a replaced tissue, with minimal toxic response. In this way, this review focuses on the different medical applications that can be achieved by 3D printing of bioinert ceramics, as well as on the latest advances in the 3D printing of bioinert ceramics. Moreover, an in-depth comparison of the different AM technologies used in ceramics is presented to help choose the appropriate methods depending on the part geometry.

Investigations into the interface failure of yttria partially stabilised zirconia - porcelain dental prostheses through microscale residual stress and phase quantification

OBJECTIVES

Yttria Partially Stabilised Zirconia (YPSZ) is a high strength ceramic which has become widely used in porcelain veneered dental copings due to its exceptional toughness. Within these components the residual stress and crystallographic phase of YPSZ close to the interface are highly influential in the primary failure mode; near interface porcelain chipping. In order to improve present understanding of this behaviour, characterisation of these parameters is needed at an improved spatial resolution.

METHODS

In this study transmission micro-focus X-ray Diffraction, Raman spectroscopy, and focused ion beam milling residual stress analysis techniques have, for the first time, been used to quantify and cross-validate the microscale spatial variation of phase and residual stress of YPSZ in a prosthesis cross-section.

RESULTS

The results of all techniques were found to be comparable and complementary. Monoclinic YPSZ was observed within the first 10μm of the YPSZ-porcelain interface with a maximum volume fraction of 60%. Tensile stresses were observed within the first 150 μm of the interface with a maximum value of ≈300 MPa at 50 μm from the interface. The remainder of the coping was in mild compression at ≈-30MPa, with shear stresses of a similar magnitude also being induced by the YPSZ phase transformation.

SIGNIFICANCE

The analysis indicates that the interaction between phase transformation, residual stress and porcelain creep at YPSZ-porcelain interface results in a localised porcelain fracture toughness reduction. This explains the increased propensity of failure at this location, and can be used as a basis for improving prosthesis design.

Why a zero CTE mismatch may be better for veneered Y-TZP structures

OBJECTIVE

Compare residual stress distribution of bilayered structures with a mismatch between the coefficient of thermal expansion (CTE) of framework and veneering ceramic. A positive mismatch, which is recommended for metal-ceramic dental crowns, was hypothesized to contribute to a greater chipping frequency in veneered Y-TZP structures. In addition, the multidirectional nature of residual stresses in bars and crowns is presented to explore some apparent contradictions among different studies.

METHODS

Planar bar and crown-shaped bilayered specimens with 0.7 mm framework thickness and 1.5 mm porcelain veneer thickness were investigated using finite element elastic analysis. Eight CTE mismatch conditions were simulated, representing two framework materials (zirconia and metal) and six veneering porcelains (distinguished by CTE values). Besides metal-ceramic and zirconia-ceramic combinations indicated by the manufacturer, models presenting similar mismatch values (1 ppm/°C) with different framework materials (metal or zirconia) and zirconia-based models with metal-compatible porcelain veneers were also tested. A slow cooling protocol from 600 °C to room temperature was simulated. The distributions of residual maximum and minimum principal stresses, as well as stress components parallel to the long axis of the specimens, were analysed.

RESULTS

Planar and crown specimens generated different residual stress distributions. When manufacturer recommended combinations were analysed, residual stresses obtained for zirconia models were significantly higher than those for metal-based models. When zirconia frameworks were combined with metal-compatible porcelains, the residual stress values were even higher. Residual stresses were not different between metal-based and zirconia-based models if the CTE mismatch was similar.

SIGNIFICANCE

Some conclusions obtained with planar specimens cannot be extrapolated to clinical situations because specimen shape strongly influences residual stress patterns. Since positive mismatch generates compressive hoop stresses and tensile radial stresses and since zirconia-based crowns tend to be more vulnerable to chipping, a tensile stress-free state generated with a zero CTE mismatch could be advantageous.

In vitro lifetime of zirconium dioxide-based crowns veneered using Rapid Layer Technology

Rapid Layer Technology (RLT) uses computer-aided design/computer-aided manufacturing (CAD/CAM) to manufacture a veneer layer that is adhesively bonded to the zirconia framework, avoiding firing steps during the fabrication process and thus preventing build-up of residual stresses. This work studied, using sliding contact fatigue, the in vitro lifetime of restorations produced using RLT compared with restorations produced using conventional veneering techniques. Zirconia copings were veneered with a conventional hand-layering method (VM9) using a fast (n = 16) or a slow (n = 16) cooling protocol, or with RLT. For the latter, the veneers were CAD/CAM fabricated using a feldspathic reinforced-glass (Vitablocs Mark II; n = 16) or a polymer-infiltrated reinforced-glass network (Enamic; n = 16) and adhesively bonded to the zirconia frameworks. Crowns thus obtained were submitted to sliding contact fatigue against a steatite indenter in a chewing simulator until failure. A Kaplan-Meier survival analysis was conducted. None of the hand-layered restorations survived after a 2 × 10⁶ -cycle interval, whereas no fractures in the RLT groups were observed. Vitablocs Mark II veneers survived for a longer testing period (3.5 × 10⁶ cycles) than their Enamic counterparts (2.5 × 10⁶ cycles) owing to their superior wear behavior. The RLT represents an efficient method to veneer zirconia frameworks by reducing processing steps and, more importantly, increasing the lifetime of the restorations.

Influence of residual thermal stresses on the edge chipping resistance of PFM and veneered zirconia structures: Experimental and FEA study

OBJECTIVE

Chipping fractures of the veneering porcelain are frequently reported for veneered all-ceramic crowns. In the present study, the edge chipping test is used to measure the toughness and the edge chipping resistance of veneered zirconia and porcelain-fused-to-metal (PFM). The aim is to describe an edge chipping method developed with the use of a universal testing machine and to verify the accuracy of this method to determine the influence of residual thermal stresses on the chipping fracture resistance of veneering porcelain. A finite element analysis (FEA) was used to study the residual stress profiles within the veneering porcelain.

METHODS

Veneered zirconia and PFM bar specimens were subjected to either a fast or a slow cooling protocol. The chipping resistances were measured using the edge chipping method. The load was applied in two different directions, in which the Vickers indenter was placed in the veneering porcelain either parallel or perpendicular to the veneer/framework interface. The mean edge chipping resistance (R_eA) and fracture toughness (K_C) values were analysed. R_eA was calculated by dividing the critical force to cause the chip by the edge distance. K_C was given by a fracture analysis that correlates the critical chipping load (F_C) regarding edge distance (d) and material toughness via K_C=F_C/(βd^1.5).

RESULTS

The R_eA revealed similar values (p>0.005) of chipping resistance for loads applied in the parallel direction regardless of framework material and cooling protocol. For loads applied in the perpendicular direction to the veneer/framework interface, the most chip resistant materials were slow cooled veneered zirconia (251.0N/mm) and the PFM fast cooled (190.1N/mm). K_C values are similar to that for monolithic porcelain (0.9MPa.√m), with slightly higher values (1.2MPa.√m) for thermally stressed PFM fast cooled and veneered zirconia slow cooled groups.

SIGNIFICANCE

The developed and reported edge chipping method allows for the precise alignment of the indenter in any predetermined distance from the edge. The edge chipping method could be useful in determining the different states of residual thermal stresses on the veneering porcelain.

Thermal cracks of implant-based zirconia four-unit restorations: A fractographic analysis of two restorations fractured during production

Two zirconia-based 4-unit restorations intended for the same patient fractured during the veneering process even though the prolonged cooling protocol recommended by the manufacturers was used. Fractographic analyses revealed that both restorations fractured as a result of thermal shock, but at different times during production. Further investigation is necessary to optimize the firing protocols for large zirconia-based restorations and avoid fracture due to thermal shock.

ADM guidance-Ceramics: guidance to the use of fractography in failure analysis of brittle materials

OBJECTIVES

To provide background information and guidance as to how to use fractography accurately, a powerful tool for failure analysis of dental ceramic structures.

METHODS

An extended palette of qualitative and quantitative fractography is provided, both for in vivo and in vitro fracture surface analyses. As visual support, this guidance document will provide micrographs of typical critical ceramic processing flaws, differentiating between pre- versus post sintering cracks, grinding damage related failures and occlusal contact wear origins and of failures due to surface degradation.

RESULTS

The documentation emphasizes good labeling of crack features, precise indication of the direction of crack propagation (dcp), identification of the fracture origin, the use of fractographic photomontage of critical flaws or flaw labeling on strength data graphics. A compilation of recommendations for specific applications of fractography in Dentistry is also provided.

SIGNIFICANCE

This guidance document will contribute to a more accurate use of fractography and help researchers to better identify, describe and understand the causes of failure, for both clinical and laboratory-scale situations. If adequately performed at a large scale, fractography will assist in optimizing the methods of processing and designing of restorative materials and components. Clinical failures may be better understood and consequently reduced by sending out the correct message regarding the fracture origin in clinical trials.

Characterisation of nanovoiding in dental porcelain using small angle neutron scattering and transmission electron microscopy

OBJECTIVES

Recent studies of the yttria partially stabilised zirconia-porcelain interface have revealed the presence of near-interface porcelain nanovoiding which reduces toughness and leads to component failure. One potential explanation for these nanoscale features is thermal creep which is induced by the combination of the residual stresses at the interface and sintering temperatures applied during manufacture. The present study provides improved understanding of this important phenomenon.

METHODS

Transmission electron microscopy and small angle neutron scattering were applied to a sample which was crept at 750°C and 100MPa (sample C), a second which was exposed to an identical heat treatment schedule in the absence of applied stress (sample H), and a reference sample in the as-machined state (sample A).

RESULTS

The complementary insights provided by the two techniques were in good agreement and log-normal void size distributions were found in all samples. The void number density was found to be 1.61μm^-2, 25.4μm^-2 and 98.6μm^-2 in samples A, H and C respectively. The average void diameter in sample A (27.1nm) was found to be more than twice as large as in samples H (10.2nm) and C (11.6nm). The crept data showed the highest skewness parameter (2.35), indicating stress-induced growth of larger voids and void coalescence that has not been previously observed.

SIGNIFICANCE

The improved insight presented in this study can be integrated into existing models of dental prostheses in order to optimise manufacturing routes and thereby reduce the significant detrimental impact of this nanostructural phenomenon.

Temperature rise during polymerization of different cavity liners and composite resins

OBJECTIVE

The purpose of this study was to evaluate the thermal insulating properties of different light curing cavity liners and composite resins during light emitting diode (LED) curing.

MATERIALS AND METHODS

Sixty-four dentin discs, 1 mm thick and 8 mm in diameter, were prepared. Specimens were divided into four groups. Calcium hydroxide (Ca[OH]2), resin-modified glass ionomer cement, flowable composite and adhesive systems were applied to dentin discs according to the manufacturers' instructions. The rise in temperature during polymerization with a LED curing unit (LCU) was measured using a K-type thermocouple connected to a data logger. Subsequently, all specimens were randomly divided into one of two groups. A silorane-based composite resin and a methacrylate-based composite resin were applied to the specimens. Temperature rise during polymerization of composite resins with LCU were then measured again. Data were analyzed using one-way ANOVA and post hoc Tukey analyses.

RESULTS

There were significant differences in temperature rise among the liners, adhesives, and composite resins (P < 0.05). Silorane-based composite resin exhibited significantly greater temperature rises than methacrylate-based resin (P < 0.05). The smallest temperature rises were observed in Ca(OH)2 specimens.

CONCLUSION

Thermal insulating properties of different restorative materials are important factors in pulp health. Bonding agents alone are not sufficient to protect pulp from thermal stimuli throughout curing.

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.

On edge chipping testing and some personal perspectives on the state of the art of mechanical testing

OBJECTIVE

The edge chipping test is used to measure the fracture resistance of dental restoration ceramics and resin composites. This paper focuses on the progress of evaluating chipping resistance of these materials and also on the progress of standardization of this test method. This paper also makes observations about the state of the art of mechanical testing of ceramic and composite restorative materials in general. Interlaboratory comparative studies ("round robins") are recommended.

METHODS

An edge chipping machine was used to evaluate dozens of materials including porcelains, glass ceramics, aluminas, zirconias, filled resin-composites, new hybrid ceramic-resin composites, laminated composite ceramics, and even polymethyl methacrylate based denture materials. Force versus distance data was collected over a broad range with different indenters. Several chipping resistance parameters were quantified.

RESULTS

Older restorative materials such as feldspathic porcelains and veneering materials had limited chipping resistance, but more modern ceramics and filled composites show significant improvements. A yttria-partially stabilized zirconia had the greatest resistance to chipping. Much of the early work on edge chipping resistance of brittle materials emphasized linear force versus distance trends obtained with relatively blunt Rockwell C indenters. More recently, trends for dental restorative materials with alternative sharper indenters have been nonlinear. A new phenomenological model with a simple quadratic function fits all data exceptionally well. It is loosely based on an energy balance between indenter work and fracture and deformation energies in the chipped material.

SIGNIFICANCE

Although a direct comparison of our laboratory scale tests on idealized simple geometries to clinical outcomes has not yet been done, anecdotal evidence suggests the procedure does produce clinically relevant rankings and outcomes. Despite the variations in the trends and indenters, comparisons between materials can easily be made by chipping convenient block-shaped specimens with sharp conical 120°, Vickers, or Rockwell C indenters at a defined edge distance of 0.5mm. Broad distance ranges are recommended for trend evaluation. This work has provided important information for standardization.

Interfacial adhesion of zirconia/veneer bilayers with different thermal characteristics

The aim of this study was to investigate how changes in the thermal characteristics of veneer ceramics with almost identical chemical and mechanical properties but with different coefficients of thermal expansion (CTE) can modify their interfacial adhesion to zirconia. 48 bilayers made of one Y-TZP ceramic and four veneer ceramics were fabricated (n=12). Thermal residual stresses were calculated on the basis of the CTE and glass transition temperatures. After defined notching all specimens were loaded in a four-point bending test and the critical loads were recorded which induced stable crack extension at the adhesion interface. The strain energy release rate (G, J/m(2)) was calculated and was taken as a measure of interfacial adhesion. The CTE of the veneer ceramics were significantly correlated with their adhesion to Y-TZP (p<0.001). Interfacial adhesion in zirconia/veneer bilayers is predominantly affected by the thermal characteristics of the veneer ceramic.