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

Functionally graded bi-material interface for Porcelain Veneered Zirconia dental crowns: A study using viscoelastic finite element analysis

OBJECTIVES

During the manufacturing of Porcelain Veneered Zirconia (PVZ) dental crowns, the veneer-core system undergoes high-temperature firing cycles and gets fused together which is then, under a controlled setting, cooled down to room temperature. During this cooling process, the mismatch in thermal properties between zirconia and porcelain leads to the development of transient and residual thermal stresses within the crown. These thermal stresses are inherent to the PVZ dental crown systems and render the crown structure weak, acting as a precursor to veneer chipping, fracture, and delamination. In this study, the introduction of an intermediate functionally graded material (FGM) layer at the bi-material interface is investigated as a potentially viable alternative for providing a smoother transition of properties between zirconia and porcelain in a PVZ crown system.

METHODS

Anatomically correct 3D crown models were developed for this study, with and without the FGM layer modeled at the bi-material interface. A viscoelastic finite element model was developed and validated for an anatomically correct bilayer PVZ crown system which was then used for predicting residual and transient stresses in the bilayer PVZ crown. Subsequently, the viscoelastic finite element model was further extended for the analysis of graded sublayers within the FGM layer, and this extended model was used for predicting the residual and transient stresses in the functionally graded PVZ crown, with an FGM layer at the bi-material interface.

RESULTS

The study showed that the introduction of an FGM layer at the bi-material interface has the potential to reduce the effects from transient and residual stresses within the PVZ crown system relative to a bilayer PVZ crown structure. Furthermore, the study revealed that the FGM layer causes stress redistribution to alleviate the stress concentration at the interfacial surface between porcelain and zirconia which can potentially enhance the durability of the PVZ crowns towards interfacial debonding or fracture.

SIGNIFICANCE

Thus, the use of an FGM layer at the bi-material interface shows a good prospect for enhancing the longevity of the PVZ dental crown restorations by alleviating the abrupt thermal property difference and relaxing thermal stresses.

Interface adhesion on layered zirconia: Effects of the veneering ceramic material and veneering technique

PURPOSE

To evaluate the effect of different veneering ceramics and veneering techniques on the bond strength to zirconia.

MATERIALS AND METHODS

3Y-TZP zirconia blocks were sliced into 60 slabs, polished, and sintered. Each slab received one ceramic cylinder (Ø = 3.4 mm, 5 mm-high), according to the veneering ceramic type (feldspathic-FEL or lithium disilicate-based-LD) and the veneering technique (file-splitting with resin-based luting agent-RC, file-splitting with fusion ceramic-FC, or heat-pressing-HT), which resulted in six groups: FEL-RC, FEL-FC, FEL-HT, LD-RC, LD-FC, LD-HT. After preparation, the samples were immersed in distilled water for 24 h before the shear bond strength (SBS) test. The failure modes were classified as adhesive, predominantly adhesive, or cohesive. Representative failure mode images were taken in a Scanning Electron Microscope. The SBS data were analyzed by two-way ANOVA and Tukey's test.

RESULTS

Both type of veneering ceramic and technique affected the bond strength. FC led to the highest SBS values. RC and HP provided similar results when compared within each veneering ceramic. Lithium disilicate achieved lower bond strength than feldspathic ceramic when the heat-pressing technique was applied. The most frequent failure modes were predominantly adhesive and adhesive for FEL and LD, respectively.

CONCLUSION

File-splitting with fusion ceramic provided the highest adhesion to zirconia when feldspathic or lithium disilicate-based ceramics were used. The heat-pressing technique for veneering with lithium disilicate significantly decreased the bond strength when compared to the feldspathic ceramic.

Fatigue and failure mode analyses of glass infiltrated 5Y-PSZ bonded onto dentin analogues

The purpose of this study was to evaluate the fatigue survival of 5Y-PSZ zirconia infiltrated with an experimental glass and bonded onto dentin analogues. Disc-shaped specimens of a 5Y-PSZ (Katana UTML Kuraray Noritake) were cemented onto dentin analogs (NEMA G10) and divided into four groups (n = 15): Zctrl Group (control, without infiltration); Zglz Group (Glaze, compression surface); Zinf-comp Group (Experimental Glass, compression surface); Zinf-tens Group (Experimental Glass, tension surface). Surface treatments were varied. Cyclic fatigue loading, oblique transillumination, stereomicroscope examination, and scanning electron microscopy were performed. Fatigue data were analyzed (failure load and number of cycles) using survival analysis (Kaplan-Meier and Log-Rank Mantel-Cox). There was no statistically significant difference in fatigue survival between the Zglz, Zctrl, and Zinf-comp groups. The Zinf-tens group presented a significantly higher failure load when compared to the other groups and exhibited a different failure mode. The experimental glass effectively infiltrated the zirconia, enhancing structural reliability, altering the failure mode, and improving load-bearing capacity over more cycles, particularly in the group where the glass was infiltrated into the tensile surface of the zirconia. Glass infiltration into 5Y-PSZ zirconia significantly enhanced structural reliability and the ability to withstand loads over an increased number of cycles. This approach has the potential to increase the durability of zirconia restorations, reducing the need for replacements and save time and resources, promoting efficiency in clinical practice.

Borosilicate glass as a surface finishing alternative for improving the mechanical properties of third-generation zirconia

OBJECTIVE

This study evaluated the effect of an experimental borosilicate glass on the mechanical and optical behavior of 5Y-PSZ zirconia and comparing it to commercial glaze and as-sintered.

METHODS

Disc-shaped specimens of a 5Y-PSZ (Zpex Smile) were prepared and sintered (1550 °C, 2 h). The zirconia discs were randomly divided according to the surface treatment: as-sintered (C), commercial glaze (G), and experimental borosilicate glass (SL). Glaze and experimental glass powders were mixed with building liquids and applied to zirconia with a brush. G specimens were fired at 950 °C and SL at 1200 °C. An extended dwell time of 20 min was applied to both groups. Biaxial flexural strength, roughness (Ra and Rz), translucency (TP00), color alteration (ΔE00), Vickers hardness, fracture toughness, residual stresses, and x-ray diffraction analyses were conducted. Statistical analyses were performed with Weibull statistics, Kruskal-Wallis, or ANOVA tests (α = 5%).

RESULTS

SL yielded the highest flexural strength (799.35 MPa), followed by G (662.34 MPa), and C (485.38 MPa). The fracture origin of SL specimens was in the bulk zirconia, while G and C showed fractures starting at the surface. As-sintered reached the highest fracture toughness and hardness. Glaze and borosilicate glass provided surface compressive stresses. Borosilicate glass application led to phase transformation (t→m). SL and G showed the lowest roughness. TP00 and ΔE00 were similar among groups.

SIGNIFICANCE

Borosilicate glass improved strength without harming the optical properties of third-generation zirconia. Toughness and roughness provided by the experimental glass were similar to those from commercial glaze.

Effect of porcelain veneering technique in bilayered zirconia on bond strength and residual stress distribution

The use of all porcelain materials in dentistry has significantly increased in recent years. However, chipping has remained a common problem that affects bilayered zirconia restorations. Bonding between porcelain and the underlying zirconia framework is crucial to the success of the restoration. The bond strength may be affected by such factors as residual thermal stress and the veneering technique. This research focuses on investigating the potential and constraints of materials through an examination of the porcelain veneering technique, particularly hand-layering and heat-pressing. Forty-two cylindrical disc samples of zirconia (n = 7/group) were fabricated in the dimensions of 10 × 1.2 mm (diameter [D] × height [H]). The zirconia specimens were milled from IPS e.max® ZirCad [Z] block and Luxen Zr [L] block (n = 21/zirconia). The zirconia cores were layered with IPS e.max® Zirliner and heat-pressed with IPS e.max® ZirPress to produce a final veneer dimension of 5 × 3 mm (D × H). Conventional layering was performed for the rest of the zirconia cores using IPS e.max® Ceram and Shofu Vintage Zr. The final study groups were Luxen-Vintage (LV), Luxen-Ceram (LC), Luxen Zirpress (LP), ZirCad-Vintage (ZV), ZirCad-Ceram (ZC) and ZirCad-Zirpress (ZP). Five samples were subjected to shear bond testing (SBS) with a universal testing machine with a 5 kN load cell and 0.5 mm/min crosshead speed (n = 5/group). A sample underwent nanoindentation, and another was sectioned using Isomet machine to study the bonding interface. One-way ANOVA was used to run the statistical analyses of the SBS test. Statistical differences were found between ZV with LC and LP (p < 0.05). Residual stress is estimated to be higher in the middle of the porcelain compared with that on the surface and the interface. FESEM imaging reveals portions of visible bare zirconia on Luxen zirconia, whilst crack propagation occurred through voids in all hand-layered groups. Heat-pressed veneering showed comparable but not superior results to conventional hand-layered veneering. Heat-pressed veneering produced similar stress distribution profiles compared with hand-layered veneering.

Transient thermal stresses developed during speed sintering of 3 mol% yttria-stabilized tetragonal zirconia polycrystals

OBJECTIVES

To investigate transient thermal stresses that developed in 3Y-TZP green compacts during speed sintering.

METHODS

A total of 312 disc-shaped green compacts (Ø17.1 ×1, 1.5, 2, 2.5, 3 mm) were cold-isostatically pressed from 3Y-TZP powder (Zpex, Tosoh Corp.) for speed sintering studies as well as compositional analysis and biaxial flexural strength measurements (both at room temperature and following heating at 90 °C/min to 500 °C). Flexural strength was determined using the piston-on-3-ball method. Phase assemblies were analyzed using the X-ray diffraction method. Effects of heating/cooling rates on transient stresses were investigated by conducting definitive sintering studies to determine the threshold for fracture. Finite element analysis (FEA) was used to validate the experimental findings using measured thermomechanical properties.

RESULTS

The bulk and relative density of the green compact were 2.95 ± 0.03 g/cm3 and 48.52% ± 0.45%. The flexural strength was drastically decreased from 10.3 ± 0.4 MPa to 1.09 ± 0.07 MPa following heating at 90 °C/min to 500 °C. The monoclinic and tetragonal contents were 54.9% and 45.1%, respectively. The threshold for fracture was located at 500 °C during the first heating stage with a 90 °C/min heating rate in specimens of 2.5 mm thickness or greater. No fractures occurred in the second heating stage and cooling phase. The FEA estimated that the principal transient tensile stress was ∼1.14 MPa at 500 °C during the heating phase, which exceeded the corresponding flexural strength (1.09 ± 0.07 MPa).

SIGNIFICANCE

Advanced FEA methods are an accurate and efficient tool to analyze the history of transient stresses during sintering of ceramic dental restorations.

The Light Reflection Changes of Monolithic Zirconia and Lithium Disilicate after Using Two External Staining Kits following by Thermocycling

BACKGROUND

The purpose of the study was to evaluate the changes of light reflection% on two materials (monolithic zirconia and lithium disilicate) after using two external staining kits following by thermocycling.

METHODS

Specimens were sectioned from monolithic zirconia (n = 60) and lithium disilicate (n = 60) then divided into six groups (n = 20). Two different types of external staining kits were used and applied to the specimens. The light reflection% was measured before staining, after staining and after thermocycling using a spectrophotometer.

RESULTS

The light reflection% of zirconia was significantly higher compared to lithium disilicate at the beginning of the study (p = 0.005), after staining with kit 1 (p = 0.005) and kit 2 (p = 0.005) and after thermocycling (p = 0.005). For both materials, the light reflection% was lower after staining with Kit 1 compared to kit 2 (p < 0.043). After thermocycling, the light reflection% of lithium disilicate increased (p = 0.027) and was unchanged with Zirconia (p = 0.527).

CONCLUSIONS

There is a difference between the materials regarding light reflection% as the monolithic zirconia showed higher light reflection% comparing lithium disilicate throughout the entire experiment. For lithium disilicate, we recommend using kit 1 as we found that, after thermocycling, the light reflection% of kit 2 was increased.

Silica infiltration on translucent zirconia restorations: Effects on the antagonist wear and survivability

OBJECTIVE

To assess potential antagonist wear and survival probability of silica-infiltrated zirconia compared to glass-graded, glazed, and polished zirconia.

METHODS

Table top restorations made of 3Y-TZP (3Y), 5Y-PSZ (5Y), and lithium disilicate (LD) were bonded onto epoxy resin preparations. Each zirconia was divided into five groups according to the surface treatment: polishing; glaze; polishing-glaze; glass infiltration; and silica infiltration. The LD restorations received a glaze layer. Specimens were subjected to sliding fatigue wear using a steatite antagonist (1.25 ×106 cycles, 200 N). The presence of cracks, fractures, and/or debonding was checked every one/third of the total number of cycles was completed. Roughness, microstructural, Scanning electron microscopy, wear and residual stress analyses were conducted. Kaplan-Meier, Mantel-Cox (log-rank) and ANOVA tests were performed for statistical analyses.

RESULTS

The survival probability was different among the groups. Silica infiltration and polishing-glaze led to lower volume loss than glaze and glass-infiltration. Difference was observed for roughness among the zirconia and surface treatment, while lithium disilicate presented similar roughness compared to both glazed zirconia. Scanning electron microscopy revealed the removal of the surface treatment after sliding fatigue wear in all groups. Compressive stress was detected on 3Y surfaces, while tensile stress was observed on 5Y.

SIGNIFICANCE

3Y and 5Y zirconia behaved similarly regarding antagonist wear, presenting higher antagonist wear than the glass ceramic. Silica-infiltrated and polished-glazed zirconia produced lower antagonist volume loss than glazed and glass-infiltrated zirconia. Silica-infiltrated 3Y and lithium disilicate restorations were the only groups to show survival probabilities lower than 85%.

Metal-ceramic and porcelain-veneered lithium disilicate crowns: a stress profile comparison using a viscoelastic finite element model

Metal-ceramics (MC) are one of the oldest dental restorative systems, which are considered to be the gold standard for full crown restoration. Porcelain-veneered lithium disilicate (PVLD), on the other hand, are newer material systems that have shown high survival rate in clinical follow-ups but needs to be studied more. This study compares the stresses developed in the single crowns made from newer PVLD system against those with MC configuration. For this comparison, influence of the layer thickness and cooling rates is also taken into consideration. An experimentally validated viscoelastic finite element model (VFEM) has been developed to predict the stress profile in these systems. Three-dimensional rotationally symmetric crowns were analyzed using this validated model for both material systems, three veneer to core thickness ratios (2:1, 1:1, 1:2), and two cooling rates: slow cooling at 1.74E-5 W/mm²K (∼30 K/min) and fast cooling at 1.74E-4 W/mm²K (∼300 K/min). PVLD showed lower values of transient and residual stresses than MC. The maximum tensile residual stresses in MC systems were observed in the cusp area, whereas those in PVLD were located in the central fossa. With the reduction in veneer layer, there was reduction in residual stress in MC; however, the veneer thickness had little to no effect in PVLD. The effect of cooling rate was also evident as slow cooling resulted in lower residual and tensile stresses for both material systems.

Prospective 5-year clinical evaluation of posterior zirconia fixed dental prostheses veneered with milled lithium disilicate (CADon)

OBJECTIVE

Bi-layer zirconia-based posterior fixed dental prostheses (FDPs) have reportedly a high incidence of veneering ceramic fractures. The CADon technique employs zirconia frameworks veneered with milled lithium disilicate glass-ceramic to overcome these shortcomings but long-term clinical studies are missing. This study evaluated the clinical efficacy of posterior 3-unit CADOn FDPs over a 5-year period.

MATERIALS AND METHODS

A total of 25 patients, 16 male and 9 female with a mean age of 55.6 years, received a total of 25 posterior three-unit CADOn FDPs. Patients were recalled for baseline and for follow-up visits at 6, 12, 24, and 60 months. Parameters evaluated were fracture, marginal adaptation, marginal discoloration, wear, polish, color match, surface staining, and retention of these restorations as well as secondary caries of the abutment teeth.

RESULTS

A total of 21 FDPs evaluated at 60-months revealed a 100% survival rate. All evaluated clinical parameters were satisfactory (alpha or bravo) and there was no statistically significant difference at different time points, except for surface staining, which was also worse at 60 months compared to baseline (p  ≤  0.001).

CONCLUSIONS

CADOn three-unit posterior FDPs demonstrate excellent fracture resistance after 5 years of clinical function. All evaluated clinical parameters were satisfactory, except for surface stain, which also increased over time.

CLINICAL SIGNIFICANCE

Conventionally veneered bi-layer zirconia-based posterior FDPs have limited clinical success. Based on this study, alternative methods such as zirconia frameworks veneered with milled lithium disilicate glass-ceramic provide a viable alternative with excellent long-term clinical outcomes.