Fatigue and fracture resistance of zirconia crowns prepared with different finish line designs

Purpose: The aim of this study was to evaluate the effect of finish line design on the fatigue, fracture resistance, and failure type of veneered zirconia restorations. Materials and Methods: A CAD/CAM system (Cercon) was used to prepare zirconia frameworks (0.5 mm thick) for a maxillary central incisor. Three finish line designs were evaluated: a complete narrow chamfer, a narrow chamfer with a lingual ledge, and a complete ledge. The prepared frameworks were veneered using a press-on ceramic (Ceram Press) and were cemented on the corresponding prepared teeth using a resin cement (Panavia F2.0). The cemented specimens were thermocycled, subjected to dynamic fatigue, and finally loaded till fracture. Fractured specimens were examined under a scanning electron microscope to assess fracture type. One-way ANOVA and Bonferroni post hoc tests were used to analyze the data (α= 0.05). Results: The finish line design did not have any significant statistical influence on the fracture resistance (F = 1.9, p= 0.346) or on the failure type of the tested specimens. Adjusted R squared value (R = 0.049) indicated a weak correlation between finish line design and fracture load of the tested specimens. All specimens failed due to cracking and fracture of the veneer ceramic. Meanwhile, the framework remained entirely intact. Three narrow chamfer finish line specimens demonstrated adhesive fracture of the veneer ceramic during dynamic fatigue testing, related to overextension of the veneer ceramic during the layering procedure. Conclusion: Within the limitations of this study, the finish line design did not influence the fatigue or the fracture resistance of veneered zirconia crowns. Selection of any of the finish line designs should be based on the clinical condition of the restored tooth

Effect of surface treatment on flexural strength of zirconia bars

Statement of problem Clinical and laboratory processing techniques induce damage to the surface of zirconia frameworks, which significantly lessens their strength. Purpose The purpose of this study was to investigate the influence of 3 surface restoration methods on the flexural strength of zirconia bars. Material and methods Bar-shaped specimens were polished and received 1 of 2 surface treatments (n=20): airborne-particle abrasion with 110-μm aluminum oxide particles at a pressure of 0.2 MPa, or grinding with a diamond point under water cooling and a load of 2 N using an air turbine. Polished specimens served as the control (n=20). The induced surface damage was restored using one of the following restoration methods: polishing of the specimens using an aluminum oxide polishing point (2-μm grit) coated with 0.5-μm diamond polishing paste, application of a thin coat of glazing porcelain according to the manufacturer's instructions, or application of a phosphate ester monomer (MDP) containing an adhesive resin. The specimens were subjected to a 4-point flexural strength test and then examined using a scanning electron microscope (SEM). Two-way ANOVA and the Bonferroni post hoc test were used to analyze the data (α=.05). Results There was a significant interaction between the type of surface damage and the restoration method (P<.001). For the ground specimens, all restoration methods resulted in a statistically significant regain in strength, with the polishing procedure being the most effective (1027 MPa). For airborne-particle-abraded specimens, application of the bonding agent was the only effective restoration method. SEM analysis of the fractured surfaces of specimens indicated that application of the bonding agent resulted in sealing of the surface damage produced by airborne-particle abrasion, while polishing was successful in removing the grinding lines produced by the diamond point. Conclusions Within the limitations of this in vitro study, restoration of surface damage improved the flexural strength of zirconia specimens

Microtensile bond strength of different components of core veneered all-ceramic restorations. Part 3: double veneer technique

Purpose: The bond strength of zirconia veneer has been considered a weak link in the layered all-ceramic restoration. In a previous study, this bond was improved using a new category of veneering ceramics adopting the pressing technique; however, the resulting esthetic quality lacked the individual characterization built using the layering technique. The aim of this work was to evaluate the effect of combining both press-on and layering veneer ceramics in one restoration on the bond strength with zirconia frameworks. Materials and Methods: Zirconia discs (19.4 mm in diameter, 3-mm thick) were veneered with 3-mm thick press-on veneer ceramic or veneered with 1-mm thick press-on veneer ceramic and an additional 2-mm thick of layering veneer ceramic. Two commercial layering veneer ceramics were tested. The specimens were sectioned into microbars, and the zirconia veneer microtensile bond strength (MTBS) was measured in a universal testing machine. Scanning electron microscopy (SEM) was used to examine core veneer interface quality and to assess failure type. Energy dispersive X-ray microanalysis (EDAX) was used to analyze the chemical structure of the tested veneer ceramics, which may affect the structural integrity of the double veneered restoration. One-way analysis of variance and Tukey HSD post hoc tests were selected to analyze the data (p < 0.05 was significant). Results: The MTBS of zirconia and press-on ceramic (34.4 ± 2.9 MPa) was not affected by the addition of a second layer of either veneer ceramic. SEM analysis revealed defect-free zirconia press-on veneer interface, while the interface between the press-on and the layering veneer ceramics demonstrated a different crystal structure and glass matrix, which did not affect wetting and contact between the two materials. EDAX analysis revealed differences, which account for the observed structural differences, in the chemical composition between the tested veneers. Conclusion: The double veneer technique combines the high bond strength and superior interface quality achieved using press-on ceramics with the superior esthetics and individual characterization obtained using layering ceramics. The technique promises superior function and performance of the double veneered restoration

Bridging the gap between clinical failure and laboratory fracture strength tests using a fractographic approach

Objective: The aim of this study was to analyze and to compare the fracture type and the stress at failure of clinically fractured zirconia-based all ceramic restorations with that of morphologically similar replicas tested in a laboratory setup. Methods: Replicas of the same shape and dimensions were made for 19 crowns and 17 fixed partial dentures, all made of veneered zirconia frameworks, which fractured during intra-oral service. The replicas were statically loaded by applying axial load in a universal testing machine. The principles of fractography were used to identify the location and the dimensions of the critical crack and to estimate the stress at failure. Failure was classified according to origin and type (P < 0.05 was considered significant). Results: Clinically fractured restorations failed due to either: delamination of the veneer ceramic (28.2 ± 9 MPa), defects at core veneer interface (27.7 ± 6 MPa), the generation of Hoop stresses (884.3 ± 266 MPa), radial cracking (831 MPa), or fracture of the connector (971 ± 343 MPa). The replicas failed by mainly by cone cracking of the veneer ceramic (52.4 ± 34.8 MPa) or by fracture of the connector (1098.9 ± 259 MPa). The estimated stress at failure was significantly higher for the replicas compared to the clinically fractured restorations (F = 6.8, P < 0.01). Significance: Within limitations of this study, careful design of fracture strength test would lead to more clinically relevant data. The performance of zirconia veneered restorations could be further improved with careful design considerations

Influence of framework color and layering technique on the final color of zirconia veneered restorations

Objective: To investigate the influence of colored zirconia frameworks on the overall color match of zirconia- veneered restorations. Method and Materials: Identical natural and colored zirconia frameworks (Cercon Base, Degudent) were layered using a veneer ceramic (IPS e.max Ceram Dentin, Ivoclar Vivadent) applied either directly on the frameworks, over a thin layer of a masking liner (IPS e.max ZirLiner 1), or over a layer of a deep chroma dentin (IPS e.max Deep Dentin) of the required target color, A1. Color parameters were obtained using a spectrophotometer (Spectro Shade Micro, MHT) and were used to calculate color difference value with the preselected required color (A1 tab according to Vita Classical shade guide). Color difference value < 2.2 was considered a clinically acceptable color match. Results: Only three combinations of tested variables had an acceptable color match compared with the target color: colored zirconia frameworks with liner material or deep chroma dentin and white zirconia frameworks directly layered with the veneer ceramic (P < .001), all tested against a black background. The combination of yellow zirconia with deep chroma dentin over a white background produced a yellowish color shift, while white zirconia frameworks with either liner or deep chroma dentin were brighter and less yellow than the target color (A1). Conclusions: When properly veneered, colored zirconia frameworks could produce clinically acceptable color match and have the capacity to mask a dark background such as a dark tooth or core buildup material