Influence of core design, production technique, and material selection on fracture behavior of yttria-stabilized tetragonal zirconia polycrystal fixed dental prostheses produced using different multilayer techniques: split-file, over-pressing, and manually built-up veneers

Deyar Jallal Hadi Mahmood, Ewa H Linderoth, Ann Wennerberg, Per Vult Von Steyern Department of Prosthetic Dentistry, Faculty of Odontology, Malmö University, Malmö, Sweden Aim: To investigate and compare the fracture strength and fracture mode in eleven groups of currently, the most commonly used multilayer three-unit all-ceramic yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) fixed dental prostheses (FDPs) with respect to the choice of core material, veneering material area, manufacturing technique, design of connectors, and radii of curvature of FDP cores. Materials and methods: A total of 110 three-unit Y-TZP FDP cores with one intermediate pontic were made. The FDP cores in groups 1–7 were made with a split-file design, veneered with manually built-up porcelain, computer-aided design-on veneers, and over-pressed veneers. Groups 8–11 consisted of FDPs with a state-of-the-art design, veneered with manually built-up porcelain. All the FDP cores were subjected to simulated aging and finally loaded to fracture. Results: There was a significant difference (P<0.05) between the core designs, but not between the different types of Y-TZP materials. The split-file designs with VITABLOCS® (1,806±165 N) and e.max® ZirPress (1,854±115 N) and the state-of-the-art design with VITA VM® 9 (1,849±150 N) demonstrated the highest mean fracture values. Conclusion: The shape of a split-file designed all-ceramic reconstruction calls for a different dimension protocol, compared to traditionally shaped ones, as the split-file design leads to sharp approximal indentations acting as fractural impressions, thus decreasing the overall strength. The design of a framework is a crucial factor for the load bearing capacity of an all-ceramic FDP. The state-of-the-art design is preferable since the split-file designed cores call for a cross-sectional connector area at least 42% larger, to have the same load bearing capacity as the state-of-the-art designed cores. All veneering materials and techniques tested in the study, split-file, over-press, built-up porcelains, and glass–ceramics are, with a great safety margin, sufficient for clinical use both anteriorly and posteriorly. Analysis of the fracture pattern shows differences between the milled veneers and over-pressed or built-up veneers, where the milled ones show numerically more veneer cracks and the other groups only show complete connector fractures. Keywords: all-ceramic FDPs, connector design radius, state-of-the-art, CAD/CAM, multilayer technique, veneering ceramic technique

Quality of written communication and master impressions for fabrication of removable partial prostheses in the Kingdom of Bahrain

The aim of this study was to examine the quality of written instructions and choice of impression trays and materials for removable partial dentures (RPDs) in the Kingdom of Bahrain. All six private dental laboratories in Bahrain were contacted and invited to participate in the study. Five laboratories participated, and submitted written instructions received by them for fabrication of both acrylic (A-RPDs) and cobalt-chromium (CC-RPDs) RPDs. These were examined for evidence of selected design variables. Types of impression trays and materials used were also recorded. One hundred and thirty-one written instructions were examined. Eleven percent (n = 14) were for CC-RPDs, 89% (n = 117) for A-RPDs. All treatments were provided on a private basis. Fifty-seven percent (n =1 8) of CC-RPD instructions requested the technician to design the prosthesis, 43% (n = 6) contained a diagram and 43% (n = 6) mentioned all design variables. Seventy-nine percent (n = 92) of A-RPDs requested the technician to design the denture, and only 1% (n = 1) mentioned all design variables. Alginate impression material was most commonly used for master impressions (83% of impressions (n = 109); 85% (n = 99) of A-RPDs, and 71% (n = 10) of CC-RPDs). Master casts were poured after a minimum of 24 h. Acrylic custom trays were used in 14% (n = 19) of cases (43% (n = 6) of CC-RPDs; 13% (n = 15) of A-RPDs). The quality of written instructions to dental laboratories for the fabrication of RPDs was found to be inadequate in Kingdom of Bahrain. There was widespread use of inappropriate impression trays and materials

The effect of zirconia sintering temperature on flexural strength, grain size, and contrast ratio

OBJECTIVE: This study investigated the effect of sintering temperatures on flexural strength, contrast ratio, and grain size of zirconia. MATERIALS AND METHODS: Zirconia specimens (Ceramill ZI, Amann Girrbach) were prepared in partially sintered state. Subsequently, the specimens were randomly divided into nine groups and sintered with different final sintering temperatures: 1,300°C, 1,350°C, 1,400°C, 1,450°C, 1,500°C, 1,550°C, 1,600°C, 1,650°C, or 1,700°C with 120 min holding time. Three-point flexural strength (N = 198; n = 22 per group) was measured according to ISO 6872: 2008. The contrast ratio (N = 90; n = 10 per group) was measured according to ISO 2471: 2008. Grain sizes and microstructure of different groups were investigated (N = 9, n = 1 per group) with scanning electron microscope. Data were analyzed using one-way ANOVA with Scheffé test and Weibull statistics (p < 0.05). Pearson correlation coefficient was calculated between either flexural strength or contrast ratio and sintering temperatures. RESULTS: The highest flexural strength was observed in groups sintered between 1,400°C and 1,550°C. The highest Weibull moduli were obtained for zirconia sintered at 1,400°C and the lowest at 1,700°C. The contrast ratio and the grain size were higher with the higher sintering temperature. The microstructure of the specimens sintered above 1,650°C exhibited defects. Sintering temperatures showed a significant negative correlation with both the flexural strength (r = -0.313, p < 0.001) and the contrast ratio values (r = -0.96, p < 0.001). CONCLUSIONS: The results of this study showed that the increase in sintering temperature increased the contrast ratio, but led to a negative impact on the flexural strength. CLINICAL RELEVANCE: Considering the flexural strength values and Weibull moduli, the sintering temperature for the zirconia tested in this study should not exceed 1,550°C

Two-body wear of monolithic, veneered and glazed zirconia and their corresponding enamel antagonists

Abstract Objective. This study tested whether the two-body wear of monolithic zirconia and their corresponding enamel antagonists was higher compared to monolithic alloy and veneered zirconia. Materials and methods. Cylindrical specimens (N = 36, n = 6) were prepared out of (A) veneered zirconia (VZ), (B) glazed zirconia using a glaze ceramic (GZC), (C) glazed zirconia using a glaze spray (GZS), (D) manually polished monolithic zirconia (MAZ), (E) mechanically polished monolithic zirconia (MEZ) and (F) monolithic base alloy (control group, MA). Wear tests were performed in a chewing simulator (49 N, 1.7 Hz, 5°C/50°C) with enamel antagonists. The wear analysis was performed using a 3D profilometer before and after 120,000, 240,000, 640,000 and 1,200,000 masticatory cycles. SEM images were used for evaluating wear qualitatively. The longitudinal results were analysed using linear mixed models (α = 0.05). Results. Materials (p < 0.001) and number of masticatory cycles (p < 0.001) had a significant effect on the wear level. The least enamel antagonist wear was observed for MAZ and MEZ (27.3 ± 15.2, 28 ± 11.1 μm, respectively). GZC (118 ± 30.9 μm) showed the highest wear of enamel antagonists. The highest wear rate in the material was observed in GZS (91.3 ± 38.6 μm). While in the groups of MA, VZ, GZC and GZS 50% of the specimens developed cracks in enamel, it was 100% in MAZ and MEZ groups. Conclusion. Polished monolithic zirconia showed lower wear rate on enamel antagonists as well as within the material itself but developed higher rates of enamel cracks