Monolithic zirconia crowns – wall thickness, surface treatment and load at fracture

Influence of different surface finishing procedures of strength-gradient multilayered zirconia crowns on two-body wear and fracture load: Lithium silicate or leucite glazing versus polishing?

OBJECTIVES

To examine the influence of different finishing procedures on the surface roughness, wear resistance and fracture load of strength-gradient multilayered zirconia.

MATERIALS AND METHODS

Zirconia crowns (Multilayer 3D pro; n = 96) were manufactured and treated with a lithium-silicate- (LISI_S), leucite-based glaze spray (LEU_S), leucite-based glaze mass (LEU_M) or polished (POL). Natural molars (CG; n = 24) acted as a control. Roughness was determined on flat glazed specimens (n = 28). Two-body wear was examined by digitalizing and matching the occlusal surface before and after thermo-mechanical aging (6,000 thermal and 1,200,000 chewing cycles). The groups were split to measure fracture load initially and after aging. Kolmogorov-Smirnov, Spearman correlation, Kruskal-Wallis-H, Levene's test, one-way ANOVA with Scheffé post-hoc and Weibull modulus were computed.

RESULTS

POL presented the lowest and LEU_S the highest roughness. Following POL, no ceramic loss was observed. LISI_S, LEU_S and LEU_M showed lower ceramic wear than the CG. The lowest wear of the antagonist was observed for the CG. LISI_S showed a lower antagonistic wear than LEU_S, LEU_M and POL. LISI_S, LEU_S, LEU_M and POL showed higher fracture load values and Weibull moduli than the CG. Artificial aging did not impact the fracture load or Weibull moduli.

SIGNIFICANCE

With glazed groups showing height losses closer to the CG, glazing should be preferred to polishing when approximating the wear behavior of a natural dentition. Finishing of monolithic zirconia by glazing with a lithium silicate-based spray is recommended to preserve the antagonists. All examined zirconia crowns presented sufficient mechanical properties to withstand masticatory forces, even after prolonged aging.

Fracture resistance and failure mode of layered translucent zirconia with different crown designs

This study investigated the impact of different veneering design on the fracture resistance and failure mode of layered translucent zirconia crowns, compared to the full monolithic zirconia crowns. Ninety crowns with different designs were divided into six groups (n=15); one fully contoured monolithic crown (FMC), one fully veneered with porcelain (FVC), or four partially veneered designs. All crowns were thermo-cycled, and then loaded to failure in a universal testing machine. The fracture resistance (N) and fracture modes were assessed. One-way ANOVA was performed followed by Tukey's multiple comparison (α<0.05). Results demonstrated that all the tested partially veneered designs exhibited enhanced fracture resistances and were comparable to that of the full-contour monolithic translucent zirconia crowns.

Effect of artificial aging on high translucent dental zirconia: simulation of early failure

Higher yttria content enhances the translucency and appearance of dental zirconia materials. Alterations in material composition also affect mechanical properties. The aim of this study was to compare the fracture load after artificial short-term aging of monolithic, full-contour zirconia crowns with different amounts of yttria-stabilization. Sixty crowns (thirty super high translucent crowns (5Y-Z) and thirty high translucent crowns (3Y-Z)) were produced to fit a model of a premolar with a shallow chamfer preparation. The crowns were cemented with self-adhesive resin cement on composite abutments. For each zirconia type, three groups of crowns (n = 10) were allocated to: (i) cyclic loading (200 N, 1 Hz, 30,000 cycles), (ii) hydrothermal aging (3 × 20 min, 134°C 3.2 bar), or (iii) no treatment (control). Surviving crowns from the aging process were quasistatically loaded until fracture. The 3Y-Z crowns had statistically significantly higher fracture values (3,449 N) than the 5Y-Z crowns (1,938 N). The aging procedures did not affect load at fracture. Fractographic analysis showed that fractures started either at the crown margin or at the occlusal intaglio area. Higher yttria content leads to a reduction in material strength and damage tolerance, and this should be reflected in recommendations for clinical use.

Effect of Marginal Designs on Fracture Strength of High Translucency Monolithic Zirconia Crowns

Introduction

Monolithic zirconia is able to achieve certain aesthetic, but its durability in resisting fracture has been questioned, as fractures often originate from margins of restoration. This study determined fracture resistance of highly translucent monolithic zirconia crowns with different margin designs in terms of marginal thickness and collar height.

Materials and Methods

Zirconia blanks (Ceramill® Zolid HT⁺) were selected for the fabrication of zirconia crowns according to different designs, including varying margin thicknesses (light chamfer, C_L; heavy chamfer, C_H) and collar heights (no collar, N_C; low collar, L_C; high collar, H_C), which resulted in C_LN_C, C_LL_C, C_LH_C, C_HN_C, C_HL_C, and C_HH_C groups (15 crowns each). The crowns were seated on a metal die and loaded vertically through round end punch (θ = 10 mm), contacting with inclined planes of cusp in a testing machine with crosshead speed of 0.2 mm/min until fracture. Videos with a rate of 50 frames/second were used to record fracture. Fracture load (N) and durable period (s) were compared for significant differences using ANOVA and Bonferroni test (α = 0.05).

Results

The mean ± sd of fracture load (N) and durable time (s) were 3211 ± 778 and 212 ± 47 for C_LN_C; 3041 ± 1370 and 188 ± 53 for C_LL_C; 2913 ± 828 and 192 ± 27 for C_LH_C; 4226 ± 905 and 245 ± 35 for C_HN_C; 4486 ± 807 and 228 ± 29 for C_HL_C; and 4376 ± 1043 and 227 ± 37 for C_HH_C. This indicated that marginal thickness had a significant influence on load-bearing capacity and durable time (p < 0.05). No significant impact of collar height was shown, either on load-bearing capacity or durable time (p > 0.05). No interaction between two factors was presented (p > 0.05).

Conclusions

Heavy chamfer margin provided stronger zirconia crown than light chamfer, but both were capable of withstanding fracture load higher than maximum masticatory force. Neither presence nor absence of collar indicated any impact on strength. Fabrication of zirconia crowns with either heavy or light chamfer margin and either presence or absence of collar, with the consideration of emergence profile, should be considered.

Marginal fit of full contour monolithic zirconia in different thicknesses and layered zirconia crowns

Aim:

Use of monolithic zirconia for fabrication of all-ceramic crowns eliminates several shortcomings of layered zirconia crowns. Long-term success of restorations highly depends on the marginal fit. The crown thickness is among the factors that affect the marginal integrity. Meanwhile, reduced thickness of crowns has several advantages such as preservation of tooth structure. The aim of this study was to evaluate the marginal fit of monolithic zirconia crowns in reduced thickness and to compare the marginal fit of full-contour monolithic zirconia in different thicknesses with layered zirconia crowns.

Materials and Methods:

In this in vitro study, two standard brass dies (7 mm × 5 mm length diameter) were prepared with a heavy chamfer finish line with 0.5 and 1 mm depth. By using a CAD-CAM system, 30 crowns were made in three groups (n = 10) of 1-mm thick layered zirconia, 1-mm thick monolithic zirconia, and 0.5-mm thick monolithic zirconia. Crowns were placed on master dies and randomly numbered. The marginal gap was measured on 18 points by using a digital microscope (×230). The mean ± standard deviation (SD) values were calculated and analyzed by Statistical Package for the Social Sciences (SPSS) software program through Kruskal–Wallis and Mann–Whitney tests (α = 0.05).

Results:

The marginal gap of 1-mm layered zirconia was significantly different from that of 1-mm monolithic zirconia (P = 0.001) and 0.5-mm monolithic zirconia (P = 0.004). Analysis of variance (ANOVA) revealed no significant difference between 0.5 and 1 mm thicknesses of monolithic zirconia (P = 0.141).

Conclusion:

Marginal gap in all the three groups was clinically acceptable. The two different thicknesses of monolithic zirconia crowns had no significant effect on the restoration marginal fit; however, layered zirconia crowns showed a significantly higher marginal gap than monolithic zirconia crowns.