Monolithic zirconia crowns: effect of thickness reduction on fatigue behavior and failure load

Abstract

Purpose: The objective of this study was to evaluate the effect of thickness reduction and fatigue on the failure load of monolithic zirconia crowns.

Materials and methods: 140 CAD-CAM fabricated crowns (3Y-TZP, inCorisTZI, Dentsply-Sirona) with different ceramic thicknesses (2.0, 1.5, 1.0, 0.8, 0.5 mm, respectively, named G2, G1.5, G1, G0.8, and G0.5) were investigated. Dies of a mandibular first molar were made of composite resin. The zirconia crowns were luted with a resin composite cement (RelyX Unicem 2 Automix, 3M ESPE). Half of the specimens (n = 14 per group) were mouth-motion-fatigued (1.2 million cycles, 1.6 Hz, 200 N/ 5 - 55℃, groups named G2-F, G1.5-F, G1-F, G0.8-F, and G0.5-F). Single-load to failure was performed using a universal testing-machine. Fracture modes were analyzed. Data were statistically analyzed using a Weibull 2-parameter distribution (90% CI) to determine the characteristic strength and Weibull modulus differences among the groups.

Results: Three crowns (21%) of G0.8 and five crowns (36%) of G0.5 showed cracks after fatigue. Characteristic strength was the highest for G2, followed by G1.5. Intermediate values were observed for G1 and G1-F, followed by significantly lower values for G0.8, G0.8-F, and G0.5, and the lowest for G0.5-F. Weibull modulus was the lowest for G0.8, intermediate for G0.8-F and G0.5, and significantly higher for the remaining groups. Fatigue only affected G0.5-F.

Conclusion: Reduced crown thickness lead to reduced characteristic strength, even under failure loads that exceed physiological chewing forces. Fatigue significantly reduced the failure load of 0.5 mm monolithic 3Y-TZP crowns.

Keywords: Ceramics; Computer-aided design; Fatigue; Monolithic crowns; Zirconium.

Fig. 1. Test set-up.

Fig. 2. Definition of the different parameters. Yellow = die, grey = varying enlargement of the die, white = monolithic crown (Screenshot, provided by Dentsply Sirona).

Fig. 3. Monolithic zirconia crowns with different layer thicknesses after loading (1.5 × magnification). The circle shows the attrition caused by the antagonists during fatigue. The arrows mark the cracks. (A) G1.5-F, (B) G1-F, (C) G0.8-F, (D) G0.5-F. Cracks were found in groups 0.8-F and 0.5-F.

Fig. 4. (A) Two-parameter Weibull contour plot (Weibull modulus [m]) vs. characteristic strength [N] for G2 (black), G1.5 (green) G1.0 (grey), G0.8 (blue) and G0.5 (red) in which solid contour lines represent non-fatigued, and dotted lines represent groups subjected to fatigue prior to single load to failure test. The only group showing no overlap between contours before and after fatigue is G0.5, meaning that fatigue significantly decreased its characteristic strength. (B) Probability of survival vs. characteristic strength shows a general trend in crown survival increase for increased thickness. G1 and G1-F (grey solid and dotted reliability lines, respectively) present an overlap due to similar sample failure distribution.

Fig. 5. Monolithic zirconia crowns after single load to failure (1.5 × magnification). (A) G1.5, (B) G1, (C) G0.8, (D) G0.5.