29 October 2022

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Thickness of lithium silicate crowns reinforced with zirconia: the effect on failure modes, survival rate

Simona Chirico

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Despite the broad use of lithium disilicate systems, in recent years there are new developments with a more reliable glass-ceramic with further increased strength by microstructure reinforcement. Zirconia-reinforced lithium silicate (ZLS) system features a complex microstructure, which is composed of lithium-metasilicate, and lithium orthophosphate crystals embedded in a glassy matrix containing zirconia. The very fine crystal size exhibited in the ZLS system (0.5–1.0 m), which is approximately 4–6 times smaller than LD crystals (2.0–3.0 m), provides a higher percentage of glass content (roughly 50%) than LD (30%). This microstructure also supports exceptional optical properties with similar and higher mean translucency values compared to feldspar blocks and LD. Despite the higher glass content, the homogeneous dispersion of zirconia in the vitreous matrix increases the mechanical properties of the ceramic structure through solidification mechanisms, so much so that the ZLS ceramic discs have shown a significantly greater flexural strength than LD systems.

Materials and methods

In an in vitro study by Bergamo and his team, the authors assessed the reliability and the degree of failure of monolithic ZLS molar crowns with different thickness using the SSALT test. The authors fabricated, modelled and milled 63 monolithic ZLS molar crowns with a thickness of 0.5 mm, 1.0 mm and 1.5 mm respectively (n = 21 / group) using a CAD / CAM system. Crowns were cemented on dentin-like epoxy resin replicas with a resin cement. The specimens were subjected to single load-to-failure test for step-stress profiles designing. Mouth-motion step-stress accelerated-life test was performed under water by sliding an indenter 0.7 mm lingually down on the disto-buccal cusp until specimen fracture or suspension. Use level probability Weibull curves and reliability were calculated and plotted. Polarized-light optical microscope and scanning electron microscope (SEM) were used to characterize fracture patterns.

Results

While 0.5 mm ZLS crowns exhibited a significant reduction in the probability of survival at 200 N, 300 N and 400 N mission loads (69%, 41% and 19%, respectively), no significant difference was observed between 1.0 mm and 1.5 mm crowns. Both thicknesses have maintained the survivability at approximately 90%. Failure primarily comprised bulk fracture where radial cracks originated from the cementation surface beneath the indenter loading trail and propagated towards the cervical margin.

Conclusions

The monolithic 1.5 mm and 1.0 mm thick ZLS crowns have a higher probability of survival than the 0.5 mm thick crowns. Bulk fracture was the chief failure mode, regardless of thickness.

Bergamo ETP, Bordin D, Ramalho IS, Lopes ACO, Gomes RS, Kaizer M, Witek L, Bonfante EA, Coelho PG, Del Bel Cury AA. "Zirconia-reinforced lithium silicate crowns: Effect of thickness on survival and failure mode." Dent Mater. 2019 Jul; 35 (7): 1007-1016. DOI: 10.1016/j.dental.2019.04.007