Purpose/aim: The use of semi-monolithic designs for translucent zirconia restorations is suggested as a way of overcoming the clinical limitations, in terms of veneer fracture and poor aesthetics, of traditional bi-layered and monolithic designs for zirconia. Therefore, the aim of the study was to describe and evaluate the fracture resistance of translucent and high-translucent zirconium-dioxide crowns with different semi-monolithic designs.

Materials and methods: One hundred zirconium-dioxide crowns were produced and divided as follows into five groups (n = 20) according to five different designs: monolithic (control), semi-monolithic with 0.3 mm micro-coating buccal veneer thickness (SM0.3), semi-monolithic with 0.5 mm-buccal veneer thickness (SM0.5), semi-monolithic with 0.5 mm-buccal veneer thickness supported by a wave design (SMW), semi-monolithic with 0.5 mm-buccal veneer thickness supported by occlusal cap design (SMC). Each group was divided into two subgroups (n = 10) according to the materials used, translucent and high-translucent zirconium-dioxide. All crowns were subjected to artificial aging: thermocycling and cyclic pre-loading. After aging, all crowns were loaded to fracture, and the load data were analyzed using two-way ANOVA test (p < .05).

Results: The results showed that there were significant differences among the groups depending on the different designs tested and the materials used (p ≤ .05). Crowns made of translucent zirconium-dioxide generally showed significantly higher fracture load values compared to those made of high-translucent zirconium-dioxide, regardless of design (p < .001). Semi-monolithic crowns with a micro-coating porcelain layer withstood the highest fracture loads (3702 N ± 312) among the semi-monolithic crowns tested, close to the loads seen in the monolithic crowns (3905 N ± 226). Semi-monolithic crowns with 0.5 mm buccal veneer thickness showed significantly lower fracture loads (2325 N ± 361) in contrast to the crowns with same buccal veneer thickness but with different supporting designs, the wave (3492 N ± 300) and the cap designs (2780 N ± 340).

Conclusions: Translucent and high-translucent zirconium-dioxide crowns can be used in combination with 0.3 mm micro-coating porcelain-layer with semi-monolithic design to enhance aesthetic properties of restorations without significantly decreasing fracture resistance of crowns. If porcelain layers thicker than 0.3 mm are needed for semi-monolithic crown, wave design or cap design can be used to enhance fracture resistance of crowns. In both cases, strength gained is clinically sufficient with the vast margin of safety in relation to expected loads under clinical use.