In vitro evaluation of fracture strength of zirconia restoration veneered with various ceramic materials

Abstract

Purpose: Fracture of the veneering material of zirconia restorations frequently occurs in clinical situations. The purpose of this in vitro study was to compare the fracture strengths of zirconia crowns veneered with various ceramic materials by various techniques.

Materials and methods: A 1.2 mm, 360° chamfer preparation and occlusal reduction of 2 mm were performed on a first mandibular molar, and 45 model dies were fabricated in a titanium alloy by CAD/CAM system. Forty-five zirconia copings were fabricated and divided into three groups. In the first group (LT) zirconia copings were veneered with feldspathic porcelain by the layering technique. In the second group (HT) the glass ceramic was heat-pressed on the zirconia coping, and for the third group (ST) a CAD/CAM-fabricated high-strength anatomically shaped veneering cap was sintered onto the zirconia coping. All crowns were cemented onto their titanium dies with Rely X™ Unicem (3M ESPE) and loaded with a universal testing machine (Instron 5583) until failure. The mean fracture values were compared by an one-way ANOVA and a multiple comparison post-hoc test (α=0.05). Scanning electron microscope was used to investigate the fractured interface.

Results: Mean fracture load and standard deviation was 4263.8±1110.8 N for Group LT, 5070.8±1016.4 for Group HT and 6242.0±1759.5 N for Group ST. The values of Group ST were significantly higher than those of the other groups.

Conclusion: Zirconia crowns veneered with CAD/CAM generated glass ceramics by the sintering technique are superior to those veneered with feldspathic porcelain by the layering technique or veneered with glass ceramics by the heat-pressing technique in terms of fracture strength.

Keywords: CAD/CAM; Fracture strength; Zirconia.

Fig. 1

Titanium die (A) and zirconia coping (B) during CAD/CAM fabrication process.

Fig. 2

Schematic design of three specimens after veneering process. A: Group LT, B: Group HT, C: Group ST.

Fig. 3

Testing setup in the universal testing machine and position of load tip of test specimen and piston.

Fig. 4

Cohesive fracture modes of a specimen from experimental groups. A: Group LT, B: Group HT, C: Group ST.

Fig. 5

SEM views of the interface between zirconia coping and veneering ceramics of Group LT.

Fig. 6

SEM views of the interface between zirconia coping and veneering ceramics of Group HT.

Fig. 7

SEM views of the interface between zirconia coping and veneering ceramics of Group ST.