Long-term fracture load of all-ceramic crowns: Effects of veneering ceramic thickness, application techniques, and cooling protocol

Abstract

Background: To evaluate, in vitro, the effects of the cooling protocol, application technique, and veneering ceramic thickness on the fracture resistance of ceramic crowns with Y-TZP frameworks.

Material and methods: 80 frameworks were made from zirconia by the CAD/CAM technique and divided into 8 groups (n = 10) according to the factors: "application technique" (stratified-L and pressed -P), "thickness" (1 mm and 2 mm), and "cooling protocol" (slow-S and fast-F) of the feldspathic veneering ceramic. After, all crowns were cemented over G10 preparations with resin cement (Panavia F, Kuraray), mechanically cycled (2x106 cycles, 200 N, 3Hz), and subjected to the axial compression resistance test (0.5 mm/min, 10 kN). The data (N) underwent descriptive statistical analysis by 3-way ANOVA and Tukey's test (5%). Fracture analysis was performed to determine the possible origin of failure.

Results: The factors "cooling protocol" (P=0.0058) and "application" technique (P=0.0001) influenced the fracture resistance of the crowns. For pressed veneer technique, the P2S (4608.9±464.5). A presented significantly higher results than that P2F(3621.1±523.0)BCD (Tukey's test). For the stratified technique, this difference was not observed (P>0.05). The thickness of the veneering ceramic was not significant regardless of the cooling protocol and technique (P>0.05). The predominant failure mode was chipping of the ceramic veneer originating in the subsurface.

Conclusions: The pressed technique, used with a slow-cooling protocol, leads to the best outcome for the veneering of all-ceramic crowns. Key words:Zirconia, ceramics, cooling protocol, thickness, application technique.

Figure 1

a) Representative stereomicroscope image (7,5x) of a cone-shaped fracture propagation (cone crack) of the veneering ceramic, b) chipping without framework failure of a crown from group V1F, c) Stereomicroscope image (10x) of a P2S crown failed by chipping. The black arrows indicate the direction of the failure in the feldspathic ceramic; the red arrow indicate the area of the probable failure’s origin and the white arrow indicate the wallner lines. F: feldspathic veneering ceramic.

Figure 2

SEM photomicrographs of a crown from P2S group failed by chipping. a) Overview of the fracture (24x). b) The red arrow indicates the area of load application; the presence of wake hackles (black arrows) indicates the direction of fracture propagation; it is possible to observe the compression curl (black rectangle) (70x). c) Closer image of the area under the load application (white ellipse); it can be seen that the pressed technique shows fewer porosities (90x). F: feldspathic veneering ceramic.

Figure 3

SEM photomicrographs of a crown from V2F group failed by chipping. a) Overview of the fracture (22x). b) The area of load application is indicated by the white arrow; the presence of wake hackles (black arrows) indicates the direction of failure propagation (60x). c) Closer view of the area under the load application; it can be observed the presence of many bubbles resulted from the stratified technique (150x). F: feldspathic veneering ceramic.