On the interfacial fracture resistance of resin-bonded zirconia and glass-infiltrated graded zirconia

Abstract

Objective: A major limiting factor for the widespread use of zirconia in prosthetic dentistry is its poor resin-cement bonding capabilities. We show that this deficiency can be overcome by infiltrating the zirconia cementation surface with glass. Current methods for assessing the fracture resistance of resin-ceramic bonds are marred by uneven stress distribution at the interface, which may result in erroneous interfacial fracture resistance values. We have applied a wedge-loaded double-cantilever-beam testing approach to accurately measure the interfacial fracture resistance of adhesively bonded zirconia-based restorative materials.

Methods: The interfacial fracture energy GC was determined for adhesively bonded zirconia, graded zirconia and feldspathic ceramic bars. The bonding surfaces were subjected to sandblasting or acid etching treatments. Baseline GC was measured for bonded specimens subjected to 7 days hydration at 37°C. Long-term GC was determined for specimens exposed to 20,000 thermal cycles between 5 and 55°C followed by 2-month aging at 37°C in water. The test data were interpreted with the aid of a 2D finite element fracture analysis.

Results: The baseline and long-term GC for graded zirconia was 2-3 and 8 times greater than that for zirconia, respectively. More significantly, both the baseline and long-term GC of graded zirconia were similar to those for feldspathic ceramic.

Significance: The interfacial fracture energy of feldspathic ceramic and graded zirconia was controlled by the fracture energy of the resin cement while that of zirconia by the interface. GC for the graded zirconia was as large as for feldspathic ceramic, making it an attractive material for use in dentistry.

Keywords: Cement bond; Feldspathic ceramic; Glass-infiltrated graded zirconia; Interfacial fracture energy; Wedge-loaded double-cantilever-beam; Zirconia.

Figure 1

The wedge double-cantilever-beam (WDCB) adhesive bond specimen used. Pairs of bars (zirconia, graded zirconia, or feldspathic ceramic) are bonded by a resin composite. A crack starter is created using aluminum foil separator. The compression load P is applied by a 60° hardened steel wedge which is positioned on a 90° beveled clearance at the specimen edge. This load has a transverse component which opens up the crack faces.

Figure 2

Normalized energy release rate G vs. normalized crack length c for the WDCB specimen of Fig. 1, ν = 0.3. Symbols are from the FEM analysis, solid line is a smooth fit to the FEM data.

Figure 3

Two frames from a video sequence corresponding to feldspathic ceramic/resin/feldspathic ceramic specimen. The bright line seen at the specimen center is the crack. The image in (a) and (b) were taken right after a noticeable crack growth occurred and just before the onset of rapid crack growth, respectively.

Figure 4

Bar chart of mean and standard deviation values of fracture energy G_C obtained from all tests performed. The unshaded and shaded bars represent the non-thermocycled/aged and thermocycled/aged specimens, respectively. The abbreviations are defined as follows: Z: zirconia, GZ: graded zirconia, P: feldspathic ceramic, SB: sandblasted, and E: chemically etched. Note that the data for zirconia are well below those for feldspathic ceramic or graded zirconia.

Figure 5

SEM micrographs of the fractured surfaces of feldspathic ceramic (a), graded zirconia (b) and zirconia (c) bonded dental ceramic specimens, all subjected to sandblasting and thermocycling/aging treatments. The images conclusively correspond to a region slightly ahead of the edge of the crack starter, where slow crack growth followed by a rapid one occurred. The lower images are magnified views of the circled areas in the upper images.