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“CAD-on” interfaces - a fracture mechanics characterization Walker, Peter David


Abstract Objective: “CAD-on” crowns, consisting of CAD/CAM milled lithium disilicate (LS2) veneers glass-fused to CAD/CAM milled yttrium oxide stabilized tetragonal zirconia polycrystal (Y-TZP) framework, have shown promise in increasing veneer fracture resistance. The glass fusion technique is purported to result in stronger bonding between veneer and framework when compared to conventional veneering. The objective of this study was to apply fracture mechanics methodology to characterize the interfaces present in “CAD-on” crowns. Methods: The notchless triangular prism (NTP) specimen fracture toughness (KIC) test was used to determine interfacial KIC. Four groups, each consisting of 6X6X6X12mm NTP specimens (n=22), were produced from IPS Emax CAD (LS2), IPS Emax ZirCAD (Y-TZP), and IPS Emax ZirPress and crystal connect™(CC) fusing glass. Groups I (Emax/CC/Emax), II (Zir/CC/Zir), and III (Zir/CC/Emax) utilized half-size (6X6X6X6mm) NTP specimens approximated under vibration with the connecting glass and sintered according to manufacturers’ guidelines. Group IV specimens were coated with ZirLiner and pressed with IPS Emax ingots. The specimens were tested using a computer controlled (Bluehill) Instron 4301. Results were analyzed with one-way ANOVA, Scheffé multiple means comparisons (α=0.05) and Weibull statistics. All fractured surfaces were characterized with a light microscope. Selected fractured interfaces were characterized under a scanning electron microscope. Results: Groups I-III demonstrated a cohesive mode of failure. Number and size of defects appeared to correlate with the variability of K1C values. There were no significant differences between the KIC values of the “CAD-on” interfaces. Interfacial KIC values were limited by KIC of CC. The “CAD-on” KIC value was significantly greater than that of the ZirPress control. Conclusion: Based on the results obtained, KIC of interfaces produced during the “CAD-on” technique appear to be limited by the interfacial KIC of the connecting glass and the defects produced during processing. In this study, “CAD-on” produced veneers had stronger interfacial KIC than a conventionally veneered control group.

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