Influence of mechanical properties of supporting die materials on fracture strength and failure behaviours of monolithic zirconia molar crowns

Abstract

Objective The aim of this study was to investigate the influence of supporting die with various mechanical properties on the fracture strength and failure mode of monolithic zirconia crowns. Methods Standard composite resin and porous titanium specimens (n= 40) with elastic modulus similar to human dentin (18.6 GPa) were fabricated. Mechanical properties, including Vicker′s hardness, flexural strength and fracture toughness were evaluated. Extracted teeth (n= 10) were prepared into standardised geometry, according to the shape of dies made of porous titanium (n= 10) and composite resin (n= 10) . Subsequently, 30 monolithic zirconia (M-Zir) single crowns were milled and cemented onto the above three types of substrates. Load-to-failure test was conducted for all samples with universal testing machine, from which fracture strength was recorded, followed by the failure mode. Fractographic analysis was done by using scanning electron microscopy. Mechanical Properties Tests of die materials were statistically analyzed with t test, whereas Load-to-failure test was done with One-way ANOVA and LSD-t test (SPSS 20.0) . Results The elastic modulus of porous titanium and composite resin was (18.50 ± 1.65) and (17.48 ± 1.70) GPa, respectively. There was no statistical difference between the two groups (t= 1.358, P= 0.191) . The Vicker′s hardness (165.60 ± 11.17) HV, flexural strength (522.47 ± 52.46) MPa and fracture toughness (3.67 ± 1.18) MPa·m1/2 of porous titanium were noted significantly higher than those of composite resin (P<0.001) . The average fracture strength of monolithic zirconia crowns cemented on the dentin, porous titanium and composite resin dies was (5 306 ± 467) , (5 273 ± 447) and (4 695 ± 583) N, respectively. No significant difference was found among the three groups (F= 4.253, P= 0.025) . The average fracture strength of the composite resin substrates was significantly lower than that on dentin (P= 0.015) and porous titanium (P= 0.021) , while no difference was found between the dentin and porous titanium group (P= 0.890) . Similar fracture mode of the M-Zir crowns that built on the three different substrates was noticed. All crowns broke into two pieces with cracks extending obliquely through cusps in the mesiodistal direction. However, most of the dentin substrates and all the composite substrates suffered catastrophic fractures after loading, while the porous titanium substrates stayed intact. Conclusions The Vicker′s hardness, flexural strength and fracture toughness of porous titanium seemed superior to those of composite resin. Substrate material may have no significant effect on failure mode. Porous titanium is expected to be a promising substrate material for the in vitro study of zirconia restorations. Key words: Flexural strength; Die materials; Mechanical properties; Zirconia; Fractographic analysis