Influence of Surface Characteristics of Four Silica-based Machinable Ceramics on Flexural Strength and Bond Strength of a Dual-curing Resin Luting Agent

Purpose: The aim of the present study was to evaluate the crystalline phase and microstructure of 4 commercial machinable ceramic blocks - Cerec Vitablocs Mark II (Vita), ProCAD (Ivoclar/Vivadent), GN-I (GC), and GNCeram (GC) - and compare flexural strength and shear bond strength between a dual-curing resin luting agent and the ceramics treated with a silane coupling agent. Materials and Methods: Specimens were examined using scanning electron microscopy/energy-dispersive x-ray spectroscopy, and x-ray diffractometry. Three-point bending tests were performed with polished specimens 20 mm long, 4 mm wide, and 1.2 mm thick. Two differently shaped specimens for each of the 4 machinable ceramics were treated with a silane coupling agent. The specimens were then cemented together with a dual-curing resin luting agent. Half of the specimens were stored in water at 37°C for 24 h and the other half were thermocycled 20,000 times. Results: Chemical composition, crystalline phase, and crystallinity were significantly different between brands. The Vitablocs Mark II material had the significantly lowest flexural strength (101.7 ± 15.3 MPa), while the GNCeram material had the highest (174.8 ± 10.3 MPa). The use of a silane coupling agent yielded high shear bond strength after 20,000 thermocycles (Vitablocs Mark II: 37.7 ± 3.7 MPa, ProCAD: 41.2 ± 3.1 MPa, GNCeram: 50.2 ± 2.1 MPa), except with the GN-I material (23.9 ± 4.4 MPa). Conclusion: It appeared that crystal distribution and particle size of leucite crystal, not crystallinity, in the feldspar glass matrix of silica-based machinable ceramics might influence the flexural strength and efficacy of a silane coupling agent in bonding between a dual-curing resin luting agent and machinable ceramics. Keywords: machinable ceramic, flexural strength, bond strength, dual-curing resin luting agent, surface characteristics