Impact of Preparation Design, Substrate Elasticity, and Prolonged Aqueous Ageing on the Mechanical Integrity of Monolithic Zirconia Restorations Read Article (PDF)
Abstract
Introduction: The paradigm of minimally invasive dentistry has driven the demand for conservative tooth preparations. While monolithic zirconia is advocated for reduced-thickness restorations, the interactive effects of crown thickness, abutment stiffness, luting agent, and long-term aqueous aging on its mechanical stability remain unclear. Objectives: This in vitro study evaluated the combined impact of restoration thickness, material elastic modulus, luting agent, and prolonged distilled water storage on the fracture resistance and modes of failure of bonded monolithic zirconia restorations. Materials and Methods: A total of 48 monolithic zirconia crowns (IPS e.max ZirCAD MT) were milled to either a minimal (0.5 mm) or conventional (1.0–1.2 mm) thickness. These were bonded to abutments simulating the elasticity of enamel (lithium disilicate) or dentin (resin composite) using either an adhesive resin or a resin-modified glass-ionomer (RMGI) cement. The four groups (n = 12) were MTER, MT-DR, CT-DR, and CT-DG. Specimens were kept in distilled water for either seven days or three years prior to compressive load-to-failure testing. Fractography was conducted via scanning electron microscopy. Results: All configurations withstood loads exceeding maximum physiological masticatory forces. At 7 days, minimal-thickness crowns on dentin-analog abutments exhibited significantly lower fracture loads than all other groups. Conversely, minimal-thickness crowns on enamel-analog abutments performed comparably to conventional crowns. After 3 years, conventional crowns (CT-DR and CTDG) demonstrated a significant increase in fracture resistance, whereas minimalthickness crowns exhibited extreme variability with no significant time effect. Cement type did not significantly influence fracture load. Conclusions: Ultrathin monolithic zirconia crowns, when bonded to stiff, enamellike substrates, demonstrate short-term fracture resistance like conventional crowns. Although all designs endure physiological load limits, substrate stiffness is crucial for the success of minimally invasive crowns, while conventional preparations provide more reliable long-term biomechanical performance.