Marginal Fit of Nanohybrid Ceramic Versus Emax Monolithic Restoration With Two Finish Line Curvatures : An In Vitro Study /

Abstract

Dental ceramics are used to restore missing or damaged structures because of their high translucency, esthetics, biocompatibility, better hardness that gives the restoration its wear resistance, resistance to disintegration in oral fluids, and low thermal diffusivity, however ceramics are brittle, which means that they display a high compressive strength but low tensile strength. This subjects such restoration for failure in the oral environment. Improvements in the mechanical properties and methods of fabrication in order to improve strength was achieved by adding different types of crystal and metal oxide which obstruct crack propagation and hence increase the strength, but as strength increased the translucency decreases which had a negative effect on the overall esthetics, so there was always a need to have an outer coat from glass ceramics. The delamination of esthetic veneer usually occur, to overcome such failure the use of monolithic restoration was introduced. By definition monolithic means it is fabricated in full contour out of a single material. Monolithic crowns can be fabricated from lithium disilicate, and full contour zirconia restorations. While zirconia offers significantly greater fracture strength (more than 900 MPa compared with 400 for lithium disilicate), lithium disilicate has generally been considered to be a better choice for use in the esthetic zone because of its high translucency and a more natural appearance. In order to combine the strength and reliability of zirconia together with esthetics of lithium disilicate, Zirconia-reinforced lithium silicate glass-ceramics was introduced. It has a homogeneous structure that offers good material quality and consistency, high load capacity and long-term reliability. Resin restorations are less expensive than other crown types. However, they wear down over time and are more subjected to fractures than porcelain fused to metal crowns. Moreover, they tend to be less esthetic than ceramic crowns. Therefore, to overcome this problem and to enhance esthetic properties, a new type of hybrid ceramics was introduced named as resin-reinforced lithium silicate glass ceramics, a hybrid dental ceramic with a dual network structure that combines ceramic and composite material properties. Due to its high strength, resinreinforced lithium silicate glass ceramics is resistant to the shear and compressive forces. Used in treatment of severely abraded dentition or erosion and with bruxers , due to its high reliability causing improved edge stability leading to finer and more accurate results than with conventional dental ceramics. Moreover, it can be easily finished and polished. It is also etchable, viscoelastic, has acceptable flexure strength and provides acceptable marginal fit. Marginal misfit has many severe outcomes which may cause prospective failure of the prosthesis. Marginal fit of the crown is defined as the gap between the prepared tooth and the inner surface of the restoration. Absolute marginal discrepancy is the distance between the cavosurface finish line of the preparation and the margin of the restoration. Marginal discrepancy can be measured through different methods such as direct view of the crown on a die, cross-sectional view, impression replica technique, and clinical examination. In order to obtain a proper marginal fit, there should be an appropriate marginal design and curvature according to the type of restoration being used. A study done to evaluate the effect of finish line curvature on marginal fit of all ceramic CAD/CAM crowns had concluded that the finish line curvature had no effect on marginal fit of all ceramic crowns. However, this study was conflicted by another study that determined the greater the finish line curvature, the wider the marginal gap widths. Marginal fit differs between different types of restorations and different finish line curvatures. The hypothesis of this study is to prove that the finish line curvature and type of ceramic restoration will affect the marginal fit.