Evaluation of Edge Strength of two Hybrid Ceramics as Compared to a Glass Ceramic Veneer with Different Marginal Thickness : "An in Vitro Study" /

Abstract

The demand for treating esthetics within anterior teeth continues to grow. Available options to restore their aesthetics include conservative treatments, such as bleaching and direct composite laminate veneers. Improvements in adhesive systems and the development of new-generation porcelain technology have supported the growing demand for treating unaesthetic teeth with porcelain laminate veneers. The problem of combining esthetics and strength has been strategic especially with the development of minimal thickness of laminate veneers. Throughout the years dentistry has gone through numerous developments in knowledge, techniques and technology. Due to increasing concerns about the aesthetics and biocompatibility of dental restorations, patients and dentists have become more and more interested in metal-free tooth-colored materials, ceramic materials were developed in response to this increasing demand. Newly introduced dental ceramics allow mimicking the natural tooth color and translucency. However, ceramics remain weak under tension due to its inherent brittleness and therefore their thickness has a major influence on their strength. Laminate veneers has greatly replaced other alternatives that were used to treat tooth discoloration. However, during long term evaluation clinical failure tends to appear. The most frequent failure that appeared in laminate veneers were incisal chipping and the development of inner flaws. This refers to the main problem that ceramics remain a brittle material. As a conclusion, due to the inherent brittleness of ceramics the material must be compensated by applying a desirable sufficient thickness to help the material perform adequately. As for edge strength, it is defined as the ability of the restorations to withstand forces at a thin edge and it is evaluated by measuring the force to fracture at 0.5 mm distance of the sample edge, since it is the most susceptible area for breakdown in the clinical performance of the restoration. A previous study has concluded that the force required for chip formation increases with distance from the edge and the process of chipping starts first with a small indentation that propagates and as a result a flake drops out. Throughout many previous papers, the edge strength was measured at a progressively increasing distance with 0.1 mm intervals, and they have shown that the most common two modes of failure were cracking and chipping. A different study has classified failure into a group that can be repaired and another group of complete replacement of the restoration. A similar classification was present in a study which graded failure from 1-3. Grade 1 represented minor chipping that can be modified by polishing, grade 2 was moderate chipping which is treated in the oral cavity by photo activated resin composite followed by grade 3 which is severe chipping or fracture by which the only line of treatment was removal of the restoration and remake. Recently introduced CAD/CAM materials offer an alternative solution, that is composed of 2 phases (resin and ceramic) with different percentage in each material. The combined solution of ceramic and polymer has improved the mechanical properties and with great attention to the improvement of the resiliency of these materials that led to fabrication of thin sections without chipping. A variety of all-ceramic systems are commercially available and can offer a new esthetic dimension, particularly in anterior teeth.(8) Recently introduced the polymer infiltrated ceramic materials like Vita Enamic that combines the properties of ceramic and polymer, it belongs

to a new substance class, known as “hybrid-ceramic.” It consists of a hybrid structure with two interpenetrating networks of ceramic and polymer, a so- called double network hybrid (DNH) that combined the advantages of both ceramic and polymer that improved the mechanical properties, the material can be milled very thin (0.2 - 0.5 mm) for the preservation of tooth structure. A previous study has concluded that the mechanical properties (flexural strength and strain at failure) of feldspar ceramic can be enhanced by infiltration of a second phase (polymer) into porous ceramic precursor and that these materials are considered to more closely imitate natural tooth properties compared with existing dental restorative materials. Another hybrid ceramic material will be used is Cerasmart (from GC Dental Products) which is a high-density composite resin material containing 71% filler particles by weight. It consists of flexible nano ceramic matrix with an even distribution of nano ceramic for better strength and force absorption, the material can be milled in thin section as it is indicated for veneers. Since there is a limitation in the previous studies about these hybrid ceramics, a control group of IPS Empress CAD, (from IvoclarVivadent) was used to compare the results as it contains a microstructure of leucite, which gives the restoration its mechanical and esthetic properties and many studies have used this glass ceramic. Thus, this study was conducted to evaluate the edge strength of the three different ceramic materials mentioned in minimal veneer thickness, as during milling, cementation and function the material is subjected to forces that should be evaluated to determine the capability of the material to withstand forces in minimal thicknesses.