Effect of shade and thickness of hybrid ceramics on the degree of conversion of a light cured resin cement

Abstract

Ceramic materials have several appealing properties making it highly on demand. As esthetics have become of major importance, ceramic’s fluorescence, natural look, chemical stability and biocompatibility made its usage favorable. The success of a ceramic restoration is mainly dependent on the bond strength between the ceramic, cement and underlying tooth structure. This bond is influenced by the strength of the adhesive cement responsible for maintaining the integrity of this connection. The optimal curing of the adhesive cement is essential to reach a high bond strength. The degree of conversion of resin cements is based upon the cross-linking of monomers to form polymers. This defines the unique mechanical and physical properties of the composite luting material. However, this mechanism is affect by several intrinsic and extrinsic factors such as, the type of monomer and the inorganic component of the cement, its shade, thickness of overlying ceramic restoration, light intensity of curing unit, distance of curing, and temperature. The amount of light energy reaching the luting layer is strongly attenuated by the thickness of the indirect restoration, the shade of ceramic used and by its absorbing characteristics reducing the total amount of light reaching the adhesive cement. This attenuation is dependent on the crystal structure, thickness and shape of the indirect ceramic restoration. Even though dual-cured resin cements have been developed to overcome the inability of light to completely reach the bonding resin underneath indirect restorations, the reduction of transmitted irradiance when light curing is performed through the ceramic restorations can influence bond strength and degree of conversion of dual-cure adhesives systems. Dual cured cements lack color stability and therefore are not the prime choice for luting esthetic anterior restorations. Recent studies aim to produce dual cured cements of superior color stability. Laminate veneer restorations are greatly demanded nowadays, as they provide a highly esthetic solution for anterior defects, through a very conservative preparation. This approach, serves to conserve most of the precious tooth enamel, maintaining the tooth integrity. The evolution in adhesive dentistry aided the advancement of these highly conservative esthetic restorations. Again, the high degree of polymerization of luting resin cements is crucial for the success of laminate veneers, in terms of mechanical success and also the final shade of the restoration. To evaluate the degree of conversion of light cured resin cements, several methods have been addressed in the literature. Direct and indirect methods have been used. In this study Fourier Transform Infrared spectroscope was the method of choice. The amount of remaining unreacted C=C after the complete polymerization of the resin cement is detected and the degree of conversion calculated. This value gives us an idea about the effect of different clinical factors on the polymerization reaction and hence the physical and mechanical properties of the cement used. These properties have huge effect on the final outcome of the dental restoration. These factors should be carefully considered on choosing the ceramic materials, luting resin cement and mode of polymerization.