Micromechanical retention and chemical bonding to polycrystalline dental ceramics : studies on aluminum oxide and stabilized zirconium dioxide

Sammanfattning: Researchers are constantly developing new dental materials to replace missing teeth. One material group receiving major focus is ceramic materials; more specifically, oxide ceramics; and, in particular, yttrium dioxide-stabilized tetragonal polycrystalline zirconium dioxide (Y-TZP). In addition, one of the major challenges is to ensure retention of oxide ceramic-based restoration in the mouth, in a tissue preserving way. Success in traditional cementation of dental restorations relies on a geometric form that establishes the macromechanical retention, the surface structure of the dental restoration, the tooth substance (micromechanical retention) and the cement itself. In clinical situations when macromechanical retention is insufficient, it may be necessary to use an adhesive cementation technique. Reliable adhesive bonding between the restoration, the cement, and the tooth substance requires micromechanical retention and cement that achieves chemical retention. In oxide ceramics, chemical retention has been difficult to achieve and unpredictable. Various techniques have been proposed for modifying the surface of oxide ceramic-based restorations making adhesive cementation technique a possible treatment option. The overall aim of this thesis is to evaluate and develop techniques for modifying the surface of oxide ceramics that enable durable bonding between the restorations and adhesive cement systems. Additionally, the thesis will inventory existing methods for achieving a bondable surface on oxide ceramics and how these methods affect the materials. Study I evaluated bond strength between several adhesive cement systems and densely sintered aluminum oxide. Two of six of the cement systems studied showed acceptable bonding to densely sintered aluminum oxide. The choice of surface treatment for the oxide ceramic should be based on the cement system to be used. Study II described a modified-additive technique for producing bondable Y-TZP and evaluated the resulting surface structure and bond strength. Surface-modified Y-TZP showed a rougher surface structure and higher bond strength than unmodified Y-TZP. Study IV extended these evaluations with additional surface analysis and flexural strength testing. The results showed increased surface roughness, with a chemical composition of glass and with a content of monoclinic phase. Compared to unmodified Y-TZP, glass-modified Y-TZP showed lower flexural strength values that increased with the use of cement. Study III was a systematic literature review to inventory existing methods for achieving a bondable surface on oxide ceramics. This study also evaluated which methods provide clinically relevant bond strength and classified the various surface treatments into seven main groups: as-produced, grinding/polishing, airborne particle abrasion, surface coating, laser treatment, acid treatment, and primer treatment. Abrasive surface treatment, as well as silicacoating treatment, combined with the use of a primer treatment can result in sufficient bond strength for the bonding of oxide ceramics. This conclusion, however, needs to be confirmed by clinical studies. There is no universal surface treatment; the choice should be based on the specific materials. Together, the results of this thesis demonstrate that different surface treatments/modifications of oxide ceramics increase the bond strength between ceramics and adhesive cement systems. Surface modification with a glass medium was particularly effective. All surface treatment, however, affects the material properties and the resulting dental restoration. Choice of surface treatment should be made based on the restoration materials: the oxide ceramics and the adhesive cement systems.