Effects of a convenient silica-coating treatment on shear bond strengths of porcelain

Significant progress has been achieved in ceramics since the late 1990s1-4). In particular, new technological innovations such as heat-pressed5,6), injection-molded7), slip-cast8), and glass-infiltrated ceramics9-11) have widened the use of ceramics in prosthetic and operative dentistry. However, properties such as brittleness limit their use in applications such as fixed partial dentures. Since the beginning of the 21st century, zirconium dioxide (ZrO2: zirconia), a new and tough ceramic material, has been receiving increasing attention in dentistry12-16). Initially, its use was restricted to metalfree fixed partial anterior dentures, but its use is being extended to a variety of applications, such as fixed partial posterior dentures, orthodontic brackets19) and dental implant abutment or fixture20,21), as well as simple dental restoration or prostheses17,18). Above all, frameworks using zirconia-based ceramics have gained popularity for use as metal-free, long-lasting, and extensive fixed dental prostheses. Zirconia-based ceramics are characterized by a tetragonal phase, a monoclinic phase, and a cubic phase, and these phases are mutually interchangeable under some kinds of stress such as heating, sandblasting, and mechanical grinding. It is also well known that volume changes occur when going from monoclinic to tetragonal crystals under the specific actions mentioned above22,23). This phase change is useful for controlling crack propagation in zirconia-based ceramics and contributes to their high fracture toughness. Formation of the tetragonal phase strongly stabilizes zirconia-based ceramics, but other crystal phases are undesirable. To maintain the tetragonal phase at room temperature, zirconia-based ceramics must be stabilized by adding oxides. Yttria is a popular stabilizing additive, giving Y-TZP12,13,16). This new ceramic is useful as a prosthetic frame material because of its high fracture toughness and its precision fabrication with dental CAD/CAM systems. A final prosthetic process is needed to deposit a porcelain veneer on zirconia-based ceramics; however, chipping of the porcelain veneer causes clinical problems. Hydrothermal degradation of the veneer with lowtemperature aging is also controversial16,24). To overcome these problems, some modification techniques have been proposed. The latest zirconia-based ceramics materials, stabilized by ceria, were developed to achieve better mechanical properties than those of Y-TZP25-28). Ceriastabilized tetragonal zirconia/alumina nanocomposites (zirconia nanocomposites) are significantly reinforced by dispersing different nanosized particles into each crystal grain. Furthermore, preparation, prior to veneering, of frameworks as thin as those of traditional porcelainfused metal (PFM) is possible because of the flexibility of these nanocomposites, so extremely light frames of thickness 0.3 mm can be obtained. In esthetic restoration, dental PFM has served the profession well for nearly a half-century, but there are many disadvantages to using metals, such as allergic reactions as a result of the release of harmful ions into the body and/or low durability. Most dentists prefer allceramic restoration to traditional PFM methods when attempting to achieve the optimum esthetic results. Generally speaking, fixed-type all-ceramic prostheses usually give better esthetic results than PFM because of the difficulty in blocking out the color and opacity of the metal substructure. Moreover, it has been reported that Effects of a convenient silica-coating treatment on shear bond strengths of porcelain veneers on zirconia-based ceramics