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Introduction

Although zirconia, especially yttria-stabilized tetragonal zirconia polycrystal (Y-TZP), has been widely used in the production of all-ceramic restorations nowadays, the relatively higher failure rate of zirconia-based prostheses compared with porcelain-fused-to-metal (PFM) restoration is still concerned . The bonding between zirconia and porcelain was also reported to be inferior to that between metal and porcelain . Such weak bonding might be due to some factors, e.g. a mismatch in the coefficients of thermal expansion, surface treatment, lack of chemical bonding, unintentional formation of microporosities at the interface, etc. .

Several in vitro studies on zirconia surface treatment have been carried out aiming at improving the quality of porcelain to zirconia bonding. Since zirconia is chemically inert, it cannot be etched by some commonly used mineral acids, such as HF, H ₃ PO ₄ , under normal atmospheric conditions . Thus, acid etching may not be an effective and convenient method. It has been reported that the application of sandblasting would help to produce a durable bonding at zirconia/porcelain interface . However, such a finding has not been supported by all published studies. One study found that, although sandblasting could significantly increase the surface roughness of zirconia, it did not result in improving the zirconia/porcelain porcelain integration . Furthermore, during the process of sandblasting, surface flaws may also be produced which might be detrimental to the clinical survival of zirconia-based restorations .

Light Amplification by Stimulated Emission of Radiation (laser) technology has been utilized as one of the industrial surface treatment methods for more than forty years. Laser beam has some advanced properties, includes high and precise monochromaticity, coherence, collimation, great brightness and output efficiency . The use of laser energy in zirconia surface treatment has now been of interest and reported in several studies. It was found that the values of zirconia surface roughness had been significantly increased after being processed with Nd:YAG laser (pulse mode) as well as the surface wettability . Another study compared the shear bond strengths of resin zirconia bonding among four different pre-treatment groups (untreated, HF etching, sandblasting and laser irradiation) and claimed that CO ₂ laser irradiation (pulse mode) at the 3 W power induced higher bond strength. Such improvement in bond strength was ascribed to the generation of micro-cracks on zirconia superficial layer induced by laser treatment. Thus, the author proposed the laser irradiation as an alternative way to surface pre-treatment of zirconia . Nevertheless, the application of laser irradiation as a method of zirconia surface conditioning is still lack of investigation. Since the same effects generated in resin zirconia bonding may not work in the same way in porcelain zirconia bonding, the final word of the influence of laser treatment on porcelain to zirconia bonding still remains questionable.

The aim of this study was to investigate the effects of laser irradiation on zirconia before veneering procedure on the shear bond strength between zirconia and veneering porcelain. The hypothesis of the study was that laser irradiation would improve bonding at zirconia/veneering porcelain interface compared to sandblasting.