Aspects of resin bonding to zirconia

Abstract

Zirconia (ZrO2) is a polycrystalline ceramic widely used in dentistry because of its excellent aesthetic, high mechanical strength, and biocompatibility, all of which demonstrate its wide range of use for partial and full all-ceramic fixed or removable dental prostheses including monolithic veneer, inlays, onlays, crowns and bridges, and even denture in both anterior and posterior regions. The commonly used CAM-millable dental zirconia are yttria-stabilized tetragonal zirconia (3Y-TZP) that contains ~3 mol% tetragonal (t) phase zirconia, and newer yttria partially stabilized zirconia (4Y and 5Y-PSZ) that includes 4 or 5-mol% zirconia with a mixture of t and cubic (c) phases of zirconia. On the other hand, additive manufacturing (AM, a.k.a 3D-printing) technologies e.g. Digital Light Processing (DLP) and Gel deposition (SGZ) techniques have also been clinically adopted. All zirconia dental prostheses rely on adhesion. Sandblasting with alumina to create micromechanical retention followed by the application of primers or cement based on MDP (10-methacryloyloxydecyl dihydrogen phosphate) for chemical interaction are commonly adopted as standard surface treatment method. In the first study, it was found that DLP zirconia is significantly rougher than other zirconia, while the sandblasting would significantly increase the surface roughness (Ra) on 3Y, 5Y and SGZ zirconia (measured by profilometer) while significantly decreasing the roughness on DLP (measured by AFM). However, sandblasting was shown to increase of Al on zirconia surfaces, lower crystallinity of t-phase ZrO2 in 3Y, and detection of m-phase in 4Y, DLP and SGZ groups, which are problematic. Further, various bond strength tests might yield different outcomes. Thus, a new all-in-one resin adhesive was used and evaluated further with different bond strength test methods, namely shear bond strength (SBS) and enclosed-mould micro shear bond strength (ME-SBS) tests, to see whether an improved resin-zirconia bonding can be achieved. Accordingly, it was found that surface topography and all-in-one adhesive with primer are essential for establishing reliable resin-zirconia bond strength, such that the surface free energy (SFE) could have an exponential effect on the initial SBS, but thermocycling would have no significant effects on SBS. The use of primer, all-in-one adhesive together with sandblasting would contribute a significant increase and stability for ME-SBS under thermocycling (up to 10000 cycles), such that Vickers hardness of the sandblasted zirconia would positively correlate with initial ME-SBS. 3Y and DLP have the highest ME-SBS in overall. In terms of test methods, ME-SBS demonstrated significantly superior bond strength and reliability compared to conventional SBS materials, particularly under thermocycling-induced aging. Ultimately, this PhD study evaluated and compared the resin-zirconia bonding, physiomechanical, and surface characteristics of milled and 3D-printed zirconia, along with susceptibility to thermal ageing of the adhesive interface with a new all-in-one adhesive. ME-SBS was proven to be a proper bond strength test method to evaluate the resin bond strength and durability of the resin-zirconia adhesion strength. (458 words)