Abstract
The increase in patients’ awareness and demands for metal-free, highly aesthetic restorations, even in the posterior area of the mouth, has contributed to the introduction of high-strength ceramics into dentistry. Yttrium-stabilised tetragonal zirconia polycrystals (Y-TZP) is by far the strongest of these compositions, however, it is opaque white in colour, which compromise its aesthetics properties, therefore, it was covered by a layer of more translucent veneering ceramic to improve the overall aesthetics of the restoration. However, chipping of the veneering layer is a frequent problem associated with these restorations, therefore, several zirconia compositions with enhanced translucency were introduced to allow the production of the full contour of the restoration from zirconia alone.
The elimination of the veneering layer brings zirconia into direct contact with the tooth material and the oral environment, increasing the risk of excessive tooth material wear as a result of the high surface hardness of zirconia. Additionally, the oral environment is an ideal place for phase transformation to happen which involves enlarging of the zirconia crystal that leads to microcracking, making zirconia more abrasive. Therefore, studying the effect of aging on wear behaviour of zirconia is of high importance.
This thesis begins with a literature review composed of two parts in which the peer reviewed literature of the various components of study was critically analysed and comprehensively presented. The first part focused on development of all-ceramic systems for the monolithic use, especially zirconia ceramics. The findings indicate that the literature lacks the answer to whether the different zirconia products of the same generation and the different generations share the same material properties. In fact, the dental research community is even unaware of this issue. Additionally, several monolithic zirconia compositions cannot be classified under the second or third generations of zirconia and yet they are being studied and referred to using broad terms like monolithic or translucent zirconia.
The second part of the literature review examined the in vivo and in vitro studies on wear performance of monolithic zirconia against human enamel antagonists, to present and analyse the factors that control and modify the wear behaviour of monolithic zirconia in this tribological system. The findings indicated that zirconia is minimally abrasive to human enamel and less abrasive than the other dental ceramics, provided that its surface was polished, however, standard polishing protocols and surface roughness value that should be achieved to reduce abrasiveness of zirconia are yet to be determined. Furthermore, the studies on the effect of aging on wear of monolithic zirconia are lacking which motivated the conducting of the in vitro experiment presented in the next chapter. Additionally, the use of the number of wear simulation cycles as the base for comparing the results between the different studies is misleading and the total sliding distance should be used instead. The findings also agreed with part one of the literature review regarding the lack of distinction between the different zirconia products.
An in vitro study was conducted to study the effect of aging on the wear performance of monolithic zirconia. It was found that aging increased abrasiveness of monolithic zirconia regardless of the type of surface finish and that the effect of aging is “latent” and only revealed under mechanical loading during wear simulation which increased surface roughness and wear by adversely affecting zirconia’s mechanical properties, making it less capable to maintain its initial surface smoothness. Also, the glaze layer may protect zirconia from low temperature degradation (LTD), however, it was susceptible to aging which further increased its abrasiveness.