Graded Ultra-Translucent Zirconia (5Y-PSZ) for Strength and Functionalities

Abstract

Ultra-translucent zirconias are drawing immense attention due to their fascinating esthetic appearance. However, the high translucency came at the expense of diminishing strength along with the reduced ability of transformation toughening due to the increased cubic zirconia content. We aim to address these issues by infiltrating glass on the surface of an ultra-translucent zirconia (5Y-PSZ). Glasses of different shades can be used and the resulting graded glass/zirconia layer is expected to improve the material's flexural strength without compromising its esthetics. We also aim to elucidate how clinically relevant surface treatments-namely, air abrasion, glazing, or polishing-affect the fracture resistance of these zirconias with a high cubic content. All surface treatments were performed on bar-shaped (2 × 3 × 25 mm) and plate-shaped (12 × 12 × 1 mm) specimens, which were then subjected to a 4-point bending test and translucency measurements, respectively. 5Y-PSZ proved to be significantly more translucent than 3Y-TZP but also much weaker. Our hypothesis was accepted, as the strength of the glass-infiltrated ultra-translucent 5Y-PSZ (582 ± 20 MPa) is over 70% higher than its uninfiltrated counterpart (324 ± 57 MPa). Its strength is also over 25% higher than the highly polished 5Y-PSZ (467 ± 38 MPa). In addition, the translucency of 5Y-PSZ (translucency parameter [ TP] = 34, contrast ratio [ CR] = 0.31) is not affected by glass infiltration ( TP = 34, CR = 0.32) when the residual surface glass is removed by gentle polishing using 6- and then 3-µm diamond grits. Finally, both air abrasion and the presence of a glaze layer on the tensile surface decreased flexural strength significantly, being 274 ± 55 and 211 ± 21 MPa, respectively. With a combined high strength and translucency, the newly developed glass-infiltrated 5Y-PSZ may be considered a suitable material for next-generation, damage-resistant, and esthetic dental restorations.

Evaluating dental zirconia

“…The absence of an abrupt internal interface avoids a potential source of weakness in the structure. Further benefits of infiltration are enhanced shade selection at the cameo surface [25] and greater bond strength at the intaglio surface [63]. The advantages of nanoindentation property mapping of this kind should become even more apparent as new biomimetic zirconias and other inhomogeneous dental ceramics are developed.…”

“…New zirconia ceramics for dental restorations are continually under development, with a goal of maintaining mechanical integrity while improving translucency [12,25,44], The trend is toward monoliths, for greater longevity and avoidance of interfacial and residual stress issues. Compositional variants—graded structures, biomimetic structures, nanostructures—are being explored.…”

“…While the strongest and toughest of the dental ceramics, Y-TZP is nevertheless not entirely immune to clinical failure over time [15-19]. This is an important consideration as new-generation zirconia materials with improved translucency (but potentially compromised strength and/or toughness) are developed [9,12,14,20-25]. Elucidation of fracture and deformation mechanisms in these materials thereby holds the key to prolonged lifetimes of next-generation dental prostheses.…”

Effect of glazing application side and mechanical cycling on the biaxial flexural strength and Weibull characteristics of a Y-TZP ceramic

“… 18 Glazing has also been applied in the internal surfaces of zirconia crowns to improve bonding to resins. 19 , 20 Furthermore, considering that one of the main areas subjected to tensile stress is the internal surface of the crowns, 21 glaze application in this area can decrease tensile stress of the zirconia. 16 A low thickness of glaze layer can also be applied under connectors areas of FDPs, especially in clinical situations where there is lack of space for veneering ceramic.…”

Toughening of highly translucent zirconia by monoclinic ZrO₂ and SiO₂ particle coating

“…Highly translucent partially stabilized zirconia (PSZ) was developed for overcoming this problem; the phase composition of this novel material is different from that of conventional Y-TZP 6,7) . Owing to a higher Y2O3 stabilizer content, this highly translucent zirconia contains a significant amount of cubic zirconia (cZrO 2) and a smaller amount of Al2O3, rendering this zirconia more translucent.…”