Masanao Inokoshi

Meta-analysis of Bonding Effectiveness to Zirconia Ceramics

Dental zirconia can no longer be considered un-bondable to tooth tissue. In literature, an increasing number of papers indeed report on the bonding effectiveness of different luting techniques to zirconia. We aimed to disclose general trends in bonding to zirconia by systematically collecting zirconia bond-strength data. A search in PubMed and EMBASE revealed 1,371 bond-strength tests reported on in 144 papers. A macro-shear bond-strength protocol was most frequently used; it revealed significantly lower bond strengths and was less discriminative than the other test methods. Regarding luting technique, the combination of mechanical and chemical pre-treatment appeared particularly crucial to obtain durable bonding to zirconia ceramics. The cement choice was not revealed as a determining factor after aging conditions, as long as composite cement was used. Regarding test protocol, a tensile test appeared more discriminative, particularly when combined with ‘water storage’ aging.

Influence of sintering conditions on low-temperature degradation of dental zirconia

UNLABELLED The effect of sintering conditions and concomitant microstructure of dental zirconia (ZrO2) ceramics on their low-temperature degradation (LTD) behavior remains unclear. OBJECTIVES Therefore, their effect on LTD of dental ZrO2 ceramics was investigated. METHODS Three commercial pre-sintered yttria-stabilized dental zirconia materials were sintered at three temperatures (1450°C, 1550°C and 1650°C) applying three dwell times (1, 2 and 4h). Grain size measurements and LTD tests were performed on polished sample surfaces. LTD tests were performed at 134°C in an autoclave. The amount of monoclinic ZrO2 on the exposed surface was measured by X-ray diffraction (XRD). RESULTS Higher sintering temperatures and elongated dwell times increased the ZrO2 grain size. Simultaneously, a larger fraction of zirconia grains adopted a cubic crystal structure, resulting in a decreased yttria content in the remaining tetragonal grains. Both the larger grain sizes and the lower average stabilizer content made the tetragonal grains more susceptible to LTD. Overall, independent on the commercial dental zirconia grade tested, the specimens sintered at 1450°C for 1h combined good mechanical properties with the best resistance to LTD. SIGNIFICANCE In general, increased sintering temperatures and times result in a higher sensitivity to low-temperature degradation of Y-TZP ceramics.

Highly-translucent, strong and aging-resistant 3Y-TZP ceramics for dental restoration by grain boundary segregation

Latest trends in dental restorative ceramics involve the development of full-contour 3Y-TZP ceramics which can avoid chipping of veneering porcelains. Among the challenges are the low translucency and the hydrothermal stability of 3Y-TZP ceramics. In this work, different trivalent oxides (Al2O3, Sc2O3, Nd2O3 and La2O3) were selected to dope 3Y-TZP ceramics. Results show that dopant segregation was a key factor to design hydrothermally stable and high-translucent 3Y-TZP ceramics and the cation dopant radius could be used as a controlling parameter. A large trivalent dopant, oversized as compared to Zr(4+), exhibiting strong segregation at the ZrO2 grain boundary was preferred. The introduction of 0.2 mol% La2O3 in conventional 0.1-0.25 wt.% Al2O3-doped 3Y-TZP resulted in an excellent combination of high translucency and superior hydrothermal stability, while retaining excellent mechanical properties.

Strength, toughness and aging stability of highly-translucent Y-TZP ceramics for dental restorations

OBJECTIVE The aim was to evaluate the optical properties, mechanical properties and aging stability of yttria-stabilized zirconia with different compositions, highlighting the influence of the alumina addition, Y2O3 content and La2O3 doping on the translucency. METHODS Five different Y-TZP zirconia powders (3 commercially available and 2 experimentally modified) were sintered under the same conditions and characterized by X-ray diffraction with Rietveld analysis and scanning electron microscopy (SEM). Translucency (n=6/group) was measured with a color meter, allowing to calculate the translucency parameter (TP) and the contrast ratio (CR). Mechanical properties were appraised with four-point bending strength (n=10), single edge V-notched beam (SEVNB) fracture toughness (n=8) and Vickers hardness (n=10). The aging stability was evaluated by measuring the tetragonal to monoclinic transformation (n=3) after accelerated hydrothermal aging in steam at 134°C, and the transformation curves were fitted by the Mehl-Avrami-Johnson (MAJ) equation. Data were analyzed by one-way ANOVA, followed by Tukeys HSD test (α=0.05). RESULTS Lowering the alumina content below 0.25wt.% avoided the formation of alumina particles and therefore increased the translucency of 3Y-TZP ceramics, but the hydrothermal aging stability was reduced. A higher yttria content (5mol%) introduced about 50% cubic zirconia phase and gave rise to the most translucent and aging-resistant Y-TZP ceramics, but the fracture toughness and strength were considerably sacrificed. 0.2mol% La2O3 doping of 3Y-TZP tailored the grain boundary chemistry and significantly improved the aging resistance and translucency. Although the translucency improvement by La2O3 doping was less effective than for introducing a substantial amount of cubic zirconia, this strategy was able to maintain the mechanical properties of typical 3Y-TZP ceramics. SIGNIFICANCE Three different approaches were compared to improve the translucency of 3Y-TZP ceramics.

Face simulation system for complete dentures by applying rapid prototyping

STATEMENT OF PROBLEM With the use of CAD/CAM technology in the fabrication of complete dentures, a face simulation program could become an integral part of the procedure. Thus far, little is known about the accuracy of face simulation programs. PURPOSE The purpose of this study was to evaluate the accuracy of a face simulation method for complete dentures. The method simulated the face after changing the arrangement of artificial teeth in complete dentures fabricated on a computer. MATERIAL AND METHODS The faces of 10 edentulous participants were simulated with integrated facial and denture data. The facial data of a participant wearing the Rapid Prototyping (RP) dentures and the corresponding simulation were compared. The normal distances at each of 10 anthropological measuring points were calculated and based on the results, the simulations were modified and then repeated. The Wilcoxon Signed Rank Test was used (α=.05). RESULTS For differences of more than 1 mm of the normal distance in the first simulation, the modified simulation improved the difference to less than 0.5 mm. CONCLUSIONS The findings of the present study indicated that the method of face simulation in the fabrication of complete dentures with a computer may be clinically useful.

Structural and Chemical Analysis of the Zirconia–Veneering Ceramic Interface

The interfacial interaction of veneering ceramic with zirconia is still not fully understood. This study aimed to characterize morphologically and chemically the zirconia–veneering ceramic interface. Three zirconia-veneering conditions were investigated: 1) zirconia–veneering ceramic fired on sandblasted zirconia, 2) zirconia–veneering ceramic on as-sintered zirconia, and 3) alumina–veneering ceramic (lower coefficient of thermal expansion [CTE]) on as-sintered zirconia. Polished cross-sectioned ceramic–veneered zirconia specimens were examined using field emission gun scanning electron microscopy (Feg-SEM). In addition, argon-ion thinned zirconia–veneering ceramic interface cross sections were examined using scanning transmission electron microscopy (STEM)–energy dispersive X-ray spectrometry (EDS) at high resolution. Finally, the zirconia–veneering ceramic interface was quantitatively analyzed for tetragonal-to-monoclinic phase transformation and residual stress using micro-Raman spectroscopy (µRaman). Feg-SEM revealed tight interfaces for all 3 veneering conditions. High-resolution transmission electron microscopy (HRTEM) disclosed an approximately 1.0-µm transformed zone at sandblasted zirconia, in which distinct zirconia grains were no longer observable. Straight grain boundaries and angular grain corners were detected up to the interface of zirconia– and alumina–veneering ceramic with as-sintered zirconia. EDS mapping disclosed within the zirconia–veneering ceramic a few nanometers thick calcium/aluminum-rich layer, touching the as-sintered zirconia base, with an equally thick silicon-rich/aluminum-poor layer on top. µRaman revealed t-ZrO2-to-m-ZrO2 phase transformation and residual compressive stress at the sandblasted zirconia surface. The difference in CTE between zirconia– and the alumina–veneering ceramic resulted in residual tensile stress within the zirconia immediately adjacent to its interface with the veneering ceramic. The rather minor chemical elemental shifts recorded in the veneering ceramic did not suffice to draw definitive conclusions regarding potential chemical interaction of the veneering ceramic with zirconia. Sandblasting damaged the zirconia surface and induced phase transformation that also resulted in residual compressive stress. Difference in CTE of zirconia versus that of the veneering ceramic resulted in an unfavorable residual tensile stress at the zirconia–veneering ceramic interface.

Influence of Alumina Addition on Low Temperature Degradation of Y2O3-Coated Powder Based Y-TZP Ceramics

The influence of the addition of 0.25, 2 and 5 wt.% alumina on the mechanical properties and low temperature degradation (LTD) behaviour of 3 mol% yttria-coated ZrO2 powder based Y-TZP ceramics was investigated, and compared to commercial powder based co-precipitated 3Y-TZPs with 0-0.25 wt.% Al2O3 addition. The ceramics were subjected to accelerated hydrothermal degradation in an autoclave in H2O at 134°C up to 40 hrs. X-ray diffraction and Raman spectroscopy were used to assess the LTD behaviour. Incorporating the Y2O3 stabilizer by means of a coating method resulted in a higher LTD resistance without compromising the higher fracture toughness, compared to the co-precipitation method. Alumina addition did not significantly influence the mechanical properties of all Y-TZPs but significantly increased the LTD resistance of the Y-TZP ceramics. The LTD resistance of 0.25 wt% Al2O3 doped TZPs was substantially higher than that of ceramics containing 2 or 5 wt.% Al2O3, which had a comparable susceptibility. The highest LTD resistance for the 0.25 wt.% alumina doped ceramics could be correlated to the solubility limit of alumina in zirconia.