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Comparative analysis on intaglio surface trueness, wear volume loss of antagonist, and fracture resistance of full-contour monolithic zirconia crown for single-visit dentistry under simulated mastication

PURPOSE This analysis aimed to evaluate the intaglio surface trueness, antagonist’s wear volume loss, and fracture resistance of full-contour crowns of (Y, Nb)-stabilized fully-sintered zirconia (FSZ), 4 mol% or 5 mol% yttria-stabilized partially sintered zirconia (4YZ or 5YZ) with high-speed sintering. MATERIALS AND METHODS A total of 42 zirconia crowns were separated into three groups: FSZ, 4YZ, and 5YZ (n = 14). The intaglio surface trueness of the crowns was evaluated at the inner surface, occlusal, margin, and axial areas and reported as root-mean-square, positive and negative average deviation. Half of the specimens were aged for 120,000 cycles in the chewing simulator, and the wear volume loss of antagonist was measured. Before and after chewing, the fracture load was measured for each group. The trueness values were analyzed with Welch's ANOVA, and the wear volume loss with the Kruskal-Wallis tests. Effect of the zirconia type and aging on fracture resistance of crowns was tested using two-way ANOVA. RESULTS The intaglio surface trueness measured at four different areas of the crown was less than 50 µm, regardless of the type of zirconia. No significant P in wear volume loss of antagonists were detected among the groups (P > .05). Both the type of zirconia and aging showed statistically significant effects on fracture resistance (P < .05). CONCLUSION The full-contour crowns of FSZ as well as 4YZ or 5YZ with high-speed sintering were clinically acceptable, in terms of intaglio surface trueness, antagonist’s wear volume loss, and fracture resistance after simulated mastication.

The Journal of Advanced Prosthodontics

INTRODUCTION
Recently, zirconia ceramics are currently widely used as the materials of choice for prosthodontic treatment, expanding their applications along with the advances of digital technology in clinical dentistry. 1 It has excellent mechanical properties, tooth-like aesthetics in case of increased yttria content, and has been optimized for computer-aided design and manufacturing for many years. 1,2 Generally, in order to reduce the wear of milling tools and chances of phase transformation, dental zirconia is used for milling as in the form of a pre-sintered or partially-stabilized block and produced as white-stage products that require further sintering to final density. 3 According to the conventional protocol, the required sintering time is in the range from 6 to 12 hours to complete. Since it is a time-consuming process, the reduction in sintering time within 90 minutes, so-called fast sintering or high-speed sintering, has recently been focused as a possible alternative pathway. [4][5][6] High-speed sintering may not significantly change the dimensional accuracy of the milled prostheses, while it could decrease the translucency of dental zirconia. 4,5 It reduced the amount of surface wear of sintered objects, revealed from the two-body wear simulation. 6 The fracture resistance of high-speed sintered product remained consistent or slightly increased compared to those of the products from conventional protocol. 6 The sintering temperature, holding time, and total sintering time may change the optical and mechanical properties of dental zirconia. 7 As an alternative to changing sintering protocol, the fully-sintered zirconia blocks could be answers for saving production time and the pathway to single-visit dentistry. 8 In comparison to partially-sintered zirconia prostheses, fully-sintered zirconia prosthesis showed decreased volume proportion of pores, increased strength, and precise fit. 9 However, due to high surface hardness of conventional fully-sintered zirconia blocks, the increased wear of machining tool was reported to be the problem. 8,10 To resolve these concerns, a fully-sintered (Y, Nb)-TZP (tetragonal zirconia polycrystal) block with low surface hardness has recently been developed for clinical use. It has excellent machining capability and dimensional accu-racy. It also minimizes the chances of phase deformation during fabrication by the presence of stabilizing dopants such as Nb. 11,12 Numerous factors should be considered in order to produce zirconia-based dental prostheses with longterm clinical success. Proper seating of dental prosthesis is important and it depends on the dimensional accuracy of the intaglio surfaces. 13,14 Failure to proper seating can result in thick cementation layers, creating residual stresses on the tensile surface due to viscoelastic deformation during cyclic loading. 15 The marginal area of fixed prosthesis is one of the most frequent source of prosthetic complication, closely related to plaque retention, secondary caries and periodontal disease in case of misfit. [16][17][18][19] To minimize marginal discrepancy, evaluation of intaglio trueness of dental prosthesis is important in terms of manufacturing accuracy. 20 The fracture resistance of dental prosthesis is an important factor in clinical situation where occlusal forces should be tolerated. 21 The fracture resistance of all-ceramic crown is reported to be affected by the crown thickness, crown shape, and axial wall height affect. 22,23 Moreover, clinical failure of all-ceramic restoration is a multifactorial event that includes patient characteristics, masticatory loading, material properties, and fatigue phenomena. 24 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) has high fracture toughness and flexural strength due to transformation toughening. 25,26 However, 4 mol% (or 5 mol%) yttria-stabilized partially sintered zirconia (4Yor 5Y-PSZ) has higher proportion of cubic phase than 3Y-TZP, which has reduced martensitic transformation ability and decreased mechanical properties. 25 Considering the clinical situations, the performance of the prosthesis should be examined under cyclic loading and hydrothermal aging. 27 The wear of occlusal surfaces of the prosthesis also needs to be simulated in the physiologic environment. 6 Fatigue failure is described as the formation of subcritical cracks in pre-existing flaws in ceramic materials as a result of intermittent stress below the material's typical fracture strength in an aqueous environment. 28 Zirconia ceramics are known as susceptible to hydrothermal aging, a process in which water molecules cross through the grain boundaries and cause phase trans-https://doi.org/10.4047/jap.2022. 14.3.173 formation, resulting in aggravation of mechanical properties. 29,30 The consequences of hydrothermal aging are surface roughening, enhanced wear rates, reduced mechanical properties and even catastrophic failure. 31,32 The purpose of this study is to evaluate the intaglio trueness, wear volume loss of antagonistic teeth, and fracture resistance of full-contour monolithic crowns, fabricated using fully-sintered (Y, Nb)-TZP and partially-sintered 4Y-PSZ (or 5Y-PSZ) with high-speed sintering (90-minute protocol). The first null hypothesis was that no difference would be found in terms of intaglio trueness of the crowns, regardless of the type of zirconia. The second null hypothesis was that the wear volume loss of antagonistic teeth would be the same, irrespective of the type of zirconia. The third null hypothesis was that no difference would be found in the fracture resistance among the different zirconia groups, regardless of artificial aging.

RESULTS
In terms of intaglio surface trueness of zirconia crowns, the measured values of three groups (FSZ, 4YZ, and 5YZ) in four different areas (inner surface, occlusal, margin, and axial area) are shown in Table 2, Figure 1 and Figure 2. The FSZ group showed significantly larger mean RMS values than the 4YZ and 5YZ groups in the margin (P < .001) and occlusal area (P <    Inner Surface J Adv Prosthodont 2022;14:173-81 Comparative analysis on intaglio surface trueness, wear volume loss of antagonist, and fracture resistance of full-contour monolithic zirconia crown for single-visit dentistry under simulated mastication The Journal of Advanced Prosthodontics groups, respectively. No statistically significant differences were detected among the groups (P = .95). In terms of fracture resistance, the fracture load values of the FSZ group were larger than those of the other groups both before and after artificial aging (Fig.   3). Both the type of zirconia (P = .001) and artificial aging (P = .026) affected significantly on the fracture resistance of the monolithic crowns; however, there was no interaction between two factors (P = .295). Regardless of artificial aging, the FSZ group showed the highest fracture resistance among the zirconia crown groups (Fig. 3, Table 3).

DISCUSSION
Based on the results of this study, the first and third null hypotheses were rejected. The FSZ group showed significantly lower intaglio surface trueness than the other zirconia groups, especially in the regions such as margin and occlusal areas. In this study, the FSZ crown was fabricated using a 4-axis milling machine according to the manufacturer's instruction, which can be less accurate than the 5-axis milling machine, and this would explain the difference in trueness from the crowns of the 4YZ or 5YZ groups. 8,37 Furthermore, if the bur diameter exceeds the curvature of the prepared abutment, particularly the transition from the axial to the occlusal surface, or the path of milling tools does not fully reproduce the morphologies of the intaglio surface of the crown, it could decrease the accuracy of milled prosthesis. 13 The higher negative average deviation in the margin area of the FSZ group than the others could be the result of the aforementioned problem. Nevertheless, the accuracy of the monolithic crowns evaluated in this study is within the clinically acceptable level. 8,38 Based on the RMS values at the margin and inner surface areas showed the interquartile range of the FSZ group was relatively smaller than those of the other groups. Aside from the wear of milling tools, 'hard milling' of dental ceramics without further sintering may be more advantageous than 'soft milling' for production of complex dental prostheses. 2 For the 4YZ and 5YZ groups, the uneven shrinkage after sintering may have contributed relatively wide interquartile range of RMS measurements for intaglio surface trueness. 39 Additionally, the high-speed sintering was reported to affect the internal fit of monolithic crown, resulting in worse fit than the conventional sintering, due to different pattern of temperature change. 4 According to the manufacturers' information, the (Y, Nb)-TZP block and 4Y-PSZ block showed similar flexural strength (MPa) values, while the 5Y-PSZ block showed relatively lower than those. However, the fracture load (N) was significantly higher for FSZ group than 4YZ group. It is speculated that the material of the support die, the loading angle, the fatigue loading, the specimen thickness, and the cementation procedure have influenced the load-to-failure testing results. 21,23 In addition, relatively high fracture load value may be attributed to the use of Co-Cr alloy for testing abutment with high elastic modulus. 21 Generally, the zirconia crowns undergone high-speed sintering could change the grain size and fracture resistance compared to the conventional sintering. 5,6,30 Despite of high-speed sintering, the fracture load of the 4YZ group was lower than that of the FSZ group, which needs further evaluation.
After artificial aging, the fracture load values changed in all three groups, but only the 4YZ group showed a significant decrease. Previous studies showed that 4Y-PSZ and 5Y-PSZ were less sensitive to hydrothermal aging than 3Y-TZP. [30][31][32] Higher stabilizer content is responsible for the high resistance to aging and for eliminating the transformation toughening mechanism, also being responsible for the appearance of a great amount of cubic crystals in its microstructure. 40 Since there was less hydrothermal degradation in the FSZ group utilizing (Y,Nb)-TZP under low-temperature aging conditions, the change in strength was relatively minimal. This is mostly attributed to the phase stability of zirconia, which is caused by the Y-Nb alignment in the tetragonal zirconia lattice. 12

CONCLUSION
Within the limitations of this study, the full-contour crowns of fully-sintered (Y, Nb)-TZP as well as partially sintered 4Y-PSZ or 5Y-PSZ with high-speed sintering were concluded as clinically acceptable for single-visit dentistry, in terms of intaglio surface trueness, antagonist's wear volume loss, and fracture resistance after simulated mastication. The fully-sintered zirconia crown showed higher fracture resistance than the partially sintered zirconia crown with high-speed sintering.