Polishing of Ground Y-TZP Ceramic is Mandatory for Improving the Mechanical Behavior

An Approach to Improve Specimen Processing for the Flexural Strength Testing of Zirconia

Measuring the flexural strength of restorative materials such as zirconia is crucial for providing proper indications for clinical applications and predicting performance. Great variations in specimen preparation for flexural strength measurements exist among laboratories. The aim was to evaluate how the processing method, surface treatment, and test method of the specimens affect the flexural strength of zirconia. Zirconia specimens (VITA YZ HT) (n = 270) were processed using CAD/CAM or were conventionally milled with three different surface treatments (machined, ground, polished) and were measured with three-point bending (non-chamfered/chamfered) or biaxial flexural strength test. Weibull statistics were conducted. The mean flexural strength values ranged from 612 MPa (conventional, machined, three-point bending non-chamfered) to 1143 MPa (CAD/CAM, polished, biaxial flexural strength). The highest reliability is achieved when specimens are prepared using thoroughly controllable processing with CAD/CAM and subsequently polished. Higher strength values are achieved with the biaxial flexural strength test method because the stress concentration in relation to the effective volume is smaller. Polishing reduces surface microcracks and therefore increases the strength values.

Antagonist enamel tooth wear produced by different dental ceramic systems: A systematic review and network meta-analysis of controlled clinical trials

OBJECTIVES

The aim of this study was to evaluate the amount of enamel tooth wear induced by different antagonistic ceramic crown materials in the posterior area within a follow-up period up to 24 months in function. A network meta-analysis was performed to assess the effect of the materials on the mean vertical loss (MVL) of the antagonist enamel tooth surface.

DATA

Main search terms used in combination: ceramic, dental materials, metal ceramic, tooth wear and dental enamel.

SOURCES

An electronic search was conducted in PubMed/Medline, Embase, and Cochrane CENTRAL plus hand-searching.

STUDY SELECTION

Eligibility criteria included clinical studies reporting on MVL on antagonist's tooth up to 24 months following the permanent crown placement. From a total of 5697 articles, 7 studies reporting on 261 crowns for 177 subjects with 3 ceramic materials (Lithium disilicate, metal-ceramic, monolithic zirconia) were included. Among all, metal-ceramic and zirconia caused significantly higher enamel tooth wear on antagonist teeth, representing 82.5 µm [54.4; 110.6]) and 40.1 µm [22.2; 58.0]) more MVL than natural teeth group. In contrast, lithium disilicate showed only 5.0 µm [-48.2; 58.1]) more MVL than occurs on opposing natural teeth.

CONCLUSIONS

This systematic review demonstrated that prosthodontic ceramic materials produced significantly more antagonist enamel tooth wear than opposing natural enamel tooth wear, and ceramic material type was correlated to the degree of enamel tooth wear. Additional well-conducted, randomized controlled trials with homogeneous specimens are required due to inadequate sample size and number of the clinical studies included in the analyses.

CLINICAL SIGNIFICANCE

The amount of wear caused by different restorative materials has a high influence on the antagonistic natural teeth and should therefore be evaluated intensively by the dentist.

A brief review on fatigue test of ceramic and some related matters in Dentistry

The characteristics of dental ceramics have been extensively studied over the years to provide highly qualified materials for use in prosthetic restorations. The ability to adhere to dental substrates, outstanding aesthetics (translucency, color, and substrate masking ability) and improved mechanical properties provide these materials with optical features and high strength to withstand masticatory stimuli. Different classifications are adopted, and it is generally considered that glass-ceramics have better optical characteristics due to the high glass content, and polycrystalline ceramics have superior strength favored by their densified and organized crystals, hampering crack growth. This knowledge was largely built-up during years of scientific research through different testing methodologies, but mainly employing static loads. It is important to not only take into account the intensity of loads that these materials will be exposed to, but also the effect of the intermittence of cyclic load application leading to mechanical fatigue and the influence of factors related to the crack origin and their propagation under this condition. Furthermore, the bonding surface of ceramic restorations requires surface treatments that improve the bond strength to luting agents; however, these treatments require caution because of their potential to produce defects and affect the structural behavior. Moreover, ceramic restorations often require internal adjustments for proper seating or external adjustments for fitting the occlusal contact with the antagonist. In this sense, finishing/polishing protocols may alter the defect population, as luting agents may also interact by filling in the superficial defects on the restoration intaglio surface. Thus, the balance among all these factors will define the performance of a restorative setup, as well as the posterior exposure to the humid environment and the masticatory stimuli (cyclical loading), which may favor developing slow and subcritical growth of cracks in ceramic materials and the degradation of the bond interface. Therefore, it is essential that the concepts which explain the fatigue mechanism are understood, as well as the crack propagation and failure patterns of restorative ceramic materials.

Should finishing, polishing or glazing be performed after grinding YSZ ceramics? A systematic review and meta-analysis

The present systematic review and meta-analysis aimed to assess the characteristics and consequences of post-processing methods after grinding procedures in YSZ ceramics on its surface roughness and flexural strength. The protocol of this review was made prospectively and is available online in the PROSPERO database (link). Literature searches on PubMed/MEDLINE, EMBASE, Lilacs, Web of Science and Scopus were conducted on December 2022 to select in vitro studies written in English, without publishing-date restrictions, that considered surface characteristics and mechanical properties of YSZ ceramics submitted to grinding and subsequent post-processing surface treatments as an attempt to revert the effect induced by grinding. Two authors independently selected the studies, extracted the data and assessed the risk of bias. Mean differences (Rev-Man 5.1, random effects model, α= 0.05) were obtained by comparing flexural strength and surface roughness values of ground surfaces with at least one post-processing surface treatment (global analysis). Subgroup analyses were performed considering the most prevalent categories of post-processing methods. A total of 33 (out of 4032) studies were eligible and included in the analysis. In the global analysis, ground surfaces showed higher flexural strength than when post-processing methodologies were employed (p< 0.0001). The subgroup analysis showed that only polishing was able to enhance the flexural strength after grinding (p= 0.001); however, when other protocols were used, the ground surface was always superior in terms of flexural strength (p< 0.0001). Post-processing techniques in both the global and sub-group analyses were able to reduce the surface roughness after grinding in YSZ ceramics (p< 0.00001). High heterogeneity was found in all the meta-analyses. Concerning the risk of bias analysis, the included studies had mixed scores for the considered factors. In conclusion, in terms of improving flexural strength and restoring surface roughness after grinding, polishing protocols can be considered the best indication as post-processing treatment after YSZ ceramics adjustments/grinding.

A statistical model of the rate-dependent fracture behavior of dental polymer-based biomaterials

An insight into the fracture behavior of dental polymer-based biomaterials is important to reduce safety hazards for patients. The crack-driven fracture process of polymers is largely stochastic and often dependent on the loading rate. Therefore, in this study, a statistical model was developed based on three-point bending tests on dental polymethyl methacrylate at different loading rates. The fracture strains were investigated (two-parameter Weibull distribution (2PW)) and the rate-dependency of the 2PW parameters were examined (Cramér-von Mises test (CvM)), arriving at the conclusion that there could be a limiting distribution for both quasi-static and dynamic failure. Based on these findings, a phenomenological model based on exponential functions was developed, which would further facilitate the determination of the failure probability of the material at a certain strain with a given strain rate. The model can be integrated into finite element solvers to consider the stochastic fracture behavior in simulations.

Overview of Several Typical Ceramic Materials for Restorative Dentistry

With the development of ceramic technology, prosthodontic ceramics are becoming a useful option for improving esthetic outcomes in dentistry. In this paper, various ceramic materials were reviewed and evaluated, and their advantages and disadvantages and indications in oral prosthodontics were analyzed objectively. The properties of resin-based ceramics, polycrystalline ceramics, and silicate ceramics were compared and analyzed. Resin-based ceramics may replace other ceramic materials in the CAD/CAM field.

3-D Surface Morphological Characterization of CAD/CAM Milled Dental Zirconia: An In Vitro Study of the Effect of Post-Fabrication Processes

Objective: To investigate the effect on zirconia surface of the post-fabrication surface treatments on the morphological characteristics and mechanical properties of CAD/CAM milled dental zirconia specimens as well as to identify the critical parameters in the measurement of oral retention under in vitro circumstances. Method: The zirconia specimens (N = 20, n = 4) were subjected to CAD/CAM milling and divided into five groups. The specifications were: Group G1—sintered; Group G2—sintered followed by a polishing process; Group G3—sintered followed by polishing and sandblasting with alumina particles Al2O3 (110 µm); Group G4—sintered followed by sandblasting; Group G5—sintered followed by sandblasting with polishing as the end process. All the groups were subjected to Fretting wear tests, 3-D surface roughness measurements, and Vickers’s Micro hardness tests. Investigation of the phase transformation using XRD, and surface feature examination using SEM were also carried out. Additionally, one-way ANOVA, Tukey, and Pearson correlations were statistically analysed. Results: The fabrication processes had a significant effect on the performance of zirconia specimens in all the groups (p > 0.05). Specimens that underwent polishing as the last process exhibited lower surface roughness. The monoclinic phase of zirconia was observed in all the specimens before and after wear except for those in the G2 and G5 groups, where polishing was the end process. In G5, the post-wear surface properties revealed lower surface roughness and hardness. Further, the SEM and 3-D topography show grooves as seen by the dale void volume (Vvv) values; shallow valley depth (Svk); micro craters; and wear track. Conclusion: Specimens in G5 that were subjected to multistep post-fabrication process, namely sandblasting followed by polishing, yielded better results when compared to those in the other groups (G1, G2, G3, and G4). G5 with an interlayer of alumina is recommended for clinical applications due to its enhanced surface properties, mechanical properties, and low wear.

Fatigue strength of 5Y-FSZ: glazing and polishing effects

OBJECTIVE

The aim of this study was to evaluate the effect of four different finishing procedures on the fatigue strength of a fully stabilized zirconia (5Y-FSZ) material.

MATERIALS AND METHODS

Disc-shaped specimens of a 5Y-FSZ (Katana UTML, Kuraray Noritake) were made (ISO 6872-2015), grinded with 600- and 1200-grit silicon carbide paper, sintered as recommended, and randomly assigned into four groups according to the finishing technique: C (control, as-sintered), P (polished with polishing rubbers), G (glaze application - powder/liquid technique), and PG (polished with polishing rubbers + glaze application - powder/liquid). Then fatigue strength (staircase method), X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses were performed.

RESULTS

The C group presented the lowest fatigue strength, while the PG group presented the highest. The P and G groups presented intermediate behavior, presenting similar statistical results. XRD showed similar crystalline phase patterns for all groups. SEM images revealed some changes in the zirconia surface, with the P group presenting some scratches on the surface, while the scratches in the PG group were filled with the glaze material.

CONCLUSION

None of the techniques analyzed in this study impaired the fatigue strength of fully stabilized zirconia. Importantly, the polishing rubbers combined with glaze application (PG group) improved its fatigue strength.

CLINICAL RELEVANCE

The polishing rubbers followed by glaze application improve the fatigue strength in ultra-translucent zirconia.

Biaxial Flexural Strength of Different Monolithic Zirconia upon Post-Sintering Processes

OBJECTIVE

 Different post-sintering processes are expected to be a reason for alteration in the strength of zirconia. This study evaluated the effect of post-sintering processes on the flexural strength of different types of monolithic zirconia.

MATERIALS AND METHODS

 A total of 120 classical- (Cz) and high-translucent (Hz) monolithic zirconia discs (1.2 mm thickness and 14 mm in Ø) were prepared, sintered, and randomly divided into four groups to be surface-treated with (1) as-glazed (AG); (2) finished and polished (FP); (3) finished, polished, and overglazed (FPOG); and (4) finished, polished, and heat-treated (FPHT) technique (n = 15). Biaxial flexural strength (σ) was determined on a piston-on-three ball in a universal testing machine at a speed of 0.5 mm/min.

STATISTICAL ANALYSIS

 Analysis of variance, and post hoc Bonferroni multiple comparisons were determined for significant differences (α = 0.05). Weibull analysis was applied for survival probability, Weibull modulus (m), and characteristic strength (σ₀). The microstructures were examined with a scanning electron microscope and X-ray diffraction.

RESULTS

 The mean ± standard deviation value of σ (MPa), m, and σ₀ were 1,626.43 ± 184.38, 9.51, and 1,709.79 for CzAG; 1,734.98 ± 136.15, 12.83, and 1,799.17 for CzFP; 1,636.92 ± 130.11, 14.66, and 1,697.63 for CzFPOG; and 1,590.78 ± 161.74, 10.13, and 1,663.82 for CzFPHT; 643.30 ± 118.59, 5.59, and 695.55 for HzAG; 671.52 ± 96.77, 3.28, and 782.61 for HzFP; 556.33 ± 122.85, 4.76, and 607.01 for HzFPOG; and 598.36 ± 57.96, 11.22, and 624.89 for HzFPHT. The σ was significantly affected by the post-sintering process and type of zirconia (p < 0.05), but not by their interactions (p > 0.05). The Cz indicated a significantly higher σ than Hz. The FP process significantly enhanced σ more than other treatment procedures.

CONCLUSION

 Post-sintering processes enabled an alteration in σ of zirconia. FP enhanced σ, while FPOG and FPHT resulted in a reduction of σ. Glazing tends to induce defects at the glazing interface, while heat treatment induces a phase change to tetragonal, both resulted in reducing σ. Finishing and polishing for both Cz and Hz monolithic zirconia is recommended, while overglazed or heat-treated is not suggested.

Fracture toughness of different monolithic zirconia upon post-sintering processes

Background

Surface treatments are expected to be a reason for alteration in fracture resistance of zirconia. This study evaluated the effect of post-sintering processes on the fracture toughness of different types of monolithic zirconia.

Material and Methods

Classical- (Cz) and high-translucent (Hz) monolithic zirconia discs (1.2 mm thickness, 14 mm in Ø) were prepared, and randomly divided for surface treatments with 1) as-glazed (AG); 2) finished and polished (FP); 3) finished, polished, and overglazed (FPOG); and 4) finished, polished, and heat-treated (FPHT) technique (n=15/group). Fracture toughness (KIC) was determined with indentation fracture toughness method at load 1 kg for AG, FPOG and 10 kg for FP, FPHT with 15 sec dwelling time. Weibull analysis was applied for survival probability, Weibull modulus (m), and characteristic toughness (K0). Microstructures were examined with SEM and XRD. ANOVA and multiple comparisons were determined for significant differences (α=0.05).

Results

The mean±sd value of KIC (MPa.m1/2), m, and K0 were 1.60±0.19, 7.27, 1.71 for CzAG; 9.57±0.89, 9.97, 10.96 for CzFP; 1.61±0.15, 10.56, 1.68 for CzFPOG; 6.45±0.31, 20.31, 6.60 for CzFPHT; 1.45±0.13, 10.91, 1.51 for HzAG; 6.58±0.24, 27.00, 6.70 for HzFP; 1.24±0.05, 23.90, 1.27 for HzFPOG; and 5.07±0.16, 30.51, 5.15 for HzFPHT. The KIC was significantly affected by the post-sintering process, type of zirconia (p<0.05). The Cz indicated a significantly higher KIC than Hz. The FP significantly enhanced KIC, while OG was unable to raise KIC. HT reduced KIC due to reverse phase transformation.

Conclusions

Post-sintering processes caused alteration in fracture resistance of zirconia. Fracture toughness was enhanced with FP, but not with either OG or HT process for both Cz and Hz. Surface treatment of zirconia through a finished-polished process is recommended, while glazing and heat-treated are not suggested.

Key words:Fracture toughness, glazing, heat treatment, polishing, post-sintering process, zirconia.

Quantitative analysis on the wear of monolithic zirconia crowns on antagonist teeth

BACKGROUND

The present study aimed to quantitate the wear of the highly transparent Yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) ceramic monolithic zirconia crown on the enamel in vivo and discuss the prone position of the wear and the underlying mechanism.

METHODS

A total of 43 patients with 43 posterior teeth were selected for full zirconia crown restoration and examined immediately, at 6 months, and at 1 year after restoration. During the follow-up visit, the fine impression of the patients' monolithic zirconia crowns, the antagonist teeth, the corresponding contralateral natural teeth, the super plaster cast, and epoxy resin model was ontained. The model of epoxy resin was observed under a stereo microscope, and the microstructure parts were observed under a scanning electron microscope.

RESULTS

After 1 year, the mean depth and volume of wearing of the monolithic zirconia crown were the smallest (all P < 0.01), while those of the antagonist teeth were significantly larger than those of the natural teeth (P < 0.0001), and no significant difference was found among the natural teeth (P = 0.3473, P = 0.6996). The amount of wear after one year was remarkably higher than that at 6 months (P < 0.0001). The microscopic observation revealed the tendency of wearing of the monolithic zirconia crown on the antagonist teeth at the protruding early contact points. Electron micrographs of tooth scars showed that the wearing mechanism of the monolithic zirconia crown on natural teeth was mainly abrasive and fatigue wear.

CONCLUSIONS

Although the self-wearing is insignificant, the monolithic zirconia crown can cause wear of the antagonist teeth via occlusal or early contact significantly; the amount of wearing is higher than that of natural teeth and increases over time. The wearing mechanism is mainly abrasive and fatigue wear.

Flexural strength and crystalline stability of a monolithic translucent zirconia subjected to grinding, polishing and thermal challenges

The objective of this study is to investigate the magnitude of structural degradation of a monolithic translucent zirconia caused by clinically relevant grinding and polishing procedures, when associated or not with low temperature degradation (LTD), induced by accelerated hydrothermal aging using autoclave or thermocycling Ninety disks (Ø12 × 1 mm) were prepared from dental zirconia for monolithic restorations (Vipi Block Zirconn Translucent, Vipi). The specimens were divided into 3 groups (n = 30) according to surface treatment: As Sintered (untreated), Grind (diamond bur), Grind + Polish (diamond bur + polish); and then subdivided according to aging method (n = 10): Baseline (no aging), Autoclave (134°C, 2.2 kgf/cm² pressure for 5 h), and Thermocycling (200,000 cycles, 5°C and 55°C, for 15 s each). Roughness, biaxial flexural strength and percentage of monoclinic phase were evaluated. Regarding surface treatment, the Grind group presented higher roughness and greater flexural strength compared to As Sintered group, while Grind + Polish showed intermediate roughness and flexural strength similar to Grind group. Aging had little effect on roughness, but yielded a significant reduction in flexural strength. Tetragonal to monoclinic phase transformation was observed in all groups, caused by both mechanical stresses (grinding and polishing) and LTD, which was similarly induced by the traditional autoclave method, as well as the thermocycling method The use of diamond burs to grind zirconia surface may result in deleterious effects on the surface quality of monolithic zirconia restorations, yet has a potential toughening effect by phase transformation. However, when zirconia is exposed to LTD, regardless of the surface treatment, degradation of the surface quality and strength are observed.

Effects of low-temperature degradation on the surface roughness of yttria-stabilized tetragonal zirconia polycrystal ceramics: A systematic review and meta-analysis

STATEMENT OF PROBLEM

A wide range of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramics have been applied for dental restorations. However, whether hydrothermal aging affects the surface roughness of Y-TZP is unknown.

PURPOSE

The purpose of this systematic review and meta-analysis was to evaluate the effects of low-temperature degradation (LTD) on the surface roughness of Y-TZP ceramics.

MATERIAL AND METHODS

This report follows the Preferred Reporting Items for Systematic Reviews and Qualitative Analyses statement. The literature search was conducted with Medline through the PubMed, Web of Science, and Cochrane Library with no publication year limits. The screening and quality assessment were performed by 2 independent reviewers. The studies comparing the surface roughness of Y-TZP ceramics after LTD by using steam autoclave aging were included in the meta-analysis. Meta-analyses were conducted with a random-effects model (α=.05) by using Review Manager Software (Cochrane Collaboration).

RESULTS

Of the 203 potentially relevant studies, 32 full texts were assessed for eligibility. A total of 17 articles were included in the systematic review, and 15 were included in the qualitative analyses. The results showed no significant difference in the arithmetic average height (R_a) values between the nonaged and aged Y-TZP (P=.670; mean difference=0.01; 95% confidence interval=-0.03 to 0.05). Subgroup analyses revealed that the aging duration (P=.003) and specimen preparation (P=.010) contributed significantly to the changes in the surface roughness of the Y-TZP ceramics.

CONCLUSIONS

Although LTD was found to have no significant effects on the surface roughness of Y-TZP ceramics, the effects of LTD depended on the duration of the steam autoclave process and the specimen preparation.

Does Diamond Stone Grinding Change the Surface Characteristics and Flexural Strength of Monolithic Zirconia?

PURPOSE

The present study evaluated the effect of grinding on the surface morphology, mean roughness, crystalline phase, flexural strength, and Weibull modulus of monolithic (MZ) and conventional (CZ) zirconias.

METHODS AND MATERIALS

CZ and MZ bars and square-shaped specimens were distributed into three subgroups, combining grinding (G) and irrigation (W) with distilled water: Ctrl (Control: no grinding, 20 × 4 × 1.2 mm and 12 × 1.2 mm), DG (dry grinding, 20 × 4 × 1.5 mm and 12 × 1.5 mm), and WG (grinding with irrigation, 20 × 4 × 1.5 mm and 12 × 1.5 mm). The grinding (0.3 mm) was performed on a standardized device using a low-rotation wheel-shaped diamond stone. The four-point flexural strength test was performed on the EMIC 2000 machine (5 KN, 0.5 mm/min). Scanning electron microscopy (SEM) was used to evaluate the surface morphology. An X-ray diffractometer (XRD) was used to obtain the crystalline structures that were analyzed by the Rietveld method. Flexural strength (FS) values were subjected to the Shapiro-Wilk test and two-way analysis of variance followed by the Tukey's test (for all tests, α=0.05).

RESULTS

Grinding, either with or without irrigation, did not change the FS of the MZ but increased the FS of the CZ. Both MZ and CZ showed similar morphologic patterns after grinding, and in the WG groups, the grinding was more aggressive. The MZ had greater monoclinic content in all groups; grinding without irrigation caused the smallest t→m transformation.

CONCLUSION

The grinding, when necessary, should be carried out without irrigation for conventional and monolithic zirconias.