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

Current status of zirconia implants in dentistry: preclinical tests

PURPOSE

This systematic review aimed to provide an overview of zirconia implants as well as regarding the outcome of the implant-restorative complex in preclinical studies.

STUDY SELECTION

An electronic search of the literature prior to July 2017 was performed to identify all articles related to preclinical research on zirconia implants. The search was conducted using MEDLINE (National Library of Medicine) and PubMed without restrictions concerning the date of publication. The search terminology included: zirconia implant, osseointegration, bone-to-implant contact, soft tissue, histology, histomorphometry, surface modification, surface roughness, surface characteristics, and restoration (connecting multiple keywords with AND, OR).

RESULTS

Fifty-seven studies were finally selected from an initial yield of 654 titles, and the data were extracted. The identified preclinical studies focused on several aspects related to zirconia implants, namely biocompatibility, mechanical properties, implant design, osseointegration capacity, soft tissue response, and restorative options. Due to heterogeneity of the studies, a meta-analysis was not possible. The most frequently used zirconia material for the fabrication of implants is yttria-stabilized tetragonal zirconia polycrystal. The resistance-to-fracture for zirconia implants ranged between 516-2044N. The mostly investigated parameter was osseointegration, which is compared to that of titanium. A lack of evidence was found with other parameters.

CONCLUSIONS

Due to its good biocompatibility as well as favorable physical and mechanical properties, zirconia implants are a potential alternative to titanium implants. However, knowledge regarding the implant-restorative complex and related aspects is still immature to recommend its application for daily practice.

Evaluating dental zirconia

OBJECTIVES

To survey simple contact testing protocols for evaluating the mechanical integrity of zirconia dental ceramics. Specifically, to map vital material property variations and to quantify competing damage modes.

METHODS

Exploratory contact tests are conducted on layer structures representative of zirconia crowns on dentin.

RESULTS

Sharp-tip micro- and nano-indentations were used to investigate the roles of weak interfaces and residual stresses in veneered zirconia, and to map property variations in graded structures. Tests with blunt sphere indenters on flat specimens were used to identify and quantify various critical damage modes in simulated occlusal loading in veneered and monolithic zirconia.

SIGNIFICANCE

Contact testing is a powerful tool for elucidating the fracture and deformation modes that control the lifetimes of zirconia dental ceramics. The advocated tests are simple, and provide a sound physical basis for analyzing damage resistance of anatomically-correct crowns and other complex dental prostheses.

Load-bearing capacity of lithium disilicate and ultra-translucent zirconias

OBJECTIVE

The aim of this study was to evaluate the load-bearing capacity of monolithic lithium disilicate (LiDi - IPS e.max CAD) and novel ultra-translucent zirconia restorative systems of various compositions: 5Y-PSZ (5 mol% yttria-partially-stabilized zirconia) and 4Y-PSZ (4 mol% yttria-partially-stabilized zirconia); relative to a 3Y-TZP (3 mol% yttria-stabilized zirconia) control.

MATERIALS AND METHODS

Experiments were carried out with 10 disc specimens (Ø12 ×1 mm) per ceramic material. The zirconia intaglio surface (as machined) was sandblasted (50 µm Al₂O₃ at 2 bar), while LiDi was etched with 5% HF for 20 s. The ceramic discs were then adhesively bonded onto a dentin-like substrate (G10, a high-pressure fiberglass material) using Multilink Automix cement and Monobond Plus primer, producing a ceramic/cement/dentin-like substrate trilayer structure. The bonded specimens were stored in water for 3 days at 37 °C prior to a Hertzian indentation flexural radial fracture test. The plate-on-foundation theory was used to validate the load-bearing capacity of the trilayer systems based on the flexural tensile stress at the ceramic intaglio (cementation) surface-a cause for bulk fracture of ceramic onlays.

RESULTS

The experiment data showed that, when bonded to and supported by a dentin-like substrate, the load-bearing capacity of LiDi (872 N) is superior to the 5Y-PSZ (715 N) and can even reach that of 4Y-PSZ (864 N), while 3Y-TZP still holds the highest load-bearing capacity (1195 N). Theoretical analyses agree with experimental observations. The translucency of 5Y-PSZ approaches that of LiDi, which are superior to both 4Y-PSZ and 3Y-TZP.

CONCLUSIONS

When adhesively bonded to and supported by dentin, lithium disilicate exhibits similar load-bearing properties to 4Y-PSZ but much better than 5Y-PSZ.

Flexural strength, fracture toughness, and translucency of cubic/tetragonal zirconia materials

STATEMENT OF PROBLEM

The development of zirconia materials with optimized properties has been rapid, and studies comparing the mechanical and optical properties of recently introduced zirconia with lithium disilicate materials are limited.

PURPOSE

The purpose of this in vitro study was to compare the mechanical and optical properties of cubic/tetragonal zirconia materials with those of a lithium disilicate ceramic.

MATERIAL AND METHODS

Specimens were fabricated from 6 different noncolored zirconia materials: Ceramill Zolid FX (CZ), CopraSmile (CS), DD cubeX² (DD), NOVAZIR MaxT (NZ), priti multidisc ZrO₂ (PD), and StarCeram Z-Smile (SC), and 1 lithium disilicate ceramic as a control, IPS e.max Press LT A2 (CG). Four-point flexural strength (N=105/n=15) and fracture toughness using the single-edge V-notched beam (N=105/n=15) were examined according to International Organization for Standardization standard 6872:2015. Translucency (N=70/n=10) was evaluated with an ultraviolet spectrophotometer. Grain size (N=6/n=1) of zirconia was investigated by using scanning electron microscopy. Data were analyzed using the Kolmogorov-Smirnov test, multivariate analysis, 1-way analysis of variance, followed by the post hoc Scheffé test and Kruskal-Wallis and Mann-Whitney U tests, and Weibull analysis, using the maximum likelihood estimation method at 95% confidence level (α=.05).

RESULTS

Zirconia materials showed higher mechanical and lower optical properties than CG (P<.001). No differences were observed among the zirconia materials with respect to flexural strength (P=.259) or fracture toughness (P=.408). CG and CS showed significantly higher Weibull modulus than SC and PD. The lowest translucency values were measured for NZ and SC, followed by CS, DD, and PD (P<.001). CZ showed the highest translucency values (P<.001). The lowest grain sizes were found for NZ, DD, and SC; the largest were shown for CS (P<.001).

CONCLUSIONS

Cubic/tetragonal zirconia showed better mechanical properties than lithium disilicate ceramic. However, the optical properties and the reliability of zirconia are lower than those of lithium disilicate ceramic.

Effects of an etching solution on the adhesive properties and surface microhardness of zirconia dental ceramics

STATEMENT OF PROBLEM

Conventional approaches to adhesive bonding are not applicable to zirconia restorations. Recently, an etching solution, Zeta Etching Solution (ZES), has been introduced for etching the surface of zirconia. The effects of this etching solution on the bond strength and mechanical properties of zirconia are unknown.

PURPOSE

The purpose of this in vitro study was to examine the effects of ZES on the bond strength and surface hardness of zirconia.

MATERIAL AND METHODS

Two different types of partially stabilized tetragonal polycrystalline zirconia (TZP), Prettau zirconia (group P) and anterior Prettau (group AP), were evaluated with and without ZES etching. Each group was bonded to a zirconia substrate by using an adhesive resin cement. After 24 hours of storage in distilled water, the bond strength of the zirconia was analyzed. Vickers hardness was determined by using a microhardness tester. Scanning electron microscopy was used to analyze the surface microstructure and determine the mode of failure for each specimen. Results were analyzed and compared using 1-way ANOVA and Student t tests (α=.05).

RESULTS

Scanning electron microscopy analysis showed that etching the surface of zirconia with ZES etching solution for 60 minutes changed the morphological characteristics and microstructure of zirconia, making the surface more irregular. The changes were more pronounced for AP specimens. Etching with ZES significantly increased the shear bond strength of zirconia (P<.05) in AP specimens. The bond strength of Prettau (P group) specimens after ZES etching did not increase significantly (P>.05). An adhesive failure mode was observed for P zirconia specimens, whereas zirconia specimens exhibited a cohesive mode of failure. No significant decrease (P>.05) was observed in the mean Vickers hardness numbers.

CONCLUSIONS

Within the limitations of this in vitro study, it was concluded that etching in ZES for 30 minutes significantly enhanced the shear bond strength of highly translucent anterior Prettau (AP) zirconia restorations. Moreover, etching with ZES did not adversely affect the surface hardness of the zirconia specimens tested.

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

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 mm³) and plate-shaped (12 × 12 × 1 mm³) 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.

Effect of different semimonolithic designs on fracture resistance and fracture mode of translucent and high-translucent zirconia crowns

Purpose

The aim of this study was to describe different designs of semimonolithic crowns made of translucent and high-translucent zirconia materials and to evaluate the effect on fracture resistance and fracture mode.

Methods

One hundred crowns with different designs were produced and divided into five groups (n=20): monolithic (M), partially veneered monolithic (semimonolithic) with 0.3 mm buccal veneer (SM0.3), semimonolithic with 0.5 mm buccal veneer (SM0.5), semimonolithic with 0.5 mm buccal veneer supported by wave design (SMW), and semimonolithic with 0.5 mm buccal veneer supported by occlusal cap design (SMC). Each group was divided into two subgroups (n=10) according to the materials used, translucent and high-translucent zirconia. All crowns underwent artificial aging before loading until fracture. Fracture mode analysis was performed. Fracture loads and fracture modes were analyzed using two-way ANOVA and Fisher's exact probability tests (P≤0.05).

Results

SM0.3 design showed highest fracture loads with no significant difference compared to M and SMW designs (P>0.05). SM0.5 design showed lower fracture loads compared to SMW and SWC designs. Crowns made of translucent zirconia showed higher fracture loads compared to those made of high-translucent zirconia. M, SM0.3, and all but one of the SMC crowns showed complete fractures with significant differences in fracture mode compared to SMW and SM0.5 crowns with cohesive veneer fractures (P≤0.05).

Conclusion

Translucent and high-translucent zirconia crowns might be used in combination with 0.3 mm microcoating porcelain layer with semimonolithic design to enhance the esthetic properties of restorations without significantly decreasing fracture resistance of the crowns. If 0.5 mm porcelain layer is needed for a semimonolithic crown, wave design or cap design might be used to increase fracture resistance. In both cases, fracture resistance gained is likely to be clinically sufficient as the registered fracture loads were high in relation to expected loads under clinical use.

Aging resistance, mechanical properties and translucency of different yttria-stabilized zirconia ceramics for monolithic dental crown applications

OBJECTIVES

The dental market moves towards high-translucency monolithic zirconia dental crowns, which are usually placed either with - or without - a thin glaze layer. The microstructural features and the mechanical performances of these materials are still controversial, as well as their susceptibility to aging. This paper aims at studying these aspects in the current generation of zirconia dental crowns showing different degrees of translucency.

METHODS

Four different commercial zirconia materials were investigated, including one standard 'full-strength' 3Y-TZP and three grades with improved translucency. The microstructural features (phase composition and assemblage, grain size) were carefully studied, as well as mechanical properties (biaxial bending strength and indentation toughness), translucency and aging behavior (in autoclave at 134°C). Aging was conducted on crowns with and without glaze to better represent clinical uses.

RESULTS

Important differences are found in terms of microstructures among the materials in terms of cubic phase content and yttria in the tetragonal phase, leading to different optical, mechanical and aging resistance properties. We show that higher cubic phase content leads to better translucency and stability in water steam, but at the expense of strength and toughness. A compromise is always inevitable between translucency and aging resistance on one side and mechanical properties on the other side.

SIGNIFICANCE

Crystallographic and morphological analysis of sandblasted highly translucent dental zirconia

OBJECTIVE

To assess the influence of alumina sandblasting on four highly translucent dental zirconia grades.

METHODS

Fully sintered zirconia disk-shaped specimens (15-mm diameter; 0.5-mm thickness) of four highly translucent yttria partially stabilized zirconia (Y-PSZ) grades (KATANA HT, KATANA STML, KATANA UTML, Kuraray Noritake; Zpex Smile, Tosoh) were sandblasted with 50-μm alumina (Al₂O₃) sand (Kulzer) or left 'as-sintered' (control) (n=5). For each zirconia grade, the translucency was measured using a colorimeter. Surface roughness was assessed using 3D confocal laser microscopy, upon which the zirconia grades were statistically compared for surface roughness using a Kruskal-Wallis test (n=10). X-ray diffraction (XRD) with Rietveld analysis was used to assess the zirconia-phase composition. Micro-Raman spectroscopy was used to assess the potentially induced residual stress.

RESULTS

The translucency of KATANA UTML was the highest (36.7±1.8), whereas that of KATANA HT was the lowest (29.5±0.9). The 'Al₂O₃-sandblasted' and 'as-sintered' zirconia revealed comparable surface-roughness Sa values. Regarding zirconia-phase composition, XRD with Rietveld analysis revealed that the 'as-sintered' KATANA UTML contained the highest amount of cubic zirconia (c-ZrO₂) phase (71wt%), while KATANA HT had the lowest amount of c-ZrO₂ phase (41wt%). KATANA STML and Zpex Smile had a comparable zirconia-phase composition (60wt% c-ZrO₂ phase). After Al₂O₃-sandblasting, a significant amount (over 25wt%) of rhombohedral zirconia (r-ZrO₂) phase was detected for all highly translucent zirconia grades.

SIGNIFICANCE

Al₂O₃-sandblasting did not affect the surface roughness of the three highly translucent Y-PSZ zirconia grades, but it changed its phase composition.

Comparison of the mechanical properties of translucent zirconia and lithium disilicate

STATEMENT OF PROBLEM

Three mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) possesses excellent mechanical properties but is relatively opaque. Five mol% yttria-stabilized zirconia polycrystal (5Y-ZP) offers improved translucency, but many of its clinical properties have not been compared with those of 3Y-TZP and lithium disilicate.

PURPOSE

The purpose of this in vitro study was to compare the flexural strength, translucency parameter, bond strength, and enamel and material wear of 5Y-ZP (Katana UTML) with 3Y-TZP (Katana HT) and lithium disilicate (e.max CAD).

MATERIAL AND METHODS

Flexural strength bars were sectioned (n=10, 25×4×2 mm), sintered or crystallized, polished, and fractured at 1 mm/min. Translucency specimens (1 mm thick) were fabricated (n=10). Their L*a*b* values were measured against a black-and-white background with a spectrophotometer, and ΔE₀₀ was calculated. Zirconia bond strength specimens were airborne-particle abraded with 50 μm alumina followed by the application of a 10-methacryloxydecyl dihydrogen phosphate-containing primer (Clearfil Ceramic Primer). Lithium disilicate bond strength specimens were etched with 5% hydrofluoric acid followed by application of a silane-containing primer (Clearfil Ceramic Primer). A Tygon tube filled with resin cement (Panavia SA) was fixed to the surface of the ceramics and light-polymerized. After 1 day or 150 days of water storage, the resin cement was debonded in a macroshear test (n=10). The cusps of extracted human molars were isolated and mounted into the University of Alabama at Birmingham wear-testing device. Wear testing was performed with a 20-N load for 300000 cycles in 33% glycerin. The volumetric wear of polished zirconia, lithium disilicate, and enamel were measured along with the wear of the opposing enamel cusps using a noncontact profilometer (n=8). The data were compared by ANOVA and Tukey-Kramer analysis (α=.05).

RESULTS

No statistical difference was seen between the bond strengths (P=.155) or the opposing enamel wear (P=.533) of different ceramics. A statistically significant difference was seen between the flexural strength (P<.001), translucency parameter (P<.001), and wear (P<.001) of the materials. The flexural strength values (MPa) were 1194 ±111 (Katana HT), 688 ±159 (Katana UTML), and 450 ±53 (e.max LT). The translucency parameter values were 6.96 ±0.53 (Katana HT), 8.30 ±0.24 (Katana UTML), 9.28 ±0.36 (e.max LT), and 12.64 ±0.48 (e.max HT). Bond strength values (MPa) at 1 and 150 days were 34.22 ±5.14 and 28.37 ±6.03 (Katana HT), 35.04 ±5.69 and 25.03 ±6.44 (Katana UTML), and 35.50 ±3.45 and 22.32 ±3.45 (e.max LT). Material and enamel wear (mm³) were 0 and 0.24 ±0.19 (Katana HT), 0 and 0.23 ±0.09 (Katana UTML), 0.28 ±0.13 and 0.31 ±0.10 (e.max CAD), and 0.09 ±0.03 and 0.31 ±0.14 (enamel).

CONCLUSIONS

5Y-TZP has a flexural strength and translucency parameter between those of 3Y-TZP and lithium disilicate. Both the short-term and long-term bond strength of 5Y-ZP and 3Y-TZP was shown to be similar to lithium disilicate. 5Y-ZP demonstrated no measurable material wear and opposing enamel wear similar to that of all the other materials tested.

Novel Zirconia Materials in Dentistry

Zirconias, the strongest of the dental ceramics, are increasingly being fabricated in monolithic form for a range of clinical applications. Y-TZP (yttria-stabilized tetragonal zirconia polycrystal) is the most widely used variant. However, current Y-TZP ceramics on the market lack the aesthetics of competitive glass-ceramics and are therefore somewhat restricted in the anterior region. This article reviews the progressive development of currently available and next-generation zirconias, representing a concerted drive toward greater translucency while preserving adequate strength and toughness. Limitations of efforts directed toward this end are examined, such as reducing the content of light-scattering alumina sintering aid or incorporating a component of optically isotropic cubic phase into the tetragonal structure. The latest fabrication routes based on refined starting powders and dopants, with innovative sintering protocols and associated surface treatments, are described. The need to understand the several, often complex, mechanisms of long-term failure in relation to routine laboratory test data is presented as a vital step in bridging the gaps among material scientist, dental manufacturer, and clinical provider.

Novel Translucent and Strong Submicron Alumina Ceramics for Dental Restorations

An ideal ceramic restorative material should possess excellent aesthetic and mechanical properties. We hypothesize that the high translucency and strength of polycrystalline ceramics can be achieved through microstructural tailoring. The aim of this study is to demonstrate the superior optical and mechanical properties of a new class of submicron grain-sized alumina ceramics relative to the current state-of-the-art dental ceramic materials. The translucency, the in-line transmission ( T_IT) in particular, of these submicron alumina ceramics has been examined with the Rayleigh-Gans-Debye light-scattering model. The theoretical predictions related very well with the measured T_IT values. The translucency parameter ( TP) and contrast ratio ( CR) of the newly developed aluminas were measured with a reflectance spectrophotometer on a black-and-white background. For comparison, the T_IT, TP, and CR values for a variety of dental ceramics, mostly measured in-house but also cited from the literature, were included. The flexural strength of the aluminas was determined with the 4-point bending test. Our findings have shown that for polycrystalline alumina ceramics, an average grain size <1 µm coupled with a porosity level <0.7% could yield translucency values ( T_IT, TP, CR) similar to those of the commercial high-translucency porcelains. These values are far superior to the high-translucency lithium disilicate glass-ceramic and zirconias, including the most translucent cubic-containing zirconias. The strength of these submicron grain-sized aluminas was significantly higher than that of the cubic-containing zirconia (e.g., Zpex Smile) and lithia-based glass-ceramics (e.g., IPS e.max CAD HT). A coarse-grained alumina could also reach a translucency level comparable to that of dental porcelain. However, the relatively low strength of this material has limited its clinical indications to structurally less demanding applications, such as orthodontic brackets. With a combined high strength and translucency, the newly developed submicron grain-sized alumina may be considered a suitable material for dental restorations.

Effects of small-grit grinding and glazing on mechanical behaviors and ageing resistance of a super-translucent dental zirconia

OBJECTIVES

The purpose of this study is to elucidate the effects of small-grit grinding on the mechanical behaviors and ageing resistance of a super-translucent dental zirconia and to investigate the necessity of glazing for the small-grit ground zirconia.

METHODS

Small-grit grinding was performed using two kinds of silicon carbide abrasive papers. The control group received no grinding. The unground surfaces and the ground surfaces were glazed by an experienced dental technician. Finally, the zirconia materials were thermally aged in water at 134°C for 5h. After aforementioned treatments, we observed the surface topography and the microstructures, and measured the extent of monoclinic phase, the nano-hardness and nano-modulus of the possible transformed zone and the flexural strength.

RESULTS

Small-grit grinding changed the surface topography. The zirconia microstructure did not change obviously after surface treatments and thermal ageing; however, the glaze in contact with zirconia showed cracks after thermal ageing. Small-grit grinding did not induce a phase transformation but improved the flexural strength and ageing resistance. Glazing prevented zirconia from thermal ageing but severely diminished the flexural strength. The nano-hardness and nano-modulus of the surface layer were increased by ultrafine grinding.

CONCLUSIONS

The results suggest that small-grit grinding is beneficial to the strength and ageing resistance of the super-translucent dental zirconia; however, glazing is not necessary and even impairs the strength for the super-translucent dental zirconia.

CLINICAL SIGNIFICANCE

This study is helpful to the researches about dental grinding tools and maybe useful for dentists to choose reasonable zirconia surface treatments.

The Effect of Resin Bonding on Long-Term Success of High-Strength Ceramics

Digital manufacturing, all-ceramics, and adhesive dentistry are currently the trendiest topics in clinical restorative dentistry. Tooth- and implant-supported fixed restorations from computer-aided design (CAD)/computer-aided manufacturing (CAM)-fabricated high-strength ceramics-namely, alumina and zirconia-are widely accepted as reliable alternatives to traditional metal-ceramic restorations. Most recent developments have focused on high-translucent monolithic full-contour zirconia restorations, which have become extremely popular in a short period of time, due to physical strength, CAD/CAM fabrication, and low cost. However, questions about proper resin bonding protocols have emerged, as they are critical for clinical success of brittle ceramics and treatment options that rely on adhesive bonds, specifically resin-bonded fixed dental prostheses or partial-coverage restorations such as inlays/onlays and veneers. Resin bonding has long been the gold standard for retention and reinforcement of low- to medium-strength silica-based ceramics but requires multiple pretreatment steps of the bonding surfaces, increasing complexity, and technique sensitivity compared to conventional cementation. Here, we critically review and discuss the evidence on resin bonding related to long-term clinical outcomes of tooth- and implant-supported high-strength ceramic restorations. Based on a targeted literature search, clinical long-term studies indicate that porcelain-veneered alumina or zirconia full-coverage crowns and fixed dental prostheses have high long-term survival rates when inserted with conventional cements. However, most of the selected studies recommend resin bonding and suggest even greater success with composite resins or self-adhesive resin cements, especially for implant-supported restorations. High-strength ceramic resin-bonded fixed dental prostheses have high long-term clinical success rates, especially when designed as a cantilever with only 1 retainer. Proper pretreatment of the bonding surfaces and application of primers or composite resins that contain special adhesive monomers are necessary. To date, there are no clinical long-term data on resin bonding of partial-coverage high-strength ceramic or monolithic zirconia restorations.