Monolithic Zirconia Specimens Research Articles

Effects of different finishing/polishing protocols and systems for monolithic zirconia on surface topography, phase transformation, and biofilm formation.

PURPOSEThe purpose of this study was to evaluate the effects of various protocols and systems for finishing and polishing monolithic zirconia on surface topography, phase transformation, and bacterial adhesion.MATERIALS AND METHODSThree hundred monolithic zirconia specimens were fabricated and then treated with three finishing and polishing systems (Jota [JO], Meisinger [ME], and Edenta [ED]) using four surface treatment protocols: coarse finishing alone (C); coarse finishing and medium polishing (CM); coarse finishing and fine polishing (CF); and coarse finishing, medium polishing, and fine polishing (CMF). Surface roughness, crystal phase transformation, and bacterial adhesion were evaluated using atomic force microscopy, X-ray diffraction, and streptococcal biofilm formation assay, respectively. One-way and two-way analysis of variance with Tukey post hoc tests were used to analyze the results (α=.05).RESULTSIn this study, the surface treatment protocols and systems had significant effects on the resulting roughness. The CMF protocol produced the lowest roughness values, followed by CM and CF. Use of the JO system produced the lowest roughness values and the smallest biofilm mass, while the ME system produced the smallest partial transformation ratio. The ED group exhibited the highest roughness values, biofilm mass, and partial transformation ratio.CONCLUSIONStepwise surface treatment of monolithic zirconia, combined with careful polishing system selection, is essential to obtaining optimal microstructural and biological surface results.

Effects of different surface finishing protocols for zirconia on surface roughness and bacterial biofilm formation.

PURPOSESurface finishing of a zirconia restoration is essential after clinical adjustment. Herein, we investigated the effects of a surface finishing protocol for monolithic zirconia on final roughness and bacterial adherence.MATERIALS AND METHODSForty-eight disk-shaped monolithic zirconia specimens were fabricated and divided into four groups (n = 12) based on initial surface treatment, finishing, and polishing protocols: diamond bur+polishing bur (DP group), diamond bur+stone grinding bur+polishing bur (DSP group), no diamond bur+polishing bur (NP group), and no diamond bur+stone grinding bur+polishing bur (NSP group). Initial and final surface roughness was measured with a profilometer, and shown using scanning electron microscope. Bacterial adhesion was evaluated by quantifying Streptococcus mutans in the biofilm. Kruskal-Wallis and Mann-Whitney U tests were used to compare results among groups, and two-way analysis of variance was used to evaluate the effects of grinding burs on final roughness (α=.05).RESULTSThe DP group had the highest final Ra value, followed by the DSP, NP, and NSP groups. Use of the stone grinding bur as a coarse-finishing step significantly decreased final Ra values when a diamond bur was used (P<.001). Omission of the stone grinding bur increased biofilm formation on specimen surfaces. Combining a stone grinding bur with silicone polishing burs produced the smallest final biofilm values, regardless of the use of a diamond bur in initial surface treatment.CONCLUSIONCoarse finishing of monolithic zirconia with a stone grinding bur significantly decreased final Ra values and bacterial biofilm formation when surfaces had been roughened by a diamond bur.

Effects of treatment with aqueous and acid-based coloring liquid on the color of zirconia

Statement of problemTo preserve the mechanical property of color-treated zirconia for optimal restoration longevity, aqueous colorants have been developed as an alternative to acid-based coloring agents. However, little is known regarding the effects of aqueous colorants on the color of zirconia. PurposeThe purpose of this in vitro study was to compare the effect of aqueous coloring liquids with acid-based coloring liquids on the color of zirconia. Material and methodsEighty monolithic zirconia specimens (10×10×2 mm) were fabricated and divided into 4 groups according to their color treatment: unshaded zirconia (control), precolored zirconia, aqueous coloring liquid on zirconia, and acid-based coloring liquid on zirconia. The shaded zirconia specimens were further divided into 3 subgroups based on the number of coloring applications used (1, 3, or 6). The International Commission on Illumination (CIELab) color coordinates were measured by spectrophotometry. ResultsSignificant differences in the CIE a∗ and b∗ values were observed between the specimen groups treated with the aqueous coloring liquid and the acid-based coloring liquid (P<.001). Increasing the number of colorings resulted in an increase in the CIE a∗ and b∗ values and a decrease in the CIE L∗ values in all the groups (P<.001). ConclusionsTreatment with aqueous coloring liquid on zirconia produced a greater redness or yellowness compared with treatment with acid-based coloring liquid. The coloring of zirconia lowered its brightness and imparted a red/yellow hue.

Effect of hydrothermal aging on the optical properties of precolored dental monolithic zirconia ceramics

Statement of problemThe long-term color stability of precolored monolithic zirconia has not been thoroughly investigated. PurposeThe purpose of this in vitro study was to evaluate the effect of hydrothermal aging on the optical properties, phase transformation, and surface topography of precolored monolithic zirconia ceramics. Material and methodsPrecolored monolithic zirconia specimens (17.0×17.0×1.5 mm, n=50) and lithium disilicate glass-ceramic specimens (16.0×16.0×1.5 mm, n=50) were artificially aged in an autoclave at 134°C under 0.2 MPa for 0, 1, 3, 5, or 10 hours (n=10). CIELab color parameters were obtained from spectral measurements. The translucency parameter (TP) and CIEDE2000 color differences (ΔE00) were calculated. The microstructural and surface properties were analyzed by X-ray diffraction (XRD), atomic force microscope (AFM), and scanning electron microscope (SEM). Data were analyzed with 2-way ANOVA and pairwise comparison (α=.05). ResultsSignificant interactions were found between aging time and ceramic material on L*, a*, b*, and TP (P<.001) as follows: b* partial eta squared [ηp2]=0.689; L* ηp2=0.186; a* ηp2=0.176; and TP ηp2=0.137. The b* values significantly decreased after aging for zirconia (P<.001), whereas TP increased after aging for zirconia (P<.014) except at 10 hours (P=.389) and for lithium disilicate (P<.001). The ΔE00 values relative to baseline ranged from 2.03 to 2.52 for aged zirconia and from 0.07 to 0.23 for aged lithium disilicate. XRD analysis revealed that hydrothermal aging promoted an increase in m-phase contents. AFM and SEM demonstrated surface alterations after aging. ConclusionsOptical properties and microstructures of precolored monolithic zirconia ceramics were affected by hydrothermal aging, and translucency increased slightly with aging time.

Effects of fabrication and shading technique on the color and translucency of new-generation translucent zirconia after coffee thermocycling

Statement of problemThe color stability and translucency of preshaded and externally shaded monolithic and veneered new generation translucent zirconia are not well known. PurposeThe purpose of this in vitro study was to investigate the effect of fabrication (veneered or monolithic) and shading (preshaded or externally shaded) techniques on the color and relative translucency of translucent zirconia after coffee thermocycling. Material and methodsSpecimens of different thicknesses (0.5 mm for veneered and 1.5 mm for monolithic) were sectioned from preshaded and externally shaded translucent zirconia. Externally shaded specimens were colored by using the dipping technique. Externally shaded (Ext Mono) and preshaded (Pre Mono) monolithic zirconia specimens were sintered and glazed. Externally shaded (Ext Vene) and preshaded (Pre Vene) 0.5-mm-thick specimens were sintered, veneered with feldspathic porcelain (1 mm), and glazed. The color coordinates of specimens were measured with a spectroradiometer before and after 10 000 thermocycles in coffee solution. Color differences were calculated using CIEDE2000, and relative translucency parameter (RTP) values were calculated using the RTPCIEDE2000 formula. Analysis of variance (ANOVA) was used to analyze the CIEDE2000 color differences and RTP values (α=.05). ResultsShading technique had a significant effect on the color difference values (P=.018). For the translucency data, the 3-way ANOVA revealed a significant interaction between the fabrication technique and shading technique (P=.002). Each pair of material subgroups within each combination of fabrication technique and shading technique was found to have a significantly different RTP, except between Ext Vene and Pre Vene (P=.115). ConclusionsExternally shaded translucent zirconia had a greater color change in coffee than the preshaded translucent zirconia, either in monolithic or veneered form. Fabrication technique significantly affected the RTP, and the monolithic zirconia was more translucent than the veneered zirconia.

Evaluation of color stability of preshaded and liquid-shaded monolithic zirconia

Statement of problemThe effect of thermal aging on the color stability of monolithic zirconia has not been thoroughly investigated. PurposeThe purpose of this in vitro study was to evaluate the color stability before and after thermocycling of monolithic zirconia specimens, which were either preshaded or characterized in the laboratory through the assessment of color parameters L*, a*, and b*. Material and methodsA total of 80 specimens (10×10×15 mm) were prepared from zirconia disks (BruxZir) and divided into 2 groups. The first group consisted of preshaded specimens from disks 100, 200, 300, and 400, and the second group consisted of white specimens characterized before the sintering stage with coloring liquids in shades A2, B2, C2, and D2. Ten specimens of each shade were used. A double-beam ultraviolet-visible light recording spectrophotometer was used to assess the color parameters L*, a*, and b* before and after thermocycling. The color difference ΔE was calculated based on ΔΕ=[(ΔL*)2+(Δa*)2+(Δb*)2]½. The reliability of the examiner was investigated by MedCalc v12.5 software, and the tests of normality, homogeneity of variances, and 1-way ANOVA statistical analysis of variance were performed by IBM Statistics SPSS v20.0 software (α=.05). ResultsNo statistically significant ΔΕ (P>.05) was observed among the different groups after thermocycling. All ΔΕ values were below the limit of the 3.7 that an untrained observer can perceive. Only for the specimens in shades 200 and 400 was ΔΕ above 1, revealing a visible color difference, which, however, was still clinically acceptable. The L*, a*, and b* values for both liquid-shaded and preshaded zirconia specimens were not significantly affected by thermocycling. A higher color stability for laboratory-characterized compared with preshaded monolithic zirconia was observed irrespective of the color shade. ConclusionsWithin the limitations of this in vitro study, monolithic zirconia specimens of any shade, either preshaded or characterized in the laboratory, can be considered as color-stable materials after thermocycling.

Optical properties of pre-colored dental monolithic zirconia ceramics

ObjectivesThe purposes of this study were to evaluate the optical properties of recently marketed pre-colored monolithic zirconia ceramics and to compare with those of veneered zirconia and lithium disilicate glass ceramics. MethodsVarious shades of pre-colored monolithic zirconia, veneered zirconia, and lithium disilicate glass ceramic specimens were tested (17.0×17.0×1.5mm, n=5). CIELab color coordinates were obtained against white, black, and grey backgrounds with a spectrophotometer. Color differences of the specimen pairs were calculated by using the CIEDE2000 (ΔE00) formula. The translucency parameter (TP) was derived from ΔE00 of the specimen against a white and a black background. X-ray diffraction was used to determine the crystalline phases of monolithic zirconia specimens. Data were analyzed with 1-way ANOVA, Scheffé post hoc, and Pearson correlation testing (α=0.05). ResultsFor different shades of the same ceramic brand, there were significant differences in L*, a*, b*, and TP values in most ceramic brands. With the same nominal shade (A2), statistically significant differences were observed in L*, a*, b*, and TP values among different ceramic brands and systems (P<0.001). The color differences between pre-colored monolithic zirconia and veneered zirconia or lithium disilicate glass ceramics of the corresponding nominal shades ranged beyond the acceptability threshold. ConclusionsDue to the high L* values and low a* and b* values, pre-colored monolithic zirconia ceramics can be used with additional staining to match neighboring restorations or natural teeth. Clinical significanceDue to their high value and low chroma, unacceptable color mismatch with adjacent ceramic restorations might be expected.

Influence of type of cement on the color and translucency of monolithic zirconia

Statement of problemWith the development of translucent zirconia, questions regarding the influence of cements on the final color of monolithic zirconia restorations have arisen. PurposeThe purpose of this in vitro study was to evaluate color changes in terms of the perceptibility and acceptability of monolithic zirconia-and-cement combinations with 2 monolithic zirconia thicknesses and 3 types of cement. The translucency parameters of these combinations were also compared. Material and methodsSixty monolithic zirconia ceramic disks were milled with 2 different thicknesses (0.6 mm and 1 mm). A conventional glass ionomer cement, a resin-modified glass ionomer cement, and a resin cement from the same manufacturer were applied to the ceramic surfaces of both thickness disks (n=10). Translucencies and color changes of the monolithic zirconia specimens after cement application were examined by using a spectrophotometer, and translucency parameters (TPs) and color changes (ΔEs) were calculated and statistically analyzed. ResultsColors and TPs of the zirconia disks changed significantly after being cemented to 0.6- and 1-mm-thick disks (P<.05). The lowest ΔE values (3.53 for the 0.6-mm and 2.23 for the 1-mm thick disks) were observed for the zirconia-resin modified glass ionomer combination, whereas the highest ΔE values (5.64 for the 0.6-mm and 5.06 for the 1-mm thick disks) were observed for the zirconia-resin cement combination. The glass ionomer cement most strongly affected the TP values of both of the thicknesses. ConclusionsCement types and zirconia thickness affected the colors and translucencies of the monolithic zirconia specimens.

Evaluation of various polishing systems and the phase transformation of monolithic zirconia

Statement of problemAlthough a well-polished monolithic zirconia surface is essential, insufficient comparative studies on the constituents and efficacy of polishing systems have been conducted. PurposeThe purpose of this in vitro study was to analyze the abrasive constituents and compare the polishing effectiveness of 6 zirconia polishing systems, including their potential influence on the phase transformation of monolithic zirconia. Material and methodsThe compositions of 6 zirconia polishing systems were analyzed using energy dispersive spectroscopy (EDS). Sintered and high-speed ground monolithic zirconia specimens were used as control groups (n=14 specimens per group). Test groups (n=168) were pretreated identically to the control group and arbitrarily divided into 12 groups to examine 6 polishing systems for 2 polishing periods (60 or 120 seconds). The surfaces were characterized by a surface profiler and various analytic techniques, including x-ray diffraction (XRD) analysis. The data were analyzed using 2-way ANOVA and multiple comparisons and the Bonferroni method (α=.05). ResultsThe abrasives were primarily composed of C, O, and Si. Diamond was identified as the main abrasive and SiC as the supplementary abrasive in the polishing systems. No significant differences were observed for the 60- or 120-second polishing times (P>.05). Four products had significantly lower surface roughness values than the other 2 (P<.001). The occlusal-adjustment-simulating pretreatment and polishing processes did not cause phase transformations in the zirconia specimens. ConclusionsThe monolithic zirconia polishing systems contained diamond and SiC abrasives. All zirconia polishing systems showed clinically acceptable results, and 4 systems exhibited lower surface roughness. Phase transformation did not occur during polishing procedure.

Effect of monolithic zirconia on the degree of conversion of two resin cements analyzed by FT-IR/ATR spectroscopy

Objective: This study aimed to evaluate the degree of conversion (DC) of two different resin cements polymerized under the monolithic zirconia specimens in different thicknesses and colors.Material and methods: Partially stabilized monolithic zirconia blocks (inCoris TZI) were cut into three different thicknesses (0.5, 1.0, and 2.0 mm) and the specimens were divided into four color groups (A1, A2, A3, and A4). The light transmittance of each specimen was measured. Panavia F 2.0 or Variolink N resin cement was applied into teflon mold and irradiated using the light emitting diode curing unit for 20 s under monolithic zirconia specimen (n = 10). The resin cement specimens were stored at room temperature under dry conditions. The DC of each specimen was measured by Fourier transform infrared-attenuated total reflection (FT-IR/ATR) spectroscopy after the 1st and 10th day. Data were analyzed with two-way analysis of variance (ANOVA), two-way repeated measures ANOVA, three-way repeated measures ANOVA, and the Tukey least significant difference (LSD) tests (α = 0.05).Results: The light-cure resin cement groups showed higher DC than the dual-cure resin cement groups (p < 0.05). The DC of both resin cements reduced with an increase in the thickness and darkening of the color of monolithic zirconia specimens. There was a statistically meaningful increase in the 10th-day values for dual-cure resin cement (p < 0.05), whereas there were no significant differences between the 1st- and 10th-day values for light-cure resin cement (p > 0.05).Conclusion: The use of light-cure resin cement can be suggested for the luting of monolithic zirconia restorations.

Effects of surface treatments on the translucency, opalescence, and surface texture of dental monolithic zirconia ceramics

Statement of problemSurface polishing or glazing may increase the appearance of depth of monolithic zirconia restorations. PurposeThe purpose of this in vitro study was to investigate the effects of surface treatments on the translucency, opalescence, and surface texture of dental monolithic zirconia ceramics. Material and methodsForty-five monolithic zirconia specimens (16.3×16.4×2.0 mm) were divided into groups I to V, according to the number of colorings each received. Each group was then divided into 3 subgroups (n=3) according to the surface treatment: N=no treatment; P=polished; and G=glazed. CIElab color coordinates were obtained relative to D65 on a reflection spectrophotometer. The translucency parameter (TP) and opalescence parameter (OP) were calculated. One specimen per subgroups I and V was selected for evaluation of surface roughness (Ra) and was examined with scanning electron microscopy (SEM). Data were analyzed with 2-way ANOVA and pairwise comparisons (α=.05). Statistical powers were verified to evaluate results (α=.05). ResultsThe interaction effects of surface treatments combined with the number of colorings were significant for TP, OP, and Ra (P<.001), as follows: Ra=partial eta squared [ηp2]=.928; OP ηp2=.184; and TP ηp2=.144. No significant differences between TP were found among surface treatments in most groups (P>.05), whereas glazing significantly decreased OP and Ra in most groups. SEM images demonstrated that surface treatments affected the surface texture of monolithic zirconia ceramics. ConclusionsSurface treatments combined with coloring strongly affect the surface texture of dental monolithic zirconia ceramics.

Effect of the amount of thickness reduction on color and translucency of dental monolithic zirconia ceramics.

PURPOSEThis study investigated the effect of amount of thickness reduction on color and translucency of dental monolithic zirconia ceramics.MATERIALS AND METHODSOne-hundred sixty-five monolithic zirconia specimens (16.3 mm × 16.3 mm × 2.0 mm) were divided into 5 groups (Group I to V) according to the number of A2-coloring liquid applications. Each group was then divided into 11 subgroups by reducing the thickness up to 1.0 mm in 0.1-mm increments (Subgroup 0 to 10, n=3). Colors and spectral distributions were measured according to CIELAB on a reflection spectrophotometer. All measurements were performed on five different areas of each specimen. Color difference (ΔE*ab) and translucency parameter (TP) were calculated. Data were analyzed using one-way ANOVA and multiple comparison Scheffé test (α=.05).RESULTSThere were significant differences in CIE L* between Subgroup 0 and other subgroups in all groups. CIE a* increased (0.52<R2<0.73), while CIE b* decreased (0.00<R2<0.74) in all groups with increasing thickness reduction. Perceptible color differences (ΔE* ab>3.7) were obtained between Subgroup 0 and other subgroups. TP values generally increased as the thickness reduction increased in all groups (R2>0.89, P<.001).CONCLUSIONIncreasing thickness reduction reduces lightness and increases a reddish, bluish appearance, and translucency of monolithic zirconia ceramics.

Shear bond strength of indirect composite material to monolithic zirconia

PURPOSEThis study aimed to evaluate the effect of surface treatments on bond strength of indirect composite material (Tescera Indirect Composite System) to monolithic zirconia (inCoris TZI).MATERIALS AND METHODSPartially stabilized monolithic zirconia blocks were cut into with 2.0 mm thickness. Sintered zirconia specimens were divided into different surface treatment groups: no treatment (control), sandblasting, glaze layer & hydrofluoric acid application, and sandblasting + glaze layer & hydrofluoric acid application. The indirect composite material was applied to the surface of the monolithic zirconia specimens. Shear bond strength value of each specimen was evaluated after thermocycling. The fractured surface of each specimen was examined with a stereomicroscope and a scanning electron microscope to assess the failure types. The data were analyzed using one-way analysis of variance (ANOVA) and Tukey LSD tests (α=.05).RESULTSBond strength was significantly lower in untreated specimens than in sandblasted specimens (P<.05). No difference between the glaze layer and hydrofluoric acid application treated groups were observed. However, bond strength for these groups were significantly higher as compared with the other two groups (P<.05).CONCLUSIONCombined use of glaze layer & hydrofluoric acid application and silanization are reliable for strong and durable bonding between indirect composite material and monolithic zirconia.

Effect of Polishing Systems on Surface Roughness and Topography of Monolithic Zirconia.

This study evaluated the effect of different chairside polishing systems on the surface roughness and topography of monolithic zirconia. Thirty-five monolithic zirconia specimens (Lava PLUS, 3M ESPE) were fabricated and divided into five groups of seven and polished with the following: Group 1 (WZ)-Dura white stone followed by Shofu zirconia polishing kit; Group 2 (SZ)-Shofu zirconia polishing kit; Group 3 (CE)-Ceramiste porcelain polishers; Group 4 (CM)-Ceramaster porcelain polishers; and Group 5 (KZ)-Komet ZR zirconia polishers. All specimens were ground with a fine-grit diamond bur prior to polishing procedures to simulate clinical finishing. Baseline and post-polishing profilometric readings were recorded and delta Ra values (difference in mean surface roughness before and after polishing) were computed and analyzed using one-way analysis of variance and Scheffe post hoc test (p<0.05). Representative scanning electron microscopy (SEM) images of the ground but unpolished and polished specimens were acquired. Delta Ra values ranged from 0.146 for CE to 0.400 for KZ. Delta Ra values for KZ, WZ, and SZ were significantly greater than for CE. Significant differences in delta Ra values were also observed between KZ and CM. The SEM images obtained were consistent with the profilometric findings. Diamond-impregnated polishing systems were more effective than silica carbide-impregnated ones in reducing the surface roughness of ground monolithic zirconia.

Flexural strength and reliability of monolithic and trilayer ceramic structures obtained by the CAD-on technique

ObjectiveTo evaluate the flexural strength, Weibull modulus, fracture toughness, and failure behavior of ceramic structures obtained by the CAD-on technique, testing the null hypothesis that trilayer structures show similar properties to monolithic structures. MethodsBar-shaped (1.8mm×4mm×16mm) monolithic specimens of zirconia (IPS e.max ZirCAD – Ivoclar Vivadent) and trilayer specimens of zirconia/fusion ceramic/lithium dissilicate (IPS e.max ZirCAD/IPS e.max CAD Crystall./Connect/IPS e.max CAD, Ivoclar Vivadent) were fabricated (n=30). Specimens were tested in flexure in 37°C deionized water using a universal testing machine at a crosshead speed of 0.5mm/min. Failure loads were recorded, and the flexural strength values were calculated. Fractography principles were used to examine the fracture surfaces under optical and scanning electron microscopy. Data were statistically analyzed using Student's t-test and Weibull statistics (α=0.05). ResultsMonolithic and trilayer specimens showed similar mean flexural strengths, characteristic strengths, and Weibull moduli. Trilayer structures showed greater mean critical flaw and fracture toughness values than monolithic specimens (p<0.001). Most critical flaws in the trilayer groups were located on the Y-TZP surface subjected to tension and propagated catastrophically. Trilayer structures showed no flaw deflection at the interface. SignificanceConsidering the CAD-on technique, the trilayer structures showed greater fracture toughness than the monolithic zirconia specimens.

Effects of airborne-particle abrasion protocol choice on the surface characteristics of monolithic zirconia materials and the shear bond strength of resin cement

This study evaluated the effect of several airborne-particle abrasion protocols on the surface characteristics of monolithic zirconia and of protocol choice on the shear bond strength of resin cement. 375 bar-shaped (45×4×3mm3) and 500 disk-shaped (Ø9×1mm2) monolithic zirconia specimens were divided into 25 groups. All specimens were abraded with one of three different sizes of alumina particles (25, 50 or 125μm), two different pressures (2 or 4bar), two distinct application times (10 or 20s) and two distinct incidence angles (45° or 90°). The bar-shaped specimens were used for 3-point bending test; Weibull parameters were calculated and transformed monoclinic phase (XM), surface characteristics were examined. The disk-shaped specimens were used to determine the shear bond strength of resin cement before and after thermocycling. All data were analyzed using 4-way ANOVA and a multiple comparison Scheffé test (α=.05). The particle size, pressure and time significantly affected the flexural strength, while the incidence angle was insignificant. The XM and surface roughness were proportional to the size, pressure, time and incidence angle. The Raman spectrum analysis showed a higher proportion of the monoclinic phase as the depth of the specimen was closer to the abraded surface. In bonding with resin cement, the highest shear bond strength after thermocycling was obtained by the abrasion with 50μm particles at 4bar for 20s, regardless of incidence angle. Surface treatment of monolithic zirconia with 50μm particle at 4bar for 20s at either 45° or 90° incidence angles is recommended.

Effects of cementation surface modifications on fracture resistance of zirconia

ObjectivesTo examine the effects of glass infiltration (GI) and alumina coating (AC) on the indentation flexural load and four-point bending strength of monolithic zirconia. MethodsPlate-shaped (12mm×12mm×1.0mm or 1.5 or 2.0mm) and bar-shaped (4mm×3mm×25mm) monolithic zirconia specimens were fabricated. In addition to monolithic zirconia (group Z), zirconia monoliths were glass-infiltrated or alumina-coated on their tensile surfaces to form groups ZGI and ZAC, respectively. They were also glass-infiltrated on their upper surfaces, and glass-infiltrated or alumina-coated on their lower (tensile) surfaces to make groups ZGI2 and ZAC2, respectively. For comparison, porcelain-veneered zirconia (group PVZ) and monolithic lithium disilicate glass–ceramic (group LiDi) specimens were also fabricated. The plate-shaped specimens were cemented onto a restorative composite base for Hertzian indentation using a tungsten carbide spherical indenter with a radius of 3.2mm. Critical loads for indentation flexural fracture at the zirconia cementation surface were measured. Strengths of bar-shaped specimens were evaluated in four-point bending. ResultsGlass infiltration on zirconia tensile surfaces increased indentation flexural loads by 32% in Hertzian contact and flexural strength by 24% in four-point bending. Alumina coating showed no significant effect on resistance to flexural damage of zirconia. Monolithic zirconia outperformed porcelain-veneered zirconia and monolithic lithium disilicate glass–ceramics in terms of both indentation flexural load and flexural strength. SignificanceWhile both alumina coating and glass infiltration can be used to effectively modify the cementation surface of zirconia, glass infiltration can further increase the flexural fracture resistance of zirconia.

Surface properties of monolithic zirconia after dental adjustment treatments and in vitro wear simulation

ObjectivesTo investigate the surface properties (roughness, composition, phase transformation) of monolithic zirconia specimens after dental adjustment procedures (grinding, polishing) and wear simulation. MethodsZirconia specimens (Cercon base, Cercon ht, DeguDent, G; n=10/material) were successively sintered, ground, and polished with an intraoral polishing kit in a three-step procedure. Sintered zirconia specimens with high surface roughness served as a reference. For each treatment step, wear simulations with steatite plates (d=10mm) as antagonists were conducted as well as surface roughness tests (Ra), EDX analysis, and X-ray diffraction (XRD) measurements. SEM pictures were taken, and data were statistically analyzed (one-way ANOVA, post hoc Bonferroni, α=0.05). ResultsGrinding significantly (p=0.000) increased the roughness of sintered zirconia up to values of 1.36±0.11μm (Ra). Polishing significantly (p=0.000) reduced Ra. The lowest roughness value after the final polishing step was 0.20±0.03μm. Wear testing resulted in a further slight decrease of Ra. After the grinding procedure, SEM pictures showed deep grooves that were progressively smoothed by polishing. The EDX spectra showed that magnesium was transferred from steatite antagonists to zirconia by wear. In the XRD-patterns, monoclinic (m) peaks were observed after grinding and polishing. The maximum intensity ratio between the m (11−1) peak and the tetragonal t (111) peak decreased after the completion of all polishing steps. Wear did not induce phase transformation. ConclusionsAdequate polishing reduced the roughness of ground zirconia. Wear had little influence on roughness and no influence on phase transformation. Clinical significanceCareful polishing is recommended to keep surface roughness and phase transformation low.

Effect of the number of coloring liquid applications on the optical properties of monolithic zirconia

ObjectiveThis study was aimed to investigate the effect of the number of coloring liquid applications on the optical properties of monolithic zirconia. MethodsEighteen monolithic zirconia specimens (27.6mm×27.6mm×2.0mm) were fabricated and divided into 6 groups (n=3). Each group was designated by the number of A2-coloring liquid applications (Group I to Group V) and Group O as a control. Color and spectral distribution of the specimens were measured with a double-beam spectrophotometer. CIE L*, a* and b* relative to the standard illuminants D65 were measured in reflectance and transmittance modes. Color difference (ΔEab*), translucency parameter (TP) and opalescence parameter (OP) were calculated. All measurements were performed on five different areas of each specimen. All data were analyzed by ANOVA and multiple comparison Scheffé test, Pearson correlations and linear regression analysis (α=0.05). ResultsWith the increase of the number of coloring liquid applications, CIE L* (R2=0.878) and OP values (R2=0.701) were decreased, but CIE b* (R2=0.938) was increased. However, TP values were not significantly changed. The color differences among groups ranged from 1.3 to 15.7 ΔEab* units. Strong correlation was found between OP and Δb* (R2=0.982, P<.01). SignificanceWithin the limitations of this study, it can be concluded that the number of coloring liquid applications with a single shade affects the lightness, yellow chromaticity and opalescence of monolithic zirconia, although its translucency cannot be controlled by the coloring procedure.

Effect of polishing and glazing on the color and spectral distribution of monolithic zirconia

PURPOSEThe aim of this study was to evaluate the effect of polishing and glazing on the color and spectral distribution of monolithic zirconia.MATERIALS AND METHODSForty-five monolithic zirconia specimens (16.3 mm × 16.4 mm × 2.0 mm) were fabricated and divided into 5 groups according to the number of A2-coloring liquid applications (Group I to V). Each group was divided into 3 subgroups according to the method of surface treatments (n=3): N: no treatment; P: polishing; G: glazing. Color and spectral distribution of five different areas of each specimen were measured according to CIELAB color space in the reflectance mode relative to the standard illuminant D65 on a reflection spectrophotometer. Data were analyzed using one-way ANOVA followed by Tukey's HSD test, Pearson correlation and regression analysis (α=.05).RESULTSThere was a significant difference in CIE L* between Subgroup N and P, and in CIE b* between Subgroup P and G in each group. Spectral reflectance generally decreased in Subgroup P and G in comparison with Subgroup N. Color differences between Subgroup P and G were within the perceptibility threshold (ΔE*ab< 3.7) in most groups. Highly significant correlation was found between CIE b*and each subgroups as the number of coloring liquid applications increased (R2>0.88, P<.001).CONCLUSIONA perceptible color difference can be detected after polishing of monolithic zirconia. Polishing decreases the lightness, and glazing also decreases the lightness, but increases the yellowness of monolithic zirconia.