Jane D. Brewer

Acceptability of shade differences in metal ceramic crowns

STATEMENT OF PROBLEM Application of technology that quantify color and color differences is currently used in esthetic restorative materials research. However, parameters for the size of perceptible and acceptable color differences in metal ceramic crowns have not been determined. PURPOSE The CIELAB colorimetric system was used to study the relationship between instrumentally measured color differences (delta E) and human observer assessment of color differences in metal ceramic crowns. MATERIAL AND METHODS Subjects were shown pairs of metal ceramic crowns whose color difference varied from indistinguishable to obvious. The subjects attempted to discern a difference in each pairs color and indicated the clinical acceptability of the shade difference between the crown pairs. RESULTS Correlations between instrumental and visual assessment of color differences in these crown pairs did not agree in all dimensions of color space. Thresholds acceptability of color differences were 1.1 delta E units for red-varying crowns and 2.1 delta E units for yellow-varying crowns. Thresholds for perceptibility judgments were significantly lower than thresholds for acceptability judgments. CONCLUSIONS Acceptability thresholds were found to be dependent on chromaticity. Observers were more sensitive and critical of crowns whose color differed in redness as opposed to crowns whose color differed to the same extent in yellowness.

The effect of zirconia surface treatment on flexural strength and shear bond strength to a resin cement.

STATEMENT OF PROBLEM Despite the expanded applications for zirconia in restorative dentistry, there is no clear recommendation in the literature regarding surface treatment before bonding. PURPOSE The purpose of this study was to evaluate the effect of mechanical surface treatment of yttria-partially stabilized zirconia on its flexural strength and the effect of mechanical and chemical surface treatments on its bond strength to a resin cement. MATERIAL AND METHODS For flexural strength evaluation, zirconia bars (4 x 5 x 40 mm) were prepared from zirconia blocks, finished using a diamond rotary cutting instrument, sintered, then assigned into 4 groups: (1) control (no treatment), (2) airborne-particle abrasion, (3) silicoating, and (4) wet hand grinding. After storage for 24 hours at 37 degrees C, flexural strength was determined using a 3-point bending test, and the results were analyzed using 1-way ANOVA (alpha=.05). For shear bond strength evaluation, zirconia rods (2.5 x 3 mm) were prepared from zirconia blocks, sintered, and assigned into 16 groups. Each group underwent a combination of the following mechanical and chemical treatments. Mechanical treatment included: (1) control (no treatment), (2) airborne-particle abrasion, (3) silicoating, or (4) wet hand grinding. Chemical treatment included: (1) control (no treatment), (2) acid etching followed by silanation, (3) silanation only, or (4) application of zirconia primer. Dentin specimens were prepared from extracted molars stored in 0.5% chloramine-T. Zirconia rods were bonded to dentin using a resin cement (Multilink Automix), then light polymerized. After storage, the specimens were loaded to failure with the notched shear bond test method in a universal loading apparatus. For artificial aging analysis, the groups that achieved the highest bond strength values were duplicated, stored at 37 degrees C and 100% humidity for 90 days, and thermal cycled before being loaded to failure. Results were analyzed using 2-way ANOVA (alpha=.05). RESULTS Airborne-particle abrasion and hand grinding significantly increased flexural strength. The highest shear bond strength values were achieved for the following groups: silicoated + silanated > hand ground + zirconia primer > airborne-particle abraded + silanated > zirconia primer > airborne-particle abraded + zirconia primer. Artificial aging resulted in significantly lower shear bond strength for the silicoated/silanated and the zirconia primer groups. CONCLUSIONS Mechanical modification of the surface increased the flexural strength of Y-TZP. The resin bond to Y-TZP was improved by surface treatment. A combination of mechanical and chemical conditioning of the zirconia surface was essential to develop a durable resin bond to zirconia.

Reliability and accuracy of four dental shade-matching devices

STATEMENT OF PROBLEM There are several electronic shade-matching instruments available for clinical use, but the reliability and accuracy of these instruments have not been thoroughly investigated. PURPOSE The purpose of this in vitro study was to evaluate the reliability and accuracy of 4 dental shade-matching instruments in a standardized environment. MATERIAL AND METHODS Four shade-matching devices were tested: SpectroShade, ShadeVision, VITA Easyshade, and ShadeScan. Color measurements were made of 3 commercial shade guides (Vitapan Classical, Vitapan 3D-Master, and Chromascop). Shade tabs were placed in the middle of a gingival matrix (Shofu GUMY) with shade tabs of the same nominal shade from additional shade guides placed on both sides. Measurements were made of the central region of the shade tab positioned inside a black box. For the reliability assessment, each shade tab from each of the 3 shade guide types was measured 10 times. For the accuracy assessment, each shade tab from 10 guides of each of the 3 types evaluated was measured once. Differences in reliability and accuracy were evaluated using the Standard Normal z test (2 sided) (alpha=.05) with Bonferroni correction. RESULTS Reliability of devices was as follows: ShadeVision, 99.0%; SpectroShade, 96.9%; VITA Easyshade, 96.4%; and ShadeScan, 87.4%. A significant difference in reliability was found between ShadeVision and ShadeScan (P=.008). All other comparisons showed similar reliability. Accuracy of devices was as follows: VITA Easyshade, 92.6%; ShadeVision, 84.8%; SpectroShade, 80.2%; and ShadeScan, 66.8%. Significant differences in accuracy were found between all device pairs (P<.001) for all comparisons except for SpectroShade versus ShadeVision (P=.033). CONCLUSIONS Most devices had similar high reliability (over 96%), indicating predictable shade values from repeated measurements. However, there was more variability in accuracy among devices (67-93%), and differences in accuracy were seen with most device comparisons.

Defining a natural tooth color space based on a 3-dimensional shade system

STATEMENT OF PROBLEM The natural tooth color space reported by a manufacturer may not represent the comprehensive spectrum of natural teeth for all population groups. PURPOSE The purpose of this study was to define a natural tooth color space within the Greater Buffalo, New York population and to compare that to the color space determined by a manufacturer. MATERIAL AND METHODS Nine hundred and thirty-three maxillary central incisors (501 patients) were measured with a shade-taking device (Vita Easyshade). For each tooth, L*, a*, b* values, chroma, hue, and the closest shade (Vita 3D-Master) were recorded. A linear regression analysis was performed to determine how well the manufacturers values predict actual values for L*, a*, and b*. Color differences (DeltaE*) between the Buffalo population and the closest shade were also calculated. A 1-sample t test was used to determine whether the color differences seen in the sample were statistically different from the perceptibility threshold, DeltaE*=3.7 (alpha=.05). RESULTS All 3 attributes of the Buffalo population displayed a broader range than those from the shade guide. However, the regression analysis revealed a significantly positive relationship between the L*, a*, and b* values of the 2 methods (P<.001). The 1-sample t test revealed a significant DeltaE* (mean DeltaE*=6.15) difference from the perceptibility threshold of DeltaE*=3.7 (P<.001). CONCLUSIONS Color differences between the Buffalo population and the shade guide were frequently above published perceptibility thresholds, but within the range of acceptability. The Buffalo population tooth color space encompassed the manufacturers color space.

Variability of porcelain color reproduction by commercial laboratories

STATEMENT OF PROBLEM Many investigations in the field of metal ceramics have examined materials, manipulative variables, and the relationship of these factors to a restorations color. However, the effect of the artistic component of restoration fabrication is not known. PURPOSE The purpose of this study was to determine through instrumental colorimetry the variability in color reproduction for metal ceramic crowns fabricated by commercial dental laboratory technicians. MATERIALS AND METHODS Fifty metal ceramic crowns were fabricated on standardized metal frameworks to the same shade specifications by 5 commercial dental laboratories (n=10). Laboratory prescriptions requested that the technician match the shade and translucency of a provided Vita Lumin A3.5 shade tab. Technicians used the porcelain and technique of their own selection to match the tab. Color differences were determined by use of a colorimeter between crowns and the prescribed shade tab at middle and incisal sites. Analysis of variance was used to determine whether differences in color reproduction existed among laboratories. Where statistically significant interactions existed, the Tukey honestly significant difference test was used to determine significant differences between laboratories according to sites (alpha=.05). RESULTS Color reproduction was significantly different (P <.0001) among laboratories for both sites. Mean color difference from shade tabs ranged from 3.5 to 11.1 DeltaE units. All laboratories were better at matching shades in the incisal third of the crown. CONCLUSION Within the limitations of this study, the ability to reproduce the color of the target shade tab differed among laboratories. Most crowns fabricated by the laboratories in this study, when compared to the prescribed shade tab, were above the clinical threshold for an acceptable shade match under intraoral conditions (DeltaE 3.7).

Clinical discrimination between autoglazed and polished porcelain surfaces

Precementation refinements of porcelain-fused-to-metal crowns often require reglazing or polishing of the porcelain surface. This study was done to determine whether visual inspection differences exist between glazed and polished porcelain surfaces. Prosthodontists, general dentists, and students (six in each group) rated esthetic properties of 12 porcelain-fused-to-metal crowns. All crowns were initially autoglazed. For phase 1 observations, six crowns were air abraded and polished and six retained their glazed surface. For phase 2 observations, the surface treatments were reversed. At both observations, crowns were rated on 5-point Likert scales for outline form, porosity, smoothness, reflectance, texture, dullness, defects, and general esthetic appearances. Phase 1 polished and glazed crowns had different means for outline form sharpness, porosity, reflectance, dullness, and general esthetic appearance. Phase 2 crowns were different for dullness. Polished and glazed crowns alike were more dull at phase 1 than at phase 2. Glazed crowns were different between phases for reflectance and general esthetic appearance. All reported differences were significant at p less than .01. Significant differences occurred among raters with polished and glazed crowns for several variables.

Comparison of fracture resistance of pressable metal ceramic custom implant abutments with CAD/CAM commercially fabricated zirconia implant abutments

STATEMENT OF PROBLEM The adequacy of the strength of dental zirconia abutments under heavy occlusal load conditions is questionable, with degradation of the mechanical properties of zirconia having been reported. Therefore, some alternatives must be considered. PURPOSE The purpose of this study was to determine the efficacy of fabricating pressable metal ceramic custom implant (Pr) abutments, and to evaluate the fracture resistance of these abutments. MATERIAL AND METHODS Two groups of implant abutment specimens were fabricated (n=10). The experimental group consisted of Pr abutments, and the control group consisted of CAD/CAM-designed zirconia-based ceramic (Zr) abutments. For the experimental group, custom metal abutments were cast using a compatible metal alloy (Lodestar); this was followed by injection molding with lithium disilicate pressable ceramic (IPS e.max Press) around the metal column of the custom implant abutment. For the control group, 10 specimens were fabricated from CAD/CAM-designed zirconia abutments (Procera Zirconia). Following scanning, all-ceramic crowns with the average dimensions of a human central incisor were fabricated for the experimental and control abutments (n=20) using lithium disilicate pressable ceramic (IPS e.max Press). Each crown was cemented to the implant abutments with a resin luting agent (Variolink II). The crown-abutment test specimens were fixed to titanium implant analogs and placed in a test stand at 30 degrees from the vertical axis of the specimens in a computer-controlled universal testing device. The independent t test was used to detect if the mean values of the fracture load differed significantly (alpha=.05) between the 2 groups. RESULTS The mean (SD) fracture load was significantly higher in the Pr group (901.67 (102.05) N) than in the Zr group (480.01 (174.46) N, P<.005). CONCLUSIONS This study found that Pr abutments are stronger than Zr abutments.

Effect of two connector designs on the fracture resistance of all-ceramic core materials for fixed dental prostheses

STATEMENT OF PROBLEM Most all-ceramic fixed dental prostheses (FDPs) fail at the connectors. PURPOSE The purpose of this study was to determine the effect of 2 connector designs on the fracture resistance of core materials used for all-ceramic FDPs. MATERIAL AND METHODS Three materials were tested: (1) heat-pressed lithium disilicate glass ceramic (IPS e.max Press (Press)), (2) milled lithium disilicate glass ceramic (IPS e.max CAD (CAD)), and (3) milled yttrium-stabilized tetragonal zirconia polycrystals (Y-TZP) (IPS e.max ZirCAD (ZirCAD)). Specimens were made into 30 x 4 x 4-mm bars to represent 3-unit FDPs. Two connector designs, round (0.60 +/-0.01-mm radius of curvature) and sharp (0.06 +/-0.001-mm radius of curvature), with a 3.00 +/-0.05-mm cross-section for each connector, were studied (n=5). Each specimen was loaded to fracture in a universal testing machine with a crosshead speed of 0.1 mm/min. Data were analyzed with a 2-way univariate ANOVA and Tukey HSD test (alpha=.05). RESULTS Mean (SD) failure loads for round connector designs were 684.2 (70.1) N for ZirCAD, 260 (7.8) N for CAD, and 172.9 (35.5) N for Press. Mean (SD) failure loads for sharp connector designs were 386.3 (51.5) N for ZirCAD, 87.9 (7.0) N for CAD, and 125.1 (15.1) N for Press. The 2-way univariate ANOVA indicated statistically significant differences (P<.005) for material and connector design, and, also, a significant interaction between material and connector design. Higher maximum failure loads were found for the round connector design when compared to the sharp connector design, for ZirCAD and CAD. However, this difference was not statistically significant for the Press groups. SEM subjective assessment of the fractured specimens revealed that the fracture initiated from the gingival surface (tensile) of the connector toward the pontic (central loading point). CONCLUSIONS Fracture resistance of ceramic core materials is affected by fabrication technique and connector design. Connector design affected fracture resistance of the milled ceramic, but not the pressed ceramic.

In vitro model to evaluate reliability and accuracy of a dental shade-matching instrument

STATEMENT OF PROBLEM There are several electronic shade-matching instruments available for clinical use; unfortunately, there are limited acceptable in vitro models to evaluate their reliability and accuracy. PURPOSE The purpose of this in vitro study was to evaluate the reliability and accuracy of a dental clinical shade-matching instrument. MATERIAL AND METHODS Using the shade-matching instrument (ShadeScan), color measurements were made of 3 commercial shade guides (VITA Classical, VITA 3D-Master, and Chromascop). Shade tabs were selected and placed in the middle of a gingival matrix (Shofu Gummy), with tabs of the same nominal shade from additional shade guides placed on both sides. Measurements were made of the central region of the shade tab inside a black box. For the reliability assessment, each shade tab from each of the 3 shade guide types was measured 10 times. For the accuracy assessment, each shade tab from 10 guides of each of the 3 types evaluated was measured once. Reliability, accuracy, and 95% confidence intervals were calculated for each shade tab. Differences were determined by 1-way ANOVA followed by the Bonferroni multiple comparison procedure. RESULTS Reliability of ShadeScan was as follows: VITA Classical = 95.0%, VITA 3D-Master = 91.2%, and Chromascop = 76.5%. Accuracy of ShadeScan was as follows: VITA Classical = 65.0%, VITA 3D-Master = 54.2%, Chromascop = 84.5%. CONCLUSIONS This in vitro study showed a varying degree of reliability and accuracy for ShadeScan, depending on the type of shade guide system used.

Comparison of porcelain surface and flexural strength obtained by microwave and conventional oven glazing.

STATEMENT OF PROBLEM Although the superior qualities of microwave technology are common knowledge in the industry, effects of microwave glazing of dental ceramics have not been investigated. PURPOSE The purpose of this study was to investigate the surface roughness and flexural strength achieved by glazing porcelain specimens in a conventional and microwave oven. MATERIAL AND METHODS Thirty specimens of each type of porcelain (Omega 900 and IPS d.Sign) were fabricated and sintered in a conventional oven. The specimens were further divided into 3 groups (n=10): hand polished (using diamond rotary ceramic polishers), microwave glazed, and conventional oven glazed. Each specimen was evaluated for surface roughness using a profilometer. The flexural strength of each specimen was measured using a universal testing machine. A 2-way ANOVA and Tukey HSD post hoc analysis were used to determine significant intergroup differences in surface roughness (alpha=.05). Flexural strength results were also analyzed using 2-way ANOVA, and the Weibull modulus was determined for each of the 6 groups. The surfaces of the specimens were subjectively evaluated for cracks and porosities using a scanning electron microscope (SEM). RESULTS A significant difference in surface roughness was found among the surface treatments (P=.02). Follow-up tests showed a significant difference in surface roughness between oven-glazed and microwave-glazed treatments (P=.02). There was a significant difference in flexural strength between the 2 porcelains (P<.005), but no significant difference in flexural strength by surface treatment (P=.48). The Weibull modulus value for the Omega 900 microwave-glazed group was the highest (1.9) as compared to the other groups. CONCLUSIONS The surface character of microwave-glazed porcelain was superior to oven-glazed porcelain. Omega 900 had an overall higher flexural strength than IPS d.Sign. Weibull distributions of flexural strengths for Omega 900 oven-glazed and microwave-glazed specimens were similar. SEM analysis demonstrated a greater number of surface voids and imperfections in IPS d. Sign as compared to Omega 900.