Dental students' preferences and performance in crown design: conventional wax-added versus CAD.

The purpose of this study was to evaluate dental students' perceptions of traditional waxing vs. computer-aided crown design and to determine the effectiveness of either technique through comparative grading of the final products. On one of twoidentical tooth preparations, second-year students at one dental school fabricated a wax pattern for a full contour crown; on the second tooth preparation, the same students designed and fabricated an all-ceramic crown using computer-aided design (CAD) and computer-aided manufacturing (CAM) technology. Projects were graded for occlusion and anatomic form by three faculty members. On completion of the projects, 100 percent of the students (n=50) completed an eight-question, five-point Likert scalesurvey, designed to assess their perceptions of and learning associated with the two design techniques. The average grades for the crown design projects were 78.3 (CAD) and 79.1 (wax design). The mean numbers of occlusal contacts were 3.8 (CAD) and 2.9(wax design), which was significantly higher for CAD (p=0.02). The survey results indicated that students enjoyed designing afull contour crown using CAD as compared to using conventional wax techniques and spent less time designing the crown using CAD. From a learning perspective, students felt that they learned more about position and the size/strength of occlusal contacts using CAD. However, students recognized that CAD technology has limits in terms of representing anatomic contours and excursive occlusion compared to conventional wax techniques. The results suggest that crown design using CAD could be considered as an adjunct to conventional wax-added techniques in preclinical fixed prosthodontic curricula.

Color match of machinable lithium disilicate ceramics: Effects of cement color and thickness.

STATEMENT OF PROBLEM: The effects of cement color and thickness on the color of machinable lithium disilicate (MLD) ceramic luted on metal foundation restorations is unknown. PURPOSE: The purpose of this study was to evaluate the effects of cement color and thickness on the shade matching of MLD restorations luted on silver-palladium (Ag-Pd) foundations. MATERIAL AND METHODS: Fifteen 1.5-mm-thick ceramic specimens were made from shade A1 LT lithium disilicate blocks. Five resin cements with different colors and opacities (Multilink Automix white opaque, Multilink Automix yellow, Nexus3 white opaque, Nexus3 white, Nexus3 yellow) of 3 thicknesses (300 µm, 100 µm, 50 µm) were sequentially luted to a roughened Ag-Pd alloy foundation restoration block. Five shade measurements were made with a portable spectrophotometer after optically connecting the ceramic specimen to each cement foundation block. The color differences (ΔE) between each specimen assembly and the target block (a 14×14×12-mm shade A1 LT crystalized e.max block) were recorded with the CIE (Commission internationale de l'éclairage) L*a*b* system. Clinical significance was determined by comparing color differences to perceptibility and acceptability thresholds with the 1-sample t test (α=.05). RESULTS: Both cement color and cement thickness significantly affected the mean values of the color difference (ΔE) of lithium disilicate ceramic assemblies to the target block (P<.001). Among the 5 cements tested, the lowest mean ΔE was observed for Nexus3 white opaque cement. Overall, the combination of Nexus3 white opaque cement and 100-µm thickness led to the lowest mean ΔE. The means of ΔE were below the clinically perceptible level (ΔE<2.6) for combinations of Nexus3 white opaque of 100-µm and 300-µm cement thicknesses (P<.001 and P=.022, respectively). The means of ΔE were below the clinically acceptable level (ΔE<5.5) for the following combinations: Nexus3 white opaque of 50-µm thickness, Nexus3 white, and Nexus3 yellow for all tested cement thicknesses and Multilink white opaque or Multilink yellow of either 50-µm or 100-µm cement thickness (P<.001). The means of ΔE were above the clinically acceptable level (ΔE>5.5) for Multilink white opaque and Multilink yellow of 300-µm cement thickness. CONCLUSIONS: The colors of machinable lithium disilicate ceramic restorations placed on Ag-Pd foundation were affected by both the color and thickness of cements. Among the 5 cements tested in this study, Nexus3 white opaque cement of 100 µm or 300 µm yielded shade matches below the clinical perceptible threshold (ΔE<2.6) relative to the target block.

Color match of machinable lithium disilicate ceramics: effects of foundation restoration.

STATEMENT OF PROBLEM: Metal or white opaque foundation restorations may negatively affect the color of machinable lithium disilicate (MLD) ceramic restorations. PURPOSE: The purpose of this study was to evaluate the effects of ceramic thickness and foundation restoration materials on the color of MLD restorations. MATERIAL AND METHODS: Forty-five ceramic slices in 3 thicknesses (1.0 mm, 1.5 mm, 2.0 mm; 15 slices in each group) were made from low-translucency (LT) shade A1 IPS e.max CAD blocks. Resin cement (Multilink yellow) of 100-µm cement thickness was bonded to 3 different foundation restoration materials: silver-palladium (Ag-Pd) (Albacast) alloy, Type III gold (Midas), and white opaque foundation resin (Paracore white) to make the cement-foundation blocks. After optically connecting each ceramic specimen to the cement-foundation block, the color of each laminated combination was measured with a portable spectrophotometer (Vita EasyShade Compact). The color differences (ΔE) between the specimen assemblies and a control target block (a 12×14×14-mm crystalized shade A1 LT e.max CAD block) were calculated. Two-way ANOVA and general linear model were used to assess the effects of ceramic thickness, foundation materials, and their interactions to the resultant ΔE (α=.05). Clinical significance was determined by comparing color differences to perceptibility and acceptability thresholds by using the t test (α=.05). RESULTS: Both ceramic thickness and foundation materials significantly affected the mean values of color difference (ΔE) of MLD restorations (P<.001). The mean value of ΔE decreased as the ceramic thickness increased. At a ceramic thickness of 1 mm, the color difference was above the clinically perceptible level (ΔE>2.6) with the 3 tested foundation materials (P<.001). As for the foundation materials, the ΔE was the lowest for Type III gold alloy, followed by Ag-Pd, then white opaque foundation resin. The color differences for Type III gold and a ceramic thickness of 1.5 or 2.0 mm were below the clinically perceptible level (ΔE<2.6) (P<.001). For Ag-Pd alloy or white opaque foundation resin, the color differences were above the clinically perceptible level (ΔE>2.6) (P<.001). Ag-Pd alloy reduced, the values of L* and b* parameters of MLD complexes, whereas the white opaque resin increased them. CONCLUSIONS: Based on the results of the study, the colors of MLD ceramic restorations were affected by both the ceramic thickness and foundation restoration materials. Increasing ceramic thickness improved the resultant shade matching. Ag-Pd alloy made the ceramic restorations darker and bluish, whereas white opaque foundation resin made restorations brighter and yellowish.

Intraoral determination of the tolerance of dentists for perceptibility and acceptability of shade mismatch.

STATEMENT OF PROBLEM: There is little agreement in the dental literature as to how much color difference constitutes an acceptable shade mismatch or how much color difference is considered perceivable to observers. Most studies attempting to determine perceptibility and acceptability of tolerances for shade mismatches have been conducted under in vitro conditions that are not applicable to clinical scenarios. PURPOSE: The goal of this study was to determine valid acceptability and perceptibility tolerances for shade mismatch in an actual clinical scenario using spectroradiometric instrumentation. MATERIAL AND METHODS: A test denture was fabricated that allowed 10 maxillary left central incisors of varying shade mismatch with the right central incisor to be interchanged within the denture base. A spectroradiometer was used to determine the CIELAB coordinates and color differences (DeltaE) between the right central incisor and the interchangeable left central incisor denture teeth. The interchangeable denture teeth ranged uniformly from 1 DeltaE unit (visually undetectable) to greater than 10 DeltaE units (an obvious shade mismatch). The test denture with each of the interchangeable teeth was modeled by a subject to 28 dentists in a clinical setting. For each of the interchangeable teeth, dentist observers were asked if they could see a difference between the central incisors and, if so, whether the difference was acceptable. A Probit regression analysis was used to predict acceptability and perceptibility tolerances with 95% confidence limits. RESULTS: The predicted color difference at which 50% of the dentist observers could perceive a color difference (50/50 perceptibility) was 2.6 DeltaE units. The predicted color difference at which 50% of the subjects would remake the restoration due to color mismatch (clinically unacceptable color match) was 5.5 DeltaE. Acceptability and perceptibility color tolerances at the 50/50 level were significantly different (P<.05), as their 95% confidence limits did not overlap. CONCLUSIONS: Tolerances for perceptibility were significantly lower than tolerances for acceptability for shade mismatch between 2 denture teeth.

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 restoration's 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).