Woong-Chul Kim

An evaluation of marginal fit of three-unit fixed dental prostheses fabricated by direct metal laser sintering system.

OBJECTIVES This in vitro study aimed to evaluate and compare marginal fit of three-unit fixed dental prostheses (FDPs) fabricated using a newly developed direct metal laser sintering (DMLS) system with that of three-unit FDPs by a conventional lost wax technique (LW) method. METHODS Ten cobalt-chromium alloy three-unit FDPs using DMLS system and another ten nickel-chromium alloy FDPs using LW method were fabricated. Marginal fit was examined using a light-body silicone. After setting, the silicon film was cut into four parts and the thickness of silicon layer was measured at 160× magnification using a digital microscope to measure absolute marginal discrepancy (AMD), marginal gap (MG) and internal gap (IG). A repeated measure ANOVA for statistical analysis was performed using the SPSS statistical package version 12.0 (α=0.05). RESULTS The mean values of AMD, MG, and IG were significantly larger in the DMLS group than in the LW group (p<0.001). Means of AMD, MG and IG in the first molars were 83.3, 80.0, and 82.0μm in the LW group; and 128.0, 112.0, and 159.5μm in the DMLS group, respectively. No significant difference between measurements for premolars and molars was found (p>0.05). SIGNIFICANCE The marginal fit of the DMLS system appeared significantly inferior compared to that of the conventional LW method and slightly larger than the acceptable range. For clinical application further improvement of DMLS system may be required.

Three-dimensional evaluation of gaps associated with fixed dental prostheses fabricated with new technologies

STATEMENT OF PROBLEM One of the most important factors in determining the clinical outcomes of fixed dental prostheses is the gap between the fixed dental prosthesis and the abutment. However, reports that investigated these gaps in the context of fixed dental prostheses fabricated with new technologies are few. PURPOSE The purpose of this study was to measure and analyze the fit of fixed dental prostheses. The fixed dental prostheses for the study were produced with the subtractive method (milling soft metal blocks), additive method (selective laser sintering), and traditional method (lost wax and casting). MATERIAL AND METHODS Ten specimens were fabricated with the soft metal block, selective laser sintering, and lost wax and casting methods. The 3-dimensional measurement method was adopted to obtain the measure gap figures of the specimens. To fabricate a digital replica, computer-aided design reference casts were prepared by scanning the study casts, and silicone replicas were fabricated for each specimen. These silicone replicas were scanned and obtained 40,000 point cloud data. The study also defined the mean gap for each specimen by averaging approximately 40,000 gap points to evaluate the fit of the specimens. Data were analyzed with 1-way ANOVA and the Tukey honestly significant difference (α=.05). RESULTS The mean gap was the smallest for fixed dental prostheses fabricated with the soft metal block, followed by the selective laser sintering, then the lost wax and casting. One-way ANOVA revealed statistically significant differences in the size of the gap among the 3 groups (P<.001), and the Tukey honestly significant difference test confirmed the specific differences among the groups (P<.05). CONCLUSIONS The fit obtained with the new soft metal block and selective laser sintering techniques was better than that obtained with the traditional lost wax and casting method. Thus, fixed dental prostheses produced by using these new techniques can be considered clinically acceptable.

Effects of the sintering conditions of dental zirconia ceramics on the grain size and translucency

PURPOSE This study aimed to identify the effects of the sintering conditions of dental zirconia on the grain size and translucency. MATERIALS AND METHODS Ten specimens of each of two commercial brands of zirconia (Lava and KaVo) were made and sintered under five different conditions. Microwave sintering (MS) and conventional sintering (CS) methods were used to fabricate zirconia specimens. The dwelling time was 20 minutes for MS and 20 minutes, 2, 10, and 40 hours for CS. The density and the grain size of the sintered zirconia blocks were measured. Total transmission measurements were taken using a spectrophotometer. Two-way analysis of variance model was used for the analysis and performed at a type-one error rate of 0.05. RESULTS There was no significant difference in density between brands and sintering conditions. The mean grain size increased according to sintering conditions as follows: MS-20 min, CS-20 min, CS-2 hr, CS-10 hr, and CS-40 hr for both brands. The mean grain size ranged from 347-1,512 nm for Lava and 373-1,481 nm for KaVo. The mean light transmittance values of Lava and KaVo were 28.39-34.48% and 28.09-30.50%, respectively. CONCLUSION Different sintering conditions resulted in differences in grain size and light transmittance. To obtain more translucent dental zirconia restorations, shorter sintering times should be considered.

Evaluation of the marginal and internal gap of metal-ceramic crown fabricated with a selective laser sintering technology: two- and three-dimensional replica techniques

PURPOSE One of the most important factors in evaluating the quality of fixed dental prostheses (FDPs) is their gap. The purpose of this study was to compare the marginal and internal gap of two different metal-ceramic crowns, casting and selective laser sintering (SLS), before and after porcelain firing. Furthermore, this study evaluated whether metal-ceramic crowns made using the SLS have the same clinical acceptability as crowns made by the traditional casting. MATERIALS AND METHODS The 10 study models were produced using stone. The 20 specimens were produced using the casting and the SLS methods; 10 samples were made in each group. After the core gap measurements, 10 metal-ceramic crowns in each group were finished using the conventional technique of firing porcelain. The gap of the metal-ceramic crowns was measured. The marginal and internal gaps were measured by two-dimensional and three-dimensional replica techniques, respectively. The Wilcoxon signed-rank test, the Wilcoxon rank-sum test and nonparametric ANCOVA were used for statistical analysis (α=.05). RESULTS In both groups, the gap increased after completion of the metal-ceramic crown compared to the core. In all measured areas, the gap of the metal cores and metal-ceramic crowns produced by the SLS was greater than that of the metal cores and metal-ceramic crowns produced using the casting. Statistically significant differences were found between cast and SLS (metal cores and metal-ceramic crown). CONCLUSION Although the gap of the FDPs produced by the SLS was greater than that of the FDPs produced by the conventional casting in all measured areas, none exceeded the clinically acceptable range.

Influence of dentin porcelain thickness on layered all-ceramic restoration color

OBJECTIVES Final aesthetic performance of all-ceramic restorations is influenced by the thickness ratio of core and porcelain ceramics. This study aimed to identify and quantify the effect of increased dentin porcelain thickness on the color and color coordinates of all-ceramic specimens. METHODS Lava and IPS e.max Press all-ceramic systems were investigated. A1, A2 and A3.5 shades of layered specimens were made with 2-mm ceramic cores and 0-2mm bevelled dentin porcelain. Color was measured at 0.05mm dentin porcelain thickness intervals with a spectroradiometer. The effect of ceramic brand, shade and dentin porcelain thickness on color change was analyzed by 3-way ANOVA. RESULTS Color changes of layered ceramics varied by ceramic brand, shade and dentin porcelain thickness. For most ceramics, CIE a* and b* values gradually increased as the thickness of the dentin porcelain increased. Conversely, such increases in dentin porcelain thickness were correlated with decreasing CIE L* values. When compared with the IPS e.max Press ceramics, Lava ceramics showed smaller color changes in response to increases in dentin porcelain thickness. CONCLUSIONS The final appearance of ceramic restorations can be manipulated by varying the dentin porcelain thickness; however, the color of the layered ceramics varied by ceramic brand, shade and dentin porcelain thickness.

Bond and fracture strength of metal-ceramic restorations formed by selective laser sintering

PURPOSE The purpose of this study was to compare the fracture strength of the metal and the bond strength in metal-ceramic restorations produced by selective laser sintering (SLS) and by conventional casting (CAST). MATERIALS AND METHODS Non-precious alloy (StarLoy C, DeguDent, Hanau, Germany) was used in CAST group and metal powder (SP2, EOS GmbH, Munich, Germany) in SLS group. Metal specimens in the form of sheets (25.0 × 3.0 × 0.5 mm) were produced in accordance with ISO 9693:1999 standards (n=30). To measure the bond strength, ceramic was fired on a metal specimen and then three-point bending test was performed. In addition, the metal fracture strength was measured by continuing the application of the load. The values were statistically analyzed by performing independent t-tests (α=.05). RESULTS The mean bond strength of the SLS group (50.60 MPa) was higher than that of the CAST group (46.29 MPa), but there was no statistically significant difference. The metal fracture strength of the SLS group (1087.2 MPa) was lower than that of the CAST group (2399.1 MPa), and this difference was statistically significant. CONCLUSION In conclusion the balling phenomenon and the gap formation of the SLS process may increase the metal-ceramic bond strength.

Accuracy and precision of polyurethane dental arch models fabricated using a three-dimensional subtractive rapid prototyping method with an intraoral scanning technique

Objective This study aimed to evaluate the accuracy and precision of polyurethane (PUT) dental arch models fabricated using a three-dimensional (3D) subtractive rapid prototyping (RP) method with an intraoral scanning technique by comparing linear measurements obtained from PUT models and conventional plaster models. Methods Ten plaster models were duplicated using a selected standard master model and conventional impression, and 10 PUT models were duplicated using the 3D subtractive RP technique with an oral scanner. Six linear measurements were evaluated in terms of x, y, and z-axes using a non-contact white light scanner. Accuracy was assessed using mean differences between two measurements, and precision was examined using four quantitative methods and the Bland-Altman graphical method. Repeatability was evaluated in terms of intra-examiner variability, and reproducibility was assessed in terms of inter-examiner and inter-method variability. Results The mean difference between plaster models and PUT models ranged from 0.07 mm to 0.33 mm. Relative measurement errors ranged from 2.2% to 7.6% and intraclass correlation coefficients ranged from 0.93 to 0.96, when comparing plaster models and PUT models. The Bland-Altman plot showed good agreement. Conclusions The accuracy and precision of PUT dental models for evaluating the performance of oral scanner and subtractive RP technology was acceptable. Because of the recent improvements in block material and computerized numeric control milling machines, the subtractive RP method may be a good choice for dental arch models.

Three-dimensional evaluation of the repeatability of scanned conventional impressions of prepared teeth generated with white- and blue-light scanners.

STATEMENT OF PROBLEM Digital scanning is increasingly used in prosthodontics. Three-dimensional (3D) evaluations that compare the repeatability of the blue-light scanner with that of the white-light scanner are required. PURPOSE The purpose of this in vitro study was to evaluate the repeatability of conventional impressions of abutment teeth digitized with white- and blue-light scanners and compare the findings for different types of abutment teeth. MATERIAL AND METHODS Impressions of the canine, premolar, and molar abutment teeth were made and repeatedly scanned with each scanner type to obtain 5 sets of 3D data for each tooth. Point clouds were compared, and error sizes per tooth and scanner type were measured (n=10). One-way ANOVA with Tukey honest significant differences multiple comparison and independent t tests were performed to evaluate repeatability (α=.05). RESULTS Repeatability (mean ±SD) of the white- and blue-light scanners for canine, premolar, and molar teeth was statistically significant (means: P=.001, P<.001, P<.001; ±SD: P<.001, P<.001, P=.003). Means of discrepancies with the white-light scanner (P<.001) were 5.8 μm for the canine, 5.9 μm for the premolar, and 8.6 μm for the molar teeth and 4.4 μm, 2.9 μm, and 3.2 μm, respectively, with the blue-light scanner (P<.001). Corresponding SDs of discrepancies with the white-light scanner (P<.001) were 15.9 μm for the canine, 23.2 μm for the premolar, and 14.6 μm for the molar teeth and 9.8 μm, 10.6 μm, and 11.2 μm, respectively, with the blue-light scanner (P=.73). CONCLUSIONS On evaluation of the digitized abutment tooth impressions, the blue-light scanner exhibited greater repeatability than the white-light scanner.

White light scanner-based repeatability of 3-dimensional digitizing of silicon rubber abutment teeth impressions.

PURPOSE The aim of this study was to evaluate the repeatability of the digitizing of silicon rubber impressions of abutment teeth by using a white light scanner and compare differences in repeatability between different abutment teeth types. MATERIALS AND METHODS Silicon rubber impressions of a canine, premolar, and molar tooth were each digitized 8 times using a white light scanner, and 3D surface models were created using the point clouds. The size of any discrepancy between each model and the corresponding reference tooth were measured, and the distribution of these values was analyzed by an inspection software (PowerInspect 2012, Delcamplc., Birmingham, UK). Absolute values of discrepancies were analyzed by the Kruskal-Wallis test and multiple comparisons (α=.05). RESULTS The discrepancy between the impressions for the canine, premolar, and molar teeth were 6.3 µm (95% confidence interval [CI], 5.4-7.2), 6.4 µm (95% CI, 5.3-7.6), and 8.9 µm (95% CI, 8.2-9.5), respectively. The discrepancy of the molar tooth impression was significantly higher than that of other tooth types. The largest variation (as mean [SD]) in discrepancies was seen in the premolar tooth impression scans: 26.7 µm (95% CI, 19.7-33.8); followed by canine and molar teeth impressions, 16.3 µm (95% CI, 15.3-17.3), and 14.0 µm (95% CI, 12.3-15.7), respectively. CONCLUSION The repeatability of the digitizing abutment teeths silicon rubber impressions by using a white light scanner was improved compared to that with a laser scanner, showing only a low mean discrepancy between 6.3 µm and 8.9 µm, which was in an clinically acceptable range. Premolar impression with a long and narrow shape showed a significantly larger discrepancy than canine and molar impressions. Further work is needed to increase the digitizing performance of the white light scanner for deep and slender impressions.

Comparison of prosthetic models produced by traditional and additive manufacturing methods

PURPOSE The purpose of this study was to verify the clinical-feasibility of additive manufacturing by comparing the accuracy of four different manufacturing methods for metal coping: the conventional lost wax technique (CLWT); subtractive methods with wax blank milling (WBM); and two additive methods, multi jet modeling (MJM), and micro-stereolithography (Micro-SLA). MATERIALS AND METHODS Thirty study models were created using an acrylic model with the maxillary upper right canine, first premolar, and first molar teeth. Based on the scan files from a non-contact blue light scanner (Identica; Medit Co. Ltd., Seoul, Korea), thirty cores were produced using the WBM, MJM, and Micro-SLA methods, respectively, and another thirty frameworks were produced using the CLWT method. To measure the marginal and internal gap, the silicone replica method was adopted, and the silicone images obtained were evaluated using a digital microscope (KH-7700; Hirox, Tokyo, Japan) at 140X magnification. Analyses were performed using two-way analysis of variance (ANOVA) and Tukey post hoc test (α=.05). RESULTS The mean marginal gaps and internal gaps showed significant differences according to tooth type (P<.001 and P<.001, respectively) and manufacturing method (P<.037 and P<.001, respectively). Micro-SLA did not show any significant difference from CLWT regarding mean marginal gap compared to the WBM and MJM methods. CONCLUSION The mean values of gaps resulting from the four different manufacturing methods were within a clinically allowable range, and, thus, the clinical use of additive manufacturing methods is acceptable as an alternative to the traditional lost wax-technique and subtractive manufacturing.