Ki Baek 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.

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.

Implementation of stabilizing receding horizon controls for time-varying systems

In this paper, a new stabilizing receding horizon control (RHC) scheme is proposed for linear discrete time-varying systems, which can be easily implemented by using linear matrix inequality (LMI) optimization. The control scheme is based on the minimization of the finite horizon cost with a finite terminal weighting matrix. The resulting stabilizing RHC scheme leads to time-varying finite terminal weighting matrices even for time-invariant systems, which is more general than in the case of using constant matrices. Based on the proposed scheme, another implementation method is also discussed for easy computation and numerical feasibility consideration of LMI optimization, although the second method does not guarantee the closed-loop stability theoretically. Through a simulation example, the effectiveness of the proposed schemes is illustrated.

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.

Receding horizon guidance laws for constrained missiles with autopilot lags

Abstract In this paper, a new stabilizing receding horizon control (RHC) scheme is proposed for linear continuous time-invariant systems with input and state constraints. The control scheme is based on the minimization of the finite horizon cost with a finite terminal weighting matrix subject to constraints. A new algorithm is suggested to implement the RHC scheme for a constrained receding horizon guidance law (CRHG). The proposed CRHG, which does not use information on the time-to-go, is obtained by using linear matrix inequality (LMI) optimization. Through simulation examples, the performance of the proposed CRHG is illustrated.

Modified receding horizon guidance law with information on small accurate time-to-go

A modified receding horizon guidance law (MRHG), which is useful when the accurate time-to-go is available only for a short time before the final time, is proposed. The proposed MRHG is based on the receding horizon strategy and optimal control theory. In the presence of arbitrary target maneuvers and an initial lateral miss distance (MD) rate between a target and a missile, it is shown that the proposed MRHG guarantees not only to keep the lateral MD less than a given value, within which the warhead of the missile is detonated, from the appropriately selected time to the intercept time, but also to render the terminal MD zero. Through simulations when the time-to-go is accurate only for a short time, the performance of MRHG is compared with the optimal guidance law (OGL) and the receding horizon guidance law (RHG) in terms of the terminal MD, the lateral MD, and the missile acceleration.

Evaluation of the color reproducibility of all-ceramic restorations fabricated by the digital veneering method

PURPOSE The objective of this study was to evaluate the clinical acceptability of all-ceramic crowns fabricated by the digital veneering method vis-à-vis the traditional method. MATERIALS AND METHODS Zirconia specimens manufactures by two different manufacturing method, conventional vs digital veneering, with three different thickness (0.3 mm, 0.5 mm, 0.7 mm) were prepared for analysis. Color measurement was performed using a spectrophotometer for the prepared specimens. The differences in shade in relation to the build-up method were calculated by quantifying ΔE* (mean color difference), with the use of color difference equations representing the distance from the measured values L*, a*, and b*, to the three-dimensional space of two colors. Two-way analysis of variance (ANOVA) combined with a Tukey multiple-range test was used to analyze the data (α=0.05). RESULTS In comparing means and standard deviations of L*, a*, and b* color values there was no significant difference by the manufacturing method and zirconia core thickness according to a two-way ANOVA. The color differences between two manufacturing methods were in a clinically acceptable range less than or equal to 3.7 in all the specimens. CONCLUSION Based on the results of this study, a carefully consideration is necessary while selecting upper porcelain materials, even if it is performed on a small scale. However, because the color reproducibility of the digital veneering system was within the clinically acceptable range when comparing with conventional layering system, it was possible to estimate the possibility of successful aesthetic prostheses in the latest technology.

Influence of various gypsum materials on precision of fit of CAD/CAM-fabricated zirconia copings

The clinical applicability of CAD/CAM-fabricated zirconia copings is tested using working models made from four different highstrength Type IV gypsum materials. Each of the four materials was used to fabricate 15 zirconia copings. Precision of fit was measured with a digital electron microscope using the silicone replica technique. The mean and standard deviation of each reference point were analyzed using the one-way analysis of the variance (ANOVA) and Tukeys honest significant difference (HSD) tests (α=0.05). The overall marginal and internal fits of the zirconia copings were as follows: GS (GS: Grey Stone) group: 91.43 μm, LS (LS: Light green Stone) Group: 87.89 μm, RS (RS: Red Stone) Group: 88.75 μm, BS (BS: Beige Stone) Group: 82.78 μm. There were no significant differences between the mean adaptations of the gypsum varieties (p>0.05). This confirmed that the type of gypsum material used does not determine the precision of fit of a prosthesis.

Receding horizon guidance laws with the current target information

Novel receding horizon guidance laws (RHG) are proposed for missiles, which are appropriate when the time-to-go is inaccurate but the current target information is known (K.B. Kim et al., 2000). In the presence of arbitrary large target maneuvers and initial relative vertical separation rates between the target and missile, the proposed RHG is shown to intercept the target by keeping the relative vertical separation less than the given value, within which the warhead of the missile is detonated from the appropriately selected time. Through simulation examples, the effectiveness of the proposed RHGs is illustrated in terms of the relative vertical separation, the missile acceleration, and the terminal miss distance.

Stability of intervalwise receding horizon controls for linear continuous time-varying systems

In this paper, intervalwise receding horizon controls (IRHC) are proposed for linear continuous time-varying systems. Terminal inequality conditions are suggested for those systems under which the closed-loop stability is guaranteed with the proposed IRHC. The results in this paper are also applicable to periodic and time-invariant systems which belong to the class of time-varying systems. In addition, the previous results of pointwise receding horizon controls (PRHC) for linear systems and intervalwise receding horizon controls for linear periodic systems are handled as special cases of the results in this paper.