Papers by Author: Nam Sik Oh

Abstract: Adhesion characteristics of base-coated single crystal brackets for orthodontics were investigated to evaluate the feasibility for the ceramic bracket applications. Sapphire single crystal ingot were machined, abraded and spray-coated with slurries containing 50 wt% of flux and 40µm, 80µm and 150µm of granules. Maximum adhesion to the base surface of the brackets was observed when the size of granule in the slurries was 80µm. It may be due to higher bonding area of adhesive to granule and to base as a result of interlocking structure which has moderate thickness and uniform distribution of granules. The one-way analysis of variance indicated that there were not significant differences in bond strength measurements between the once and twice coating condition in 80µm and 150µm granule groups. But ARI values exhibited large differences between once and twice coating groups and showed different median or ranges. Adhesive remnant of adhesive resin was minimized when the slurry of 80µm granule was coated once. Reduction in adhesive remnant is likely due to the higher bonding of adhesive resin to granules. No teeth and bracket damages were found in any teeth de-bonded by shear force. The base-coated brackets with higher strength (12.9 MPa) and no adhesive remnant were newly proposed for the orthodontics applications.

Abstract: In Vitro Bond strength of base-coated (Y, Fe)-TZPs (yttria and iron co-doped tetragonal zirconia polycrystals) were investigated to evaluate the feasibility for the ceramic bracket applications. Pre-sintered (Y,Fe)-TZP brackets were machined, abraded and spray-coated with granule slurries containing 10~50 wt% of flux. Better adhesion to the base surface of the brackets was observed by raising the amount of flux in the slurries. It may be because of the fusion and the collapse of the granules as a result of the high amount of flux. The one-way analysis of variance indicated that there were significant differences in bond strength measurements between the experimental groups except 30% and 50% flux content groups. The Weibull distribution revealed that 50% flux content group is the best with a high characteristic strength value. Mean ARI values exhibited large differences and ranged from 1.6 for 50% flux content group to 3.0 for non-coated group. Adhesive remnant of adhesive resin was decreased by increasing the flux up to 50%. Reduction in adhesive remnant is likely due to the higher bonding of adhesive resin to granules. No teeth and bracket damages were found in any teeth de-bonded by shear force. The base-coated brackets with higher strength (11.5 MPa) and no adhesive remnant were newly proposed for the orthodontics applications.

Abstract: Yttria and iron co-doped tetragonal zirconia polycrystals ((Y, Fe)-TZPs) were investigated to evaluate the feasibility for the ceramic bracket application. Presintered the (Y,Fe)-TZPs brackets were machined, abraded and coated with granule spray containing 10~50 wt% of flux. Finally, the base-coated brackets were sintered. Roughness of the coated surface was decreased to 13 um and 10 um when the flux content was 30 wt% and 50 wt%, respectively. It may be due to the fusion and the collapse of the granules as a result of the high amount of flux. Better adhesion to the base surface of the brackets and irregular shape were observed by raising the amount of flux in the slurries. Wetting angle of ethylene glycol droplet on the (Y,Fe)-TZP bare surface was determined to be 41.85°, however, the angle became 0° after the granule spraying treatments. The (Y,Fe)-TZP brackets exhibited uniform embossed base and good wetting. The newly developed base-coated bracket could be highly applicable to enhance retention and to reduce adhesive resin remnant during the bracket debonding.

Abstract: This study was designed to evaluate mechanical properties and the marginal fitness of glass infiltrated alumina core fabricated from aqueous alumina tape. Utilizing the automatic Dr. Blade model (DP-150 of Japan JinChungjungGi manufacturer) fabricated the slurry into 0.5 ㎜ thickness aqueous-based alumina tape. The coefficient of thermal expansion and biaxial flexural strength of alumina composite produced from alumina tape were investigated. Three upper central resin incisors were prepared with 90o, 110o, 135o shoulder margin for all-ceramic crown. Individual tooth model’s impressions were taken 15times each to make epoxy resin die. Cores were made of alumina tape to each and every dies. Crown setting was done on the epoxy resin dies. The specimens were evaluated for measuring for marginal gaps of glass infiltrated alumina core fabricated from aqueous alumina tapes under 180 magnifications with Kan Scope(Sometech Vision, Korea) was done. The retained measurements were analyzed with Turkey’s Studentized Range Test for marginal fitness of each specimen. Coefficient of thermal expansion of alumina tape was 7.5x10-6/°C, and biaxial flexural strength was observed to be 498±32MPa. The marginal fits of alumina cores made of alumina tapes showed the least marginal gap of 41.5 ㎛ in the 110o shoulder margin, and increasingly with 135o, 90o shoulder margin. Marginal fitness should be better in angles larger than 90o.

Abstract: The aim of this paper was the HA and β-TCP powers were synthesized by a new wetchemical method using eggshell and phosphoric acid. The biocompatibility of synthesized natural HA, HA/β-TCP(50:50) and β-TCP derived from eggshell was compared with those of as commercial chemical powder with mesenchymal stem cells derived from human bone marrow. Development of crystalline phases of the mixtures was studied as functions of mixing ratio and temperature using X-ray diffractometer. The morphological characteristics of the calcined eggshell and synthesized powders were examined by scaning electron microscopy. The in-vitro cytotoxicity and cell attachment of sintered disks were examined using human bone marrowderived multipotent stem cells(hBMSCs). Cell response was characterized by MTT assay , Alkaline phosphatase stain and RT-PCR analysis. Pure HA was synthesized in the mixing ratio of 1:1.1 wt% at 900°C for 1h. the crystallization of HA was started at 800°C in the 1:1.1 mixing ratio, ant the HA phase was continued up to the high temperatures. In the ratio of 1:1.3 and 1:1.5 wt%, β-TCP was effectively synthesized at 900°C. In the 1:1.5 ratio, β-TCP phase was detected at 700°C, and complete crystallized β-TCP was observed above 900°C. At the higher temperature than 1000°C, the β-TCP was gradually decreased and α-TCP was observed. The HA and β-TCP disk does not exert cytotoxic effect on the hBMSCs undergoing osteoblastic differentiation. In addition, the hBMSCs are adhered on the surface of synthesized natural HA and β-TCP disk as successfully as on the culture plate or as commercial chemical HA and β-TCP disk. The hBMSCs adhered on either synthesized natural HA, β-TCP or as commercial chemical HA, β-TCP disk displays undistinguishable actin arrangement and cellular phenotypes, indicating that synthesized natural HA, β-TCP does not disrupt normal cellular responses. Analysis of differentiation of the hBMSCs cultured on culture plate, synthesized natural HA, β-TCP and as commercial chemichal HA, β-TCP disk shows that three matrices are able to support osteoblastic differentiation of the hBMSCs as accessed by alkaline phosphatase staining.

Abstract: Feldspathic porcelain paste system using organic vehicle was prepared and applied to fabricate the porcelain fused to metal fixed partial denture. The mean linear fired shrinkage of paste porcelain system was 15.2% that is 0.8% lower than that of conventional build-up method. The color difference between two methods (􀁕E*) was 0.156. Mean biaxial flexural strength of sintered preformed laminating porcelains of this system and conventionally build-up porcelains were 74±3 and 70±4 MPa respectively. There was no deterioration of physical properties of feldspathic porcelain system by introducing organic vehicle, and this new build-up method can be applied to fabricate the porcelain fused to metal restorations.