Effects of surface treatments on the susceptibilities of low temperature degradation by autoclaving in zirconia.

The aim of this study was to investigate the effect of surface treatments on the aging susceptibilities by autoclaving in zirconia ceramics. Four commercially available tetragonal zirconia polycrystals and one zirconia-alumina composite were tested. Disk-shaped specimens were prepared and the grain sizes were analyzed using a scanning electron microscope and image analyzer. The specimens were divided into three groups based on surface treatments including heat treatment subsequent to mirror polishing, grinding, and sandblasting. Specimens in each group were autoclaved at 134°C for 1, 3, 5, 10, and 15 h. The phases of the specimens were analyzed using an X-ray diffractometer, and the relative amount of the monoclinic phase was calculated and analyzed using Student's t-test and Newman-Keuls multiple comparisons test. Single routine autoclave treatment for sterilization did not promote the phase transformation in zirconia. The phase transformations of all specimens by autoclaving were correlated with grain size, except for the zirconia-alumina composite. Grinding or sandblasting treatments gave rise to increased formation of the monoclinic phase, especially for the zirconia-alumina composite, which showed the highest fraction for the monoclinic phase. The effects of surface treatments on the aging susceptibilities by autoclaving were different in the experimental groups. It is notable that not all zirconia ceramics show similar phase transformation by autoclaving after surface treatments.

In vitro cytocompatibility of N-acetylcysteine-supplemented dentin bonding agents.

INTRODUCTION: Cytotoxic resin components of dentin bonding agents are known to cause oxidative damage and suppress odontogenic differentiation of dental pulp cells. Because antioxidants were found to protect cells from cytotoxicity of resin monomers in previous studies, we investigated the effect of N-acetylcysteine (NAC) on cytotoxicity and anti-differentiation activity of bonding agents. METHODS: Human dental pulp cells were treated with the extracts of dentin bonding agents (Prime & Bond NT, Adper Single Bond, and Dentin Cement), and then cell viability, alkaline phosphatase (ALP) activity, and matrix mineralization were observed. To assess the effects of NAC, NAC was directly added into culture media or mixed with bonding agents before extraction. Release of NAC from bonding agents was also observed by high-performance liquid chromatography. RESULTS: NAC enhanced ALP activity and mRNA expression of dentin sialophosphoprotein gene, whereas extracts of dentin bonding agents inhibited the induction of ALP activity. When the cells were treated with extracts of the bonding agents, the NAC in the culture media reduced the cytotoxicity of the bonding agents. When NAC was incorporated into bonding agents, a protective effect was only seen for Prime & Bond NT containing more than 1% NAC. The disruption of ALP activity and matrix mineralization in pulp cells was partially reversed by NAC only in Prime & Bond NT-treated cells. High-performance liquid chromatography analysis of NAC showed that the amount of NAC effluxed from Prime & Bond NT was not greater than that effluxed from Adper Single Bond. CONCLUSIONS: NAC was useful for reversing cytotoxicity and anti-differentiation effects of Prime & Bond NT on human dental pulp cells.

Surface characteristics of orthodontic materials and their effects on adhesion of mutans streptococci.

OBJECTIVE: To test the hypothesis that there are no significant differences in the adhesion of mutans streptococci (MS) to various orthodontic materials based on their surface characteristics. MATERIALS AND METHODS: Surface roughness (SR) and surface free energy (SFE) characteristics were investigated for nine different orthodontic materials (four orthodontic adhesives, three bracket raw materials, hydroxyapatite blocks, and bovine incisors) using confocal laser scanning microscopy and sessile drop method. Each material, except the bovine incisors, was incubated with whole saliva or phosphate-buffered saline for 2 hours. Adhesion assays were performed by incubating tritium-labeled MS with each material for 3 or 6 hours. RESULTS: Orthodontic adhesives had higher SFE characteristics and lower SR than bracket materials. Orthodontic adhesives showed a higher MS retaining capacity than bracket materials, and MS adhesion to resin-modified glass ionomer and hydroxyapatite was highest. Extended incubation time increased MS adhesion, while saliva coating did not significantly influence MS adhesion. SFE, specifically its dispersive and polar components, was positively correlated with MS adhesion, irrespective of saliva coating. CONCLUSIONS: The hypothesis is rejected. This study suggests that SFE characteristics play an important role in the initial MS adhesion to orthodontic materials.

Quantitative analysis of adhesion of cariogenic streptococci to orthodontic raw materials.

INTRODUCTION: Knowledge of adhesion patterns of cariogenic streptococci to orthodontic materials can provide valuable information on the cause of enamel demineralization during orthodontic treatment. The purpose of this study was to investigate the adhesion of 2 cariogenic streptococci strains to 7 orthodontic raw materials (3 light-cured orthodontic adhesives, 3 bracket raw materials, and hydroxyapatite) with respect to bacterial species, incubation time, and saliva coating. METHODS: Each material was incubated with unstimulated whole saliva or phosphate-buffered saline solution for 2 hours. Binding assays were then performed by incubating tritium-labeled cariogenic streptococci with each raw material for 3 or 6 hours. RESULTS: The degree of adhesion varied by material type. Generally, adhesion of cariogenic streptococci was significantly higher for bonding adhesives than for bracket materials, and adhesion to resin-modified glass ionomer was the highest. A longer incubation time generally increased bacterial adhesion, whereas saliva coating did not significantly influence bacterial adhesion. CONCLUSIONS: Bonding adhesives around brackets should be removed carefully during the bonding procedure to avoid enamel decalcification.