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PubMed1988

Possible methylation of inorganic mercury by silicones in the environment.

Organosiloxanes (silicones) are being released to the environment in increasing quantities, and there are questions concerning the reactivity of silicones as methylating agents under environmental conditions. Specifically, the interaction of inorganic mercury and polydimethylsiloxanes, and other silicones, to give methylmercury is important in this content. To this end a series of laboratory experiments, in which inorganic mercury, short and long chain silicones, cyclic siloxanes, oxidized silicone gums, and industrial formulations containing silicones, were conducted. In all cases, there was evidence of the production of methylmercury, a finding of potential significance. Analyses for methylmercury and related silicon-containing degradation products were conducted using a combination of gas chromatography, mass spectrometry, and head-space gas analysis.

Environmental Pollution↗
PubMedDate not supplied

Effects of local cooling rate and processing variables on leucite in dental porcelain.

PURPOSE: This research determined whether there is a measurable effect of local geometry factors on leucite content of dental porcelain in fixed partial dentures (FPD). MATERIALS AND METHODS: Four-unit FPD frameworks (n = 36) were fabricated using a nickel-chromium alloy (Rexillium III). Body porcelain (Crystar, shade A2) was applied in one increment and subjected to two simulated body firings, followed by a simulated glaze firing to achieve a thickness of 1.5 to 2.0 mm. The completed FPD specimens were randomly assigned to three groups of 12 specimens each: (1) simulated post-soldering, (2) multiple firing, and (3) control. The FPDs from each test group were sectioned into individual units: canine retainer, premolar pontic, molar pontic, and molar retainer. The porcelain was removed from each unit, and the leucite content was measured via quantitative x-ray diffraction. RESULTS: Porcelain cracking indicated that the soldering simulation had successfully reproduced conditions in the dental laboratory that result in porcelain cracking during soldering. The leucite content was not significantly different between the retainer and pontic units for either the soldering simulation or control FPDs, although the canine retainer units did have a slightly lower leucite content than the pooled values of the other units. Comparison of the pooled data for the three groups indicated statistically significant differences among the leucite contents. CONCLUSION: Compared to the control, the simulated post-soldering procedure produced a significant increase in leucite, and the multiple firing group exhibited a significant decrease in leucite. Increases in leucite weight fraction during post-soldering operations-and the larger thermally induced stresses that accompany these leucite increases-are responsible for the cracking that occurs.

Aluminum Silicates↗
PubMedDate not supplied

Bond strength of self-etching primers to enamel and dentin of primary teeth.

PURPOSE: The objective of this in vitro study was to evaluate the tensile bond strength of three self-etching primers to human primary enamel and dentin. METHODS: Forty (40) freshly extracted primary molars were sectioned bucco-lingually and embedded in self-curing acrylic resin with the facial or lingual surfaces exposed. The specimens were wet ground to 600 grit SiC paper to expose a flat enamel or dentin surface. The materials tested were: Prompt L-Pop (LP2, ESPE), Clearfil SE Bond (CSE, Kuraray America), Etch and Prime 3.0 (EP, Degussa) anda control, Prime and Bond NT (NT, Dentsply/Caulk). The adhesive systems were applied according to the manufacturers' instructions. An inverted, truncated cone of composite (Pertac II, ESPE) with a 2-mm bonding diameter was cured using a polytetrafluoroethylene jig. The specimens were debonded in tension using a universal testing machine (Instron) at a crosshead speed of 0.5 mm/min. RESULTS: Analysis of variance (ANOVA) showed significant differences existed between the four systems and two surfaces. To enamel of primary teeth, Prime and Bond NT had significantly higher bond strength (25.9 MPa) than when bonding with the three acidic primers Prompt L-Pop (18.5 MPa), Etch and Prime 3.0 (19.3 MPa) and Clearfil SE Bond (18.7 MPa). Complete bond failures occurred with Prompt L-Pop and Etch and Prime 3.0 to dentin of primary teeth. With a mean of 39 Mpa, the bond strength to dentin of primary teeth with Clearfil SE Bond was significantly higher than with Prime and Bond NT (12.5 MPa). CONCLUSIONS: The results of this in vitro study show that the four adhesive systems tested bonded effectively to enamel of primary teeth, but only CSE achieved adequate bond strengths to dentin of primary teeth.

Acid Etching, Dental↗
PubMedDate not supplied

Curing light intensity effects on wear resistance of two resin composites.

This in vitro study evaluated the wear resistance of resin composite polymerized using four different light-curing systems. For this, a well-defined cylindrical cavity preparation (4.0 mm in diameter x 3.0 mm in depth) was made in a ceramic block (n=4 per material/light condition). Uncured material, either a universal hybrid composite (Herculite XRV) or a flowable hybrid composite (Revolution Formula 2), was packed and light-cured from the top surface only with one of the four light-curing units: 1) a conventional quartz-tungsten-halogen light, 2) a soft-start light, 3) an argon-ion laser or 4) a plasma-arc curing light. After storing the specimens in deionized water at 37 degrees C for 24 hours, the excess cured material was ground through successive grits up to a final 1200-grit SiC abrasive. The specimens were placed in deionized water at 37 degrees C for an additional 24 hours. Wear simulation was performed using a four-station Leinfelder-type three-body wear device. A slurry of water and unplasticized polymethylmethacrylate beads, simulating an artificial food bolus, was placed on the surface of each resin-composite-restored ceramic block. The entire cycling procedure was carried out 400,000 times. Impressions of each resin composite surface were taken with polyvinylsiloxane and epoxy replicas were made. Wear analyses were conducted by generating tracings across the worn surface of epoxy replicas using profilometer scans. For the universal hybrid composite and the flowable hybrid composite, the lowest wear occurred in specimens that were cured using the conventional quartz-tungsten-halogen light, and the highest wear was detected on those specimens made using the argon-ion laser. For both resin composites, the mean wear for specimens cured using the argon-ion laser was significantly higher than that of the specimens cured with the three other lights, which were statistically similar.

Analysis of Variance↗
PubMed2002

Evaluation of thermal cycling and mechanical loading on bond strength of a self-etching primer system to dentin.

OBJECTIVE: The aim of this study was to evaluate the in vitro durability and fracture modes of the tooth-resin interface following thermal cycling, fatigue load cycling (FLC) and micro-tensile bond strength testing (MTBS). METHODS: Twenty-four human molars were divided into two groups. In group A, coronal dentin was ground flat with #600-grit SiC paper, Clearfil Linerbond.2V (LB.2V) adhesive was applied and a crown fabricated with Clearfil AP-X resin composite. Samples were loaded in the FLC simulator and thermal cycle device. In group B, Class I cavities were prepared with a diamond bur and the cavity restored with LB.2V/AP-X. Samples were simultaneously subjected to FLC and thermal cycling. After storage of the specimens in water for 1 week, MTBS tests were performed, and fracture modes examined by SEM. RESULTS: For group A, MTBS were approximately 40 MPa with cohesive failures in the bonding resin of all specimens. Neither thermal nor mechanical stressing altered bond strengths on flat dentin surfaces. In group B, MTBS in the unstressed control group was 21 MPa with cohesive failures in the resin. As both thermal and mechanical stresses increased, there was a significant decrease in bond strength (16 MPa, P<0.05) and the fractures were seen in the resin-dentin interface and hybrid layer. SIGNIFICANCE: Surface preparation, C-factor, cavity depth, the dentin substrate and character of the smear layer strongly influence the bond strength values after thermal and fatigue loading. The FLC simulator in combination with MTBS testing provides meaningful in vitro evaluation of dentin bonding durability.

Acid Etching, Dental↗
PubMedDate not supplied

Durability of enamel bond strength of simplified bonding systems.

The purpose of this study was to investigate the influence of thermal cycling on enamel bond strength of 2-step bonding systems. The systems used were self-etching primer systems; Imperva Fluoro Bond, Clearfil Liner Bond II, and Mac Bond II; and self-priming adhesive systems; One Step, OptiBond Solo, Prime & Bond 2.0, and Single Bond. Bovine mandibular incisors were mounted in self-curing resin and the facial surfaces were wet ground with #600 SiC paper. Enamel surfaces were treated following each manufacturer's instructions. Adhesives were applied and composites were condensed into a Teflon mold (phi 4 x 2 mm) and light cured. Bonded specimens were divided into four groups; stored in 37 degrees C water for 24 hours, followed by thermal cycling between 5 degrees C and 60 degrees C for 3,000, 10,000, and 30,000 cycles. Ten samples per test group were tested in shear mode at a crosshead speed of 1.0 mm/min. One-way ANOVAs followed by Duncan's multiple range test (p < 0.05) were done. For self-etching primer systems, a significant decrease in bond strength was observed for the thermal cycling groups. For the self-priming adhesive systems, a small decrease in bond strength was observed but no significant differences were found for thermal cycling groups. The changes in bond strength after thermal cycling were different between the bonding systems.

Acid Etching, Dental↗
PubMed1997

The swelling phenomenon of acrylic resin polymer teeth at the interface with denture base polymers.

PURPOSE: This study examined the interface between acrylic resin polymer teeth and denture base polymers. MATERIAL AND METHODS: An autopolymerized denture base polymer was cured either at 30 degrees C, 50 degrees C, or 70 degrees C, and a heat-cured denture base polymer was cured at 100 degrees C in contact with acrylic resin polymer teeth. The specimens were ground wet and polished to a thickness of 0.21 mm for the examination of the interface with a light microscope. The surface of the specimens was then treated with the solvent tetrahydrofuran, and the specimens were examined with a scanning electron microscope (SEM). RESULTS: As a result of grinding, the inner parts of the beads at the interface of the teeth were exposed to the denture base polymer. The inner part in these polymer beads of the polymer teeth swelled to a depth of 3 microns when the autopolymerized polymer was cured at 30 degrees C. When cured at 70 degrees C, the thickness of the swelled layer was 21 microns (p < 0.001). Microscopically, the interface between the heat-cured denture base polymer and the polymer teeth appeared diffuse in the region of the interpenetrating polymer networks (IPN) and the matrix. The IPN appeared as a separate zone in the outer parts of the beads of the teeth, both with the light microscope and with the SEM. CONCLUSIONS: The results suggest that by increasing the polymerization temperature, the monomers of the denture base polymers diffused more effectively into acrylic resin polymer teeth. This increases the bond strength between the polymer teeth and the denture base polymer.

Acrylic Resins↗
PubMed2002

Durability of the dual-cure resin cement/ceramic bond with different curing strategies.

PURPOSE: To evaluate the effects of different curing strategies on the durability of the dual-cure resin cement/ceramic bond. MATERIALS AND METHODS: Machinable ceramic blanks were cut into pairs of 3-mm-thick slices, which were then polished using wet 600-grit SiC paper. The slices were silanated using one of two ceramic priming systems: (1) Tokuso Ceramic Primer (TCP), and (2) K-etchant/Clearfil Liner Bond 2V Primer (LB2V)/Porcelain Bond Activator (PBA), and bonded with one of two dual-cure resin cements (Bistite II, Panavia F), to make four experimental groups. Each group was subjected to one of three curing strategies: (1) no light, (2) 20 s light exposure from one direction, and (3) 20 s light exposure from each of six directions. After 24 h water storage at 37 degrees C, 0.7-mm-thick slabs were produced by serially sectioning perpendicular to the bonded interface. Immediately thereafter, and after one and six weeks of water storage, two slabs were randomly selected and sliced into beams for the microtensile bond strength (microTBS) test. Data were evaluated using Kruskal-Wallis and Wilcoxon rank tests (p < 0.05), and failure modes determined using a laser-scanning confocal microscope. RESULTS: After priming with TCP, microTBS of Bistite II significantly increased over time when exposed to light, whereas the microTBS of the no-light group significantly decreased over time (p < 0.05). After priming with TCP, microTBS of Panavia F increased over time, and after 6 weeks water storage, there were no significant differences in microTBS between the no-light and light-exposed groups (p > 0.05). Increases in microTBS were associated with increases in the number of cohesive failures in resin cement. After phosphoric acid treatment, priming with LB2V/PBA, and light exposure, microTBS of Bistite II remained stable, whereas that of Panavia F significantly reduced over time (p < 0.05). The microTBS of no-light LB2V/PBA groups reduced significantly over time (p < 0.05). CONCLUSION: The durability of the dual-cure resin cement/ceramic bond depends upon the multicomponent ceramic primer and the amount of light received by the resin cement.

Acid Etching, Dental↗
PubMed2004

Comparison of in vitro tensile bond strengths of luting cements to metallic and tooth-colored posts.

The aim of this study was to compare the tensile bond strength of tooth-colored ceramic and carbon-fiber post materials as well as titanium and stainless steel post materials luted with three different kinds of luting cements. Disks of alloy post materials were polished with 600-grit SiC paper, air abraded, and ultrasonically cleaned. Ceramic surfaces were pretreated with hydrofluoric acid and silanized. Panavia F, Vitremer, and ProTec Cem cements were bonded to the post specimens and placed in a humidor for 24 hours. Specimens were placed in a jig, and the debonding values were obtained using a universal testing machine. Means and standard deviations were analyzed by two-way ANOVA. Panavia F provided the highest bond strengths for all types of post materials. ProTec Cem bonded more strongly to stainless steel and titanium than to zirconium oxide. Vitremer results were the lowest. Bonds to carbon-fiber post materials were weaker than to metallic post materials, but stronger than to zirconium oxide. In general, higher bond strengths resulted in a higher percentage of cohesive failures.

Acid Etching, Dental↗
PubMed1978

An anatomical study of the microcirculation in the rabbit femur.

A technique is described which will allow a study of the microcirculation to a long bone such as the femur. It involves use of a silicone rubber compound (Microfil) in combination with the Spalteholz clearing technique. Preliminary results in the normal rabbit skeleton suggest that this technique can be used to study the changes in vascularity which occur under various pathological conditions.

Angiography↗
PubMed1995

Failure of 29Si NMR to detect increased blood silicon levels in silicone gel breast implant recipients.

We have compared directly the results of atomic absorption (AA) spectroscopy and a 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) technique reported in the literature by Garrido et al. (Garrido, L.; et al. Magn. Reson. Med. 1994, 31, 328-330) for analyzing blood silicon levels in control patients versus patients with silicone gel breast implants. AA spectroscopy yielded blood silicon levels in the nanogram per milliliter range for control patients, while somewhat higher values were found in patients with implants. The 29Si MAS NMR technique applied to the identical blood samples was unable to detect silicon in any of the samples. Sensitivity calculations demonstrate that 29Si MAS NMR should not be expected to detect silicon at the levels determined by AA spectroscopy under the spectroscopic conditions employed and that the concentration of silicon-containing compounds would need to 10(4) times the level detected by AA in order to be detected by this NMR method.

Breast Implants↗
PubMed1997

Cellular immune reactivities in women with silicone breast implants: a preliminary investigation.

BACKGROUND: Surgical implantation of silicone breast prostheses has been conducted and considered safe for over 30 years. Some implant recipients, however, complain of a group of symptoms similar to those observed in connective tissue disorders, rheumatoid arthritis, systemic lupus erythematosus, or polymyositis. To date, immunologic sequelae have not been confirmed and remain controversial. OBJECTIVE: To examine an autoimmune-like basis for the "silicone associated disease" reported by some women with silicone breast prostheses. METHODS: Proliferative responses of peripheral blood mononuclear cells against a panel of control and connective tissue proteins and to compounds common to silicone prostheses were measured in 26 women who received silicone breast implants (with implants in place an average of 166.4 [standard deviation (SD) 58.3] months), and 23 age-matched and sex-matched healthy controls. RESULTS: The frequency and intensity of cellular immune responses against collagen I, collagen III, fibrinogen, and fibronectin were significantly increased in silicone breast implant recipients versus controls. In implant subjects, the highest frequency of immune reactivity was directed against collagen I (11/26, 42%) with collagen III being the most immunostimulatory self-antigen with a mean stimulation index (SI) of 8.2 [95% confidence interval (95% CI) 3.2]. In addition, 10/26 (39%) of the implant recipients responded to more than one of the connective tissue antigens versus 0/23 (0%, P = .0007) healthy controls. Immunologic reactivities to other antigens, including silicone-based compounds, were remarkably similar. CONCLUSIONS: The identification of self-reactivity towards these connective tissue antigens may provide important information for attempts at associating silicone breast implants with disease.

Adult↗
PubMed2005

Platelet adhesion and protein adsorption on silicone rubber surface by ozone-induced grafted polymerization with carboxybetaine monomer.

Platelet adhesion and protein adsorption on the silicone rubber film grafted with N,N'-dimethyl-N-methacryloyloxyethyl-N-(2-carboxyethyl) ammonium (DMMCA) was studied. The grafting was carried out by means of ozone-induced method and was confirmed by ATR-FTIR and XPS investigations. The grafted films possessed relatively hydrophilic surface revealed by contact angle measurement. The blood compatibility of the grafted film was evaluated in vitro by platelet adhesion in platelet-rich plasma (PRP) and protein absorption in bovine fibrinogen (BFG) using silicone film as the reference. No substantial platelet adhesion was observed for the grafted films incubated in PRP for 60 and 180 min. The protein absorption was also significantly reduced after incubated in bovine fibrinogen for 60 min. Both the results indicated that the blood compatibility of silicone rubber was greatly improved by ozone-induced grafting of carboxybetaine zwitterionic polymer onto its surface.

Animals↗
PubMed1996

Microcracks in dental porcelain and their behavior during multiple firing.

Dental porcelains rely on the high-thermal-expansion mineral leucite to elevate their bulk thermal expansion to levels compatible with dental PFM alloys. The microcracks that form around these leucite particles when cooled during porcelain manufacture are a potential source of change in bulk porcelain thermal expansion during fabrication of porcelain-fused-to-metal crowns and bridges. The purpose of the present study was to determine whether multiple firings of commercial dental porcelains could produce changes in microcrack density. Specimens of six commercial porcelains and the "Component No. 1" of the Weinstein patent were fabricated and subjected to 1, 2, 4, 8, and 16 firings. The microcrack densities were determined by quantitative stereology, whereby intersections of microcracks were counted with a test grid. The microcrack data were subjected to linear regression analysis and analysis of variance. The microcrack densities of four of the six porcelains and the Component No. 1 frit were not significantly affected by the number of firings (p > 0.05). One porcelain exhibited a weak but highly significant positive correlation between microcrack density and multiple firings (r2 = 0.24, p = 0.0003), while the remaining porcelain exhibited a weak but statistically significant negative correlation between microcrack density and multiple firings (r2 = 0.15, p = 0.006). The results of this study indicate that even for porcelains that exhibit a measurable change in microcrack density as a function of multiple firings, the magnitude of the increase or decrease in microcrack density after several firings is sufficiently small to cause only negligible shifts in porcelain bulk thermal expansion.

Aluminum Silicates↗
PubMed1995

Novel assay system favorable for the study of cell-to-cell transmission of HIV-1 and its application to the evaluation of anti-HIV drugs.

The cell-to-cell transmission of human immunodeficiency virus type 1 (HIV-1) was studied using MOLT-4 cells chronically infected with a variant strain of HIV-1SF-2 (MOLT-4/HIV-1SF-2H) and CD4+ human lymphoid MT-4 cells. MOLT-4/HIV-1SF-2H cells produced less than 1 TCID50 infectious particles per day as determined by the cytopathogenicity in MT-4 cells. However, the expression of envelope glycoproteins gp120 and gp41 on the MOLT-4/HIV-1SF-2H cell membrane was satisfactory for syncytium formation with the uninfected MOLT-4 cells. When MOLT-4/HIV-1SF-2H and MT-4 cells were co-cultured, severe cytopathogenicity was observed in MT-4 cells without being accompanied by the formation of multi-nucleated cells. Thus, the system consisting of MOLT-4/HIV-1SF-2H and MT-4 cells is convenient for exclusive study of the mechanism of cell-to-cell transmission of HIV-1. Using various compounds, it was confirmed that cell-to-cell transmission required both gp120/gp41-CD4 binding and de novo DNA synthesis.

Acquired Immunodeficiency Syndrome↗