Gordon C. Hall

Dental soft polymers: plasticizer composition and leachability

Abstract The objective of this investigation was to identify the type and quantify the leachability of plasticizer constituents from dental soft polymer gel systems. The clinical implications are that some plasticizers are known to be potentially toxic and in addition, loss of plasticizer results in changes in physical properties of dental soft polymers. A total of 7 different constitutents were quantitatively identified in the 8 solutions analysed by GC and GC-MS. Specimens of gel material (plasticized polymer) were evaluated for loss of plasticizer while subjected to both static and cyclic tests (1 Hz, 5 min, ± 12 N) in H 2 O at 37°C. With one exception, cycled specimens did not yield greater extraction of ester. The use of 1- 14 C EtOH indicated complete diffusion of ethanol from the gel within 24 h. The data indicate that a typical denture soft lining would leach between 10–40 times the level of ester that would normally be obtained from food and the environment, but would leach about one-tenth of the most stringent ADI value.

Chemical and Molecular Weight Analyses of Prosthodontic Soft Polymers

A number of studies have been made investigating the properties of dental prosthodontic soft polymer materials. In such materials, which are used as short-term denture soft liners, the polymer component is complexed with one or more plasticizers. This lowers the glass transition temperature, Tg, of the polymer by allowing greater chain mobility and produces a more flexible material. Little information is available relating to the polymer components of such soft-lining materials. The chemical composition and molecular-weight distribution of a polymer help to determine its ability to form a plasticized soft polymer-gel. In the present work, both pyrolysis depolymerization/gas chromatography (GC) and size exclusion chromatography (SEC) have been used to characterize 11 commercial polymer powders. By use of SEC and polymer standards having a narrow molecular-weight range, it has been possible for accurate molecular-weight distributions to be determined for all 11 powders. The peak molecular weights were found to be between 1.79 x 105 and 3.28 x 105. These values are above the molecular weight at which chain entanglement occurs. The polydispersity ratios (Mw/Mn) were between 2.09 and 4.48. Methacrylate polymers can be readily decomposed thermally to their constituent monomers, which, once recovered, can be analyzed by gas chromatography. The simple pyrolysis apparatus used in the present study was easily assembled and was capable of producing both qualitative and quantitative results. None of the commercial polymers analyzed gave methacrylate decomposition products with pendant chains greater than ethyl (-CH2-CH3). All of the commercial polymers successfully studied by the pyrolysis test method proved to be poly(ethyl [100-79%]/methyl [0-21%] methacrylate) polymers or co-polymers.

Crystallization of experimental bioactive glass compositions

Crystallization kinetics studies for six experimental glass formulations in the system Na2O-CaO-SiO2-P2O5 synthesized by wet chemistry were conducted by means of differential thermal analysis. These glasses had CaO/P2O5 and SiO2/ (CaO + Na2O) ratios ranging from 8.74-3.38 and 0.92-3.03, respectively. Samples of each glass (n = 30 were heated from 23 to 1250 degrees C under N2 atmosphere at heating rates ranging from 10 to 50 degrees C/min. Glass-ceramics were obtained after heat treating the initial glasses at temperatures determined from their DTA exotherms. The activation energy of crystallization for each glass composition was calculated from an expression-relating log-heating rate and the reciprocal of the exothermic peak temperature. The compositions of the six glasses were significantly different (p = 0.05). The activation energy of crystallization (Q) values ranged from 196 to 782 kJ/mole. A correlation was obtained between Q and CaO/P2O5 and between Q and the Youngs modulus (P < 0.001). Two of the six glasses exhibited bulk crystallization. X-ray diffraction studies showed that four of the six glasses exhibited different proportions of crystalline phases following heat treatment. These phases were wollastonite (CaSiO3), Na2CaSi3O9, combeite [Na4Ca3SI6O16(OH)2], and some unidentifiable phases. Two of the six bioceramic materials had a mixture of unknown crystalline phases.

Materials Science The Response of Dental Amalgam to Dynamic Loading

A conventional (γ 2-containing) dental amalgam was fatigue-tested at 1800 and 80 cycleslmin, employing uni-axial, sinusoidal loading, with R = -8. Compressive, tensile, and creep tests were conducted to characterize the alloys static mechanical behavior. Tests were performed at 37°C on specimens which were aged for seven days, at 37°C. Fatigue-tested specimens were microscopically examined for fracture surface appearance and crack path. The amalgam demonstrated a frequency dependence and a significant reduction in fracture strength due to fatigue loading. The fatigue crack path was primarily intergranular in the γ 1 phase and inclined at approximately 45° to the principal stress axis. These observations are characteristic of some metals when subjected to low-frequency, elevated temperature testing where significant grain boundary sliding occurs, and therefore suggest a creep-fatigue interaction for this alloy.A conventional (gamma 2-containing) dental amalgam was fatigue-tested at 1800 and 80 cycles/min, employing uni-axial, sinusoidal loading, with R = -8. Compressive, tensile, and creep tests were conducted to characterize the alloys static mechanical behavior. Tests were performed at 37 degrees C on specimens which were aged for seven days, at 37 degrees C. Fatigue-tested specimens were microscopically examined for fracture surface appearance and crack path. The amalgam demonstrated a frequency dependence and a significant reduction in fracture strength due to fatigue loading. The fatigue crack path was primarily intergranular in the gamma 1 phase and inclined at approximately 45 degrees to the principal stress axis. These observations are characteristic of some metals when subjected to low-frequency, elevated temperature testing where significant grain boundary sliding occurs, and therefore suggest a creep-fatigue interaction for this alloy.

Gelled Calcium Polyphosphate Matrices Delay Antibiotic Release

Introducing a gelling step during antibiotic incorporation has previously been found to delay vancomycin delivery from a calcium polyphosphate matrix intended for local treatment of bone infections. This study examined the general applicability of this approach using cefuroxime, a lower-molecular-weight antibiotic with different charge characteristics compared with those of vancomycin. A calcium polyphosphate/cefuroxime paste was “gelled” in disk form in a humid environment for 5 or 24 hours prior to drying. Antibiotic release in Tris-buffered saline under gentle agitation was monitored over a seven-day period. While non-gelled samples clearly exhibited a burst release, the gelling process significantly retarded early antibiotic release from five- and 24-hour gelled matrices, yielding a constant release rate over the first four days. Cefuroxime incorporation did not appear to alter matrix structure or degradation. Overall, this non-aggressive process effectively trapped cefuroxime and reduced its release rate, suggesting its potential applicability with molecularly diverse therapeutic agents.

Crevice Corrosion Products of Dental Amalgam

The objective of this study was to determine the in vitro corrosion products that resulted from crevice corrosion of low-and high-copper dental amalgams. Specimens were potentiostatically polarized in a chloride-containing electrolyte while set against a PTFE surface to form a crevice. After 16 h, corrosion products were examined by light microscopy, SEM, EDS, and XRD. Analysis showed the presence of three previously reported products [Sn4(OH)6Cl2, SnO, and Cu2O] and a new product, CuCl, which formed on high-copper, γ2-free amalgams. Thermodynamic considerations show that CuCI is stable for the reported in vivo potentials of amalgam restorations and the high acidity and high chloride ion concentration associated with crevice corrosion.

Correlation of Dental Amalgam Crevice Corrosion with Clinical Ratings

In vitro corrosion behavior of γ2-containing and γ2-free dental amalgams was examined under crevice conditions which simulated the amalgam-tooth interface. By means of potentiostatic testing, currentdensity/time behavior and integrated anodic current were measured for 16 hr. All of the amalgams demonstrated crevice corrosion susceptibility. Crevice corrosion propagation for γ2-free vs. γ2-containing amalgams was characterized by lower acceleration and maximum rates during the most dynamic period. Integrated anodic current results were correlated with marginal breakdown ratings reported in two clinical investigations. Strong and significant coefficients were found between integrated current and marginal breakdown ratings when γ2-containing and γ 2-free amalgams were correlated as a single group, but not when correlated separately.

Corrosion and cytotoxicity evaluation of thermafil endodontic obturator carriers

The purpose of this study was to evaluate the in vitro cytotoxicity of two metallic and two plastic Thermafil carrier materials, and to separately evaluate the in vitro corrosion behavior of the two metallic carriers. Stainless-steel, Ti-6AI-4V, Polysulfone, and Vectra carriers were evaluated for cytotoxicity using the ISO-recommended agar overlay test. The two metallic carriers were additionally corrosion tested in 0.9% NaCl solution for 174 wk, at 37 degrees C. Elemental analyses of the corrosion solutions were periodically conducted using inductively coupled plasma-mass spectroscopy. Agar overlay results showed that the four carrier materials were not cytotoxic to L929 fibroblasts. Inductively coupled plasma-mass spectroscopy results showed that the metallic materials had small mass loss rates. Scanning electron microscopy showed no evidence of pitting or crevice corrosion.

A spectrophotometric assay of trout (Salvelinus fontinalis) liver diphenyl ether hydroxylase activity

Abstract Subcellular fractions of trout ( Salvelinus fontinalis ) liver homogenate metabolized diphenyl ether mainly to the 4-hydroxy derivative, but with traces of other compounds, including the 3-hydroxy derivative and possibly the 4,4-dihydroxy derivative. An ultraviolet spectrophotometric method for the determination of 4-hydroxydiphenyl ether is described.