Analysis of four dental alloys following torch/centrifugal and induction/ vacuum-pressure casting procedures.

STATEMENT OF PROBLEM: Previous studies have shown casting methodology to influence the as-cast properties of dental casting alloys. It is important to consider clinically important mechanical properties so that the influence of casting can be clarified. PURPOSE: The purpose of this study was to evaluate how torch/centrifugal and inductively cast and vacuum-pressure casting machines may affect the castability, microhardness, chemical composition, and microstructure of 2 high noble, 1 noble, and 1 base metal dental casting alloys. MATERIAL AND METHODS: Two commonly used methods for casting were selected for comparison: torch/centrifugal casting and inductively heated/ vacuum-pressure casting. One hundred and twenty castability patterns were fabricated and divided into 8 groups. Four groups were torch/centrifugally cast in Olympia (O), Jelenko O (JO), Genesis II (G), and Liberty (L) alloys. Similarly, 4 groups were cast in O, JO, G, and L by an inductively induction/vacuum-pressure casting machine. Each specimen was evaluated for casting completeness to determine a castability value, while porosity was determined by standard x-ray techniques. Each group was metallographically prepared for further evaluation that included chemical composition, Vickers microhardness, and grain analysis of microstructure. Two-way ANOVA was used to determine significant differences among the main effects. Statistically significant effects were examined further with the Tukey HSD procedure for multiple comparisons. Data obtained from the castability experiments were non-normal and the variances were unequal. They were analyzed statistically with the Kruskal-Wallis rank sum test. Significant results were further investigated statistically with the Steel-Dwass method for multiple comparisons (α=.05). RESULTS: The alloy type had a significant effect on surface microhardness (P<.001). In contrast, the technique used for casting did not affect the microhardness of the test specimen (P=.465). Similarly, the interaction between the alloy and casting technique was not significant (P=.119). A high level of castability (98.5% on average) was achieved overall. The frequency of casting failures as a function of alloy type and casting method was determined. Failure was defined as a castability index score of <100%. Three of 28 possible comparisons between alloy and casting combinations were statistically significant. The results suggested that casting technique affects the castability index of alloys. Radiographic analysis detected large porosities in regions near the edge of the castability pattern and infrequently adjacent to noncast segments. All castings acquired traces of elements found in the casting crucibles. The grain size for each dental casting alloy was generally finer for specimens produced by the induction/vacuum-pressure method. The difference was substantial for JO and L. CONCLUSIONS: This study demonstrated a relation between casting techniques and some physical properties of metal ceramic casting alloys.

Porosity of dental gypsum-bonded investments in setting and heating process.

The porosity of gypsum-bonded investments for set and heated compacts was measured and theoretically computed quantitatively, because porosity is an effective factor for determining the strength, setting/heating expansion, and permeability of compacts at casting. A helium gas pycnometer was used to measure the solid volume of fine powders, powder-water mixtures, and porous compacts. The compositions of the conventional cristobalite investment and rapid-heating type investment were estimated from the measured solid densities of the as-received powders and the set investments. The porosity and water content of the set investments were determined from the experimental data. Excess water content in the set investment was calculated in relation to the elapsed time from the start of mixing with water. The experimental porosities of the set and heated investments were about 40% for dry set >compacts and about 50% for fired compacts, which well agreed with the numerically computed estimations, respectively.

Corrosion behavior of as-received and previously cast type III gold alloy.

PURPOSE: The rationale for using gold alloys is based largely upon their alleged ability to resist corrosion, but little information is available to determine the corrosion behavior of recast alloys. This study characterized the elemental composition of as-received and recast type III gold alloy and examined the in vitro corrosion behavior in two media using a potentiodynamic polarization technique. MATERIALS AND METHODS: Seventy-eight disk-shaped specimens were prepared from a type III gold alloy under three casting protocols according to the proportion of as-received and recast gold alloy (n = 26). (1) Group as received (100% as-received metal), (2) group 50% to 50% (50% wt. new metal, 50% wt. once recast metal), and (3) group recast (100% once recast metal). The surface structures of 20 specimens from each group were examined under scanning electron microscopy, and their elemental compositions were determined using X-ray energy-dispersive spectroscopy. Further, the potentiodynamic cyclic polarization between -1000 and +1000 mV (SCE) were performed for six specimens from each casting protocol in 0.09% NaCl solution (n = 3) and Fusayama artificial saliva (n = 3) at 37 degrees C. Zero-current potential and corrosion current density were determined. The data were analyzed with 1-way ANOVA and the Ryan-Einot-Gabriel-Welsch multiple-range test t (alpha= 0.05). RESULTS: Elemental composition was significantly different among the casting groups (p < 0.001). The mean weight percentage values were 72.4 to 75.7% Au, 4.5 to 7.0% Pd, 10.7 to 11.1% Ag, 7.8 to 8.4% Cu, and 1.0 to 1.4% Zn. The mean values for Zero-current potential and corrosion current density for all casting protocols were not significant (p > 0.05); however, the difference between the electrolytes was significant (p < 0.001). Fusayama artificial saliva seemed to offer the most corrosive environment. CONCLUSIONS: Type III gold alloy in any casting protocol retained passivity under electrochemical conditions similar to the oral environment. Moreover, high-gold type III alloys from reputable manufacturers and recasting protocol tested should produce acceptable corrosion-resistant castings.

Surface and elemental alterations of dental alloys induced by electro discharge machining (EDM).

OBJECTIVES: To evaluate the surface and elemental alterations induced by electro discharge machining (EDM) on the surface of dental cast alloys used for the fabrication of implant retained meso- and super-structures. METHODS: A completed cast model of an arch that received dental implants was used for the preparation of six wax patterns which were divided into three groups (Au, Co and Ti). The wax patterns of the Au and Co groups were invested with conventional phosphate-bonded silica-based investment material and the Ti group with magnesia-based investment material. The investment rings of the Au and Co groups were cast with an Au-Ag alloy (Stabilor G) and a Co-Cr base alloy (Okta C), respectively, while the investment rings of group Ti were cast with cp Ti (Biotan). One casting of each group was subjected to electro discharge machining (EDM); the other was conventionally ground and polished. The surface morphology and the elemental compositions of conventionally and EDM-finished surfaces were studied by SEM/X-ray EDS analysis. Six spectra were collected from each surface employing the area scan mode and the mean value of each element between conventionally and EDM-finished surfaces was statistically analyzed by t-test (a=0.05). Then the specimens of each group were cut perpendicular to their longitudinal axis and after metallographic grinding and polishing the cross-sections studied under the SEM. RESULTS: The EDM surfaces showed a significant increase in C due to the decomposition of the dielectric fluid during spark erosion. Moreover, a significant Cu uptake was noted on these surfaces from the decomposition of the Cu electrodes used for EDM. Cross-sectional analysis showed that all alloys developed a superficial zone (recast layer) varying from 2 microm for Au-Ag to 10 microm for Co-Cr alloy. SIGNIFICANCE: The elemental composition of dental alloy surfaces is significantly altered after EDM treatment.

Age hardening by dendrite growth in a low-gold dental casting alloy.

Commercial low-gold dental casting alloy composed of Ag-Pd-In-Au-Zn was studied to clarify the age-hardening mechanism and related microstructural changes. The hardness of solution-treated specimen began to increase and reached the maximum value with ageing time, and then the maximum hardness value decreased by further ageing. The changes of X-ray diffraction (XRD) pattern during isothermal ageing revealed that the age hardening was not caused by phase transformation. By comparing the age-hardening curve with the changes in full-width at half-maximum of the XRD peaks at each ageing time, it was revealed that the coherency strains were formed in the Ag-rich matrix, which contributed to the hardness increase during ageing. From scanning electron microscopic observation and electron probe microanalysis, it was clarified that fine particle-like structures composed of InPd containing small amount of Zn gathered by diffusion in the Ag-rich matrix, and the coherency strains which formed during that time caused the hardness increase in the early stage of age-hardening process. The coherency strains were released by the progress of coarsening of Zn-containing InPd dendrite during further ageing, which caused the overaging in the later stage of age-hardening process.

Artifacts from dental casting alloys in magnetic resonance imaging.

The potential advantage of magnetic resonance imaging (MRI) has been limited by artifacts due to the presence of metallic materials. For quantitative evaluation of the magnitude of artifacts from dental casting alloys and implant materials in MR imaging, 11 dental casting or implant materials were imaged by means of 1.5 T MRI apparatus with three different sequences. Mean and standard deviation of water signal intensity (SI) around the sample in the region of interest (1200 mm(2)) were determined, and the coefficient of variation was compared for evaluation of the homogeneity of the SI. A variety of artifacts with different magnitudes was observed. Only one of the samples, composed mainly of Pd, In, and Sb, showed no artifacts in all imaging sequences. We concluded that selection of specific dental casting alloys according to their elemental compositions could minimize the metal artifacts in MRI; however, titanium alloys currently pose a problem with respect to causing MRI artifacts.

Metal content of biopsies adjacent to dental cast alloys.

Single case reports indicate that components of dental alloys accumulate in the adjacent soft tissue of the oral cavity. However, data on a wider range of dental alloys and patient groups are scarce. Therefore, the aim of the present study was to examine the metal content of oral tissues adjacent to dental alloys showing persisting signs of inflammation or other discoloration (affected sites) and of healthy control sites with no adjacent metal restoration in 28 patients. The composition of the adjacent alloys was analyzed and compared to the alloy components in the affected sites. Tissue analysis was performed using atomic absorption spectroscopy. Alloy analysis was performed with energy-dispersive X-ray analysis. In the affected sites, the metals Ag, Au, Cu, and Pd prevailed compared to control sites, reflecting the frequency distribution of single metals in the adjacent alloys. In most cases (84%), at least one of the analyzed metals was a component of the alloy and also detected in the tissue. Metal components from almost all dental cast alloys can be detected in adjacent tissue.

Toothbrushing causes elemental release from dental casting alloys over extended intervals.

The release of elements from dental alloys has been linked to alloy biocompatibility. Much of the research measuring elemental release has been done in vitro under passive conditions. The current study supplements a previous report that measured elemental release from dental alloys during and after the equivalent of 1 week of toothbrushing. In the current study, toothbrushing times were extended to the equivalent of 2 years, and elemental release was measured during and after brushing, with and without toothpaste. The results showed that for the major classes of dental alloys, brushing alone caused no significant elemental release during the brushing, and only minor increases after brushing. Brushing with toothpaste caused significant increases in elemental release for all elements of all alloys, but the largest increases were for the two nickel-based alloys. Nickel released during brushing with toothpaste reached 600-800 microg/cm(2) of alloy surface. Both beryllium-containing and non-beryllium-containing nickel-based alloys behaved similarly, refuting claims that non-beryllium alloys are superior in this regard. Thus, brushing with toothpaste under these extended in vitro conditions appears to increase the biological liabilities from elemental release for all alloys, but primarily for nickel-based alloys.

Assessment of dental material degradation product toxicity using a bioluminescent bacterial assay.

OBJECTIVES: This study examined dental material degradation product toxicity using the Microtox bacterial bioluminescence assay as well as the effects on toxicity of selective leaching, chelation with protein, the physical form of the products, and synergistic/antagonistic interactions among released ions. METHODS: Polarization was used to produce ionically dissolved (ID) and precipitated corrosion products from Litecast B alloy specimens, which were then chemically analyzed to determine their composition and to identify metal valence states. Corrosion product toxicity, as well as that of the individual alloying elements, alone and in the presence of mucin, was analyzed using Microtox. A mathematical approach identified synergistic/antagonistic interactions and determined element contribution to product toxicity. The mechanism by which the Microtox test bacterium interacts with solid products was explored. The toxicity of methyl methacrylate (MMA) monomer was also examined. RESULTS: Precipitated corrosion products were found to be more toxic than ID products. The metals in the precipitate have been shown to be available to the test bacterium. Be and Ni were the most toxic elements in the products and contributed significantly to their toxicity. Synergistic and slightly antagonistic interactions were observed in the ID and precipitated products, respectively. Mucin decreased toxicity of all elements except Be. MMA monomer toxicity was found to be low compared to metal toxicity. SIGNIFICANCE: Microtox is useful for evaluating dental degradation product biocompatibility and has significant promise for use in other types of studies, such as determining the effectiveness of antimicrobial treatments.

Inductively coupled plasma-mass spectroscopy measurements of elemental release from 2 high-palladium dental casting alloys into a corrosion testing medium.

STATEMENT OF PROBLEM: The biocompatibility of high-palladium alloy restorations has been of some concern due to the release of palladium into the oral environment and sensitivity reactions in patients. PURPOSE: This study measured the in vitro elemental release from a Pd-Cu-Ga alloy and a Pd-Ga alloy into a corrosion testing medium. MATERIAL AND METHODS: Both alloys were cast into 12-mm-diameter x 1-mm-thick disks, subjected to heat treatment that simulated porcelain firing cycles, polished to a 0.05-mm surface finish, and ultrasonically cleaned in ethanol. Two specimens of each alloy were immersed 3 times (at 7, 70, and 700 hours) in an aqueous lactic acid/NaCl solution used for in vitro corrosion testing and maintained at 37 degrees C. The specimens were removed after each immersion time, and the elemental compositions of the solutions were analyzed with inductively coupled plasma-mass spectroscopy (ICP-MS). Elemental concentrations for the 2 alloys at each immersion time were compared with Student t test (alpha=.05). RESULTS: No significant differences in palladium release were found for the 7- and 70-hour solutions, but significant differences were found for the 700-hour solutions. Mean concentrations of palladium and gallium in the 700-hour solutions, expressed as mass per unit area of alloy surface, were 97 (Pd) and 46 (Ga) microg/cm(2) for the Pd-Cu-Ga alloy and 5 (Pd) and 18 (Ga) microg/cm(2) for the Pd-Ga alloy. CONCLUSION: Relative proportions of the elements in the solutions were consistent with the release of palladium and breakdown of microstructural phases found in the alloys. The results suggest that there may be a lower risk of adverse biological reactions with the Pd-Ga alloy than with the Pd-Cu-Ga alloy tested.

Evaluation of the new TAMZ titanium alloy for dental cast application.

OBJECTIVE: To reveal the potential of the new titanium alloy as dental prosthodontic materials. METHODS: Dental castings of TAMZ alloy were investigated in the casting machine specially designed for titanium. A mesh pattern was used to count the castability value. The mechanical properties were measured by means of a universal testing machine. Optical micrography was done on the exposed cross-section of TAMZ alloy casting. From the surface to the inner part the Knoop hardness in reacted layer of TAMZ alloy casting was measured. The structure and elemental analyses of the reacted layer were made by SEM and element line scanning observation. RESULTS: The castability value (Cv = 98%) and the tensile test (sigma b = 850 Mpa, sigma 0.2 = 575 Mpa, delta = 7.33%) data were collected. The castings microstructure showed main alpha phase and small beta phase. Knoop hardness in the surface reacted layer was greater than that in the inner part. From the SEM and element line scanning observation, there are three different layers in the surface reacted layer of the TAMZ alloy castings, and higher level of element of O, Al, Si and Zr were found in the reacted layer while the Si permeated deeper than others. CONCLUSION: TAMZ alloy can be accepted as a material for dental alloy in prosthodontics.

Avaliaçäo da rugosidade de fundiçäo de ligas de níquel-crômio em funçäo da liga, do revestimento, temperatura de estufagem e da respectiva temperatura de fusäo / Evaluation of the roughness of the nickel chrome binders casting

O ajuste cervical das restauraçöes fixas é o principal fator de substituiçäo deste tipo de prótese dentária. Várias säo as causas que podem determinar estes desajustes. Entre estas, dependendo da sua magnitude, podemos citar as rugosidades superficiais. O objetivo desta pesquisa foi avaliar a influência dos revestimentos fosfatados Microfine 1700 (R1) e Termocast (R2), às ligas de níquel-crômio Verabond (L1) e Durabond (L2), submetidas as temperaturas normais e de 100ºC acima da normalidade para estufagem de anéis e fusäo das ligas, na rugosidade superficial. Foram utilizados 80 corpos de prova de acrílico que foram divididos em 16 grupos, em funçäo das variáveis, com 5 repetiçöes cada. Os corpos de prova foram fundidos em centrífuga, por induçäo. Para proceder à leitura da rugosidade, foi utilizado o rugosímetro DENTAK II. Foram realizadas cinco leituras em cada superfície externa e interna dos corpos de prova, perfazendo 800 leituras. Os principais resultados indicam que as variáveis utilizadas determinam graus variáveis de rugosidade superficial. As superfícies externas e internas comportaram-se de forma semelhante. A melhor combinaçäo do revestimento/liga foi a do revestimento Microfine 1700 (R1) associado à liga Durabond (L2). As rugosidades encontradas, possivelmente näo determinariam desajustes cervicais de restauraçöes metálicas fundidas

Avaliaçäo da fundibilidade de uma liga de cobalto-cromo / Castability evaluation of a cobalt-chromium alloy

Neste trabalho propusemo-nos a valiar a fundibilidade de uma liga de cobalto-cromo (VERA-PD1) em funçäo da utilizaçäo de três revestimentos: Knebel (aglutinado por sílica), Termocast e Wirovest (aglutinados por fosfato) e duas temperaturas de aquecimento para o molde (900ºC e 950ºC). Para a execuçäo do teste foi utilizado o método descrito por HINMAN et al. (1985). O método de fundiçäo foi o de cera perdida sob chama de gás-oxigênio. Os dados obtidos foram submetidos a análise estatística e demonstraram näo haver diferença estatisticamente significante para os revestimentos Knebel e Wirovest, e diferença estatisticamente significante ao nível de 0,1 por cento para o revestimento Termocast quando da variaçäo da temperatura de aquecimento do molde...

Interfacial reactions of cast titanium with mold materials.

PURPOSE: Dental casting requires replication of complex shapes with high fidelity. To achieve this objective, the problem of scale formation on titanium dental castings must be overcome. Scaling occurs readily at high temperatures because of the high reactivity of molten titanium with investment materials. The purpose of this study was to examine the effectiveness of using stable oxide coatings on the mold surface to reduce the interfacial reactions. MATERIALS AND METHODS: A traditional phosphate-bonded dental investment, a commercial titanium investment, and an experimental oxide coating were used for the molds. Pure titanium samples were cast, divested, and prepared for scanning electron microscopic and energy dispersive x-ray spectroscopic analyses. RESULTS: Layers of 7- to 10-micron-thick scale were formed on titanium castings during reaction with traditional mold materials. Interface reaction was reduced between the molten titanium and the investment material when yttrium oxide or zirconium oxide coatings were applied to the mold before casting. CONCLUSION: Less titanium-mold interfacial reactions occurred when protective coatings were used as diffusion barriers for titanium casting. Y2O3-coating oxide particles applied without binder were entrapped in the cast titanium surface layer. Further study of a binder system for Y2O3 coating is needed.

Effect of burnout temperatures on strength of phosphate-bonded investments--Part II: Effect of metal temperature.

OBJECTIVES: It had previously been found that the strength of phosphate-bonded investments is temperature sensitive. However, while the effect of heat from the cast metal is expected to have some effect on continued reactions and melting, there is no published report dealing with this. It was the purpose of this study to consider further the effects of burnout temperature on strength and the contribution of metal casting temperature and investment composition. METHODS: The disc-rupture test (Luk, H. W.-K. and Darvell, B. W., Strength of phosphate-bonded investments at high temperature. Dental Materials 1991, 7, 99-102) was employed to determine the effect of burnout temperature (ranging from 400 to 1000 degrees C) on the 'actual' strength of six phosphate-bonded investments; the cast metal temperature was controlled to be the same as that of the mould. Atomic absorption was employed to investigate aspects of the composition of the investments. RESULTS: The 'actual' strength obtained in this test was always higher than the 'service' strength observed in similar testing with the metal at a fixed temperature (1460 degrees C). The 'actual' strength was also temperature sensitive, but with a different pattern of variation compared with the 'service' values. All investments tested were found to behave plastically at high temperatures. Calcium, sodium, zinc and iron were found in appreciable quantities in the investments. CONCLUSION: Heat from the high-temperature casting metal has a material effect in decreasing the strength of phosphate-bonded investments. Such heating increases the plastic behaviour of the investment and this, together with the casting pressure, may result in a distorted mould and thus inaccurate castings, a hitherto unrecognized source of error. 'Actual' strength gives no guide to the 'service' strength, emphasizing the need for tests under service conditions. Composition affects investment high-temperature strength substantially.

The influence of alloy composition and casting procedure upon the corrosion behaviour of dental alloys: an in vitro study.

The influence of the composition and the casting process upon the electrochemical corrosion of a selection of palladium-based alloys and non-precious metal alloys has been investigated in vitro using an electrochemical method. No influence of the casting process upon the corrosion resistance was observed with palladium-based alloys. Changes in the crystallographic structure effected by the different composition of the alloys were found, the casting process did not cause significant effects. With nickel-chromium alloys rather low corrosion stabilities were observed.

Classificaçäo das ligas para fundiçäo com alto conteúdo de cobre de uso odontológico, em conseqüência da capacidade de sofrerem branidura / Classification of high copper casting alloys in function of their ability to be burnished

Estabeleceu-se uma escala numérica para a classificaçäo de cinco ligas com alto conteúdo de cobre e uma experimental, contendo berílio, usadas em fundiçäo, em conseqüência da capacidade de sofrerem branidura. Para tanto, obteve-se o número de branidura de cada uma das ligas por meio da divisäo da dureza Brinell pela porcentagem de alongamento. Segundo este método, quanto maior o número, maior a dificuldade de se brunir a liga. Os valores foram obtidos sob duas condiçöes de tratamento durante o resfriamento, um rápido, em que, após a solidificaçäo, a liga era resfriada bruscamente em água, e um lento, em que se deixava a liga esfriar lentamente até a temperatura ambiente. Verificou-se que as ligas apresentaram números de brunidura diferentes, e que os tratamentos durante o resfriamento näo os influenciavam, com exceçäo da liga Idealloy, cujos valores säo diferentes

Characterization of noble metal implant cylinders: as-received cylinders and cast interfaces with noble metal alloys.

A common procedure in the fabrication of implant prostheses is the use of premade wrought cylinders in cast frameworks. Although manufacturers outline some precautions in the use of these components, detailed information about the metal interface between cylinders and cast alloys is lacking. This article, following a previous report that compared titanium-based implant cylinders used with two different classes of cast alloys, compares conventional noble metal cylinders from three different manufacturers combined with these two classes of cast noble alloys. Analysis of the as-received cylinders revealed that the implant cylinders as a group are predominantly composed of metals commonly found in noble dental alloys, namely, platinum, palladium, gold, and silver. The interfaces created by casting both high-fusing and low-fusing alloys around the cylinders exhibited a general elemental concentration variability compared with the bulk alloy regions, but continuous concentrations for shared elements suggested alloy-cylinder compatibility. Vickers hardness values, which ranged from 212 to 276 for the as-received cylinders, decreased from 12% to 43% for the various cylinders after casting. This study suggests characteristics of an ideal cast interface that include maintenance of the cylinder and casting alloy microstructures up to the interface, absence of interfacial reaction regions, lack of porosity created by volatilization of components from either alloy or the casting process, and sufficient strength to maintain anticipated loads.

Recycling of calcium sulphate dihydrate.

This research was carried out to prepare dental stone from the previously fabricated stone casts. The study included reheating of the used casts at different conditions in an autoclave. Results indicated that calcium sulphate hemihydrate can be reproduced using the previously fabricated stone casts.

Viscosidade e escoamento de géis fluoretados in vitro: consideraçöes sobre sua utilizaçäo em moldeiras pré-fabricadas / Fluoride gels viscosity and flowing off in vitro: considerations about their use with application trays

O objetivo do trabalho foi observar o comportamento de alguns géis fluoretados, relacionado à sua viscosidade e escoamento, quando utilizados em moldeiras pré-fabricadas. Foram analisados seis produtos comerciais disponíveis na clínica de Odontopediatria da FO/UERJ. Verificou-se que eles podem escoar totalmente pelas moldeiras sob a açäo da gravidade ou permanecer "in situ", suportando seu próprio peso. Esta condiçäo poderá servir de parâmetro, quando se eleger o produto mais conveniente para a utilizaçäo na terapia tópica fluoretada