Marginal microhardness of corroded amalgams: a comparative in vitro study

Abstract – One conventional and two high-Cu amalgams were tested for marginal microhardness after 2 months' corrosion in an 85 mM NaCl solution. Amalgams immersed in 200 mM phosphate buffer solution were used as controls. The microhardness tests were conducted on cross-sections of the amalgams 50μm from the surface edges. The microstructure of the amalgams was studied in SEM and the amounts of Sn, Cu, Zn, Ag, and Hg dissolved in the solutions were analyzed with an atomic absorption spectrophotometer. For the amalgams immersed in the NaCl solution the depth of corrosion after 2 months was between 50 and 400μm. The specimens immersed in the phosphate solution showed no signs of subsurface corrosion. The marginal microhardness of all the amalgams was reduced after corrosion in the NaCl solution. The greatest microhardness in both the uncorroded and corroded states was shown in the two high-Cu amalgams. The reduction in marginal microhardness after corrosion can probably be attributed mainly to degradation of the gamma-2 phase for the conventional amalgam and to degradation of the η' phase for the two high-Cu amalgams.     

Cytotoxicity of corroded and non-corroded dental silver amalgams

The cytotoxicity of one conventional and four non-γ₂-amalgams was studied in a cell culture system, using the Millipore filter method. Before testing set amalgam specimens were kept in distilled water or in artificial saliva at pH 4, 5 or 7 for up to 28 wk to produce a corrosion layer on the test surface. Non-corroded set amalgam specimens were also tested. None of the non-corroded, set amalgams showed any sign of surface accumulation of cytotoxic products whereas the corroded amalgams showed varying degrees of cytotoxicity. Generally, the non-γ₂-amalgams gave a more pronounced cytotoxic effect than the conventional amalgam. When the corrosion procedure was carried out at pH 7, the various non-γ₂-amalgams showed different degrees of cytotoxicity. It appears that the difference in cytotoxic effect between the non-γ₂-amalgams and the conventional amalgam as well as the differences among the various non-γ₂-amalgams could be related to variation in the retention of corrosion products deposited on the amalgam surface.     

Electrochemical properties of corroded amalgams

Three types of amalgam, one conventional, ANA 68, and two with high copper content, Dispersalloy (dispersed type) and ANA 2000 (single composition type), were investigated. The amalgams were immersed for periods of 7 wk at a time, up to 35 wk, in 23 ml (37 degrees C) of 0.9% NaCl aqueous solutions and in 0.9% NaCl solution buffered with NaH2PO4 (8.8 mM) and Na2HPO4 (1.2 mM). The amalgam specimens were embedded in epoxy resin. The surface area of amalgam exposed to the solutions was 0.2 cm2 for each specimen. Every 7 wk the corrosion potential was measured, the amalgam specimens lightly brushed with a soft toothbrush, and the solutions renewed. After 14-21 wk and 35 wk the currents during anodic polarization sweeps over the amalgams were recorded. The corrosion potential for the high-Cu amalgams was somewhat more positive (noble) in the phosphate buffered solution than in the non-buffered solution during the 35 wk of corrosion. The phosphate buffer reduced the reactivity of the amalgams during anodic polarization. Corrosion made the amalgams more passive during the anodic polarization. However, all the elements leached from the amalgams into the solutions throughout the entire experiment.     

Corrosion of dental amalgams in solutions of organic acids

A conventional and two high copper amalgams were tested in 0.5% aqueous solutions of acetic, formic, lactic and succinic acid. The corrosion behavior of the amalgams in the different solutions was evaluated by analyzing the soluble corrosion products using an atomic absorption spectrophotomeler every month during a 6-month experimental period. The high copper amalgams showed a high dissolution rate in formic and lactic acid solutions from the initial stages of immersion when compared to the conventional. Later a marked decrease of the dissolution rate could be observed but it still remained at high levels. In acetic acid the amounts of elements dissolved from high copper amalgams were much less. Conventional amalgam released much smaller amounts of elements in almost all solutions tested except in the case of silver in lactic acid. Finally, in succinic acid solution, the amounts of elements dissolved were unexpectedly small considering the low pH of the solution and the dissolution rates of the amalgams in the other organic acid solutions.     

Marginal fracture of dental amalgams

abstract – The purpose of this study was to determine the marginal fracture of different amalgams, using a semiclinical method which facilitates standardization and minimizes the observation time, in 10 patients receiving complete dentures. Cavities were cut occlusally in acrylic premolars and molars. The margin was then beveled to 45° with a cone-shaped diamond. Two dispersion-strengthened amalgams and one conventional amalgam were used in each mouth. The cavities of each quadrant were filled with the same amalgam. Thus, conventional amalgam fillings were always placed in opposing quadrants to those filled with a dispersion-strengthened amalgam. The degree of marginal fracture was evaluated from the prints (magnification × 5–7) by five dentists separately and blindly. After 6 months' service, severe marginal fracturing was frequently seen.     

Use of potentiodynamic polarization technique for corrosion testing of dental alloys

Abstract— The number and in particular the diversity in types and composition of alloys used in dentistry are increasing, thus enhancing the need for predictive corrosion testing. No corrosion test is generally accepted as being both applicable to and relevant for all dental alloys. However, some methods have gained a certain recognition, and among these are the potentiodynamic polarization techniques. The purpose of the present work was to study the application, the reproducibility and the influence of some methodological variables on the results of potentiodynamic polarization corrosion testing. Embedded specimens of 18 different dental alloys, representing all major types, were subjected to anodic polarization scan within the potential range of – 700 mV to 1000 mV (SCE). Artificial saliva, with and without organic compounds, and sodium sulfide solution were used as electrolytes. The results were processed on a microcomputer using locally developed software. The curves of current density versus potential showed fair reproducibility, with great differences between some of the alloys, showing a clear relationship to nobility or passivity of the alloy. There were marked differences between the results obtained in artificial saliva and in sodium sulfide solution, while addition of organic components to the artificial saliva only had a minor effect. The potentiodynamic polarization corrosion test supplies detailed information such as anodic charge, and open circuit, rupture, and passivation potential. Furthermore, it indicates the passive range and sensitivity to pitting corrosion. These are all useful when evaluating the corrosion properties of a dental alloy. An analysis of variance of the results in this study showed that possible differences between the alloys were clearly detectable with this method. However, for alloys releasing non-ionic products during corrosion/degradation, the method may be inappropriate. It is thus concluded that the method appears applicable as an in vitro corrosion test for most dental alloys, with the possible exception of amalgams.     

Electrochemical potentials of amalgam restorations in vivo

Abstract The potentials of 407 amalgam restorations have been determined in vivo. The measurements were performed with very high impedance equipment, and relative to a Ag/AgCl reference electrode. The readings varied from — 23 mV to —595 mV, with 90% of the readings confined to within — 127 mV to —431 mV and a mean value of —226.1 mV. Of the restorations, 394 were measured twice, and no significant difference could be found between the first and the second reading. During the study, eight new restorations were inserted. Their potentials varied from —180 mV to —565 mV, with a mean of —339.4 mV, which was significantly lower than that of the older restorations.     

Pulpal response to dental amalgams

Abstract – The pulpal reactions to amalgams with relatively high Cu content were compared with a conventional amalgam placed in unlined cavities prepared in intact monkey teeth. After a 1-week observation period the pulpal responses from one of the Cucontaining amalgams and the conventional amalgam were of the same magnitude, whereas the two other amalgams gave more pronounced reactions. After observation periods of 1 and 2 months only one o( the amalgams with high Cu content caused more pronounced reactions than the conventional amalgam.     

Accelerated corrosion analysis of dental amalgams

Abstract – The corrosion of powdered conventional and high Cu dental amalgam was studied in vitro under fixed conditions (100% oxygen, pH 4, and constant weak mechanical action). Results were retrieved from X-ray diffraction of samples of amalgam and solid corrosion products Formed, in combination with recording of the HC1 consumed to maintain the fixed pH. In the conventional amalgam no corrosion of γ₁ occurred until all γ₂ had corroded, whereas in the high Cu amalgam corrosion of γ₁ occurred from the beginning, concurrent with corrosion of ε and ń. Corrosion products found were AgCl, Hg₂Cl₂, CuCl₂-3Cu(OH)₂, and SnO₂. The results may be interpreted as follows: in the conventional amalgam the matrix phase γ₁ is anodically protected against corrosion as long as any γ₂ remians; in the high Gu amalgam the least noble phases ε and ń do not protect γ₁ in a similar way.     

Initial corrosion of amalgams in vitro

Abstract – The amounts of copper, mercury, silver or zinc released from two brands of freshly prepared, lathe cut amalgams and from one brand of dispersed phase type amalgam into artificial saliva have been measured. Samples were immersed in the solution a few minutes after the end of trituration and exposed statically for periods of up to about 4 days. The initial mercury release from such specimens could exceed the long term mercury release from old amalgams by more than two orders of magnitude. The measurements indicate that during the first day after insertion of two amalgam fillings, each with an assumed surface area of 1 cm² and under presumably static conditions, mercury at the level of more than twice the mercury food and drink intake could be released in the oral cavity. This is, however, a situation which appears infrequently, e.g. following dental treatment once a year. The amounts of copper and zinc released initially were considerably lower than the corresponding food and drink intake values, while silver might be on the same level. The measurements were conducted using nuclear tracer techniques.     

Corrosion of dental amalgams in solutions of sodium chloride, sodium sulfide and ammonia

Abstract — Specimens were prepared from three different dental amalgams and were immersed in 0.5% aqueous solutions of sodium sulfide, ammonia and sodium chloride. Every month and over a 6-month experimental period the solutions were replaced with fresh electrolyte and were analyzed in an atomic absorption spectrophotometer with respect to their content in silver, mercury, copper, tin and zinc. In sulfide solutions large amounts of tin and mercury were released from the amalgams while none of the other elements could be detected. Copper, tin and mercury were mostly dissolved in ammonia solutions. An increased silver dissolution could also be observed. Zinc was the first element to be released in sodium chloride solutions. After a 4-month immersion, considerable amounts of copper and mercury could also be found in the same solutions.     

Maximum contents of mercury in dental silver amalgams

The maximum content of mercury forming intermetallic compounds has been determined in dental amalgams prepared from alloys with different copper contents. In amalgams made from alloys with silver contents less than about 42% there is a risk of occurrence of free mercury if an effective condensation technique has not been used.     

Dimensional and phase changes of dental amalgams

Abstract The relationship of long-term expansion to phase changes of dental amalgams was examined. Using multiple regression analysis, a predictive equation was found which showed that the linear dimensional change was dependent on the alterations in the content of γ₂ and β.     

Corrosion behaviour of high copper dental amalgams

This study evaluated the corrosion behaviour of two high copper dental amalgam alloys [Dispersalloy (Dentsply-Caulk) and Tytin (Kerr)] in different electrolytes. Amalgam specimens were prepared, coupled to a copper wire, cemented into glass tubes and polished to a 600-grit finish. A corrosion cell was prepared using a carbon counter-electrode, a standard calomel electrode as the reference and amalgam as the working electrode. The alloys were tested in the following mediums at 37 degrees C: (i) artificial saliva based on Fusayama's solution (FS), (ii) artificial saliva with citric acid adjusted to pH 4.0 (FC) and (iii) 1% sodium chloride solution (SC). Corrosion potentials (E(corr)) and corrosion rates (I(corr)) were determined using potentiostatic and impedance spectroscopy methods. Data was subjected to anova/Scheffe's post hoc test at 0.05 significance level. For both alloys, the corrosion potential in FS was significantly greater than in SC. Corrosion potential of Tytin in FS and SC was also significantly greater than in FC. The corrosion rate of Dispersalloy in FC was significantly greater than in FS and SC. For Tytin, corrosion rate in SC was significantly greater than in FS and FC. Although no significant difference in corrosion potential/rate was observed between the alloys when tested in FS, significant differences were observed when electrochemical testing was carried out in FC and SC. The corrosion behaviour of high copper amalgam alloys are both material and environment dependent. Certain food substances may increase the corrosion of high copper amalgams.     

Effect of free mercury on the strengths of dental amalgams

A previous study of several different Ag?Sn, Ag?Cu, and Ag?Sn?Cu alloys revealed that some of these alloys were susceptible to mercury embrittlement. The present study was undertaken to determine the extent to which embrittlement of alloy particles in amalgam affects the strength of the material. Specimens of low copper amalgam (O), high copper admixed amalgam (D), and high copper single composition amalgams (S and T) were condensed into 2 mm×4 mm×15 mm molds using a pressure of 14 MPa. After aging the specimens for 7 days at 37°C, mercury was coated on each specimen either by electro-plating mercury or by immersion in mercury. In each method, mercury was applied for 15 s or 1 min. At 30 s after coating, each specimen was placed in a 3-point fracture fixture and immediately loaded at 0.25 and 2.54 mm/min until fracture occurred. As controls, specimens of each amalgam without mercury coating were also tested. Ten specimens per experimental condition were tested. Fractured surfaces were examined using a scanning electron microscope. Strength reductions as high as 60% for O amalgam, 44% for D amalgam, and 31% for T amalgam were found. Two observations suggest that embrittlement of unconsumed alloy particles is primarily responsible for the strength reduction: 1) the amalgams showing susceptibility to mercury embrittlement in the present study contain alloy particles of alloys that were shown to be susceptible to embrittlement in an earlier study, and 2) the amalgam S, which showed the least susceptibility to mercury embrittlement, contains unconsumed alloy particles of an alloy that showed no significant embrittlement in the earlier study. Experimental evidence supports the hypothesis that free mercury released during corrosion or aging of amalgam restorations can contribute to the weakening of these restorations.     

Corrosion behavior and microhardness of three amalgams

Abstract – The marginal microhardness of three different types of amalgam was tested after 2 months' immersion in an aqueous solution of NaCl (85 mM) and phosphates (Na₂HPO₄ 100 mM and NaH₂PO₄ 100 mM). Amalgams immersed in distilled water were used as controls. The microhardness tests were conducted at a distance of 50 μm from the margins and at the bulk of each specimen. The solutions were analyzed for Sn, Cu, Zn, Ag, and Hg by means of atomic absorption spectrophotometry (AAS). A statistically significant reduction in the marginal microhardness after immersion in the test solution was found for the conventional and the high-Cu single composition amalgam but not for the high-Cu blended amalgam. SEM-examination of cross-sections of the amalgams revealed small areas of subsurface grain boundary corrosion, no deeper than 10 μm for all the amalgams. The SEM-examination of the specimens and AAS analysis of the solutions indicated that the reduction in marginal microhardness was attributed mainly to corrosion of the Cu-rich phases for the high-Cu single composition amalgam and to corrosion of the γ₂ phase for the conventional amalgam. The phosphates reduced the corrosion of the amalgams in the presence of NaCl. It is concluded that the marginal strength of dental amalgams in a corrosive environment is largely dependent upon their corrosion resistance.     

Phases in preamalgamated silver Samalgam alloy

abstract — The phase content of six preamalgamated alloys available on the dental market has been determined by means of X-ray diffraction. The β₁-phase was found in five alloys.     

Test of long-term corrosion of dental amalgams

Abstract — Seven non-gamm-amlgams and one conventional amalgam were used in the study. Cylindrical specimens were kept in a test solution for 1 yr. The depth of corrosion was measured and leakage of copper, zinc an mercury was determined by atomic absorption spectrophotometry. EDX was used to analyze th corrosion products. The results showed that copper leakage from the non-gamma-amalgams was higher than from the conventional amalgam. The amount of copper in the solution was not related to the concentration in the alloy. Dissoltion of this ion was low during the last 6 months of the rate seemed to be less time dependent and leakage probably had not stopped after 1 yr. The depth of corrosion in hte amalgams were more corroded than the conventional amalgam.     

Impact of nocturnal bruxism on mercury uptake from dental amalgams

The mercury (Hg) release from dental amalgam fillings increases by mechanical stimulation. The aim of this study was to investigate the possible impact of nocturnal bruxism on Hg exposure from dental amalgams and to evaluate the effect of an occlusal appliance. 88 female patients from an orofacial pain clinic with a complete maxillary and mandibuiar dentition, a normal frontal vertical overbite with cuspid guidance, and at least 4 occlusal amalgam fillings in contact with antagonists in intercuspidal position, were examined with the Bruxcore bruxism monitoring device to measure the level of on-going nocturnal bruxism. Based on the degree of abrasion recorded, the subjects were divided into a group denned as bruxists, ( n = 29), another group defined as non-bruxists, ( n = 32), serving as controls, the intermediate group being discarded. The Hg exposure was assessed from the Hg concentration in plasma and urine, corrected for the creatinine content. In a regression model with bruxism as the only explanatory variable, no significant effect of bruxism was found, but when the number of amalgam fillings, chewing gum use, and other background variables were taken into account, there was a limited impact of bruxism on Hg in plasma. The nocturnal use of an occlusal appliance did not, however, significantly change the Hg levels. This study indicates that mechanical wear on amalgams from nocturnal bruxism may increase the Hg uptake, but the magnitude of this effect seems to be less than from the use of chewing gum.     

Corrosion current and pH rise around titanium coupled to dental alloys

Abstract – Corrosion reactions around titanium, usually considered biologically inert, might be provoked by coupling it galvanically with more corrodible dental alloys. Experiments in vitro simulating the conditions of a titanium dental implant or root canal post coupled to an amalgam filling, demonstrated corrosion current densities up to 31 μA/cm² anodic pH values around the amalgam down to 2, and cathodic pH values around the titanium up to 10. The amounts of tin released by the enhanced corrosion, of amalgam might contribute measurably to the daily intake of this element; the corrosion current generated reached values known to cause taste sensations. If the buffer systems of adjacent tissues in vivo are not able to cope with the high pH generated around the titanium, local tissue damage may ensue; this relationship is liable to be overlooked, as it leaves no evidence in the form of corrosion products.