Existence of an electrically insulating layer in amalgam-containing galvanic couples.

OBJECTIVE: Avoiding the placement of amalgam and noble metal restorations in interproximal contact is recommended due to anticipated galvanic corrosion of the amalgam. There is a similar concern for amalgam/amalgam galvanic couples. It was the objective of this study to determine if an electrically insulating layer forms in the contact area of these galvanic couples. The existence of an electrically insulating layer, which could reduce the galvanic corrosion rate, would be indicated by different corrosion potentials for the two restorations of the couple. METHODS: Using a convenience sample of 158 human subjects, corrosion potentials were measured on each restoration of three types of galvanic couples: amalgam/noble metal (n=8), amalgam/amalgam (n=93) and noble metal/noble metal (n=7). Measurements were made with a Ag/AgCl micro-reference electrode and a high impedance voltmeter. All restorations were at least 6 months old. RESULTS: Statistical analysis showed that the mean absolute corrosion potential differences and the simultaneous confidence intervals of the couples were, respectively, amalgam/noble metal: 62 (31)mV and (27, 99)mV, amalgam/amalgam: 11 (14)mV and (7, 15)mV and noble metal/noble metal: 7 (10)mV and (0, 19)mV. SIGNIFICANCE: The amalgam/noble metal couples had consistent and mostly large corrosion potential differences between their restorations, which indicated the presence of an electrically insulating layer. An electrically insulating layer was also indicated for the amalgam/amalgam and noble metal/noble metal couples. The layer is probably composed of non-metallic corrosion products, biofilms, and possibly, dental calculus, which could reduce galvanic corrosion rates to small or negligible values.

Corrosion potential recovery of dental amalgam restorations following prophylaxis.

OBJECTIVE: Dental amalgam restorations are subjected to abrasion during selective prophylaxis that can damage or remove the protective oxide and result in increased rates of corrosion and chemical dissolution of mercury. It was the objective of this research to study the corrosion potential change of dental amalgam restorations to obtain an indication of the time required for in vivo repassivation following prophylaxis. METHODS: The corrosion potentials of 27 Class I and Class II amalgam restorations were measured pre- and post-prophylaxis using a high impedance voltmeter and a Ag/AgCl micro-reference electrode. Prophylaxis was performed for approximately 2s on each amalgam surface using a slow-speed handpiece with a rubber-cup and commercial abrasive paste. Subjects thoroughly rinsed before the post-prophylaxis corrosion potentials were measured. The data were analyzed using a confidence interval, a t-test and correlation analysis. RESULTS: The pre- and post-prophylaxis mean corrosion potentials were, respectively, -132 (27)mV and -126 (27)mV. The mean of the differences between the pre- and post-prophylaxis corrosion potentials was 6.1 (28)mV, with an associated 95% confidence interval of (-4.8, 17)mV. A t-test showed the mean absolute difference in corrosion potential was less than 50 mV (p<0.0001). SIGNIFICANCE: The results of this study show that the post-prophylaxis recovery of the corrosion potential of amalgam restorations occurred by at most 10-44 min, indicating that the period of elevated corrosion rate and elevated chemical dissolution rate of mercury, due to oxide damage or removal, may be short-lived.

Time-dependent corrosion potential of newly-placed admixed dental amalgam restorations.

OBJECTIVE: It was the objective of this study to measure the corrosion potential over time of newly-placed admixed dental amalgam restorations as a possible indicator of corrosion activity. METHODS: The corrosion potentials of 271 amalgam restorations, 4-min to 24-month old were measured in a convenience sample of 81 subjects. The selected restorations had no occlusal or interproximal contact with other metallic restorations. Eighty-one percent of the restorations aged 4 min to 7 months were made from zinc-containing admixed high-copper amalgam. The remaining amalgams were an unknown mixture of admixed high-copper and single-composition-alloy amalgams. RESULTS: The data were fit to a mixed-effects model with random patient effects to allow for the correlation, using the maximum likelihood method. The early data best fit a quadratic model with an initially rapidly rising corrosion potential that leveled off to a constant mean value of -146 (60)mV (versus Ag/AgCl, 3M KCl at 35 degrees C) by approximately 7 months. Most of the ennoblement occurred within the first 4 months after restoration placement. SIGNIFICANCE: This in vivo study observed a slower rate of corrosion potential ennoblement for admixed amalgam restorations than would be predicted from in vitro studies. The slower rate is believed due to the combination of mechanical, chemical and biological forces in the mouth that are generally absent using simulated conditions. The slower rate of ennoblement suggests a slower rate of achieving maximum corrosion resistance than would be predicted using in vitro studies.

Corrosion potential variation of aged dental amalgam restorations over time.

OBJECTIVE: The corrosion potential of a dental amalgam restoration is generally determined using a single measurement, even though environmental factors and abrasion can continuously alter the surface state and reactivity of this alloy. It was, therefore, the purpose of this study to determine the maximum variability of the corrosion potential of aged dental amalgam restorations, for 28 days. METHODS: The corrosion potentials of 148 aged dental amalgam restorations in 12 human subjects were measured at t = 0 and 4 h, and 4, 7, 14, 21 and 28 days. Measurements were made with a high impedance voltmeter and a Ag/AgCl micro-reference electrode. The subjects were instructed not to alter their usual eating and oral hygiene routines. RESULTS: Corrosion potential changes occurred throughout the 28 days. They were both positive and negative for the same restoration, and were sometimes very large. Only 4 h after the initial measurement, the absolute value of the corrosion potential changes ranged between 18 and 287 mV for 50% of the restorations. The largest maximum absolute corrosion potential change for each subject's restorations ranged between 85 and 329 mV. Statistical analysis showed that the overall mean maximum absolute corrosion potential change for the subjects' restorations was 74 mV. SIGNIFICANCE: It was shown that the corrosion potential of aged dental amalgam restorations varies substantially over time, and that a single measurement is not representative of short- or long-term electrochemical behavior. This finding has implications regarding the corrosion rate of dental amalgam restorations, particularly those that are part of a galvanic couple.

In vivo galvanic currents of intermittently contacting dental amalgam and other metallic restorations.

OBJECTIVE: It was the objective of this study to determine the magnitude of in vivo galvanic currents produced by simulating electrical contact between occluding metallic restorations, and to examine the influence of restoration age, difference in pre-contact corrosion potentials, and surface area. METHODS: A convenience sample of 106 human subjects was studied. A Ag/AgCl micro-reference electrode and a high impedance voltmeter were used to measure the pre-contact corrosion potentials. Galvanic couples (n = 194) were next formed by simultaneously contacting occluding restorations with gold-plated probe tips for 15 s. The resulting current-time transients were measured with a zero-resistance ammeter and recorded with an electronic data acquisition program. The vast majority of couples measured contained at least one dental amalgam restoration. RESULTS: Galvanic current-time transients were typically characterized by an immediate and rapid rise to a peak current, followed by an exponential decay to a much lower value at 15 s. For couples with a restoration < or = 12 mo old, median peak current and median current at 15 s were 2.26 microA (range 0.24-13.06 microA) and 0.57 microA (range 0.03-6.47 microA), respectively. For couples with a restoration >12 mo old, median peak current and median current at 15 s were 1.40 microA (range 0.24-12.09 microA) and 0.37 microA (range 0.00-3.05 microA), respectively. Couples with fresh amalgams (< or = 6 mo) generally had elevated currents (range 2.56-102.54 microA). SIGNIFICANCE: A wide range of galvanic currents resulted from electrical contact of restorations in vivo. These currents were influenced by restoration age and total surface area of the galvanic couple. For amalgam-amalgam couples, the difference in the pre-contact corrosion potentials may be useful in predicting galvanic currents, when the difference is at least 24 mV.