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Journal of Oral Rehabilitation, 1990, Volume 17, pages 419-424
Electrical potentials of restorations in subjects without oral complaints
A.W.J. MULLER, L.A.J. VAN LOON and C.L. DAVIDSON Department of Dental Materials Science, Faculty of Dentistry, Academic Centre of Dentistry Amsterdam. University of Amsterdam, Amsterdam, The Netherlands
Summary
The electrical potentials of 183 amalgam and 11 precious metal restorations, and one set of brackets, were measured. None of the 28 subjects had galvanism, leukoplakia, oral lichen planus, or toxic or allergic reactions to restorations. The potentials of the amalgam restorations increased with age, from about -350 mV NHE at 30 days, to about + 100 m V NHE after more than 1000 days. In most subjects potential differ- ences of more than 50 m V were present between restorations; this phenomenon is therefore assumed to be common in healthy populations.
Introduction
The electrical potential of a restoration (EPR) is highly relevant to improvement of our understanding of in vivo corrosion. It may also be important in the aetiology of galvanism (pain, battery, taste) (Axell, Nilner & Nilsson, 1983), leukoplakia (Inovay & Banoczy, 1961), oral lichen planus (B3n6czy et al., 1979), and toxic and allergic reactions to corrosion products (Van Loon et al., 1984). A detrimental effect of metallic restorations results from frequent remission of leukoplakia and oral lichen planus after removal of restorations (Banoczy et al., 1979); in this context, a difference of more than 50 m V between EPR values has been described as pathological (Inovay & Banoczy, 1961). To investigate this claim and, more generally, the link between EPRs and these illnesses and complaints, EPRs were initially measured in healthy subjects.
For this purpose, the EPRs of all metallic restorations in 28 healthy dental students were measured using a recently developed method (Muller, De Groot & Davidson, 1989), which differs from previous measurement techniques (Lukas, 1973; Bergman, Ginstrup & Nilner, 1978; Nomoto, Ano & Onose, 1979; Cohen & Levy, 1986; Yontchev et al., 1986) in that the restoration is not polished or cleaned before measurement, and the probe tip is insulated by soft wax to avoid electrochemical interaction with the saliva. Polishing and abrasion were avoided because they can disturb the potential, causing either an increase (Shimizu, Fusayama & Horibe, 1971), or a decrease (Marxkors, 1965; Marek, 1984; De Melo, Gjerdet & Erichsen, 1985); a scratch can result in a passivated metal becoming active (Fontana & Greene, 1983).
Materials and methods
The study sample consisted of 28 dental students at the Academic Centre of Dentistry in Amsterdam. They were questioned about complaints resembling galvanism, i.e. pain, battery taste, 'burning mouth', or toxic or allergic reactions that could be attributed to oral metallic restorations. The presence of leukoplakia and oral lichen planus was checked visually. All questions and examinations gave negative results. In the 28 subjects 183 amalgam restorations and 11 restorations cast from noble alloys were found. In one subject orthodontic brackets were present; the potentials of these brackets were also measured.
Details of the manufacture and use of the probe and reference electrode were given in the previous study (Muller et al., 1989). The probe and the tip of the reference electrode were disposable. The tip of the probe consisted of a gold-plated header-and-socket-pin*. The previous use of a composite resin in the probe was found to be unnecessary and was therefore discontinued in the present study. The probe was pressed through the wax on to the restoration prior to measurement. The contact with the restoration was preferentially made at the occlusal surface. The reference electrode consisted of a modified MI-402 Ag/AgCI micro-reference electrode, the potential of which was determined relative to a saturated calomel reference (SCE) electrode before measurement. A Hewlett-Packard 34703A Yoltmeter/34750A Display combination (input impedance > 10^10 ohm) measured the potential difference between the probe and the reference electrode. The latter was placed in the oral cavity, under the tongue. The EPR value relative to the Normal Hydrogen Electrode (NHE) was obtained by adding the potential difference between the Ag/ AgCI and SCE reference electrodes, and the SCE potential (+241 mV NHE), to the measured potential.
Results
Table 1 shows the EPR values of 10 subjects. In 20 of the 28 subjects, potential differences of more than 50mY were present between restorations, e.g. 37 (-147 mV) and 45 (-299mV) in subject 2: 152mY, between 14 and 26 in subject 3:310mY, between 17 and 46 in subject 4, etc. In 11 subjects such differences were present between restorations in antagonistic teeth, e.g. between 27 and 37 in subject 7 and between 15 and 45 in subject 10.
The average potential (plus or minus S.D.) of the 183 amalgam restorations was 17 plus or minus 111mV. Figure 1 shows a histogram of these EPR values, most of which lie in the range -50 mV to +150 mV, but a few are as low as -400 mV, the lowest being -417 mV.
In the case of 48 amalgam restorations the date of placement and the brand name of the alloy used were available. All brand names concerned non-y 2 alloys. Figure 2 shows the EPR values of these restorations plotted against age on a logarithmic scale. The increasing trend is clear, the maximum being about 125 mV; wide scattering with a bandwidth of 150 m Y is apparent.
Figure 2 shows that only one of the EPR values of amalgam restorations of age > 1000 days was lower than -100 mV. However, one subject categorically stated that an amalgam restoration that had an EPR value of -388 mV was more than 8 years old, the precise date of placement being unknown.
Fig. 1. Histogram of the measured potentials of amalgam restorations. (not shown)
Fig. 2. Potentials of amalgam restorations plotted against restoration age. (not shown)
Nine crowns and two bridges consisting of noble alloys were present; their average potential (plus or minus S.D.) was l54 plus or minus 55 mV.
In one subject brackets were present. The six brackets in the upper jaw had an average potential (plus or minus S.D.) of 71 plus or minus 9 mV, while those in the lower jaw had an average potential (plus or minus S.D.) of 74 plus or minus 3.3 mV.
Table 1. Potentials of restorations placed in the oral cavity of 10 subjects, according to standard tooth notation (not shown)
Discussion
It is concluded that differences between EPR values of more than 50 mV are common in healthy individuals. Thus it is unlikely that such a large difference is pathological. Whether or not it is a predisposing factor warrants further research.
Restorations consisting of noble alloys
The average EPR value (plus or minus S.D.) observed, 154 plus or minus 55 mV, is of the same magnitude as the average EPR found by Yontchev et al. (1986), namely 192 plus or minus 91 mV. Because there were only 11 noble restorations in the present study it is not possible to draw further conclusions apart from the assessment of a rather high EPR for noble restorations in the oral cavity.
Amalgam restorations
The average EPR of 17 plus or minus 111 mV is in good agreement with the previously reported value of 8 plus or minus 153 mV of 1293 restorations (value obtained by adding the SCE potential of 241 mV NHE to the given average) (Yontchev et al., 1986).
Nilner and Holland (1985) found that younger restorations have lower potentials. Another study (Cohen & Levy, 1986) stated that the EPR is approximately -760 mV directly after placement, but gradually increases to 40 mV ( -200 mV SCE) or to a 'more positive' value. This increase is confirmed by the present study, and is in contrast with the stationary and decreasing potentials reported in an in vitro study of 245 days duration (Moberg, 1987).
The increase can be explained by selective corrosion of less noble components, resulting in a more noble behaviour of the alloy, which in turn causes an increase in the corrosion potential (Kaesche, 1979; Cohen & Levy, 1986).
In the previous study, the root mean square deviation of EPR values determined at 30-min intervals was 14 mV. This measure of the accuracy of the measurement method is much smaller than the scattering bandwidth of 150 m V shown in Fig. 2. Such wide scattering can be attributed to the effect of (marginal) fracture or of abrasion, which can occur during chewing. These processes can intermittently bring previously unexposed amalgam into contact with saliva, resulting in a temporary drop in potential: the less noble amalgam components at the freshly exposed surface will show accelerated corrosion, which would result in a rapid return to the earlier corrosion potential. The scattering may also be related to the distinct behaviour of different alloys, the effects of polishing or abrasion (Marxkors, 1965; Shimizu et al., 1971; Marek, 1984; De Melo et al., 1985), or the idiosyncracy of patients encountered in in vivo tarnish studies (Mezger, 1989).
Figure 1 shows that the distribution of the EPR values is skewed towards lower potentials. This can be explained by the time behaviour of the EPR illustrated in Fig. 2, which shows that restorations of age > 1000 days have an EPR of approximately +50 mV, but that younger restorations have much lower EPR values. The probability of encountering a recently placed restoration is much lower than that of encountering an older one, accounting for the small number of low EPR values.
Brackets
We are not aware of any previous determination of the in viva potential of brackets. The high potential of the brackets in the jaws, 71 and 74 m V, respectively, shows that they are in the passive and not the active state; in the passive state, the release of possibly discolouring corrosion products is minimal.
As the brackets in one jaw are electrically connected, their potentials must be equal. Hence the standard deviations of the EPR values of the brackets in the jaws, 9 and 3.3 m V, respectively, provide a measure of the accuracy of the method. These values are comparable to the root mean square deviation of EPR values determined at 30-min intervals, i.e. 14 m V, that was found in the previous study (Muller et at. , 1989).
Conclusion
In most of the subjects potential differences of more than 50 mV were present between restorations. From this frequent occurrence, and the rarity of adverse reactions attributed to metallic restorations, it is concluded that the presence of the latter does not necessarily indicate a health hazard that should be terminated by removal of restorations. Most of the scattering in potential values of amalgam restorations can be explained by the effect of differences in age of restorations.
Acknowledgements
We thank the subjects for their participation, their dentists for providing information on their restorations, and Jeanne Kruchowski for proofreading.
References
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Correspondence: Professor C.L. Davidson, Department of Dental Materials Science, ACTA, Louwesweg 1, 1066 EA Amsterdam, The Netherlands.
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