Droplets on the surface of non-ɣ2-amalgams
Polishing the surface of many high copper amalgams stimulates the formation of droplets rich in mercury, see Figs. 2 and 3. This formation happens even if the polishing takes place under cold water to avoid any rise in temperature and continues a number of hours after the polishing has stopped.
This phenomenon was first described by Rehberg and Scharschmidt in 1976 and has since been verified by a number of researchers (Rupp et al. 1979; Schneider and Sarkar 1982; Sarkar et al. 1991). Publication has mainly been done in the form of scientific meetings abstracts but to our knowledge no dental scientific journal paper has ever been devoted to this most striking phenomenon alone. Some abstracts are not even possible to get from the dental organization, who initially held the meetings. However, there has obviously been internal discussions taking place and some regard this as a polishing artefact. Observations of droplets have however been made on clinical fillings contradicting this notion (Fredin 1994).
One of the very few pictures of these droplets in the dental scientific literature can be seen in one of the big standard encyclopedias of dental materials accompanied by the text: “The small, very light, drop-shaped areas on particle phase are high in mercury owing to the freshly polished specimen (×1000).” (Anusavice et al. 2012). No further discussion of the phenomenon is offered. Another picture of droplets without comment is presented by Herö et al. (1983).
A few papers, published outside of the dental community, have however dealt with this phenomenon. Both the formation of droplets and documentation of them is presented by a corrosion expert, outside of the dental community (Pleva 1994).
In another study, the investigator has indeed seen small “globules” on all surfaces of fillings from extracted teeth, indicating that this is not just an in vitro phenomenon but indeed occurs in clinical situations. Unfortunately the type of amalgam was not accounted for (Fredin 1994).
In 1985, one of the authors (UB) contacted the National Board of Health and Welfare in Sweden about findings of droplets on the surface of modern amalgams. The Swedish Institute for Metals Research was given the task of stripping these small droplets from the surface to determine their content of mercury. Through an extraction replica technique, five droplets were lifted from the surface and measurements ranged from 44.1 to 85.4% mercury (Lehtinen 1985).
These findings gave rise to the suspicion that the formation of these droplets was accompanied by an increased emission of mercury vapor. A final examination project was initiated at Linköping University to study mercury vapor emission in amalgams, previously found to produce droplets, with low copper amalgams used as controls. It was concluded that, indeed, non-ɣ2 amalgams exhibit an increased emission of mercury vapor (Toomväli 1988).
One would expect that droplets rich in mercury found on high copper fillings should have been published and discussed in journals commonly read by dental personnel, especially in an issue involving safety. As far as we can find, this has not happened.
This is one of two phenomena of instability, introduced with the new non-ɣ2-amalgams. The other is described below.
Increased emission of mercury vapor in non-ɣ2-amalgams
In 1994, it was shown that the amount of tin in the ɣ1-phase is related to the emission of mercury vapor (Mahler et al. 1994). Based on this paper, it is possible to identify the brands tested: conventional amalgams, amalgams with reduced amount of ɣ2- and non-ɣ2-amalgams. The result is clear; non-ɣ2-amalgams emit substantially more mercury vapor than the old, conventional ones used before the 1970s, see Fig. 4. Using the highest emitter of the low copper amalgams as a baseline, the high copper amalgams emits 3–43 times as much mercury vapor depending on brand. One of the most wide spread amalgams, DIS, emits ten times the amount of mercury vapor as compared with the highest emitter of the conventional amalgams, OPT, under the experimental conditions used.
Also Ferracane (1995) compared losses of mercury as related to the amount of ɣ1-phase. He confirmed the pattern of differences in mercury vaporization from amalgams of different composition. Using the highest emitter of the low copper amalgams as a baseline, the high copper amalgams emitted 3–62 times as much mercury vapor depending on brand and the high copper amalgams had by far the highest emission of mercury vapor (Ferracane 1995).
In an investigation measuring differences in mercury vapor emission in corroded and uncorroded samples, only one non-ɣ2-amalgam and one low copper amalgam was used. The pattern is once again confirmed with the non-ɣ2-amalgam emitting substantially more mercury vapor than the conventional one (Boyer 1988). Corroded samples emitted more mercury vapor than not corroded ones (Boyer 1988). In another investigation, using the same brands of amalgam as Mahler et al. (1994), the specimens were abraded, immersed in artificial saliva and mercury was then measured in the solution after 2h (Marek 1997). Also in this investigation, the mercury loss decreased with increasing tin content in the ɣ1-phase. In a second part of the test, when the specimens were treated differently in order to generate an oxide layer before testing, there was no relation between mercury loss and tin content.
In the four investigations above, the main researchers in dental amalgam are all reaching similar results. When the reducing oxide layer is removed, the emission of mercury is inversely related to the amount of tin in the gamma-1 phase. This oxide layer is very fragile, so touching the surface with a piece of cotton wool will result in higher levels of mercury vapor.
Unfortunately, we cannot find any openly published information/discussion on increased emission of mercury vapor from modern amalgams in any journal commonly read by dental personnel. On the contrary, several big national and international dental organizations have stated that mercury fillings are stable.
Thereby, this is the second phenomenon of instability, introduced with the new non-ɣ2-amalgams, which needs to be considered when evaluating exposure and losses of mercury from dental amalgam. Increased emission of mercury vapor may be provoked by a slight touch of the filling surface as by chewing or polishing or by a slight increase of temperature such as consuming hot beverages or hot food.