Effect of corrosion on the strength of dental silver amalgam

Abstract

Objectives

To characterize the effect of crevice corrosion on the strength of dental silver amalgam as determined by the Hertzian ‘ball on disc’ method, with a view to providing a test method for use in standards compliance testing.

Materials & methods

Sixteen dental silver amalgam products were tested using the ‘ball on disc’ protocol at 30 d after setting at 37 °C in air or immersed in artificial saliva at pH 6.2. The mixed materials were packed into a tapered steel disc mold (10 mm diameter, 3 mm thick) resting on a glass surface, slightly overfilled and carved level with a sharp edge, then ejected at ∼10 min and placed immediately into an incubator at 37 °C. For corrosion specimens, the disc was laid on a flat polystyrene surface, immersed in artificial saliva, to create a spontaneous crevice corrosion cell. Testing was in Hertzian mode, using a 20 mm steel ball, with the specimen resting on a disc of glass-filled polyamide ( E = 10 GPa) at a cross-head speed of 0.2 mm/min on a universal testing machine (E3000, Instron). The load at first crack was recorded, as was the number of radial cracks produced.

Results

Radial cracking into 2–4 pieces, in a clinically relevant (non-explosive) mode was observed in all cases. On average, corrosion caused a decrease in load at failure of ∼10%, although the interaction with alloy (analysis of variance) was significant ( P ∼ 0.03) indicating variation between products. Comparison of the 30 d dry (uncorroded) results with those at 24 h obtained earlier showed that there was highly significant increase on average ( P ∼ 5 × 10 −12 ), but again a significant variation between products ( P ∼ 5 × 10 −6 ), the maximum effect being +22%.

Significance

The ball-on-disc test provides a facile means of ascertaining the sensitivity of dental silver amalgam to corrosion under clinically relevant conditions, and is viable as a standards compliance test.

Introduction

As has been recently discussed, dental silver amalgam retains an important place in the dental armamentarium . Broadly speaking, it may be considered a ‘mature’ technology, as is indicated perhaps by the rather low and declining rate of publications dealing with the material, despite one or two attempted rejuvenations, such as with gallium or indium additions . Nevertheless, because of the continued volume of use, it is important to maintain performance standards, even if (and perhaps especially because) in countries where more expensive substitutes are affordable, despite their poorer performance, it is being banned or abandoned on presumed environmental grounds (which is another long and separate debate, by no means clear cut).

Dental silver amalgam is a polyphase material, and the electrode potential differences between the various phases is substantial. Galvanic corrosion of the most electropositive phase ( i.e. γ 2 -Sn–Hg) therefore, is guaranteed . This corrosion is assisted by the oxygen concentration cell that forms in the inevitable marginal crevice but, providing the crevice is sufficiently narrow and the corrosion rate sufficiently high to drive the precipitation of corrosion products in, as opposed to diffusion of the metal ions (predominately tin) out of the crevice, such restorations are self-sealing in a relatively short time . This is beneficial. Indeed, it is likely that should that seal be disturbed by flexure of the tooth it will promptly reform. On the other hand, if the crevice is large, this cannot occur and recurrent caries is a possibility.

High-copper alloys were introduced on the basis of improved strength, although subsequently, when the cause was understood to be due to the great reduction or elimination of the γ 2 , the consequent reduction in corrosion rate was considered an advantage . This notwithstanding the now lower rate of sealing the crevice, and the now extended period for which corrosion (now of the η′-Cu 6 Sn 5 [Hg] phase, the next most electropositive) would occur, as shown by the long-continued brightness of the anodically protected free surface . This interaction of rate and crevice size has recently been recognized .

Much has been done on the corrosion of dental silver amalgam, for example from the point of view of the release of mercury and other metals , electrochemistry , being under tension , metallography , discoloration and other aspects . The study of the effect of corrosion on strength, in contrast, has been very limited. That is, in practical terms, what is the effect of the unavoidable corrosion on the longevity of a restoration? In many other fields, corrosion of metals is viewed from the point of view of structural integrity and load-bearing capacity. This is rarely the focus in dental research.

The corrosion release of mercury causing embrittlement of silver amalgam has been discussed, revisiting a concept introduced some time before and said to account for amalgam restoration marginal fracture. A reduction in tensile strength to due galvanic corrosion has been noted , as well as effects on fatigue behavior . However, there seems to be very little in the way of a practical test to establish the susceptibility of an amalgam product to mechanical deterioration. Certainly, from this author’s involvement with the relevant committee of ISO TC106 (Dentistry), it can be said that the challenge of a suitable test to establish the acceptability of a product in terms of corrosion has proved formidable.

Recently, an attempt has been made to address this deficiency using a circumferentially notched cylinder . A thread tied in the notch was used to induce crevice corrosion, and after immersion in 1% NaCl solution with hydrogen peroxide added for 14 days at 37 °C, the specimen was fractured in three-point bend. In principle, this has all the required elements: crevice corrosion and failure initiated in tension. In practice, however, it may prove more problematic. The precision required for the notch is very high, the crevice corrosion conditions sensitive to the exact conditions created by the thread, and the specimen itself prepared according to the one-shot method criticized earlier . The hydrogen peroxide was presumably added as an accelerant, although its decomposition would be expected to be fairly rapid. Thus, despite initial air bubbles being removed by reduced pressure, some oxygen bubble formation would be expected, especially on the thread, interfering just as would air bubbles with the uniformity of the crevice conditions. Acceleration is, however, of dubious value without demonstrated equivalence to normal service conditions: clearly the deposition of corrosion products in the crevice and porosity (whether pre-existing or created by the corrosion process) is sensitive to the rate of corrosion, consequent pH gradients and solution concentrations of metal ions. The comparison with the self-sealing behavior described above is important. While the test span was not specified, the depth to span ratio is unusually high (judging from the figure), and the need for accurate centring of the notch evident (the load element must seat exactly in the notch, and this must produce some wedging action) . Furthermore, the coefficient of variation was very high: 13–53%, RMS ∼31%, indicating that a large sample size would be required.

Noting, then, that if the corrosion process is self-limiting if the conditions are appropriate, the concern must be whether it has progressed far enough by that time to compromise the strength of the material. Clearly, also, the comparator specimen, the failure load type and the failure mode must be clinically relevant. Accordingly, the Hertzian ball-on-disc test was adapted to test the effect of crevice corrosion on failure load in that mode.

Materials and methods

The dental silver amalgam products tested are shown in Table 1 . The procedure followed was identical to that described earlier , with appropriate modifications. For convenience, the relevant parts are repeated here. Pre-dosed encapsulated products were mixed as supplied. Where instructions were available for bulk powder and tablet products, they were followed using a screw-cap reusable capsule (SS White) and a 7.97 mm diameter steel ball as a pestle for efficiency. Some were very old ‘archival’ materials and no instructions were available, in which case a suitable mixing ratio was identified by preliminary trials. All products were mixed under the same conditions (Silamat Plus; Vivadent, Liechtenstein) for 8 s, on the ‘slow’ setting, having ascertained that a clinically usable mix was obtained in each case. The mixed material was packed by hand into a steel disc mold, 10.0 mm diameter, 3.00 mm thick, resting on a glass plate (‘matrix’), using a conventional 2 mm diameter condensing point, overfilling slightly, then carving back smooth and level using the edge of a glass microscope slide. In the case of Dispersalloy, Megalloy EZ and Septalloy NG50 the product was available in both 1- and 2-spill capsules, and a full series of specimens was prepared for each. The material was allowed to set for ∼10 min, then carefully ejected from the mold.

Table 1
Dental silver amalgam products tested.
Brand Manufacturer’s description Manufacturer Batch id.
1. a Amalcap Plus 1 Spill, non-gamma2, regular Ivoclar Vivadent, Sweden F55151, NT4008
2. Ease Dispersed phase, pellets, type 2 Caulk, USA 112177, 011178
3. DCL Amalgam Cap 2 Spill, gamma 2-free, dispersed phase Dental Composite, UK 037335
4. Dispersalloy 2 Spill, regular set, admixed Dentsply Caulk, USA 090828
5. Dispersalloy Dispersed phase, fast set, zinc, tablets Johnson & Johnson, USA 6E063
6. Epoque 80 Non-spherical, non-gamma 2 Scania Dental, Sweden 7-217
7. Megalloy EZ 1 Spill, high strength, high copper, spherical Dentsply Caulk, USA 0803241, 091102
7. Megalloy EZ 2 Spill, high strength, high copper, spherical Dentsply Caulk, USA 1101251
8. Scanalloy 68 Fine grain Scania Dental, Sweden
9. Septalloy NG 50 1 Spill, single composition, non-spherical, non-gamma 2 Septodont, France 07079C, 09002A
9. Septalloy NG 50 2 Spill, high copper, single composition, non-spherical, non-gamma 2 Septodont, France 07084B
10. Supralloy High copper, spherical blend, type 1, class III Unident, USA 032282
11. Token No 1 Zinc, non-spherical Toyo Chemical Laboratories, Japan 2042
12. Toyo DP Dispersed phase, spherical, non-zinc Toyo Chemical Laboratories, Japan 910-1021
13. Toyo Atomized Spherical fine cut, non-zinc Toyo Chemical Laboratories, Japan 891-1112
14. Vivacap 2 Spill, high copper, single composition, non-gamma2 Ivoclar Vivadent, Sweden MT4065, MT 4072

a Key numbers for Figs. 3 and 6 .

Control specimens were then placed immediately into an incubator, in air, at 37.0 °C. Corrosion test specimens were placed, ‘matrix’ surface downwards, into a polystyrene blood dilution vial ( Fig. 1 ) (V130; Simport, Beloeil, QC, Canada) containing 15 mL of artificial saliva adjusted to pH 6.2 (to represent nominal resting conditions). The base of the vial was flat, and established a crevice immediately. The vial was then capped and placed into an incubator 37.0 °C and left completely undisturbed for the duration of the exposure time, which was 30 d.

Fig. 1
Vial used for immersion of the specimen and to create the spontaneous crevice corrosion cell.

Specimens were removed from the incubator 10 min before the time of test in order to cool, removed from the solution and rinsed with deionized water without disturbing the adherent corrosion product layer. They were then tested, without drying, using the apparatus shown in on a universal testing machine (Model E3000; Instron, High Wycombe, UK) at 0.2 mm/min cross-head speed in air at 23 °C, resting freely on the substrate, a 5 mm-thick disc, 10 mm diameter, of 30% glass fiber-filled polyamide (LS360362 SJP; Goodfellow Cambridge, Huntington, UK), 1

1 The catalog number appears to have been changed; it is now at the time of writing AM367910 (but the same order code, 791-942-57), described as “Polyamide – Nylon 6, 6 – 30% Glass Fiber Reinforced – Rod, PA 6, 6 30% GFR”.

faced flat. The relative dimensions of specimen, substrate and ball were chosen to ensure radial cracking initiated from the lower surface of the specimen ( i.e . at the interface with the substrate). That is, Hertzian cone-cracks and ring cracks must not (and do not) occur for validity. Testing was all started within −0, +2 min of the target time. Substrate disks were replaced if any wear or deformation was detected (which was rare). The data acquisition rate was set to 100 s −1 . This enabled the detection of the first crack through the ringing of the load signal . In some cases, with no discernible pattern, the fracture was not abrupt enough to cause the ringing, and the change of slope gradual, so as to preclude the identification of a failure load sufficiently precisely. This was often marked by progressing to an extraordinarily high load (and test termination), although inspection showed that fracture had indeed occurred. As before, the number of pieces into which the specimen fractured was recorded, confirming radial dissection in each case, i.e . bottom-initiated cracking. This required the use of some finger pressure as the pieces were often held together by the intimate tortuosity of the fracture surfaces; incomplete fracture could not be excluded in such cases (the crack was often not detectable visually).

Statistical analysis was in software (SigmaPlot 11.2; Systat Software, San Jose, CA, USA).

Materials and methods

The dental silver amalgam products tested are shown in Table 1 . The procedure followed was identical to that described earlier , with appropriate modifications. For convenience, the relevant parts are repeated here. Pre-dosed encapsulated products were mixed as supplied. Where instructions were available for bulk powder and tablet products, they were followed using a screw-cap reusable capsule (SS White) and a 7.97 mm diameter steel ball as a pestle for efficiency. Some were very old ‘archival’ materials and no instructions were available, in which case a suitable mixing ratio was identified by preliminary trials. All products were mixed under the same conditions (Silamat Plus; Vivadent, Liechtenstein) for 8 s, on the ‘slow’ setting, having ascertained that a clinically usable mix was obtained in each case. The mixed material was packed by hand into a steel disc mold, 10.0 mm diameter, 3.00 mm thick, resting on a glass plate (‘matrix’), using a conventional 2 mm diameter condensing point, overfilling slightly, then carving back smooth and level using the edge of a glass microscope slide. In the case of Dispersalloy, Megalloy EZ and Septalloy NG50 the product was available in both 1- and 2-spill capsules, and a full series of specimens was prepared for each. The material was allowed to set for ∼10 min, then carefully ejected from the mold.

Table 1
Dental silver amalgam products tested.
Brand Manufacturer’s description Manufacturer Batch id.
1. a Amalcap Plus 1 Spill, non-gamma2, regular Ivoclar Vivadent, Sweden F55151, NT4008
2. Ease Dispersed phase, pellets, type 2 Caulk, USA 112177, 011178
3. DCL Amalgam Cap 2 Spill, gamma 2-free, dispersed phase Dental Composite, UK 037335
4. Dispersalloy 2 Spill, regular set, admixed Dentsply Caulk, USA 090828
5. Dispersalloy Dispersed phase, fast set, zinc, tablets Johnson & Johnson, USA 6E063
6. Epoque 80 Non-spherical, non-gamma 2 Scania Dental, Sweden 7-217
7. Megalloy EZ 1 Spill, high strength, high copper, spherical Dentsply Caulk, USA 0803241, 091102
7. Megalloy EZ 2 Spill, high strength, high copper, spherical Dentsply Caulk, USA 1101251
8. Scanalloy 68 Fine grain Scania Dental, Sweden
9. Septalloy NG 50 1 Spill, single composition, non-spherical, non-gamma 2 Septodont, France 07079C, 09002A
9. Septalloy NG 50 2 Spill, high copper, single composition, non-spherical, non-gamma 2 Septodont, France 07084B
10. Supralloy High copper, spherical blend, type 1, class III Unident, USA 032282
11. Token No 1 Zinc, non-spherical Toyo Chemical Laboratories, Japan 2042
12. Toyo DP Dispersed phase, spherical, non-zinc Toyo Chemical Laboratories, Japan 910-1021
13. Toyo Atomized Spherical fine cut, non-zinc Toyo Chemical Laboratories, Japan 891-1112
14. Vivacap 2 Spill, high copper, single composition, non-gamma2 Ivoclar Vivadent, Sweden MT4065, MT 4072
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Effect of corrosion on the strength of dental silver amalgam
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