Effect of corrosion on the strength of dental silver amalgam.

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 24h 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.