ACID-ASSISTED CONSOLIDATION OF SILVER ALLOYS FOR DIRECT FILLINGS

Silver-rich metal powders cold-welded by consolidation have been inves tigated as possible direct dental filling material. The surface of the silver powder must undergo an acid treatment to remove existing conta minants and prevent formation of new surface contaminants during conso lidation. This study was designed to investigate the effect of the aci d treatment on the strength of the consolidated alloy, its reactivity with the surrounding tooth structure, and its reactions with certain c avity liners. This study investigated the effects of pH and concentrat ion of fluoboric acid on the flexural strength of silver powder consol idated into rectangular beams in a partial 4 x 4 design. The study als o assessed, by visible and scanning electron microscopy, what effect t he acid-treated powders had on dentin that had been pre-coated with di fferent cavity liners. Mean flexural strengths for beams consolidated using dental hand instruments were in the range (77.0 +/- 9.28) MPa to (166.1 +/- 17.6) MPa, where the quoted uncertainties are standard unc ertainties (i.e., one standard deviation estimates). ANOVA indicated t hat fluoboric acid pH was highly significant (p <0.0001) with lower pH values resulting in higher flexural strength. Concentration alone was not a significant factor for flexural strengths, but there was a sign ificant interaction between concentration and pH (p < 0.0001). Microsc opy revealed that the acid-treated silver powder demineralized approxi mately 2 mu m of dentin when used with no liner. The use of copal or p olyamide varnishes eliminated most of this demineralization, but the u se of a dentin adhesive liner resulted in some dislodgment and breakdo wn of the adhesive film by the acid. The results of this study indicat e that this silver powder when treated with dilute fluoboric acid at a pH of approximately 1.0 can result in a filling material with strengt h equivalent to currently used conventional amalgam. The demineralizat ion of tooth structure appears to be minimal and can be eliminated wit h the use of cavity liners.