High copper alloys for dental amalgam.

The nature, physical properties and clinical performance of amalgams made from alloys containing copper in excess of 6 per cent by weight are reviewed. There are two basic types of high copper alloy: (a) mixtures of 'spherical' silver/copper or silver/tin/copper particles with low copper low copper lathe-cut or 'spherical' and (b) single composition silver/tin/copper spherical or lathe-cut particles. Copper contents range from 8.5 per cent to 30 per cent. In amalgams made from high copper alloys the soft corrodible gamma 2 (Sn7Hg) phase is reduced or eliminated by preferential formation of the eta (Cu6Sn5) phase. Improved clinical performance (less marginal breakdown) has been related to low creep, little or no gamma 2 phase and the presence of zinc. The possible roles played by these factors in the mechanism of marginal breakdown are discussed. Physical properties are not a reliable guide to the clinical performance of amalgams. Although high copper amalgams as a group show 'superior' physical properties and clinical performance to low copper amalgams, a high copper content does not necessarily mean improved clinical performance. Indeed, certain well-manipulated low copper amalgams can show clinical results comparable with some high copper amalgams, but not as good as the best high copper amalgams. In most clinical studies the silver/copper plus lathe-cut (dispersed phase) alloys and some of the single composition high copper alloys show the greatest clinical durability. The most significant factor in clinical performance is the choice of alloy.