Early and Transversal Measurement of Amalgam Expansion

¹ Tokyo Medical and Dental University, and National Institute of Hygienic Science in Japan, Tokyo, Japan

By using an air micrometer and a self-devised accessory, vertical and transversal dimensional changes of cylindrical amalgam specimens were measured from 5 minutes after the start of trituration namely, immediately after packing. Residual mercury [see figure in the PDF file] contents and Knoop hardnesses of top, middle, and bottom portions of those specimens were also investigated in correlation with difference in dimensional change. The following findings were obtained.

Remarkable initial contraction was found during the first 10 minutes, which was neglected by the conventional tests, which start at 15 minutes after the start of trituration. Early measurement thus resulted in greater initial contraction and smaller final expansion. This was considered more realistic. Such an early measurement can also show greater or reverse difference between the dimensional changes by different techniques in comparison with the conventional measurement, owing to considerable change in early initial contraction.

In transversal measurement, top portions of cylindrical specimens showed earlier and smaller initial contraction, and delayed and greater subsequent expansion. Bottom portions were reverse. Amalgam expansion seemed uniform in both vertical and transversal directions in individual portions. Transversal expansion of the top layers really affecting marginal closure of fillings was greater than vertical expansion of the whole length previously used.

Residual mercury content was highest in top portions and lowest in bottom portions. Hardness was, however, lowest in the bottom portions with the lowest mercury content and highest in the top portions.