Film thickness of dental luting cements

The film thickness as a function of time after start of mixing was measured for several dental luting cements (one zinc phosphate, 2 polycarboxylate and 3 glass ionomer cements) with 2 different methods, i.e., between 2 plane plates as used to control film thickness in several specifications/standards (Method 1), and in a crown-tooth model (Method 2) simulating clinical conditions. The results showed different film thickness values for the various cements, and these values differed depending on the time after start of mixing in the 2 systems. The minimum film thickness was, in general, lower when measured using Method 2 than using Method 1. For the zinc phosphate cement a shorter time was recorded in Method 1 than in Method 2 for the time lapse from commencement of mixing until a rapid increase of the film thickness started. For the polycarboxylate cements, an opposite relation was recorded for this time-dependent change. The results for the glass ionomer cements were variable. The start of the rapid increase in film thickness was in general more distinct in Method 2 than in Method 1. The results question the validity of defining working time for dental luting cements by a specified film thickness value as measured between 2 plane plates.