Influence of polymerization mode and C-factor on cohesive strength of dual-cured resin cements

Objectives
The aim of this study is to determine the influence of the C-factor and the mode of polymerization on the cohesive strength of various dual-cure resin cements.

Methods
Three curing conditions were tested; chemical curing with free shrinkage conditions (C = 0), and constraint shrinkage conditions (C = 25), and dual-curing with free shrinkage conditions (C = 0). Opaque polyethylene, brass (pretreated with Clearfil SE bond), and transparent polyethylene tubes respectively, were filled with the different cements. The tubes were 20 mm long with an inner diameter of 1.6 or 1.8 mm. Five cements, DC Core Automix, Panavia F 2.0, Maxcem, Multilink, and RelyX Unicem, were tested with ten specimens per group. The specimens were trimmed to an hour-glass shape with a neck diameter of 1 mm, stored in water (37 °C, 24 h), and subjected to microtensile testing (1 mm min−1). SEM analysis was carried out on chemically cured samples of DC Core Automix C = 0 and C = 25. Data were statistically analyzed (Two-way ANOVA, Tukey's post hoc test, p < 0.05).

Results
Most cements showed no significant differences between the curing modes. A high C-factor negatively influences the cohesive strength of some cements. SEM analysis shows that chemical curing of DC Core Automix in a high C-factor environment leads to more and larger microvoids in the cement.

Significance
Constraint shrinkage conditions, i.e. a high C-factor, can negatively influence the physical properties of a dual-cured resin cement, which would clinically be the case in the confined space of a root canal or post space preparation.