Sorption, solubility, shrinkage and mechanical properties of "low-shrinkage" commercial resin composites

This study determined the volumetric shrinkage, degree of conversion, sorption, solubility, and flexure properties (before and after aging) of commercial dental composites identified as "low shrinkage".

Six Bis-GMA-based composites (Point 4, ELS, Filtek Supreme, Aelite LS Posterior, Filtek Z250 and Heliomolar), a silorane-based (Filtek LS) and a dimer dicarbamate dimethacrylate-based composite (N'Durance) were studied. Total shrinkage was measured in a mercury dilatometer (n=3). Elastic modulus and flexural strength was determined by the three point bending test before and after 4 months aging in 75% ethanol solution (n=10). Sorption and solubility in water was assessed according to ISO 4049. Additionally, composite degree of conversion was determined using near-IR spectroscopy (n=3). Data were analysed using one-way/two-way ANOVA or Kruskal-Wallis, and Tukey's test. Student's t-test was used to compare storage periods (α=0.05).

The volumetric shrinkage values ranged between 1.5% (Filtek LS) and 3.4% (Point 4). The materials presented different behaviors regarding aging in ethanol, as evidenced by the large range in values of percentage reduction for elastic modulus (26-75%) and flexural strength (25-86%). Sorption values ranged from 8.2μg/mm(3) (Aelite LS Posterior) to 38.2μg/mm(3) (Point 4). Solubility values ranged from -1.5μg/mm(3) (Filtek LS) to 5.7μg/mm(3) (Aelite LS Posterior). The degree of conversion values at 10min ranged between 28% (Filtek LS) and 73% (Point 4). At 72h after curing the values ranged between 39% (Filtek LS) and 83% (Point 4).

Among the materials identified by their manufacturers as "low shrinkage", only Filtek LS presented statistically lower values of shrinkage compared to composites based on conventional dimethacrylates, but lower degree of conversion as well. Overall materials with higher filler content presented higher initial values of flexural properties. However, their resistance to ethanol degradation seems to be influenced by different compositional factors and, therefore, cannot be directly related to water sorption results.