Effect of low Elastic Modulus Liner and Base as Stress-Absorbing Layer in Composite Resin Restorations

Objectives. The aim of this study was to evaluate the effectiveness of liner and base materials to reduce the stress resulting from polymerization shrinkage. The null hypothesis tested was that the presence of liner and base low viscosity materials under composite resin restoration reduces the polymerization shrinkage stress. Methods. A quasi-three-dimensional photoelastic model of a second premolar with a class I preparation was restored using four experimental groups (n = 7): RC, resin composite (Filtek Z250); FLRC, flowable liner (Filtek Flow) + resin composite restoration, VLRC, resin-modified glassionomer liner (Vitrebond) + resin composite restoration, and VBRC, resin-modified glass-ionomer base + resin composite restoration. The maximum shear stresses (τmax) were calculated along the adhesive interface in 13 predefined and standardized point locations. Data were submitted to one-way ANOVA analysis, followed by a Tukey’s post-hoc test (p < 0.05). Results. A significant difference was found among the experimental groups (p = 0.001); therefore, the null hypothesis was rejected. The mean maximum shear stress was: 38.0 KPa for RC, 52.1 KPa for FLRC, 72.8 KPa for VLRC, and 90.2 KPa for VBRC. The polymerization shrinkage stress level from least to greatest was: RC < FLRC < VLRC < VBRC. The overall stress distribution in class I restoration indicated that stresses were primarily accumulated at the cavosurface and internal line angles. Significance. Using a flowable composite or resin-modified glass-ionomer as liner or base material under composite resin restoration increases the polymerization shrinkage stresses at the adhesive interface leading to a possible adhesive failure.