Shrinkage Stress Dependency on Configuration Factor for Several Flowable Composites

Objectives: To investigate the effect of configuration cavity factor (C-factor, the ratio of bonded to unbonded surface area) on normalized shrinkage stress, otherwise known as polymerization shrinkage development (PSD, MPa/g) produced by volumetric contraction of flowable composite resin during light activated polymerization.
Methods: Three flowable composite resins were studied, Danville Materials Bulk EZ, Nanova Biomaterials Novapro flowable, PrimeDent flowable. A universal mechanical testing machine (Instron 5566A) with a 100N load cell was used. Cylindrical resin was made between the gap of two fused silica rods mounted on load cell chunks with adjusted height of 0.5 mm, 1.0 mm, and 1.5 mm to achieve different C-factors. Five duplicated specimens (n=5) for each condition were tested. The contraction forces were recorded by connected computer at a data acquisition interval of 0.1 s for 25 minutes after light activation and the shrinkage stresses were calculated. The statistical analysis was conducted by ANOVA and Tukey’s post hoc test, and linear regression via JMP Pro12.
Results: There was significant difference in polymerization shrinkage stress among three tested composite (Figure 1). Direct linear correlations between PSD and C-factor was found for all tested flowable composites (Figure 2).
Conclusions: PSD decreased linearly with increasing height of the specimen and hence with corresponding decrease in C-factor. Increasing thickness, which is associated with decreasing C-factor and thus increased free surface area of the specimen perimeter, allows for some slight radial inward deformation and stress-relief during polymerization.