Effect of thickness on shrinkage stress and bottom‐to‐top hardness ratio of conventional and bulk‐fill composites

This study evaluated the effect of the material thickness on shrinkage stress and bottom‐to‐top hardness ratio of conventional and bulk‐fill composites. Six commercial composites were selected based on their different technologies: Two conventional (C1, C2), two high‐viscosity bulk‐fill (HVB1, HVB2), and two low‐viscosity bulk‐fill (LVB1, LVB2). Shrinkage stress was analyzed for five specimens with 2 mm thickness (C‐factor 0.75 and volume 24 mm3) and five specimens with 4 mm thickness (C‐factor 0.375 and volume 48 mm3) for 300 s in a universal testing machine. Bottom‐to‐top hardness ratio values were obtained from Knoop microhardness measurements in specimens with 2‐ and 4‐mm thickness (n = 5). Thickness increase resulted in significantly higher shrinkage stress for all materials with the exception of HVB2 and LVB1. C1, C2, HVB2, and LVB1 showed lower bottom‐to‐top hardness ratios at 4 mm than at 2 mm. Only LVB2 presented a bottom‐to‐top hardness ratio lower than 80% at 2 mm, while HVB1 surpassed this threshold at 4 mm of depth. The results suggest that the increase of composite thickness affected the shrinkage stress values. Also, thickness increase resulted in lower bottom‐to‐top hardness ratio. HVB1 showed better behavior than other bulk‐fill materials, with low stress and adequate bottom‐to‐top hardness ratio at 4 mm thickness.