Polymerization contraction stress in light-cured packable composite resins

Objective: Determination of the polymerization contraction stress of packable composites (ALERT, Surefil, Solitaire, Solitaire 2) and a packable ORMOCER material (Definite) in comparison with a conventional hybrid composite (Tetric Ceram). Methods: Contraction force generated by the test materials (10 replications each) was measured by polymerizing the composites filled in a plastic tray between two aluminum attachments mounted in a Stress–Strain-Analyzer testing machine (specimen size: 4×4×2 mm, C-factor=0.33). Contraction force was recorded for 300 s under a standard exposure condition (40 s, 800 mW/cm2). Maximum contraction stress (MPa), force rate (N/s), relative force rate (%/s) of each material were statistically analyzed by ANOVA (α=0.05) and post-hoc Tukeyʹs test. Results: Maximum contraction stresses of the packable materials were 4.60±0.32 MPa (ALERT), 4.16±0.18 MPa (Definite), 3.36±0.08 MPa (Solitaire 2), 3.33±0.23 MPa (Solitaire) and 3.13±0.18 MPa (Surefil), which were significantly higher than that of Tetric Ceram (2.51±0.14 MPa). Tetric Ceram exhibited the significantly lowest force rate. Force/time curves were S-shaped. Solitaire especially showed a longer pre-gelation phase before contraction force was recorded. Significance: High contraction stress and rapid contraction force development can lead to failure of bond to tooth structure. This study suggested that, packable composite resins are less capable of reducing the contraction stress during the early setting stage, thus not superior in maintaining the bond with cavity walls to conventional hybrid composite Tetric Ceram.