Long-term Volumetric Analysis of Resin Composites stored in PBS

Objectives: To evaluate the volumetric change (polymerization shrinkage/hygroscopic expansion) of 4 bulk-fill restorative composites and 2 incremental/conventional fill composites over 60 days of storage in 0.2M physiologic phosphate buffered saline (PBS).
Methods: Composite specimens (n=15) were created on keyed pedestals following manufacturer recommendations, and stored in PBS at 37 °C until the appointed testing time. Volumetric measurements were recorded using a video imaging analysis instrument (AcuVol, BISCO Inc.). Volumetric measurements were captured at following intervals: pre-polymerized, immediate post-polymerized, 1, 3, 7, 14, 21 and 30 days. Statistical analysis: Friedman with post stepwise step-down test (p = 0.05) and Kruskal-Wallis with post stepwise step-down test (p = 0.05).
Results: All materials underwent significant polymerization shrinkage from their pre-polymerized volumes; conventional composites peaked at day 1, bulk-fill on day 3. ACTIVA BioACTIVE, Filtek One, Filtek Supreme Ultra (FSU), and Revolution2 Flowable returned to a volume statistically similar to their pre-polymerized volume. SonicFill2 demonstrated expansion, but not statistically similar to its pre-polymerized volume. Fill-Up! did not show significant expansion from peak shrinkage. FSU showed earliest hygroscopic expansion, statistically similar to pre-polymerized volume by day 3.
Conclusions: The conditions of this study found that five resins exhibited hygroscopic expansion in PBS, and 4 returned to similar pre-polymerized volumes. One product did not demonstrate any expansion. Volumetric shrinkage has been reported to correlate with increased shrinkage stress that has been associated with sensitivity and bond degradation. Composites that can relieve shrinkage stress via hygroscopic expansion may prove to possess a greater potential for clinical success.