Simple optical method for measuring free shrinkage

Objectives A simple optical method for measuring polymerization shrinkage of dental composites is compared with an established dilatometer. Methods Five restorative composites were used to test the methods: Filtek Supreme Ultra (3M ESPE), Filtek LS (3M ESPE), Premise (Kerr), Gradia Direct (GC), and GC Kalore (GC). Uncured composites were attached to sandblasted silane-treated glass slides. The slides were placed sample side inside a mercury-filled dilatometer (ADAF). The mercury levels were recorded as the materials were light-cured through the glass-slides (40 s). Mercury levels, which correlated with volumetric shrinkage, were recorded for 60 min (N = 6). For the optical method, uncured composite was placed on a smooth silicone platform. A pre-polymerization image was captured under a stereomicroscope, and the specimen was light-cured (40 s). Post-polymerization images were captured at 2, 10, 60, and 90 min (N = 10). Composite outlines were traced to obtain projected surface areas (ImageJ) and volumetric shrinkage was calculated. Results were analyzed using two-way ANOVA (α = 0.05) and Pearson Correlation tests. Shrinkage deformation for both methods was modeled using finite element analysis. Results Volumetric shrinkage at 60 min ranged between 1.24% and 2.24% for dilatometer and 1.35-2.68% for optical methods. Optical method shrinkage was consistently higher than the dilatometer (P =.0001), but the ranking of the composites was the same (Pearson Correlation Coefficient 0.9997). Finite element analysis showed that lower shrinkage values of the dilatometer method could be attributed to bonding of its samples. Significance The optical method using a general-purpose stereomicroscope and public-domain software is a simple and accurate alternative to measure free shrinkage.