Shrinkage vs conversion in low-shrinking composite restoratives

Objectives: The aim of the present study was to measure the setting shrinkage strain (%SS) and curing efficiency of five low-shrinking composite resins. Methods: The materials tested were: Ceram-X Mono/CX (Dentsply), Premise/PR (Kerr), Clearfil Majesty/CM (Kuraray), ELS/EL (Saremco), Filtek Silorane/FS (3M/Espe) all of A2 shade. Shrinkage strain was calculated using the bonded-disk method on specimens (n:5, h:1mm, dia:6mm) irradiated directly from bottom surfaces (standard mode, 40s, 850mW/cm²). Shrinkage was recorded for 10m after initial exposure by an LVDT connected to a computer. The curing efficiency was measured on the top and bottom surfaces of composite specimens (n:3, h:2,5mm) cured as above with ATR-FTIR spectroscopy. The amount of remaining C=C bonds (%RDB for all except FS) or the residual amount of epoxy rings (%RER for FS) was calculated immediately after irradiation and normalized versus the unset pastes. One-Way ANOVA and Tukey's multiple comparison test (α=0,05) were used to assess the statistically significant differences in shrinkage among the materials. Two-Way ANOVA and Bonferroni multiple comparison test (α=0,05) were used for the statistical assessment of the curing efficiency. Pearson's correlation analysis was used to determine the correlation between %SS and %RDB or %RER. Results: The %SS values for each composite were: CX 2,29 ±0,27^a, PR 1,82 ±0,13^b,c, CM 1,54 ±0,08^c,d, EL 2,03 ±0,07^a,b, FS 1,34±0,18^d. The %RDB values (T=top, B=bottom) were: CX T=56,92±3,17^a,b B=70,06±4,21^c,d , PR T=65,65±5,9^a B=73,52±3,69^c , CM T=50,61±3,25^b B=62,59±3,99^d , EL T=62,71±6,71^a B=72,2±8,32^c and the %RER were: FS T=32,46±2,2 B=44,72±2,15. (same letters show mean values with no statistically significant differences). No correlation was found between the %SS and %RDB or %RER. Conclusion: The composites tested varied in polymerization shrinkage, showing medium to low percentages and also varied in their curing efficiency. Polymerization shrinkage was not found to depend solely on the setting conversion.