Polymerization Shrinkage Stress of Bulk-Filling, Nano- and Microhybrid-Composites

Objectives: Nano-composites and bulk-filling composites claim to have low polymerization shrinkage and therefore to be superior to microhybrid-composites. The purpose of this study was to determine and compare polymerization shrinkage stress, which is clinically more relevant, of different bulk-filling, nano- and microhybrid-composites. Methods: The bulk-filling/nano-/microhybrid-composites tested were: x-tra fil U (XF) (Voco) and Quixfil U (QF) (Dentsply)/ Filtek Supreme XT A2 (FS) (3M ESPE) and Grandio A2 (GR) (Voco)/ Spectrum TPH3 A2 (TPH) (Dentsply), Filtek Z250 A2 (Z250) (3M ESPE) and KRIS A3 (KR) (Heraeus Kulzer), a chemically improved Charisma^®-update. To determine polymerization shrinkage stress, cylindrical cavities (Æ 4mm) were prepared in Araldit B epoxy resin plates (40x40x4mm) and pretreated with the Rocatec system (3M ESPE) to ensure bonding of the resin composites. The resin composite specimens (n=10) were exposed to light for 60s with a QTH curing device (Translux Energy, Heraeus Kulzer). The samples were stored dark and dry (23°C) during the first hour and after that in distilled water (37°C). Polymerization shrinkage stress data (MPa) 1h and 24h post exposure were calculated based on the diameter of the isochromatic curves of first order, obtained from the Araldit plates. Data was statistically analyzed with ANOVA (p=0.05, LSD adjustment). Results: After 1h/24h the following mean values for shrinkage stress in MPa were recorded: XF 3,97^A/4,91^b,c, QF 4,33^B/5,04^c, FS 4,02^A/4,23^a, GR 4,73^D/5,69^e, TPH 4,92^E/5,48^d, Z250 4,53^C/4,96^b,c, KR 3,95^A/4,84^b. Letters indicate statistical significance. Conclusions: Within the limits of this study, there were significant differences in polymerization shrinkage stress after 1h and 24h between all composites tested whereas bulk-filling and nano-composites did not necessarily show the best results. The nano-composite FS showed the best results after 24h, but GR and the bulk-filling materials only showed equivalent or even worse results than the microhybrid-composites Z250 and KR.