Photoelastic determination of polymerization shrinkage stress in low-shrinkage resin composites

Low-shrinkage resin composites are in the focus of research in posterior resin composite restoratives. The aim of the study was to examine the polymerization shrinkage stress of new composites (Venus Diamond/ Heraeus Kulzer; SDR/DENTSPLY) and an experimental low-shrinkage resin composite (Ormocer/VOCO) in comparison to established low-shrinkage resin composites (Filtek Silorane/ 3M ESPE; els/Saremco; Filtek Supreme XT/3M ESPE; Clearfil Majesty Posterior/Kuraray). Cylindrical cavities (∅ 4 mm) in Araldit-B epoxy resin plates (40×40×4 mm) were pretreated with the Rocatec system to ensure bonding of the resin composites. The resin composite specimens (n = 10) were exposed to light for 60 s with a QTH curing device (Translux energy, Heraeus Kulzer, Germany). The samples were stored dark and dry (23 °C). Polymeri-zation shrinkage stress data (MPa) 4 min and 24 h post exposure were calculated based on the diameter of the first-order isochromatic rings, obtained from the Araldit plates. The statistical analysis of the obtained data was carried out with the Wilcoxon test (p = 0.05). After 24 h, the following mean stress values and standard deviations were obtained: Venus Diamond 3.4 ± 0.27 MPa; SDR 3.3 ± 0.26 MPa; exp. Ormocer 4.0 ± 0.18 MPa; Filtek Silorane 2.8 ± 0.19 MPa; els 2.5 ± 0.09 MPa; Filtek Supreme XT 6.0 ± 0.20 MPa; and Clearfil Majesty Posterior 5.6 ± 0.15 MPa. For all materials, higher polymerization stress values were recorded after 24 h. All differences in the shrinkage data obtained after 24 h were statistically significant (p < 0.05) except Venus Diamond/SDR. Venus Diamond, els and SDR showed shrinkage data closer to that of Filtek Silorane.