Post-irradiation surface viscoelastic integrity of photo-polymerized resin-based composites

ObjectiveA class of ultra-rapid-cure resin-based composites (RBCs) exhibited immediate post-irradiation surface viscoelastic integrity using an indentation-creep/recovery procedure. The aim of this study was to determine whether such behavior is more generally characteristic of a wider range of RBCs.MethodsEight representative RBCs were selected based on different clinical categories: three bulkfills (OBF, Filtek One Bulk Fill; VBF, Venus Bulkfill; EBF, Estelite Bulkfill), three conventional non-flowables (XTE, Filtek Supreme XTE; GSO, GrandioSo; HRZ, Harmonize) and conventional flowables (XTF, Filtek Supreme XTE Flow; GSF, GrandioSo Flow). Stainless steel split molds were used to fabricate cylindrical specimens (4 mm (dia) × 4 mm). These were irradiated (1.2 W/cm²) for 20 s on the top surface. Post-irradiation specimens (n = 3), within their molds, were centrally loaded with a flat-ended 1.5 mm diameter indenter under 14 MPa stress: either immediately (<2 min) or after 24 h delayed indentation. Stress was maintained for 2 h, then – after removal – recovery measurements continued for a further 2 h. Indentation depth (%) versus time was measured continuously to an accuracy of <0.1 μm. Data were analyzed by One-way ANOVA and Tukey post-hoc tests (α = 0.05).ResultsTime-dependent viscoelastic indentation was observed for all RBCs. For immediate indentation, the maximum indentation range was 1.43–4.92%, versus 0.70–2.22% for 24 h delayed indentation. Following 2 h recovery, the residual indentation range was 0.86–3.58% after immediate indentation, reducing to 0.22–1.27% for delayed indentation. The greatest immediate indentation was shown by VBF followed by XTF and GSF. OBF, HRZ, XTE and GSO had significantly lower indentations (greater hardness). XTE showed a significantly reduced indentation maximum compared to OBF (p < 0.05). Indentations delayed until 24 h post-irradiation were reduced (p < 0.05) for most materials.SignificanceThe indentation-creep methodology effectively characterized resin-based composites within several categories. Viscoelastic properties evaluated by the indentation-creep method confirmed that highly filled RBCs were more resistant to indentation. Indentations were reduced after 24 h post-irradiation due to further matrix-network development.