Light-curing protocol effects on margin continuity and conversion degree of adhesive dual-curing cements

Purpose: to investigate the marginal continuity of adhesive crowns using an universal dual curing cement subjected to different polymerization protocols, different adhesive approaches and thermal aging procedure. Materials & Methods: intact human single-rooted teeth (n=48) were prepared for a full-crown restoration with chamfer margin. After performing an optical scan, full crowns were designed with a CAD software and full adhesive crowns were milled using a lithium silicate material (Celtra Duo, Dentsply). Once completed the sintering and glazing/polishing procedures crowns were luted using an universal dual-curing cement (G-Cem One, GC) with different approaches (n=24 each): self-adhesive mode without preparing tooth surface (SAG); adhesive mode with sandblasting and primer application (CG). Samples were then divided in 4 subgroups (n=6 each) according to the curing protocol employed: remove excess with brush, wait for 1 minute and light cure for 20 s per side (SG1); remove excess with brush, self-cure of the cement (SG2); tack-cure (5 s per side), remove excess with a scaler, wait for 1 minute and light-cure for 20 s per side (SG3); remove excess with brush, wait for 1 minute and light curing for 60 seconds per side (SG4). Specimens were scanned using a Micro-CT before and after 10.000 cycles of thermocycling (5°C-55°C, dwell time 1 minute). DICOM files were processed with a segmentation software. To reveal interfacial gap progression between tooth margin and crown external gaps, internal gaps and absolute discrepancy were measured. Data, expressed in μm, were collected and statistically analyzed with three-way ANOVA and Tukey’s post hoc tests. Results: analyzed data showed that the curing protocol had a statistically significant influence on any parameter of marginal continuity, while the adhesive approach had a significant influence only on internal gap (p < 0,01) and absolute discrepancy (p < 0,01). Thermal aging didn’t significantly influence any parameter (p = 0,25). Conclusions: tack-curing technique seems to negatively affect the marginal discrepancies when luting lithium silicate adhesive crowns, while the self-curing process showed the lowest gaps. Considering the obtained results and the limitations of this study the best performance can be achieved by the self-adhesive approach combined with pre-treatment.