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Background: This in vitro study was conducted to compare the effect of universal bonding
application strategy (i.e., self‑etch and etch‑and‑rinse) on marginal adaptation of bulk‑fill and
conventional composite resins in Class II restorations.
Materials and Methods: In this in vitro study sixty sound premolars extracted for orthodontic
reasons were selected. The samples were allocated to four groups based on the universal bonding
application strategy (self‑etch and etch and rinse) and type of composite (bulk‑fill and conventional).
In each group, boxes were prepared with a depth of 4 mm on the mesial surfaces. Finally, the marginal
adaptation of the samples was evaluated under a stereomicroscope. Two‑way ANOVA was used
to compare the marginal adaptation data in the study. Statistical significance was set at P < 0.05.
Results: Considering the type of universal bonding application strategy, there was a statistically
significant difference in marginal adaptation. Etch‑and‑rinse strategy showed better marginal
adaptation compared to self‑etch strategy (P < 0.001). However, there was no statistically significant
difference in marginal adaptation between the two composite resins (P = 0.829). Furthermore, the
interaction between the two factors (type of universal bonding application strategy and type of
composite resin) was not statistically significant (P = 0.629).
Conclusion: Etch‑and‑rinse bonding application strategy in both the bulk‑fill and conventional
composite resins exhibited better marginal adaptation compared to self‑etch bonding application
strategy. However, the difference of marginal adaptation between the two types of composite
resins (bulk and conventional) was not significant.