Background: To compare the cervical adaptation of paste-like bulk-fill resin composites placed with either conventional or pre-heating insertion techniques to that of flowable bulk-fill resin composites.

Methods: Six paste-like resin composites (one conventional, five bulk-fill) and five flowable resin composites (one conventional, four bulk-fill) were placed into Class II box-only cavities. Paste-like composites were placed with either conventional insertion or after pre-heating (68 °C). Flowable composites were placed only with the conventional insertion technique; no pre-heating was performed. After cavities were restored, specimens were subjected to thermal aging. Then, ammoniacal silver nitrate (AgNO₃ ) tracer was used followed by micro-computerized tomography scanning. AgNO₃ penetration volume was calculated. Data was analyzed with 2-way ANOVA followed by Tukey's multiple comparison test (α = 0.05).

Results: Pre-heating improved the cervical adaptation of all composites compared to conventional placement (p < 0.05) except for the fiber-reinforced composite. All flowable resin composites showed higher AgNO₃ penetration than their paste-like pre-heated counterparts (p < 0.05). AgNO₃ penetration volume at the cervical margin was significantly lower when paste-like bulk-fill resin composites were pre-heated to 68 °C before they were placed into Class II box-only cavities.

Conclusions: Pre-heating significantly enhanced the cervical adaptation of paste-like bulk-fill resin composites, offering superior marginal integrity compared to both their non-heated counterparts and flowable composites.