Objective: This study aimed to determine whether the clinical performance of (Sonicfill 2) and (Fill Up) bulk fill resin composite is comparable according to the modified United State Public Health Service (USPHS) criteria.

Methods: A total of 40 class II restorations were done following the manufacturer's instructions, one side of each patient's mouth received both types of restorations (SonicFill 2) and (Fill Up) in two adjacent posterior teeth. The restorations' Color match (CM), Marginal adaptation (MA), Marginal discoloration (MD), Anatomic form (AF), and Secondary caries (SC) were evaluated based on Ryge's criteria (modified USPHS) at baseline (after 1 week), as well as 6 month, 12 months, and after 18 months of follow-up by two calibrated examiners. The statistical analysis utilizing the Friedman and Wilcoxon tests, A p-value below 0.05 was deemed to be statistically significant.
Results: There were no significant differences between the two types of bulk fill resin composite at baseline, and after six months, as 100% of both restorations had Alpha (A) score. Following 18-month period, 60% of (Fill Up) rein composite restorations displayed a decline in the (A) score and revealed Bravo (B) score regarding color match and anatomical form criteria, a statistically significant difference was observed between the two restorations (p ≤ 0.05), while in the other assessed criteria, both restorations displayed the (A) score.
Conclusion: Within 18 months clinical follow up period, the two tested bulk-fill resin composite restorative materials exhibited satisfactory clinical performance as a direct restoration for class II cavity preparations.