Flexural strength of bulk-fill resin composites: A Bayesian hierarchical network meta-analysis

This study aimed to systematically synthesize existing in‑vitro data on clinical variability and mechanical performance differences of bulk‑fill materials.

PubMed, Scopus, and Web of Science were searched up to January 1, 2025. Inclusion criteria was restricted to peer-reviewed in-vitro studies evaluating direct resin composites intended for posterior use. Primary outcome was flexural strength (MPa). Treatment arms were hierarchically specified as composite classes (bulk-fill flowable, bulk-fill sculptable, fiber-reinforced, sonic/heat-activated, chemically activated/alkasite bulk-fill, and conventional sculptable) and their respective sub-brands. Results were synthesized through Bayesian hierarchical model.

In total 44 studies (187 arms; 2134 specimens) were analyzed, with specimen numbers per arm ranging from 10 to 140. At class level, conventional sculptables ranked highest, followed by fiber-reinforced and sonic/heat-activated bulk-fills. Bulk-fill flowable and chemically activated/alkasite occupied the lowest ranks. Conventional sculptable demonstrated significant superiority to bulk-fill flowable and exhibited higher flexural strength relative to chemically/alkasite. At brand level; among conventional microhybrids, Grandio and Filtek Z250, among sculptable bulk-fills, Filtek One Bulk Fill Restorative and SonicFill, showed higher mean flexural-strength estimates. In contrast, flowable bulk‑fills such as Surefil SDR Flow and Tetric EvoFlow Bulk Fill tended to show lower values. Filtek One Bulk Fill Restorative demonstrated significantly higher flexural strength compared to SDR Flow and Tetric EvoFlow Bulk Fill. However, confidence in most class- and brand-level comparisons was rated as low or very low, primarily due to heterogeneity, indirectness, and imprecision.

While conventional composites generally outperformed bulk-fill composites, some sculptable bulk‑fills exhibited comparable strength. Due to high heterogeneity, current testing and reporting practices provide limited support for clinically meaningful differentiation among bulk-fill composite classes.

Flexural strength data in isolation are insufficient for product selection among bulk-fills; clinical decisions should rely on comprehensive evidence, including multiple in vitro properties and clinical performance data rather than single laboratory rankings.