Bioactive Fillers in Bulk-Fill Composite Resins: A Comprehensive Review of the Effects on Polymerization Shrinkage Behavior and Mechanical Performance

Polymerization shrinkage remains a primary cause of marginal failure in posterior composite restorations, contributing to interfacial gap formation and secondary caries development. Bioactive filler technologies represent a paradigm shift, offering simultaneous stress reduction and therapeutic ion release through engineered matrix–filler interactions. This narrative review synthesizes current evidence on how bioactive glass (including 45S5), amorphous calcium phosphate, and surface pre-reacted glass-ionomer fillers modulate polymerization shrinkage dynamics and mechanical performance in bulk-fill systems. These systems exhibit distinct mechanisms of bioactivity, with bioactive glass (45S5) promoting ion release and apatite formation, amorphous calcium phosphate (ACP) enabling rapid calcium phosphate ion delivery for remineralization, and surface pre-reacted glass-ionomer (S-PRG) fillers providing sustained multi-ion release with buffering and antibacterial potential. A comprehensive literature search was conducted in PubMed/MEDLINE, Scopus, and Web of Science for studies published up to June 2025, including experimental investigations and reviews assessing bioactive filler integration, with studies selected based on predefined inclusion and exclusion criteria focusing on relevance and reported outcomes. The available evidence indicates that optimized bioactive formulations reduce shrinkage stress by approximately 25–40%, decreasing from 35–40 MPa in conventional systems to 22–32 MPa in bioactive bulk-fill composites while maintaining flexural strength above 100 MPa and elastic modulus within clinically acceptable ranges (11–13 GPa). However, substantial heterogeneity in filler chemistry, loading protocols, and testing methodologies limits cross-study comparisons. This variability also reflects differences in testing conditions, material compositions, and evaluation protocols across studies.