Impact of bonding protocols and physical-mechanical properties of composite on durability and failure rate of fixed orthodontic retainers: a systematic review of laboratory studies

Background: Adhesive failure is the most common complication in fixed orthodontic retention. Understanding the mechanical properties of composites used in bonding fixed retainers and finding proper bonding protocols are essential to achieve proper durability of the adhesive interface and therefore, reliable fixed retention. The aim of the systematic review is to evaluate and summarize the available evidence on composite materials used for fixed orthodontic retention bonding, particularly in relation to their shear bond strength, wear resistance, microleakage, and adhesive stability within different bonding protocols. Materials and methods: A comprehensive search was conducted across five databases: PubMed, Web of Science, Scopus, Embase, and EBSCO Dentistry & Oral Sciences Source. The final search was performed on 10th July 2024. The keywords used were: “orthodontic” AND (“retention” OR “retainer” OR “fixed retainer”) AND (“composite” OR “adhesive”). Inclusion criteria were: in-vitro studies on debonding of fixed retainers, comparing multiple composites based on material type, applied forces, structural homogeneity, or bonding procedures. Exclusion criteria were: incomplete studies, case reports, editorials, short communications, and reviews. Studies published after 2000 were included with no language restrictions. The QUIN assessment tool was employed to evaluate the risk of bias. PROSPERO registration number is CRD42023487320. This research did not receive any external funding. Results: The search identified 927 records, with 504 duplicates removed, leaving 423 articles for analysis. Ultimately, 39 studies were included in the qualitative synthesis. Eleven studies exhibited low risk of bias, while the remaining presented medium risk of bias. It has been found that composite with higher filler content are more resistant to mechanical stress. Urethane dimethacrylate resin demonstrated superior adhesion. Etch and rinse bonding protocols yielded higher adhesion values and reduced composite microleakage, whereas self-adhesive composites necessitated more frequent check-ups. There is no direct relationship between the power of the polymerization lamp and composite conversion rate. To optimize fixed retention outcomes, composites should have a minimum islet width of 4–4.5 mm and a thickness of at least 1 mm. Conclusions: Bonding protocols to enhance the longevity and effectiveness of orthodontic retainers should comprise etch and rinse protocol, and composite islets of at least 4–4.5 mm width and 1 mm thickness.