Flowable vs. conventional composite for mandibular fixed retainers: bond failure, chairside time, and association with bracket debonding — a 6-month randomized clinical trial

Abstract Background Flowable composites have gained popularity in fixed orthodontic retention due to their ease of application and reduced technique sensitivity. However, clinical evidence comparing their performance to conventional composites in fixed retainer bonding remains limited, and it is unknown whether a “debond‑prone phenotype” connects bracket and retainer failures across phases. This study aimed to evaluate debond failure rates, chairside bonding time efficiency, and the exploratory association between bracket and retainer bond failures. Methods In this randomized clinical trial, 56 patients were allocated to receive mandibular fixed retainers bonded with either GC Flowable or GC Gradia composite. Retainer failures were recorded at 0–1, 1–3, and 3–6 months. Bracket failure counts from the active treatment phase were also collected. Chairside bonding time was recorded with a digital stopwatch. Retainer failure analysis and bracket association were conducted per protocol (52 patients), while time analysis followed intention-to-treat (56 patients). Results No statistically significant differences in bond failure rates were observed between the two composites at either the tooth level or retainer level. GC Flowable showed a 46% reduction in chairside bonding time. No significant association was observed between bracket and retainer failures (RR = 1.279, P = 0.07); larger studies are needed to draw any firm conclusion. Conclusions GC Flowable offered a clinically meaningful reduction in chairside time and represents an acceptable alternative for bonding mandibular fixed retainers. Trial registration Iranian Registry of Clinical Trials (IRCT), IRCT20230430058031N1. Registered on 04 November 2023.