Effects of Humidity Variability and Acid Etching on Immediate and Aged Dentin Bond Strength of Three Universal Adhesives

Background Limited research exists on the effects of dentin moisture levels and etching protocols on immediate and aged microshear bond strength (μSBS) of universal adhesives. Objective To investigate how dentin moisture (over-wet, wet, dry) and etching conditions (etched/non-etched) influence μSBS of Prime Bond Universal (PBU), Clearfil Universal Bond Quick (UBQ), and Single Bond Universal (SBU). Methods One hundred eighty human molars were prepared to expose dentin, divided into nine groups, and treated with three moisture states on etched or non-etched surfaces. After bonding with adhesives and Filtek Z350 XT resin, specimens were sectioned into 1 mm×1 mm×10 mm rods. Immediate μSBS was measured (20 rods/group); 20 rods/group underwent 10,000 thermal cycles (5–55°C) for aging. Fracture patterns and resin tags were analyzed via stereomicroscopy and scanning electron microscopy (SEM). Data were compared using Multi-way ANOVA and t-tests ( a = 0.05 ). Results 1. Moisture effects: PBU showed no significant differences in immediate μSBSacross moisture levels (etched/non-etched). UBQ exhibited significant variations between all moisture states ( P<0.05 ), whereas SBU maintained consistent strength on etched surfaces regardless of moisture ( P>0.05 ). 2. Etching impact: Etching significantly improved μSBS for all adhesives ( P < 0.05 ) except SBU in wet/over-wet groups. 3. Aging: Thermal cycling reduced bond strengths universally, but only SBU showed significant declines on etched dentin ( P<0.05 ). 4. Adhesive comparison: SBU outperformed PBU in immediate strength under most conditions but matched PBU on etched wet/over-wet dentin. UBQ surpassed SBU on etched wet dentin ( P < 0.05 ) but underperformed under non-etched over-wet settings. PBU had lower baseline strength than SBU but superior moisture tolerance. 5. Fracture modes: Adhesive failure dominated with no inter-adhesive differences. 6. SEM analysis: Etched dentin displayed dense, elongated resin tags; non-etched surfaces showed sparse, short tags. Conclusions 1. Etching universally enhanced bond strength via mechanical interlocking. 2. SBU exhibited optimal performance with strong humidity tolerance post-etching. 3. UBQ required strict moisture/etching control due to variable adaptability. 4. PBU, though initially weaker, proved reliable in variable humidity.