Addition of metal chlorides to a HOCl conditioner can enhance bond strength to smear layer deproteinized dentin

Objectives: To evaluate the effect of smear layer deproteinization using hypochlorous acid (HOCl) with/without metal chlorides (SrCl₂ and ZnCl₂) on the microtensile bond strength (µTBS) of two simplified adhesives to dentin. Methods: Human dentin surfaces with a standardized smear layer were pretreated using a 105 ppm HOCl solution with/without SrCl₂ (0.05 M, 0.1 M, 0.2 M, 0.4 M) or ZnCl₂ (0.05 M, 0.1 M, 0.2 M) for 5 s, 15 s, or 30 s. After the deproteinizing solution was washed out with water for 5 s, 15 s, or 30 s, pretreated surfaces were bonded with one-step self-etch adhesive Bond Force II or universal adhesive Clearfil Universal Bond Quick, and µTBS was measured after 24 h. Additionally, the deproteinizing effects of HOCl solutions with/without the metal chlorides were compared by measuring changes in the amide:phosphate ratio using attenuated total reflection Fourier transform infrared spectroscopy. Statistical analysis was performed using multifactor ANOVA, Tukey's post hoc tests and t-tests (p < 0.05). Results: Pretreatment with pure HOCl for 15 s and 30 s significantly decreased the amide:phosphate ratio (p < 0.05), indicating effective deproteinization, but the µTBS of both adhesives increased significantly only if HOCl was washed out for 30 s (p < 0.05). Increasing the concentrations of metal chlorides enabled shortening of the wash-out time down to 5 s while maintaining the improved µTBS (p < 0.05). The deproteinizing effect of HOCl was not significantly altered by the addition of metal chlorides (p > 0.05).