Influence of sodium hypochlorite concentration and treatment time on enamel surface morphology and shear bond strength for orthodontic attachments: an in-vitro study

Background: Clear aligner therapy is widely used because of its aesthetics and comfort; however, repeated debonding of resin attachments remains a clinical concern. This study evaluated the effects of sodium hypochlorite (NaOCl) pretreatment applied prior to phosphoric acid etching on enamel surface morphology and shear bond strength (SBS). Methods: Sixty extracted premolars were randomly assigned to ten groups (n = 6 per group) in a staged experimental design. The enamel surfaces were pretreated with NaOCl at concentrations of 0%, 3%, 5.25%, or 10% for 30 s or with 5.25% and 10% for 20, 40, or 60 s before acid etching. Surface morphology was examined using scanning electron microscopy (SEM), and surface characteristics were semi-quantitatively analyzed using the ImageJ software. Standardized resin attachments were bonded, and the SBS was measured using a universal testing machine (UTM). Data were analyzed using one-way and two-way analysis of variance (ANOVA) with Tukey’s post-hoc test (α = 0.05). Results: NaOCl pretreatment altered enamel surface morphology and increased SBS in a concentration- and time-dependent manner. SBS values were significantly higher in the 5.25% (226.60 ± 22.17 N; 113.30 ± 11.09 MPa) and 10% (239.21 ± 51.52 N; 119.61 ± 25.76 MPa) groups than in the control (P < 0.05). The highest SBS was observed with 10% NaOCl for 60 s (276.01 ± 63.67 N; 138.01 ± 31.84 MPa), which was significantly greater than shorter application times at the same concentration (P < 0.001). Two-way ANOVA demonstrated significant effects of both concentration and exposure time, as well as their interaction (P < 0.001). Conclusion: NaOCl pretreatment applied prior to phosphoric acid etching significantly enhanced the SBS of resin attachments under controlled in vitro conditions. These findings suggest that NaOCl may influence enamel surface characteristics and bonding performance; however, further studies are warranted to clarify the underlying mechanisms and evaluate its clinical applicability.