Abstract
<title>Abstract</title> <p> <bold>Objective</bold> : To investigate the effect of incremental saliva contamination mixed directly into the orthodontic primer on the shear bond strength (SBS) of stainless-steel brackets and to analyze the associated failure modes. <bold>Methods</bold> : A total of 150 freshly extracted bovine mandibular incisors were randomly allocated into five groups (n = 30 per group) based on the volume ratio of artificial saliva to Transbond XT™ Primer (Primer:Saliva, v/v): G0 (100:0, control), G5 (95:5), G10 (90:10), G20 (80:20), and (50:50). Brackets were bonded following standard protocols, including acid etching, primer application, and light curing. After 24 hours water storage and 500 thermocycles (5–55℃), SBS was tested using a universal testing machine. Adhesive Remnant Index (ARI) scores were evaluated under a stereomicroscope. Data were analyzed using one-way ANOVA with Tukey’s HSD post-hoc test and the Kruskal-Wallis H test with Dunn-Bonferroni correction. <bold>Results</bold> : The Shapiro-Wilk test confirmed normal distribution across all groups, and Levene’s test indicated homogeneity of variances ( <italic>P</italic> > 0.05). One-way ANOVA revealed significant intergroup differences in SBS ( <italic>F</italic> = 145.67, <italic>P</italic> < 0.001). No significant difference was observed between G0 (16.76 ± 1.87 MPa) and G5 (15.53 ± 2.16 MPa) ( <italic>P</italic> = 0.053). However, SBS decreased significantly in groups with ≥ 10% contamination (G10: 10.24 ± 1.59 MPa, G20: 7.07 ± 1.34 MPa, G50: 4.26 ± 1.42 MPa, all <italic>P</italic> < 0.001). ARI scores shifted significantly toward lower values with increasing contamination ( <italic>H</italic> = 63.82, <italic>P</italic> < 0.0001), indicating a transition from cohesive/adhesive failure to interfacial failure at the enamel-resin junction. <bold>Conclusions</bold> : Inadvertent mixing of saliva into orthodontic primer significantly compromises bracket bond strength in a dose-dependent manner. While minimal contamination (≤ 5% v/v) may be clinically tolerable, contamination levels of 10% or higher result in a statistically and potentially clinically significant reduction in bond integrity. Clinicians should strictly avoid cross-contamination between the primer brush and saliva, as this form of contamination creates an irreversible weak interface. </p>