Shear-strength of Self-etch Bondings Produced by Inexperienced Experimenters

Objectives: The aim of the present study was to evaluate the shear strength to dentin of four self-etch bonding agents applied by inexperienced experimenters.
Methods: The experiments were conducted by dental students (16 students in 2017 and 14 students in 2018, during their regular classes). The dentin bond strengths of two so-called self-etch “universal bonding agents“ (SBU, Scotchbond universal, 3M Espe; PBactive, Prime&Bond active, Dentsply), a self-etch/etch-and-rinse bonding agent applied in self-etch mode (XS, Xeno Select, Dentsply) and a two-step self-etch bonding agent (OBXTR, Optibond XTR, Kerr) were evaluated. 288 (2017) and 252 (2018) composite-cylinders were fixed onto bovine teeth with the bonded-systems perpendicular to the exposed dentin surfaces. Shear-tests were conducted with a universal testing-machine (Zwick). Data were analysed by mixed-model ANOVA.
Results: OBXTR and SBU showed significantly higher shear-strengths than XS (2017) or PBact (2018) (p<0.001). In 2017 shear-strengths of OBXTR were significantly higher than with SBU (p<0.001), whereas there was no significant difference between OBXTR and SBU in 2018. In 2018 shear strengths of all tested materials were slightly higher than in 2017 (Table 1). Compared to a previous study (2015) with the same experimental setting in which XS showed high initial testing failures (33 %) almost no (max 1%) initial testing failures were observed with all materials tested in 2017 and 2018 (Table 2).
Conclusions: Elimination of testing failures and constant improvement in shear strengths were most likely caused by more extensive and precise instructions given to the students especially regarding the air-drying technique. In the hands of inexperienced experimenters, over the years, the two-step system OBXTR produced the highest shear strengths (and thus least technique sensitivity) followed by SBU.