Purpose: To investigate the effect of radiotherapy, doxycycline and adhesive systems on the microtensile bond strength (mu TBS) of the dentin-composite interface. Methods: 60 human third molars were sectioned to expose middle dentin surface and distributed according to: (1) adhesive system (Adper...

Purpose: To investigate the effect of radiotherapy, doxycycline and adhesive systems on the microtensile bond strength (mu TBS) of the dentin-composite interface. Methods: 60 human third molars were sectioned to expose middle dentin surface and distributed according to: (1) adhesive system (Adper Scotchbond MP and Clearfil SE Bond) applied, (2) application or not of doxycycline, and (3) submission to 60 Gy total radiation (2 Gy daily doses, 5 days/week for 6 weeks) before restoration procedure (RtRes); after restoration procedure (ResRt) or not submitted to radiotherapy (Control group). Specimens were tested for mu TBS and mode of failure were evaluated under optical microscopy. The bonding interface was evaluated with a scanning electron microscope (SEM). Data was submitted to three-way ANOVA and Tukey's test (alpha= 0.05). Results: There was no significant difference between the mu TBS (MPa) of Adper Scotchbond MP (25.5 +/- 11.1) and Clearfil SE (27.6 +/- 9.1). Control (30.5 +/- 10.9) and ResRt (29.1 +/- 10.4) presented mu TBS significantly higher than RtRes (23.1 +/- 7.2). Doxycycline (21.7 +/- 7.6) significantly reduced mu TBS compared to groups without doxycycline application (33.6 +/- 8.6). Dentin cohesive failure mode was predominant for RtRes and mixed failure mode for ResRt. Mixed and adhesive failures were frequently observed in control groups. SEM showed adhesive penetration in dentin tubules in all groups, regardless of the radiotherapy and the application of doxycycline. The radiotherapy before composite restoration procedure decreased the mu TBS. No statistical difference was observed between the adhesive systems. The doxycycline reduced mu TBS regardless of the other conditions

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