Influence of repeated cycles of full-strength sodium hypochlorite activation on the mechanical behavior of root canal-treated teeth: a laboratory study

To assess the effects of different activation protocols of full-strength sodium hypochlorite (NaOCl), including stepwise intraoperative activation (SIA), conventional ultrasonic irrigation (CUI), and their combination (SIA + CUI), compared to conventional syringe irrigation (CSI), on the fracture resistance of root canal-treated teeth. Ninety extracted maxillary second premolars were assigned to four experimental groups (n = 15 each) based on the irrigation protocol: CSI, SIA, CUI, and SIA + CUI, along with two control groups (n = 15): intact teeth (positive control) and saline irrigation (negative control). Standardized access cavities were prepared, followed by uniform root canal instrumentation, while irrigation protocols were conducted as assigned. Root canals were filled, and access cavities were sealed with resin composite. Specimens underwent compressive fracture testing using a universal testing machine to determine the maximum load to fracture after thermo-mechanical fatigue, and failure modes were assessed. Data were statistically analyzed, with the significance threshold set at p < 0.05 and a 95% confidence interval (CI). All experimental groups, along with the negative control group, exhibited significantly reduced fracture resistance and notable irreparable failure patterns in comparison to intact teeth (563.92 ± 28.52 N; CI: 548.13–579.71) (p < 0.001). No statistically significant differences in the maximum load to fracture were observed among the CSI (473.90 ± 13.94 N; CI: 466.18–481.62), CUI (470.94 ± 12.5 N; CI: 464.02–477.87), SIA (468.15 ± 9.68 N; CI: 462.79–473.51), SIA + CUI (467.61 ± 6.74 N; CI: 463.88–471.35), and the negative control group (474.45 ± 5.62 N; CI: 471.34–477.56), despite increased volumes and longer exposure to NaOCl in activation groups (p > 0.05). The majority of failure modes were catastrophic across the tested and NC groups, though distribution did not vary significantly (p = 0.06). The repeated activation of full-strength NaOCl via SIA, CUI, or their combination did not compromise fracture resistance compared to CSI. Root canal treatment itself reduced structural integrity relative to intact teeth, with a noted increase in the likelihood of unfavorable fracture types, underscoring the necessity for meticulous preservation of tooth structure during root canal procedures.