INTRODUCTION: The aim was to compare the compressive strength of dentin after irrigation with different concentrations of sodium hypochlorite (NaOCl).

METHODS: Two hundred and fifty-nine extracted human tooth roots with no root caries or previous endodontic treatment were collected. They were sorted by size and length, and assigned to 7 groups (n=37): negative control (no instrumentation, no irrigant), positive control (saline irrigant), 1%, 2.5%, 4%, 5.25%, and 10% NaOCl. A total of 13 mL NaOCl or saline (in 1mL increments) was used per canal while being instrumented to #35 files (approximately 13 min). EDTAC (15%) was then used to remove smear layer (2 min), followed by NaOCl as a final rinse. The roots were sectioned into 2mm thick discs and subjected to compression testing.

RESULTS: There was a significant difference between the negative control group and all instrumented groups (P < .05). The positive control group was significantly different to the 5.25% group (P < .05) and had a low P-value when compared to the 2.5% group and the other higher concentrations (P ≅ .1). When the NaOCl groups were compared to each other, there was a significant difference between 1% NaOCl and all higher concentrations (P < .05), but no significant difference when 2.5% NaOCl was compared to higher concentrations.

CONCLUSION: A concentration of 1% NaOCl had the least effect on the compressive strength of dentin and this was significantly different to all higher concentrations tested. Therefore, the use of concentrations of NaOCl above 1% should be reconsidered.