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This study was designed to evaluate the effect on root dentin microhardness of 2.5% and 6% sodium hypochlorite solutions for various irrigation periods. Forty-two bovine roots were divided into seven groups. The control group was irrigated with saline. The experimental samples were continuously irrigated with 2.5% or 6% NaOCl for 5, 10, or 20 min. Microhardness was measured at depths of 500 mum, 1000 mum, and 1500 mum from the lumen. A decrease in microhardness was found at 500 mum between the control and samples irrigated with 6% NaOCl and 2.5% NaOCl (p = 0.352, p = 0.084 respectively) at all irrigation periods. There also was a significant difference in groups irrigated for 10 or 20 min (p = 0.001, p < 0.001 respectively). At all distances, the decrease in microhardness was more marked after irrigation with 6% NaOCl than 2.5% NaOCl.

Fifty‐eight bleached pulpless teeth were re‐examined after periods of 1–8 years. Recall examination included recording of the esthetic results, clinical findings and radiographs. External root resorption was found in 4 of the cases (6.9%), and was progressive in 2 cases and arrested in 2 cases. There had not been any pre‐ or postoperative trauma in any of the 4 cases. The occurrence of resorption was not related to the bleaching technique used. Resorptive lesions were found to have been initiated apical to and not at the cemento‐enamel junction. Esthetically, the bleaching was considered successful in only 50% of the cases, acceptable in 29% and failed in 21%. These results caution against indiscriminate use of bleaching with hydrogen peroxide, and emphasize the importance of preventive measures and postoperative follow‐up of bleached pulpless teeth.

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