In this in vitro study, we investigated the efficacy of sodium hypochlorite (NaOCl) and calcium hydroxide (Ca(OH)(2)) in dissolving necrotic tissue and cleaning root canals. In the first part of the study, 0.5% NaOCl solution and Ca(OH)(2) paste and solution were tested with samples of necrotic bovine muscle in different treatment modes and for different periods. The necrotic tissue was weighed before and after the test and the percentage of weight change calculated. In the second part of the study, 40 extracted single-rooted human teeth were hand instrumented and then subjected to different irrigation regimens. The cleansing efficacy in root canals of 0.5% NaOCl with Ca(OH)(2) pretreatments and ultrasonics was examined using scanning electron microscopy. A solution of 5% NaOCl was significantly more effective than 0.5% NaOCl as a solvent of necrotic tissue. Calcium hydroxide was an effective solvent for necrotic tissue as a paste but not as a solution. Pretreatment of necrotic tissue with Ca(OH)(2) increased its solubility in 0.5% NaOCl. While 5% NaOCl plus ultrasonic irrigation produced cleaner root-canal, walls at the middle and apical thirds, 0.5% NaOCl used with the same technique achieved no root-canal cleaning. However, pretreatment of root canals with Ca(OH)(2) paste increased the effectiveness of 0.5% NaOCl plus ultrasonic irrigation, except in the coronal third of the root canal.
