The Effect of Heating to Intracanal Temperature on the Stability of Sodium Hypochlorite Admixed with Etidronate or EDTA for Continuous Chelation

Introduction: Tetrasodium etidronate (Na-4 etidronate) and tetrasodium EDTA (Na-4 EDTA) are chelators that can combine with sodium hypochlorite (NaOCl) as a 1-mix endodontic irrigant in a process called continuous chelation. The therapeutic window of these mixtures is determined by the chemical reaction between NaOCl and the chelator. At room temperature, this window is 60 minutes for Na-4 etidronate and 30 minutes for Na-4 EDTA. Because reaction kinetics are influenced by heat, this study assesses the influence of heating to an intracanal temperature of 35 degrees C on the therapeutic window in continuous chelation. Methods: The loss of free available chlorine (FAC) in NaOCl mixtures with Na-4 etidronate or Na-4 EDTA was determined by iodometric titration at 23 degrees C +/- 0.7 degrees C (23 degrees C) and 34.6 degrees C +/- 0.3 degrees C (35 degrees C) at 1, 20, 40, and 60 minutes after mixing. The pH and temperature of the mixtures were measured. Results: At 23 degrees C, 18% Na-4 etidronate/5% NaOCl solutions at 20, 40, and 60 minutes lost 4%, 9% and 18% FAC, and at 35 degrees C, they lost 20%, 68% and 92% FAC; 5% Na-4 EDTA/2.5% NaOCl solutions at 20, 40, and 60 minutes at 23 degrees C lost 88%, 94%, and 97% FAC, and at 35 degrees C, they lost 96%, 99%, and 100%. Decreases in FAC were accompanied by pH declines. Conclusions: The effect of heating to 35 degrees C from a room temperature of 23 degrees C on 18% Na-4 etidronate/5% NaOCl solutions reduces its therapeutic window to 20 minutes. Solutions of 5% Na 4 EDTA/2.5% NaOCl are not useful in the continuous chelation technique. Tracking pH changes could be used to estimate NaOCl degradation.