The Effects of Temperature on Sodium Hypochlorite Short-Term Stability, Pulp Dissolution Capacity, and Antimicrobial Efficacy George Sirtes, DMD, Tuomas Waltimo, DMD, PhD, Marc Schaetzle, DMD, and Matthias Zehnder, DMD Abstract The purpose of this study was to test some effects of preheating NaOCl solutions using a commercially avail- able syringe heating device. Irrigating solution temper- atures in 10-ml syringes were measured. Stability of 5.25, 2.62, and 1% NaOCl solutions for 60 min at 20, 45, and 60°C was assessed using iodine/thiosulfate titration. Human pulp tissue dissolution capacity of a 1% NaOCl solution was gauged at the latter temper- atures, and compared to corresponding values with a 5.25% solution at 20°C. Killing efficacy of diluted NaOCl solutions against 48-h incubations of Enterococ- cus feacalis ATCC 29212 was compared at 45°C and 20°C. Using the heating device, a 20°C solution reached 45°C and 60°C in 7 and 20 min, respectively. Solutions remained stable during the observation pe- riod. The 1% NaOCl solution at 45°C dissolved pulp tissues as effectively as the 5.25% solution at 20°C, while the 60°C/1% solution was significantly more effective (p  0.05). A 100-fold increase in killing efficacy was observed between corresponding NaOCl solutions at 20°C and 45°C. Key Words Sodium hypochlorite, temperature, pulp dissolution, antimicrobial From the Department of Preventive Dentistry, Periodon- tology, and Cariology, University of Zurich Center for Dental Medicine Zurich, Switzerland (Drs Zehnder and Sirtes); Depart- ment of Orthodontics and Pediatric Dentistry, University of Zurich Center for Dental Medicine, Zurich, Switzerland (Dr Schaetzle); and the Institute of Preventive Dentistry and Oral Microbiology, University of Basel Center for Dental Medicine, Basel, Switzerland (Dr Waltimo). Address requests for reprints to Dr. Zehnder, Department of Preventive Dentistry, Periodontology, and Cariology, University of Zu ¨ rich Center for Dental Medicine, Plattenstrasse 11, CH 8028 Zu ¨ rich, Switzerland; E-mail: matthias.zehnder@zzmk.unizh.ch. Copyright © 2005 by the American Association of Endodontists S odium hypochlorite (NaOCl) was first recommended as an antiseptic solution by Henry Dakin to irrigate open wounds in World War I (1). Several years later, the use of chlorinated soda solutions was advocated for root canal therapy (2). Today, NaOCl solutions are still the most favored root canal irrigants, based on their antibacterial, tissue-dissolving, and lubricating properties (3). In addition, they have a good shelf life if correctly stored, are inexpensive, and easily available from many sources (4). The choice of concentration of NaOCl is still a matter of debate. Dakin originally used an aqueous 0.5% NaOCl solution. Later, NaOCl solutions of higher concentrations were advocated for root canal debridement (5). The antibacterial efficacy of hypochlo- rite solutions is a function of their concentration (6), as is their tissue dissolution capacity (7), and on the other hand, their caustic potential (8). Serious incidents have been reported when concentrated hypochlorite solutions were inadvertently forced into periodontal tissues (9), or when such a solution leaked through the rubber dam onto the patient’s skin (10). Simply increasing hypochlorite concentrations in irrigating solutions over 1% NaOCl to render them more effective may not be advisable. One alternative approach to improve the effectiveness of hypochlorite irrigants in the root canal system could be to increase the temperature of low-concentration NaOCl solutions. This appears to improve their immediate tissue-dissolution capacity (11, 12). At the same time, the systemic toxicity of preheated NaOCl irrigants, once they have reached body temperature, should be lower than the one of more concentrated non- heated counterparts with similar efficacy in the root canal. However, there is only little data available on features of heated hypochlorite solutions relevant to the endodontist. Previous studies provide some insight into chemical stability, dissolution action and antimicrobial efficacy of NaOCl preparations, but the findings appear to be somewhat contradictory. In addition, available data pertaining to heated hypochlorite solutions were not always obtained using endodontically relevant tissues and/or microbiota, or at temperatures higher than 37°C. Despite this lack of knowledge, heating devices for endodontic irrigating syringes have recently been introduced to the dental market. Consequently, the purpose of this in vitro study was threefold: (1) to evaluate the short-term chemical stability of preheated NaOCl solutions using a commercially avail- able heating device for irrigating syringes; (2) to compare dissolution capacities of NaOCl solutions on necrotic human pulp specimens at different temperatures; and (3) to assess the efficacy of preheated NaOCl solutions on E. faecalis, a species associated with failed endodontic therapy (13). MATERIALS AND METHODS Solutions In this study 1, 2.62, and 5.25% sodium hypochlorite solutions (NaOCl in water, wt/wt) were used. The aforementioned solutions were prepared by diluting a pure 14% NaOCl solution (Thommen & Co. AG, Ru ¨ ti bei Bu ¨ ren, Switzerland). The available chlo- rine (OCl - and HOCl) content of NaOCl solutions was measured using a standard iodine/ thiosulfate titration method (14). The solutions were protected from oxidation in tightly covered amber glass bottles and stored in a refrigerator at 5°C between experiments. Basic Research—Technology JOE — Volume 31, Number 9, September 2005 Effects of Preheating NaOCl 669