LETTERS TO THE EDITOR C o m m e n t s on heat steritization To the Editor: The article, "Comparison of Convection Heat Sterilization Units for the Orthodontic Office" by Johnston, Moore, and Rodu (AJO-DO, January 1991), compares two kinds of equipment engineered and marketed for completely different purposes. The household oven is designed to heat turkeys, casseroles, and cakes, whereby air is moved throughout the chamber to reduce tem'perature differences. The instrument sterilizers are designed to kill microorganisms with fast-moving heated air. Two synergistic principles, well-known to the art, are engaged: (1) energy transfer from heated air to contami~ating microbes, and (2) dehydration, a process accelerated by fast removal of moisture through rapidly moving air. The fundamentally different purposes of ovens versus contemporary heated air sterilizers is reflected in their dissimilar construction; whereas the entire oven chamber is used for cooking, only part of the sterilizer chamber is dedicated to destroying microbes. Specifically, air is warmed by heating elements and driven by a fan, both components located outside the chamber. Hot air enters the chamber, passes through the load, and exits at yet a different opening, No attempt is made to heat the chamber uniformly. To do so would defeat the operational design. In the Cox unit operated in accordance with the manufacturer's instructions, instruments are exposed to laminar air heated to 375 ° +_ 3° F and moving at 2000 linear feet per minute. In peripheral regions, temperatures and airflow rates diverge from those within the laminar airflow. Regarding the Cox sterilizer, the cardinal flaw in the Johnston study was the reporting of temperature and spore destruction as though the device was an oven. No distinction was made between points of measurement lying within the domain of sterilization and points external. As a referee to this article in the draft stages, I informed the authors of the necessity to distinguish misuse data from user-relevant information. Efficacy studies of the Cox sterilizer were carried out per grant in my laboratory at Baylor College of Dentistry. This endeavor was the first application of the FDA standard for dry heat sterilization. Accordingly, 106 spores of Bacillus subtilis were inoculated onto plier hinges. Double the time required by the device to reduce viability to 10 ° formed the antimicrobial basis of official permission to market thissterilizer. Both the application and the federal sign-off are available on request to the FDA. Regarding sporestrips, these biologic indicators are destroyed in the Cox sterilizer within 60 seconds after timing begins (American Association of Dental Research Abstract No. 1918, 1990). The shortest FDA-registered exposure is 6 minutes, representing the calculated 18A

time required to kill 1042 spores on unpackaged instruments. Dr. Robert Kolstad Baylor College of Dentistry Dallas, Texas

Reply To the Editor:

In response to Dr. Kolstad's criticism of the Farberware oven (Comparison of Convection Heat Sterilization Units for the Orthodontic Office, AM J ORTHODDENTOFAC ORTHOP 1991;99:57-63), our evaluation was intended neither to undermine the sterilizer industry nor to compare it to a household appliance. Rather, our study was a clinical evaluation of two sterilizers and an oven capable of circulating hot air in its chamber. Dr. Kolstad points out in his letter that "No attempt is made to heat the chamber uniformly." This is an interesting point. With the Cox sterilizer, there are no racks in which to place the instruments. Our understanding was that instruments were to be laid on the tray that is then inserted into the oven. It would seem to me that it is possible that the tips of some of these pliers may be in "peripheral" regions that were not in areas of the best "laminar flow." In an orthodontic office, what is the likelihood of orthodontic staff always placing the pliers in the ideal location for sterilization? While the concept of laminar air flow appears to have merit, I have not seen any evidence that such amounts of forced air (22 laminar feet per minute) are necessary. The concept of convection heat is to maintain a target temperature in the oven. This temperature is sufficient enough to be bacteriocidal. Our experimental design was a valid technique for this assessment. Dr. Kolstad also implied a flaw to our study in that no distinction was made between points lying within the domain of the sterilizer and points external. On the contrary, we specified precisely the areas in which there was spore growth (top front left, 24 hours; top front right, 24 hours; lower front middle, 24 hours). These areas were further illustrated in Fig. 5 of the spore strip placement. In the three trials, several of these areas consistently showed growth. While these areas of spore growth were not in the center of the pliers tray, they were in areas that a contaminated pliers tip could be placed. In that sense, our study implied that unless the manufacturers' sterilization procedures are strictly followed, complete sterilization may not be obtained. As pointed out in the study, both the Cox sterilizer and the Dentronix DDS 5000 are very convenient and useful sterilizers. The Orthodontic Clinic at the University of Alabama is using one of these sterilizers. It was our recommendation that a Farberware oven be used primarily as a"backup" unit in the event of a regular sterilizer needing servicing. For most offices, if there was a breakdown, an unacceptable alternative such as alcohol wipes