Efficacy of Microwave Oven Sterilization of Gypsum Casts

Objective: To evaluate the efficacy of microwave oven sterilization of gypsum dental casts. Methods: The dental casts (Die-Keen, Heareus Kulzer Inc., Armonk, New York) were divided into five test groups of 15 casts each and 5 casts each for the controls (inoculated/no microwave exposure and no inoculation/no microwave exposure). Dental casts were soaked for two hours in an overnight culture (~10^6 cfu/ml) of S. aureus, E. faecalis or C. albicans respectively in a Tryptic Soy Broth. The casts were air-dried for ten hours after which they were individually exposed in a 1200 watt household microwave oven at full power for 1, 2 or 3 minutes. Each cast was then immersed in 325 ml of PBS and stirred vigorously for 30 minutes. A 2 ml sample was collected, serially diluted (1:10 dilution) and a 100 µl of the specimen of both undiluted and diluted were plated in Blood Agar Plates. The plates were incubated at 37 Celsius degrees for 24-48 hours. The surviving colonies were counted and computed for the cfu/ml. Kruskal-Wallis rank test and Wilcoxon-Mann-Whitney rank sum test were used for data analysis at the significance level (á=.05). Results: Considerable growth of bacteria occured in the inoculated/no microwave control whereas the uninoculated/no microwave had no growth. Three minutes of microwave oven exposure showed 100% killing rate for all three organisms tested compared to 91% against S. aureus, C. albicans and 86 % against E. faecalis for the 1 minute exposure; 99% against S. aureus, C. albicans and 85% against E. faecalis for the 2 minute exposure. The 3 minute exposure was significantly different (p value <0.0001) from both 1 and 2 minute exposure. Conclusions: The tested microorganisms on the dental casts can be effectively eliminated by using a 1200 watt microwave oven at full power for 3 minutes.