High level microwave disinfection of dental gypsum casts

Chemical disinfection of impressions is strongly recommended but is problematic, environmentally and because the disinfectants may not be compatible with all impression materials. However, cross-contamination can also be avoided by disinfecting the gypsum cast. Objective: This study tested whether bactericidal microwave radiation could be generated in a household microwave oven to achieve disinfection of gypsum casts that complies with the requirements of the European Standard EN 1040, and if this procedure would be as effective as one based on recommended chemical disinfection of impressions. Methods: In 2 in vitro experiments, samples of 5 irreversible hydrocolloid (Blueprint Cremix) impressions were contaminated with suspensions of Staphylococcus aureus and Pseudomonas aeruginosa. In an in vivo experiment, 10 impressions were made of 10 subjects, poured, and the resulting casts cut in two halves. One half of each cast was exposed to 5 minutes irradiation in a microwave oven at 2450 MHz, 900 W. The other halves were left untreated as controls. In a second in vivo experiment, 10 impressions were disinfected by immersion in a 0.06 % solution of NaOCl for 3 minutes, and then poured. All casts were incubated aerobically at 37° for 6 hours and assessed for bacterial growth by counting colony forming units per ml (cfu/ml). The results were analyzed using Wilcoxon sign rank test on differences, using P < 0.05 as significance level. Results: Untreated gypsum casts showed cfu/ml counts with a median log value of 6. The microwave irradiated ones had cfu/ml counts of 0. Casts poured from chemically disinfected impressions demonstrated cfu/ml counts with a median log value of 4. Conclusion: Microwave disinfection of contaminated casts as described above demonstrated a 10⁵ or more reduction in bacterial viability, which is in compliance with EN 1040 and current infection control guidelines for the dental laboratory.