Application of automated thermal disinfection instead of sterilisation procedures for treatment of rotating dental instruments: efficacy against viruses?

In dentistry it is of primary importance to take into consideration microbial transfer due to the nature of the construction of rotating dental instruments. This aspect was the starting point for our research with the question whether or not sterilisation is fundamentally necessary for slow and high speed hand pieces to make them "safe" out of a virological point of view, or whether a thermal disinfection could also possibly be adequate for this purpose. In this context, we tested the efficiency of the cleaning and disinfection capacity of an automated steam disinfection and sterilisation unit (Sirona Hygiene Center, Siemens, AG, Bensheim) intended to the hygienic treatment of dental instruments with respect to viruses. In model tests the corresponding instruments were experimentally infected with herpes simplex virus type 1 (HSV) and simian vacuoling virus (SV40). As indicator systems we used for both cell cultures (measurement of the degree of infectiosity) and (for HSV) polymerase chain reactions (PCR; determination of viral nucleic acids). In the tests for (residual) infectiosity after thermal disinfection (as an isolated step of the Hygienic Centre) and also for a combination of cleaning and subsequent thermal disinfection (also after protein application), no infectious virus could be found in the interior of the slow handpieces and turbines tested. In opposite to this, infectious HSV and SV40 could be found after completion of every isolated cleaning program in the turbine (in all three ducts) and in the slow handpiece (only in the gearbox duct in the case of HSV, and in the case of SV40 also in the water and air ducts in very small amounts). The PCR analyses showed that no nucleic acids could be found in both instruments (in the air and water ducts) following a practice-relevant combination of cleaning and disinfection, but that PCR-positive signals were obtained for the larger-volume gearbox and drive and return air ducts in 1 or 2 of 3 test samples. The detection of viral nucleic acid proves that it is not a matter of complete removal of the infectious agents from the instruments, but rather that they are simply inactivated. Since the test samples did not show similar amounts of viral nucleic acid, it demonstrates the strong influence of the basic parameters (efficiency of the virus contamination, flushing conditions and the like). Our experimental model demonstrate that thermal disinfection may be adequate to prevent virus contamination of rotating dental instruments, while compulsory sterilisation is not mandatory. Further studies are necessary to demonstrate whether the present data are applicable for rotating dental instruments from other manufactures.