Evaluation of the Effectiveness of Decontamination of Dental Syringes

Aim Steam autoclaving is the gold standard for decontaminating dental instruments, but worldwide disinfection is still widely employed. We have evaluated a range of procedures for their ability to inactivate duck hepatitis B virus contaminating dental syringes.Methods Residual infectivity of virus suspensions following 2% glutaraldehyde treatment, ultrasonication or steam sterilisation at 121° or 134° was assayed by injecting day-old ducklings and examining their livers for viral DNA 2.5 weeks later. Dental syringes were contaminated with DHBV positive blood, then treated by the same methods. An anaesthetic cartridge containing water was loaded into the syringe and 400μl aliquots used to inject day-old ducklings. Used dental syringes were examined by Scanning Electron Microscopy.Results Suspension test:- ultrasonic treatment failed to inactivate DHBV in suspension, but complete inactivation was achieved by 2% glutaraldehyde and autoclaving. Syringe test:- neither ultrasonic treatment nor glutaraldehyde inactivated DHBV. Autoclaving at 134° (3 minutes) permitted transmission to 1/16 ducklings but steam sterilisation at 121° (15 minutes) was effective. Electronmicroscopy demonstrated organic debris (biofilm) in the lumen of used syringes.Conclusion Short autoclaving cycles, albeit at raised temperatures, may fail to inactivate the virus because of poor steam penetration, inadequate heat transfer and the accumulation of protective biofilm.Infection control practices have come under increasing public scrutiny over the last decade in the light of significant medical problems associated with the acquisition of blood borne pathogens following both medical and dental procedures.Successive revisions of official guidelines have made progressively more stringent recommendations for decontamination of instruments in office practice, to bring them into line with current 'best practice' procedures in hospitals. The National Health and Medical Research Council of Australia (NHMRC) recommends that all instruments, materials and medications introduced into sterile tissue must be sterile. For instruments and equipment, this should be achieved by using single-use items only, or, if the items are designed for multi-use, they must be scrupulously cleaned and adequately sterilised.The level of risk for invasive surgical, including dental procedures is ranked by Spaulding's classification scheme as 'critical' and therefore all instruments used must be sterile. Items that come in contact with intact mucous membranes however are classified as semi-critical and must be subjected to high-level disinfection with an approved disinfectant such as glutaraldehyde. In contrast to Spaulding's classification The British Dental Association (BDA) Advisory Service recommends sterilisation of all instruments that become contaminated with oral and other body fluids. Sterility can be achieved by: autoclaving (steam sterilisation under pressure at 121° to 134°C), or by dry heat at 160°C. Large scale systems of radiation, ethylene-oxide, or chemical treatment are also effective. Autoclaving is generally accepted as the method of choice to render contaminated instruments safe for reuse. Currently steam sterilisation using portable autoclaves is recommended for sterilising dental instruments for use in critical sites but it has been reported that some practitioners are still utilising high-grade disinfection such as buffered alkaline glutaraldehyde although this is now recommended for use in semi-critical areas only. This lack of compliance may arise because there is only limited direct scientific evidence about the effectiveness of various disinfection/sterilisation methods in the dental practice.Blood borne viruses are of major concern in the health care setting. Hepatitis B virus (HBV) is the most resistant blood borne virus in the environment and can persist for extended periods on contaminated surfaces or under the fingernails of providers. It is also present in high concentration in the blood and saliva and currently there are many more carriers of HBV in the community than there are carriers of HIV. Therefore, the effective elimination of HBV is the most critical indicator for assessing efficacy of infection control procedures.Unfortunately the lack of suitable infectivity models for HBV, such as tissue culture or animal inoculation, makes assessment of disinfectant efficacy difficult. The polymerase chain reaction (PCR) is a sensitive and specific test for the detection of viral nucleic acids and has been successfully employed to trace the fate of viruses in the environment. However, since nucleic acids survive treatments with heat, solvents and fixatives known to destroy microbial infectivity, PCR results obtained from disinfected or sterilised instruments may be misleading.Resistance to chemical and physical agents is very similar for members of each virus family, and this similarity is very useful when selecting test organisms for evaluation of disinfection and sterilisation procedures. Studies of the duck hepatitis B virus (DHBV) in particular have revealed similar biological and structural characteristics as HBV. The DHBV model also appears to have similar disinfectant inactivation kinetics to those reported in the very limited chimpanzee transmission studies of HBV. DHBV reaches high titre >10 ID/ml (10 times the amount of virus required to infect half of the experimental animals is present in each ml of blood) in the blood of infected ducks, and day-old ducklings are exquisitely sensitive to infection. These advantages have led to adoption of DHBV as a model for disinfectant and antiviral testing by the worlds regulatory bodies.Education of the dental health care professionals about the parameters that influence disinfectant efficacy is of paramount importance in gaining informed compliance with regulatory guidelines. It is also important that the guidelines are built on firm scientific evidence that takes into account the multiple parameters such as shape of instrument, amount and type of biological contaminant which influence efficacy in the field as opposed to results obtained using artificial test protocols. With this in mind we have used DHBV to determine inactivation efficacy of autoclaving and glutaraldehyde disinfection of blood soiled dental syringes.