Dental impressions are an integral part of many common dental procedures. They are also a potential source of cross infection as they may be contaminated with saliva or blood, which can contain both viral and bacterial pathogens, including HIV and hepatitis A, B, and C viruses. Although most pathogens do not survive for long periods of time outside the body, pathogens can survive for several days in protein-containing body fluids. Microorganisms can be transferred from contaminated impressions to dental casts, and oral bacteria may remain viable in set gypsum materials for several days.
Pathogenic microorganisms have the potential to cause both airborne and blood borne infections if dental impressions are not effectively decontaminated; meaning that direct transmission of infection could occur from patients to dental staff.
A study which examined the dental impressions of 54 patients prior to disinfection, showed infection frequencies of Streptococci (100 per cent), Staphylococci (55.6 per cent), Candida (25.9 per cent), methicillinresistant Staphylococcus aureus (25.9 per cent) and Pseudomonas aeruginosa (5.6 per cent).
Although less common, impressions may become contaminated with respiratory pathogens, which are coughed into the mouth from the lungs. Dental impressions from a patient previously diagnosed with tuberculosis were found to harbour the causative agent Mycobacterium tuberculosis. This further demonstrates the microbial contamination which may be found on dental impressions and the need for effective decontamination procedures.
The responsibility for ensuring impressions have been cleaned and disinfected before dispatch to the dental laboratory lies with the dentist. HTM01-05 states in section 7.1 that all impressions must be decontaminated prior to sending to the laboratory.
A postal survey of dentists registered with the UK General Dental Council found that 95 per cent of dental technicians had received bloodcontaminated impressions and 15 per cent had encountered bloodfilled voids upon trimming back the peripheries of impressions. A wide range of solutions, often at different dilutions of the same product, were used by the dentists to disinfect dental impressions; 37.2 per cent rinsed the impressions with water. Of the dentists completing the questionnaire, only 50 per cent claimed to disinfect all impressions and only 24 per
cent actually communicated their disinfection procedures to the dental laboratory receiving the impressions. The authors concluded that: “Compliance with good practice is less than ideal and education in impression disinfection for both dentists and dental technicians is required to address this.”
Infection prevention recommendations are not always performed to the highest standards in other EU countries. A survey undertaken in Sweden, found a 72 per cent bacterial contamination level in alginate impressions delivered to a large dental laboratory and only half of the dental clinics had any kind of disinfection routine.
This uncertainty about the disinfection of dental impressions is reflected in the decontamination policies of many dental laboratories, which contain specific guidance for cleaning and disinfecting dental impressions received from dental practices. Uncertainty about impression disinfection poses potential risks to the health of both dental practitioners and the receiving dental technicians.
Guidelines in section 7 of HTM01- 05 give clear instructions about the management of dental impressions. Decontamination involves a multistep process which should be carried out in accordance with the device or material manufacturer’s instructions. As impressions cannot withstand sterilisation, adequate cleaning and disinfection must be conducted.
The following procedure is recommended [HTM01-05]:
a) Immediately after removal from the mouth, any device should be rinsed under clean running water. This process
should continue until the device is visibly clean.
b) All devices should receive disinfection according to the manufacturer’s instructions. This will involve the use of specific cleaning materials noted in the CE-marking instructions.
c) After disinfection, the device should again be thoroughly washed.
d) If the device is to be returned to a supplier/laboratory or in some other fashion sent out of the practice, a label to indicate that a decontamination process has been completed.
Disinfection is the process used to reduce the number of viable microorganisms but may not inactivate certain viruses and bacterial spores. Dental impressions do not tolerate heat treatment so should always be chemically disinfected to eliminate the risk of cross contamination. However, the selected disinfectant should not compromise the precision of the impression.
When selecting a disinfectant for heat sensitive appliances like dental impressions, check that it is active against bacterial spores, M. tuberculosis and viruses including HIV, HCV and HBV. The disinfectant must be compatible with the material and not have a damaging effect.
Products that are suitable for the disinfection of impressions are CE marked to demonstrate conformity to European directives. The manufacturers’ recommendations for the dilution of the disinfectant and immersion time should be followed. Ideally, disinfectants should not be sprayed onto the surface of impressions as this could reduce efficacy and may create an inhalation risk. When disinfectant sprays are used for alginate impressions, they can result in air bubbles in the cast, which may distort the quality of the impression. So long as the recommended immersion period is strictly followed, immersion disinfectants are unlikely to negatively affect the quality of the impression surface.
It is good practice to agree the cleaning and disinfection process with the laboratory and label the device to indicate disinfected status. This removes uncertainty and, for impressions, also removes the possibility of repeated disinfection, which may detract from the quality of the impression.
Obtaining accurate dental impressions is key to success in a wide range of clinical restorative procedures. However, regular training is essential for all dental staff to ensure an adequate understanding of the risks associated with handling dental impressions and the correct procedures for their decontamination, including the safe use of disinfectants.
References available on request.