Disinfecting Impressions for Infection Prevention
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Impression taking is a frequently-performed procedure in the dental office that requires selection of an appropriate impression material and technique for any given procedure. Following the procedure, the impressions will be poured for use as study models, or models and dies for the fabrication of appliances, indirect restorations and prostheses. Irrespective of the purpose, an appropriate infection prevention protocol must be followed before, during and after impression taking to avoid cross-contamination and the risk of disease transmission. The focus of this paper is on the disinfection of set impressions.
A Source of Infection
Set impressions are a source or reservoir for pathogens. They have been found to contain microorganisms – bacteria, fungi and viruses – following their removal from the patient’s mouth, through transport to the laboratory and have also been shown to transmit microorganisms into stone and plaster while models are being poured. As such, they represent a risk for disease transmission to dental healthcare workers, transporting personnel, and laboratory personnel through indirect contact. Therefore, an appropriate infection control protocol must be followed to prevent crosscontamination and the transmission of disease.
Types of Impression Materials
Impression materials include the use non-elastic (such as compound) and elastic impression materials. The vast majority of procedures use elastic impression materials – aqueous hydrocolloids (typically alginate) and non-aqueous elastomeric materials including vinylpolysiloxanes (VPS or addition reaction silicones) (e.g., Aquasil), condensation reaction silicones (e.g., Xantopren), polysulfides (e.g., Permlastic) and polyethers (e.g., Impregum). The characteristics of these materials with respect to their hydrophilicity, the presence or absence of surfactant and their tolerance of immersion in water or other fluids are key elements in understanding disinfection protocols for impression materials. For example, VPS is hydrophilic and will absorb water and other liquids, changing its dimensions, and surfactant may also leach out affecting the wettability of the impression; polyether is also hydrophilic and can leach; and alginates are sensitive to wet and dry environments.
An Overview of the Disinfection Procedure
Impression materials cannot tolerate heat and must therefore be chemically treated, as with heat-sensitive instruments (except handpieces). However, while instruments can be cold sterilized by immersing them in a hospital-level disinfectant for several hours (the number depending on the chemical), impression materials are sensitive to long-term immersion and are therefore disinfected, as opposed to cold-sterilized which takes longer.
The manufacturer’s directions for disinfection must be followed. The general steps in the disinfection procedure are as follows:
- Blood and bioburden must first be cleaned off the impression prior to disinfection
- Cleaning should be performed immediately after taking the impression, before blood and bioburden has a chance to dry onto the impression material. While instruments can be pre-soaked to loosen debris, impression materials have limited tolerance of immersion in liquids and this should therefore be minimized and controlled
- The impression material is next disinfected using an EPA-registered disinfectant
- After disinfecting the impression, it must be rinsed under running water before being further processed or sent to the laboratory. Failure to properly rinse the disinfectant off the impression can result in a substandard model due to incorporation of residual disinfectant into the mixed stone or plaster
Disinfecting Solutions and Sprays
These include iodophors, sodium hypochlorite (5.25%, diluted 1:10), 7.5% hydrogen peroxide, chlorine dioxide, and dual or synergized quaternaries. The disinfectant must have a tuberculocidal claim (and will provide a TB kill time on the labelling). The required exposure time for a disinfectant is an important element in its selection for disinfecting impressions. The impression is soaked by either spraying or immersing the impression material. Both techniques have been found to be effective in controlling bacterial contamination and preventing cross-contamination.
Immersing a given impression material in disinfecting solutions and other fluids for too long a period risks dimensional and surface changes – and, therefore, inaccuracies in dies and models. Studies have assessed immersion and the use of a number of disinfectant solutions and their effects on the impression. In comparing each category of elastomeric material, it was found in one study that there were no statistically significant differences after immersion for up to 20 minutes with either 0.5% sodium hypochlorite or 2% glutaraldehyde.
Another study compared the effect of 3.4% glutaraldehyde (Cidex Plus), 17.2% isopropanol (Cavicide) and 7.5% hydrogen peroxide (Sporox) on polyether, addition reaction silicone and condensation reaction silicone impressions. The dimensions of the medium-body condensation reaction silicone (Xantopren) impression were statistically similar before and after immersion with 3.4% glutaraldehyde only. Use of the other disinfectants resulted in significant dimensional changes for this impression material and all three disinfectants affected the light-body condensation reaction silicone. Conversely, no statistically significant differences were found for the tested VPS materials following immersion in any of the disinfectants tested. For the polyether (Impregum), only 7.5% hydrogen peroxide resulted in no statistically significant dimensional changes.
Alginates can be sprayed with disinfectant. If alginates are disinfected by immersion, the time must not exceed 10 minutes. Using the spray technique, the impression is cleaned, sprayed with the recommended disinfectant and then placed in a sealed plastic bag for the recommended time (10 minutes) before then removing and thoroughly rinsing the impression. The impression can also be wrapped in a disinfectant-soaked towel before being placed in the bag. Disinfectants recommended by manufacturers include 5.25% sodium hypochlorite diluted 1:10, and iodophors.
Non-aqueous Elastomeric Materials
Non-aqueous elastomeric impression materials can be immersed in an EPA-registered disinfectant, provided the recommended disinfection time meets the immersion time required for disinfection and is compatible without incurring dimensional changes in the impression material. A spray technique may also be used. Disinfectants variously recommended by manufacturers include a standard glutaraldehyde disinfectant solution, iodophors, 5.25% sodium hypochlorite diluted 1:10, and 7.5% hydrogen peroxide. For polyether impressions, an immersion time of not more than 10 minutes is typically recommended. For VPS materials, a hospital level disinfectant except neutral glutaraldehyde is recommended and water-based disinfectants have been recommended as being preferable. For zinc oxide-eugenol impression paste (and the non-elastomeric impression compound), 2% glutaraldehyde and iodophors have been recommended for immersion. The manufacturer’s recommendations must be followed.
As with other procedures, the dental healthcare worker must wear personal protection equipment (gloves, mask, protective eyewear and clinical clothing) while disinfecting impressions. All staff must be trained in infection prevention procedures and offices must be OSHA-compliant. The dental office should provide written communication to the dental laboratory outlining the method used for cleaning and disinfection of the impression (or other laboratory-bound item).
The disinfection of impressions is a requirement to prevent the risk of disease transmission and cross-contamination. Both spray and immersion techniques are available. Considerations in selecting a disinfectant and technique include the type of impression material and preference. The manufacturer’s recommendations for both the impression material and the disinfectant should be followed.
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3. Centers for Disease Control and Prevention. Guidelines for Infection Control in the Dental Healthcare Setting, 2003.
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5. Estafanous EW, Palenik CJ, Platt JA. Disinfection of bacterially contaminated hydrophilic PVS impression materials. J Prosthodont. 2012 Jan;21(1):16-21.
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7. Otero N, Nathanson D. Effect of a new disinfection solution on the dimensional stability of elastomeric impression materials. Abstract #0892, AADR 2003, San Antonio, TX.
About the Author
Dr. Fiona M. Collins has authored and presented CE courses to dental professionals and students in the US and internationally, and is a consultant in the dental industry. She is a member of the American Dental Association and the Organization for Asepsis, Safety and Prevention. Fiona earned her dental degree from Glasgow University and holds an MBA and MA from Boston University. Dr. Collins can be reached at fionacollins [at] comcast.net