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Historical documented exposure of dental healthcare professionals and their patients to a variety of bacterial, viral, and other microbial pathogens led to the development of infection control guidelines by the Centers for Disease Control and Prevention (CDC) and the American Dental Association (ADA).1-4 The first published recommendations for infection control in the dental laboratory were written by the ADA in 1978.1 The CDC introduced its "universal precautions" document in 1986 to protect all healthcare personnel and patients from pathogens spread by blood and other secretions of bodily fluid. This was later expanded in scope and referred to as "standard precautions."4 The Federal Occupational Safety and Health Administration (OSHA) mandated regulations describing a series of protocols and practices called the Bloodborne Pathogens Standard in December 1991.5 As a result, effective infection control measures must be implemented and routinely used across the range of dental practices, including the dental laboratory.
Occupational Risks
Dental laboratory personnel are at risk of infection transmission by the same mechanisms as other dental healthcare professionals. Potential routes of transmission include direct contact with infected saliva or blood through cuts and abrasions, indirect accidental percutaneous exposure when using knives and other sharps items, and airborne infection from microbial-laden aerosols and spatter created during laboratory procedures.9 The greatest exposure risk for dental laboratory personnel is indirect contact through cross-contamination (Figure 1).11 Most of the cases being handled, manipulated, and processed in the dental laboratory move back and forth between the operatory and the laboratory setting. Impressions, prostheses, and appliances may be contaminated with saliva and blood and facilitate the transmission of microorganisms from operatory to laboratory. For example, microorganisms on a dental impression may be transferred to dental casts and remain in set gypsum for up to 7 days.6,7 Thus, the potential for cross-contamination may occur from dental office to laboratory, and back to the dental office. Cross-contamination may also crop up within the laboratory from case to case, and may also occur from surface contact, contaminated handpieces (Figure 2), burs, rag wheels, pumice pans (Figure 3), and hands.
In order for an infection to be transmitted along any of these routes, specific conditions must be present: a viable microorganism, a reservoir that allows the microorganism to survive, a mode of transmission, a portal of entry, and a susceptible host. This is termed "the chain of cross-infection." A primary goal of infection control is to minimize the spread of infection by breaking as many links in the chain as possible. In the dental laboratory, this would incorporate adherence to principles of aseptic technique, appropriate immunizations for laboratory personnel, the use of barrier techniques, and implementation of standard precautions. Standard precautions dictate that all laboratory cases are handled the same way and are treated as if contaminated and infectious.
As mentioned above, dental laboratories are required to comply with all relevant federal, state, and local regulations that affect their operations and employees. As a component of the OSHA regulations and CDC recommendations, all personnel must be evaluated for potential occupational exposures.4,5 At-risk employees must be offered the Hepatitis B vaccination, and all employees are to be provided initial and annual training regarding bloodborne pathogens. If an employee's responsibilities change such that their exposure risk increases, the individual must be given additional training consistent with the new duties. It is also important to note that infection control policies in the dental laboratory must be written, as they are for dental practices. They should be clear and concise, in a manner such that all laboratory personnel can clearly understand the policies. Written infection programs should be reviewed at least annually and periodically updated. All occupational exposure incidents should be recorded. Documentation of accidents should include the type of first aid administered, provision of healthcare professional counseling, post-exposure evaluation, and any indicated follow-up.5-9
Infection Control Precautions
Given that the dental office and the laboratory are generally not in the same physical facility and the high potential for cross-contamination between the two settings, excellent communication between the two is essential for effective infection control.8,9,12 The laboratory should clearly delineate infection control requirements for case submission to the dental office. The dental office and laboratory must reliably communicate the disinfection status of each incoming and outgoing case (Figure 4). If the status is uncertain, the process should be repeated. Thus, prevention of cross-contamination should always be a prime consideration in the dental laboratory. Prevention measures include: organization of the dental laboratory into separate receiving, production, and shipping areas; appropriate personal protective equipment; frequent hand hygiene; use of unit-dosing principles where appropriate; and proper laboratory case disinfection.9
Designated receiving, production, and shipping areas provide assistance in organizing and implementing an effective infection control plan. A receiving area should handle all items entering the laboratory and ought to have running water and hand-washing facilities. Countertops in this area should be covered with impervious paper or cleaned and disinfected on a regular basis (Figure 5). The receiving area technician must wear appropriate personal protective equipment (PPE) when receiving and disinfecting laboratory cases. After the items have been cleaned and disinfected, they can then be safely transferred to the production area. Because items in this area have already been disinfected, they no longer require special handling. The production area should be monitored to ensure that no contaminated items are allowed to enter. If an occupational injury occurs in this area of the laboratory, the involved prosthesis must be disinfected before being moved to another area of the laboratory.
Lastly, the shipping area is designated for final inspection of items leaving the dental laboratory. This portion of the facility should be cleaned at least once a day and all case pans need to be cleaned before being used for another case. The disinfection of items to be shipped is only required if they have been contaminated to the point where handling is not safe. Receiving dental offices should disinfect all impressions and prostheses before insertion in a patient's mouth (Figure 6). Please note that the shipping area of a dental laboratory should not function as the receiving area, unless it has been properly cleaned and disinfected after receiving cases.
Personal Protective Equipment
PPE must be worn in the dental laboratory when the potential for injury or exposure to bloodborne pathogens exists. It is recommended that disposable gloves be worn when in direct contact with contaminated items. Heavy-duty utility gloves are more puncture- and chemical-resistant than examination gloves, and should be worn when cleaning and disinfecting equipment and surfaces. When the potential for splashes, spatter, and aerosols is present, lab coats also must be worn as well as masks covering the nose and mouth, eyewear (with protective side shields or goggles), or face shields. PPE should always be worn when cleaning and disinfecting cases and opening received items. Clinic jackets or lab coats should be changed daily or more frequently if soiled, and should not be worn outside of the dental laboratory.
Hand Hygiene
Hand hygiene is the single most important measure that healthcare personnel can take to prevent the transmission of infectious diseases in any healthcare setting. Hand hygiene in a dental laboratory may be accomplished using soap and water, an antimicrobial soap and water, or waterless alcohol-based hand rubs. The procedure should be performed before and after donning gloves, when hands are visibly soiled, after touching contaminated items, and prior to leaving the laboratory work area. Alcohol-based waterless products are not appropriate when hands are visibly soiled with blood, bloody saliva, or other biological debris. In those instances, hand washing using water provides effective cleaning. Dental laboratories may consider installing automatic hand-hygiene dispensers for convenience and to encourage frequent use (Figure 7).
Dental Laboratory Areas
All clinical materials being sent to a laboratory should be cleaned and disinfected by the dental office that sent it, and identified as such. If the dental laboratory is uncertain about whether disinfection has been performed, the laboratory should carry out the process with an EPA-registered, intermediate-level disinfectant following the manufacturer's instructions. The chemical disinfectants must be compatible with the dental impression materials used and must not affect the dimensional accuracy or surface texture. If the accuracy of the impression is compromised, the resulting gypsum casts will also be compromised, which may result in ill-fitting and non-functional prostheses. No single disinfectant is compatible with all impression materials, so the manufacturer of the impression material and disinfectant should be consulted. A laboratory might perform a "test-run" if a new combination of impression material and disinfectant is being used.
Cleaning and Disinfection Procedures
The first step in any infection control procedure is cleaning. With regard to dental laboratories, impressions may be cleaned by scrubbing gently and rinsing to reduce bioburden, and subsequently disinfected with an antimicrobial agent (Figure 8).10 Dental impressions may be disinfected by spraying, dipping, or immersing. The advantages of the spray method are that less of the disinfectant product is used, and often the same product may be used to disinfect environmental surfaces. However, the spray method may not be as effective as immersion due to the potential for pooling of the disinfectant in some areas, while other areas may remain dry secondary to undercuts in the impression. The spray method also releases chemicals into the air, increasing the potential for occupational exposures. The dipping or immersion technique requires complete coverage of the impression in the disinfectant. Shorter exposure times minimize possible distortion and deterioration of the surface quality of the resulting stone casts. However, the manufacturer's instructions should always be consulted for their recommended procedures.
A contaminated dental impression will result in a contaminated dental cast. Because of potential damage to the model, it is more difficult to disinfect a cast than to disinfect an impression. Therefore, it is preferable to disinfect the impression prior to the fabrication of a cast. If a model must be disinfected, spraying with an EPA-registered, intermediate-level disinfectant followed by rinsing is recommended. The cast should then be placed on its end for drainage. If the cast is being disinfected for shipping, it should be allowed to dry before wrapping for shipment.
Contaminated prostheses should also be disinfected prior to being worked on in the dental laboratory. The prosthesis should be scrubbed with a brush using an antimicrobial soap to remove debris. If there is calculus adherent to the prosthesis, it should be placed in a sealed plastic bag or beaker filled with ultrasonic cleaning solution or calculus remover, and placed in the ultrasonic cleaner for the required time (Figure 9). The ultrasonic unit should be covered to reduce splashes and spatter, followed by cleaning in a detergent and then appropriate disinfection. After rinsing and drying, it is safe to continue performing laboratory work on the prosthesis.
Dental prostheses should not be stored in disinfectants prior to insertion. If a manufacturer's recommended contact time is exceeded, there are potential corrosion risks for metal components. Dental prostheses may be stored in diluted mouthwash or water if desired.
The lathe in a dental laboratory presents unique safety and infection control risks. When using a lathe, dental technicians must wear protective eyewear. In addition, a Plexiglas® shield should be in position and the vacuum should be activated (Figure 10). The optimal pumice solution is made by adding surfactant soap and possibly a disinfectant solution to the mix. The pumice solution should be changed daily or more frequently if contaminated. All brushes, rag wheels, other laboratory tools, and the lathe should be cleaned and sterilized daily.
Conclusion
Protection of dental laboratory and dental office employees from bloodborne pathogens is mandated by OSHA regulations. The same infection control protocols must be followed in the dental laboratory as in the dental office. Adhering to Standard Precautions, use of aseptic technique, proper disinfection and sterilization procedures, and wearing appropriate personal protective equipment can prevent disease transmission from contaminated items entering the dental laboratory. Effective communication and coordination of efforts between the laboratory and dental office are critical components of a successful infection control program.
References
1. ADA Council on Dental Therapeutics. Infection control in the dental office. J Am Dent Assoc. 1978; 97(4):673-677.
2. ADA Council on Dental Therapeutics, Council on Prosthetic Services and Dental Laboratory Relations. Guidelines for infection control in the dental office and the commercial dental laboratory. J Am Dent Assoc. 1985;110(6):969-972.
3. ADA Council on Scientific Affairs, Council on Dental Practice. Infection control recommendations for the dental office and the dental laboratory. J Am Dent Assoc. 1996; 127(5):672-680.
4. CDC. Guidelines for infection control in dental health-care settings, 2003. MMWR. 2003;52(RR-17):1-66.
5. Department of Labor, Occupational Safety, and Health Administration. Occupational exposure to bloodborne pathogens; final rule. Fed Reg. 1991;56:64004-64182.
6. Hussain SM, Tredwin CJ, Nesbit M, et al. The effect of disinfection on irreversible hydrocolloid and type III gypsum casts. Eur J Prothodont Rest Dent. 2006;14(2):50-54.
7. Ivanovski S, Savage NW, Brockhurst PJ, et al. Disinfection of dental stone casts: antimicrobial effects and physical property alterations. Dent Mater. 1995;11(1):19-23.
8. Kugel G, Perry RD, Ferrari M, et al. Disinfection and communication practices: a survey of U.S. dental laboratories. J Am Dent Assoc. 2000;131(6):786-792.
9. Merchant VA. Infection Control in the Dental Laboratory. In: Molinari JA. Practical Infection Control in Dentistry. 3rd ed. Philadelphia, PA: Lippincott, Williams and Wilkins; 2010:246-260.
10. Naylor WP. Infection control in fixed prosthodontics. Dent Clin North Am. 1992;36(3):809-831.
11. Plummer KD, Wakefield CW. Practical infection control in dental laboratories. Gen Dent. 1994;42(6):545-548.
12. Plummer KD, Dental Laboratory Relationship Working Group, Organization for Safety and Asepsis Procedures. Laboratory asepsis position paper. Annapolis, MD: OSAP Foundation.
About the Authors
Marie T. Fluent, DDS
Assistant Editor
The Dental Advisor
Ann Arbor, Michigan
John A. Molinari, PhD
Director of Infection Control
The Dental Advisor
Ann Arbor, Michigan
