Dental Waterlines: A Decade in Review
Shannon E. Mills, DDS, FAGD, FICD
In August 1995, the American Dental Association (ADA) Council on Scientific Affairs convened a panel of experts in Chicago to review the scientific evidence regarding the microbiological quality of water produced by dental units and other dental equipment. The recommendations developed by the panel were published as a white paper in February 1996, setting an agenda for research and challenging the dental industry to develop methods to improve the quality of water used for dental treatment.1
As we approach the 10-year point since the publication of the ADA statements, how have the profession, the scientific community, and the dental industry responded? Have scientific investigations revealed any new information about dental waterlines, biofilms, and the potential health effects on dental healthcare professionals and patients? In this article, the events of the past decade on this topic will be reviewed and the current status of science, technology, and public policy related to dental water quality will be described.
THE ADA WATERLINE PANEL: SETTING THE STAGE
The panel that met in Chicago consisted of representatives of the dental profession, various governmental agencies, academia, research, and industry. Scientific evidence accumulated over 3 decades had conclusively demonstrated that water produced by dental units and other dental devices was frequently colonized with high numbers of bacteria.2-4 Although most of the species recovered were common water bacteria with limited pathogenic potential, Pseudomonas aeruginosa,5,6 aquatic Mycobacteria,7 and Legionella8 had been recovered from dental water delivery systems. Two articles published in the 1980s revealed that dental office personnel were significantly more likely to carry antibody markers of exposure to Legionella bacteria than the general population. The most likely source of this exposure was conjectured to be the water provided by dental units.9,10
When it came to dental water quality, the ADA was eager to avoid the type of unfavorable media coverage that had followed the Florida HIV transmission in 1990 and concerns about health risks from mercury in silver amalgam a few years earlier. Dental public health experts from the Centers for Disease Control and Prevention (CDC) also encouraged the ADA to take a proactive stance and actively participated in the deliberations of the panel.
Although there were relatively few well-designed studies on the subject and there was only a single peer-reviewed case report of disease transmission in a dental setting,6 the panel quickly reached a consensus that the use of water with high levels of microbial contamination for dental treatment was inconsistent with basic principles of infection control practice in healthcare. No other healthcare discipline routinely used water that failed to meet standards for public drinking water for invasive clinical procedures. Moreover, studies had shown that the numbers of bacteria in the water used for dental treatment were alarmingly high (sometimes exceeding 100,000 colony forming units per milliliter [CFU/mL]).11,12
Bacterial biofilms—complex microbial communities attached to solidsurfaces in contact with water—were recognized as the source of this contamination.13,14 The ability of biofilm organisms to resist efforts at dislodgement or disinfection made effective biofilm control a more complicated process than might have been expected. Flushing of lines for several minutes as had been recommended in the 1993 Dental Infection Control Guidelines from the CDC had been shown to be unreliable as a means of improving water quality.2,12 In addition, the narrow bore tubing in dental units created unfavorable geometry that put small quantities of water in contact with a relatively large surface area colonized by biofilms—greatly amplifying the numbers of bacteria in effluent when compared with the water entering the system.
Several questions then had to be answered: “How clean must dental water be to protect the health and safety of dental healthcare professionals and patients?” “Are there any reliable means currently available to improve the quality of water used for dental treatment?” “With whom does the responsibility for improving dental quality lie?”
While dozens of different microbes with the potential to act as opportunistic pathogens had been recovered from dental units, evidence of illness as a consequence of exposure during dental treatment was limited. This made it extremely difficult to answer the question of “how clean.” There were however, existing standards for drinking water, for fluids used during surgery, and for renal dialysis that could be used by inference to develop a standard for dentistry.
After much deliberation, the panel chose 200 CFU/mL—which mirrors the recommendations for water used for hemodialysis—for nonsurgical dental procedures. When bacterial colony counts exceeded this amount in dialysate fluids, dialysis patients often experienced symptoms including tachycardia, chills, or fever during treatment. These “pyrogenic reactions” were the result of exposure to bacterial endotoxin that originated in the cell walls of bacteria living in biofilms on the narrow bore tubing used in dialysis systems.15 Although such reactions had not been observed in dental patients or dental healthcare professionals, an upper limit of 200 CFU/mL in unfiltered output was viewed as attainable from an engineering standpoint and low enough to provide a reasonable assurance of safety for patients and staff. Standardized microbiological test methods and devices specifically designed to test water to meet the 200 CFU/mL level were available. The panel also agreed with the updated CDC guidelines for dental infection control that had been published in 1993, which recommended that only sterile water should be used for oral surgery procedures that involved the cutting of bone.16
As for the question of how dental water quality could be improved, there were decidedly fewer options. Studies dating to the mid 1960s had shown that use of independent water reservoirs and treatment of waterlines with household bleach, iodophors, and other chemicals could reduce the numbers of bacteria in water from dental units.2-4 Few US dental unit manufacturers, however, had publicly acknowledged that water used in dental treatment might be of questionable quality and there was very little information available to help guide the profession. Very few commercial products were available and most dental units were not equipped with independent reservoirs. The effects of chemical agents such as iodophors and household bleach on the many different types of dental units were not well understood.
The panelists understood that improving the quality of water used in dentistry would require changes on the part of clinicians as well as dental manufacturers. As to the answer to the final question of “who,” it was clear that only the manufacturers could create the necessary engineering controls and products to limit the adherent microbial biofilms that colonized the small diameter tubing used to provide water for dental treatment. For this reason, the 1996 ADA white paper challenged researchers and manufacturers of dental equipment to develop new approaches to improve dental water quality by the year 2000.1
MEETING THE YEAR 2000 GOAL
Four years after the initial ADA waterline panel met, the ADA Council on Scientific Affairs reconvened the waterline panel in 1999 to consider the progress made since the publication of the challenge. Results of this meeting were published in a statement from the Council in November of 1999.17 The progress had by that time been significant. The dental research community had responded to the challenge issued in the ADA waterline statement. During the 3 decades between the publication of Blake’s landmark report in 1963 of bacterial contamination in dental water reservoirs and the ADA waterline statement, 55 articles on dental water contamination had been published in peer-reviewed English-language journals. Unfortunately, many of these studies suffered from a failure to understand the significance of biofilm as the underlying cause of dental waterline contamination and were of limited value in solving the problem. Between 1995 and 2000, at least 25 additional studies or review articles had answered many questions about how biofilms form in dental equipment, the potential risks of exposure to contaminated water, and various methods to improve dental water quality. Armed with a better understanding of the true nature of the phenomenon, these studies helped establish a sound basis for technological solutions.
The dental industry responded as well. Most dental unit manufacturers now offered independent reservoir systems as an option and at least 2 major manufacturers made them standard equipment. A few also provided explicit recommendations for treating dental waterlines to control biofilms. A number of waterline treatment products using intermittent and continuous chemical treatment with various chemical agents, microfiltration, and adjunctive use of ultraviolet radiation were now commercially available.18 Although the response of the dental industry was heartening, many dentists remained either uninformed about or disinterested in the issue.
RISKS, RECOMMENDATIONS, AND REGULATIONS
Martin’s 1987 description of Pseudomonas aeruginosa postoperative infections in 2 immunocompromised patients and asymptomatic carriage of the bacteria in 78 additional patients remains the only peer-reviewed case report of dental waterline mediated infection.6 This supports the conjecture that such infections are either rare, or that when they occur, they are of such limited clinical significance that they do not result in case reports. The actual number of infections related to contaminated water may be greater than the paucity of reports suggest. Dentists rarely culture postoperative infections and most are self-limiting. Moreover, infections or physiological reactions secondary to inhalation or ingestion of contaminated water are unlikely to be linked to dental treatment.
In recent years, researchers have increasingly directed their investigations toward assessment of the risks that continuous exposure to aerosols containing bacteria or bacterial by-products may have on dental healthcare professionals.19-21 Endotoxin is a lipopolysaccharide component of the cell walls of gram-negative bacteria, which has potent physiological effects including the capacity to cause pyrogenic reactions in hemodialysis patients, delayed wound healing, and exacerbation of asthma in susceptible individuals. Endotoxin levels as high as 2,560 endotoxin units per milliliter (EU/mL) have been found in water from biofilm-colonized dental units.22,23 These levels are much higher than usually seen in tap water and are most likely the results of the unfavorable surface area-to-volume ratios seen in small-bore tubing. By comparison,the United States Pharmacopeia (USP) permits no more than 0.25 EU/mL of endotoxin in USP sterile water for irrigation. A recent study from the United Kingdom included dental water quality among potential risk factors for the development of asthma among dentists. The authors concluded that smoking and colony counts in dental water >200 CFU/mL were independent risk factors for new onset asthma among dentists.19
In 2003, the CDC issued updated Guidelines for Infection Control in Dental Health-Care Settings—2003.24 Included in the new guidelines were specific recommendations for the quality of water used for both surgical and nonsurgical dental treatment. The definition of surgery, however, was expanded from the 1993 guideline to include all procedures that involved “the incision, excision,or reflection of tissue that exposesthe normally sterile areas of the oralcavity.” This definition encompasses most invasive procedures with the exception of deep scaling and tooth extractions that do not involve reflection of mucoperiosteal flaps or manipulation of bone. While acknowledging that the oral cavity is extensively colonized with endogenous bacteria, the potential for introduction of microorganisms into the vascular system orotherwise sterile areas of the body with an increased potential for localized or systemic infection led to the recommendation that only sterile water or saline be used for these procedures. These recommendations bring dentistry in line with other healthcare disciplines that perform invasive clinical procedures.24
The new CDC guidelines recommend that water used for nonsurgical dental treatment contain no more than 500 CFU/mL of bacteria to ensure that it meets the standards set by the American Public Health Association and American Water Works Association for levels of heterotrophic bacteria in drinking water.25 Both this number and the 200 CFU/mL level put forth in the 1996 ADA statement are engineering standards and do not represent a threshold value above or below which human disease is or is not likely to occur. The equation that determines infection is complex and includes bacterial pathogenicity, virulence, and host resistance as well as colony counts. To optimize patient and healthcare worker safety, the CDC guidelines also recommend that both manufacturers and clinicians strive to keep levels of bacteria as low as are reasonably achievable. The guidelines, like the ADA waterline statement, clearly assign responsibility for developing safe and effective methods for ensuring good water quality on the dental manufacturing industry.
While the CDC guidelines are recommendations and are not enforceable by any federal agency, many state boards are adopting them as standards of care. In some states, compliance with CDC guidelines may be required by state dental practice acts. Nationally recognized standards are also frequently used by both sides in malpractice litigation.
CURRENT WATERLINE TECHNOLOGY
When the ADA waterline panel first convened in 1996, there were very few options for dentists who wanted to improve dental water quality. A mere handful of manufacturers even acknowledged the issue and provided information or instructions to users on maintaining good water quality. In contrast today, the dental marketplace includes at least 22 commercial products using a range of different technological approaches and with varying degrees of convenience and cost. Almost all major manufacturers of dental units and other devices now include recommendations to control or eliminate biofilm formation in dental water systems.
Currently available dental waterline products can be classified as:
? Independent reservoir systems (must be used with chemical agents to control biofilms).
? Chemical germicides or cleaners that inactivate or remove biofilms (sometimes described as intermittent or “shock” treatment).
? Chemical germicides or cleaners that prevent attachment of biofilm in new or cleaned systems (sometimes described as continuous treatment. These agents may be added to or used as the irrigating solution for clinical treatment.)
? Slow-release resins or metering devices that release low concentrations of agents designed to prevent the attachment of biofilm, including antimicrobial reservoirs and tubing.
? Bottled water and water conditioning systems that treat water by filtration, sedimentation, and/or ultraviolet irradiation to remove bacteria and other contaminants entering the water system (these devices generally have no direct effect on biofilms).
Independent Reservoirsand Chemical Treatment
When English dentist GC Blake published the first report of bacterial contamination in a dental water system in 1963, he also described his efforts to remediate the problem using chemical germicides. Blake’s dental unit used a separate water reservoir to provide coolant water to a new state-of-the-art, air-driven, high-speed handpiece. He discovered that the water contained in the reservoirs quickly became contaminated with bacteria. Although he knew nothing about biofilms (it would be many years before the term biofilm was used or its role in dental waterline contamination was understood), he used several germicidal agents to reduce bacterial counts.4
For many years, independent reservoir bottles provided the only practical way to introduce chemicals to control bacterial contamination. As Blake learned 4 decades ago, without chemical treatment separate reservoirs alone are virtually useless for improving dental water quality. Over the years, dozens of chemical agents in various formulations and concentrations have been evaluated for the control of microbes in dental treatment water, including sodium hypochlorite,26-29 super-oxidized water,28 chlorine dioxide,28,30,31 peroxide-based solutions,28,32-35 iodophors,2,28 chlorhexidine gluconate,28,36,37 commercial oral mouthrinses,38,39 and silver-based formulations.40,41
Chemical agents are typically applied in either intermittent or continuous fashion to remove or prevent formation of biofilms.18 Intermittently applied agents are usually in higher concentrations and are intended to inactivate or remove established microbial biofilm attached to waterlines and other water-bearing surfaces. Some of these products can be irritating, toxic, caustic, or corrosive if mishandled.29
Continuously applied products are generally used to prevent the formation of biofilms on tubing surfaces and are formulated to be safe for patients and dental healthcare professionals when used as a coolant for nonsurgical dental procedures. The concept is identical to that used by municipal water systems in which chlorine is added to drinking water to inhibit the growth of bacteria in the water delivery system and slow the formation of biofilm. The standard 2 to 3 part per million (ppm) chlorine residual in drinking water, however, may not be effective in controlling biofilm in dental water systems because of the small diameter of the tubing. The biofilm present in the lines quickly absorbs the remaining chlorine, leaving the majority of the lines (which may total more than 30 feet in some systems) unprotected.26 Adding a small amount of diluted household bleach to the water reservoir to restore the chlorine residual has been shown to be effective in controlling biofilm formation in dental waterlines.26
Today, at least 2 dozen commercial products can be purchased that can be used either intermittently or continuously to decontaminate dental water systems. These products come in liquid, powder, or tablet form and are applied according to varying treatment intervals.42,43 With continuous treatment agents, the premixed product either serves as the coolant for nonsurgical procedures or is added to the water each time the reservoir is filled. To be successful, these regimens require compliance on the part of the dental staff and may be technique-sensitive.
Noncompliance by staff with treatment protocols has been identified as a cause of treatment failure in clinical studies.27 One approach taken by some manufacturers to reduce the need for daily or weekly intervention involves the use of cartridges containing slow-release resins or metering devices to provide continuous chemical treatment of dental water systems.
Slow-Release Resin and Metering Devices
Continuous introduction of agents to prevent attachment of biofilm can be accomplished using either passive or automated systems. A passive system is one that continuously releases the chemical agent without a mechanical pump or metering system. Antimicrobial substances such as iodine or silver can be impregnated into in-line cartridges and filters as well as water reservoirs and tubing. In-line cartridge systems can be used to treat either incoming municipal water or water contained in separate reservoirs. The user only needs to shock the system at the time of installation to remove biofilms (most devices of this type are designed to maintain water quality rather than to eliminate well-established biofilms) and to replace the cartridges, filters, or water reservoir pick-up tubes according to manufacturer recommendations. Some products in this category require specially conditioned water for optimal efficacy. The section on water conditioning systems provides more detailed information on this topic.
Water reservoirs and tubing can also be impregnated with materials to prevent biofilm formation. These compounds can be eluting (depleted over time during use) or noneluting (the antimicrobial action is integral to the material and does not dissolve into the solution). Elution has posed a significant problem for the development of effective antimicrobial materials. Antimicrobial materials currently on the market address this problem by using materials that can be recharged using a concentrated solution of the active ingredient. Water hardness may have an impact on the success of antimicrobial materials since deposition of mineral deposits on water contacting surfaces will block the effect of the antimicrobial agent.
Automated systems use some type of mechanical or electronic device to deliver measured amounts of antibiofilm agents at timed intervals. Automated systems are usually built into the dental unit by the manufacturer. Some devices of this type also include other built-in water conditioning features such asfiltration and removal of minerals.
Bottled Water and Water Conditioning Systems
The quality of the water introduced into the dental unit—whether via water reservoirs or from municipal water systems—can be an important determinant of the success of water treatment programs.36,44 Municipal water often contains varying amounts of organic and inorganic substances as well as the bacteria that seed the formation of biofilms in dental water systems. The use of bottled water or devices that treat the water to remove some or all of these contaminants may help improve and maintain dental water quality.
Water conditioning devices may use a number of approaches including filtration, sedimentation, reverse osmosis, and ultraviolet germicidal irradiation (UVGI).18 Despite advertised claims by some makers of water purificationsystems, however, these devices cannot improve the quality of water used for treatment unless biofilms are controlled within the dental unit. For the samereason, bottled water alone—even if sterile—will not remove biofilm or eliminate bacterial contamination. Nevertheless, improving the overall quality of the water entering the unit can have a number of beneficial effects. For example, by removing high levels of dissolved calcium, problems with scale buildup in tubing and devices can be avoided. Some water treatment systems require water with low levels of minerals or other impurities to provide optimum results.
For units that remain connected to municipal water systems and use intermittent chemical treatment regimens, UVGI treatment of incoming water can potentially reduce the numbers of organisms in incoming water, but like other water conditioning systems, cannot affect biofilms. With units that use an intermittent chemical treatment approach, the quality of the output water is unlikely to be improved by passage through the dental unit. While an empiric case can be made for the potential benefit of reducing bacteria and contaminants in incoming water, no scientific studies have been published that validate the effectiveness of water conditioning devices or UVGI on dental water quality.
CONCLUSION
Ten years have passed since the ADA published its statement on dental waterlines in 1996. The proactive stance taken by the Association on the issue of dental water quality has stimulated scientific inquiry, technological advancement, and the development of consensus standards of care to ensure a safe environment for the delivery of oral healthcare. The challenge issued to researchers and the dental industry has resulted in significant advances in our understanding of the phenomenon of biofilm formation in dental waterlines and the development of engineering solutions to address it. Questions about the potential risks of chronic exposure to airborne bacteria and endotoxin by dental healthcare professionals, however, still remain. Hopefully, future investigations will help define the nature of such hazards and develop appropriate and cost-effective solutions.
Disclosure
The author is the patent holder forthe Air Controlled Sterile Irrigation System, licensed by the US Air Forceto Aseptico, Inc.
REFERENCES
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18. Mills S. The dental unit waterline controversy-defusing the myths, defining the solutions. J Am Dent Assoc. 2000;131:1427-1441.
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21. Pankhurst CL. Risk assessment of dental unit waterline contamination. Prim Dent Care. 2003;10(1):5-10.
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43. Harte JA, Browning C. Synopsis of dental unit waterline treatment products and devices (Project 04-03) 2006, US Air Force Dental Evaluation and Consultation Service. Available at: https://decs.nhgl.med.navy.mil/2QTR04/PRODUCTEVALUATIONS/waterlinesynopsis.htm. Accessed: February 5, 2006.
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Selecting Products to Maintain Dental Treatment Water QualityOnce the dentist has made the decision to address the issue of dental waterline contamination, he or she must contemplate a wide range of choices that did not exist a decade ago. With so many choices, how do dentists determine the best course of action for a particular dental practice? The first consideration involves the age and configuration of existing dental units and other devices that use water, such as ultrasonic scalers. Can the unit or device be easily retrofitted with an independent reservoir or other water treatment device? Is the unit or device nearing its operational life expectancy? Are there other reasons to consider replacement as an alternative to modification? Can the dental unit manufacturer provide recommendations to improve dental water quality with the existing equipment? If the decision is to keep the existing unit or device and install a water treatment system, the following checklist can help the dentist ask the right questions of the dealer or manufacturer to guide the decision-making process: Dental Water Treatment Agent or Device Checklist
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Use the following checklist when considering new dental units or other equipment that uses water as a coolant or irrigant (eg, ultrasonic scalers, dental lasers).Dental Unit or Device Water Quality Checklist
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Internet Resources on Dental Waterline Science, Policy, and Products
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