Dental Unit Waterlines (DUWL): An Update

Headnote

Abstract

Infection control in healthcare sector especially dentistry is an area of major concern. The water used for irrigation and cooling of devices has the potential to get contaminated due to the passage ofwater through thin long pipelines commonly referred as dental unit water lines (DUWL). It has been proven that contaminated water of DUWL can lead to formation of a biofilm which can host pathogenic micro-organisms and has the potential to infect patients and health professionals. This paper reviews the factors responsible for contamination of DUWL, bio film composition and provides a detailed update regarding maintenance of DUWL by various means thereby leading to infection control in dental clinics.

Keywords: Dental unit waterlines; infection control; dentistry.

Introduction

Infection control in dentistry remains an area of major con- cern. Clinical infection control procedures concentrate on breaking the chain of infection that primarily consist of pa- thogens in sufficient numbers, a susceptible host and cond- ucive environment. The pathogenicity of organism and su- sceptibility of host are 2 links on which we have least con- trol. But we can concentrate our efforts to reduce the num- ber of organisms in clinical environment. Considerable effort and expenses are incurred to accomplish this goal everyday through various methods of sterilization and ase- psis. One aspect of infection control very vital to the safety of patient and health care worker is prevention of infection through contaminated dental unit waterlines (DUWL).

Dental treatment water is non sterile water used during de- ntal treatment for irrigation of non-surgical operative sites and cooling of high speed rotary and ultrasonic instrume- nts. The water leaving the waterlines of dental units is fre- quently contaminated by the pathogenic and opportunistic micro-organisms.12 Detachment of micro-organisms or their DNA from dental unit biofilm flushed into the oral cavity could infect the patient. Splatter and aerosols from contaminated water during dental procedures may possi- bly affect the patient and health care personnel. 2 main sources of contamination by micro-organisms are: from the patients by suck-back or from the incoming potable tap water; the presence of bio film attached to the inner surfa- ces of the tubes enhances the survival and reproduction of the bacteria.3 4 These organisms are primarily naturally occ- urring slime producing bacteria and fungi that form bio- films adherent to tubings in the dental unit. Small particles of this biofilm may break off as the water passes through the tubing there by contaminating the water which is expr- essed in patient's mouth and aerosolized into the dental operatory environment. Microbial load in this water often exceeds 104-105CFU/ml and this greatly exceeds the reco- mmended drinking water standards of less than 500 CFU/ ml of non-coliform types.5 So it makes sense that dental treatment water should atleast meet the standards of drink- ing water.

Dr. GC Blake first reported the existence of contaminated water in dental units.6 Dozens of literature confirmed this finding and offered solutions to combat this problem in the next 3 decades. But it failed to impact dentistry till the world wide epidemic of HIV infection in 1990's propelled dental fraternity to show concern in this matter.7 8 Reports of contaminated waterlines and role of biofilms led the Center for Disease Control and Prevention to first address the top ic of water quality in its 1993 infection control guidelines.9 The American Dental Association for the first time publ-ished formal statement about dental unit waterlines in 1996.10

Biofilm formation and composition: Biofilm similar to plaque on teeth can be found in domestic plumbing such as shower heads, faucets and drinking taps. Similarly biofilm is formed in the water tubing of dental units. It is basically an accumulated mass of bacteria and extracellular material tightly adhered to a surface which cannot be easily remo- ved. It is heterogeneous in species and morphology and enveloped in a glycocalyx slime layer which protects them from desiccation and chemicals. Symbiotic relationships between organisms make the environment conducive to the proliferation of a wide variety of micro life including fungi, algae, protozoa and nematodes.1112 It is important to understand how DUWL gets contaminated when they are supplied by well maintained municipal water systems. 3 important physical factors namely surface colonization, la- minar flow and surface:volume ratio play a role in the bio- film formation in DUWL.13

Surface colonization: DUWL provide a particularly favo- rable environment for biofilm formation.12 Stagnant water at the tubing walls allows bacteria to adhere and colonize the tubing surfaces. Molecules ofwater may adhere to lum- en surfaces by utilizing physical adsorption and chemisor- ption mechanisms. Once the conditioned substratum is for- med, it attracts other molecules and different micro colon- ies grow within the matrix. Thus coaggregation of different microbes with each other and matrices increase the depth of the biofilm promoting adhesion of suspended bacteria leading to continuous sheet of bacteria on the surface pro- tected by glycocalyx.

Laminar flow: Fluids moving through narrow bore tubing assume a hydrodynamic pattern known as laminar flow. In this, closer to the tubing surface, frictional forces slow the movements of fluids which create a conducive environm- ent to the formation of biofilm with minimum risk of being dislodged.13

Surface:volume ratio: As the diameter of the tube decre- ases and length increases, the surface area available for the same amount of water increases. Hence larger quantities of organic matter with small quantity of water in tubes, with small diameter and long length result in increased amount ofbiofilm formation.

Mature biofilm provide hospitable environment for fungi, protozoa and other organisms surviving in drinking water. Gram-negative aerobic heterotrophic bacteria of low path- ogenicity comprise the majority ofmicrobial species found in DUWL output water.1415 While most have minimum pat- hogenic potential in immuno-competent hosts, some prot- ozoa host parasitic bacteria like Legionella, Pseudomonas Aeroginosa, are associated with opportunistic infections in immune compromised patients. Costerton and colleagues proved that biofilm fragments are inhaled or aspirated and bacteria persist because phagocytes cannot process them thus leading to pulmonary insult.19There is potential risk of infection, especially from water, particularly in patients who are immune compromised due to drug therapy, alcoh- ol abuse, systemic diseases, and old age.17 Unless procedu- res specifically designed to prevent, eliminate, trap or kill biofilm are performed there is little reason to believe that any dental unit can avoid being colonized by bacteria.

Proven health risk associated with contaminated dental unit water lines can be answered by epidemiologic studies, clinical case reports, legal cases and environmental invest- igations.13 Clark recovered some species of gram negative Pseudomonas species from dental units and nasal floor of 14 out of 30 dentists, while there were no clinical sympt- oms reported.18 Seeding of respiratory tract with gram neg- ative bacteria has been identified as an antecedent event in development of pneumonia in hospitalized patients. There are opportunistic and true human pathogens such as Pseud- omonas Aeruginosa, Legionella Pneumophila, Mycobact- erium species and Staphylococcus species detected in DUWL.19 A case report describing Psuedomonas Aerog- inosa wound infection in 2 immune compromised patients isolated from infected sites matched bacteria from dental unit.20 2 legal cases reported claimed that bacterial endocar- ditis and the need for subsequent prosthetic heart valve sur- gery resulted from dental treatment with contaminated wa- ter and another claimed brain abscess episode after under- going dental treatment with contaminated water.13 Enviro- nmental investigations also have confirmed the presence of bacterial endotoxins in dental unit waterlines. A study by Putnins et ah, indicated that endotoxin present in DUWL output water might stimulate the release of proinflamma- tory cytokines in gingival tissue during oral surgery and adversely affect healing.21

Quality control in DUWL: Centre for Disease Control and Prevention recommended Infection Control Guideli- nes for Dentists in 1993 to install and maintain antiretract- ion valves to prevent oral fluids from being drawn into DU WLs; flushing waterlines daily for several minutes and for 20-30 seconds in between patients; sterile solutions to be used for surgical procedures involving bone cutting.10 At present, there appear to be no state or local laws or regul- ations that specifically address the dentists' obligations to ensure dental treatment water quality. Existing rales for drinking water quality may be enforceable in the dental 1- * 13,22 clinics.

The subsequent section reviews the methods to improve and maintain the quality of water in DUWL as per the curr- ent literature.

Anti-retraction valves: Anti-retraction valves prevent re- aspiration of contaminated fluid and hence, reduce the risk of transfer of potential infective material back into DUWL. However, it seems that the majority of the anti-retraction devices do not prevent retraction when the turbine stops running, particularly after it has been used for some time, and this leads to a contamination of the waterlines.23 Also, as with other components of the water supply line, the val- ves are subject to clogging owing to biofilm deposition and fatigue. In order to ensure adequate mechanical function- ing they require regular maintenance and programmed rep- lacement.13 Backflow prevention devices like anti-retract- ion valves present on hand pieces can prevent back-sipho- nage of contaminated oral fluids into the DUWL and are recommended by ADA.24

Filtration: Use of filters on the DUWL to eliminate bact- eria from the water entering the hand piece was first desc- ribed in 1978.25 Filters offer a physical barrier to the pass- age ofmicro-organisms when installed near the handpiece. Filters will have little influence on microbial output of the already formed biofilms. Certain filters are impregnated with iodine, a strong antibacterial agent, which is gradually released into the water during the use of the handpieces. This measure will reduce bacterial counts.29 Studies have demonstrated high levels of re-contamination of DUWL occur within 24 hours asa result of trapping and growth of bacteria on filters.27 Therefore, disposable filters are recom- mended and must be changed daily.7 Disposable filters are a promising method of improving water quality but their clinical effectiveness has not yet been fully established.28 Hence, filters should be used in combination with other methods of preventing or controlling biofilm formation in DUWL. Filters must be installed close to the point of use. Though they reduce the microbial contamination in the delivered water, it doesn't have any role in eliminating bio- film.15

Waterline flushing: Flushing of the waterlines may redu- ce the level of bacterial contamination of DUWL but its eff- icacy is not proven.29 31 The recommendations of the US Centers for Disease Control and Prevention (CDC), the American Dental Association (ADA) and the British Den- tal Association (BDA) are that waterlines should be flushed through for "several minutes" at the beginning of each cli- nical day to expel the overnight build up of microbial load in stagnant areas and for 20-30 seconds between patients to remove material that may have been retracted during treat- ment.3233 It is suggested that flushing discharges stagnant water and improves the quality ofthe water andreduces the odour and bad taste imparted to the water by microbial con- tamination. Barbeau et al., noted that flushing for 20 minu- tes would reduce bacterial count to zero, but this would be impractical in most dental surgeries.34 Although, between- patient flushing of waterlines may have a transient benefit. Simply flushing waterlines without chemical treatment, fil- tration or other scientifically validated intervention to imp- rove water quality should bean interim measure until more effective methods can be applied.35 It is essential to flush DUWL everyday morning and in between patients. Flush- ing high speed hand piece, air water syringe and ultrasonic scaler for 2 minutes or more at the beginning of each day temporarily reduces number of organisms suspended in DUWLs.

Independent Reservoirs: These isolate the DUWL from municipal water and allow the use of water of known mic- robiological quality. But without chemical treatment inde- pendent reservoirs are of little value in improving the qua- lity of water delivered. These reservoirs cannot deliver ste- rile irrigating solutions unless the water pathway is sterile. But they allow introduction of germicides to control or eli- minate biofilm formation within the water delivery system by relatively inexpensive methods.13 Independent water systems can be equipped with a bypass switch that allows the clinician to switch back and forth from municipal water or can be refilled manually with the tap water. An excellent source of water for use in dental water systems is bottled sterile water for irrigation, as it not only is free of viable micro-organisms but also has very low levels of minerals and organic compounds that can encourage re-establish- ment of biofilm. If an autoclave with a fluid sterilization cycle is available, sterile water can be prepared in the den- tal clinic for use in independent reservoirs. Heating water to boiling also can produce water free from viable veget- ative bacteria. However, minerals and organic compounds still will be present in sterile or boiled tap water. Continu- ous ultraviolet germicidal irradiation of incoming tap wat- er may reduce the numbers of viable microbes in incoming water.36 While distillers and deionizers can reduce mineral or organic content, they are less reliable in eliminating bac- terial contamination. Distiller hoses and holding tanks mu- st be cleaned on a regular basis to ensure water of accept- able quality. Chemical treatment using independent reserv- oirs is the water treatment method with the strongest supp- ort in the scientific literature.613

Use ofpolytetraflorethylene (PTFE) tubing is preferable to polyethylene on account of its ability to inhibit the coloni- sation and growth of Pseudomonas aeruginosa.37 It was also noted that polyvinylidene fluoride, were more effect- ive at resisting biofilm formation and in reducing the level of contamination in DUWL output water than conventi- onal DUWL tubing made ofpolyurethane.38

Chemical Treatment: Early attempts at reducing water contamination included diluting the water with bleach sol- utions. Although effective, these solutions tend to corrode the dental unit's internal metal components and can dam- age rubber gaskets.39

Depending upon their chemical nature, biocides may be used intermittently ("shock" treatments) or continuously to treat dental waterlines. For example, some waterline tre- atments are used as irrigant/coolant solutions in place of water. Others are delivered to the line via a metering device that releases low levels of a chemical germicidal solution such as ozone or hydrogen peroxide.40 Although reservoir bottles containing germicidal agents to DUWL can be bist ailed on virtually any unit, the consequences of chemical exposure on materials used on the construction of the unit are not always predictable. More data are needed on the eff- ects of these biocides and disinfectant bio-products upon oral tissues and dental materials. Anumber of studies repo- rted that some DUWL treatment products containing citric acid, sodium hypochlorite and Chlorhexidine gluconate co- mbined with 12% ethyl alcohol, may adversely affect bon- ding of composite material to dentine and enamel.4142

Chlorine, as sodium hypochlorite or chlorine dioxide is the most commonly employed biocide for water treatment.43 Chlorine compounds have been studied more extensively than any other class of chemical agents intended to control or eliminate biofilm in dental unit water systems. Most inv- estigators have found sodium hypochlorite in varying con- centrations to be effective, chloramine-T and elemental chlorine have also been tested. Chlorine dioxide waterline cleaners are effective in decontaminating DUWL biofilm. Chlorine dioxide has advantages over other chlorine prod- ucts. Controlling DUWL bio film may have beneficial effe- cts on nosocomial infections.43

Independent reservoir cleaning water systems can also be used to deliver chlorine flushes to the dental waterline.14 Chlorine can be used intermittently in high doses at 50 ppm ("shock" treatment or hyperchlorination) every 6 months. Alternatively, a continuous chlorination system can be ins- talled with an automatic dosing mechanism that provides 1 ppm of available chlorine at the dentist's chair.

Povidone iodine (10%) left standing in the DUWL over- night will reduce (but not eliminate) DUWL bacterial con- tamination.44 It has therefore been suggested that povidone iodine (10%) with subsequent use of sterile water reserv- oirs may offer a means of effectively reducing microbial contamination in DUWL. Furthermore, the only reported contraindication to the use of a povidone iodine mouthw- ash solution in humans is a known allergy to iodine. How- ever, there are no substantial supportive data on the effect of such a regimen in clinical practice.

Gluteraldehyde is a highly effective disinfectant, with bac- tericidal action against most vegetative bacteria, mycobac- teria and viruses, but it is highly toxic and its sensitization of the human lung and skin has limited its use for DUWL decontamination.45 Chlorhexidine gluconate is a common- ly used antimicrobial in dentistry, and hence may be of pot- ential effect in the cleaning of DUWL. Disinfection of DU WL and reservoirs with Chlorhexidine at concentrations of 1:5000 or 1:10,000 for 24 hours resulted in no bacterial growth.46

Chemical treatment uses either low concentrations of bio- cidal agents or less toxic substances in the water used for patient treatment.47

- Intermittent chemical release: Delivering potential bio- cidal concentration of germicide for a specified contact time and frequency using an independent water reservoir.

- Continous chemical treatment: Low concentration of biocidal agents or less toxic substances continuously in tre- atment water

The use of ozone as the method of disinfection would offer the best solution as part of an integrated approach to dental care.48 Indeed, O, would seem to offer the opportunity of unit sterilisation which is a very different approach to dent- al unit care. Ozonated water exceeds all current standards for water quality in DUWL's at high enough concentrat- ions. Disadvantages include chemicals get aerosolized alo- ng with water, effect of chronic exposure to healthcare per- sonnel and enamel/dentin bond strength of dental adhesive material may be affected.50

Using source water of known microbiological quality can be a simple and efficient method wherein sterile water can be prepared in office using autoclave with fluid sterilizat- ion cycle. Boiled water, water treated with UV germicidal irradiation, distilled or deionised water can also be used. Sterile Water Delivery Systems bypass or replace the den- tal unit water to provide sterile irrigants for powered surg- ical instruments, eg., Piezo dispenser for implants. Other important considerations to ensure the efficacy of treatm- ent include ensuring that there are no lines within the dental unit attached to unused ports, such as extra air/water or handpiece lines not in use. The treatment of dental lines requires the water to pass through the lines to ensure effic- acy. If these so-called "dead legs" are present in the dental unit, clamp them off to prevent recontamination of the lines that are in use.

The study by O'Donnell et al., concluded that the long term, a variety of features can each be a factor in DUWL disin- fection failure including inadequate compliance with DU WL disinfection protocols, adverse effects of disinfectants on dental unit components and selection of intrinsically disinfectant-tolerant bacteria.51 Dental practitioners need to understand the limitations of available DUWL treatments, and to consider the use of sterile water for non-surgical, as well as surgical purposes in treatment of immune compro- misedpatients.2152

How to test dental unit waterlines?

Though the clinicians might be curious to know the quality of treatment water in their clinics, current CDC Guidelines do not recommend routine microbiological culturing of environmental and medical device surfaces. Keep yourself updated on new developments and be prepared to answer questions from patient and staff. Use only sterile fluids for surgical procedures. Contact the equipment manufacturer or dealer to obtain current recommendations for improving and maintaining water quality

Guidelines to cleaning dental unit waterlines

1. Identify the source ofwater for your dental unit: If water from municipal water supply is used directly, install filter and anti-retraction valve close to the point of use. Install dental unit components that allow delivery of cleaning age- nts at the junction box. In case of reservoirs add cleaning agents to the bottle regularly.

2. Identify products that fit your needs and are compatible with your dental unit (contact the dental unit manufact- urer).

3. Develop a schedule for waterline maintenance and assi- gn the duty to a particular person.

4. Establish a protocol for monitoring the quality of dental unit water.

Conclusion

DUWL is not a public health crisis. Nevertheless, water that is unfit to drink as defined by international standards is unsuitable for therapeutic use in dentistry. This can under- mine public confidence in delivery of quality dental care. On another note, dentists have an ethical obligation to pro- vide patients and employees alike with a safe clinical envi- ronment. In this context to provide uncontaminated dental treatment water in clinics is of utmost importance in infec- tion control. Staff training programs on the correct use of control procedures in work environments can be a valuable suggestion.

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