Ozone Information For Clinicians

Management Of Dental Water Lines With Ozone

© Dr Julian Holmes

Ozone Information

Management Of Dental Water Lines With Ozone
  • Abstract:
  • Clinical Relevance:
  • Objectives:
  • Introduction:
  • The Role Of Biofilms:
  • Biofilm Formation:
  • Source Of Contamination:
  • Methods To Control DUWL Contamination:
    Treatment Of The Dental Water:
    Improving Unit Design:
  • Summary:

  • Management Of Dental Water Lines With Ozone

    Management Of Dental Water lines
    Hisham Al Shorman, Layla Abu Naba’a, Wilson A Coulter, Caroline L Pankhurst and Edward Lynch, 2004

    Dental Unit Water Lines harbour considerable amounts of bacteria that are derived form the biofilm on the inner surface of these lines. This continuous reservoir of bacteria carries the potential of causing infection to the patients and dental workers. This article describes this problem and reviews the different methods of control and provides the recent recommendations that should be adopted in the dental clinic to ensure that a satisfactory quality of water is delivered to the patient.

    Clinical Relevance:
    Dental practitioners should be aware of the problem of dental unit water lines contamination and the appropriate methods to control it as part of their duty of care to their patients.

    After reading this article, the reader should be able to appreciate the importance of dental unit water line contamination and be able to decide upon the appropriate methods of control to provide the patients with water of acceptable microbiological standards.

    Advances in medicine and technology have increased longevity and permitted the continued survival of the immunologically most vulnerable members of society. Therefore, it is well recognised that immunocompromised patients, may require special cross infection control procedures, when undergoing invasive medical and dental procedures were the mucosa is breached. Such procedures may include using sterile water and as providing the patient with antiseptic mouthwash to use immediately before the treatment. Dental surgery is unique compared to medicine in that the majority of procedures require the use of water for cooling instruments, irrigation and oral rinsing. So it is extremely important that the water used in dental surgery is of a high quality.

    Contamination of dental unit water lines (DUWL) by microorganisms is a real problem facing modern dentistry which has yet to be completely resolved even though the first report of contaminated water in the dental lines was as far back as 1963.1 Blake working in the United Kingdom recovered high concentrations of bacteria from dental waterlines, and he reported on the effectiveness of disinfectants as water decontaminants.

    The problem, however, does not lie merely with the presence of bacteria in the water but with the number or bacterial load and the opportunistic pathogens, which may be present. The concern about this issue is growing, as more studies are conducted. DUWL contamination in untreated systems often exceeds 1,000 colony-forming units per millilitre (CFU/mL). Counts ranging between 10,000 and 1,000,000 CFU/mL may be common place.2-4 The European Union potable-water guidelines on microbial load stated that the drinking water should not harbour more bacteria than 100 CFU/mL.5 In the USA, 500 CFU/mL of aerobic heterotrophic (air tolerating, free living) bacteria is the accepted maximum limit for drinking water, according to the Safe Drinking Water Act-USA. Similar, water quality standards are recommend for recreational waters such as swimming pools and spas.6 It is clearly apparent that the microbial quality of dental unit water should be at the same standard or preferably better than that prescribed for drinking water. Thus in 1996 the American Dental Association proposed the goal of 200 CFU/mL as the upper limit for dental unit water line contamination.7

    Although the number of proven reported cases of infection resulting from exposure to contaminated dental waterlines dental unit water are limited, a large body of scientific evidence exists documenting waterborne cross-infection in hospital settings. So, every effort should be made to guarantee a low risk and safe environment in the dental surgery. Contact of open wounds, mucous membrane or the oral cavity with water of poor microbiological quality simply is inconsistent with patient expectation of safety standards in modern dentistry8. This situation becomes even more relevant when considering the growing number of immuno-compromised patients, as these “high- risk patients” are more prone to infections and associated complications. Those considered to be at higher risk of acquiring respiratory infections include people seen every day in general practice e.g. smokers, diabetics, asthmatics on systemic steroids, as well as profoundly immunosuppressed patients with conditions such as AIDS or leukaemia, although the latter group are more likely to be treated in tertiary referral centres.

    Studies show that contaminated dental water also poses a risk to dental professionals because the dental procedures generate large amount of aerosols that may be inhaled. A proportion of dentists experience occupational exposure to Legionella pneumophila, a waterborne pathogen that can cause Legionnaire’s disease and a related condition known as Pontiac fever. Antibody titres to Legionella are significantly higher among dental workers than the general population. Fortunately, in the vast majority of cases there is no evidence of it leading to pulmonary infection. Although, a single fatal case occurred in a Californian Dentist. Reported data demonstrated there was strong circumstantial evidence linking the death from Legionella dumoffi sp. with exposure to aerosols from his dental unit water lines.9,10 The health and Safety Commission have recently issued a new Approved Code of Practice regarding the control of Legionella in water systems. It stated that water temperature can be used as a method of thermal control. The guidelines state that hot water should be stored at 60?C and the cold water should be kept below 20?C. The temperature of hot and cold tap should be checked monthly. Once a year a representative number of taps on a rotational basis should be checked. If taste or odour problems are noted then a microbiological investigation may be required as this could signal development of conditions that could promote growth of Legionella. The specific HSE guidelines on dental equipment state that they should be "drained down and cleaned at the end of each working day".

    The Role Of Biofilms:
    The DUWL contamination is not just about microorganisms suspended in the water, the real problem lies in the presence of a reservoir that provides a constant and continuous source of bacteria, namely the biofilm.11 It consists of a microbial community that adheres to the inner surface of the dental waterline tubing. Most biofilms are heterogeneous in species composition and morphology, characteristics similar to those found in another biofilm with which dentists are more familiar with, dental plaque.

    On average the biofilm is 30-50 microns thick and is enclosed in a polysaccharide layer known as the glycocalyx, which provides the biofilms with the property of resistance to chemical agents. Bacteria derived from the incoming main water are intrinsically resistant to most biocides. These organisms become the primary source for continued contamination of the system.12 The mature biofilm eventually becomes populated with a wide variety of species of bacteria, fungi and amoebae. In addition growth within the confines of the biofilm enhances bacterial proliferation by binding and retaining a supply of nutrients13 which permits a higher level of metabolic activity.13,14 Whilst protecting bacterial population from biocidal agents and flushing of the waterline.

    Biofilm Formation:
    Biofilm formation is reversible in the early stages, as the adhesion to the surface is not strong. As occurs on tooth surface with pellicle formation preceding development of dental plaque, the inner surface of the tubing is first conditioned by absorbed macromolecules and low molecular weight hydrophilic molecules from the main water. This stage enhances the efficacy of bacterial adhesion. Initial colonisation and adhesion is by microorganisms that produce attachment structures (e.g. extracellular polymeric substances, fimbriae). Later adhesion events involve other species of bacteria that need prolonged exposure to a surface in order to attach firmly. Irreversible adhesion and colonisation is achieved through the secretion of extracellular polysaccharides (EPS) and subsequent microbial multiplication, which is a time-dependent process.

    As the water is continuously moving through the tubes of the working dental unit, how do the microorganisms find enough time to form biofilms? The answer lies in the properties of fluid dynamics and geometry of dental lines. A fluid in a tube moves in layers (laminar flow). At the centre of the lumen it travels fastest; the further away from this centre layer the slower the movement becomes as a result of friction. Water at the tubing walls is virtually stagnant allowing bacteria to adhere and colonise the internal surface (fig.1). In addition, the dental unit water lines represent an ideal place for biofilm formation because of the high surface-to-volume ratio in the tubing system. As the diameter of the water line decreases (as is the case for the microbore tubing deployed in dental units) an increasingly larger surface area relative to volume becomes available for colonisation allowing the bacteria to be in contact with the water lines surface for a long time.15

    Source Of Contamination:
    There are two sources of DUW contamination; the water supply and oral fluids from the patients' mouths.
  • a. Water supply Studies have shown that the main source of contamination is the incoming water.16-18 The capacity of rapid development of biofilm on the dental water supply lines combined with the generation of potentially contaminated aerosols are a unique feature of dental chair water lines. Table 1 shows micro-organisms isolated from dental unit water lines. Emphasis is concentrated on clinically relevant waterborne microorganisms responsible for serious diseases. Pseudomonas aeruginosa is associated with a wide range of opportunistic infections and is a cause of pneumonia in hospitalised patients. Martin et al (1987) reported two cases of wound infection in immunocompromised patients by P. aeruginosa, which was also isolated from the dental water lines.19 Nevertheless, studies from Denmark and the UK on patients with cystic fibrosis undergoing dental treatment found no increased risk from contaminated DUW.20,21 Legionella organisms can be found in most types of water systems including dental waterlines.22,23 Aquatic non-tuberculous Mycobacterium species associated with pulmonary disease and opportunistic wound infections also have been recovered in DUWL.24 Since the dominant organisms in DUW are Gram-negative bacteria, which synthesise endotoxins, some authors have investigated the potential pathogenic role that endotoxins might play. Dentists are familiar with the action of endotoxins in plaque and periodontal disease. These endotoxins may be responsible for immune reactions ranging from mild inflammation to serious toxic shock. Indeed, the DUW was found to contain as high as 500 endotoxin units per millilitre (EU/mL) with an average of 80 EU/mL.25 Currently, there is no Quality Standard for endotoxin concentration in drinking, recreational or dental water.
  • b. Oral fluids: Bagga et al stated that each time a handpiece airturbine fitted with a retraction valve is stopped while the bur is still in the patient's mouth, almost 1 ml of microbe-laden oral fluids might be aspirated back into the dental unit water line. This fluid may contain an average in excess of 54,000 microorganisms per millilitre, including both facultative and obligate anaerobic bacteria of medium to high virulence. In vitro experiments have demonstrated that HIV and hepatitis B virus particles can be sucked back into the waterline. However, the introduction of antiretraction valve in the manufacture of dental handpieces has virtually eliminated the aspiration of microorganisms from the patient's mouth. Recent DUWL microbial analysis studies of dental waterlines report a low recovery rate of oral bacteria, the latter act as a marker of retrograde aspiration from the mouth. Thus supporting the value of anti-retraction valves. If failure of the valve occurs then potentially microbe-laden water could then be sprayed into the mouth of the next patient.4 Indeed, Walker et al reported the detection of Candida spp., oral streptococci and Fusobacterium spp. from dental units, suggesting back siphonage and failure of anti-retraction devices.26 Anti-retraction valve failure can be circumvented by flushing the waterline between patients and sterilisation of the handpiece. Non-pathogenic environmental water bacterial flora that predominates in waterline biofilms are adapted to growth in cold temperatures. Therefore the advice is not to heat DUWL, even though this is more comfortable for patients with dentine sensitivity. Heating preferentially selects and enhances the growth of the pathogenic bacteria such as Pseudomonas species, adapted to life in a warm-blooded host.22, 27

    Methods To Control DUWL Contamination:
    A wide variety of different measures and recommendations for controlling the quality of the water of dental units have been proposed. They concentrate on two aspects of control; treatment of dental water and improving dental unit design.

    a. Treatment Of The Dental Water:
  • Flushing water lines. This considerably reduces bacterial counts but does not allow levels to fall to those acceptable for drinking water.28-30 Furthermore, such flushing must be carried out in such a fashion as to avoid misting, which can contaminate the ambient air.31 Flushing is however, insufficient to reduce bacterial counts below the 500 CFU/ml limit unless unrealistically long clinical times are used.32 The physics of laminar flow of fluids through the water lines allows the frictional forces to slow the movement of fluids until flow rate at the surface is stabilised.33 Due to this phenomena, results from studies using dye elimination from DUWL demonstrate, that it takes at least 3 minutes of flushing to eliminate suspended bacteria from a 3-metre water line.34 Nevertheless, flushing the water lines for a period of two minutes before treating the first patient in the morning and for 20-30 seconds between patients can decrease the number of microorganisms in the water of dental unit.35-37 The BDA (2000) and ADA (1999) recommendations consider that simple flushing of water lines is an expedient interim measure for use by all dentists until more effective methods are introduced.
  • The use of sterile or distilled water per se is of no use in solving this problem if it is used within the dental lines already contaminated with a biofilm. Mills et al (1986) reported that sterile water alone didn't effectively reduce levels of microbial contamination in DUWL.38 For minor oral surgery procedures the guidance from both BDA and ADA is to irrigate with sterile water or saline. Importantly, for these sterile solutions to remain uncontaminated they need to be delivered through devices other than the dental unit waterline. The disadvantage of these system is that they can be expensive to purchase and more cumbersome and often less convenient to use than conventional delivery systems. Although when small volumes are required a sterile disposable syringe filled with saline is a suitable alternative.
  • Control of water storage temperatures. The average temperature of most DUWs is 23? C, which supports Legionella growth. To prevent colonisation of cold-water systems, water in a DUW should be kept below 20?C. Contamination of surgery hand basins with Legionella can be avoided by heating hot water to 55-60? C (Legionella are normally killed at this temperature).
  • Biocides are effective at eliminating bacteria suspended in the water but do not work efficiently on the biofilm, which is the major source of contamination.39 Karpay et al (1999) studied the efficacy of combined intermittent and continuous treatment with diluted sodium hypochlorite to improve DUW quality in a clinical setting. They found that NaClO was able to reduce CFU/mL sufficiently, to attain the proposed American Dental Association goal of less than 200 CFU/ml. Scanning electron microscopy at the end of the study demonstrated removal or inhibition of biofilm formation.40 In a series of trials, Meiller et al (1999) investigated the recurrence of microbial growth after treating DUWLs with sodium hypochlorite (bleach); glutaraldehyde; or isopropanol 15.3 ?. These agents reduce microorganisms in effluent water but do little to destroy the biofilm matrix in the DUWL, even with periodic treatments.41 Povidone-iodine 10 % prevented recovery of microorganisms for 3 to 14 days when used in combination with sterile water reservoir.38 In a study to examine the effect of a non-corrosive solution of 4 per cent Tween 80 coloured with Ponceau, Kelstrub et al (1977) found the solution to be effective in removal of microorganisms from the walls of the tubing system.42 The effect of these chemicals on biofilms was not mentioned and thus cannot be guaranteed. Reports about the use of ozone (O3) in the treatment of DUWLs showed promising results43 yet, further investigations are being conducted. An important consideration when employing biocides for purging DUWL, are the potential for adverse reactions arising from the by-products of the biological /chemical actions of the biocide. Unfortunately, most published studies failed to report or assess whether such adverse reactions occurred, therefore our knowledge of the impact of such events is limited. However, it should be noted that certain anti-biofilm agents were shown during in vitro testing to reduce the strength of both enamel and dentine adhesive bonds.32, 44 Whether the effects are clinically significant is unclear. Manufacturer’s instructions on use of biocides compatibility with their equipment should be followed, as agents containing hypochlorite are corrosive and may damage components of the dental unit. Mills (2000) cites several proprietary agents, which have received FDA clearance for marketing as waterline cleaners (not biocides). These include a glycerine-based bur lubricant (introduced continuously) that contains 0.12 % chlorhexidine gluconate, 12% ethanol and flavouring agents, a hydrogen peroxide-based solution used intermittently and a citric acid-based product.27

    b. Improving Unit Design:

  • Independent bottled water systems. These systems have been available as optional accessories to the dental unit for many years. Their main advantage is they bypass the municipal water supply. Instead, fluids are drawn from the reservoir bottle(s) holding either sterile or distilled water or alternatively a dilute aqueous biocide solution. However, such systems alone cannot reliably improve the quality of treatment water, as biofilms are able to form within the bottles unless carefully managed. Regular cleaning of the inside of the bottles and flushing of the waterlines with a disinfectant is required to reduce adherent microbial biofilms.6
  • Automatic treatment devices: Fayle (1996) stated that a manufacturer produces equipment which can be programmed to automatically flush disinfecting solution through the water/suction system between patient visits (Castellini Autosterile). This system has many disadvantages and limitations of use.45 A chlorhexidine acetate based slow-release device designed to be placed in DUWL reservoirs has been described. The device was successful in keeping units free of bacteria over a 3-month period, but effective cleaning of the tubing within the units was found to be essential for success.46 DentaPure iodinated resin cartridges (MLRB international) continuously releases 2 to 6 ppm free iodine into treatment water to control biofilm; their use life ranges from one week to one year. Other devices include Odyssey I (Tuttnauer USA) which generates an ozone and silver germicide via electrolytic action on incoming water. Periodic treatment regimens also can be automated using devices such as the Clean Source1 (Aquarius Technologies) or the PortaPurge (Micrylium Laboratories).
  • The use of antibacterial filters sited near the outlets can prevent contaminated water reaching the handpiece. However, they are prone to rapid clogging with bacteria and have to be changed frequently. Some products do not filter out the minerals in the water and may not be suitable for surgical procedures. Nevertheless, studies conducted suggest that filters can improve water quality47 to meet or exceed the 200 CFU/mL goal established by the FDA for nonsurgical procedures when used according to manufacturer's recommendations. Among the potential advantages of filters are the reduction or elimination of reliance on chemicals, the potential for damage to dental units and possible staff exposure to chemical residues.27
  • Tubes with antimicrobial properties48 or composed of materials that do not promote the formation of a biofilm are potentially an interesting solution as biofilm formation is affected by the composition of the DUWLs. Unfortunately, no studies have demonstrated the effectiveness of such modified tubes over the long term. However, it is important to ensure that the DUWLs comply with the British Standards for water pipes.
  • Anti retraction valves are recommended to prevent the aspiration of contaminated fluid and reduce the risk of transfer of potentially infective material.4,49-51 Montebugnoli and Dolci performed a study to evaluate 2 operating systems recently equipped with antiretraction devices installed inside the handpiece (Kavo) or inside the unit (Castellini) and to compare the results with those obtained from an operating system without any anti-suckback device (Bien Air). They used potassium bichromate dye to simulate fluid contamination. They concluded that antisuction devices could reduce but not prevent- dye penetration into the air chamber of a high speed handpiece.52

    In conclusion, the only way to lower bacterial counts in dental water to acceptable levels is to permanently eliminate the existing biofilm inside the tubes and prevents its formation in the new units. 29 Except for the fully autoclavable waterline with a sterile water reservoir system, all other methods are open to occasional failure. Introduction of any water decontamination system ideally should be validated to insure that water delivered to the patient is of an acceptable and safe standard. Chairside microbiological monitoring of water supplies may play an essential part of improving the safety of DUWs. Routine testing of the microbiological quality of water is unnecessary provided a system is in place which can be relied upon to deliver high quality water. Testing may be of value when a GDP is taking steps to establish a reliable water delivery system. Maintaining a drinking water standard in the DUWLs should be an integral component of the surgery’s infection control and quality assurance programme.

    It will be useful to summarise what the general dental practitioner needs to do to reduce the problem in his dental unit. According to BDA recommendations, the following measures should be implemented:

  • Flushing of the water lines for 2 minutes every morning before treating the first patient.
  • Flushing of the water lines for 30 seconds between each two patients.
  • The use of independent water system.
  • The use of disinfectants such as H2O2 and NaClO.
  • Avoidance the use of warm water in the DUWLs as the water should be kept below 20° C.
  • Draining of the DUWLs at the end of each day.
  • Making sure that the anti-retraction valve in the handpiece is functional by regular maintenance.


    January 2015
    The-O-Zone © Dr Julian Holmes