Co-editor; Dr Caroline Pankhurst
formerly Senior Lecturer in Oral Microbiology, Kings College London, UK
The previous chapter outlined the key principles involved in personal protection of the dental health care worker such as the personal protection equipment (PPE), how to avoid sharps injuries in the clinic and the protocol and strategic planning related to injuries so sustained and concluded with vaccines that are important for dental health care workers. This chapter outlines sterilization, disinfection and antisepsis as applied in dentistry, an equally important aspect of infection control for all involved in the practice of clinical dentistry.
Sterilization, disinfection and antisepsis
The reader should clearly bear in mind the following basic definitions of sterilization, disinfection and antisepsis as these terms are frequently used in clinical dentistry.
■ Sterilization is a process that kills or removes all organisms (and their spores) in a material or an object.
■ Disinfection is a process that kills or removes pathogenic organisms in a material or an object, excluding bacterial spores, so that they pose no threat of infection.
■ Antisepsis is the application of a chemical agent externally on a live surface (skin or mucosa) to destroy organisms or to inhibit their growth. Thus all antiseptics could be used as disinfectants, but all disinfectants cannot be used as antiseptics because of toxicity.
In general, sterilization involves extensive treatment of equipment and materials, and is costly and labour intensive. It is dependent on:
■ knowledge of the death curves of bacteria or spores when they are exposed to the inactivation process. Spores vary in their resistance to sterilizing agents: spores of Bacillus stearothermophilus are used to test the efficacy of steam autoclaves and unsaturated chemical vapour, whereas
Bacillus subtilis spores are used to test the efficacy of dry heat and ethylene oxide sterilization
■ the penetrating ability of the inactivating agent: steam penetrates more effectively than dry heat
■ the ability of the article to withstand the sterilizing process, with no appreciable damage to instruments and other materials (e.g., corrosion of sharp, cutting edges of instruments)
■ a procedure that is simple but efficient and relatively quick (so that there is a readily available supply of sterile instruments and materials): thus the temperature of sterilization is of crucial importance, as is the period for which the instrument or material is held at a given temperature; both these factors dictate the efficacy of the chosen sterilization method
■ the effects of organic matter, such as saliva and blood, which enhance the survival of bacteria and interfere with the sterilization process. All articles must be clean before sterilization.
All instruments and appliances used in dentistry should ideally be sterilized, although some items of equipment and certain surfaces (e.g., bracket tables attached to the dental chair) do pose problems. In such circumstances, the best alternative is to disinfect the items or surfaces concerned. In some instances, hard-to-decontaminate dental equipment can be replaced with equivalent single-use devices. Single-use devices are labelled by the manufacturer for only a single use and therefore do not have reprocessing instructions. Use single-use devices for one patient only and dispose of appropriately.
Decontamination (synonym: reprocessing)
Decontamination is the process by which reusable items are rendered safe for further use and for staff to handle. Decontamination is required to minimize the risk of cross infection between patients and between patients and staff. The term decontamination (as opposed to sterilization and disinfection) has gained popularity particularly in European regions and is less widely used in North America. Decontamination is a complex and an exacting process and entails:
■ cleaning
■ disinfection
■ sterilization (Fig. 38.1A).
Decontamination of instruments
Receiving, cleaning and decontamination
The removal of contaminated instruments and equipment from the treatment area should follow a set routine, avoiding crosscontamination between the soiled and sterilized instruments. Once an effective method of instrument or equipment flow has been worked out, this method should be strictly adhered to.
Reusable instruments, supplies and equipment should be received, sorted, cleaned and decontaminated in one section of the processing area. Cleaning should precede all disinfection and sterilization processes and should involve removal of debris as well as organic and inorganic contamination.
Removal of debris and contamination is achieved either by:
■ cleaning using a thermal washer disinfector (most preferred method)
■ manual combined with ultrasonic cleaning
■ manual cleaning (the least preferred).
If visible debris, whether inorganic or organic matter, is not removed, it will interfere with microbial inactivation and can compromise the disinfection or sterilization process. After cleaning, instruments should be rinsed with water to remove chemical or detergent residue.
Considerations in selecting cleaning methods and equipment include:
■ efficacy of the method, process and equipment
■ compatibility with items to be cleaned
■ occupational health and exposure risks.
Note that the use of automated cleaning equipment such as an ultrasonic cleaner or thermal washer disinfector does not require presoaking or scrubbing of instruments. These instruments therefore:
■ increase cleaning efficacy and productivity
■ reduce danger of aerosolization of infectious particles
■ reduce incidence of sharps injuries and are hence safer
■ reduce manual labour.
Presterilization cleaning
Whenever possible, cleaning should be performed using an automated and validated process in preference to manual cleaning. Manual cleaning should only be considered where manufacturer's instructions specify that the device is not compatible with automated processes. Heavy-duty household utility gloves must be used when cleaning instruments; plastic aprons, eye protection and face masks are also desirable. Instruments should be cleaned as soon as possible after use.
Fig. 38.1 (A) The instrument decontamination cycle. (B) A thermal washer disinfector. (Part A from Department of Health. (2013). Decontamination in primary care dental practices (HTM 01-05) (2nd ed.). London: Department of Health. Available from: https://www.gov.uk/government/publications/decontamination-in-primary-care-dental- practices, with permission; Crown Copyright.)
If immediate cleaning is not feasible, placing instruments in a puncture-resistant container and soaking them with detergent, a disinfectant/detergent or an enzymatic cleaner will prevent drying of patient material and make cleaning easier and less time-consuming. Use of a liquid chemical sterilant/high-level disinfectant (e.g., glutaraldehyde) as a holding solution is not recommended.
Reusable dental hand instruments with sharp or a cutting edge should be handled with extreme care during scrubbing to prevent injury to the hands. Cements should be removed in the surgery before setting solid. Uncapped needles should never be left on the instrument tray, and after use, these and other single-use disposable sharps should be placed directly in puncture-resistant containers.
Automated cleaning using washer disinfectors
A thermal washer disinfector (Fig. 38.1B) is the preferred method for cleaning dental instruments as it offers the best option for the control and reproducibility of cleaning; a typical thermal washer disinfector cycle for instruments includes the following five stages:
■ Flush: removes gross contamination, including blood, tissue and solid debris, bone fragments and other fluids.
A water temperature of less than 45°C is used to prevent protein coagulation and fixing of soil to the instrument.
■ Wash: removes any remaining soil. Mechanical and chemical processes loosen and break up contamination adhering to the instrument surface. Detergents should be compatible with the instruments used in order to avoid discolouration, staining, corrosion and pitting.
■ Rinse: removes detergent used during the cleaning process. This stage can contain several substages. The quality of water used is important as otherwise it may lead to long-term problems such as spotting of instruments.
■ Thermal disinfection: the temperature of the load is raised and held at the preset disinfection temperature for the required disinfection holding time: for example, 80°C for 10 min or 90°C for 1 min.
■ Drying: purges the load and chamber with heated air to remove residual moisture.
Preparation and packaging
In a separate section of the processing area, cleaned instruments and other supplies should be inspected; assembled into sets or trays; and wrapped, packaged or placed into container systems as appropriate for sterilization. Instruments used in dentistry may be packaged for sterilization using:
■ an open-tray system sealed with a see-through sterilization bag
■ perforated trays with fitted covers wrapped with sterilization paper
■ individual packaging in commercially available sterilization bags.
Prior to packaging, all hinged instruments should be opened and unlocked. An internal chemical indicator should be placed in every package. In addition, an external chemical indicator (e.g., chemical indicator tape) should be used when the internal indicator cannot be seen from outside the package.
For unwrapped loads, at a minimum, an internal chemical indicator should be placed in the tray or cassette with items to be sterilized. Dental practices should refer to the manufacturer's instructions regarding use and correct placement of chemical indicators. Critical and semicritical instruments that will be stored should be wrapped or placed in containers (e.g., cassettes or organizing trays) designed to maintain sterility during storage.
Sterilization
The sterilization process
In dentistry, sterilization is usually achieved by moist heat (steam under pressure in an autoclave).
Other sterilization methods, not used in dentistry, are ethylene oxide gas and gamma-irradiation (employed by commercial suppliers of plastic goods) and filtration (used for sterilization of injectable drugs). Hot air ovens and chemiclaves were once popular in dentistry but they are no longer used or recommended owing to quality control and environmental issues.
Moist heat sterilization (steam under pressure)
Steam is a very effective sterilizing agent as it:
■ liberates latent heat when it condenses to form water, potentiating microbicidal activity
■ contracts in volume during condensation, thus reinforcing penetration.
When water is heated in a closed environment, its boiling point is raised, together with the temperature of the generated steam; for example, at 104 kPa (15 psi), the steam temperature is 121°C. This phenomenon is utilized in steam sterilization by the autoclave (Fig. 38.2). Put simply, an autoclave is a glorified domestic pressure cooker with a double-walled or jacketed chamber; steam circulates under high pressure inside the chamber, in which the objects for sterilization (the load) have been placed. Once the sterilization cycle is complete, drying the load is accomplished by evacuating the steam. Drying can
Fig. 38.2 Principal features of a small autoclave used in dentistry.
Table 38.1 Examples of sterilization times and temperatures for packaged items
|
Method |
Holding time (min) |
Temperature °C (°F) |
Biological monitoring agent |
|
Steam autoclave |
Bacillus stearothermophilus |
||
|
• Gravity displacement |
3-4 |
121 (250) |
|
|
• Pre-vacuum sterilizer |
3-4 |
134-138 (272) |
be accelerated by the suction of warm, filtered air into and through the chamber. It is important to expel the air in the chamber at the beginning of a sterilization cycle because:
■ the temperature of an air-steam mixture at a given pressure is lower than that of pure steam
■ air pockets interfere with steam penetration.
There are two types of autoclaves:
1. Vacuum autoclaves in which air is evacuated from a metal chamber by vacuum pump are now becoming popular in dentistry due to wide availability of small, bench-top units. In central sterile supply units in hospitals they are sometimes referred to as porous load autoclaves. These vacuum autoclaves are more desirable for routine dentistry than the gravity displacement type for the sterilization of hollow devices such as dental handpieces.
2. Gravity displacement autoclaves are small, automatic bench-top autoclaves. They work on the principle of downward displacement of air as a consequence of steam entering at the top of the chamber. Examples of sterilization times and temperature for autoclaves are shown in Table 38.1. Of the options given, a sterilization cycle of 134°C for 3-4 min at 207 kPa is recommended for both wrapped and unwrapped dental instruments.
Autoclaves used in dentistry
Three different types of autoclaves used in dentistry; these are:
■ Type N: air removal in type N sterilizers is achieved by passive displacement with steam. They are non-vacuum sterilizers designed for non-wrapped solid instruments.
■ Type B (vacuum): these sterilizers incorporate a vacuum stage and are designed to reprocess load types such as hollow, air-retentive and packaged loads. A number of different cycles may be provided. Each cycle should be fully validated and used in accordance with instructions provided by both the sterilizer manufacturer and the instrument manufacturer(s).
■ Type S: these sterilizers are specially designed to reprocess specific load types. The manufacturer of the sterilizer will define exactly which load, or instrument, types are compatible, and should be used strictly in accordance with these instructions.
Types B and N are most frequently used in dental practices.
Fig. 38.3 The stages of a full sterilization cycle. (A) Heat up; (B) holding time; (C) cooling time.
The sterilization cycle
The sterilization cycle can be divided into three periods (Fig. 38.3): the heating-up period, the holding period and the cooling period. For the N-type non-vacuum bench-top autoclave (routinely used in dentistry), this entails:
1. removal of air by a vacuum pump or downward displacement of air by incoming steam while the chamber is heated to the selected temperature
2. 'holding' the load, which is sterilized, for the appropriate period at the selected temperature and pressure
3. drying the load to its original condition by a partial vacuum (this is assisted by the heat from the jacket)
4. restoration of the chamber to atmospheric pressure by rapid exhaustion of steam.
Notes on the proper use of bench-top autoclaves
■ Autoclaves should not be overloaded with instruments.
■ The water reservoir should be filled at the start of the day with fresh distilled water or reverse osmosis water to prevent build-up of residues or lubricant. Do not use tap water, as this can lead to biofilm formation in the reservoir and accumulation of lime scale. Sterilizers are designed either with an automated single-use water cycle and drainage or the water reservoir should be drained down manually at the end of the day, the reservoir cleaned with fresh purified water and left dry overnight.
■ Autoclaves should be validated and serviced annually, and a logbook of daily process monitoring, validation, autoclave maintenance and defects should be kept (in the UK these records must be maintained for a minimum of 2 years).
■ The mechanical indicators of the autoclave (temperature, pressure and holding time) should be monitored routinely at the start of the day and the results recorded in the machine log book as part of the practice's quality control procedures.
■ When using a vacuum autoclave a drying cycle should be used for bagged instruments.
Fig. 38.4 Chemical indicators (1, 2) and a biological indicator (3) used for autoclave monitoring.
Monitoring sterilization
Achievement of the requisite temperature and pressure, as indicated by the gauges of the autoclave (or any other sterilizer), does not guarantee that the entire load has been sterilized. All sterilization procedures must therefore be carefully and regularly monitored so that failures are detected and sterility is assured. The indicators used for checking sterility are (Fig. 38.4):
■ process monitoring using mechanical indicators (i.e., the temperature, holding time and pressure gauges of the autoclave)
■ chemical indicators
■ biological indicators/monitors.
Chemical indicators are materials (either liquid or paper) that change colour on exposure to the appropriate sterilization cycle, indicating that the load has been processed. Note that process indicators do not prove sterilization but merely verify that the items have been subjected to the processing conditions; thus the main function of a process indicator is to assure the operator that the material has gone through a sterilization cycle. At least one process indicator should be cycled with every sterilization load, and the results should be documented in a sterility control file.
In contrast to process indicators, biological monitors are designed to prove sterilization. The indicators used in this system are bacterial spores (Table 38.1), which require high temperatures for extended periods to lose their viability (the corollary is that, if the spores are killed, then less-resistant microbes are killed more readily and sterility is achieved).
Biological monitoring or spore tests should be used on a weekly basis in dentistry. The monitor should be placed in the sterilizer at a point where sterilization is most difficult to achieve (e.g., inside bags or trays). After cycling, each strip should be sent for culture or cultured in the clinic according to the manufacturer's instructions. The results of biological monitoring should be routinely recorded and kept in a sterility control file. Spore tests should also be done when commissioning a new autoclave, after servicing or repairs and as part of the training of new staff.
Quality control of small bench-top autoclaves
Small autoclaves should be operated to ensure that they are:
■ compliant with the local safety requirements, as well as the manufacturer's instructions
■ installed, commissioned, validated, maintained and operated appropriately in compliance with the manufacturer's instructions.
Daily tests of small autoclaves
The daily tests should be performed by the user and will normally consist of:
■ a warm-up cycle before instruments can be processed (for some autoclaves)
■ a steam penetration test: Helix or Bowie-Dick tests (vacuum sterilizers only)
■ an automatic control test according to manufacturers' instructions.
■ the above outcomes to be recorded in the logbook together with the date and signature of the operator.
The Bowie-Dick or Helix test is used in vacuum autoclaves to check the steam penetration into the centre of the autoclave load and to signal the presence of any air pockets.
Storage and care of sterile instruments/devices
Once sterilized, the instruments or devices should be maintained in a sterile state until they are used again. The proper storage of sterile instruments is therefore as important as the sterilization process itself; improper storage would break the 'chain of sterility' and introduce the possibility of pathogenic recolonization risk. A barrier(s) should be maintained between the instruments and the general practice environment. The following guidelines should be followed in storing sterile instruments/devices:
■ maintain rigorous records to identify all instruments, packs and their contents, and their storage times
■ use a 'first-in first-out principle' when removing instruments from the store
■ store sterilized instruments in purpose-built storage cabinets that can be easily cleaned
■ the instrument storage area should be dedicated for the purpose and situated ideally in the clean area of the decontamination room (Fig. 38.5)
■ instruments should be stored above floor level away from direct sunlight and water in a secure, dry and cool environment
■ appropriately coded labels should be used to indicate the contents, cycle number and expiry dates where packs are non-transparent
■ before using the stored instruments, check them to ensure that the packaging is intact and dry
■ do not reuse wrapped instruments stored for more than one year from the date of sterilization. These have to be cleaned and re-sterilized (Fig. 38.1).
Disinfection
Methods of disinfection consist of:
■ heat (pasteurization; boiling in water)
■ physical methods (ultrasonics)
■ chemical methods.
Fig. 38.5 Example layout for a single decontamination room. (From Department of Health. (2013). Decontamination in primary care dental practices (HTM 01-05) (2nd ed.). London: Department of Health. Available from: https://www. gov.uk/government/publications/decontamination-in-primary- care-dental-practices, with permission; Crown Copyright.)
Disinfection by heat
Pasteurization
Pasteurization is named after Louis Pasteur's discovery that mild heating prevents the spoilage of wine by selective killing of unwanted microbes. A similar treatment is now applied to milk to delay souring due to microbial activity. Milk is raised to a temperature of either 63-660°C for 30 min or (in the flash method) to 72°C for 15 s. This procedure renders the milk safe from contamination with Mycobacterium tuberculosis, Campylobacter and other pathogens. It should be noted that pasteurization is not a sterilization process.
Boiling water
If the boiling period is short, bacterial spores can survive; boiling water is therefore inadequate for sterilization of dental instruments.
physical methods: ultrasonics
Ultrasound is an effective way of disrupting microbial cell membranes and is used for removing debris before autoclaving. The ultrasonic bath generates high-frequency sound waves that create alternating regions of high and low pressure within the bath. The bath is filled with a detergent or enzymic solution, and the sound waves produce bubbles in the detergent under low-pressure, which implode when the pressure changes from low to high, thereby dislodging debris adhering to the instrument. Ultrasonic baths are particularly useful when cleaning complex, hinged or serrated devices.
Chemical methods
Choosing a chemical disinfectant should be done carefully because a disinfectant used for one purpose may not be equally effective for another. Further, the antimicrobial activity of a chemical disinfectant falls drastically in the presence of organic debris. Products that usually disinfect items or surfaces may not do so when there is heavy contamination, particularly with resistant microbes in large numbers. The residual levels of organisms following disinfection may still represent an infection risk to unusually susceptible patients.
Mode of action of chemical disinfectants
The chemicals used as disinfectants generally behave as 'protoplasmic poisons' in three different ways.
1. Membrane-active disinfectants damage the bacterial cell membrane with resultant egress of the cell constituents; examples are chlorhexidine, quaternary ammonium compounds, alcohols and phenols.
2. Fixation of the cell membrane and blockage of egress of cellular components appear to be the mode of action of formaldehyde and glutaraldehyde.
3. Oxidizing agents oxidize cellular constituents; examples are halide disinfectants such as hypochlorite and bromides (the former is more active than the latter).
Conditions determining the effectiveness and choice of a disinfectant Spectrum of activity
Disinfectants vary widely in their activity; for example, some are more active against Gram-positive than Gram-negative bacteria (Table 38.2).
Satisfactory contact
All contaminated surfaces should come into contact with the disinfectant for the specified period. Organic debris, air and greasy material may prevent this, hence the importance of thorough cleaning of the material or instrument before disinfection.
Concentration
Adequate concentration of disinfectants is essential, and they should always be accurately dispensed. It is important to use the manufacturer's stated dilution of the disinfectant.
Table 38.2 Properties of disinfectants used in dentistry
Table 38.3 Categories of patient-care items and how they should be processed after usage
pH
The activity of a disinfectant is often pH dependent (e.g., glutaraldehydes act only at alkaline pH, whereas phenols work best at acid pH).
Neutralization
A wide range of substances, including blood and saliva, hard water, soaps and detergent, may neutralize the disinfectant.
Stability
Not all disinfectants are stable, especially when diluted, and may deteriorate with age or storage. Solutions should be freshly prepared for use and marked with an expiry date.
Speed of action
In general, disinfectants act slowly, and their activity depends on the concentration used. Hypochlorites have a rapid action but are corrosive at high concentrations. Glutaraldehyde is slow acting but is an effective sporicidal agent.
Absence of odour and toxicity
These attributes are desirable for disinfectants used in dentistry.
Cost
This is an important factor when choosing a disinfectant, although inexpensive disinfectants should not be used at the expense of those with desirable properties.
Biodegradability and environmental impact
These should also be considered when choosing a disinfectant.
Potency of disinfectants and their uses
Disinfectants can be generally categorized as having high, intermediate or low potency, depending on their ability to kill various groups of organisms.
■ High-level disinfectants are active against Gram-positive and Gram-negative bacteria, spores and M. tuberculosis (Table 38.2).
■ Intermediate-level disinfectants destroy M. tuberculosis, vegetative bacteria, most viruses and fungi, but few, if any, spores.
■ Low-level disinfectants kill most bacteria and most fungi, but not M. tuberculosis or spores.
A rough guide to the use of these three categories of disinfectants is as follows.
Step 1
Categorize the items that require disinfection or sterilization into three groups (Table 38.3):
■ critical items are those that penetrate the skin or mucosa and/or touch exposed tissues including bone (e.g., surgical instruments and periodontal scalers burs)
■ semicritical items are those that come in contact with mucous membranes or non-intact skin such as exposed
skin that is abraded or has dermatitis (e.g., mouth mirrors, amalgam condensers)
■ non-critical items only come into contact with skin (e.g., radiograph head/cone, blood pressure cuff, face bow).
Step 2
Use the appropriate technique (Table 38.3):
■ steam sterilization for all critical items as they have the greatest risk of transmitting infection
■ steam sterilization for heat-tolerant semicritical items (e.g., dental handpieces); for heat-sensitive items replace with a heat-tolerant or disposable alternative. If there is no alternative, use high-potency disinfectants
■ intermediate (intermediate-level (i.e., tuberculocidal claim)) or low-potency hospital disinfectants for non-critical items.
Disinfectant and antiseptic agents commonly used in dentistry
Alcohols
Ethyl alcohol or propyl alcohol (70%) in water is useful for skin antisepsis prior to cannulation, injection and surgical hand-scrubbing. Alcohol combined with disinfectants is used in dentistry for surface disinfection, but authorities in the USA do not recommend alcohol for this purpose as it evaporates relatively quickly and leaves no residual effect. Other disadvantages are its flammability, limited sporicidal activity and ready inactivation by organic material. Yet, alcohols are still popular because they are cheap, fast acting, readily available and water soluble.
Aldehydes
Glutaraldehyde is perhaps the most popular disinfectant used in dentistry in some regions, whereas it is banned in others. It is both a skin irritant and a sensitization agent, which results in both long-term and short-term health effects. It is mainly used for so-called cold sterilization or the high-level disinfection of equipment (such as fibre-optic instruments) that does not withstand autoclaving procedures. All aldehydes are high- potency disinfectants.
The free aldehyde groups of glutaraldehyde react strongly with the free amino groups of proteins in a pH-dependent manner. This leads to the effective microbicidal activity, sensitization of skin and incidentally, cross-linking with proteins such as collagen when used as a component of dentine-bonding systems. Hence as the pH decreases, the activity of glutaralde- hyde declines while its stability increases. Conversely, when the pH is alkaline, the activity is higher and it becomes less stable. Hence, in practice, glutaraldehyde is commercially available as a 2% acidic solution, to which an 'activator' has to be added to bring the solution to the 'in-use' alkaline pH of 8.0. Although the activated solution has a shelf-life of up to 14 days, this should be interpreted with caution as the solution may become prematurely ineffective due to other factors. In regions where glutaraldehyde is banned, then alternative, non- glutaraldehyde containing products are used for high-level disinfection (e.g., ortho-phthalaldehydes, peracetic acid and hydrogen peroxide).
Bisguanides
Chlorhexidine is an example of a bisguanide disinfectant; it is widely used in dentistry as an antiseptic and a plaque-controlling agent. For example, a 0.4% solution in detergent is used as a surgical scrub (Hibiscrub); 0.2% chlorhexidine gluconate in aqueous solution is used as an antiplaque agent (Corsodyl) and at a higher concentration (2%), it is used as denture disinfectant. It is a cationic bisguanide molecule, usually prepared as salts of acetate, digluconate, hydrochloride and nitrate.
As chlorhexidine has two positive charges at its polar ends, it is highly active against both Gram-positive and Gram-negative organisms. (Note: all bacteria possess negatively charged cell walls in nature.) It also kills Candida (but not M. tuberculosis). Owing to ingress of the disinfectant, the cell membrane permeability is altered with resultant leakage of cell contents and precipitation of the cytoplasm leading to cell death. Its substantivity (i.e., prolonged persistence) in the oral cavity is mainly due to absorption on to hydroxyapatite and salivary mucus.
Halogen compounds
Hypochlorites are oxidizing agents and act by releasing halide ions. Although cheap and effective, hypochlorites readily corrode metal and are quickly inactivated by organic matter (examples of proprietary preparations are Chloros and Domestos). Note: available chlorine is a measure commonly used to indicate the oxidizing capacity of hypochlorite agents and is expressed as the equivalent amount of elemental chlorine. Thus the equivalence of 1% available chlorine corresponds to 10 000 ppm available chlorine. Chlorine-releasing granules (e.g., sodium hypochlorite/sodium dichloroisocyanurate) or a liquid solution of hypochlorite at a concentration of 10 000 ppm (1%) is used to disinfect surfaces contaminated by blood and body fluid spills. Disposable chlorine-releasing wipe (equivalent to 1000 ppm or 0.1% free chlorine) are employed to clean blood spots from dental chairs.
Phenolics
Phenolic disinfectants are clear, soluble or black/white fluids (black/white fluids are not used in dentistry). They do not irritate the skin and are used for gross decontamination because they are not easily degraded by organic material. They are poorly virucidal and sporicidal. As most bacteria are killed by these agents, they are used widely in hospitals and laboratories. Examples are ClearSOL and Stericol.
Chloroxylenol is also a non-irritant phenolic used universally as an antiseptic; it has poor activity against many bacteria, and its use is limited to domestic disinfection (e.g., Dettol).
A sterilization and disinfection guide for items commonly used in dentistry is given in Table 38.4.
Environmental disinfection
The dental clinic setting should always be kept free of potential pathogens by appropriate environmental infection control measures. In general, when using environmental disinfectants:
■ the manufacturers' instructions for correct use of cleaning and disinfecting products must be strictly adhered to
■ high-level disinfectants for disinfection of environmental (clinical contact or housekeeping) surfaces should not be used as they pose a health hazard to workers
Table 38.4 Sterilization and disinfection guide for items commonly used in dentistry
Table 38.4 Sterilization and disinfection guide for items commonly used in dentistry—cont’d
■ always use appropriate personal protective equipment when cleaning and disinfecting environmental surfaces (e.g., puncture- and chemical-resistant gloves, disposable plastic apron, protective eyewear/face shield and mask).
Clinical contact surfaces
Clinical contact surfaces can be directly contaminated from patient materials either by direct spray or spatter generated during dental procedures or by contact with contaminated gloved hands of the dental personnel. These surfaces can subsequently contaminate other instruments, devices, hands or gloves. Examples of such surfaces include:
■ light handles
■ switches
■ dental radiograph equipment
■ dental chairside computers
■ reusable containers of dental materials
■ drawer handles
■ faucet handles
■ countertops
■ doorknobs.
Barrier protection of surfaces and equipment can prevent contamination of clinical contact surfaces, but is particularly effective for those that are difficult to clean. Barriers include clear plastic wrap, bags, sheets, tubing and plastic-backed paper or other materials impervious to moisture. Because such coverings can become contaminated, they should be removed and discarded between each patient, with gloved hands. After removing the barrier, the surface needs to be cleaned and disinfected. The barrier makes cleaning easier and faster. After removing gloves and performing hand hygiene, clean barriers on these surfaces should be replaced before the next patient.
If barriers are not used, surfaces should be cleaned and disinfected between patients by using either a low-level or an intermediate-level disinfectant wipe. Disinfectant sprays are to be avoided where possible as repeated use of sprays can lead to hypersensitivity reactions in staff or trigger allergic reactions in susceptible patients.
Housekeeping surfaces
■ The dental practice should train staff to undertake daily cleaning of exposed housekeeping surfaces (e.g., floors, walls, cupboard doors and sinks) with a detergent and warm water or registered hospital disinfectant/detergent. An alternative cleaning method widely used in hospitals is cleaning with a microfibre cloth and water. Microfibre cloths are formed of very fine fibres at a high density producing a vast surface area that acts to remove and trap dirt and microbes. The cloths, which are reusable, are then laundered in a washing machine at 60-90°C to remove and destroy the pathogens.
■ Clean mops and cloths after use and allow to dry before reuse, or use single-use, disposable materials. Cloths and mops used to clean housekeeping surfaces in heavily contaminated patient treatment areas should be separated from those used to clean non-clinical areas to prevent cross contamination. This can be achieved by colour coding of cleaning materials (e.g., mops, cloths and buckets) according to the room where they are to be used.
■ It is critical that fresh cleaning or disinfecting solutions are made daily or according to manufacturer's instructions.
■ Walls, blinds and curtains in patient-care areas should be cleaned weekly to avoid build-up of dust or soil.
■ A nominated person in the dental practice should train staff, inspect and audit the cleaning undertaken in the dental premises.
Dental unit waterlines: disinfection and management
The question of the quality of water in dental unit waterlines (DUWLs) attached to handpieces, ultrasonic sealers and air/ water syringes has been debated widely. The source of water to the dental unit is either directly from municipal supply or via water reservoir bottles usually filled with distilled or reverse osmosis water. After entering the unit, it passes through a multichannel control box that distributes the water to hoses (DUWLs) feeding various attachments such as the high-speed handpiece, the air/water syringe and the ultrasonic scaler. Unfortunately, certain properties of the DUWLs promote bacterial growth. DUWLs have a very small bore, but a high surface area, and this together with the intermittent, slow flow rate may result in the whole column of water in the DUWL becoming stagnant for long periods. In addition, suck-back of oral fluids from the mouth can occur during use. Hence bacteria tend to form biofilms within a few hours on the internal surfaces of the DUWL unless they are regularly cleaned and disinfected (Chapter 31). The main risk to dental staff and patient health from DUWL contamination comes from opportunistic and respiratory pathogens such as Legionella spp., non-tuberculous mycobacteria (NTM) and Pseudomonads. These organisms can be amplified in the biofilm to reach infective concentrations, with the potential for inhalation-associated respiratory infections or direct contamination of surgical wounds. Although the majority of microbes isolated from DUWL are innocuous environmental saprophytic bacteria, legislation has provided guidelines for the upper limits of bacteria and hence the quality of the water resources that service the DUWL. Generally, the water entering the DUWL contains very few organisms: 0-100 colony-forming units (CFUs)/ml. However, water exiting the handpiece may contain up to 100 000-1 000 000 CFU/ml, mainly because of the organisms that are picked up from the bacterial biofilms growing within the lines.
The guidelines from the Centers for Disease Control and Prevention (CDC) in the USA recommend that water delivered to patients from DUWL during non-surgical dental procedures should meet drinking water standards (i.e., <500 CFU/mL of aerobic, mesophilic heterotrophic water bacteria). In the UK the current recommendation for dental unit water quality is more stringent and is set at 100-200 CFU / ml of aerobic heterotrophs at 22°C. To allow dentists to have better control over the quality of the water used in patient care, independent reservoirs or water-bottle systems are recommended, which are independent of the public water supply. These systems are not sufficient on their own to supply suitable water for dental treatment. Commercial products and devices are available that can improve and maintain water quality and are discussed in the next section. Furthermore, independent systems act as a type A air gap, a physical gap that prevents back-siphonage of contaminated water into the municipal mains supply.
Recommendations on care of waterlines
■ The quality of water used for routine dental treatment should match that of the national standards for drinking water (i.e., <100-200 CFU/ml or <500 CFU/ml of heterotrophic water bacteria).
■ Independent reservoir and water-bottle systems are recommended to be filled with freshly produced (i.e., less than 12-h old) reverse osmosis or distilled water. These purified waters are not sterile but are less likely to contain legionella, non-tuberculous mycobacterium and pseudomonads found in potable tap water.
■ In the UK, national guidelines recommend that dental surgeries/offices should 'drain down, clean, flush and disinfect all system components, pipework and bottles twice daily. Independent water storage bottles should be cleaned, rinsed with reverse osmosis or distilled water, dried and stored dry and inverted overnight'.
■ All DUWLs should be flushed for 2 min at the beginning of each day, prior to commencing treatment and at the end of the day.
■ The DUWL should be flushed for 20-30 s between patients to reduce temporarily the microbial count, as well as to clean the waterline of materials that may have entered from the patient's mouth. This includes handpieces, ultrasonic scalers and air/water syringes.
■ All DUWLs should be fitted with non-retractable devices, to prevent suck-back (backflow/back-siphonage) of material into the municipal water supply.
■ Water from DUWL should never be used as an irrigant in procedures involving breaches of the mucosa and bone exposure. During surgical procedures use sterile solutions of coolant/irrigant administered by an appropriate delivery device, for example, sterile bulb syringe, sterile tubing that bypasses DUWLs or sterile single-use devices.
■ The dental unit manufacturer should be consulted for appropriate methods and equipment to maintain the recommended quality of dental unit water and their recommendations followed for monitoring and sustaining water quality; the need for periodic maintenance of antiretraction mechanisms should also be verified with the manufacturer.
Maintaining quality of dental unit water
This could be achieved currently using antiretraction valves, filters, flushing, chemicals or water purifiers.
Antiretraction valves (check valves)
These are now the norm in all modern dental units and dental handpieces and prevent the re-aspiration (or suck-back or back- siphonage) of fluid contaminated with oral flora of patients into the waterline. However, it is now known that the antiretraction valves are very inefficient unless they are regularly maintained and replaced periodically. Check valves on the dental unit help prevent back-siphonage into the potable water system.
Filters
Filters may be installed, for instance, between the waterline and the dental instrument. These have no effect on the biofilm in the waterlines but will remove microorganisms as the water is delivered to the patient. Filters are inefficient as they must be replaced periodically, and the frequency depends on the amount of biofilm in the waterlines.
Flushing
This is a simple and efficient means of temporarily reducing the bacterial load in the waterline. It is recognized that regular flushing prior to patient treatment will discharge the stagnant water, reduce the impact of oral suck-back and draw up fresh biocide into the DUWL, facilitating disinfection of the waterline. Staff should be alert to signs of change in water quality such as malodour, cloudiness and bad taste imparted to the water by microbial contamination, which are particularly noticeable after periods of stagnation. These signal that conditions may be appropriate to support the growth of legionella. The dental practice should seek further advice on microbial sampling for legionella detection.
Although flushing can reduce the numbers of bacteria in expelled water, the effect is transient and has no impact on the waterline biofilm. Care should be taken to avoid splatter and aerosol exposure during DUWL flushing and masks and eyewear should be donned. Irregularly used dental units should be flushed on a routine basis (at least weekly) to prevent stagnation.
Biocides and chemicals
These remove, inactivate or prevent the formation of biofilm. Chemicals can either be continuously infused into or be intermittently added to the dental unit water by varying technologies. Disinfectants deployed in DUWLs must be active against the range of microorganisms found in the DUWL, including Legionella spp., be non-toxic to patients and not damage the waterlines or handpieces; examples include, amongst others, alkaline or hydrogen peroxide, hydrogen peroxide/silver ions, peracetic acid formulations, tetrasodium ethylenediamine- tetraacetic acid (EDTA), chlorhexidine formulations, iodine, quaternary ammoniums and chlorine dioxide. Hypochlorite, once popular for disinfecting DUWL, leads to corrosion of handpieces and is now used for 'shock treatment' eradication of microbiologically proven legionella contamination. Concerns here are the possible development of bacteria resistant to the chemicals and environmental pollution.
Water purifiers
Water purifiers treat the water coming into the dental unit (source water). These treat the source water and kill or remove microorganisms by methods such as filtration, electrolyzed water or ultraviolet light. One advantage of this method is that they may delay biofilm formation on waterlines or synergize other treatment methods.
Miscellaneous
Other, rather expensive, methods for delivery of quality water include the use of sterile water and autoclavable systems.
Boil-water advisories
Boil-water advisory is issued by authorities when the public water supply is likely to be contaminated with pathogenic organisms or the numbers of microbes in the system are above that which is compatible with health. During such periods, the following apply:
■ Do not deliver water from the public water system to the patient through the dental unit, ultrasonic scaler or other dental equipment connected to the public water system.
■ Do not use water from the public water system for dental treatment, patient-rinsing or hand-washing. For the latter purpose, antimicrobial-containing products that do not require water can be used (e.g., alcohol-based hand rubs). If hands are visibly contaminated, use bottled water and soap for hand-washing or an antiseptic hand towel.
■ Once the advisory is cancelled, follow guidance given by the local water utility on adequate flushing of waterlines.
If no guidance is provided, flush dental waterlines and faucets for 1-5 min before resuming patient care. Disinfect dental waterlines as recommended by the dental unit manufacturer.
Recommendations on care of handpieces and other devices attached to air and waterlines
■ Clean and steam-sterilize handpieces and other intraoral instruments that can be removed from the air and waterlines of dental units after each patient treatment session. Their surfaces should be cleaned, and the internal elements cleaned and lubricated according to the manufacturer's instructions before sterilization.
■ Do not surface-disinfect or use liquid chemical sterilants or ethylene oxide on handpieces and other intraoral instruments that can be detached from the air and waterlines of dental units.
■ The handpiece should be stored as appropriate according to national guidelines and run to remove excess lubricant immediately before use on patients.
Dental radiology
■ Always wear gloves when exposing radiographs and handling contaminated film packets. If spattering of blood or other body fluids is likely, use appropriate protective wear such as eyewear and mask.
■ Use heat-tolerant or disposable intraoral devices whenever possible (e.g., film-holding and positioning devices). Clean and heat-sterilize heat-tolerant devices between patients. If heat-sensitive material is used, then high-level disinfection for semicritical items must be employed.
■ Transport and handle exposed radiographs in an aseptic manner to prevent contamination of developing equipment.
■ Digital radiography sensors: depending on the manufacturer's recommendations, either clean and heat-sterilize or high-level disinfect the sensor between patients. The sensor is usually a barrier-protected, semicritical item. If the item cannot tolerate these procedures, then a recommended barrier system has to be employed or cleaned and disinfected with an intermediate-level (i.e., tuberculocidal) activity. Manufacturer's recommendations must be adhered to for disinfection and sterilization of digital radiology sensors and for the protection of related computer hardware.
Laboratory asepsis
Dental practitioners regularly send clinical material to the laboratory: impression material, dentures sent to the technology laboratory or pathological samples such as pus or biopsy specimens referred to pathology laboratories, for example. The dentist is obliged to deliver all such items in a manner that obviates infectious hazards, whether during transport or within the laboratory. Blood and saliva must be carefully cleaned from the impressions and denture work by washing under running water and disinfection, and, if appropriate, placed in plastic bags before transport to the laboratory. Proprietary disinfectant soaking solutions are preferred to sprays for decontaminating the microbes retained on impression surfaces.
The dental laboratory itself should be regarded as a clean (not contaminated) area, and appropriate protocols for disinfection of surfaces and material, as well as regular and timely renewal of disinfectant solutions, should be established. Smoking and eating should be prohibited.
Microbiological specimens sent to the laboratory should be securely bagged to avoid contamination of personnel who handle the items. The request form should be separately enclosed to prevent contamination. Biopsy specimens should be put in a sturdy container with a secure lid to prevent leakage during transport. Care should be taken when collecting specimens to avoid contamination of the external surface of the container. If specimens are to be transported by post or courier, then they should be placed in an outer padded leak-proof postal packet clearly marked with the words 'PATHOLOGICAL SPECIMEN - FRAGILE WITH CARE' and the name and address of the sender (and person to be contacted in case of leakage or queries) and that of the recipient.
Office/surgery design and maintenance
Proper office or surgery design is the cornerstone of an effective infection control programme (Fig. 38.6). Major features of such a design are:
1. There is a clear demarcation between the contaminated or dirty and clean zones, that is, the surgery and the sterilizing and storage areas, respectively.
2. Treatment areas and the laboratory should have few, if any, wood surfaces, porous or heavy draperies, or textured wall coverings, in order to facilitate cleaning and disinfection.
3. No eating or smoking is allowed in contaminated zones.
4. Carpets should not be used in the treatment areas, where flooring should be covered with seamless, disinfectant-resistant vinyl in order to minimize dust and microbial burden and to withstand frequent cleaning.
5. Ideally, ventilation in the surgical and peripheral areas should be centrally controlled (air renewal 12-15 air changes per hour) and planned to minimize cross-currents of air from one area to another. The air filter, if any, should be periodically changed, and special venting should be installed to scavenge noxious chemical vapour.
Fig. 38.6 Floor plan of a dental clinic designed to minimize cross infection
Infection control requirements should always be borne in mind when selecting new equipment.
Instrument recirculation and office design
In order to conduct an efficient and routine sterility programme, it is important to organize the various arms of the infection control programme outlined earlier in the most effective manner. Therefore it is essential to design the dental office and instrument decontamination areas (washing up, sterilizing and storage) to achieve this aim. The instrument decontamination area should be organized in order to:
■ separate contaminated objects from sterile or clean objects
■ store sterile items until required
■ facilitate easy cleaning and disinfection
■ facilitate a smooth flow of items between contaminated and clean zones.
■ design the ventilation of the air so that it flows from clean to the dirty area.
A suitable instrument recirculation profile is shown in Fig. 38.7. Other noteworthy points are:
■ If possible, the instrument decontamination centre should be close to the clinic for ease of use.
■ The work surfaces of the area should be smooth, non-porous and seamless.
■ An air evacuation system (low volume) with continuous movement of air upwards from the working surface should be operational to reduce airborne microbes and noxious chemical vapours (these should be regularly serviced, and filters should be replaced as appropriate).
Disposal of clinical waste
Any waste material that has been in contact with human sources is contaminated with potentially pathogenic microbes or will possibly support their growth.
General recommendations
Develop a clinical waste management programme. Disposal of regulated clinical waste—both hazardous and non-hazardous waste—must follow local and federal regulations. Ensure that health care workers who handle and dispose of potentially infective wastes are trained in appropriate handling and disposal methods and informed of the possible health and safety hazards. In the UK, all dental workers who handle waste including cleaners must be vaccinated against hepatitis B.
Clinical waste in dental health care facilities
Use a colour-coded and/or labelled container that prevents leakage (e.g., biohazard bag) to contain non-sharp regulated medical waste according to national guidelines on waste classification before disposal and making safe by either incineration or alternative temperature technologies. Incineration is required for the safe disposal of medicines waste.
Fig. 38.7 A suggested scheme for instrument recirculation. *See text for other options.
All sharp items (especially needles), tissues or blood should be considered as particularly dangerous and should be handled and disposed of with special precautions. Disposable needles, scalpels or other sharp items must be placed intact into puncture-resistant containers before disposal and incineration.
If permitted by local regulations, discard blood, suctioned fluids or other liquid waste carefully into a drain connected to a sanitary sewer system. Wear appropriate protective attire while performing this task. Clinical waste should never be mixed with domestic waste, as this is a dangerous practice; it may also lead to litigation, therefore national clinical waste disposal guidelines should be strictly adhered to.
• Decontamination is the process by which reusable items are rendered safe for further use and for staff to handle. Decontamination is required to minimize the risk of cross infection between patients and between patients and staff. Decontamination includes cleaning, disinfection and sterilization steps.
• Sterilization is a process that kills or removes all organisms (and their spores) in a material or an object.
• Disinfection is a process that kills or removes pathogenic organisms in a material or an object, excluding bacterial spores, so that they pose no threat of infection.
• Antisepsis is the application of a chemical agent externally on a live surface (skin or mucosa) to destroy organisms or to inhibit their growth (all antiseptics are disinfectants but not all disinfectants are antiseptics).
• Sterilization can be divided into four stages: presterilization cleaning, packaging, the sterilization process and aseptic storage.
• In dentistry, sterilization is usually achieved by moist heat (steam under pressure in an autoclave). The sterilization cycle (either in an autoclave or in a hot-air oven) can be divided into the heating-up period, the holding period and the cooling period.
• The indicators that must be routinely used for checking sterility are mechanical process indicators (i.e., the temperature and pressure gauges of the autoclave), chemical indicators and biological indicators/monitors.
• Regular decontamination audits should be conducted of all areas of the dental premises where instrument and device reprocessing occurs.
• The key modes of disinfection are physical (ultrasonics) and chemical (disinfectants) methods (most used in dentistry).
• Disinfectants can be generally categorized as having high, intermediate or low potency, depending on their ability to kill various groups of organisms.
• Water in dental unit waterlines (DUWLs) for non-surgical procedures should not contain more than 100-200 colonyforming units (CFU)/ml of aerobic, heterotrophic bacteria.
• When sending clinical material to the laboratory, obviate infectious hazards during transport and within the laboratory.
• Dispose of clinical waste, including sharps, medicines, personal protective equipment, in a safe manner.
• Proper office/surgery design is the cornerstone of an effective infection control programme.
Review questions (answers on p. 368)
Please indicate which answers are true, and which are false.
38.1 Which of the following modes of sterilization are permitted by legislation for use in a small dental clinic?
A. steam (autoclave)
B. dry heat
C. unsaturated chemical vapour
D. radiation
E. glutaraldehyde exposure for 30 min
38.2 Which of the following statements on disinfectants is/are true?
A. alcohol is active against bacterial spores
B. glutaraldehyde is active against both Gram-positive and Gram-negative organisms
C. chlorhexidine is not inactivated by either soap or proteins
D. glutaraldehyde is a medium-level disinfectant
E. hypochlorites kill organisms by their reducing action
38.3 Which of the following statements related to disinfection/sterilization is/are true?
A. a dental mirror could be classified as a critical item
B. all critical items must be sterilized
C. semicritical items may be used in dentistry after high-level disinfection
D. the head rest of a dental chair could be classified as a semicritical item
E. a high-level disinfectant must be used for disinfecting housekeeping surfaces
38.4 With regard to dental unit waterlines (DUWLs):
A. the quality of water used for dental treatment must match that of the standards of drinking water
B. water from DUWL may be used for cleansing the wound during a surgical removal of a third molar tooth
C. the water of DUWL could be contaminated up to 500 CFU/mL of heterotrophic bacteria
D. Legionella infection can be transmitted to patients via DUWL
E. the biofilms in DUWL can be removed by flushing the waterline regularly
Further reading
Centers for Disease Control and Prevention (CDC). (2016). Summary of infection prevention practices in dental settings: Basic expectations for safe care. Atlanta: CDC. Available from: http://www.cdc.gov/ oralhealth/infectioncontrol/pdf/safe-care.pdf.
Department of Health (2013). Decontamination in primary care dental practices (HTM 01-05) (2nd ed.). London: Department of Health. Available from https://www.gov.uk/government/publications/ decontamination-in-primary-care-dental-practices.
Franco, F. F. S., Spratt, D., Leao, J. C., et al. (2005). Biofilm formation and control in dental unit water lines. Biofilms, 2, 9-17.
Health & Safety Executive. (2013). Legionnaires' disease: The control of legionella bacteria in water systems. Approved Code of Practice L8 (4th ed.). Available from: http://www.hse.gov.uk.
Pankhurst, C. L., & Coulter, W. A. (2017). Basic guide to infection prevention and control (2nd ed.). Chichester: Wiley-Blackwell.
Pankhurst, C. L., Scully, C., & Samaranayake, L. (2017). Dental unit water lines and their disinfection and management. Dental Update, 44, 284-292.
Samaranayake, L. P., Scheutz, F., & Cottone, J. (1991). Infection control for the dental team. Copenhagen: Munksgaard.