From Volume 40, Issue 7, September 2013 | Pages 543-548
HTM 01-05 guidelines state that decontamination of handpieces remains a challenge, in particular the lumen, due to oil impeding access for steam sterilization. This paper discusses important aspects of cleaning and sterilization of the handpiece lumen and critically appraises the literature found on this topic. The paper is not intended to cover precleaning methods in detail.
The aim of the cleaning process is to ensure a surface free of tissue debris to allow sterilization to be effective. If the surface is not clean then sterilization is not possible. The presence of tissue debris and body fluids will impede access of steam to the surface which needs to be sterilized, particularly if baked on as a consequence of multiple exposures to steam sterilization without removal.
Different components make up the total tissue debris, which includes oil and prions and, together with the complex internal surface, makes the decontamination of handpieces particularly challenging. HTM 01-05 guidance recognizes that sterilization of handpieces remains a challenge and states ‘whilst steam sterilization alone will provide a small log reduction in infectivity, the most effective method of decontamination is rigorous cleaning to remove all proteinaceous material from surfaces.’
Whilst the cleaning phase will normally deal with most tissue debris, oil coating has caused controversy in the literature. The use of oil after the cleaning phase results in the internal surface of the handpiece becoming coated in hydrophobic material, which impedes the contact of saturated steam on the surface.
The principles of sterilization require a clean surface, so the logical sequence should be the complete removal of oil during the cleaning phase and then oiling can take place after sterilization and before clinical use. A dilemma arises, however, as manufacturers' reprocessing instructions require oiling of handpieces prior to sterilization in order to help protect the moving parts of the handpiece during repeated sterilization cycles.
In practical terms, internal cleaning of the lumen aims to reduce the bioburden to facilitate successful sterilization and this should be extended to include prions. In recent years, with the very low but not insignificant threat of transmissible spongiform encephalopathies, the modern aim of cleaning should be up to a level which will prevent disease transmission by prion type diseases.
Chan-Myers et al investigated the bioburden level of lumened medical devices after clinical use and after cleaning in a hospital setting.1 The levels after clinical use were found to be relatively low, ranging from 10 to 104 colony forming units (CFU) per device. After the instruments were cleaned, none of the devices studied contained bioburden levels greater than 104 CFU. Eighty-three per cent had bioburden levels less than or equal to 102 CFU.1 Bacterial contamination comprised mainly vegetative bacteria and normal oral microflora, rods and cocci. Typically, the bioburden would also expect to comprise serum and saliva, made up of a proportion of protein and fat and, if originating from dental unit water lines (DUWL), sometimes Legionella and Pseudomonas species.
No detailed studies have been performed on the proportions of each type of contamination, though there is ongoing work on the subject of biofouling of dental handpieces at Glasgow University.
The types of different contaminants are listed in Table 1.
| Contaminant | Method of Decontamination |
|---|---|
| Bacteria |
Steam sterilization using cycles prescribed by steam tables for 100% saturated steam |
| Endotoxins | Control of water quality to maintain levels below 30 EU/ml |
| Dissolved ions (hard water) | Chemical water softeners to achieve less than 50 mg/L (calcium carbonate) |
| Protein | Rinse with water 45–50 °C |
| Fat | Rinse with water 85–90 °C |
| Prions | Enzymatic detergents and steam sterilization |
Bacterial contamination during intra-oral use entering from the ‘working end’ of the handpiece has been demonstrated with low speed handpieces during prophylaxis2 and during pulpotomies on primary teeth.3 During clinical use, bacteria and tissue debris, including prions from the oral environment, can be pulled internally via the head of the handpiece as the bur rotates, particularly when the bur is submerged in oral fluids. If sterilization procedures are inadequate, there is the potential for bacteria that remain within the handpiece to be ejected back out into patients treated subsequently. Montebugnoli and Dolci demonstrated, with dye solution, that anti-retraction valves fitted to handpieces have been shown to reduce internal contamination.4 Transmission of bacteria can also occur from the DUWL via handpieces directly into the oral cavity, which can present a potential health risk to patients, particularly to those who are immunocompromised. Aerosol and splatter from handpieces can also present a risk to dental staff.5 During clinical use, the dental handpiece becomes a detachable extension of the dental unit waterline (DUWL). Proper clinical maintenance of the DUWL and ensuring quality control of the water supply play an essential part in minimizing the risk of inoculation of the internal lumen of the handpiece with biofilm. Biofilm sloughing from surfaces of the DUWL can be inoculated into the handpiece during clinical use. Tissue debris entry has also been shown to be influenced by handpiece design as described in a study by Barnes et al who demonstrated that less tissue debris was found in sealed versus unsealed dental prophylaxis angles.6
Guidance documentation on the subject of infection control does not wholly agree with the literature on the specific topic of oil lubrication. The body of work on this topic comprises six papers, which support the idea that oil does not prevent sterilization.
Guidance for cleaning followed by sterilization is provided by a number of documents. Local guidelines are produced for separate regions within the UK. Content is broadly similar for England, Wales and Northern Ireland, which have their own versions of HTM 01-05 guidelines and Scotland has the Scottish HTM 2010 guidelines. These documents all acknowledge the difficulties associated with handpieces, and make special reference to prion contamination and oil. In Scotland, specific further guidance on the subject of vacuum bench-top sterilizers considers that evidence in terms of patient outcomes is required rather than biological measures of contamination. This advice should be seen in the context of the entire document.
Handpieces are not the only instruments considered difficult to sterilize. The presence of oil represents only one of many sources of contamination, including prion contamination. A good analogy of efforts in risk reduction of these devices is found in guidance related to endodontic files. UK Department of Health guidance considers the risk of transmission of disease from prion cross-contamination to be very low, provided that optimal standards of cross-infection control are maintained. One study has attempted to quantify this risk. Bourvis et al estimated the risk of infection from endodontic instruments in the absence of adequate prion inactivation. The risk of being infected was estimated to be one in 3–13 million procedures, but much lower if standard decontamination procedures are followed.7 This compares to the probability of matching six numbers and winning the UK National Lottery (1 in 14 million). Is it easier or more difficult to sterilize a handpiece lumen (a complex internal surface) compared to an endodontic file (a complex external surface)? This is a pertinent question when it is considered that the Department of Health have deemed the use of endodontic files as single use because they cannot be reliably sterilized. It is also worth noting that endodontic files are not normally contaminated with oil.
Pong et al demonstrated, with the use of dye marked oil tests, that output water from a handpiece during use can contain oil droplets for up to 40 minutes of continuous use. Further gravimetric tests showed that this oil discharge continued for up to 240 minutes.8 This could cause a potential hazard to patients and staff from inhalation of oil droplets from aerosol. This may be of concern, particularly as face masks may not provide a sufficient barrier to nebulized oil.9
The potential clinical effect of oil droplets in handpiece output water has been assessed by Powers et al in 1995. It was considered that dentine bonding, being a technique sensitive procedure, was an appropriate test of water quality and that the presence of oil may have a measurable and detrimental clinical effect. Using the total etch technique, it was shown by laboratory tensile bond strength testing that contamination by handpiece oil had little effect on the bond strength between enamel and composite (Pekafill), using Gluma 2000 dentine-bonding agent. However, dentine bond strength was reduced by 40% by oil contamination. Treatment of the contaminated surface with the use of acid etchant had the effect of restoring bond strengths to their original levels. Whilst rinse and etch systems can overcome this problem, the expectation is that self-etching systems would be more vulnerable since they do not contain a rinsing step.10 Results would expect to vary with the amount of contamination and type of dentine-bonding agent.
The effectiveness of moist steam sterilization processes for devices with narrow lumens is assessed by subjecting a heat resistant micro-organism to lethal conditions of predetermined temperature, pressure and time so that a very high proportion of good quality 100% saturated steam is generated. The test subject (handpiece) is seeded with a known quantity of these organisms in internal locations considered difficult to sterilize. The organisms are standardized preparations of bacterial endospores, chosen for their exceptional heat resistance, and because the numbers reduce as they are killed in a predictable manner. Organisms are often prepared with serum or saliva to create a ‘test soil’. The validity of the results is dependent on knowledge of the sensitivity of the endospores; precise control of the physical parameters of temperature, time and pressure of a validated bench-top sterilizer; standard protocols for loading of the handpiece into the sterilizer; appropriate microbiological techniques for preparation, incubation and recovery of micro-organisms using an aseptic technique; and use of positive and negative controls.
One hundred per cent saturated steam is highly predictable and microbicidal and destroys organisms by direct contact, resulting in coagulation of cell structures at the precise point of condensation of water from its vapour form to liquid form. Energy, known as latent heat of condensation, is released. Precise control of physical parameters ensures a cyclical process of change between vapour to liquid and back to vapour which makes the process as efficient as possible, with maximum and continual release of energy. An early work by Perkins in 1956 established a series of steam tables indicating physical conditions optimal for the creation of saturated steam for a range of temperature and pressure.11 The Medical Research Council (MRC) later published work in the Lancet in 1959 which accepted Perkins’ recommendations but introduced modified parameters to increase the safety margins for the intended purpose of sterilization. A table of this information is shown which forms the basis of steam sterilization cycles today (Table 2).
| Perkins | MRC | ||
|---|---|---|---|
| Time | Temp | Time | Temp |
| Mins | °C | Mins | °C |
| 2 | 132 | 3 | 134 |
| 8 | 125 | 10 | 127 |
| 12 | 121 | 15 | 121 |
The endospores used for the validation of steam sterilization processes are Geobacillus stearothermophilus. Typically, a spore sample is obtained from a manufacturer as a spore preparation with a viable count of 106 organisms. Rate of kill follows a predictable relationship over time such that the log number of organisms reduces linearly with time. Endospore sensitivity is expressed as a D value which is defined as the time taken for the number of organisms to reduce by 1 log, eg 106 to 105, or 90% for a given temperature. Absolute sterility cannot be demonstrated so is commonly expressed in terms of probability. If a sterilization process is able to destroy 106 organisms and reduce their number to zero, the process will have achieved a 6 log reduction. This level of reduction is said to equate to the probability of finding a single viable organism on one of 106 handpieces, otherwise expressed as a sterility assurance level (SAL) of 10-6. In light of knowledge of microbicidal kinetics, raising the temperature results in an exponential rise in lethality and a corresponding shorter time to kill a given number of organisms.
Hollow devices, such as handpieces and 3 in 1 aspiration tips with their narrow, complex lumens, are a challenge as they are effectively a secondary semi-closed chamber within the autoclave into which steam must penetrate. Entrapped air within narrow lumens adversely affects the penetration of steam. A vacuum cycle ensures the evacuation of the lumen to remove the hindrance caused by air so that steam can pass into the lumen with the least resistance. A gravity cycle is less efficient because, without this assistance, the steam must enter passively into the lumen. It is unlikely that steam entering passively into the internal lumen of a handpiece would be effective in contacting the total surface area of such a complex structure.
This effect varies in relation to lumen diameter, increased length and orientation of the device when subjected to laboratory conditions simulating a non-vacuum autoclave.12 Air/steam mixes will have some microbicidal effect but the stability of such systems is difficult to predict, hence the desire to have saturated steam which is highly predictable, in fact the presence of air has been shown to enhance the lethality of steam on a certain strain of Geobacillus stearothermophilus spores.13 The principles of steam sterilization support the use of vacuum cycles but the evidence in the peer reviewed literature is limited. In the light of this, and with their substantially greater costs, this has limited their widespread use in practice.
The application of oil into a handpiece after cleaning and before sterilization is a necessary part of a manufacturer's reprocessing instructions. Oil coats the lumen surface and acts as a barrier which may hinder successful sterilization. The use of bench-top sterilizers using a gravity cycle further increases the risk of incomplete sterilization as the presence of air may act as a further hindrance to steam from contacting the surface of the lumen. Despite this, the literature on this topic, presented in the next paper, supports the idea that oil does not prevent sterilization. Aerosol generated from handpiece water expelled during clinical use contains oil droplets and can adversely affect technique-sensitive procedures, which depend on a clean surface, such as dentine bonding. These clinical hazards may be sufficient reason to move towards alternative methods by the development of handpieces which do not rely on oil lubricants.
