Introduction

Dental personnel should consider every patient as a potential source of infection, regardless of whether the patient is known to have an infection or not.1 Over the years, the target of infection control has always been to prevent transmission of infectious diseases, but new challenges have appeared and have stimulated new research. In the mid-1990s, cases of Transmissible Spongiform Encephalopathy (TSEs), also known as prion diseases, were described in humans with different findings to previously reported cases.2 TSEs are incurable fatal neurological diseases, appearing as vacuoles within the grey matter of the brain.3 The big challenge of this new disease was the unique resistance of the prions to inactivation by current universal precautions used for infection control in dentistry and the strong surface affinity of the prion protein.3,4,5,6

There was concern in the UK over the possible transmission of prions via contaminated surgical instruments.7 Since prions are particularly founded in neural tissues3 and have been detected in the oral and dental tissues of animals, this issue heightened the concern of the transmission of the prions by reusable dental instruments, especially endodontic instruments. During endodontic therapy, various instruments can come into intimate contact with terminal branches of trigeminal nerve when vital pulp extirpation is performed.7,8,9 As a precautionary measure, the Chief Dental Officer at the Department of Health recommended in April 2007 that endodontic files and reamers be treated as single-use instruments.10 He also stressed that the highest standards of decontamination be observed for all dental instruments.

Reduction of the initial bioburden on instruments, prior to sterilisation, is believed to reduce the risk of transmission of iatrogenic Creutzfeldt-Jakob Disease (CJD).11 This leads to the importance of the cleaning step in the sterilisation process. Cleaning can be accomplished manually, with hand scrubbing, or mechanically, by using an ultrasonic cleaner or a washer disinfector.

A washer disinfector is a machine intended to clean and disinfect medical devices, instruments and other articles used in the context of medical, dental, pharmaceutical and veterinary practice. Automatic dishwashers have been used for some time in the field of food and industry as a piece of decontamination equipment.12 Many hospitals and universities have modified these washers to decontaminate surgical instruments and linens.12 By adding a drying cycle, the washer disinfector eliminates the corrosion of instruments.13,14 Little information exists on the effectiveness of the washer disinfector, although some research has showed that it is an effective cleaning method for dental instruments15 and dental burs.16 There has been no specific study on the cleaning of endodontic files using a washer disinfector. Although endodontic files are now regarded as single-use in the United Kingdom, their re-use is common in other countries and therefore the issue of how best to clean them is still pertinent.

The manufacturers of the washer disinfector provide wire-mesh cleaning baskets of different sizes to suit different dental instruments. There is no evidence that the use of these baskets to hold endodontic files inside the machine will give better results in terms of the cleaning of files rather than the use of the conventional file holders. There is only one study comparing the cleaning effect between two holding mechanisms (baskets and cassettes) for other dental instruments in the washer disinfector, and this study recorded that there was no difference.15 This area needs further investigation regarding the use of other types of dental instruments eg endodontic files, and other types of holding mechanisms such as endodontic file holders. Endodontic file holders are designed to allow water penetration but cannot allow as much penetration to the files as when they are loose in a wire basket.

The aim of this study was to compare the cleanliness of endodontic files that had been through a washer disinfector according to the holding mechanism – either a file holder or a wire basket.

Method

Extracted permanent teeth with fully formed apices were collected from clinics in the School of Dentistry at the University of Manchester. They were stored in containers with 10% formalin until they were used in the study. The endodontic preparations were all done by one operator to avoid differences in the use of the files between different operators. Access cavities were prepared using diamond burs and the canals located with an endodontic explorer. A K-file size 10 was used to ascertain patency and determine the working length - when the file appeared at the apical foramen. Canals were selected which showed tightness of the file size 10 to ensure that all the successive files would work actively inside the canals. The coronal one-third of each canal was prepared using Gates Glidden drills sizes 5, 4, 3, and 2 respectively. The canals were irrigated with 3.5% sodium hypochlorite solution. The apical and middle-third of the canals were prepared using the step-back technique using a separate set of K-Flexofiles® (Maillefer, Dentsply, Weybridge, UK) for each canal starting from size 15, 20, 25, 30, 35 and 40. Each file was used with a quarter-clockwise rotation and the file was pulled out until it rotated freely inside the canal to the full working length. Sodium hypochlorite irrigation was used after each file. Progressively larger sizes were then used until the whole set of files had been worked with the same method.

Following a small pilot study to collect initial data, a power calculation showed that the number of files needed to give an 80% power of detecting a difference in the debris score of 1 at a significance level of 0.05 was 192 files – 81 in each of the two experimental groups and 30 in a control group.

Each set of six used files was placed in either a file holder or an instrument basket directly after having being used. The holders and baskets were then placed in a DS50 washer disinfector (Peacocks Medical Group, Newcastle-upon-Tyne, UK) and cleaned using the intensive programme which includes an intensive pre-wash, a wash with detergent and thermal disinfection at 93°C for ten minutes. The whole washer disinfector cycle took 90 minutes. The files were then packed and wrapped in sterilisation packs and autoclaved at 134°C for three minutes at above atmospheric pressure. The control group files were packed and sterilised without being cleaned in the washer disinfector.

The packs of sterilised files were then opened and each file was examined for debris under a light microscope at x45 magnification (Meiji EMZ-TR, Meiji Techno Co. Ltd, Tokyo, Japan). Collimated light from a halogen lamp (Schott, KL 1500, Germany) was used for illumination. All the files were examined by one operator to determine the percentage of the debris on each file under the microscope. In order to examine the files under the microscope the file's plastic head was inserted in a cubic box full of wax in order to make it stable under the microscope and to examine each part of the file from four different angles according to the surfaces of the box. The data from each surface were collected and the mean of the four data recordings was taken from the tip and shaft of each file. The tip was defined as the apical half (8 mm) of the cutting element of the file and the shaft was defined as the coronal half (8 mm) of the cutting element of the file. A piece of graduated paper was used as a backfield for the file in order to determine the limit between the tip and the shaft for each file. A computer with a camera (Pulnix PEC 3010, Japan) was attached to the microscope in order to take photographs of each file. The debris was scored according to a previously used scale.17 The scale was as follows:

Score = 0 if there was no debris.

Score = + if 0-5% of the file surface was contaminated with visible debris.

Score = ++ if 6-15% of the file surface was contaminated with visible debris.

Score = +++ if 16-25% of the file surface was contaminated with visible debris.

Score = ++++ if >26% of the file surface was contaminated with visible debris.

Non-parametric statistical tests were used to compare the debris scores between files in the different groups and also between the tips and the shafts of files in the same group.

To check for intra-examiner reliability, a random sample of 30 files was re-examined a week later and the kappa statistic calculated.

Results

The debris on each file was scored from 0-4 and mean scores were calculated for purposes of description. Intra-examiner reliability was substantial with weighted Kappa scores of 0.80 for the tip of the file and 0.88 for the shaft.

The mean scores for the tip and the shaft of the files for each of the three groups are shown in Figure 1. The control group files were all heavily contaminated with scores of either 3 (+++) or 4 (++++) (Fig. 2). The files in both the file holder group and the basket group were significantly cleaner than those in the control group (Mann-Whitney p = 0.000), showing the benefit of cleaning in the washer disinfector (Fig. 3).

Figure 1
figure 1

Mean debris scores (range 0–4) for the tip and the shaft for files in the file holder (FH), basket (BA) and control (CON) groups by the size of the file (15–40)

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Figure 2
figure 2

Control group (uncleaned) file scored 4 (++++)

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Figure 3
figure 3

File cleaned in basket group – scored 1 (+)

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Although none of the cleaned files were totally free of organic debris, the files in the basket group were generally cleaner than the files in the file holder group. This difference became more noticeable as the file size increased and was statistically significant (p = 0.000 at the tip and 0.000 at the shaft) with lower debris scores in the basket group. In order to take the clustering of the files within the cycles into account a generalised estimating equation (GEE) model was fitted with tip as the continuous outcome variable and file size and type of holder as dependent variables, clustering for cycles. The results showed that there was a significant difference between the types of holding mechanisms (p = 0.000) with the tips of the files in the baskets being cleaner than those in file holders. File size was associated with the amount of debris (p = 0.000) with the tips of larger files having more debris present.

The same analysis was performed for the shaft of the files. In this case, file size was not associated with the amount of debris (p = 0.164) but the holding mechanism was (p = 0.000), with more debris on the shaft of the files cleaned in the file holders than those cleaned in the baskets.

Debris scores at the tip and shaft of the files were compared using the Wilcoxon Signed Ranks Test. In the file holder group there was a significant difference between the two segments of the file (z = -3.346, p = 0.001) with the shafts having higher debris scores. The same analysis was applied to files in the basket group but the results showed no significant difference (z = −1.232, p = 0.218).

Discussion

This study aimed to establish whether a washer disinfector could be used to remove organic debris from endodontic files prior to sterilisation and whether the holding mechanism within the machine made any difference to the level of cleanliness achieved.

The results showed that the washer disinfector is a fairly effective device in the cleaning of the endodontic files prior to sterilisation. None of the files were completely free of debris, but there were significant differences in the results related to the holding mechanism used. Files cleaned in baskets showed significantly less debris than files cleaned in the file holders.

The washer disinfector manufacturer's instructions recommend the use of the cleaning baskets provided but there is an increased risk of needlestick injury to clinical staff when loose files are handled. Endodontic file holders are designed so that files placed back in the holder by the operator after use do not have to be handled again during the cleaning and sterilisation process, thus minimising the risk of needlestick injury.

The manufacturers of the file holder used in this study claim in their literature that it is suitable for use in a washer disinfector but the design of the product clearly precludes water penetration from various directions (Fig. 4). One of the walls has slits in it but the other is almost completely metal. Inside the holder, the white plastic inner covers some of the cutting element of the files at both the tip and the shaft where it holds them in position.

Figure 4
figure 4

4 Nichrominox Endo Pro 12 endodontic file holder

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If washer disinfectors become more commonly used in general dental practice, further development is needed to produce instrument holders that are compatible with the working mechanism of the machine by allowing sufficient penetration by the water jets to the instruments being cleaned. The small size and complex geometric shape of many dental instruments, including endodontic instruments, pose problems in this regard. Although a previous study has shown that over 99% of instruments, such as hand scalers, suction tips and artery forceps, were bacteria-free after cleaning in a washer disinfector,15 the problem of residual debris remains, even if it is sterile. The fact that none of the endodontic files were totally free from debris, whatever holding mechanism was used, supports the argument that endodontic files should be single-use only. Although there is no evidence that these instruments are an infection hazard,18 dentists have a moral, ethical and legal responsibility to ensure that all contaminated instruments are clean prior to re-use, however difficult that may be. With a careful technique, it is possible to render endodontic instruments completely clean19 and this should therefore also be possible with all dental instruments. However, present evidence shows that standards in general dental practice in the UK still leave room for improvement.20

Conclusion

Endodontic files cannot be rendered totally free of organic debris after being cleaned in a washer disinfector but the holding mechanism is significant. Files cleaned loose in a basket had less debris than files held in an endodontic file holder. The design of the file holder appears to be the major contributory factor in this difference.