Evaluation of the mechanical effect of single instruments with reciprocating motion (WaveOne system) in contaminated root canals

The endodontic treatment has undergone many changes in recent decades, particularly regarding the technical aspects of therapy; in fact, the instrumentation technique has been the most discussed topic due to the significant technological innovations that have occurred in this field.

Rotary systems have been developed utilizing a continuous rotational mechanical movement that offers various advantages such as a reduced instrumentation time compared to manual instruments, a high percentage of reduction of the bacterial flora present within the canals, and a more centered preparation in the canal lumen. In any case, the manual instrumentation technique has been the most used for years and remains, to this day, the most taught method in university courses. Recently, a new endodontic instrumentation system has been developed and proposed worldwide, namely the WaveOne automatic system by Dentsply-Maillefer, a single instrument with reciprocating motion capable of cleaning and shaping the root canal. The kinematics of the instrument is very simple: it features a wide rotation radius in the cutting direction and a small radius in the opposite direction, allowing the instrument to proceed along the canal pathway (Figure 1).

This reciprocating motion reduces the cyclic fatigue to which the instrument is subjected, and furthermore, the files are made with a special Ni-Ti alloy, modified with a thermal process, called M-Wire, which has superior mechanical resistance properties compared to conventional Ni-Ti alloys.

The system is also more practical and faster than traditional ones, consequently reducing the effort in performing the work and the stress imposed on the patient. However, only a few studies have been conducted on systems with a single reciprocating instrument, and there are no articles analyzing its mechanical action on contaminated canals. The aim of this study was therefore to evaluate the reduction of bacteria present within the mechanically instrumented canals using the WaveOne system, comparing them with manual instrumentation.

Materials and methods

In this study, 32 disto-buccal canals of straight upper molars were analyzed, single, with a standard length of 12 mm. The canals were instrumented to a working length of 11 mm, starting with a K file #8 up to a K file #15 (Dentsply Maillefer, Ballaigues, VD, Switzerland). The apex was covered with resin, and the outside of the root, including the apex, was sealed with epoxy resin (Araldite, Brascola, Joinvile, SC, Brazil).

The samples were then fixed onto polystyrene supports for 24 test tubes and secured with acrylic resin. Both supports with the installed samples were sterilized with ethylene oxide (Acecil, Campinas, SP, Brazil).

A suspension of Enterococcus faecalis (ATCC 29212) was prepared in tryptic soy broth (TSB, Difco, Le Pont de Claix, RA, France) and standardized to a value of 4 on the McFarland scale, and under a filtered air hood, the canals were contaminated with the E. faecalis suspension using an insulin syringe. The supports were sealed and incubated at 37° for 21 days, and the contents of the canals were irrigated with fresh TSB every 48 hours.

After the incubation period, the canals were filled with sterile water, and samples of contaminated water were collected using a sterile #15 paper cone (Dentsply Maillefer, Ballaigues, VD, Switzerland) inserted into the canal for one minute. Subsequently, the sterile tips were stored in tubes containing 500 µL of TSB, and predetermined dilutions were prepared for bacterial presence analysis.

Bacteria were quantified based on the number of colonies formed per mL (CFU/mL) after inserting the different dilutions of the collected samples into agar culture media with m-Enterococcus (Difco, Le Pont de Claix, RA, France) and incubated at 37° for 48 hours.

The samples were then divided into two groups (n=15) according to the different instrumentation techniques:

Group 1 was prepared with the WaveOne system (Dentsply Maillefer, Ballaigues, VD, Switzerland) according to the guidelines provided by the manufacturer.

The “primary file” (25.08) was used with the reciprocating engine, penetrating first into the coronal third and then extracting it in the same manner in the middle third and subsequently the cervical third, until reaching the determined working length. Irrigation was performed with distilled water and a manual exploration with K file #15 after each insertion and extraction of the reciprocating file. The final irrigation was performed with 5 ml of distilled water using a 29-gauge NaviTip syringe (Ultradent Products, South Jordan, UT, USA).

Group 2 was prepared using a manual instrumentation technique (Dentsply Maillefer, Ballaigues, Switzerland) as described by Machado et al.14. The cervical and middle thirds were enlarged with Gates-Glidden drills sized from 1# to 3# (Dentsply Maillefer, Ballaigues, VD, Switzerland) and then the canals were prepared with a sequence of increasing diameter files until reaching the working length with K file #35. The root canals were then irrigated with distilled water, which was reapplied after each single step with manual instruments, and finally irrigated with another 5 mL of solution once the preparation was completed, as in Group 1.

The control group consisted of two uncontaminated samples, one of which was instrumented as in Group 1, while the other was instrumented as in Group 2. After the final irrigation, the canals were again filled with distilled water and samples were collected with sterile paper points to quantify the residual bacterial colonization as previously described. The bacterial count was then verified both before and after instrumentation, and the percentage of reduction was recorded. The collected data were statistically processed using the Mann-Whitney U test to evaluate the level of statistical significance (significance at 5%, p<0.05).

Results

The evaluation of the difference between pre- and post-instrumentation values showed an average reduction in bacterial load of 95.76% in the WaveOne Group and 92.97% in the Manual Technique Group. No statistically significant differences were noted between the two groups (p>0.05). The statistical data are shown in Table 1.

The presence of bacterial population in the control group was not highlighted, thus confirming the sterility conditions under which the experiment was conducted.

Discussion

A reduction in working time is certainly a great advantage of modern dentistry techniques, but it is of fundamental importance to evaluate whether these new techniques lead to better or similar results compared to conventional techniques.

The effect of the mechanical action of endodontic instruments on the removal of existing bacteria must be carefully evaluated. Some bacterial strains such as Enterococcus faecalis can indeed persist in the canal system for long periods, even in areas poor in nutrients for bacteria, leading to the failure of endodontic therapies. The contamination technique used in this study, with 21 days of persistence of Enterococcus faecalis, allows for an increase in bacterial presence within the canal and its organization into biofilm.

The endodontic treatment is a chemical-mechanical procedure that consists of a combination of channel instrumentation action and the use of chemical products such as irrigants, antibacterial dressings, and cements.

The instrumentation technique presented in this study was not associated with the use of irrigants, as the aim was to compare the mechanical action of bacteria removal of two different instrumentation techniques, without the interference due to the effect of chemical solutions.

The results of the study highlighted how the reduction of the bacterial population obtained with the WaveOne system was similar to the manually instrumented group, thus agreeing with previous works that showed similarities between manual techniques and automated mechanical instrumentation.

Both techniques, WaveOne and manual instrumentation, utilize the kinematics proposed by Roane et al., namely the use of a wide counterclockwise rotation of the instrument to cut the dentin and a small clockwise arc to proceed with the file inside the canal system. The main difference between the two systems is that the WaveOne technique relies on a mechanical system based on a specific motor that produces a reciprocating automated movement during the shaping of the canal.

This system is available with three files of different sizes: “small” (21.06), “primary” (25.08), and “large” (40.08); the choice depends on the diameter of the canal. In this study, the “primary” file was used because the exploration probe initially used was a K file #15.

Many studies have been conducted on premolars to evaluate the reduction of present bacteria, but molars are part of that group of teeth that is most often subject to endodontic therapies; for this reason, they were used in the study to get as close as possible to clinical reality.

Moreover, premolars often have a wider diameter than molars, which is why in certain cases the WaveOne “large” file would have been more appropriate. It was chosen to use the “primary” file of WaveOne because, according to the manufacturer, it is the one used for the majority of root canals.

The mesiovestibular and palatal canals of the molars were instead excluded as it would have been difficult to isolate and decontaminate them properly, thus risking influencing the bacterial count. Additionally, the standardization of the mesiovestibular canal would have been particularly complex due to the presence of the fourth canal.

The shaping of the root apex in the Manual technique Group was performed with a K file #35, while in the WaveOne Group the tip of the instrument was of a different size (#25). The use of different tip diameters is justified by the fact that with shaping techniques using automatic instruments, such as ProTaper, it is possible to proceed with the closure even with master cones in gutta-percha that have a larger apical diameter than the last instrument used because the shaping of the rotary instruments is wider than the last file used with automatic movement.

WaveOne has the same characteristic; this observation, in fact, seems to be consistent with the results obtained in this study.

The different shaping can also be motivated by the different taper, which for manual instruments is 2% while it is 8% in the case of the “primary” WaveOne instrument.

As for the shaping of the middle third and coronal third of the canal, in the case of the manual technique, Gates-Glidden drills were used, thus producing a greater flaring, however not affecting the differences between the two groups in terms of reduction of bacterial presence.

No sample was found to be completely free of bacteria, in contradiction to what Coldero et al. expressed, who observed that 81% of samples instrumented with manual technique had a bacterial presence equal to zero. This apparent contradiction should be evaluated carefully: it may be that the observed differences are actually related to methodological limitations; a small bacterial growth cannot be observed with standard cell culture methods.

It is interesting to note how the automatic WaveOne system was able to remove 95.76% of the bacteria present without the additional use of chemical products, achieving a result comparable to conventional techniques but in a shorter time and without losing quality.

Conclusions

The mechanical action of the WaveOne system on infected root canals produces a reduction in bacterial load comparable to that obtained with the classic manual instrumentation technique.

Manoel Eduardo de Lima Machado, Cleber Keiti Nabeshima, Mário Francisco de Pasquali Leonardo, Felipe Britto de Lima Machado, Maria Leticia Borges Britto, Silvana Cai

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