The fundamental goal of root canal therapy is disinfection of the root canal system followed by proper coronal and apical sealing, while the role of shaping is to remove infected dentin and provide space for irrigant penetration and subsequent obturation. Irrigation during shaping does not eliminate half of the debris that accumulates throughout instrumentation especially in the apical third [10]. Consequently, different agitation techniques have been recommended to enhance the efficiency and diffusibility of the irrigating solutions [11].
The copper cube allowed the investigation of root cross sections at the different stages of root canal preparation and irrigation so that each tooth served as its own control for a true analysis of the change in anastomosis cleanliness [2, 6, 7, 12]. Although the copper cube and sectioning method is older, and requires lots of steps, it reflects both soft and hard tissues debris while previously published studies that used micro-computed tomographic examination (micro-CT) measured only hard tissues debris[13].
This study compared three final irrigation techniques. Hence, one irrigant (NaOCl) was chosen to eliminate the added effect of any chelating agents such as EDTA, also, smear layer removal was out of the scope of this study. Mesial roots of mandibular first molars were chosen for this study because it is the most common root to have anastomoses specially in the apical 6 mm[1]. Tissue debris tends to accumulate in the anastomoses during instrumentation which represent a major challenge for adequate cleaning and disinfection, which in turn impacts outcome of the root canal treatment particularly in infected root canals [14].
Samples were analyzed under a stereomicroscope at 30x magnification because it allowed clear visualization of both root canals and anastomoses in the same frame, in contrast to other studies that employed scanning electron microscope in which only limited areas of the section could be scanned[15].
All the final irrigation techniques used in this study resulted in significant improvement in anastomosis cleanliness. As increasing the irrigant volume and activating it results in improved cleanliness [2, 5, 9, 13, 16–18]. However, none of the techniques were able to fully clear the root canal system of debris.
The least improvement in anastomosis cleanliness occurred in the control group (FNI) (Figs. 1–2). This was in accordance with other investigations [16, 19]. At 2 mm, EDDY significantly improved cleaning compared to PUI (Figs. 1–2). Therefore, the null hypothesis was rejected. This can be explained by the fact that the narrowest part of the root canal is the most apical part and that the streaming effect of ultrasonic tips decreases significantly when it touches the root canal walls, unlike sonic tips, which do not lose their effect upon contacting canal walls, rendering it more effective in cleaning the narrow 2 mm level [4]. Contrarily, other studies found no significant difference at the 2 mm level between sonic activation and PUI [6, 20]. It is worth noting that they used EndoActivator as the sonic activation device, which produces less oscillation frequency with subsequent lower agitation capability than the airscaler that operates the EDDY tip, which can explain the superior cleaning of EDDY in the present study. Also, in those studies apical enlargement was done up to 40.04, while in this study it was 35.04, to create a more conservative preparation without compromising the biological objectives. Furthermore, mesial root canals of mandibular molars are small-sized canals, thus #35 is already larger than #30 master apical file which is the recommended size for the irrigant to reach the apical third [21]. This difference in the apical enlargement could have affected the ability of the PUI tip to vibrate freely thus producing these incompliant results.
At the 4 mm and 6 mm levels, the difference between EDDY and PUI was insignificant. This can be attributed to the larger canal diameter at these levels, which allowed less-restricted vibration of both tips. This came in harmony with the previously published data comparing sonic to ultrasonic activation [5, 12, 20]. However, conflicting results were reported by Linden et al.[18], which could be attributed to the difference in methodology since they used micro-CT. Also, they admitted that debris moved from one canal to the other connected one which may have affected their results.
Regarding the intragroup analysis of FNI, the 2 mm level showed the highest CD. This can be explained by the fact that after cleaning and shaping, this area is the one with the most packed residual debris[22]. So, even the slight addition of a non-activated irrigant as a final flush showed more improvement in this area compared to the wider 4 mm and 6 mm levels.
PUI at the 2 mm level was less effective than 4 and 6 mm. This can be attributed to the narrowness of this area which results in a more significant file-to-wall contact than the wider middle and coronal levels, which provide more room for a stronger streaming effect. Some of the samples even showed signs of canal deformation due to the ultrasonic tip touching the walls while vibrating (Fig. 1). The creation of irregularities from wall contact was supported by other studies [23, 24]. In the EDDY group, there was no significant difference between all levels. This can be explained by the fact that for the entire length of the tip, sonic energy generates just one single node and antinode. As a result, touching with dentinal walls has little effect on tip amplitude and hence tip movement [25].
It is important to emphasize that in this study, 1 mL of irrigant was activated for 30 seconds to investigate the exclusive effect of the activation technique, as increasing the irrigant volume and activation time and the final apical enlargement size may have different implications on the overall anastomosis cleanliness [11, 14].