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Correspondence: Prof. Matthias Zehnder, Division of Endodontics, Clinic of Conservative and Preventive Dentistry, University of Zurich, Center of Dental Medicine, Plattenstrasse 11, CH-8032 Zurich, Switzerland, Phone: +41 44 634 3284, FAX: +41 44 634 4308,
The in-vitro efficacy of irrigant activation devices has not been contrasted to their safety. This was attempted in this study, using apically closed versus patent simulated root canal systems in epoxy resin models; the latter containing a simulated periapical lesion.
Methods
All 72 models had two joining canals connected by an isthmus, which was filled with dentin debris. The simulated periapical lesion was filled with colored gelatin in the 36 respective models. Canals were irrigated with 1.3% NaOCl. Samples were divided into four subgroups per system (n = 9): conventional irrigation, sonic low (EndoActivator) and high frequency (EDDY), and ultrasonic agitation of the irrigant; always applying 3 cycles of 20 s. Total cleared surface areas (mm2) in the simulated isthmus and periapical lesion were compared between systems and devices using parametric tests (P < .05).
Results
The cleaning of the isthmus was more effective in the apically open compared to the closed systems, and was also significantly influenced by the agitation method (P < .001). In the closed systems, EDDY and ultrasonic agitation achieved the significantly (P < .05) best cleaning of the isthmus. In the open systems, ultrasonic agitation showed the single best result (P < .05). EDDY caused by far the highest, and ultrasonic activation the lowest dissolution of the gelatin in the simulated periapical lesion.
Conclusions
Under the conditions of this study, passive ultrasonic agitation of a sodium hypochlorite irrigant was more laterally targeted and thus safer and more effective than sonic agitation methods.
Root canal irrigation is performed clinically in closed systems with an apical terminus blocked by hard tissue and open systems when there is apical patency. These two situations were simulated in epoxy resin models, and resulted in significantly different isthmus cleaning efficacy versus apical sodium hypochlorite extrusion exerted by common endodontic irrigant agitation/activation devices.
Effective root canal irrigation is an essential aspect of cleaning and shaping. There is ample clinical evidence that mechanical instrumentation by itself cannot achieve a level of debridement that results in predictable healing of endodontically caused periapical lesions. When compared to mechanical preparation alone, the bacterial load is dramatically reduced when root canals are additionally irrigated with an antibacterial solution
. This so-called chemo-mechanical root canal debridement process can be executed in various ways, yet it fails to predictably reach potentially infected endodontic niches such as isthmuses, fins, oval extensions, and lateral canals
Biofilm removal from a simulated isthmus and lateral canal during syringe irrigation at various flow rates: A combined experimental and computational fluid dynamics approach.
Microbial status of apical root canal system of human mandibular first molars with primary apical periodontitis after “one-visit” endodontic treatment.
. Hence, to increase the efficacy of root canal debridement in complex anatomies, agitating or activating a sodium hypochlorite (NaOCl) solution with sonic and ultrasonic equipment, respectively, has been recommended
Influence of 1-hydroxyethylidene-1,1-diphosphonic acid on the soft tissue-dissolving and gelatinolytic effect of ultrasonically activated sodium hypochlorite in simulated endodontic environments.
. For the sake of simplicity, the term "agitation" for all the irrigation methods described was used in the following text.
While irrigation is considered mandatory to achieve good treatment outcomes, increased apical positive pressure during irrigation can cause accidental NaOCl extrusion
. Therefore, efficacy versus safety assessments become an issue. Clinically, this is not easy to study, and retrospective analyses on the matter have been inconclusive
. Reports on NaOCl accidents are retrospective by nature, and consequently not free from reporting bias. Nevertheless, there appears to be a consensus that in root canal systems with an open apex or perforations, the risk to inadvertently irrigate beyond their natural confines is increased
. Creating pressure in the apical aspects of the irrigated root canal system is a key parameter related to irrigant extrusion, as has been shown in natural single-rooted teeth
, transparent resin models filled with either dental debris, soft tissue, or a soft tissue surrogate have been used to study and illustrate various aspects of root canal irrigation and irrigant agitation. While the results obtained from such studies may lack direct clinical meaning, they may nevertheless be useful to study clinically observed phenomena in a controlled environment. However, the choice of such models may not always reflect common clinical situations. A root canal system in clinics can be "closed", i.e. blocked by calcification or debris in its apical portion, or "open". Closed systems have been used to study the lateral cleaning of irrigant agitation devices
A novel methodology providing insights into removal of biofilm-mimicking hydrogel from lateral morphological features of the root canal during irrigation procedures.
Adjunctive steps for the removal of hard tissue debris from the anatomic complexities of the mesial root canal system of mandibular molars: A micro-computed tomographic study.
Hard tissue debris removal from the mesial root canal system of mandibular molars with ultrasonically and laser-activated irrigation: A micro-computed tomography study.
. However, in terms of procedural safety, the extrusion of the NaOCl solution upon its agitation is the core issue. Therefore, it may not be enough to simply test irrigant agitation devices or methods merely in a model with a closed (simulated) apical root canal terminus.
In this study, we report on the development of a closed and an open, i.e. apically patent, root canal system with an isthmus, mimicking the complex anatomical situation commonly found in mandibular molars
. Bovine dentin debris was filled into the simulated isthmus of these models, and in the apically patent models, colored gelatin was used to simulate periapical soft tissues. The specific aim of this study was to compare four irrigation methods used to agitate the NaOCl solution regarding their cleaning effiacy in the isthmus versus their propensity to extrude hypochlorite beyond the apex.
MATERIALS AND METHODS
Power Analysis
Calculation of the sample size was carried out after a triplicate pilot experiment. The main outcome used for this calculation was the area cleaned from debris in the simulated isthmus of the models. The effect size in this preliminary trial was 2.0. With an alpha error probability of 0.05 and 80% power, this resulted in a minimum of 8 samples required per group (ANOVA, fixed effects, omnibus, one-way; G*Power 3.1, Heinrich Heine Universität Düsseldorf, Germany). With a minimum requested number of specimens of 8 per group, the sample size for the experiment was set to 9 samples (n = 9) per agitation method and model type.
Model Fabrication
Seventy-two models were made for this study. To that end, 9 molds of each model type were created and cast 4 times, each.
All these models contained a standardized two-canals system connected by an isthmus. Half of the models had a closed apex (closed systems, n = 36), while the other half (open systems, n = 36) contained a simulated endodontically induced periapical lesion of 180 mm3 and had an apical opening of 0.45 mm in diameter. The design of the two canals covered possible variations, with a straight canal and a curved one, joining the straight canal 2 mm from the apical opening. The simulated isthmus had a thickness of 0.09 mm and covered the whole space from the simulated pulp chamber down apically to where the canals joined. To facilitate the placement of debris, the models were designed in two parts fixed with screws and guided with an index (Fig.1).
Figure 1- Model Fabrication (see also Supplement): A&B: silicon mold containing a positive impression of the simulated canal system and acrylic supporting skeleton prior to casting with epoxy resin; C&D: closed system model; E&F: open system model.
Model fabrication followed a multistep molding process (for exact instructions, see Supplement). In brief, canals were simulated with two gutta-percha points (WaveOne Gold Conform Fit Gutta-Percha Points, size large, Dentsply Sirona, Charlotte, NC). These canals joined 2 mm from their apical terminus. The curved canal formed a 45° angle with the straight counterpart. The straight canal length was set to 14 mm while the curved canal was set to 13 mm. The simulated isthmus space was modeled using a double layered metal matrix band (Tasma Metalova Metal Tape Stainless Steel, AnGer G&A, Szczyglice, Poland) to a total thickness of 0.09 mm. Its width was 4.5 mm coronally and 0 mm apically. The parts were glued together with Super Glue 101 (Scotch Brand, 3M, Saint Paul, MN). The Pulp chamber was simulated with a rubber heavy body impression material (Hydrorise Maxi heavy viscosity A Silicone, Zhermack, Italy). It was 3 mm in depth, 6.5 mm in width and 7.5 mm in height. The simulated root canal system was then cast in an acrylic two-piece reference models (Vertex Dental, Soesterberg, The Netherlands), one with and one without a periapical lesion. The latter was milled in its reference model to a length of 15 mm, a width of 8 mm and a depth of 0.75 mm on each model side. The resulting simulation was used for further production of the silicone molds (Hydrorise Maxi high viscosity A-Silicone, Zhermack, Badia Polesine, Italy) (Fig.1 and Supplemement).
To prevent fracture of the epoxy resin upon screw-connecting the model parts, a supporting acrylic (Vertex Dental) frame in a U-shaped form was implemented in both pieces (Supplement). The acrylic frame was positioned with a space 1 mm from the casting mold to allow a thin layer of epoxy resin (Art Epoxy Resin, EPOKE, Bengaluru, India) as an internal interface of the model. The positioning was achieved with assistance of the metal pins in the holes. The epoxy resin was then mixed according to the manufacturer's instructions and poured in the casting models. These were left for 48 hours until the resin had fully set. Metal pins of 3 mm diameter and 30 mm height were placed in the holes of the rubber template molds. These pins transferred the index holes to the final epoxy model to facilitate repositioning and fixation with screws. After setting, paired parts were joined together with M3 screws of 35 mm length and nuts before being trimmed from their sides.
Filling the Simulated Spaces
Dentin powder was ground from the inner aspects of bovine tooth roots using a carbide bur without water cooling. The dentin powder was mixed with water at 2:1 by volume ratio was applied to the isthmus to simulated clinically accumulated dentin debris
while securing the canal space with matching gutta-percha cones. The model interface outside the canal system was coated with a thin layer of Vaseline to enhance the sealability when connecting the two parts using M3 screws. The simulated periapical lesion was filled with 20% gelatin (Merck, Darmstadt, Germany) dissolved in deionized water and stained with 1% red food dye (2 mL). The models were kept in a water container until the experiments were performed.
Experimental Set-up
For transillumination, the test model was attached to an LED light (LED light engine, CHINLY, Guangzou, China) using a custom-made acrylic holder. The light source provided a light intensity of 3310 lux. The test model was trans-illuminated with the simulated canal anatomy at the same measuring reference plane (Fig. 2). The canals were positioned at the same plane of the outer front holder surface. At the same plane on the holder surface, a millimetric metal scale was fixed as a calibration reference. A macro lens (VR Macro-Nikkor 105 mm, Nikon, Japan) attached to a camera (D7200, Nikon, Japan) and held on a camera tripod was used to photographically document the effect of irrigation before and after irrigation and irrigant agitation (Fig.2).
Figure 2- Model loading and activation: A: gutta-percha cones placement to secure the canals; B: loading of isthmus with dentin debris; C: closed system test model loaded; D-G: representative samples after irrigation in closed system model: D: without agitation; E: Endoactivator, F: EDDY, G: ultrasonic tip. H: Open system test model loaded; I – L: representative samples after irrigation in open system: I: without agitation; J: Endoactivator; K: EDDY; L: ultrasonic tip.
The irrigation for each sample was performed over three cycles. At each of these, 1 mL of sodium hypochlorite (Hypex, Jordan Chemical Industries CO., Jordan) diluted to 1.3% NaOCl with distilled water was delivered to each canal using a side-vented 30G needle (Endo-Top, Cerkamed, Stalowa Wola, Poland). The NaOCl was delivered at 10 mL/min rate using a custom-made automatic irrigation device. In the agitation groups, the delivery of 1 mL of NaOCl to 2 mm from apical foramen was followed by agitation for 20 s per cycle using: a plastic tip, (EndoActivator medium, Dentsply Sirona) run in a proprietary hand piece at the "high power" setting of 190 Hz
, a plastic tip (EDDY, VDW, Munich, Germany) run at 6000 Hz and 120 μm amplitude in an air scaler (SONICflex 2003, KaVo, Biberach, Germany), or a size-25 titanium alloy tip (Coxo, Guangdong, China) run in a 38-kHz ultrasonic device (Ultra Smart Endo, Coxo).
In the conventional needle irrigation (negative control) group, only 1 ml of 1.3% of NaOCl was delivered to canal 1 mm (as opposed to 2 mm in the other groups) from apical foramen with 20 s contact time before the next interval.
Calculation of Removed Debris and Dissolved Gelatin
The thickness of the isthmus and the simulated periapical lesion was fixed in the models used for this study. We therefore simply calculated the cleared surface area (mm2) in each of these spaces (Fig. 2). The obtained photographs were analyzed using a Java-based image analysis software (ImageJ, National Institutes of Health, Bethesda, MD). The millimetric scale implemented in the models was used as a calibration reference for the measurements.
Statistical Analysis
The cleared areas in both the simulated isthmus and periapical lesion (both in mm2) were tabulated and statistically analyzed (JMP Version 15, SAS Institute, Cary, NC). The data distribution was tested for normality with the Kolmogorov Smirnov test and Shapiro-Wilk tests. To assess the impact of the models (closed versus open) and the type of irrigant agitation, two-way ANOVA was applied. Subsequently, isthmus cleaning ("efficacy") and dissolution of the gelatin in the simulated periapical lesion ("safety") was compared within each model type using one-way ANOVA followed by Tukey's HSD test (P ˂ .05).
RESULTS
In terms of cleaning the dentin debris from the simulated isthmus, there was a statistically highly significant (P ˂ .001) effect by both type of simulated root canal system (closed versus open) and the type of irrigant agitation device that was applied (Table 1). In addition, there was a statistically highly significant effect of the interaction between these two factors, indicating that the effect of the system differed between the different types of irrigant agitation device. The ultrasonic device reached the highest cleaning, especially in the open system, while in the closed system the effect of the EDDY tip was similar to that of the ultrasonically driven titanium alloy tip (Table 2). Moreover, the ultrasonically driven tip cleaned the isthmus areas significantly (P ˂ .001) better in the open than in the closed system.
Table 1Two-way ANOVA for the Effects of the Model (Closed/Open) and Irrigant Agitation Devices on Cleaned Area (mm2) in the Canals and Isthmus of the Simulated Canal Systems
Table 2Differences between Cleaned Area (mm2, Means ± SD) in the Simulated Isthmus Area according to Type of Simulated Root Canal System and Irrigant Agitation Device
Agitation device
Closed system
Open system
None
0.2 ± 0.2 A
0.6 ± 0.2 AB
Endo Activator
1.3 ± 0.7 AB
1.6 ± 0.6 B
EDDY
5.1 ± 1.7 C
5.7 ± 0.6 C
Ultrasonic tip
4.9 ± 1.2 C
7.6 ± 0.9 D
Sets of values that share a superscript letter did not differ at the 5% level (P >.05, ANOVA and Tukey’s HSD).
In terms of apical extrusion of the irrigant as assessed by the gelatin area that was dissolved by the NaOCl, there were highly significant differences between the groups also (Table 3). The EDDY tip caused by far the highest dissolution of the gelatin, while the ultrasonic tip caused the lowest apparent apical irrigant extrusion, which was even lower than that caused by conventional needle irrigation delivered 1 mm from the apex (P ˂ .05). The apparent irrigant extrusion was statistically similar between the EndoActivator and conventional irrigation, but also higher than that observed with the ultrasonic tip (P ˂ .05).
Table 3Area of Dissolved Gelatin (mm2, Means ± SD) in the Simulated Periapical Lesions according to Type of System and Irrigant Agitation Device
Agitation device
Dissolved gelatin
None
4.4 ± 1.5 AB
Endo Activator
7.6 ± 1.4 AB
EDDY
33.9 ± 6.6 C
Ultrasonic tip
0.3 ± 0.4 A
Sets of values that share a superscript letter did not differ at the 5% level (P >.05, ANOVA and Tukey’s HSD).
The current bench-top experiments took a novel approach in that the laterally directed cleaning efficacy of irrigant agitation tools was contrasted to their propensity to press the NaOCl irrigant beyond the apex into a simulated periapical lesion. Moreover, apically closed and open systems were used to compare their impact on cleaning the simulated isthmus from dentin debris. The results suggest that under the conditions of these experiments, ultrasonic irrigant activation using a metal tip had the best efficacy and also the highest safety among the tested methods. Isthmus cleaning tended to be better in open compared to closed root canal models, especially when ultrasonic activation was applied (Table 2).
It should be cautioned that the results reported in in this study were obtained in models in a laboratory setting. It would therefore be false or premature to draw any clinical conclusions. Nevertheless, especially the extrusion of irrigant caused by the EDDY system described in this study and the apical pressure that system exerts in 3D-printed teeth
may raise some questions, which should be followed up with clinical assessments.
Based on the current finding that a small change in the model caused a significant impact in outcome, in this case the cleaning of the simulated isthmus from dentin debris, may raise further doubts. The critical reader of the endodontic literature may question the type of model that has been used to test specific devices. Two prominent examples are the use of closed systems to create a "vapor lock" in the irrigated root canal system
, and, in resin blocks, the choice of two-canal systems with an isthmus and a high irrigant volume in the simulated access cavity when laser-activated irrigation was tested
Dynamic movement of intracanal gas bubbles during cleaning and shaping procedures: The effect of maintaining apical patency on their presence in the middle and cervical thirds of human root canals-an in vivo study.
, these models appear to highlight advantages of selected systems, while they fail to reflect the variance of potential clinical situations.
In the current approach, we tried to create models that would allow to single out a potentially critical point that can be encountered in clinics, and that is whether the apex of a root canal system is patent or not
. The size of the simulated apical opening created in these models was comparable to that of natural teeth with apical periodontitis and the frequently occurring inflammatory apical root resorption associated with such teeth
. Moreover, we decided to create a simulated root canal system with two joining canals and an isthmus between these, as this is a situation in which the use of additional irrigation tools is recommended
Radiographic healing after a root canal treatment performed in single-rooted teeth with and without ultrasonic activation of the irrigant: A randomized controlled trial.
. The build-up of apical pressure appears to be reduced in (simulated) root canal systems containing canal complexities as opposed to single, straight canals
Effect of canal anastomosis on periapical fluid pressure build-up during needle irrigation in single roots with double canals using a polycarbonate model.
. In that sense, the open models used here did not reflect a worst-case scenario, but rather a common situation that could be encountered clinically. The current results regarding apical irrigant extrusion concur with published results in 3D-printed single-rooted teeth with an open apex, in which the EDDY system created considerably higher apical irrigant pressure than passive ultrasonic irrigation
Our results confirm earlier observations in that the cleaning of lateral root canal spaces appears to be a function of the power that is applied to the tip that is used to agitate the irrigant
. Under the current study conditions, it was observed that an EndoActivator had minute effect on debriding the isthmus and presented an effect similar to that of conventional irrigation. This somewhat contradicts findings in extracted human teeth, in which all of the final irrigant activation schemes under investigation reduced accumulated debris in the isthmus to a statistically similar extent
Micro-ct evaluation of sonically and ultrasonically activated irrigation on the removal of hard-tissue debris from isthmus-containing mesial root canal systems of mandibular molars.
. These differences may be due to the natural variance in isthmus areas in natural teeth, which can make it hard to find statistically significant differences between groups. Moreover, this simulated isthmus area in the current model was larger than that in natural teeth, allowing to detect more subtle differences.
Our results on extrusion of the irrigant through the apical foramen by the EDDY tip and the absence of such extrusion by ultrasonic irrigant activation concur with published results in extracted teeth
. As mentioned above, whether that has any clinical significance remains to be investigated. Interestingly, ultrasonic irrigant activation appears to be correlated with reduced post-operative pain after root canal treatment
Postoperative pain in root canal treatment with ultrasonic versus conventional irrigation: A systematic review and meta-analysis of randomized controlled trials.
. However, whether this is related to a reduced exposure of the periapical tissues to the irrigant is not known. Data on post-operative pain induced by sonic irrigation, especially using EDDY, are sparse and did not reveal any potential problems
Based on these results, and without extrapolating too much from the current in vitro conditions to clinics, it nevertheless appears to be advisable to use sonic irrigant activation devices with caution. It may be better to apply these tools in earlier phases of instrumentation, when there is little danger of inadvertently propelling the NaOCl irrigant beyond the apex. As a final irrigation device, the targeted lateral action of an ultrasonically driven metal tip appears to be the best among the options investigated here, both in terms of isthmus cleansing and safety.
Biofilm removal from a simulated isthmus and lateral canal during syringe irrigation at various flow rates: A combined experimental and computational fluid dynamics approach.
Microbial status of apical root canal system of human mandibular first molars with primary apical periodontitis after “one-visit” endodontic treatment.
Influence of 1-hydroxyethylidene-1,1-diphosphonic acid on the soft tissue-dissolving and gelatinolytic effect of ultrasonically activated sodium hypochlorite in simulated endodontic environments.
A novel methodology providing insights into removal of biofilm-mimicking hydrogel from lateral morphological features of the root canal during irrigation procedures.
Adjunctive steps for the removal of hard tissue debris from the anatomic complexities of the mesial root canal system of mandibular molars: A micro-computed tomographic study.
Hard tissue debris removal from the mesial root canal system of mandibular molars with ultrasonically and laser-activated irrigation: A micro-computed tomography study.
Dynamic movement of intracanal gas bubbles during cleaning and shaping procedures: The effect of maintaining apical patency on their presence in the middle and cervical thirds of human root canals-an in vivo study.
Radiographic healing after a root canal treatment performed in single-rooted teeth with and without ultrasonic activation of the irrigant: A randomized controlled trial.
Effect of canal anastomosis on periapical fluid pressure build-up during needle irrigation in single roots with double canals using a polycarbonate model.
Micro-ct evaluation of sonically and ultrasonically activated irrigation on the removal of hard-tissue debris from isthmus-containing mesial root canal systems of mandibular molars.
Postoperative pain in root canal treatment with ultrasonic versus conventional irrigation: A systematic review and meta-analysis of randomized controlled trials.
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