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Address requests for reprints to Dr Christos Boutsioukis, Department of Endodontology, Academic Centre for Dentistry Amsterdam, University of Amsterdam and Vrije Universiteit Amsterdam, Gustav Mahlerlaan 3004, 1081 LA Amsterdam, The Netherlands.
Department of Endodontology, Academic Centre for Dentistry Amsterdam (ACTA), University of Amsterdam and Vrije Universiteit Amsterdam, Amsterdam, The Netherlands
Department of Endodontology, Academic Centre for Dentistry Amsterdam (ACTA), University of Amsterdam and Vrije Universiteit Amsterdam, Amsterdam, The Netherlands
The aim of this study was to compare the irrigant flow in curved root canals prepared to various apical sizes by constant-taper or variable-taper instruments during syringe irrigation with 3 endodontic needles at 2 different flow rates.
Methods
Two matched curved mesial root canals of human mandibular molars were imaged by micro–computed tomographic imaging after preparation to apical size 20, 25, and 30/.06 taper either by constant-taper or variable-taper instruments. A Computational Fluid Dynamics model was used to simulate the irrigant flow in the 2 root canals prepared to each apical size during syringe irrigation with a 30-G open-ended needle and 30-G and 31-G closed-ended needles at 0.05 and 0.15 mL/s.
Results
The irrigant could not penetrate up to the working length in root canals prepared to apical size 20 or 25/.06 taper. The 30-G open-ended needle combined with the low flow rate allowed the irrigant to reach the working length in size 30/.06 taper root canals while maintaining a relatively low apical pressure, but the wall shear stress was very low. The 31-G closed-ended needle combined with the high flow rate also delivered the irrigant to the working length in size 30 root canals and developed higher wall shear stress, but the apical pressure was also higher.
Conclusions
Syringe irrigation using 30-G and 31-G needles was compromised in minimally shaped root canals.
Irrigant flow is a prerequisite for the cleaning and disinfection of the root canal. The flow created by syringe irrigation could not reach the working length in minimally shaped root canals irrespective of the needle type and size and the flow rate.
Minimally invasive endodontics promotes the preservation of as much healthy hard dental tissue as possible in an effort to maintain the strength and function of the tooth
Impacts of conservative endodontic cavity on root canal instrumentation efficacy and resistance to fracture assessed in incisors, premolars, and molars.
Impacts of conservative endodontic cavity on root canal instrumentation efficacy and resistance to fracture assessed in incisors, premolars, and molars.
. Apical enlargement at least to size 30–35 is considered necessary in order to achieve debridement and disinfection of the root canal system because it allows 27- to 30-G irrigation needles to be inserted closer to the working length (WL) and the irrigant to overcome viscosity-related effects that limit its penetration
The efficacy of dynamic irrigation using a commercially available system (RinsEndo) determined by removal of a collagen 'bio-molecular film' from an ex vivo model.
, but the flow created by these needles has not been evaluated so far. Furthermore, numerical models have been used in the past to investigate root canal irrigation
, but previous studies adopted a univariate approach. Thus the effect of any interactions between the irrigant flow rate; the apical root canal size and taper; and the needle type, size, and insertion depth remains largely unexplored.
Therefore, the aim of this study was to compare the irrigant flow in curved root canals prepared to various apical sizes by constant-taper or variable-taper instruments during syringe irrigation with a 30-G open-ended and 30-G and 31-G closed-ended irrigation needles at 2 different flow rates using a Computational Fluid Dynamics (CFD) model.
Materials and Methods
Geometry of the Root Canals and Irrigation Needles
The use of extracted teeth was approved by the institutional ethics committee (2018042). Two human mandibular molars with separate, moderately curved (20°–40°)
mesial root canals were obtained from a pool of extracted teeth. Two of these mesial root canals were matched based on their length, curvature, and diameter in the apical 5 mm, so they were as similar as possible. These specimens were debrided and stored in a 0.05% thymol solution.
The specimens were scanned by a micro–computed tomographic scanner (μCT 40; Scanco Medical AG, Brüttisellen, Switzerland) operating at 70 kV and 114 μA using a 10-μm voxel size before any intervention (initial scan). The scans were reconstructed, filtered, and segmented using Fiji 1.49m
to obtain 3-dimensional renderings of the 2 root canals.
Standard access cavities were prepared in both specimens. The working length (WL) was set 0.5 mm short of the major apical foramen. The apical 5 mm of the roots was sealed with cyanoacrylate (Pattex; Henkel, Dusseldorf, Germany). A glide path was prepared using stainless steel K-files of size 08–15 (Dentsply Maillefer, Ballaigues, Switzerland). One of the 2 matched root canals was randomly allocated (www.randomizer.org) to the constant-taper protocol, and it was prepared by Mtwo rotary nickel-titanium files (VDW, Munich, Germany) size 15/.05 taper, 20/.06, 25/.06, and 30/.06. The second matched root canal was prepared by variable-taper V-Taper 2H rotary nickel-titanium files (17/.04, 20/.06, 25/.06, 30/.06; SS White Dental, Lakewood, NJ), which have a decreasing taper from their tip toward their shaft in order to remove less dentin in the middle and coronal third of the root canal
. After every instrument, the root canal was rinsed with 2% sodium hypochlorite (Orphi Farma, Lage Zwaluwe, The Netherlands) and distilled water. The distal root canals were left unprepared. The selection and chemomechanical preparation protocol is described in more detail in Supplemental Appendix S1 (available online at www.jendodon.com). Each specimen was scanned 3 more times by the micro–computed tomographic scanner after preparation of the matched root canals to apical size 20, 25, and 30.
The geometry of 30-G irrigation needles has been described previously
Clinical relevance of standardization of endodontic irrigation needle dimensions according to the ISO 9,626:1991 and 9,626:1991/Amd 1:2001 specification.
. The geometry of the 31-G closed-ended double-side-vented needle (Navitip; Ultradent Products Inc, South Jordan, UT) was obtained through examination under a stereoscopic microscope (Stemi SV-6; Zeiss, Göttingen, Germany).
was used in this study. In order to cope with the irregular geometry of real root canals, the hexahedral mesh originally used in the root canal was replaced by a hybrid mesh. The effect of this modification on the predictions of the model was examined in a preliminary study as detailed in Supplemental Appendix S2 (available online at www.jendodon.com). The comparison of the time-averaged velocity magnitude and vectors along the root canal between the new case with the modified mesh, the original case used for the validation of the model, and experimental particle image velocimetry measurements
revealed a close agreement, thereby confirming that the mesh modification did not affect the results.
The 3-dimensional geometry of the 2 matched root canals after preparation to apical size 20, 25, and 30 was imported into ANSYS Design Modeler 14.5 (ANSYS Inc, Canonsburg, PA). The flow domain (Fig. 1) included the complete root canal from the orifice until the apical end point of instrumentation (WL), where a wall was defined. The length of the root canals was standardized to 8.9 mm by the removal of excess coronal structure. The 3 selected needles, a 30-G flat open-ended needle and 30-G and 31-G closed-ended double-side-vented needles, were modeled using the actual needles as references (Navitip [30 G], Ultradent Products Inc; Endo-Irrigation Needle [30 G], Transcodent, Neumünster, Germany; and Navitip [31 G], Ultradent Products Inc). The external and internal diameters were standardized to 308 μm and 196 μm for the 30-G needles and 254 μm and 156 μm for the 31-G needles, respectively. The length of all needles was set to 31 mm.
Figure 1(Left) Geometry of the matched root canals after preparation to apical size 20, 25, and 30 and (right) charts depicting the diameter of the root canals in the apical 5 mm. The slope of each line represents the root canal taper. A dashed line indicating a constant 0.06 taper has been added to the charts as a reference.
The positioning and bending of each needle was based on a preliminary in vitro experiment that determined the maximum attainable insertion depth (until binding) for each needle in curved molar root canals prepared by constant-taper or variable-taper instruments to apical size 20, 25, and 30 (Supplemental Appendix S1 is available online at www.jendodon.com). The maximum attainable insertion depth was converted to the minimum attainable distance from the WL. The average values per case were increased by 1 mm in order to prevent needle binding during irrigation, and they were used to define the fixed position of each needle inside the modeled root canals. The needles were positioned in the root canals as centered as possible.
ANSYS Mesh 14.5 (ANSYS Inc) was used to create the hybrid mesh of the flow domain (1.1–1.8 million cells). The mesh was refined near the walls and in areas where high velocity gradients were anticipated. Grid independence of the results was verified. No-slip boundary conditions were applied to all the walls, which were assumed to be rigid, smooth, and impermeable. A velocity inlet boundary condition was applied at the inlet of the needle, and flat velocity profiles corresponding to a flow rate of 0.05 or 0.15 mL/s were prescribed. Atmospheric pressure was imposed at the root canal orifice. Sodium hypochlorite 2.5% was used as irrigant (density = 1060 kg/m3; viscosity = 1.073·10−3 Pa·s
), and it was modeled as an incompressible Newtonian fluid. The needle and the root canal were filled with the irrigant. Gravity was defined so as to mimic the orientation of the mesial root canals of a mandibular molar when the patient is lying horizontally during treatment.
ANSYS Fluent 14.5 (ANSYS Inc), a finite volume solver, was used to solve the time-dependent Navier-Stokes equations in 3 dimensions. An unsteady isothermal flow was assumed, and no turbulence model was used. A steady-state solution was obtained first and then used as the initial condition for the unsteady simulations. All transport equations were discretized to be at least second-order accurate. For temporal discretization, a first-order implicit formulation was used. The convergence criterion was set to 10−4 of the maximum scaled residuals. Pressure, velocity, and vorticity were also monitored to ensure adequate convergence in every time step. A time step of 10−6 seconds was used throughout the calculations, which were carried out for a real flow time of 20 milliseconds on a workstation with a 14-core Intel Xeon 2.5 GHz processor (Intel, Santa Clara, CA) and 32 GB of RAM. The flow fields calculated for each of the 36 cases were compared in terms of irrigant velocity, wall shear stress, and apical pressure.
Results
Root Canal Anatomy
The initial total length of the matched root canals assigned to the constant- and variable-taper preparation protocols was 11.1 and 9.8 mm, respectively. This was reduced to 10.4 and 9.3 mm after preparation to apical size 30. The curvature of the root canals was 37.0° and 36.7°
before preparation, and it decreased to 29.6° and 31.8° after preparation, respectively. Details on the diameter of the root canals in the apical 5 mm are provided in Figure 1.
Flow Pattern
A steady flow was developed by all needles in low flow rate cases and also by the 30-G closed-ended needle in the high flow rate cases. The flow was unsteady in the high flow rate cases that involved the 30-G open-ended or the 31-G closed-ended needle. The 30-G open-ended needle created a high-velocity irrigant jet directed apically with a slight diversion toward the outside of the root canal curvature. The closed-ended needles created smaller jets at both outlets directed toward the apex at an angle of almost 45° to the needle axis. Most of the irrigant flowed through the outlet most proximal to the tip (30-G needle, 0.05 mL/s: 61.0% [1.3%] and 0.15 mL/s: 70.4% [3.2%]; 31-G needle, 0.05 mL/s: 59.1% [2.5%] and 0.15 mL/s: 72.1% [1.5%]; data presented as time average [standard deviation]). The jet velocity in the 31-G needle cases was higher than in the 30-G needle cases. All jets were more intense in high flow rate cases. The main flow pattern was not affected by the apical preparation size or the instrument type (Fig. 2).
Figure 2Triads of time-averaged irrigant velocity contours (top) and vectors (middle) along the z-y plane and streamlines indicating the route of massless particles released downstream from the needle inlet and colored according to time-averaged velocity magnitude (bottom) in the root canal prepared with Mtwo files to apical size 25. Particle trajectories provide visualization of the main fresh irrigant flow in 3 dimensions. Needles are colored in red.
None of the needles was able to deliver the irrigant to the WL in size 20 or 25 root canals irrespective of the flow rate (Fig. 3). The 30-G open-ended and the 31-G closed-ended needles performed similarly, and both were more effective than the 30-G closed-ended needle in these canals. The irrigant penetrated up to the WL in size 30 root canals when the 30-G open-ended needle was used regardless of the flow rate. The same was noted for the 31-G closed-ended needle but only at the higher flow rate; a stagnation area was evident apically when the lower flow rate was used. To the contrary, the 30G closed-ended needle could not deliver the irrigant until WL even in the size 30/.06 taper root canals. The higher flow rate improved irrigant penetration when the closed-ended needles were used but not in the case of the open-ended needle. Irrigant penetration was comparable in root canals prepared with either type of instruments with minor differences that were attributed to the slightly different needle insertion depths.
Figure 3Maximum time-averaged axial irrigant velocity in the apical 5 mm of the root canal as a function of distance from the WL for each needle type, instrument type, apical preparation size, and flow rate, indicating the irrigant penetration front. The scale of the vertical axis has been adjusted to 0–0.5 m/s to highlight differences in the area apically to the needles. Colored dots on the horizontal axis indicate the position of the needle tip for each root canal size. Velocities higher than 0.1 m/s (dotted horizontal line) were considered to indicate clinically relevant penetration.
Low wall shear stress was developed in the apical 1–2 mm of root canals prepared to size 20 or 25. Slightly higher values were calculated in size 30 root canals (Fig. 4). The 30-G open-ended needle developed high shear stress in a small area within 0.5 mm apically to its tip. The 2 closed-ended needles developed a different pattern with 2 local maxima next to their outlets on the 2 opposite sides of the root canal wall. The 30-G open-ended and the 31-G closed-ended needles developed higher wall shear stress in the apical part of the root canals compared with the 30-G closed-ended needle. The wall shear stress was considerably higher in the high flow rate cases compared with the low flow rate ones, but it was not affected by the instrument type.
Figure 4The maximum time-averaged irrigant shear stress applied on the root canal wall as a function of distance from the WL for the various needle types, instrument types, apical preparation sizes, and flow rates. Colored dots on the horizontal axis indicate the position of the needle tip for each root canal size. Examples of contours of time-averaged irrigant shear stress on the root canal wall of size 25 root canals irrigated at 0.15 mL/s are provided above each graph to illustrate the shear stress pattern. A shadow of the needle is shown behind the root canal wall. The color map on the right side refers to the contours.
The irrigant pressure at the apical end of the root canal increased from size 20 to size 25, but in most cases, it decreased from size 25 to 30 (Fig. 5). The maximum values were similar for the 30-G open-ended needle and the 31-G closed-ended needle; the 30-G closed-ended needle developed lower maximum pressure. High flow rate cases resulted in higher apical pressure than the low flow rate ones. The pressure was similar in root canals prepared by either type of instruments.
Figure 5Time-averaged irrigant pressure at the apical end of the root canal as a function of needle distance from the WL for the various needle types, instrument types, apical preparation sizes, and flow rates.
The main research hypothesis tested in this study was that irrigant penetration and the wall shear stress developed in the apical third during syringe irrigation are compromised in minimally shaped root canals when 30-G or 31-G needles are used. The results of the computer simulations confirmed this hypothesis.
Curved mesial root canals of mandibular molars were selected as a sufficient challenge for root canal preparation and needle placement in the apical third. Modeling the irregular geometry of real root canals required modifications of the mesh that could affect the validity of the model. Therefore, the predictions of the current model were compared both to the predictions of the original model and to the experimental measurements used for its validation
to confirm that the modified meshing strategy did not introduce additional error to the results. Previous research has shown that different subtypes of open-ended needles create very similar irrigant flow, and the same applies to closed-ended needles
. Consequently, only one 30-G open-ended and one 30-G closed-ended needle were included in this study to represent each type and also to allow comparisons with the 31-G closed-ended needle that had not been evaluated before. The selected irrigant flow rates fall within the range applied by clinicians when 30-G needles are used
. Because there was no such information concerning 31-G needles, the same flow rates were used in order to facilitate comparisons.
Fresh irrigant is carried apically to the needle only by the axial component of the irrigant velocity (the component in the direction of the root canal axis at each level), so the magnitude of this component was used to determine the extent of irrigant penetration in the apical third, similarly to earlier studies
. Recently, the CFD-calculated magnitude of the total irrigant velocity, which contains information about both the chemical and mechanical effects of irrigation, was correlated to biofilm removal from artificial isthmuses and lateral canals in vitro
Biofilm removal from an artificial isthmus and lateral canal during syringe irrigation at various flow rates: a combined experimental and Computational Fluid Dynamics approach.
. Nevertheless, it should be emphasized that without a validated pressure threshold for sodium hypochlorite accidents, it is not possible to determine in which of these cases an accident would actually occur.
Several interesting results were generated by the numerical model. First, the irrigant could not penetrate until the WL in root canals prepared to apical size 20 or 25 irrespective of the type and size of the needle and the flow rate. Clinically, a small amount of irrigant may still be transported along the files during instrumentation, but both the chemical and mechanical effects will be very limited in this area. Thus, it is very unlikely that the apical third of minimally shaped root canals can be sufficiently cleaned and disinfected by syringe irrigation even when a 31-G needle is used. A minimum apical size 30 was required in order for the irrigant to reach the WL. These findings are in agreement with earlier studies that modeled simplified root canals and 30-G needles
and are also consistent with the trend reported by in vitro studies that evaluated 27- to 30-G needles regarding the effect of apical size on irrigant penetration, root canal cleaning, and disinfection
, and the irrigant did not penetrate until the WL in any of the simulated cases. The flow developed by the 30-G open-ended needle could reach the WL in size 30 root canals even at the lower flow rate while maintaining a relatively low apical pressure, but the wall shear stress was also very low. Under these conditions, the 30-G open-ended needle appeared to be less likely to extrude irrigant through the apical foramen than the closed-ended needles examined here, a finding that contradicted earlier studies
. Nonetheless, only a single flow rate was evaluated in these earlier studies, and all needles were inserted at the same distance from the WL. This contradiction highlighted the importance of the concurrent evaluation of several parameters in the same study and accounting for differences in the attainable insertion depth between the needles. The 31-G closed-ended needle combined with the higher flow rate was the most effective option when considering both the chemical and mechanical effects. In this case, the irrigant penetrated until the WL in size 30 root canals, and the wall shear stress was maximized. However, the apical pressure was higher than any of the low flow rate cases, which may indicate an increased risk of irrigant extrusion through the apical foramen.
An interaction was found between the flow rate and the needle type regarding irrigant penetration. A higher flow rate improved irrigant penetration for the closed-ended needles but not for the open-ended one, which is in agreement with an earlier in vitro study
. However, increasing the flow rate from 0.05 to 0.15 mL/s led to a large increase in the wall shear stress in all cases; thus, a higher flow rate may still provide an advantage even when an open-ended needle is used. It is noteworthy that irrigant flow rate had a significant effect on biofilm removal from artificial isthmuses and lateral canals during syringe irrigation with an open-ended needle in vitro
Biofilm removal from an artificial isthmus and lateral canal during syringe irrigation at various flow rates: a combined experimental and Computational Fluid Dynamics approach.
The flow was not distributed equally between the 2 side vents of the closed-ended needles. Most of the irrigant passed through the vent closer to the tip, which is in line with earlier findings
. Still approximately 30%–40% flowed through the second vent, with more irrigant flowing through in the low flow rate cases. This was considerably higher than the previously reported 6.5%
. The difference could be attributed to the lower flow rate used in this study and to the smaller root canals that resisted the flow through the vent most proximal to the tip, thereby favoring the path through the second vent.
The apical pressure decreased in size 30 root canals compared with size 25 when the 31-G closed-ended needle was used. This decrease could be due to the 25%–50% additional space that was available around the tip of the needle for the backflow of the irrigant
. A similar yet less pronounced decrease was also noted in the root canal prepared with variable-taper instruments when a 30-G needle was used.
One of the strengths of this study compared with earlier work was that the needles were inserted as close as possible to the WL without binding based on in vitro measurements rather than assumptions in order to study irrigation under optimum clinically realistic conditions. As a result, they were placed up to 0.5 mm closer to the WL in the root canal prepared with constant-taper instruments compared with the one prepared with variable-taper instruments. This difference may have improved irrigant penetration slightly in these cases
, but the current study design could not provide conclusive evidence about the effect of the insertion depth on the flow because other parameters were varied at the same time.
Constant-taper and variable-taper instruments created similar shapes in the apical third of the matched root canals. Size 20 and 25 variable-taper instruments were slightly more conservative than the corresponding constant-taper instruments, but the opposite was true for size 30 instruments. These observations should be interpreted with caution because only 2 root canals were evaluated, but they may indicate that using an instrument of variable taper does not always result in less dentin removal or a variable-taper root canal shape, at least in the apical third. However, it should be noted that the variable-taper instruments used in this study had a constant 0.06 taper near their tip
, which could explain the similarities in the shape of the apical third. The lack of significant differences in the flow between the 2 root canals could also be attributed to this fact because the shape of the apical third determines irrigant penetration to a large extent
and the current analysis of the flow focused on that area. On the other hand, the variable-taper instruments were more conservative 5 mm away from the WL. This did not seem to affect the flow, but it could explain the differences in the attainable needle insertion depth between the 2 root canals.
Conclusions
Preparation of curved root canals to apical size 20 or 25/.06 taper did not allow sodium hypochlorite to reach the WL. When the apical size was increased to 30/.06 taper, a 30-G open-ended needle allowed the irrigant to reach the WL even when irrigation took place at a low flow rate (0.05 mL/s). The 31-G closed-ended needle combined with the high flow rate (0.15 mL/s) also delivered the irrigant up to the WL in size 30 root canals. Although this gave rise to higher wall shear stress, it also developed higher apical pressure.
Acknowledgments
Supported by the European Society of Endodontology Annual Research Grant in 2017.
The authors deny any conflicts of interest related to this study.
Impacts of conservative endodontic cavity on root canal instrumentation efficacy and resistance to fracture assessed in incisors, premolars, and molars.
The efficacy of dynamic irrigation using a commercially available system (RinsEndo) determined by removal of a collagen 'bio-molecular film' from an ex vivo model.
Clinical relevance of standardization of endodontic irrigation needle dimensions according to the ISO 9,626:1991 and 9,626:1991/Amd 1:2001 specification.
Biofilm removal from an artificial isthmus and lateral canal during syringe irrigation at various flow rates: a combined experimental and Computational Fluid Dynamics approach.
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