Wall shear stress effects of different endodontic irrigationtechniques and systems

Narisa Goode a,1, Sara Khan a,1, Ashraf A. Eid b, Li-na Niu c, Johnny Gosier a,Lisiane F. Susin a, David H. Pashley d, Franklin R. Tay a,d,*aDepartment of Endodontics, Georgia Regents University, Augusta, GA, USAbDepartment of Dental and Biomedical Material Sciences, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, JapancDepartment of Prosthodontics, School of Stomatology, Fourth Military Medical University, Xi’an, ChinadDepartment of Oral Biology, Georgia Regents University, Augusta, GA, USA

j o u r n a l o f d e n t i s t r y 4 1 ( 2 0 1 3 ) 6 3 6 – 6 4 1

a r t i c l e i n f o

Article history:

Received 22 March 2013

Received in revised form

11 April 2013

Accepted 12 April 2013

Keywords:

Apical fluid pressure

Calcium hydroxide

Canal fin

Fluid flow rate

Sodium hypochlorite

Two-phase gas–liquid flow

a b s t r a c t

Objectives: This study examined debridement efficacy as a result of wall shear stresses

created by different irrigant delivery/agitation techniques in an inaccessible recess of a

curved root canal model.

Methods: A reusable, curved canal cavity containing a simulated canal fin was milled into

mirrored titanium blocks. Calcium hydroxide (Ca(OH)2) paste was used as debris and loaded

into the canal fin. The titanium blocks were bolted together to provide a fluid-tight seal.

Sodium hypochlorite was delivered at a previously-determined flow rate of 1 mL/min that

produced either negligible or no irrigant extrusion pressure into the periapex for all the

techniques examined. Nine irrigation delivery/agitation techniques were examined: Navi-

Tip passive irrigation control, Max-i-Probe1 side-vented needle passive irrigation, manual

dynamic agitation (MDA) using non-fitting and well-fitting gutta-percha points, EndoActi-

vatorTM sonic agitation with medium and large points, VProTM EndoSafeTM irrigation

system, VProTM StreamCleanTM continuous ultrasonic irrigation and EndoVac apical nega-

tive pressure irrigation. Debridement efficacies were analysed with Kruskal–Wallis ANOVA

and Dunn’s multiple comparisons tests (a = 0.05).

Results: EndoVac was the only technique that removed more than 99% calcium hydroxide

debris from the canal fin at the predefined flow rate. This group was significantly different

( p < 0.05) from the other groups that exhibited incomplete Ca(OH)2 removal.

Conclusions: The ability of the EndoVac system to significantly clean more debris from a

mechanically inaccessible recess of the model curved root canal may be caused by robust

bubble formation during irrigant delivery, creating higher wall shear stresses by a two-

phase air–liquid flow phenomenon that is well known in other industrial debridement

systems.

# 2013 Elsevier Ltd. All rights reserved.

* Corresponding author at: Department of Endodontics, College of Dental Medicine, Georgia Regents University, Augusta, GA 30912-1129,USA. Tel.: +1 706 7212152; fax: +1 706 7218184.

E-mail address: ftay@gru.edu (F.R. Tay).1

Available online at www.sciencedirect.com

journal homepage: www.intl.elsevierhealth.com/journals/jden

These authors contributed equally to this work.0300-5712/$ – see front matter # 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jdent.2013.04.007

j o u r n a l o f d e n t i s t r y 4 1 ( 2 0 1 3 ) 6 3 6 – 6 4 1 637

1. Introduction

In root canal therapy, irrigants function as lubricants during

canal instrumentation.1 Some irrigants also help to eradicate

canal wall biofilms via various antimicrobial strategies

ranging from cell death2 to complete hydrolysis.3 As an

irrigant moves through the root canal system, it produces a

shear force parallel to the surface of the canal wall which is

known as wall shear stress (WSS). The latter is responsible for

mechanical debridement of the root canal space.4 Wall shear

stress is affected by a variety of conditions such as the canal

taper.5 The use of computer fluid dynamics has demonstrated

a close relationship between calculated and observed results

when side-vented irrigation needles are employed for root

canal debridement.6 For example, Boutsioukis et al. reported

that WSS generated by a side-vented irrigation needle is the

highest at the port’s opening and drops rapidly towards the tip

of the needle.7 These results are in agreement with histologi-

cal observations that side-vented needles are not very

effective in cleaning instrumented root canals in the area

1 mm from the working length.8

Although various methods and techniques such as acoustic

microstreaming,9 manual dynamic agitation10 and sonic

agitation11 have been used to enhance WSS, the ultimate

magnitude of WSS is limited by patient safety issues arising

from irrigant extrusion. Boutsioukis et al.7 opined that from a

clinical point of view, the prevention of irrigant extrusion

should precede the requirement for adequate irrigant replace-

ment and wall shear stress. Even though those authors7

reported generation of an apically-directed pressure of about

75 mmHg when a 30-gauge side-vented needle was placed

3 mm from the working length (WL), a region of irrigant

stagnation beyond the needle’s termination was still apparent

with an irrigant flow rate of 15.6 mL/min, confirming the

earlier observation by Chow.12

Several new irrigation techniques have been developed

in the past decade,13 while classic strategies such as

ultrasonic activation have been modified14 and/or combined

with new techniques15 to enhance WSS. Jiang et al.

evaluated a variety of these advancements in a straight

canal model by placing the irrigation needle at 1 mm short

of the WL and reducing the flow rate to 6 mL/min.16 Khan

et al. further tested several irrigant delivery needles,

techniques and flow rates by placing the needles at 1 mm

short of WL and insuring they were not bound in the canal.17

In that study, a safety limit for root canal irrigant delivery

was proposed to minimise extrusion of cytotoxic irrigants

such as sodium hypochlorite into the periradicular regions.

The proposed safety limit was defined as the point where

the apically-directed pressure would not exceed the central

venous pressure (CVP; 5.88 Hg). Adoption of such a safety

limit avoids potentially fatal intravenous infusion, as

reported during canal drying18 and implant placement.19

Although Khan et al. demonstrated the use of apical

negative pressure as an irrigant delivery mechanism never

exceeded CVP at any irrigant flow rate, all commercially

available positive pressure root canal irrigant delivery

systems produced apically-directed pressure in excess of

the CVP at flow rates greater than 1 mL/min.

Since the efficacy of irrigant agitation is inversely propor-

tional to the extent of wall contact of an irrigant delivery or

agitation device,20 canal curvature must also be considered

when assessing WSS. Thus, the objective of the present study

was to examine the effects of WSS in a curved canal, by

comparing the efficacy of debris removal by nine irrigant

delivery and/or agitation techniques in an inaccessible recess of

a curved root canal model, using an irrigant flow rate of 1 mL/

min that was previously determined to produce apically-

directed pressure that is less than the CVP. Calcium hydroxide

(Ca(OH)2) paste was placed in the recess as an inert hydrophilic

marker to simulate canal wall debris. The null hypothesis tested

was that the method of irrigant delivery or agitation does not

influence the efficacy of mechanical debridement when sodium

hypochlorite is delivered at the universal flow rate of 1 mL/min.

2. Materials and methods

2.1. Root canal model

A reusable, curved root canal cavity was milled into mirrored

medical-grade (Grade II) titanium blocks (Figs. 1 and 2) with

the aid of a computer-aided 3-D design software (Dassault

Systemes SolidWorks Corp., Waltham, MA, USA). The canal

had a primary curvature of 178, a secondary curvature of 248

and a tertiary curvature of 688. The blocks were precision-

lapped to form a fluid-tight seal when bolted together. This

fluid-tight seal was verified via under-water testing. The WL of

the cavity was 17 mm. Canal geometry was equivalent to

having taken a size-30, 0.06 taper rotary instrument to WL, and

then clearing the apical seat with a size 40, 0.02 taper hand

instrument. An inaccessible groove was milled in one titanium

block (Fig. 2) to simulate a canal fin. It measured 0.2 mm wide,

0.5 mm deep and 4.0 mm long, commencing 2 mm coronal to

the apical termination and was located between the secondary

and tertiary curvatures of the canal.

2.2. Fin loading and predefined parameters

Due to its uniform particle size, predictable flow and definite

opacity, Ca(OH)2 paste (UltraCal XS, Ultradent Products Inc.,

South Jordan, UT, USA) was used as the test debris and marker.

It was loaded into the fin by using a 30-gauge NaviTip (Ultradent)

attached to the UltraCal XS syringe, starting from the apical

aspect with the needle slowly advancing coronally. After the fin

was filled, the opposing block was aligned with precision

positioning pins; bolting of the blocks was performed exactly

60 s after Ca(OH)2 loading to prevent drying of the paste. Sodium

hypochlorite (2.6%) was used as the sole irrigant at the

predefined flow rate of 1 mL/min. The rationales for using

sodium hypochlorite include: it does not corrode titanium, does

not chemically react with UltraCal and is a clinically-relevant

intracanal irrigant. Irrigant delivery was controlled using an

Aladdin precision syringe pump (World Precision Instruments,

Sarasota, FL, USA) connected to all test groups via polyethylene

tubings and Luer connectors. A master delivery tip derived from

the EndoVac system (Sybron Dental Specialties, Orange, CA,

USA) was permanently mounted over the access opening to

aspirate overflowing irrigant during all testing.

Fig. 1 – (A) Complete working fixture of the test unit including the removable titanium block (double headed arrow) is shown

indexed and locked to the stationary-mirrored titanium block. The canal orifice is indicated by top arrow with the NaviTip

extending to 1 mm short of working length and receiving NaOCl from the precision syringe pump at an irrigant flow rate of

1 mL/min. The excess irrigant is aspirated away by the EndoVac’s master delivery tube attached to the operatory’s high

vacuum suction system. During EndoVac irrigant delivery, the Luer fitting from the positive pressure type needle is moved

to the master delivery tube delivery needle. (B) When the removable titanium block containing the simulated canal fin is

removed, the curved root canal (arrow) is apparent in the stationary-mirrored titanium block.

j o u r n a l o f d e n t i s t r y 4 1 ( 2 0 1 3 ) 6 3 6 – 6 4 1638

2.3. Irrigant delivery techniques

2.3.1. I and II: Passive irrigationIn group I (control), the precision syringe pump was set at the

predefined flow rate of 1 mL/min, attached to a 30-gauge, flat,

open-ended NaviTip and placed into the canal 1 mm short of

the WL (Fig. 1A). The NaviTip was left undisturbed for 60 s

during NaOCl delivery, resulting in irrigant delivery of 1.0 mL.

In group II, the procedures in group I were repeated by

replacing the NaviTip with a 30-gauge side-vented needle

(Max-i-Probe1; Dentsply-Rinn, Elgin, IL, USA).

2.3.2. III and IV: Manual dynamic agitationIn group III, the NaviTip was used as in group I for three 20 s

intervals and then removed. After each interval, the irrigant-

filled canal space was manually agitated with a size 30, 0.06

taper (i.e. well-fitting) gutta-percha point for 10 s, with up-

down strokes extending from WL to 5 mm short of WL at 3 Hz.

This sequence was repeated 3 times, resulting in 60 s of

irrigant delivery (1.0 mL) and 30 s of MDA. Group IV was

identical to group III except that a non-fitting, size 40, 0.02

taper gutta-percha point was employed.

2.3.3. V and VI: Automated agitation with a sonic deviceThe procedures in group III were followed with the

exception that agitation was performed with the EndoActi-

vatorTM (Dentsply Tulsa Dental Specialties, Tulsa, OK, USA),

using a size 25, 0.04 taper (group V) and a size 35, 0.04 taper

(group VI) non-cutting polymer sonic tip operated in mode

III (190 Hz).

2.3.4. VII: VProTM EndoSafeTM

The VProTM EndoSafeTM (Vista Dental, Racine, WI, USA) was

used as described in group I.

2.3.5. VIII: Continuous ultrasonic irrigationThis technique was performed with the 30-gauge VProTM

StreamCleanTM Tip (Vista) attached to a piezoelectric MTS-1

control unit and endodontic handpiece (Spartan, Fenton, MO,

USA) set at a power setting of three. Irrigant was delivered at

the predefined rate of 1 mL/min. The tip was used in an up-

down motion at a frequency of 2 Hz, from 4-mm to 1-mm short

of WL. This action lasted for 60 s, resulting in 1.0 mL total

delivery.

2.3.6. IX: Apical negative pressure irrigation (EndoVac)Per manufacturer’s instructions, the irrigant was delivered via

the EndoVac’s master delivery tip at the predefined rate. The

macro-cannula was used in an up-down motion at a frequency

of 1 Hz, from the point at which its apical progression was

blocked and then up 5 mm; this action lasted 30 s. The micro-

cannula was then inserted and left undisturbed at WL for 30 s,

as irrigant delivery from the MDT continued. Total combined

irrigant delivery time was 60 s resulting in 1.0 mL total delivery.

After completion of each irrigation protocol, the titanium

blocks were dissembled for taking a digitised image of the

Fig. 3 – Boxplots showing the percentage cleanliness

achieved by the 9 groups within the curved simulated

canal fin (N = 10). Abbreviations: NT = NaviTip; MP = Max-

i-ProbeW; MDA = Manual Dynamic Agitation;

EdAc = EndoActivatorTM. For each group, the boxplot

graphically depicts the sample minimum, lower quartile,

median upper quartile and the sample maximum. Groups

labelled with the same upper case letter are not

significantly different ( p > 0.05).

Fig. 2 – Representative high magnification images of the

curved simulated canal fin in the titanium root canal

model, showing varying extents of calcium hydroxide

paste placement and removal. (A) Complete removal

showing empty groove between arrows; (B) completely

filled groove between arrows; (C) partially cleaned groove

indicated by arrow.

j o u r n a l o f d e n t i s t r y 4 1 ( 2 0 1 3 ) 6 3 6 – 6 4 1 639

simulated canal fin (Fig. 2). A metal ruler was included in the

image for calibration during image analysis. The canal fin was

then cleaned with a tooth brush and 95% ethanol, and

reloaded with Ca(OH)2 paste. The titanium blocks were re-

assembled for subsequent testing. Experiments in each group

were repeated ten times (N = 10).

2.4. Data collection and statistical analysis

Images were analysed using the ImageJ software (NIH,

Bethesda, MD) by a blinded collaborator who was not involved

in the irrigation procedures. The outline of the simulated canal

fin was traced to delineate its surface area. Likewise, the area

covered by Ca(OH)2 paste within the canal fin was determined

for calculating the percentage area occupied by residual debris

after irrigation. Evaluation via volumetric analysis was

impossible because it was previously determined that the

Ca(OH)2 paste dried quickly. In addition, once the blocks were

separated, an indeterminate amount of residual irrigant

always adhered to the test block.

As the data were not normally distributed (Shapiro–Wilk

test) and exhibited heterogeneous variances (modified Levene

test), they were analysed non-parametrically using

Kruskal–Walls analysis of variance to examine the effect of

delivery or agitation technique on canal fin debridement

efficacy. Post hoc multiple comparisons were performed using

the Dunn’s method. For all tests, statistical significance was

set at a = 0.05.

3. Results

Representative images of completely empty, full and partially

cleaned fins are shown in Fig. 2. The method of irrigant delivery

or agitation significantly affected Ca(OH)2 removal from the

simulated canal fin ( p < 0.001). Debridement efficacies of the

nine groups are summarised in Fig. 3 EndoVac was the only

delivery technique that consistently removed more than 99%

Ca(OH)2 from the canal fin at the flow rate of 1 mL/min (median

percentage cleanliness = 99.88%; p < 0.05). This group was

significantly different from the other groups; the other 8 groups

resulted in incomplete Ca(OH)2 removal at the irrigant flow rate

of 1 mL/min. VProTM StreamCleanTM (continuous ultrasonic

irrigation), being the second highest group (median percentage

cleanliness = 10.32%), was not significantly different from the

two EndoActivatorTM groups and Manual Dynamic Agitation

using size 30, 0.06 taper gutta-percha points. Except for VProTM

StreamCleanTM, there were no significant differences among

these other groups and Manual Dynamic Agitation using size 40,

0.02 taper gutta-percha points, VProTM EndoSafeTM, the Max-i-

Probe1 and the NaviTip control. The NaviTip control had the

lowest median canal cleanliness (0%).

4. Discussion

A basic tenet of in vitro modelling is to reduce substrate

variability and increase reproducibility. The artificial canal

j o u r n a l o f d e n t i s t r y 4 1 ( 2 0 1 3 ) 6 3 6 – 6 4 1640

groove model created in split human root-halves employed in

previous studies21–23 is accurate and reproducible. However,

each re-assembled root can only be used 8 times. For 90 tests,

this requires the use of 12 roots. Although finding human teeth

with straight canals is easily achieved, it would be very

difficult to collect 12 curved roots with identical primary,

secondary and tertiary curvatures. Thus, a reusable, non-

leaking canal was milled in split titanium blocks.

Understandably, there are difference between the surface

of intraradicular dentine and that of milled titanium. The

surface energies of titanium24 and dentine,25 and hence their

wetting characteristics, depend on how those surfaces are

treated. Also, the titanium surface is devoid of dentinal

tubules, which also affects the contact angle made by an

irrigant with that surface.26 As un-etched titanium is more

hydrophobic than un-etched dentine, a titanium canal could

have hampered irrigant flow and produced less desirable

results. Despite these limitations, it should be emphasised

that the same simulated canal fin was used throughout the

experiments. Thus, any undesirable outcomes should have

affected all groups.

The canal geometry employed in the present study is

virtually identical to the model employed by Jiang et al.16 in

terms of apical shape, groove placement and size, but differs

in the canal curvature. Utilisation of the present canal

geometry permits assessment of debridement efficacy of

different irrigant delivery and agitation techniques in a curved

simulated canal fin. This has never been investigated in depth

in previous studies. Other differences from the Jiang et al.

study include testing of three previous untested groups: the

EndoActivator (size 25, 0.04 taper medium tip and size 35, 0.04

taper large tip) and the Max-i-Probe groups. In addition, the

macro-cannula was used with the micro-cannula in the

EndoVac group, in succession, to conform to the manufac-

turer’s instructions for that irrigant delivery system. The

EndoVac macro-cannula is capable of delivering aspiration

pressure at �250 mmHg (unpublished results), while the

micro-cannula maintains a negative apical fluid pressure in

the range of �35 mmHg at flow rates from 0.5 to 8 mL/min.17

Based on recent findings that all positive pressure irrigant

devices generate apical fluid pressure in excess of the CVP, the

irrigant injection rate in the present study was reduced from

6 mL/min to 1 mL/min.17 Apart from this parameter, other

parameters such as irrigant delivery time, relative volume of

irrigant, and agitation times were similar to those employed by

Jiang et al.16 It is noteworthy that the Passive Ultrasonic

Irrigation (PUI) technique20 was not investigated in the study

by Jiang et al. Similarly, continuous ultrasonic irrigation CUI

(group VIII) was also investigated in the present study instead

of PUI. This is because CUI was found to be a more superior

technique for introducing irrigants into lateral canals than

PUI.15

The present study demonstrated nearly 3 orders-of-

magnitude difference in debridement efficacy between the

EndoVac irrigation technique and the other irrigation techni-

ques. This remarkable improvement in debridement efficacy

of the EndoVac system cannot be only attributed to the fact

that irrigant agitation efficacy is inversely proportional to

canal wall contact.20 Recent computational fluid dynamics

studies by Gao et al.,6 Boutsioukis et al.7 and Kocharian27

examined different factors to enhance WSS (needle design,

placement and flow) in basically a side-vented needle that is

similar to the Max-i-Probe tested in the present study. The

conclusions are consistent, in that maximum irrigant flow

occurs at the side-vented port opening. Irrigant flow dramati-

cally decreases to a ‘‘dead zone’’ of irrigant flow within a few

millimetres. In his thesis, Kocharian suggested the study of a

phenomena known as ‘‘two-phase flow’’ (air mixed with

irrigant) in which the density of the two phases can differ by a

factor of about 1000.27 How does gas–liquid two-phase flow

enhance the cleaning dynamics inside the root canal system?

As described by investigators in dental waterline and hand-

piece cleaning,28 two-phase flow involves the use of a mixture

of gas (usually air) and a liquid to create a mixed-phase flow

along a channel wall, which produces shear stresses to remove

biofilm, debris and contaminants from the wall surface.

Turbulence in the gas enhances the ongoing reformation of

liquid droplets and also enhances the impact of liquid droplets

against the channel wall. By means of random turbulent

fluctuations of local velocity, a velocity component is devel-

oped perpendicular to the wall to create the necessary WSS to

completely debride the channel wall. Channels with internal

diameters as small as 200 mm and as large as 20 mm have been

cleaned completely using this technology.28

The two-phase flow technology has been used extensively

in the milk industry. Since WSS is proportional to the velocity

of the liquids flowing across a wall, a standard method for

increasing the velocity for the cleaning debris in milk lines is to

introduce air along with the liquid cleansing solution.29 Air

introduction is produced via vacuum pressure; admission of

air reduces the volume of liquid in the milk line and increases

the liquid flow velocity when compared to fully-flooded

operation without air admission.29 Studies in microchannels

as small as .025 mm in diameter have demonstrated that the

two-phase flow effect between air–water to be bubbly, slug,

liquid ring and liquid lump flows.30–33 Gulabivala et al.

reported the typical irrigant injection rate for the EndoVac

is 5 mL/min,4 which is much faster than the 1 mL/min flow

used in the present study. This ‘‘starvation’’ of irrigant flow

caused the EndoVac to concurrently draw both irrigant and air

bubbles through the EndoVac evacuation lines, thus creating a

visually-apparent two-phase flow phenomenon that is other-

wise impossible to achieve via constant positive pressure

irrigant injection.

5. Conclusion

Within the limits of the present study, the null hypothesis that

the method of irrigant delivery or agitation does not influence

the efficacy of mechanical debridement when sodium

hypochlorite is delivered at the universal flow rate of 1 mL/

min has to be rejected. The EndoVac system is able to

significantly clean more debris from a mechanically inacces-

sible recess of the root located in the apical third of a curved

root canal model. This may be caused by robust bubble

formation during irrigant delivery, as the EndoVac system is

aspirating NaOCl faster than the latter is applied, thus setting

up a natural air induction system and two-phase flow fluid

dynamics. The purpose of this study was not to quantify the

j o u r n a l o f d e n t i s t r y 4 1 ( 2 0 1 3 ) 6 3 6 – 6 4 1 641

WSS or two-phase flow dynamics for any endodontic irriga-

tion technique, but to assess the debridement efficacy of the

various irrigation techniques. Thus, further studies are

required to quantify the WSS created by the EndoVac system,

in order to identify the most optimal apical negative pressure

required along with air induction rates to increase two-phase

flow fluid dynamics effect on WSS.

r e f e r e n c e s

1. Schilder H. Cleaning and shaping the root canal. DentalClinics of North America 1974;18:269–96.

2. Siqueira Jr JF, Paiva SS, Ro cas IN. Reduction in the cultivablebacterial populations in infected root canals by achlorhexidine-based antimicrobial protocol. Journal ofEndodontics 2007;33:541–7.

3. Clegg MS, Vertucci FJ, Walker C, Belanger M, Britto LR. Theeffect of exposure to irrigant solutions on apical dentinbiofilms in vitro. Journal of Endodontics 2006;32:434–7.

4. Gulabivala K, Ng YL, Gilbertson M, Eames I. The fluidmechanics of root canal irrigation. Physiological Measurement2010;31:R49–84.

5. Boutsioukis C, Gogos C, Verhaagen B, Versluis M,Kastrinakis E, van der Sluis LW. The effect of root canaltaper on the irrigant flow: evaluation using an unsteadyComputational Fluid Dynamics model. InternationalEndodontic Journal 2010;43:909–16.

6. Gao Y, Haapasalo M, Shen Y, Wu H, Li B, Ruse ND, et al.Development and validation of a three-dimensionalcomputational fluid dynamics model of root canalirrigation. Journal of Endodontics 2009;35:1282–7.

7. Boutsioukis C, Verhaagen B, Versluis M, Kastrinakis E,Wesselink PR, van der Sluis PR. Evaluation of irrigant flow inthe root canal using different needle types by an unsteadycomputational fluid dynamics model. Journal of Endodontics2010;36:875–97.

8. Nielsen BA, Baumgartner CJ. Comparison of the EndoVacsystem to needle irrigation of root canals. Journal ofEndodontics 2007;33:611–5.

9. Ahmad M, Pitt Ford TR, Crum LA. Ultrasonic debridement ofroot canals: acoustic streaming and its possible role. Journalof Endodontics 1987;13:490–9.

10. Huang TY, Gulabivala K, Ng YL. A bio-molecular film ex-vivomodel to evaluate the influence of canal dimensions andirrigation variables on the efficacy of irrigation. InternationalEndodontic Journal 2008;41:60–71.

11. Jiang LM, Verhaagen B, Versluis M, van der Sluis LW.Evaluation of a sonic device designed to activate irrigant inthe root canal. Journal of Endodontics 2010;36:143–6.

12. Chow TD. Mechanical effectiveness of root canal irrigation.Journal of Endodontics 1983;9:475–9.

13. Gu LS, Kim JR, Ling J, Choi KK, Pashley DH, Tay FR. Review ofcontemporary irrigant agitation techniques and devices.Journal of Endodontics 2009;35:791–804.

14. Jiang LM, Verhaagen B, Versluis M, Zangrillo C, Cuckovic D,van der Sluis LW. An evaluation of the effect of pulsedultrasound on the cleaning efficacy of passive ultrasonicirrigation. Journal of Endodontics 2010;36:1887–91.

15. Castelo-Baz P, Martın-Biedma B, Cantatore G, Ruız-Pinon M,Bahillo J, Rivas-Mundina B, et al. In vitro comparison ofpassive and continuous ultrasonic irrigation in simulatedlateral canals of extracted teeth. Journal of Endodontics2012;38:688–91.

16. Jiang LM, Lak B, Eijsvogels LM, Wesselink P, van der SluisLW. Comparison of the cleaning efficacy of different finalirrigation techniques. Journal of Endodontics 2012;38:838–41.

17. Khan S, Niu L-N, Rid AA, Looney SW, Didato A, Roberts S,et al. Periapical pressures developed by non-bindingirrigation needles at various irrigation delivery rates. Journalof Endodontics 2013;39:529–33.

18. Rickles NH, Joshi BA. A possible case in a human and aninvestigation in dogs of death from air embolism duringroot canal therapy. The Journal of the American DentalAssociation 1963;67:397–404.

19. Davies JM, Campbell LA. Fatal air embolism during dentalimplant surgery: a report of three cases. Canadian Journal ofAnaesthesia 1990;37:112–21.

20. van der Sluis LW, Versluis M, Wu MK, Wesselink PR. Passiveultrasonic irrigation of the root canal: a review of theliterature. International Endodontic Journal 2007;40:415–26.

21. van der Sluis LW, Wu MK, Wesselink PR. The evaluation ofremoval of calcium hydroxide paste from an artificialstandardized groove in the apical root canal using differentirrigation methodologies. International Endodontic Journal2007;40:52–7.

22. Rodig T, Vogel S, Zapf A, Hulsmann M. Efficacy of differentirrigants in the removal of calcium hydroxide from rootcanals. International Endodontic Journal 2010;43:519–27.

23. Rodig T, Hirschleb M, Zapf A, Hulsmann M. Comparison ofultrasonic irrigation and RinsEndo for the removal ofcalcium hydroxide and Ledermix paste from root canals.International Endodontic Journal 2011;44:1155–61.

24. Kilpadi DV, Lemons JE. Surface energy characterization ofunalloyed titanium implants. Journal of Biomedical MaterialsResearch 1994;28:1419–25.

25. Attal JP, Asmussen E, Degrange M. Effects of surfacetreatment on the free surface energy of dentin. DentalMaterials 1994;10:259–64.

26. Ramos SM, Alderete L, Farge P. Dentinal tubules drivenwetting of dentin: Cassie-Baxter modelling. The EuropeanPhysical Journal E Soft Matter 2009;30:187–95.

27. Kocharian T. Root canal irrigation – an engineering analysisusing computational fluid dynamics. Masters thesis.Department of Mechanical and Industrial Engineering.Faculty of Applied Science and Engineering, University ofToronto. November 23, 2010. https://tspace.library.utoronto.ca/handle/1807/25188.

28. Labib ME, Lai C-Y, Tabani Y, Qian Z, Dukhin SS, Murawski JJ.Method for cleaning passageways such an endoscopechannels using flow of liquid and gas. United States PatentOffice: 8,226,774. January 24, 2012.

29. Reinemann DJ, Grasshoff A. Two-phase cleaning flowdynamics in air injected milklines. Transactions of theAmerican Society of Agricultural Engineers 1994;37:1531–6.

30. Triplett KA, Ghiaasiaan SM, Addel-Khalik SI, Sadowski DL.Gas–liquid two-phase flow in microchannels. Part I: Two-phase flow patterns. International Journal of Multiphase Flow1999;25:377–94.

31. Triplett KA, Ghiaasiaan SM, Addel-Khalik SI, LeMouel A,McCord BN. Gas–liquid two-phase flow in microchannels.Part II: Void fraction and pressure drop. International Journalof Multiphase Flow 1999;25:395–410.

32. Chen WL, Twu MC, Pan C. Gas–liquid two-phase flow inmicro-channels. International Journal of Multiphase Flow2002;28:1235–47.

33. Serizawa A, Feng Z, Kawara Z. Two-phase flow inmicrochannels. Experimental Thermal and Fluid Science2002;26:703–14.