Bram Verhaagen –
Christos Boutsioukis –
Michel Versluis –
Physics of Fluids group
University of Twente
P.O. Box 217
7500AE Enschede, The Netherlands

Lei-Meng Jiang –
Ricardo Macedo –
Department of Conservative and Preventive Dentistry, Section Endodontology
Academic Center for Dentistry
Gustav Mahlerlaan 3004
1081LA Amsterdam, The Netherlands

Damien Walmsley –
School of Dentistry
University of Birmingham
St Chad’s Queensway
Birmingham B4 6NN, United Kingdom

Luc van der Sluis –
Department of Conservative Dentistry and Endodontics
Paul Sabatier University
3 chemin des Maraîchers
31062 Toulouse, France

Popular Version of Paper 4aPA8 presented May 17, 2012 at the 163rd ASA Meeting in Hong Kong

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A trip to the dentist should be pleasant but root canal treatments may occasionally leave some pain after the treatment. This is a result of an inflammation in or around the tooth, caused by infection, which are bacteria hiding inside the tooth away from the bodys defences. Also, the bacteria hide themselves in a self-produced slime layer called the biofilm.

Removal of the biofilm requires an invasive treatment by a dentist, who attempts to kill and flush out the bacteria. For dentists, this is a routine procedure performed daily, using a needle and a syringe filled with bleach (NaOCl). The flow from these needles has been investigated with Computational Fluid Dynamics simulations and high-speed imaging experiments (Boutsioukis et al. 2010, see video 2 below), which have shown that this flow is not very effective in delivering the bleach near the bacteria.

This explains the clinical finding that in 40% of root canal treatments the infection and the pain persists (de Cleen et al. 1993), leading to further problems with the same tooth, requiring retreatment, surgery and eventually extraction. The main difficulty for complete cleaning is the very complex geometry of the root canal system. It is not a simple cylindrical canal, but contains oval extensions, isthmuses, apical ramifications, lateral and accessory canals and tubules, running in all directions, with diameters down to 1 5m (Cohen & Hargreaves 2006, Weng et al. 2009). All of these remote locations can allow bacteria to remain hidden and therefore the canals need to be cleaned.

Video 1: the complex geometry of a root canal (from:

Video 2: computer-simulated flow from a side-vented needle inside a root canal (from: Boutsioukis et al. 2010)

To improve the distribution of the bleach into such remote locations, Passive Ultrasonic Irrigation (PUI) has been introduced that makes use of a miniature file, vibrating at ultrasonic frequencies around 30 kHz (Van der Sluis et al. 2007), see figure 1. This file stirs the fluid violently, leading to acoustic streaming (Duck & Smith 1979) and the formation of cavitation bubbles (Brennen 1995). Both contribute to the cleaning of the complex root canal system.

Figure 1: sketch of the stirring done by a file inside a root canal.

The exact cleaning mechanism, acoustic streaming, cavitation, a chemical effect, or a combination of these, is not yet known. In this study we attempt to reveal the cleaning mechanisms, using knowledge and expertise from physics, acoustics and dentistry.

First, the oscillation characteristics of the miniature file have been investigated with scanning laser vibrometry experiments and computer simulation (Verhaagen et al. 2012), showing a pattern of nodes and antinodes along the file. This is similar to observing the movement of a guitar string.

The acoustic streaming is measured using high-speed photography, capable of recording the motion of the fluid up to 1,000,000 frames per second. The resulting slow-motion video shows a complex flow pattern that leads to a more effective distribution and mixing of the bleach than the flow from a needle. The flow consists of both an oscillatory flow and a steady flow, which together help in exerting a force on the bacteria and in moving the bleach to areas where the bacteria may then be killed.

A computational fluid dynamics model is being developed to investigate the influence of the confinement imposed by the narrow root canal channels on the flow.

The formation and collapse of small cavitation bubbles is also observed in the high-speed movies, as can be seen in video 3. Their contribution to cleaning is limited, because these bubbles rarely collapse onto the wall of the root canal.

Video 3: flow and cavitation around the tip of an ultrasonic file inside a root canal (recorded at 250.000 frames/second; the time is indicated in the bottom-right, in microseconds)

In order to evaluate the cleaning efficacy of this acoustic streaming and cavitation, we have produced a hydrogel as a biofilm substitute. This has similar viscoelastic properties to a real biofilm that would be found in the root canal. Our special camera has filmed the behaviour of the hydrogel and how it is removed from a surface, see movie 4. The recordings show that the steady part of the flow does most of the removal, but the narrow root canal limits this effect. Interestingly, stable bubbles, not necessarily created by the oscillating file, were observed to be able to remove the hydrogel even more efficiently and this may lead to a more effective root canal treatment.

Movie 4: removal of a hydrogel (biofilm) from glass, recorded at 15 frames per second. First the hydrogel is deformed due to the steady flow, but towards the end of the movie a stable cavitation bubble suddenly enters and slices the hydrogel off the glass

This ultrasonic device is already employed, mainly by specialized dentists (endodontists), and appears to lead to a better treatment outcome. Currently there is a clinically randomized controlled trial ongoing, in collaboration with the Beijing University School and Hospital of Stomatology in China, This will allow us to clinically measure the improvement in treatment outcome.

This research is new and exciting as it shows for the first time, in slow motion, that the use of a vibrating miniature file induces acoustic streaming and cavitation. This will improve the cleaning of the root canal and remove the bacteria causing the infection. Clinically this will mean a pain free outcome when you go to your dentist in the future.

M. J. De Cleen, A. H. Schuurs, P. R. Wesselink, and M. K. Wu, Periapical status and prevalence of endodontic treatment in an adult dutch population, Int Endod J 26, 112119 (1993)

C. Boutsioukis, B. Verhaagen, M.Versluis, E. Kastrinakis and L.W.M. van der Sluis, Irrigant flow in the root canal: experimental validation of an unsteady Computational Fluid Dynamics model using high-speed imaging, Int Endod J 43, pp. 393-403 (2010)

S. Cohen and K. M. Hargreaves, Pathways of the pulp, 9th edition (Mosby Elsevier) (2006)

L.W. M. Van der Sluis, M. Versluis, M. K.Wu, and P. R.Wesselink, Passive ultrasonic irrigation of the root canal: a review of the literature, Int Endod J 40, 415426 (2007)

P. Duck and F. Smith, Steady streaming induced between oscillating cylinders, J Fluid Mech 91, 93110 (1979)

C. E. Brennen, Cavitation and bubble dynamics (Oxford University Press) (1995)

B. Verhaagen, S. C. Lea, G.J. de Bruin, L.W.M. van der Sluis, A.D. Walmsley and M. Versluis, Oscillation characteristics of endodontic files: numerical model and its validation, submitted to Physics in Medicine and Biology (2012)

X.-L. Weng, S.-B. Yu, S.-L. Zhao, Wang, H.-G., Mu, T, R.-Y. Tang and X.-D. Zhou, Root canal morphology of permanent maxillary teeth in the Han nationality in Chinese Guanzhong area: a new modified root canal staining technique, J Endod 35, 51-656 (2009)