Introduction

Irrigant delivery by a syringe and a needle during root canal treatment dates back more than a century [91]. Despite its long history and the development of newer and more sophisticated irrigation systems, it is still recommended for use [51, 76]. In fact, a recent survey indicated that

C. Boutsioukis, DDS, MSc, PhD (*) Department of Endodontology, Academic Centre for Dentistry Amsterdam (ACTA),

Gustav Mahlerlaan 3004, Amsterdam 1081 LA, The Netherlands

e-mail: c.boutsioukis@acta.nl

L.W.M. van der Sluis, DDS, PhD

Department of Conservative Dentistry, University Medical Center Groningen, Groningen, The Netherlands

approximately half of the responding AAE members only used conventional syringe irrigation in their practices [31].

Over the years, the interest to investigate and optimize the various parameters related to this technique has diminished. Nowadays, most publications primarily aim to evaluate new irrigation techniques, so syringe irrigation is frequently used just as a control regarded a priori not effective and unnecessary bias is introduced. It seems rather unlikely that syringe irrigation will be totally replaced by other delivery techniques any time soon. Therefore, this chapter will focus on the specific aspects of syringe irrigation that need to be optimized and will also highlight its advantages and limitations. An interdisciplinary approach combining endodontics and fluid

dynamics will be employed, and essential background knowledge on fluid dynamics will also be provided to facilitate comprehension.

Redefining the Aims

The traditional long list of aims of root canal irrigation can be found in every endodontic textbook and also elsewhere in this book. This list has been refined several times in the past but has always reflected the clinician’s and microbiologist’s point of view, undoubtedly because of the decisive role of microorganisms in the development of apical periodontitis [57, 64, 99]. However, most of the aims and objectives mentioned in this list can be grouped together since they are actually realized by two simultaneous but distinct effects:

•The chemical effect, i.e., chemical disruption or inactivation of biofilms, killing of microorganisms and inactivation of endotoxin, dissolution of pulp tissue remnants, dentin debris and of the smear layer by the active chemical component(s) of the irrigant. Clearly, the chemical effect can only be exerted by chemically active solutions (e.g., sodium hypochlorite).

•The mechanical effect, i.e., mechanical disruption, detachment and removal of microorganisms/biofilms, pulp tissue remnants, and dentin debris from the root canal system via forces applied by the flowing irrigant. Mechanical effects can be exerted by both chemically active and inert irrigants (e.g., water, saline) [42, 45, 88, 117].

Evidently, both effects cannot take place unless the irrigant comes into close contact with the targeted microorganisms and tissue remnants [45, 90]. The chemical effect strongly depends upon the concentration of the active component(s) of the irrigant, the area of contact, and the duration of interaction with the targeted material. During the interaction, most commonly used irrigants are rapidly consumed, so the concentration decreases [44, 65, 78, 79]. Thus, apart from the

initial delivery and penetration, frequent refreshment of the irrigant in all areas of the root canal system is also of utmost importance for an optimum chemical effect.

Irrigants can reach the sites of interest preferably by the flow developed during delivery (or during agitation). This way, chemically active particles (molecules/ions) are transported quickly and efficiently by the fluid motion, a process termed convection. In addition, while flowing, the irrigant applies forces on the targeted material, thus exerting the mechanical effect. In areas of the root canal where a flow cannot be created, irrigant transport may still take place by diffusion, the random movement of particles in a fluid, but this process is markedly slower than convection, and its rate is further affected by the size of the particles, temperature, and concentration gradients [104]. Moreover, no mechanical effect is exerted by diffusion.

At the moment, there is no consensus on the relative importance of each one of these effects (chemical and mechanical) for the overall success of root canal treatment. Both effects are primarily produced by the flow of a chemically active irrigant and require its penetration to the full extent of the root canal system. Thus, efforts to obtain additional insight and optimize irrigant flow seem justified, and this can be achieved by understanding the fluid dynamics of root canal irrigation.

Fluid dynamics is the study of fluids in motion and the subsequent effects of the fluids upon the boundaries, either solid surfaces or interfaces with other fluids. Fluids are substances that cannot withstand any attempt to change their shape when at rest; they include both liquids and gases, as both have the ability to flow [113]. A flow is caused by the action of externally applied forces, like pressure difference, gravity, or buoyancy [4, 7, 34]. Applications of fluid dynamics in the biomedical field are anything but uncommon. An increasing number of challenging problems have been investigated by interdisciplinary approaches involving fluid dynamics. Notable examples include blood flow in the cardiovascular system and air flow in the respiratory system [23, 30, 54, 59, 71, 75, 114].

Root canal irrigation can be viewed as the microscale flow of a liquid (irrigant) inside an irregularly shaped domain of very small dimensions (root canal system). Consequently, it falls clearly within the scope of fluid dynamics and especially microfluidics. The need to investigate in detail the flow of the irrigants inside the root canal has been stressed repeatedly [2, 24, 29, 42, 82, 93, 117]; however, speculations have dominated this aspect of root canal irrigation for decades. For example, the limited performance of syringe irrigation has been attributed to its inability to deliver the irrigant into all the parts of the complicated root canal system, but without strong experimental evidence [29, 82, 84]. This lack of scientific data may still be reflected on the way this procedure is described in endodontic textbooks as well as taught in dental schools. Wide variations have been found among endodontists in the way they perform syringe irrigation ex vivo [8]. Only recently have the abundant data from experiments on the removal of microorganisms, tissue remnants, and dentin debris been coupled with detailed numerical and experimental evaluation of the irrigant flow to provide new insights into root canal cleaning and disinfection.

Based on such an interdisciplinary approach, the basic aims of root canal irrigation can be restated briefly as follows:

•Flow of the irrigant to the full extent of the root canal system and subsequently to the canal orifice, in order to come in close contact with microorganisms/biofilm, debris, and tissue remnants, carry them away and provide lubrication for the instruments. (Flow)

•Frequent refreshment of the irrigant, in order to retain a high concentration of its active component(s) at the sites of interest and compensate for their rapid consumption (applicable only to chemically active irrigants). (Chemical effect)

•Application of force on the root canal wall (wall shear stress), in order to detach/disrupt microorganisms/biofilm, debris, and tissue remnants. (Mechanical effect)

•Restriction of the flow within the constraints of the root canal system and prevention of irri-

gant extrusion towards the periapical tissues. (Safety) [11]

The remainder of this chapter will focus on the first three aims; safety aspects will be discussed in more detail in a separate chapter.

Syringes

In order to perform irrigation, syringes of variable capacity ranging from 1 to 20 mL have been suggested for use (Fig. 3.1) [2, 24, 46, 56, 66, 86, 93, 94]. Although little attention has been put on the size of the syringe used, this can affect the tactile force needed to irrigate at a certain flow rate [8]. Elementary fluid dynamics can provide an explanation for this effect.

During syringe irrigation, a clinician applies tactile force to the syringe plunger. This force is transmitted to the irrigant into the syringe, where pressure is built up (Text Box 3.1). A clinician will need to apply different amounts of force and will feel different levels of difficulty to push the plunger when syringes of a different size are used, even if the actually developed pressure inside the syringe is identical; this results from the definition of pressure. Larger syringes are more difficult to depress and control. For the same reason, the clinician cannot draw reliable conclusions about the pressure.

Fig. 3.1 Syringes of variable capacity (from top to bottom: 20, 12, 5 and 2.5 mL) used for root canal irrigation. All syringes have a Luer Lock threaded fitting (arrow)

Text Box 3.1

Pressure

The pressure (P) developed inside the syringe barrel is defined as:

P = F

A

where F is the tactile force applied to the syringe plunger and A is the cross-sectional area of the plunger. Pressure acts uniformly in all directions. In an irrigant confined by solid boundaries (e.g., the wall of the syringe or the root canal), pressure acts perpendicular to the boundary [67].

Flow rate

The flow rate (Q) of an irrigant is defined

as:

Q = V t

where V is the volume of the irrigant delivered in the root canal within a time period t [67]. The irrigant flow rate is frequently expressed in mL/s or mL/min (1 mL/s = 60 mL/min); in most cases, mL/s is more relevant to clinical practice, since irrigation rarely continues for a whole minute.

Assuming a laminar flow (see Text Box 3.3), the flow rate through a needle is described by the equation:

= p D4 P

Q

128mL

where D is the internal diameter of the needle, P is the pressure difference along the needle, μ is the viscosity of the irrigant (see Text Box 3.5), and L the length of the needle [103]. Evidently, the needle diameter influences the flow rate much more than the other parameters.

While depressing the plunger, the pressure inside the syringe barrel remains considerably higher than the ambient pressure around the tip of

the needle (which is nearly atmospheric). This pressure difference drives the irrigant through the needle and into the root canal, and that is why syringe irrigation is categorized as a positivepressure technique [21]. The irrigant flow rate is proportional to this difference, but is also affected by the size of the needle and several other parameters (Text Box 3.1). So, for the same pressure difference, the flow rate through a smaller needle will be much less than through a larger needle. In other words, a larger pressure difference is required to achieve the same flow rate through a smaller needle.

A common mistake among clinicians which is also reproduced in several publications is that delivery of the irrigant at high flow rate is erroneously termed forceful delivery or delivery under pressure. Using a very large syringe combined with a fine-diameter needle would require a large tactile force, but the flow rate would still be low. In addition, it must be emphasized that the pressure of the irrigant delivered inside the root canal is always much lower than the pressure inside the syringe, because a significant pressure drop occurs along the needle. Thus, neither “force” nor “pressure” is an appropriate term to describe how fast the irrigant is delivered. Such information can only be provided by the flow rate [8, 10], which can also be estimated clinically.

A 5-mL syringe has been recommended as a reasonable compromise between less-frequent refilling and ease of use. This syringe can be used to reach flow rates at least up to 0.20– 0.25 mL/s even when combined with fine irrigation needles [8]. Because of the very high pressures developed inside the syringe, a Luer Lock threaded fitting (Fig. 3.1) is always necessary to avoid accidental detachment of the needle during irrigation [8].

Needles

Over the years, several types of needles have been used to deliver irrigants into the root canals [13, 37, 38, 50, 56, 66, 95, 112, 115]. These needles mainly differ in the presence of an open or closed tip and one or more outlets (Fig. 3.2).

Fig. 3.2 Various types of 30G needles used for root canal irrigation [open-ended needles: flat (a), beveled (b), and notched (c); closed-ended needles: side-vented (d), double-side-vented (e), and multi-vented (f)]. Variable

views and magnifications were used to highlight differences in tip design. The multi-vented needle is not commercially available at the moment for use with a syringe. Reprinted with permission from Elsevier (Ref. [13])