Similarly to all other medical needles, the sizes of irrigation needles are most frequently described by the “gauge” system (Table 3.1) and seem to conform well to the relevant ISO specification [9]. However, the “gauge” units cannot be directly compared to the size of instruments and obturation materials. The adoption of the millimeter as the standard metric unit to express needle size already recommended by the ISO for more than a decade [52], and the development of a color code corresponding to that of the endodontic instruments could greatly assist clinical procedures [9].

In the past, large needles (21–25G) were commonly employed for irrigant delivery [20, 24, 82, 87, 102]. Such needles could hardly penetrate beyond the coronal third of the root canal, even in wide root canals. More recently, the use of finerdiameter needles (28G, 30G or 31G) has been advocated, mainly because they can reach farther into the canal, even to working length (WL) [6, 14, 19, 49, 69, 92, 117]. The effect of needle type and size on root canal irrigation will be discussed in more detail further on.

Physical Properties of Irrigants

Apart from the equipment (syringe and needle), the flow of irrigants is also affected by their physical properties, mainly density and viscosity (Text Box 3.2) [67, 103, 113]. For commonly used endodontic irrigants, these properties are very similar to those of distilled water [41, 105] because most irrigants are sparse aqueous solutions. The surface tension of endodontic irrigants (Text Box 3.2) and its decrease by wetting agents (surfactants) have also been studied extensively, under the assumption that they may have a significant effect on irrigant penetration in dentinal tubules and accessory root canals [1, 36, 100] and on dissolution of pulp tissue [97]. However, while density and viscosity affect the flow in all cases, the effect of surface tension is important only at the interface between two immiscible fluids [58, 113]. Such an interface is formed between the irrigant and an air bubble, but not between the irrigant and the dentinal fluid, because these two

liquids are miscible. Recent studies have confirmed that surfactants do not enhance the ability of NaOCl to dissolve pulp tissue [25, 27, 55] or the ability of common chelators to remove calcium from dentin [116] or to remove the smear layer [26, 62]. In addition, bubble entrapment in the apical part of root canals is an unlikely event provided that certain guidelines are followed, as it will be discussed further on.

Text Box 3.2

Density

Density (ρ) is defined as:

r = m V

where m is the mass of a certain quantity of the irrigant and V is its volume [67, 113].

Viscosity

Viscosity describes the resistance of the irrigant to motion [67, 103, 113]. A more elaborate definition will be given in Text Box 3.5, together with the definition of wall shear stress.

Surface tension

The interface between two immiscible fluids in contact (e.g., irrigant and air) is found to behave as if it were under tension, like a stretched membrane. The origin of such tension at an interface is due to the intermolecular attractive forces within each fluid. The net effect of these forces is for the interface to contract and it is called surface tension. It depends on the pair of fluids in contact and other factors, such as the temperature and the presence of wetting agents or surfactants [58, 113].

Irrigant Flow During Syringe

Irrigation

Evaluating irrigant flow even in a simple straight and uniformly tapered root canal can be a very demanding task. It has been underlined that during irrigation, the root canal behaves mostly like

a closed-ended system, so in ex vivo and in vitro experiments the apical foramen should be sealed [10, 18, 47, 73, 101]. This seemingly minor detail has been overlooked in many experimental studies in the past, giving rise to doubt about the validity of their results, as pointed out by Tay et al. [101]. In fact, a closed apical foramen results in a significantly more complicated flow pattern and adds considerable obstacles for irrigant penetration compared to a root canal open from both sides, even if no air bubbles are entrapped apically [12, 109, 113].

Fluid flows are broadly categorized into laminar and turbulent ones (Text Box 3.3). In the case of root canal irrigation, turbulence would greatly assist irrigant penetration and refreshment due to the far more effective mixing [34]. However, the development of turbulence inside root canals during syringe irrigation has not been verified experimentally yet. When the irrigant is delivered at very low flow rates (~0.01 mL/s) through a 30G needle, a steady laminar flow is developed within the root canal. At higher flow rates (up to at least 0.26 mL/s), the flow becomes unsteady, but it remains laminar [10, 12, 109], contrary to previous reports [56]. An unsteady flow changes smoothly over time, but it is not necessarily turbulent. It is likely that the formation of vortices (Text Box 3.4) and the unsteady flow during syringe irrigation could have been mistaken for turbulence in the past due to limitations of the visual assessment in real time. According to computer simulations, a higher, yet clinically unrealistic, flow rate (0.53–0.79 mL/s) may lead to the development of turbulence mostly close to the tip of the needle [10]; however, these results have not been verified in experiments.

Text Box 3.3

Laminar and turbulent flow

The type of flow occurring within the root canal depends primarily on the balance between the inertia (driving) forces and viscous (frictional) forces affecting the irrigant. This balance is expressed by the

Reynolds number (Re), which combines

four parameters influencing the flow: fluid density (ρ) and viscosity (μ), characteristic velocity scale (υ), and characteristic length scale (D).

Re = ruD m

At low Reynolds numbers, viscous forces are dominant over inertia forces, and the flow remains laminar, characterized by a smooth variation of the velocity with position and/or time. If the Reynolds number increases further than a critical value (usually taken to be around 2200–3000 for flows in pipes), a complicated series of events leads to a radical change of the flow, which becomes turbulent. In such a case, inertia forces are dominant over viscous forces, except adjacent to solid surfaces [39, 58, 63, 77, 80].

Turbulent flows possess a number of characteristic properties that distinguish them from laminar flows. They are random, unpredictable, and chaotic. Moreover, they are highly unsteady and generally vary along the three spatial directions. Visualizations of turbulent flows reveal rotational flow structures of various sizes, called turbulent eddies (not to be confused with the more stable vortices – see Text Box 3.4). The kinetic energy is continuously transferred from large eddies to progressively smaller eddies until it is dissipated and converted into thermal energy. This dissipation results in increased energy losses associated with turbulent flows [39, 60, 77, 111]. Turbulent flows are also characterized by substantially more effective mixing than laminar flows because of the eddying motions. As a consequence, heat, mass, and momentum are very effectively exchanged [39, 60, 77, 96, 111], and this can be an important advantage for certain chemical or biological applications [34].

Text Box 3.4

Jet

A jet is a high-velocity fluid stream forced out of a small-diameter opening or nozzle [103, 113].

Vortex

A vortex is a relatively stable rotating flow structure [103, 113]. It should be distinguished from the eddies formed in turbulent flows.

The type of the needle has also a substantial effect on the basic flow pattern developed in the root canal during syringe irrigation (Fig. 3.3), while other parameters such as needle insertion depth, root canal size, and taper have only a limited influence [12–16, 109]. Based on the needle design and the resulting flow, the available types of needles can be categorized into two main groups, namely, the open-ended and the closedended [13]. All needles create a jet (Text Box 3.4) at their outlet, but the exact position and shape of

the outlet determines the orientation and, to some extent, the intensity of the jet.

In the case of the open-ended needles (flat, beveled, notched), the jet is very intense and extends along the root canal, apically to their tip (Fig. 3.3a–c). Within a certain distance, which depends on the geometry of the root canal, the insertion depth of the needle, and the flow rate, the jet appears to break up gradually. Reverse flow towards the canal orifice occurs near the canal wall. The jet formed by the flat and beveled needle is slightly more intense and extends further apically than that of the notched needle [13, 109].

When closed-ended needles are used (sidevented, double-side-vented), the jet is formed near the apical side of the outlet (the one proximal to the tip for the double-side-vented needle), and it is directed apically with a slight divergence (Fig. 3.3d–e). The irrigant mainly follows a curved path around the tip and then towards the coronal orifice. A series of counterrotating vortices are formed apically to the tip, extending almost to the WL. Their size, position, and

Fig. 3.3 Time-averaged contours (left) and vectors (right) of irrigant velocity in the apical part of a size 45, 0.06 tapered root canal during syringe irrigation by different types of needles, according to computer simulations [open-ended needles: flat (a), beveled (b), and notched

(c); closed-ended needles: side-vented (d), double-side- vented (e), and multi-vented (f)]. All needles are positioned at 3 mm short of WL and are colored in red. Reprinted with permission from Elsevier (Ref. [13])

number may differ according to needle insertion depth, root canal shape, and flow rate. Despite the fact that irrigant can flow from one vortex to the next, the velocity decreases significantly towards the apex, so irrigant penetration becomes slower. The distal outlet of the double- side-vented needle has only a minor influence on the overall flow pattern because most of the irrigant (93.5 %) flows out through the proximal outlet; thus, it doesn’t provide any important advantage [13, 109].

A special case of closed-ended needle is the multi-vented needle, suggested for root canal irrigation many years ago [37, 38, 66]. Although this needle is not commercially available at the moment, it appears to develop a distinct flow pattern (Fig. 3.3f); several small jets are formed by the irrigant exiting the needle from the outlets proximal to the tip and they are directed perpendicularly to the canal wall. The most intense jets (73 % of the total flow) are formed through the pair of outlets most proximal to the tip. Most of the flowing irrigant is directed towards the canal orifice, while very low velocities are noted apically to the tip [13].

Irrigant Refreshment

As already mentioned, irrigant exchange in the various parts of the root canal system is a crucial requirement for an adequate chemical effect [29, 45, 65]. The type of needle also appears to have a significant effect on the extent of apical irrigant exchange. Earlier reports argued that closedended needles are more efficient than open-ended ones [56, 112]. However, recent studies have clarified the limitations in the irrigant refreshment apically to closed-ended needles and clearly proven their inferiority [10, 13–16, 109, 117]. Under the same conditions, closed-ended needles are always less effective in exchanging the irrigant apically than open-ended needles. Very limited differences have been detected between various types of closed-ended or between various types of open-ended needles [13, 112].

A general trend has been well-documented in the literature that needle placement closer to WL

results in more efficient irrigant exchange, irrespective of other parameters (Fig. 3.4) [14, 19, 24, 48, 93]. Furthermore, an increase in the preparation size or taper also improves irrigant refreshment [15, 16, 18, 24, 33, 48, 49], in addition to allowing needle placement closer to WL [2]. Increasing the flow rate also seems to have a similar effect. It has been found that hardly any irrigant refreshment is achieved apically to a closed-ended needle when irrigating at a very low flow rate (~0.01 mL/s), but an optimal flow rate (0.26 mL/s) can provide refreshment up to 1 mm apically to the needle [10]. A similar effect has been noted for the open-ended needles, although in this case, refreshment always extends farther compared to the closed-ended ones [109].

Even when an optimal flow rate is attained, it seems that root canal preparation to apical size 25, 0.06 taper does not allow adequate irrigant flow and apical refreshment (Fig. 3.5) [15, 48]. Apical enlargement to size 30 leads to effective exchange 2 mm apically to an open-ended needle when combined at least with 0.06 taper [16], while size 35 combined with 0.05–0.06 taper results into significant irrigant refreshment almost 3 mm apically to the needle [15, 48]. Regarding the closed-ended needles, it appears that irrigant exchange occurs almost 1 mm apically to their tip in a root canal of size 30 and at least 0.06 taper, while further increase of the size or taper has only a minimal additional effect [15, 16, 47]. Therefore, these needles need to be placed within 1 mm from WL, and in order for a 30G needle to reach this position, a minimum apical size 30 or 35 is required. If a multi-vented needle were to be used for syringe irrigation, it would also have to be placed almost at WL, since irrigant exchange apically to its tip is very limited [13]. Interestingly, a minimally tapered root canal preparation (size 60, 0.02 taper) may present an advantage over the usual tapered ones in terms of irrigant refreshment [16]. However, the resistance to root fracture, the possibility of iatrogenic errors, and the requirements of the obturation technique should also be taken into account when deciding the instrumentation strategy.

It has been reported that a dead-water or stagnation zone may exist apically to the tip of

Fig. 3.4 Triads of time-averaged velocity contours (left) and vectors (middle), and streamlines (right) in the apical part of a size 45, 0.06 tapered root canal for a closedended (top) and an open-ended needle (bottom) positioned

at 1–5 mm short of WL, according to computer simulations. Needles are colored in red. Reprinted and modified with permission from Elsevier (Ref. [14])

closed-ended needles, where no irrigant refreshment takes place [35, 74, 95]. This zone has been observed while irrigating at a medium flow rate (~0.1 mL/s) through closed-ended needles positioned 3–5 mm short of WL. Given the limited irrigant exchange apically to closed-ended needles and the flow rate used, it is possible that a zone of inadequate refreshment may indeed exist near WL in these cases. However, the real-time

visual evaluation of dye clearance that was employed has only a very limited ability to detect irrigant flow and true exchange. More detailed studies using high-speed imaging combined with computer simulations have shown that there are no areas in the main root canal where the irrigant is completely stagnant during syringe irrigation at an optimal flow rate (0.26 mL/s), even if closed-ended needles are positioned at 3 mm

Fig. 3.5 Triads of time-averaged velocity contours (left) and vectors (middle) and streamlines (right) for a closedended (top) and an open-ended needle (bottom) positioned at 3 mm short of WL in the apical part of root canals of

various sizes and tapers, according to computer simulations. Needles are colored in red. Reprinted and modified with permission from Wiley (Refs. [15, 16])

short of WL. However, the flow may be very slow near WL, not being able to ensure adequate irrigant exchange within the time limitations of a root canal treatment; such areas exist when the needle is placed too far away from WL [12–16, 109]. Increasing the flow rate, delivering additional

volume of irrigant or inserting the needle closer to WL could help to improve refreshment in these cases [14, 19, 92, 93].

Most of the data on irrigant flow and refreshment have been obtained from experiments and computer simulations of simple straight root