canal. Nonetheless, many root canals are curved in reality. The effect of root canal curvature on irrigant exchange has been studied indirectly by Nguy and Sedgley [69], who evaluated the removal of bioluminescent planktonic bacteria. Based on their results, a curvature up to 24°–28° according to Schneider’s method [89] doesn’t seem to create additional obstacles for irrigant flow even when a low flow rate is used, provided that a closed-ended needle is placed at 1 mm short of WL [69]. It can be assumed that if needles are positioned within 1–3 mm short of WL in a curved root canal, in many cases they have already bypassed most of the curvature and the remaining curvature apically to their tip is limited. Small size (30G or 31G) flexible irrigation needles widely available nowadays can facilitate placement near WL in many cases, even in severely curved canals.

In addition to improving irrigant exchange in severely curved root canals, fine-diameter needles can reach further and earlier even into straight root canals without the need of excessive enlargement; this way they satisfy better the requirements for irrigant refreshment [14]. Indeed, it has been verified in ex vivo studies that finer needles result into improved irrigant exchange and cleaning [24, 29, 82], but this is true even when positioned at the same insertion depth as larger needles [19, 24]. The latter finding probably relates to the space available around the needle for the reverse flow of the irrigant towards the canal orifice. Evidently, a larger needle occupies more space inside the root canal and leaves less space for the reverse flow compared to a finer needle. The development of an effective reverse flow improves irrigant refreshment in the apical part of the root canal and is also necessary for refreshment coronally to the needle tip. Moreover, the reverse flow carries away microorganisms, tissue remnants, and dentin debris detached from the walls by the shear stress [15, 16]. Larger needles also increase the risk of wedging and irrigant extrusion [81]. On the other hand, finer-diameter needles require more effort by the clinician during irrigation [8].

Wall Shear Stress

During irrigant flow, frictional forces occur between the flowing irrigant and root canal walls. These forces give rise to wall shear stress (Text Box 3.5), which is of particular interest to irrigation because it can detach material from the root canal wall, so it determines the mechanical effect. At the moment, there are no quantitative data on the minimum shear stress required for the removal of dentin debris, tissue remnants, isolated microorganisms, or biofilm from root canal walls; thus, the overall distribution of wall shear stress can be useful mainly for comparisons of the relative mechanical effect.

Text Box 3.5

Wall shear stress and viscosity

Frictional forces occurring within a flowing irrigant and between flowing irrigant and root canal walls tend to resist its motion. In order to explain this phenomenon, the irrigant is considered to consist of individual layers of infinitely small thickness, which can slide over each other. As the irrigant moves, the layers farther away from the wall tend to move faster than the ones closer to the wall and a shear stress is developed. Shear stress (τ) is defined as the force (F) required to slide one layer of the fluid over another divided by the area of

contact between the two layers (A):

G tG = F

A

For most irrigants, the wall shear stress is proportional to the difference of the velocity (u) between the adjacent irrigant layers close to the wall, also called as the velocity gradient (du/dy, where y is the distance from the wall), according to the equation:

t = m du dy

The viscosity of the irrigant (μ) describes its resistance to motion and could be

regarded as a measure of its internal friction. It is a property of the irrigant, depending mainly on temperature [67, 103, 113] Obviously, irrigants with higher viscosity will develop a higher wall shear stress, but they will also resist flow and require more effort to deliver

Similarly to the developed irrigant flow, two basic wall shear stress patterns can be distinguished during syringe irrigation (Fig. 3.6) [13]. Regarding the open-ended needles, an area of increased shear stress is developed apically to the needle tip, in the region of jet breakup. This area is approximately symmetrical around the needle and is slightly smaller for the beveled and

notched needles, which develop local maxima on the side of the root canal wall not facing the outlet. On the other hand, the closed-ended needles (side-vented and double-side-vented) lead to almost twice as high maximum shear stress, but limited near their tip, on the wall facing the needle outlet (the proximal outlet for the dou- ble-side-vented needle) [13]. An area of slightly increased shear stress is also identified opposite to the distal outlet of the double-side-vented needle, but has only a minimum influence on the overall stress pattern [13]. The unidirectional performance of the side-vented and double-side- vented needles has also been reported in ex vivo studies that investigated the influence of needle orientation in the debridement of the root canal [49, 115]. Being a special case of closed-ended needles, the multi-vented needles show a slightly

Fig. 3.6 Time-averaged distribution of shear stress on the root canal wall in the apical part of a size 45, 0.06 tapered root canal during syringe irrigation using various types of needles [open-ended needles: flat (a), beveled (b), and notched (c); closed-ended needles: side-vented

(d), double-side-vented (e), and multi-vented (f)], according to computer simulation. Only half of the root canal wall is shown to allow simultaneous evaluation of the needle position. Needles are colored in red. Reprinted with permission from Elsevier (Ref. [13])

Fig. 3.7 Time-averaged distribution of shear stress on the root canal wall for the a closed-ended (left) and an open-ended needle (right) positioned at 1–5 mm short of the WL in a size 45, 0.06 tapered root canal, according to

computer simulation. Only half of the root canal wall is shown to allow simultaneous evaluation of the needle position. Needles are colored in red. Reprinted and modified with permission from Elsevier (Ref. [14])

different pattern. Maximum wall shear stress can be up to seven times more than the other types of needles, but the stress is mainly concentrated on a very limited area opposite to the many needle outlets [13].

Needle insertion depth, root canal size, and taper do not seem to affect the distribution of wall shear stress to a large extent [14–16]. The maximum shear stress decreases as needles move away from WL or with increasing size or taper, because more space is available for the reverse flow of the irrigant, so the irrigant velocity decreases; at the same time, the area affected by maximum shear stress becomes larger. Based on these findings, it could be hypothesized that overenthusiastic enlargement of the root canal further than a certain size or taper may in fact reduce the mechanical effect of irrigation. Currently, no data are available on the effect of irrigant flow rate on wall shear stress. Based on the definition of wall shear stress (Text Box 3.5) and the relation of the flow rate to the velocity distribution in the root canal [10], it is very likely that an increase in the flow rate results in a direct increase in wall shear stress.

Optimum debridement seems to be achieved only in a limited part of the root canal wall near the tip of the needle, irrespective of other parameters [13–16, 49]. Consequently, it appears advantageous to move the needle inside the root canal during syringe irrigation, so that the limited area of high wall shear stress affects as much of the root canal wall as possible (Fig. 3.7). It must also be emphasized that wall shear stress may lead to the detachment of biofilm, tissue remnants, or dentin debris from the root canal wall, but it is not enough to ensure their removal from the root canal space; a favorable reverse flow is needed to carry them towards the canal orifice, as mentioned above.

Apical Vapor Lock

Most of the experiments and simulations already described in this chapter have assumed that the root canal is completely filled with a liquid (single-phase system). Recently, it has been demonstrated that air bubbles may be entrapped in the apical part of the root canal during syringe irriga-

Fig. 3.8 Bubble entrapment (vapor lock) in the apical part of size 50, 0.04 tapered root canals, according to computer simulations and in vitro experiments. The irrigant was delivered through a 30G closed-ended needle at a flow rate of 0.083 or 0.260 mL/s. The blue surface depicts the air-irrigant interface in the computer simulations. Only large bubbles occupying completely a part of the

apical root canal should be considered a vapor lock (stars). Smaller bubbles floating in the irrigant or moving with the irrigant towards the coronal orifice (arrows) are of minor importance because they cannot block irrigant penetration to any part of the root canal. Reprinted and modified with permission from Wiley (Ref. [17])

tion and totally block irrigant penetration in that area (Fig. 3.8), a phenomenon also termed apical vapor lock [17, 28, 101, 107, 108]. The presence of an air bubble results in the formation of a two- phase system (irrigant – air) (Text Box 3.6).

Despite earlier claims [28, 40, 101], bubble entrapment doesn’t seem to be a major issue during syringe irrigation. The formation and extent of apical vapor lock is dependent on the same parameters that affect irrigant penetration in general: an increase in the flow rate, use of an openended needle, insertion of the needle closer to WL, and enlargement of the root canal all seem to result into a smaller apical vapor lock. In addition, an entrapped bubble can be easily removed during syringe irrigation either by brief insertion of a closed-ended needle to WL or by increasing the flow rate to 0.26 mL/s. So, there seems to be no need for the use of negative pressure systems or agitation techniques to reach this goal [17].

Earlier studies probably overestimated the frequency and importance of apical vapor lock by positioning the needles too far away from WL and irrigating only at a very low flow rate.

In view of these recent findings, it appears that the poorer performance of syringe irrigation in closed-ended root canals (sealed apical foramen) as compared to open-ended ones [28, 40, 73, 98, 101] should not be directly attributed to the presumed apical vapor lock without demonstrating its presence. A more likely explanation is the large differences in irrigant flow between these two cases [12, 109, 113], as explained above.

Anatomical Challenges

Overall, it appears that the ex vivo cleaning efficiency of syringe irrigation in the main root canal may be similar even to that of ultrasonic activation,