canal [7, 15, 72]. A larger apical preparation may lead to a further unnecessary removal of sound dentin [44, 52]. All these are avoided when using the minimally invasive concept.

Mode of Irrigation by the SAF System

Positive Pressure Irrigation

The delivery of the irrigant to the apical part of the canal has been traditionally achieved using syringe and needle irrigation [35, 72, 93]. This mode of irrigation applies positive pressure to deliver the irrigant and has several limitations. The irrigant cannot be delivered further than 1Ð2 mm beyond the tip of the needle; thus, effective irrigation requires the tip of the needle to be 1Ð2 mm from the working length [10, 16]. The application of positive pressure close to the apical foramen involves the potential risk of pushing the irrigant beyond the apex, commonly termed a Òsodium hypochlorite accidentÓ [35]. Consequently, many operators avoid inserting a needle up to the required length, thus compromising the efÞcacy of the irrigation of the apical cul-de-sac area.

Sonic and Passive Ultrasonic

Irrigation

Sonic and passive ultrasonic irrigations are designed to induce agitation or streaming movements of the irrigant to increase the efÞciency of its action [18, 47, 48]. Sonic irrigation operates at a low frequency (1Ð6 kHz) and high amplitude and generates small shear stresses, which have been shown to be efÞcient for root canal debridement. Studies reported that the sonic instruments may contribute to the cleanliness of the canals but can leave residual debris attached to the canal walls in hard-to-reach areas of long oval canals, isthmuses, and recesses.

Passive ultrasonic irrigation is the use of a smooth metal Þle that vibrates in the canal at an ultrasonic frequency. The vibrating Þle induces acoustic streaming, which is a very effective

cleaning method [48, 93]. Nevertheless, some studies have not supported these Þndings. To be effective, the Þle must have free movement in the canal, without making contact with the canal walls. Consequently, this method may be applied effectively only after canal instrumentation and may be ineffective when applied in curved canals in which the Þle touches the wall at the canal bend. When either sonic or passive ultrasonic irrigations are used, the canal is Þlled with irrigant using a syringe and needle.

Negative Pressure Irrigation

The above limitations led to the introduction of irrigation systems that use negative pressure to deliver the irrigant to the desired area [16, 79]. The access cavity is continuously ßooded with the irrigant, and a small cannula, through which negative pressure is applied, is inserted in proximity to the working length. This causes the continuous ßow of the irrigant into the apical part of the canal while the irrigant is aspirated by the small cannula [60, 79]. This irrigation system is applied after the instrumentation of the canal. For a full effect, this method requires an enlargement of the canal to #40/0.04 or #40/0.06 [12, 26], which makes the method useful in straight canals but of limited value in thin, curved canals in which such enlargement may not be safely achieved.

No-Pressure Irrigation

The SAF may be deÞned as a no-pressure irrigation system that is applied throughout the instrumentation process [53Ð57]. Once the irrigant enters the SAF, any pressure that may have existed in the tube disappears due to the lattice structure of the Þle. The irrigant is continuously delivered into the root canal, and the vibrations of the Þle, combined with the pecking motion applied by the operator, result in the continuous mixing of the irrigant that is present in the root canal with fresh, fully active irrigant. This mode of action raises two questions: (a) will the freshly

applied irrigant be able to reach the apical part of the canal and (b) what is the potential of the pecking motion, which is applied to the working length, to push the irrigant beyond the apex?

The setup in Fig. 11.6a was used to answer the Þrst question. The simulated canal in the transparent block was Þlled with green liquid, representing the irrigant that is present in the canal (Fig. 11.6b). The SAF was operated with vibrations and pecking motions. At a given time, a red liquid, representing fresh, fully active sodium hypochlorite, was injected into the tube, and the time required for the apical part of the canal to turn completely red was measured. The total replacement of the irrigant in the apical section occurred within 30 s (Fig. 11.6c).

When using the SAF system for 4 min, as required by the manufacturerÕs instructions, the sodium hypochlorite in the apical part of the canal is continuously replaced with a fresh, fully active solution at least 8 times.

The setup in Fig. 11.7 was used to answer the second question. The tooth was mounted at the bottom of a plastic container with its tip protruding below the container. The canal was prepared to a working length with a #20 K Þle, and the patency of the apical foramen was veriÞed by passing a #15 K Þle through it (Fig. 11.7a). The SAF was used in the canal for 4 min with continuous irrigation, and the apical foramen was visually checked for any liquid passage. No liq-

Fig. 11.6 Measuring the time needed for the replacement of the irrigant in the apical part of a simulated canal. A simulated canal was Þlled with a green liquid, and the SAF was used in this canal. At a given time point, a red liquid was fed into the irrigation tube, and the time required for the liquid in the apical part of the canal to turn red was measured. (a) The setup. (b) Before SAF operation; (c)

30 s after SAF operation. The red liquid represented fresh, fully active sodium hypochlorite. During 4 min of SAF operation, the full replacement of the irrigant at the apical part with fresh, active irrigant occurred eight times

a

Fig. 11.7 The SAF system vs. syringe and needle irrigation. The tooth was prepared with a #15 K Þle, which passed through the apical foramen (a) and with a #20 K Þle to a working length of 1 mm short of the apical foramen. The SAF was used in the canal for 4 min with continuous irrigation (b). No irrigant passed through the

apical foramen. (c) A short needle was inserted into the canal to a distance of 12 mm from the apical foramen. The needle was free in the canal and did not touch its walls. When irrigated with this needle, a ßow of irrigant traveled through the apex, even though the needle was at a distance from the apex and was free in the canal

uid passed through the apical foramen throughout the procedure (Fig. 11.7b). When syringe and needle irrigation was applied in the same canal immediately after the SAF, keeping the needle at approximately 12 mm from WL, the liquid passed freely beyond the apex (Fig. 11.7c).

Why did the pecking motion not cause liquid extrusion? Why did the syringe and needle cause a free ßow of irrigant beyond the apex? Fluid mechanics analyses provide the answers to these questions. Even with a much larger apical foramen with a diameter of 0.35 mm, the liquid is contained in the canal by surface tension. The bursting pressure needed to break this surface tension is 832 Pa. The hydrostatic pressure of a 20 mm column of water is 195 Pa, and the stagnating pressure, caused by 5,000 vibrations per min within the liquid, is 196 Pa. The piston pressure caused by the SAF pecking motion is only 3 Pa. The total pressure in the root canal (394 Pa) is not large enough to reach the bursting pressure, and therefore the liquid remained in the canal [39].

The reason for such a low piston pressure is due to the shape of the apical motion of the SAF (Fig. 11.8). Even in the extreme case of a diameter of 0.2 mm in the apical part of the canal (created by a #20 K Þle), the fully compressed tip of the SAF has a cross section in the shape of a rectangle of 0.16 by 0.12 mm (Fig. 11.8). This leaves 38 % of the canal cross section open for the backßow of irrigant; thus, the potential piston is ineffective [39].

When calculating the pressures caused by syringe and needle irrigation in a canal similar to the one above and keeping the needle in a position in which 38 % of the canal cross section is free for backßow, the syringe and needle create a pressure of more than 1,270 Pa. Such a pressure is generated by the ßow of the liquid, even though the needle is not tightly Þtted to the canal walls. The total pressure in the canal will reach in this case 1,465 Pa, which is above the eruption pressure and allows the free passage of liquid [39].

Fig. 11.8 The tip of the SAF within a canal prepared with a # 20 Þle. A schematic presentation of the tip of a SAF (rectangle) when inserted into a canal, which was prepared with a #20 K Þle (circle). Thirty-eight percent of the cross section of the canal is free for backßow. This explains why the SAF is not pushing debris or irrigant through the apical foramen, as presented in Fig. 11.6 (Adapted from Hof et al. [39])

The above experiment (Fig. 11.7) was completed in a canal with an open apical foramen with only air surrounding the apex. One may assume that if no liquid passed through the apex during the operation of the SAF system, even in such conditions, the chance that the irrigant will be pushed beyond the apex under clinical conditions, in which the tissues surround the apex, is low.

In this context, when a #20 hand Þle is moved toward the apex in a tight canal with a diameter of 0.2 mm, the calculated piston pressure may reach the range of hydraulic pressure: 199,700 Pa. This may explain some of the postoperative pain that patients often experience because such pressures are likely to push small volumes of irrigant and/ or debris beyond the apical foramen [39].

Mode of Action of Sodium

Hypochlorite

As an irrigant, sodium hypochlorite is used (a) to dissolve vital or necrotic pulp tissue that remained in the recesses of the canal after instrumentation and (b) to kill bacteria that may be present in the canal. However, during this process, sodium hypochlorite is gradually inactivated [14, 19, 34].

When pulp tissue is inserted into a test tube containing sodium hypochlorite, sodium hypochlorite quickly dissolves the tissue [75, 96]. In such conditions, the volume of sodium hypochlorite is inÞnitely larger than that of the pulp tissue, and the inactivation of the solution may not be noticeable. However, in vivo, in the presence of inßammatory exudate, pulp tissue, and microbial remnants, the action of sodium hypochlorite on such substances may consume, weaken, and inactivate sodium hypochlorite [35].

When placed in a root canal, the volume of the sodium hypochlorite is rather limited (~10 μL in the maxillary central incisors), and when pulp tissue or bacteria are present, sodium hypochlorite may be quickly consumed and inactivated. Therefore, simple ßooding the canal with sodium hypochlorite during the procedure may be ineffective. Frequent replacement of the irrigant is commonly suggested to maintain the desired activity [6, 35]. When a syringe and needle irrigation is applied, fresh, fully active sodium hypochlorite may be present during the irrigation process, but only up to 2 mm from the distance at which the needle can be inserted. This implies that as long as the needle cannot be safely inserted to WL, no fully active sodium hypochlorite will be present at the apical part of the canal [64]. Any amount of sodium hypochlorite that seeps into this area will be readily inactivated. Thus during traditional endodontic procedures, with intermittent irrigation, the total time that fully active sodium hypochlorite is present at the apical part of the canal is limited.

When negative pressure irrigation is considered, the size of the canal during the instrumentation process is also a limiting factor. Only when the apical part is sufÞciently enlarged and the small cannula is inserted to WL can the fully active sodium hypochlorite reach this area.

Mode of Action of EDTA

EDTA is often used in endodontic treatment protocols. Some use it to soften the dentin walls of the canal to facilitate instrumentation, while