IRRIGATION SAFETY VS. EFFECTIVENESS
The dentist wants to irrigate effectively in order to have a maximal cleaning and antimicrobial effect. At the same time, there is a safety concern as extrusion of an irrigant—NaOCl in particular—in the periapical area would cause severe pain to the patient, as well as tissue destruction, even in large areas. Irrigation can be described as a balance between effectiveness and safety; in other words, a balance between apical clearance and apical pressure within the root canal (Figure 18).
Figure 18. During positive pressure irrigation the irrigants are exchanged 1-3 mm beyond the needle tip with most needle types. Optimally, the clearance should take place to working length (WL). However, the closer to WL the tip is, the higher apical pressure is created.
Recent studies that used different types of needles and different flow rates have shown some rather definitive results. For example, using a flow rate of 4-6 ml/min will reach a maximum clearance effect, while increasing the flow rate above 4-6 ml/min will not increase the clearance beyond the needle tip.25 However, apical pressure continues to grow in a linear fashion, even with flow rates higher than 6 ml/min. Therefore, it seems that from a combined safety and effectiveness point of view, irrigant flow rates should never be over 6 ml/min. Other safety factors include:
Keeping the needle tip free in the canal.
Avoid going any closer than 1-3 mm from the WL—depending on canal anatomy, needle type and flow rate.
Side vented “safety needles” (Figure 19) seem to give considerably lower apical pressures than open ended needles do. In curved canals, flexible needles with a fiber tip may offer an advantage in order to reach proper length with the needle (Figure 20). 25
Figure 19. An irrigation needle with a side vent and closed end produces low apical pressure, yet the clearance effect is reasonably good.
Figure 20. ”Flexi-Glide” needle has a flexible tip made of fiber that can easier reach proper depth in a curved canal.
NEGATIVE PRESSURE IRRIGATION
Most dentists are using the traditional “positive pressure” irrigation in their clinical work. It works quite well, but has the potential risk (although rare) of experiencing a hypochlorite accident. Therefore, a negative pressure approach has been developed, where the irrigants placed into the pulp chamber are sucked down the root canal, and back up again through the irrigation needle. In other words, the direction of irrigant flow has been reversed. “Endovac” is a commercial product using this approach.26,27 A number of published studies have demonstrated negative pressure irrigation to be both effective in cleaning the apical root canal, and safe as it works to completely prevent the possibility of irrigant extrusion into the periapical area. 26,27
ALTERNATING NaOCl & EDTA
Many dentists alternate NaOCl and EDTA rinsing during the root canal treatment. This may potentially have some less than optimal effects. First, EDTA inactivates NaOCl so that the tissue dissolving capability of NaOCl weakens considerably, or may be completely abolished.28 Secondly, if NaOCl is used again after EDTA has removed the smear layer (as the final disinfecting rinse), NaOCl will then cause erosion to the root canal wall dentin (Figure 21).29 Presently, it is not known whether this erosion has positive effects as the canal wall becomes even cleaner, or negative effects by weakening the dentin, which could potentially contribute to an increased possibility of vertical root fracture. Research on this matter is ongoing; at this point, though, it may be wise to avoid extensive and long-lasting NaOCl irrigation after EDTA.
Figure 21. Root canal wall irrigated with NaOCl followed by EDTA (left) or with NaOCl – EDTA – NaOCl (right). Final NaOCl irrigation after EDTA has caused strong erosion of the root canals wall. At the moment the importance of this erosion is now fully understood, but in worst case it may weaken the root.
Irrigation requires time to be effective. As improvements in instrumentation help the dentist to finish the preparation quicker than before, it is important to use this increased amount of “free time” for irrigation. For example, the disinfection of dentin with NaOCl requires this specific amount of time (Figure 22). Also, dissolving tissue debris and biofilms in the untouched areas do not happen very quickly.
Figure 22. Demonstration of NaOCl penetration into dentin. The dentin was first stained and the bleached with the NaOCl solution.
Commonly used, good irrigation protocols are the following combinations:
1) NaOCl during and after instrumentation, 2) EDTA as the final rinse.
1) NaOCl during and after instrumentation, 2) QMiX as the final rinse.
1) NaOCl during and after instrumentation, 2) Smear Clear as the final rinse.
1) NaOCl during and after instrumentation, 2) EDTA as the second rinse, 3) CHX as the final rinse.