_Influence of Passive Sonic Irrigation Systems in Eliminating Canal Bacteria_Summary

12/20/13 /Influence of passive sonic irrigation systems in eliminating canal bacteria/Summary

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Canal Cleaning Techniques

As stated earlier in this chapter, irrigants and other intracanal medicaments are necessary adjuncts that enhance theantimicrobial effect of mechanical cleansing and thus augment overall clinical efficacy.[79-81] It is well established thatlarge areas of canal walls, particularly in the apical third but also in ribbon-shaped and oval canals, cannot be cleanedmechanically,[188,308,310,408,456,464] meaning that microbiota present in these untouched areas could survive (see Figs.9-40, 9-45, and 9-47). Residual bacteria and other microorganisms exist both in these hard-to-reach spaces and indentinal tubules.[171,300,342] Chemical disinfection is an important cornerstone of a successful outcome because it isdirected towards elimination of microorganisms present in dentinal tubules and in the crevices, fins, and ramifications of aroot canal system.[287],[449] In one study, investigators prepared root canals, irrigated with saline solution, and sampledbefore, during, and after instrumentation.[112] They then cultivated and counted colony-forming units. These researchersfound that with instrumentation alone, progressive filing reduced the number of bacteria regardless of whether rotary orstainless steel hand instrumentation was used. However, no technique resulted in bacteria-free canals. Siqueira et al.[380]

confirmed this finding; they found that instrumentation combined with saline irrigation mechanically removed more than90% of bacteria in the root canal. Many authors have stressed the importance of using antimicrobial irrigants duringchemomechanical preparation to ensure complete disinfection.[384]

Substances that have been used to rinse and chemically clean root canals have different purposes, such as dissolution ofsoft and hard tissues, antimicrobial effect against bacteria or other microorganisms in the root canal, and inactivation ofbacterial lipopolysaccharides. These substances also should be as nontoxic as possible to protect the periradiculartissues. Unfortunately, solutions that are toxic for bacterial cells frequently are toxic for human cells as well, so caremust be taken to avoid extrusion of irrigants into periapical regions.[70],[190]

Several factors are important for efficient root canal irrigation. One critical factor is the volume of irrigant. In a studyevaluating the effect of different amounts of fluids, the volume of irrigant was found to affect the cleanliness of the rootcanal.[435],[470] NaOCl and EDTA administered in larger volumes produced significantly cleaner root canal surfaces thansmaller volumes.[470]

Irrigation Modes and Devices

There is a common consensus that root canal irrigants are indispensable aids in dissolving and inactivating organic debrisand destroying microorganisms. In addition, some agents allow removal of a postpreparation smear layer in order to allowaccess to dentinal tubules. Several methods of employment of an irrigant inside the canal space are available.

Syringe Delivery

Application of an irrigant into a canal by means of a syringe allows exact placement, replenishing of existing fluid, rinsingout of larger debris particles, as well as allowing direct contact to microorganisms in areas that are reached by the needletip. The actual exchange of irrigant is restricted to 1 to 1.5 mm apical to the needle tip, with fluid dynamics taking placenear the needle outlet.[481] This was the case even if the needle tip diameter was three ISO sizes smaller than thediameter of the apical preparation.[64] Volume and speed of fluid flow are proportional to the cleansing efficiency inside aroot canal. Therefore, both the diameter and position of the needle outlet determine successful chemomechanicaldébridement; placement close to working length is required to guarantee fluid exchange.[64,185,262]

The choice of an appropriate irrigating needle, therefore, is important. Although larger-gauge needles allow the irrigant tobe flushed and replenished more quickly, the wider needle diameter does not allow cleaning of the apical and narrowerareas of the root canal system (Fig. 9-70). Excess pressure or wedging of needles into canals during irrigation with nopossibility of backflow of the irrigant should be avoided under all circumstances[190] to prevent extrusion of the irrigant intoperiapical spaces. In juvenile teeth with wide apical foramina or when the apical constriction no longer exists, special caremust be taken to prevent resorption or overpreparation of the root canal.[107]

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FIG. 9-70 Irrigation needles inserted into prepared root canals. A-B, A 27-gauge needle barely reaches the middle third. C-D, A 30-

gauge, side-venting needle reaches the apical third (see Figure 9-43).

Another aspect is closeness of the needle tip or outlet to the apical endpoint of canal preparation to allow direct proximityof fresh irrigant to canal walls.[64],[366] In that respect, size of the irrigation needle,[97] as well as apical size and taper ofroot canal preparation,[89,264,280] play a role in allowing contact of irrigants to adjacent canal areas. Most root canals thathave not been instrumented are too narrow to be reached effectively by disinfectants, even when very fine irrigationneedles are used (see Figs. 9-44 and 9-70). Therefore, effective cleaning of the root canal must include intermittentagitation of the canal content with a small instrument[258],[437] to prevent debris from accumulating at the apical portion ofthe root canal (see Fig. 9-42).

Preparation size[264] and taper[102] ultimately determine how close a needle can be placed to the final apical millimetersof a root canal. Some needles and suction tips may be attached to the air/water syringe to increase both the speed ofirrigant flow and the volume of irrigant. Examples include the Stropko Irrigator (Vista Dental Products), which is an adapterthat connects to the air/water syringe and accepts standard Luer-lock needle tips for irrigant removal and application aswell as air drying.

Manually Activated Irrigation

Irrigant that is placed inside the root canal more effectively reaches crevices and mechanically untouched areas if it isagitated inside the root canal. Coronoapical movements of the irrigation needle,[190] stirring movements with smallendodontic instruments,[258],[437] and manual push-pull movements using a fitted master gutta-percha cone have beenrecommended.[186]

Sonically Activated Irrigation

In one study,[341] investigators suggested that both passive sonic or ultrasonic irrigation rendered root canals significantlycleaner than manual preparation. Débridement of root canals supported by sonically and ultrasonically activated irrigation



Update: New Content Added Date Added: 30 December 2010

was superior to passive needle application of irrigants.[77],[439] However, in comparison to sonic activation, ultrasonicirrigation produced significantly cleaner canals.[200],[341] Other investigators discovered no significant difference indébridement between sonic or ultrasonic fluid activation inside a root canal. The difference lies in the oscillatingmovements: sonic devices range between 1500 Hz and 6000 Hz, and ultrasonic equipment requires vibrations greaterthan 20,000 Hz.[200,241,401] Sonic or ultrasonic irrigation may be carried out with activated smooth wires or plastic inserts,endodontic instruments, or activated irrigation needles. Examples include EndoSonor (DENTSPLY Maillefer) andEndoSoft ESI (EMS, Nyon, Switzerland) inserts, the EndoActivator System (DENTSPLY Tulsa Dental), and the Vibringesonic syringe (Vibringe, Amsterdam, Netherlands).

Ultrasonically Activated Irrigation

Ultrasonically powered instruments have become indispensable now, with well-adapted tips from various manufacturers.During preparation, ultrasonic tips are able to remove minimal amounts of dentin, conserving as much tooth structure aspossible. Visibility is better than with burs, and the tips can be diamond coated to increase their efficiency. However, alltips develop significant heat that is transferred through dentin walls and can cause necrosis of surrounding bone if usedwithout coolant. The temperature elevation also takes place during use of ultrasonic power during root canal irrigation[10],

[195] and improves the antibacterial effect through warming of the irrigating solution.[486]

Ultrasonic action is most effective if the file is able to oscillate freely inside a given root canal.[240] Passive ultrasonicirrigation is defined as activation of the rinsing agent without simultaneous preparation of the root canal walls.[436],[486]

Passive ultrasonic irrigation is believed to promote tissue removal and tissue dissolution and may be done with a smoothwire insert that will avoid damaging canal walls and altering the shape in an undesirable way.[439] This strategy allowscleaning of isthmus areas, fins, or C-shaped canals by acoustic streaming and to a lesser extent cavitation, as well as (tosome degree) other hard-to-reach areas such as dentinal tubules or lateral canals. Disinfection is rendered more effective,an important consideration in necrotic cases.[10],[90] Kuah et al.[217] demonstrated that to eliminate smear layer anddebris in the apical region of a prepared root canal, a 1-minute application of EDTA with ultrasonics followed by a finalflush of NaOCl was the most proficient method.

Influence of passive sonic irrigation systems in eliminating canal bacteriaLouis H. Berman, DDS, FACD: Web Editor

Summary

Clinical adjuncts to assist in bacterial elimination during endodontic irrigation have been plentiful over the years.Ultrasonically activated files were introduced about 30 years ago. Because of the corrosive nature of sodiumhypochlorite, however, devices have attempted to take advantage of the effectiveness of activating the irrigant withoutactually running the irrigant through the device. The EndoActivator provides a subsonic activation of the irrigant byplacing an active plastic file-shaped device into the canal in the presence of an irrigant. Following manufacturer'sinstructions, the EndoActivator was unable to produce canals with negative cultures.

References

1. Huffaker SK, Safavi K, Spangbert LS, et al: Influence of a passive sonic irrigation system on the elimination ofbacteria from root canal systems: A clinical study. J Endod 2010; 36(8):1315-1318.

Negative- and Positive-Pressure Irrigation

Another approach to afford better access of irrigation solution is so-called negative-pressure irrigation. Here, irrigant isdelivered into the access chamber, and a very fine needle connected to the dental unit's suction device is placed into theroot canal. Excess irrigant from the access cavity is then transported apically and ultimately removed via suction. First, amacrocannula, equivalent to an ISO size #55, .02 taper instrument, removes coronal debris. Subsequently, amicrocannula, equivalent to a size #32, .02 taper, removes particles lodged close to working length. Such a system iscommercially available (EndoVac, Discus Dental) and may prove a valuable adjunct in canal disinfection.[281]

Another device that makes use of pressure-suction technology is the RinsEndo system (Dürr Dental, Bietigheim-Bissingen, Germany). It aspirates the delivered rinsing solution into an irrigation needle that is placed close to workinglength and at the same time activates the needle with oscillations of 1.6 Hz amplitude. This system has been investigatedby several authors and was superior to customary needle irrigation in cleaning and disinfection.[67],[262] Researchers[262]

used 30-gauge irrigation needles in canals shaped to ISO size #40 and compared irrigation with the RinsEndo system toconventional needle irrigation and so-called manual-dynamic irrigation that involves pumping action of a fitted mastergutta-percha cone inside a root canal; in this study, 200 push-pull strokes were delivered. The cleaning effect of theRinsEndo unit was found to be superior to conventional needle irrigation, but the least amount of residual debris was found



after hand agitation of a master cone.

Eradication of Microorganisms

Various types of microorganisms—bacteria,[205,215,270,272] yeasts,[449],[450] and possibly viruses[339],[340]—can infect thepulp and may lead to apical periodontitis (see Chapters 14 and 15 and Fig. 9-7). These microorganisms must bereduced or eliminated to reestablish periradicular health. When bacterial samples test negative after treatment, theprognosis is improved.[381],[387] During mechanical root canal preparation, endodontic instruments are used to clean andenlarge root canal systems. Rotating instruments have an additional advantageous “Archimedes screw” effect by whichdebris is transported in an apicocoronal direction.[112] Even when simple saline was used as an irrigant, a 10-fold to 1000-fold reduction of the bacterial load through mechanical instrumentation was demonstrated.[80,112,288]

However, as noted earlier, instrumentation alone does not produce a bacteria-free root canal. In one study, dentinsamples tested positive in most of the teeth after mechanical instrumentation, even though bacteria had been eliminatedfrom the root canals in some cases.[80] In that study, bacteria persisted in seven root canals despite mechanical cleaningand saline irrigation during five consecutive appointments. Moreover, teeth with a high number of bacteria in the initialsample remained infected despite being treated five times.[80] In another study, teeth that caused symptoms tended tohave more bacteria than teeth with no clinical symptoms.[288]

Other researchers[287] investigated the effect of endodontic irrigants and dressings in standardized bovine dentinspecimens that were infected with test bacteria. They found that bacteria were capable of colonizing the canal lumen anddentinal tubules. In the specimens used, E. faecalis rapidly infected the whole length of the tubules, whereas Escherichiacoli penetrated approximately 600 µm. They also found that IKI appeared to be more effective at destroying bacteria thanNaOCl, which was more effective than CHX.

Other investigators have explored the effects of NaOCl (with and without EDTA), CHX, and hydrogen peroxide in varyingconcentrations when used in sequence or in combination as endodontic irrigants.[181] They found that CHX and NaOClwere similarly effective in eliminating the bacteria tested. Synergistic effects were observed for some of the irrigants (e.g.,CHX and IKI).

Both of the preceding studies used infected dentin specimens. When evaluating literature about antimicrobial efficacy,clinicians must keep in mind that most disinfecting solutions are inhibited or even inactivated by contact with dentin ordentin powder during root canal preparation.[169],[317] Moreover, chemical interactions occur between irrigation solutions;for example, NaOCl can become ineffective if it comes in contact with EDTA[161] (Fig. 9-71).

FIG. 9-71 Interaction of sodium hypochlorite w ith various factors that determine its eff icacy.

Some of the more difficult to remove endodontic pathogens that can cause treatment failure are enterococci,Actinomyces, and Candida organisms[47,276,277,378,462] (see Chapter 15). Table 9-1 presents the results of a number ofstudies evaluating the effectiveness of some commonly used antimicrobial agents.

Currently, the endodontic irrigation solution with the best proteolytic effect is NaOCl,[481] even though it does not meet allthe requirements of an ideal irrigant (Box 9-6). It is readily available, inexpensive, and consequently a widely usedirrigation solution. As mentioned previously, necrotic tissue and debris are dissolved by the breakdown of proteins intoamino acids through free chlorine in NaOCl. However, because unbound chlorine is the important component, the solutionmust be replenished frequently during preparation to compensate for lower concentrations and to constantly renew the



fluid inside the root canal. This is even more important when the root canal is narrow and small and files must carry theNaOCl to the apical third during instrumentation (see Fig. 9-44). A 1% solution is effective at dissolving tissue andproviding an antimicrobial effect. The use of 6% commercial household bleach in its undiluted form causes substantialnecrosis of wound surface areas and may result in serious clinical side effects (Fig. 9-72). It is diluted in 1:1 or 1:3 ratioswith water to produce a 2.5% or 1% solution; both are suitable for clinical endodontic use.[398,477,482]

BOX 9-6

Properties of an Ideal Irrigant for Root Canal Treatment

The irrigant should:

♦ Be a highly effective disinfectant

♦ Be nontoxic locally and nonallergenic

♦ Differentiate between necrotic and vital host tissue

♦ Retain its effectiveness with dental hard tissue and when mixed with other irrigants

FIG. 9-72 Toxic effect of sodium hypochlorite on periradicular tissues. After root canal treatment of tooth #3, the patient reported pain. A, On a

return visit, an abscess w as diagnosed and incised. B, Osteonecrosis w as evident after 3 w eeks.

Importantly, to avoid extrusion and serious damage to periapical tissues, irrigation needles should never be wedged intocanals during irrigation.[70] Higher concentrations of NaOCl are more aggressive toward living tissue and can cause severeinjuries when forced into the periapical area (see Fig. 9-72).

Such accidents can be prevented by marking the working length on the irrigation needle with a bend or a rubber stop andby passively expressing the solution from the syringe into the canal (see Fig. 9-70). The needle should be continuouslymoved in an up-and-down motion. It should remain loose in the canal, allowing a backflow of fluid. The goal is to rinse thesuspended, concentrated dentinal filings out of the pulp chamber and root canals as new solution is brought down into themost apical areas by the endodontic instrument and the capillary effect.

As stated before, patency files should be used carefully and should not be extended farther than the periodontal ligament,because they are possible sources of irrigant extrusion.

In one study, heating increased the antibacterial action of NaOCl.[104] Heating can be done in several ways; for example,after the solution has been drawn into the irrigating syringe, a syringe warmer can be used (e.g., Syringe Warmer [VistaDental Products, Racine, WI]) (Fig. 9-73). Heating also enhanced the antibacterial effectiveness of CHX and Ca(OH)2solutions.[131] A 0.5% NaOCl solution heated to 113° F (45° C) dissolved pulp tissue as efficiently as a 5.25% solutionused as the positive control (Fig. 9-74).[386] Heating to 140° F (60° C) resulted in almost complete dissolution of tissue.Studies have shown that 1 minute at 116.6° F (47° C) is the cutoff exposure at which osteoblasts can still survive;however, higher temperatures may in fact be sufficient to kill osteoblasts and other host cells.[128],[129] Also, warming ofNaOCl to 122° F (50° C)[53] or 140° F (60° C)[5] increases collagen dissolution and disinfecting potential, but it may alsohave severely detrimental effects on NiTi instruments, causing corrosion of the metal surface after immersion for 1 hour[51]

(Fig. 9-75).



FIG. 9-73 Device for heating syringes f illed w ith irrigation solution (e.g., sodium hypochlorite) before use.

FIG. 9-74 Effect of heating on the ability of 0.5% sodium hypochlorite (NaOCl) to dissolve pulp tissue: NaOCl heated to 113° F (45° C) dissolved

pulp tissue as w ell as the positive control (5.25% NaOCl) did. When the NaOCl w as heated to 140° F (60° C), almost complete dissolution of

tissue resulted.

(Modified from Sirtes G, Waltimo T, Schaetzle M, Zehnder M: The effects of temperature on sodium hypochlorite short-term stability, pulp dissolution capacity, and antimicrobial efficacy. J Endod 31:669–671, 2005.)



FIG. 9-75 Corrosion of nickel-titanium files in heated sodium hypochlorite (NaOCl). A, Rotary instrument immersed for 2 hours in NaOCl heated

to 140° F (60° C). B, Magnif ication of rectangular area in A, show ing severe corrosion.

As stated before, an increase in the temperature of the irrigant may be another reason to include ultrasonic devices incanal irrigation[10],[486]; using these devices may also increase the tissue-dissolving capabilities of NaOCl via temperatureelevation,[5] but this effect seems to be limited to the main canal.[10] In one study, peak irrigant temperatures duringultrasonic irrigation reached 113° F (45° C) near the file tip but remained at 89.6° F (32° C) on the outer root surface ofteeth prepared to a size #45.[86] The effect seems to depend on the insert,[486] possibly due to its oscillation patterns.

Yet another reason for using ultrasonic devices might be enhancement of canal débridement. However, some authorshave reported no significant effect with ultrasonics, neither in débriding root canal walls[96,108,258] nor in reducing bacterialcounts.[118],[382] However, the majority of available studies appear to find both bacterial reduction[194] and improveddébridement,[167] as reviewed recently.[438]

Smear Layer Management

EDTA is a decalcifying chelating agent used as a 15% to 17% buffered solution during instrumentation of root canals. Thedecalcifying efficacy of EDTA-containing pastes is variable.[160],[441] EDTA acts as a chelator with calcium ions andremoves the dentinal debris produced on the root canal walls during preparation. It thus opens dentinal tubules, promotingbetter penetration of disinfectants.[154,191,422,485] Whenever the wall of a root canal is instrumented, whether by hand orrotating instruments, the parts of a dentin wall touched by an instrument are covered by a surface layer called the smearlayer.[261],[296] The smear layer, which consists of dentin shavings, cell debris, and pulp remnants,[369] can be describedas itself having two separate layers: a loose, superficial deposit and an attached stratum that extends into the dentinaltubules, forming occluding plugs.[87]

For some time, clinicians and researchers paid little attention to the smear layer, partly because it was a thin superficial



layer (1 to 5 µm) that might be present or not, depending on the type of instrument and the sharpness of its cuttingblades.[369] Also, because acids and chelating agents dissolve the smear layer, it was removed and escaped attention inroutinely processed specimens (Fig. 9-76).[109] Smear layers are not seen in unprepared canal areas, which may havecalcospherites, buttonlike structures that are abundant on intracanal surfaces.

FIG. 9-76 Prepared root canal surfaces after irrigation, show ing varying degrees of smear layer. A, Scanning electron micrograph (SEM, ×25)

show ing prepared areas w ith and w ithout open dentinal tubules. The presence of calcospherites indicates that no mechanical preparation has

been done laterally (arrow). B, SEM, ×400, show ing thin, homogenous smear layer and scattered debris in a canal that received a f inal

sequence of a high-volume flush of 17% ethylenediamine tetra-acetic acid (EDTA) follow ed by 2.5% sodium hypochlorite (NaOCl).

Some authors have reported that an overlying smear layer delays but does not eliminate the effect of medicaments.[287]

Others contend that a smear layer may adversely affect disinfection and may also increase microleakage after canalobturation.[369] Although organic substrate in a smear layer may serve as a nutrition source for some species of bacteria,[65],[297] some have suggested the converse: that a smear layer can act as a beneficial barrier, preventing microorganismsfrom entering the dentinal tubules when a root canal is colonized by bacteria between appointments.[121] The potential ofintracanal disinfectants has been evaluated in vitro after removal of the smear layer with a combination of 5.25% NaOCland 17% EDTA.[171] The decalcifying effect of EDTA is self-limiting, so the solution must be replaced at intervals.[191]

EDTA can help open very narrow root canals and can decalcify to a depth of approximately 50 µm. Because the smearlayer consists of organic and inorganic components, the combined use of NaOCl and EDTA, with time frames of 30seconds to 60 seconds each, is considered most effective for its removal.[83] In general, it seems beneficial to remove thesmear layer in the later phases of endodontic therapy rather than during the early phases. Research continues on ways toimprove the effectiveness of irrigation. For example, tensides were added to irrigants more than 20 years ago to reducetheir surface tension, thereby improving wetability.[6] The rationale for this increased wetability was to improve thepenetration of irrigants into the dentinal tubules,[409] and this concept is still pursued with MTAD today. One irrigation“cocktail” investigated was a mixture of 5% NaOCl and 17% EDTA with the tensioactive chemical Triton X-100; thissolution was used with ProFile instruments.[7],[144] The study reported that apical smear layer scores were significantlylower compared with those of control groups when the tensioactive agent was used throughout the preparation process.However, a more recent study found no effect of a wetting agent on the action of EDTA, measured as calcium in eluatesfrom root canals in vitro.[484] Rather than surfactant, ultrasonic irrigation appears to promote smear layer removal byEDTA.[238]

Liquid disinfectants were effective against E. faecalis in dentinal tubules up to depths of 400 µm. Microbiologic analyses



of split root halves showed that early removal of the smear layer resulted in significantly higher bacteria counts.[121] Incontrast, other researchers have acknowledged that the smear layer, while acting as a barrier, might block irrigationsolutions from entering the dentinal tubules.[42] Moreover, some bacteria (e.g., Bacteroides gingivalis and Treponemadenticola) have the potential to dissolve smear layer proteins,[432] thereby producing gaps which could promote bothcoronal and apical microleakage and bacterial multiplication.

Fig. 9-77 shows root canal cross sections with very little debris; irrigating solutions can penetrate the dentinal tubules inthis example. Some reported that the presence of the smear layer had no significant effect on apical leakage in dyepenetration testing.[132],[247] Others described an improved seal after removal of the smear layer.[43],[411] The latter study,which used a coronal leakage model, found a significantly decreased incidence of bacterial penetration (30% versus 70%)when the canals were irrigated with 17% EDTA and 5.25% NaOCl before obturation. In obturated root canals, a remainingsmear layer led to bacterial leakage in 60% of the samples versus no leakage when the smear layer was removed.[101]

Other authors had similar results after smear layer removal with EDTA solution alone. Another investigation[421] found thatmany lateral canals in the apical third of root canal systems cleaned with a barbed broach wrapped with MTAD-soakedcotton showed less erosion than when EDTA was used. Other studies have found that a stronger bond was present whenthe smear layer was removed,[148] and a statistically significant reduction of microleakage was also measured.[124,349,350] Still another investigation reported increased apical microleakage of the filled root canal after removal of thesmear layer.[419]

FIG. 9-77 Example of canals w ith minimal smear layer. A, Middle third after irrigation w ith 17% ethylenediamine tetra-acetic acid (EDTA) and

2.5% sodium hypochlorite (NaOCl). B, Apical third w ith some particulate debris.

As shown in Fig. 9-43, dye staining is improved when the dentinal tubules are opened and the smear layer is removedwith EDTA, at least in the two more coronal levels. However, this effect is at least partly due to tubular sclerosis and theoverall lower number of tubules apically, resulting in fewer diffusion pathways across dentin.[292]

Although the effect of the smear layer on leakage has been widely studied, its removal from root canal walls remainscontroversial. The apparently conflicting results of studies could stem from differences in the various microleakage testmodels[467] and from different obturation and irrigation techniques. The problem of coronal leakage has received muchattention as a major factor in determining the success or failure in root canal therapy.[47,327,330,331,428]

In two in vitro studies, the noninstrumentation technique, which relies on activated irrigation solutions rather than



mechanical preparation, produced excellent canal cleanliness[244],[245] (see Fig. 9-10, A). However, preliminary clinicalstudies identified a need for improvement before this system can be used routinely to clean root canals.[27]

For the time being, root canals must be mechanically enlarged before irrigation. Larger apical preparations enhance theefficacy of irrigation, and the additional use of ultrasonic energy during cleaning and shaping may also increase theefficacy of endodontic irrigants.[87,88,105,224] Ultrasonics used passively in canals with sufficiently large apicalpreparations may reach and better clean any uninstrumented canal areas.[241,463,465] One investigation studied thedébriding ability of 2.5% NaOCl in canal recesses.[105],[106] In 10 of 11 cases, these researchers found significantlycleaner histologic sections after ultrasonically activated irrigation. In the ultrasonically treated group, the bacteria countwas reduced by 99.8%, but hand filing alone reduced the bacteria count by 99.3%, so the improvement from ultrasonictherapy was limited. With ultrasonics, root canals are débrided by shear stresses produced between the irrigant and thecanal wall, with subsequent cell disruption.[9]

Acoustic streaming of the irrigation fluid through ultrasonic treatment has been suggested as a method of improvingcleanliness. However, this effect occurs mainly in the most coronal levels; the apical areas were least affected byactivated irrigation[96],[258] (see Fig. 9-43). Because the amplitude of the oscillation is greatest at the instrument's tip,attenuation and constraint most significantly affect the apical part,[447] where the diameter of the canal is smallest.

One investigator reported that the most effective regimen with ultrasonic energy was to activate every dose of irrigantplaced in the canal.[85] With this approach, roughly 18 minutes of irrigation is required per canal. Other investigators usedan irrigation time of 1 minute each for EDTA and NaOCl, which seems clinically more practical.[83],[258] These authorsstated that the use of ultrasonic energy for irrigant activation did not improve débridement compared with control groups.However, bacterial species show varying degrees of susceptibility to ultrasonication.[8,38,255,390]

Because of the conflicting evidence concerning the effectiveness of ultrasonics in root canal therapy, other methods ofdisinfecting and débriding canals properly must be studied. Such research might include better ways to deliver irrigantsand disinfecting solutions.

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_Influence of Passive Sonic Irrigation Systems in Eliminating Canal Bacteria_Summary