Table 9-1. Activity of Various Irrigants against Microorganisms*
Enterococci 3 min at 0.0005% solution in filter paper specimens
15 min at 0.25% solution in contaminated dentin blocks331 30 min at 0.5% solution and 2 min at 5.25% solution in direct contact with bacteria214
7 days of 0.5% dressing resulted in complete killing in dentin blocks up to full depth of 950 μm259 24 hours to reduce cultured bacteria below detection limit211
24 hours of iodine (2%) exposure in potassium iodide (4%) resulted in complete killing in dentin blocks up to a depth of 700 μm259 1 hour to reduce bacteria under 0.1% and 24 hours to reduce bacteria below detection limit; however, loss of activity noted through dentin powder211
5 min application resulted in no growth on infected dentin251 MTAD was as efficient as 5.25% NaOCl in cultures290
24 hours to reduce cultured bacteria below detection limit, but activity was inhibited by dentin powder, hydroxyapatite, and serum albumin211 7 days to render canals bacteria free50 but showed little effect on Enterococcus faecalis50; resulted in complete killing in dentinblocks up to full depth of 950 μm259 7 days of Ca(OH)2 in 0.5% chlorhexidine acetate dressing resulted in complete killing in dentin blocks up to full depth of 950 μm259
1 min at 1% solution20 10 sec at 0.5% solution in direct contact with bacteria214
No growth directly after rinsing with 2% CHX in patients with necrotic pulps and/or apical granuloma85
1 hour at 1% or 5% solution on root dentin with smear layer246 30 sec for both the 0.5% solutions to kill all cells in culture305
1 hour at 0.12% solution on root dentin with smear layer246 10 sec at 0.5% solution in direct contact with bacteria214 5 min at 0.5% solution to kill all yeast cells and 1 hour at 0.05% solution; less effective than IKI and NaOCl305
30 sec for both 2% and 4% solutions to kill all cells in culture; 0.2% and 0.4% solutions were as effective as 0.5% CHX305
After 1 hour and 24 hours only a small reduction of CFU was observed305
*Effects achieved by killing through antimicrobial action.
Figure 9-38 Remaining potentially infected tissue in fins and isthmus configuration after preparation with rotary instruments. A, Cross section through a mesial root of a mandibular molar, middle to coronal third of the root. Both canals have been shaped; the left one is transported mesially. (×10.) B, Magnified view of rectangle in A. Note the presence of soft tissue in the isthmus area. (×63.) (Courtesy Professor H. Messer.)
Commercially available household bleach contains 5.25% NaOCl, has an alkaline pH of 12 to 13, and is hypertonic.271,332 Some authors recommend dilution of commercially available NaOCl with 1% bicarbonate instead of water to adjust the pH to a lower level.74,271 Others do not see any reduction of the aggressiveness on fresh tissue by buffering NaOCl and recommend diluting solutions of NaOCl with water to obtain less concentrated irrigation solutions.332
NaOCl only minimally removes dentin or smear layer (Fig. 9-39); therefore, some recommend concurrent use of demineralizing agents to enhance cleaning of difficult-to-reach areas, such as dentinal tubules and lateral canals.50,185
Chlorhexidine (CHX) is a broad-spectrum antimicrobial agent effective against gram-negative and gram-positive bacteria (see Table 9-1). It has a cationic molecular component that attaches to negatively charged cell membrane areas, causing cell lysis. CHX has been used in periodontal therapy for many years. Its use as an endodontic irrigant59,88,123 is based on its substantivity and long-lasting antimicrobial effect, which arises from binding to hydroxyapatite. However, it has not been shown to have clinical advantages over NaOCl.
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Figure 9-39 Surface textures of an unprepared root canal at various levels. A, Ground section of a mandibular premolar. Areas viewed by scanning electron microscopy (SEM) are indicated by black lines. B, Canal surface at middle section, showing open dentinal tubules and typical calcospherites. (×500.) C to E, Coronal, middle, and apical areas as compound scanning electron micrographs. Note the numerous open tubules in C and D, whereas fewer tubules are visible in E. (×200.)
Some researchers found that CHX had significantly better antibacterial effects than calcium hydroxide Ca(OH)2 when tested on cultures.156 Effective combinations of CHX and Ca(OH)2 are available and show strong antimicrobial activity against obligate anaerobes, the combination augmenting the antibacterial effect of either medicament on certain species.209,260 The addition of CHX or iodine potassium iodide to an intracanal dressing of Ca(OH)2 in vitro did not affect the alkalinity (and hence the efficacy) of the calcium hydroxide suspensions.260
Iodine Potassium Iodide
Iodine potassium iodide (IKI) is a traditional root canal disinfectant. IKI kills a wide spectrum of microorganisms found in root canals (see Table 9-1) but shows relatively low toxicity in experiments using tissue cultures.270 Iodine acts as an oxidizing agent by reacting with free sulfhydryl groups of bacterial enzymes, cleaving disulfide bonds. E. faecalis often is associated with therapy-resistant periapical infections (see Chapter 15), and combinations of IKI and CHX may be able to kill calcium hydroxide-resistant bacteria more efficiently. A recent study by Sirén et al260 evaluated the antibacterial activity of a combination of calcium hydroxide with IKI or CHX in infected bovine dentin blocks. Although calcium hydroxide alone was unable to destroy E. faecalis inside dentinal tubules, calcium hydroxide mixed with either IKI or CHX effectively disinfected dentin. An obvious disadvantage of iodine is a possible allergic reaction in some patients.
New chemicals for irrigating root canals are constantly developed, including solutions based on antibiotics. Use of these irrigants is controversial, however, because of the emergence of increasingly resistant strains of bacteria (e.g., therapy-resistant enterococci), which may be due to overprescription of antibiotics in general. The increased risk of host sensitization by local antibiotics can be circumvented to some degree by using the antibiotic as a dressing. Because exposure to vital tissues is limited, higher microbicidal concentrations may be used.177 A number of antibiotics, including erythromycin, chloramphenicol, tetracycline, and vancomycin, have been tested successfully against enterococci. In one study, investigators evaluated microbial susceptibility to different antibiotics in vitro; they found that enterococcal isolates were resistant to benzylpenicillin, ampicillin, clindamycin, metronidazole, and tetracycline but sensitive to erythromycin and vancomycin.73 MTAD (Dentsply-Tulsa), a recently introduced irrigation solution, contains doxycycline, citric acid, and a surface-active detergent (Tween 80).291 In vitro experiments indicate that MTAD has potential for removing the smear layer,26,251,289 but clinical benefits have yet to be demonstrated.
Ethylenediamine Tetra-Acetic Acid
EDTA came into use in endodontics in 1957,185 whereas NaOCl has been in use for more than 70 years.303 Chelators such as EDTA create a stable calcium complex with dentin mud, smear layers, or calcific deposits along the canal walls. This may help prevent apical blockage (Fig. 9-40) and aid disinfection by improving access of solutions through removal of the smear layer.
Neutral EDTA showed a higher degree of decalcification of dentin surfaces than RC-Prep, although its effect was reduced in apical regions.301 Similar to MTAD, RC-Prep did not erode the surface dentin layer.289
The effect of chelators in negotiating narrow, tortuous, calcified canals to establish patency depends both on canal width and on the amount of active substance available as the demineralization process continues until all chelators have formed complexes with calcium.129 Calcium binding results in the release of protons, and EDTA loses its efficiency in an acidic environment. Thus the action of EDTA is thought to be self-limiting.246 In one study, demineralization up to a depth of 50 μm into dentin was demonstrated for EDTA solutions129; however, reports demonstrated significant erosion after irrigation with EDTA.289
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Figure 9-40 Presence of dentin dust as a possible source of microbial irritation. Tooth #18 underwent root canal therapy. The clinician noted an apical blockage but was unable to bypass it. Unfortunately, intense pain persisted and at the patient's request, the tooth was extracted a week later. A, Mesial root of tooth #18; mesial dentin has been removed. B, Magnified view (×125) of rectangle in A shows an apical block (gradation of ruler is 0.5 mm).
A comparison of bacterial growth inhibition showed that the antibacterial effects of EDTA were stronger than citric acid and 0.5% NaOCl but weaker than 2.5% NaOCl and 0.2% chlorhexidine.257 EDTA had a significantly better antimicrobial effect than saline solution; it exerts it strongest effect when used synergistically with NaOCl, although no disinfecting effect on colonized dentin could be demonstrated.123
Recent reports have indicated that several disinfecting agents such as Ca(OH)2, IKI, and CHX are inhibited in the presence of dentin.112,211,212 Moreover, chemical analyses indicated that chlorine, the active agent in NaOCl, is inactivated by EDTA.107,333
In light of these facts, in addition to the unproven effect of lubricants containing EDTA on rotary instrument torque, use of these solutions probably should be limited to hand instrumentation early in a procedure. Moreover, an EDTA solution preferably is used at the end of a procedure to remove the smear layer.322,333 This and/or sufficient volume of NaOCl ensures high disinfecting efficacy by enabling NaOCl to penetrate even into deeper dentin layers (Fig. 9-41).