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    removal of the smear layer /rotary endodontic courses by indian dental academy removal of the smear layer /rotary endodontic courses by indian dental academy Document Transcript

    • Removal vs Retention 121 REMOVAL OF SMEAR LAYER IN CONSERVATIVE DENTISTRY: The smear dilemma: Inside the cavity, the smear layer is “good news and bad news” or “damned if you do and damned if you don’t”. Microleakage is increased if the smear layer remains, whereas dentin permeability is increased if the smear layer is removed. While there is little equivocation concerning the necessity to remove the smear layer so as to optimize the bonding of restorative materials to enamel and dentin, an important dilemma exists concerning what is viewed as the protective role of such layers. Compromise may be possible in which the biological integrity of the pulp and dentin may be preserved by developing unique chemical formulations compatible with adhesive biomaterials. The protective effect of smear layer in tubule apertures and the consequence of removing the plugs: Vojinovic et al, (1973) reported that etching the cavity prior to the placement of composite resin resulted in a massive invasion of bacteria in the dentinal tubules The corresponding cavities, cleaned by water and with the smear layer left, had a bacterial layer on cavity walls but practically no invasion into the dentinal tubules. Obviously, smear plugs in the apertures of the tubules had prevented bacterial invasion Inflammation was present under all infected cavities being somewhat more pronounced under the etched cavities, but the difference was not great. The conclusion from Vojinovic's study was that smear plugs did not prevent bacterial toxins from diffusing into the pulp. The degree of inflammation in the pulp seems to depend on the amount and type of toxin, from both live and dead bacteria, reaching the pulp, rather than the presence of bacteria within the tubules. However, toxins, sometimes in combination with an unduly intense reaction, may lead to a local necrosis. From opened tubules, the bacteria may easily reach the pulp and multiply (Brannstrom, 1982). Therefore, removal of the smear plugs should be avoided. Pashley (1984) has also demonstrated that the smear plugs reduces the dentinal permeability.
    • Removal vs Retention 122 Another important consequence of etching and the removal of smear plugs and peritubular dentin at the surface is that the area of wet tubules may increase from 10-25% of the total (Garberoglio & Brannstrom, 1976; Johnson & Brannstrom, 1974). Subsequently, it is difficult to get the dentin dry because fluid continues to be supplied from below through the tubules. This moisture does not favor adhesive or mechanical bonding dentin. When a resin varnish, liner or restoration is allowed to set slowly, droplets and “lakes” may appear on the inner surface of the resin (Nordenvall, 1978). In sensitive dentin, the tubules are open all the way. It is better to keep them occluded with disinfected smear and with peritubular dentin preserved at the surface. The permeability is reduced and the cut dentin can be desiccated more easily with a blast of air. Pulpal irritation due to smear layer removal: An application of 50% citric acid or 37% phosphoric acid for even 5 seconds is sufficient to remove the smear plugs and the peritubular dentin at the surface (Brannstrom and Johnson, 1974, Nordenvall & Brannstrom, 1980) (Fig. 42). Other investigations have shown that even weaker acids may have the same capacity, especially if applied for 30-60 seconds (Bowen, 1978; Pashley, Michelich & Kehl, 1981). It was found that, 37% phosphoric acid or 50% citric acid applied for 15 seconds or one minute does not result in any appreciable pulpal reaction, inflammation or necrosis. This is true, even if we are very near the pulp or apply the acid to an exposed pulp for 15 seconds (Brannstrom & Nordenvall, 1978; Nordenvall, Brannstrom, & Torstenson, 1979; Torstenson, Nordenvall & Brannstrom, 1982). Acid etchants, detergents, a thin mix of phosphate cement, silicates, glass-ionomer cement and resins do not produce any appreciable damage and inflammation to the pulp, not even when applied to exposed pulps (Brannstrom, 1982, 1984). However, for reasons already mentioned, the cut dentin should not be treated with acid or EDTA in such a way that the tubules become open and widened about three fold at the surface.
    • Removal vs Retention 123 Fig. 42 (A) A surface treated with 50 per cent citric acid for 60 seconds. There is a smooth intertubular area peritubular dentin is removed. Note that border between the intertubular and the absent peritubular dentin with circumferential fibrils (X 5000). (B) The edge between the fracture (below) and the ground surface treated with 50 percent citric acid for five seconds. Tubules are funnel shaped and widened about 10µ into the dentin (X2,100) Removal of smear layer: The smear layer is far more tenacious than one would expect. Brannstrom’s group had published several articles describing the use of water, hydrogen peroxide, benzalkonium chloride, EDTA, and other agents to remove the smear layer. Brannstrom & Johnson (1974) found that common cleansing procedures such as peroxide followed by 95% alcohol, or other solvents, did not remove the superficial smear layer. They found that a nondemineralizing, microbicidal fluoride solution gave a good cleaning effect without opening/enlarging the dentinal tubules. Only various acids (50% citric acid, 50% phosphoric acid) and EDTA were capable of removing the smear layer but, unfortunately, they also removed the smear plugs and peritubular dentin. All the above factors favour the opinion that smear layer should be removed, and the remaining smear in tubule apertures should be disinfected. Several investigations were performed to find a suitable cleanser that would retain the smear plugs and remove only the superficial smear layer (Brannstrom, Glantz &Nordenvall, 1979; Brannstrom & others, 1980). Brannstrom has formulated several commercially available products (Tubulicid Blue Label,
    • Removal vs Retention 124 Tubulicid Red Label) for this purpose. A detergent should remove the superficial smear layer, so that an antiseptic component in the cleanser can reach and kill, any bacteria present in the smear plugs. It was found that a combination of detergent and 0.2% EDTA, including benzalkonium chloride as an antibacterial component (Tubulicid Blue Label-same as Red Label but without sodium fluoride), has the ability to remove most smear layers without opening tubule apertures or removing peritubular dentin (Fig. 43). It has good antibacterial effect and is non-irritant to the pulp. Cleansing was better when the surface active agent was combined with EDTA than when either of them was used separately. Moreover, it was found that EDTA potentiates the antibacterial action of benzalkonium chloride. The solution should be applied for 1 min with an initial and final scrubbing for 5 sec. One acceptable solution contained a surfactant combined with 0.2% EDTA and benzalkonium chloride to which 1% sodium fluoride was added (Tubulicid, Red Label). Fluoride in this concentration is antibacterial and we may get a fluoride impregnation of cavity walls and remaining smear plugs. Also, it was seen that this cleanser did not irritate the pulp. (Fig. 44, 45). However, Brannstrom’s (1982) concept of removing most of the smear layer over the tubules without removing the smear plugs n the tubules is an ideal that is difficult to achieve clinically because of the complex geometry of many cavities and the difficulty of obtaining adequate access.
    • Removal vs Retention 125 Fig. 43 Surface ground with high speed and a diamond cylinder and then cleansed with an antibacterial detergent to which 0.2% EDTA was added (Tubulicid improved). The surface cleaned for 1 minute and 10 seconds rubbing. The peritubular dentin is fairly intact and amorphous material remains in the tubule apertures X5300 Fig. 44 Fig. 45 The edge between the fracture (on the right) and the surface, ground in the same way. The surface was initially scrubbed for five seconds with a pellet soaked in a surface active detergent containing an antibacterial component and sodium fluoride (Tubulicid, Red Label). No debris layer remains. A plug of smear layers is seen in the tubule X6,500 Surface scrubbed for 60 seconds with a pellet soaked in the same solution as the tooth in B. A clean surface and many opened tubules are seen, an effect that is not desirable. Too energetic scrubbing may remove plugs from the tubule apertures. A plug in the outer aperture of the tubule reduces permeability of the dentin and may, at least initially, prevent ingrowth of bacterial X 2,400
    • Removal vs Retention 126 Shortall (1981) compared various cavity cleansers and found that Tubulicid was better at removal of smear layer than 3% hydrogen peroxide or Cavilax. However, studies also proved otherwise and gave contradictory results. Jodaikin & Austin (1981) used EDTA instead of acids to remove the smear layer, as the acids produced loss of peritubular dentin and widening of distal ends of cut tubules. They found that 17% EDTA removed smear layer consistently compared to Tubulicid Blue and Red Labels. Meryon et al (1987) compared several smear removal agents and ranked them in order of increasing effect and opening of the orifices of the dentinal tubules as follows: 3% H2O2 < Tubulicid < 37% phosphoric acid < 50% lactic acid < 25% polyacrylic acid < 50% citric acid < 10% EDTA. Duke et al (1985) found that polyacrylic acid gave the best result in removing smear layer. Powis et al (1982) recommended use of surface conditioners, i.e.: polyacrylic acid, tannic acid or Tubulicid as they gave highest bond strengths whereas citric acid, EDTA and ferric chloride were found to be much less effective. Gunday & Ibak (1990) found that conditioning with citric acid for 30 sec is more effective than the application of 40% polyacrylic acid in removing the smear layer, but 40% polyacrylic acid in contrast to citric acid application, did not enlarge the dentinal tubules. Other agents tried included oxalates. Greenhill and Pashley (1981) used 30% potassium oxalate as a desensitizing agent as it rapidly reacted with the dentin to form crystals of calcium oxalate. Pashley & Galloway (1985) found that dentin surfaces treated with neutral/acidic potassium oxalate became less permeable and acid-resistant as they removed the original smear layer and replaced it with an acid-resistant layer of oxalate crystals. Brown et al (1983) and Pashley & Depew (1986) agreed that tubules if opened, must be reoccluded by oxalates, or in case of amalgams, jacket cementation, Barrier or 4-META may be used. Hamlin et al (1990) evaluated a conditioner which is a solution of oxalic acid and aluminum nitrate in a weak mineral acid and has a pH of 2.06; they found that as little as 5 seconds was sufficient to remove most of the smear layer. There was complete removal of the surface smear layer in specimens treated for
    • Removal vs Retention 127 10 seconds or more. Either a 20 second or a 30 second application removed the smear layer completely and could also widen the exposed tubule orifices. Hayakawa et al (1990) evaluated the etching of dentin surface with ammonium citrate aqueous solution, the pH of which was7.3-7.4, and the concentration from 5% to 30%. The smear layer was removed and the tubules were not generally opened with 10% ammonium citrate etching. Silberman et al (1994) found that CO2 laser treatment prior to acid etching with 10% maleic acid could increase the resistance of the smear layer to acid removal. Cagidiaco et al (1997) found that both 36% phosphoric acid and 10% maleic acid were equally effective in completely removing the smear layer and demineralization of the dentin, leaving a porous collagen network layer. However, Goes et al (1998) found that phosphoric acid gels (35% and 10%) and the 10% maleic acid gel applied for 15 and 60 seconds removed the smear layer and opened the dentinal tubule orifices; however, the dentinal surface etched for 15 or 60 seconds with 10% maleic acid gel showed residues of the smear layer. Breschi et al (2001) compared maleic acid and citric acid at pH 0.7 and 1.4 and found that maleic acid at pH of 0.7 gave the highest depth of demineralization. In the present scenario, phosphoric acid, a strong inorganic acid (30-50%) is the most commonly used for etching. Strong acids at low pH denature collagen, and produce greater depth of penetration into the dentinal tubules. The adhesives are not able to penetrate completely, leading to nanoleakage. Milder acids are capable of removing smear layer such as maleic acid, citric acid and EDTA. Bogra & Kaswan (2003), very recently evaluated the effectiveness of EDTA in etching so as to replace phosphoric acid as an etchant and found that 3 min was required for complete smear layer removal with 25% EDTA gel, and that it resulted in negligible, non-uniform effect on enamel and hence is not recommended.
    • Removal vs Retention 128 Fig. 46 Scanning electron micrograph of etched dentin showing exposed collagen fibers. A higher magnification shows the characteristic collagen banding in intertubular collagen. Superficial collagen was dissolved by collagenase to remove the most superficial collagen fibers that were damaged by tooth preparation. Spencer et al (2001) found that smear layers are not totally removed by current etchants and that the disorganized collagen within the smear layer was not removed but was denatured by the acid treatment, and that mineral was trapped in this gelatinous matrix and shielded from complete reaction. Similar result was obtained by Wang & Spencer (2002) who evaluated this using 10% citric acid, 35% H3PO4, or 0.5M EDTA. Another important consideration is that the effect of the smear layer treatment on dentin is different according to the cavity wall (dentin on pulpal/lateral wall) and to the applied treatment (Luz et al, 2001)
    • Removal vs Retention 129 REMOVAL OF SMEAR LAYER IN ENDODONTICS: Endodontic success depends heavily on effective chemomechanical debridement of the root canal through the use of instruments and irrigating solutions. Scanning electron microscopic studies have shown that debris is retained in root canals prepared to clinical standards. This layer is the “smear layer” which has been described in the previous sections. Apart from the controversy regarding the effect of the smear layer on the quality of instrumentation and obturation, several investigators have found that the smear layer may itself be infected and may protect bacteria already present in the dentinal tubules. Also, despite the smear layer being considered beneficial, since it reduces the permeability of dentin and decreases bacterial penetration into the tubules, it is also considered deleterious because it prevents the penetration of irrigants, medications and filling materials into the dentinal tubules or it may even impede contact with the canal wall. If the smear layer is not removed, the bacteria within the layer may be detrimental if they survive flushing and obturation. Therefore, endodontic treatment cannot be limited to the removal of pulp remnants and the widening of root canals, but should also focus on smear layer removal. Chemicals, ultrasonic instruments, lasers, and chelating agents have been recommended for chemical and mechanical debridement during root canal treatment for the removal of smear layer. However, root canals have been filled for years without smear layer removal and a 95% rate of success has been achieved. So, is it important to really remove the smear layer? The clinical implications of the smear layer are still not fully understood. Additional research allows dentistry to further “kick the ball around” without coming to any rigid conclusions about the importance of smear layer removal. But, most of the investigations have proved (as has been discussed in the previous sections) that there is closer adaptation of sealers and obturation materials, greater penetration of intracanal medicaments and lesser apical/coronal leakage when the smear layer has been removed. It is now deemed prudent to remove the smear layer in infected root canals (Torabinejad et al, 2002).
    • Removal vs Retention 130 Mechanical instrumentation alone does not completely eliminate bacteria from the root canal system (Baker et al, 1975) and in order to eliminate them it is necessary to use the supporting action of disinfecting agents such as irrigants (Bystrom & Sundqvist, 1981) and intracanal medication (Bystrom et al, 1985). Irrigating solutions have been used during and after instrumentation to increase cutting efficiency of root canal instruments and to flush away debris. The efficacy of the irrigating solution is dependant not only on the chemical nature of the solution, but also on the quality and temperature, the contact time, the depth of penetration of the irrigation needle, the type and gauge of the needle, the surface tension of the irrigating solution and the age of the solution (Ingle, 1985). However, instrumentation accompanied by copious irrigation will succeed in building up the smear layer, instead of eliminating one. The debris formed by hand instrumentation is granular in contrast with that formed by automated instruments, which is finer and caked. Various irrigating solutions and their combinations have been studied for efficacy of smear layer removal from root canal walls. However, no single irrigant has been found to both dissolve organic pulpal material or to demineralise the inorganic calcific portion of the canal wall. Various irrigant combinations are recommended for these goals (Baumgartner & Mader, 1987). NORMAL SALINE: Normal (physiological) saline solution does not have any effect on, the removal of dentinal debris and smear layer (Baumgartner & Mader, 1987; Berg et al, 1986; Cengiz et al, 1990; Wayman, 1979; Yamada, 1983). The saline solution produces a sludge layer made up of residual debris that occludes the dentinal tubules. Bystrom and Sundqvist, (1981) reported significant reduction in the number of bacteria present in the canals, but not enough so or so much that negative cultures are achieved during one appointment.
    • Removal vs Retention 131 HYDROGEN PEROXIDE: Another irrigating solution that has had extensive use in root canal irrigation is hydrogen peroxide (H2O2). The mechanism of action of this oxidizing solution involves the reaction of super oxide ions to produce hydroxyl radicals which are the strongest radicals known. This radical can attack membrane lipids, DNA and other essential cell components for its antimicrobial action. But, hydrogen peroxide flushes are ineffective in removal of smear layer. Extended exposure of the smear layer to peroxide will cause a dense amorphous precipitate to form on the smear layer (Titley et al, 1988). CHLORHEXIDINE: Chlorhexidine (CHX) is a common irrigant in periodontal treatment, and has been suggested for use in Endodontics. The antimicrobial effect of CHX is mediated by several mechanisms. It binds electrostatically to negatively charged sites on bacteria. By attaching to the bacterial cytoplasmic membrane, CHX causes the osmotic balance to be lost, resulting in leakage of intracellular material. It also binds to hydroxyapatite and soft tissues, changing their electrical field to compete with bacterial binding. SODIUM HYPOCHLORITE: The organic tissue dissolving capacity of NaOCl is well known (Rubin et al, 1979; Wayman et al, 1979; Goldman et al, 1982) and increases with rising temperatures (Moorer & Wesselink, 1982). However, the capacity to remove smear layer from the instrumented root canal walls has been found to be insufficient. Many authors have concluded that the use of NaOCl during or after instrumentation produces superficially clean canal walls with the smear layer present (McComb & Smith, 1975; McComb et al, 1976; Baker et al, 1975; Wayman, 1979; Goldman et al, 1981; Yamada, 1983; Rome et al, 1985; Berg et al, 1986; Kennedy et al, 1986; Baumgartner & Mader, 1987, Cengiz et al, 1990). Rome et al (1985) showed that neither Gly-Oxide (a mixture of 10% urea peroxide and glycerol) nor NaOCl were able to prevent smear layer formation effectively in hand-instrumented root canals.
    • Removal vs Retention 132 The alternating use of Hydrogen peroxide and NaOCl solutions was often advocated in the past. McComb & Smith (1975); Baumgartner & Mader (1987) and Bitter (1989) showed that this combination was not more effective in the removal of the smear layer than NaOCl alone and produced canal surfaces similar to that formed with water. Adding surface active reagents to NaOCl to increase its action proved to be ineffective (Cameron, 1986). Sodium lauryl sulphate was also tried; it was used alone or in combination with NaOCl. The result showed little difference and smear layer was noted. However, Berutti & Marini (1996) showed that though the smear layer was not removed by NaOCl, by increasing the temperature of the solution to 50°C, it resulted in a smear layer that was thinner, and made of finer, less well-organized particles than when it had been used at 21°C. CHELAT1NG AGENTS: The most common chelating solutions are based on ethylene diamine tetracetic acid (EDTA) which reacts with calcium ions in dentin and forms soluble calcium chelates (Grossman et al 1988). While Fehr & Nygaard-Ostby (1963) found that EDTA decalcified dentin to a depth of 20-30 m in 5 min, Fraser (1974) stated that the chelating effect was almost negligible in the apical thirds of root canals. The original Nygaard-Ostby formula for 15% EDTA (pH: 7.3) was: disodium salt of EDTA, 17g; distilled water, 100ml; 5N sodium hydroxide, 9.25 ml. The use of chelating agents also softens the smear layer, allowing its successful removal. Although the agents themselves are not bactericidal, they can be considered antibacterial to the extent that they eliminate the bacteria- contaminated smear layer. Different preparations of EDTA have been used as a root canal irrigant. In one combination, urea peroxide was added to float the dentinal debris from the
    • Removal vs Retention 133 root canal (Stewart et al, 1969). However it appeared that despite further instrumentation and irrigation, a residue of this mixture (RC-Prep, Medical Products Laboratories, Philadelphia, PA, USA) was left on the canal walls (Zurbriggen et al, 1975). This may be a disadvantage in hermetic sealing of root canals (Cooke et al, 1976; Biesterfeld & Taintor, 1980). Cooke et al (1976) showed that RC-Prep allowed maximum leakage into filled canals - over 2.6 times that of the controls. Hill (1959) added the cationic surfactant, Cetavlon® (cetyltrimethylammonium bromide) to EDTA solution, lowering the surface tension and obtaining a bacteriostatic action: this solution is called EDTAC. Fehr & Nygaard-Ostby (1963) added a quaternary ammonium bromide (cetrimide) to EDTA solutions (EDTAC) to reduce surface tension and increase permeability of the solution. McComb and Smith (1975) reported that when this combination (REDTA, Roth International Ltd, Chicago IL, USA) was used during instrumentation, there was no smear layer except in the apical part of the canal. After in vivo use of REDTA, it was shown that the root canal surfaces were uniformly occupied by patent dentinal tubules with very little superficial debris (McComb et al, 1976). Goldberg and Abramovitch (1977) observed that circumpulpal surface had a smooth structure and that the dentinal tubules had a regular circular appearance with the use of EDTAC, i.e: EDTA mixed with 0.84g of Cetavalon (Farma-Dental Labs, Buenos Aires, Argentina). They showed that EDTA increases permeability of dentinal tubules, accessory canals, and apical foramina. Goldman et al (1981) showed that smear layer is not removed by NaOCl alone, but is removed with the combined use of REDTA. When used during and after instrumentation, remnants of odontoblastic processes could still be seen within the tubules even though there was no smear layer present. The optimum pH for the demineralizing efficacy of EDTA on dentin was shown by Cury & Valdrighi (1981) to be between 5.0 and 6.0. Because it is a chelating agent, EDTA is not dependant on a high hydrogen ion concentration to accomplish decalcification, and is effective at neutral pH (Patterson, 1963).
    • Removal vs Retention 134 It was indicated that the optimal working time of EDTAC in the root canal was 15 min and no more chelating action could be expected after this period (Goldberg & Spielberg, 1982). This study indicates that EDTA solutions should perhaps be renewed in the canal every 15 mins. It was also found that REDTA was the most efficient irrigating solution in this study in the removal of the smear layer. Contrary to this study, Yamada et al (1983) suggested that a few seconds of EDTA administration are sufficient. Meryon et al (1987) reported that the smear layer was completely removed in vivo with 10% EDTA for 60 sec. Cergneux et al (1987) applied 15% EDTA in the canals for 4 min and reported that the tubule foramina are enlarged, and the thickness of intertubular dentin is decreased. According to Calt & Serper (2000), to inhibit the erosion of dentin by 17% EDTA solution, it has to be applied for a shorter period of time (< 2 min) or in a low volume (< 10ml). Calt & Serper (2002) also showed in another study that 1minute of EDTA irrigation was effective in removing the smear layer, whereas a 10 min application caused excessive peritubular and intertubular erosion. In another study in 2002, they found greater demineralizing effect of EDTA with increasing concentration and time of exposure and that it was more effective at neutral pH than pH 9.0. They suggested that to reduce erosive effects of EDTA solutions during prolonged cleaning and shaping, lower concentrations of EDTA should be preferred at neutral pH. Garberoglio & Becce (1994) found that 3% EDTA solution was as effective as 24% phosphoric acid-10% citric acid combination and 17% EDTA. Also, this EDTA did not show as marked demineralization of dentinal walls and tubules as the acid solution. Nakashima & Terata (2005) proposed that the 3% EDTA is more useful for clinical applications when they evaluated the influence of smear layer removal with 3% EDTA solution (pH of 9.0) on the dentin in terms of the permeability of root canal disinfectants into the dentin, wetting by endodontic sealer, and adhesive strength of the sealer. However, Menezes et al (2003)
    • Removal vs Retention 135 concluded that the use of 17% EDTA was necessary to enhance cleanliness of the root canals when they evaluated the smear layer removal capacity of different disinfectant solutions (2% chlorhexidine, 2.5% NaOCl, saline) used with and without EDTA for the irrigation of canal. Recently microbrushes (Fig. 47) have been introduced to optimally finish the root canal preparation. They can be used in rotary or ultrasonic handpieces in the presence of 17% EDTA. Use of these has been shown to significantly enhance the cleanliness of the preparation (Keir et al, 1990). Fig. 47 An ultrasonically or rotary activated endodontic microbrush may be used in the presence of 17% EDTA to finish the preparation. Aktener & Bilkay (1993) observed the effectiveness of 17% EDTA and ethylenediamine (ED 5%) mixtures in removing the smear layer and found that smear layer can be totally removed by using 10 ml of a four-to-three by volume mixture of EDTA and ethylenediamine for irrigation. Different salts of EDTA too
    • Removal vs Retention 136 have been evaluated (O’Connell et al, 2000) and it was found that the alkaline tetrasodium salt, pH adjusted with HCl, is more cost effective and performed equally as well as the more commonly used disodium salt. Scelza et al (2003) found that EDTA-T (17% EDTA plus 1.25% sodium lauryl ether sulphate) had lesser decalcifying action than 10% citric acid and 17% EDTA. Other chelators are Calcinase, CDTA, Largal Ultra, Decal, Tubulicid Plus, Hypaque (liquid chelators) and Calcinase slide, Glyde File, FileCare EDTA, File- EZE (paste-type chelators). These may also be used to remove the smear layer (Hulsmann et al, 2003). SODIUM HYPOCHLORITE AND EDTA: The purpose of irrigation is two fold to remove gross debris originating from pulp tissue and possible bacteria, the organic component, and to remove the smear layer, the mostly inorganic component. Because there is not single solution which has the ability to dissolve organic tissues and to demineralise the smeared layer, a sequential use of organic and inorganic solvents have been recommended (Koskinen et al, 1980; Yamada et al, 1983; Baumgartner et al, 1984). Since Goldman et al’s landmark research in 1981, reporting the efficacy of EDTA and NaOCl to remove the smear layer, numerous authors have agreed that the removal of smear layer as well as soft tissue and debris can be expedited by the alternate use of EDTA and NaOCl (Yamada et al, 1983; White et al, 1984; Berg et al, 1984, 1986; Goldberg et al, 1985, 1986; Baumgartner & Mader, 1985, 1987; Alacam, 1987; Cengiz et al, 1990). A summary of the various concentrations and amounts of EDTA and NaOCl used by the various authors is given in the table below (Table 4). In this respect, validity of the term irrigation solution should be reevaluated as these solutions may be used during and after instrumentation. According to Kaufman & Greenberg (1986), a working solution is the solution, which is used to clean, and shape the canal, and an irrigating solution is the one, which is
    • Removal vs Retention 137 essential to remove the debris and smear layer that is created by the instrumentation process. Goldman et al (1982) examined the effect of various combinations of EDTA and NaOCl as a working and/or irrigation solution during and after instrumentation According to their results, the most effective working solution was 5.25% NaOCl and the most effective final flush was 10ml of 17% EDTA followed by 10ml of 5.25% NaOCl, which was also confirmed by Yamada et al (1983). Table 4: Summary of methods for removal of smear layer: Gutmann, International Endodntic Journal, 26:87, 1993 AUTHOR SOLUTION AMOUNT Goldman et al (1981) REDTA 17% Goldman et al (1982) REDTA 17% NaOCl 5.25% 10 ml 10 ml Yamada et al (1983) REDTA 17% NaOCl 5.25% 10 ml 10 ml White et al (1984) REDTA 17% NaOCl 5.25% 10 ml 10 ml Ciucchi et al (1989) NaOCl 3% EDTA 15% 1 ml 2 ml Gettleman et al (1991) EDTA 17% NaOCl 5.25% - - In a study done by Tam & Yu (2000), 2 new canal lubricants, i.e. Canal Lubricant and Glyde™ File Prep (which were made of 17% and 15% EDTA respectively) were tested and found to completely remove the smear layer when used in conjunction with 2.5% NaOCl. Grandini et al (2002) also showed that Glyde™ File Prep when used with 2.5% NaOCl removed the smear layer and opened dentinal tubules. Setlock et al (2003) found that irrigation of 5.25%
    • Removal vs Retention 138 NaOCl and 17% EDTA by the Quantec-E irrigation system did not produce significantly cleaner canals as compared to irrigation with syringe. EGTA (ETHYLENE GLYCOL-BIS (-AMINO ETHYL ETHER) - N,N,N1 ,N1 - TETRA ACETIC ACID): It is widely accepted that the most effective method to remove the smear layer is to irrigate the root canals with 10 ml of 17% EDTA followed by 10 ml of 5% NaOCl (Yamada et al, 1983; Goldman et al, 1982). EDTA chelates with Ca2+ and other divalent cations, demineralizes dentin, and removes the inorganic components of the smear layer but causes erosion of the walls. Baumgartner & Mader (1987) reported that the combination of NaOCl and EDTA caused a progressive dissolution of dentin at the expense of peritubular and intertubular areas, so that the diameters of tubular orifices on the instrumented canal wall were enlarged to 2.5 to 4 µm. EGTA is a chelator which has been introduced recently for root canal irrigation and is reported to bind to Ca2+ more specifically (Schmidt & Reilley, 1957) without inducing any erosion; and thereby removes the inorganic component of the smear layer effectively. To remove the smear layer on the canal wall, EGTA was used as an alternative to EDTA, i.e. 10ml of 17% EGTA followed by 10ml of 5% NaOCl (Calt & Serper, 2000, Viswanath et al, 2003). It was found to be effective in removing the smear layer without inducing any erosion. Also, demineralization of the hard tissue was more effective at neutral pH (i.e.:7.5) than at acidic or alkaline pH. However, the results of their study showed that EGTA was not as effective as EDTA in the important apical third. Further it is still not clear whether the erosion and joining of orifices from EDTA action is deleterious or not. These results seem to indicate that EDTA action is simply stronger than that of EGTA, but EGTA can also be used as an effective root canal irrigant.
    • Removal vs Retention 139 Fig 48 Peritubular and intertubular dentinal erosion is seen after EDTA and NaOCl administration on the middle third of the root canal. Configuration of two or more tubules is seen with reduction of intertubular distance (X3000). Fig. 49 Effects of EGTA and NaOCl administration on the middle third of the root canal. The instrumented root canal is clean, the smear layer is completely removed, and sharply defined orifices of the dentinal tubules are observed Fig. 50 Dentinal tubule after placement of EDTA in the root canal for 5 minutes.
    • Removal vs Retention 140 ORGANIC ACIDS: The smear layer is vulnerable to all acids, especially those used in various aspects of restorative dentistry. Both 37% phosphoric acid and 6% citric acid (15 sec for phosphoric acid and 60 sec for citric acid) will remove the smear layer and smear plugs in the tubules. Morgan & Baumgartner (1997) showed that the quantity of smear layer removed by a material is directly related to its pH and time of exposure. A) Citric acid: 50% citric acid appeared to be an effective root canal irrigant and showed better penetration of rosin sealer into the tubules and improved adaptation of gutta-percha than in untreated canals (Loel, 1975). It was more effective than NaOCl alone in the removal of the smear layer (Tidmarsh et al, 1978; Baumgartner et al, 1984). Salama & Abdelmegid (1994) found that 6% citric acid for 15 or 30sec (in comparison to 5.25% NaOCl and 3% hydrogen peroxide) was quite effective in removing all the components of the smear layer. Wayman et al (1979) showed that the canal walls treated with 10%, 25% and 50% citric acid solution were generally free of the smear layer, but they had the best results in removing the smear layer with sequential use of 10% citric acid solution and 2.5% NaOCl solution, then again followed by 10% solution of citric acid. Rawlinson (1989) also showed that ultrasonic preparation with 0.25% NaOCl and final ultrasonic agitation for 1 min with 50% citric acid produced canal walls free of smear layer along with severe erosion of coronal dentin. The working time necessary to obtain complete removal of the smear layer by EDTA was 2-3 min or more for each irrigation, which prolongs the endodontic procedure (Cantatore et al, 1996; Di Lenarda et al, 1997). Also that, several authors have demonstrated the cytotoxicity of EDTA solutions and the relatively lower toxicity of citric acid (Di Lenarda, 1997). Several studies have shown the biocompatibility of 10% citric acid, 17% EDTA and EDTA-T, indicating that citric acid was the most biocompatible solution of these (Scelza, 2001;
    • Removal vs Retention 141 Malheiros, 2000), which could suggest that 10% citric acid may be more suitable for clinical use. Another relevant aspect is the erosion of dentinal tubules caused by EDTA as reported by Calt & Serper (2002). It is believed that this could lead to the weakening of tooth structure especially when EDTA is used for young patients. The findings of Scelza et al (2004) suggest that 10% citric acid can be used in young patients as it does not weaken the tooth structure. Substitution of EDTA with an aqueous citric acid solution as an endodontic irrigant has recently been proposed by Yamaguchi et al (1996) who demonstrated its better calcium extraction and antibacterial activity than 10% EDTA. The simple preparation of citric acid solutions, their low cost, good chemical stability if correctly used, and their effectiveness with short application times suggest this irrigant is suitable for clinical use (Cernaz et al, 1998). However, one of the main problems associated with using citric acid is its very low pH (whilst an EDTA solution is almost neutral- pH: 7.2), which may have an irritant effect on periapical tissues (Garberoglio & Becce, 1994). Citric acid is said to have maximum effectiveness at a pH of 1.2 (Hennequin et al, 1994). Haznedaroglu (2003) found that lower concentrations (5%, 10%) of citric acid with its original pH were as effective as higher concentrations (25% and 50%) in removal of smear layer, and that high concentrations with low pH caused more destruction of peritubular dentin. Scelza et al (2004) found that 10% citric acid, EDTA and EDTA-T were effective in removal of smear layer at the shortest time tested (i.e.:3 min) and did not demonstrate an improved effect with increase in time (10 and 15 min). In a study done by Di Lenarda et al (2000), it was found that NaOCl followed by 1 mol L-1 citric acid solution was as effective in removing smear layer as 15% EDTA and cetrimide solution. Machado-Silveiro et al (2004) found that citric acid at 10% was the most effective decalcifying agent, followed by 1% citric acid, 17% EDTA and 10% sodium citrate (neutral pH). However, it has also
    • Removal vs Retention 142 been observed that the 25% citric acid-NaOCl group was not effective as the 17% EDTA-NaOCl combination (Yamada et al, 1983). Besides citric acid precipitated crystals in the root canal which might be disadvantageous in the root canal obturation. Citric acid, in addition to removing the smear layer, is a powerful antimicrobial agent, but its antimicrobial action is not as great as that of 5.25% NaOCl. Combining the two, NaOCl followed by 6% citric acid would give an ideal endodontic irrigant (Shorelin et al, 1982; Smith & Wayman, 1986). Alternate strengths of these two solutions have been suggested for debriding canals in the Sargenti N-2method of canal preparation. The potency of the NaOCl was reduced to 2.5%, and that of citric acid was doubled to 12.5%. B) Lactic acid: With 50% Lactic acid, the canal walls were generally clean, but the openings of the dentinal tubules did not appear to be, completely patent. Also that 50% lactic acid was less effective than 50% citric acid for removal of the smear layer. This might be attributed to the viscosity of the lactic acid (Wayman et al, 1979). C) Tannic acid: Bitter (1989) introduced the use of 25% tannic acid solution as a root canal irrigant cleanser. It was demonstrated that the canal walls irrigated with this solution appeared significantly cleaner and smoother than the walls treated with a combination of H2O2 and NaOCl, and that the smear layer was removed. Sabbak & Hasanin (1998) refuted these findings and explained that tannic acid increased the cross-linking of exposed collagen within the smear layer and within the matrix of underlying dentin, thus increasing organic cohesion to the tubules. D) Polyacrylic acid: McComb and Smith (1975) compared the efficacy of 20% polyacrylic acid with REDTA and found that it was no better than REDTA in removing or preventing the build up of smear layer, probably owing to its higher viscosity.
    • Removal vs Retention 143 McComb et al (1976) also used 5% and 10% polyacrylic acid as an irrigant and observed that it could remove the smear layer only in accessible regions. Polyacrylic acid at 40% (Durelon and Fuji II liquid) was reported by Berry et al (1987) to be very effective for the removal of smear layer but because of its potency, the application of polyacrylic acid should not exceed 30 seconds. OXINE DERIVATIVES: Derivatives of oxine (8-hydroxy-quinoline) were known to possess antiseptic properties as early as 1895. Dequalinium compounds, which belong to this group, have been widely used in medicine against infections of bacteria, molds and fungi. Bis-dequalinium-acetate (BDA) has been shown by Kaufman et al (1978) and Kaufman (1981) to remove the smear layer throughout the canal, even in the apical third. BDA is well tolerated by the tissues within the periodontium and has a low surface tension that allows penetration into spaces that instruments cannot reach. It has surface active properties similar to materials of the quaternary ammonium group and possesses the combined actions of chelation and organic debridement. BDA is also considered less toxic than NaOCl. While Kaufman et al (1978) reported that Salvizol had better cleansing properties than EDTA containing cetavlon (EDTA-C, Frenstiller or Wyegaard and Co., Norway), Berg et al (1986) found that REDTA surpassed Salvizol and other solutions in its cleansing action. It is not known whether Cetavlon in EDTA-C or cetrimide in REDTA has caused these differences in effect. Kaufman & Greenberg (1986) compared Salvizol (a commercial brand of 0.5% BDA) with 5.25% NaOCl and found both comparable in their ability to remove organic debris, but only Salvizol was able to open dentinal tubules. SUCCIMER (brand name Chemet) , and TRIENTENE HCl (Syprine): Succimer is taken orally to remove excess lead from the body (acute lead poisoning) especially in small children. Trientene HCl is taken orally to treat Wilsons disease, a condition manifested by the accumulation of too much copper in the body. Both materials are available in capsules, which can be transformed
    • Removal vs Retention 144 into solution by mixing with deiodized water. Both these agents and EDTA are effective in the removal of the smear layer and widening of the dentinal tubules. In fact, Succimer even provided a greater overall widening when compared with EDTA (Hottel, 1999). Because they are both used in medicine as oral medication in pediatrics, they may be considered safe when used in the root canal system. These two (products) irrigants increase the available medication that can be effectively used to remove both the smear layer and also widen the dentinal tubules of the root canal system of the human teeth. TETRACYCLINES: Tetracyclines (including tetracycline-HCl, minocycline, and doxycycline) are broad-spectrum antibiotics that are effective against a wide range of microorganisms. Tetracyclines have many unique properties in addition to their antimicrobial effect. They have a low pH in concentrated solution and thus can act as a calcium chelator, and they can cause enamel and root surface demineralization (Bjorvatn, 1982). The surface demineralization of dentin is comparable with that of citric acid. The substantivity of these antibiotics allows them to be absorbed and released gradually from tooth structures such as dentin and cementum. Gutierrez et al (1982) have demonstrated that EDTA used during the mechanical preparation of the root canals favors the diffusion of microorganisms within the dentinal tubules and have noted that complete removal of smear layer with EDTA increases the risk of rapid decay formation, if the coronal dentin of the endodontically treated tooth is accidentally exposed to the oral fluids (Gutierrez et al, 1990). In addition, according to Sen et al (1995), once the smear layer is removed, there is always a risk of dentinal tubule reinfection, if the seal fails. SEM studies have revealed that chelating agents or organic acids not only eliminate this layer, but also remove peritubular dentin (Karagoz-Kucugay & Bayirli, 1994; Garberoglio & Becce, 1994) subsequent to a marked demineralization of the dentin surface.
    • Removal vs Retention 145 The ability of tetracycline family of antibiotics to remove smear layers has been studied. They have been used to demineralize dentin surfaces, uncover and widen the orifices of dentinal tubules, and expose the dentinal collagen matrix. These effects provide a matrix that stimulates fibroblast attachment and growth. Barkhordar et al (1997) showed that doxycycline HCl (100 mg/mL) is more effective than EDTA or NaOCl-EDTA combination in removing the smear layer from the surfaces of instrumented canals and root-end cavity preparations. They speculated that a reservoir of active antibacterial agents might be created because doxycycline readily attaches to dentin and can be subsequently released. Haznedaroglu & Ersev (2001) showed that 1% tetracycline hydrochloride or 50% citric acid can be used to remove the smear layer from the surfaces of instrumented root canals. Although they reported no differences between these 2 groups, it appears that the tetracycline demineralized less peritubular dentin than did 50% citric acid (Fig. 51, 52). Fig. 51 Specimen treated with 50% citric acid. High magnification shows peritubular dentin is totally removed and dentinal tubules are open (X5000) Fig. 52 Specimen treated with 1% tetracycline HCl. High-power view of the surface shows that the smear layer is completely removed, and the dentinal tubule apertures are slightly enlarged. (X5000)
    • Removal vs Retention 146 One of the side effects of tetracycline is the staining of teeth (Bridges et al, 1969). Therefore, how using this drug group with the aforementioned properties in endodontic therapy relates to clinical situations remains to be investigated. MTAD (MIXTURE OF TETRACYCLINE ISOMER, AN ACID, AND A DETERGENT): Various organic acids, ultrasonic instruments, and lasers have been used to remove the smear layer. Based on available evidence, it seems that these agents and methods do not provide complete disinfection of the root canal spaces in all cases when used for one-visit root canal therapy. A search of the endodontic literature showed the absence of any research regarding the ability of an irrigant capable of removing the smear layer and disinfecting the root canal system. Torabinejad et al (2003) investigated the effect of a new irrigating solution (MTAD) containing a mixture of a tetracycline isomer, an acid, and a detergent on the surfaces of instrumented root canals. They found that 5 ml of a mixture of doxycycline and citric acid for 1-5 min was most effective in removing the smear layer compared to mixtures of doxycycline with acetic acid or polyacrylic acid. They also added a detergent to lower the surface tension and increase the penetrating ability of the irrigating solution and found that a mixture of doxycycline, citric acid, and Tween-80 was capable of removing the smear layer from the surfaces of instrumented root canals better than a combination of doxycycline and citric acid alone. The results of this study showed that MTAD is an effective solution for the removal of smear layer and is also less destructive to the tooth structure compared with EDTA when used as a final irrigant. In contrast to destructive effects of 5-min EDTA exposure, they observed no significant dentinal erosion when MTAD was in contact with root canal dentin from 1-20 min (Fig. 53, 54).
    • Removal vs Retention 147 Fig 53 Greater erosion of the dentinal tubules is present in the coronal root canal treated with NaOCl as a root canal irrigant and EDTA as a final irrigant for 5 min (X 5000) Fig. 54 Instrumentation of a root canal with 5.25% NaOCl as root canal irrigant and treatment with 5 min of MTAD as a final rinse resulted in the removal of the smear layer in the coronal portion of the root canal (X5000) In another study Torabinejad et al (2003) showed that although MTAD removes most of the smear layer when used as an intracanal irrigant, some remnants of the organic component of the smear layer remain scattered on the surface of the root canal walls. The effectiveness of MTAD to completely remove the smear layer is enhanced when low concentrations of NaOCl are used as an intracanal irrigant before the use of MTAD as a final rinse. Beltz et al (2003) found that the solubilizing effects of MTAD on pulp and dentin were somewhat similar to those of EDTA, indicating that EDTA (pH: 8.0) maybe capable of removing not only the inorganic portion of the smear layer but also a portion of the organic component quantitatively equivalent to that removed by MTAD (pH:
    • Removal vs Retention 148 2.2). The major difference between their actions was a high binding affinity of doxycycline present in MTAD for the dentin. Thus, MTAD is an effective solution for the removal of smear layer when used as a final rinse. Studies are in progress to determine the antibacterial effectiveness of these solutions. OXIDATIVE POTENTIAL WATER (OPW): OPW has been developed in Japan and is defined as electrolytically obtained highly acidic water having accumulated in the anode-containing compartment after sodium chloride-added water has consumed OH- ions. It constitutes the counterpart of alkaline water forming in the cathode-containing compartment after the water therein has consumed H+ ions. The Japanese technology makes use of the special patented Russian anode-cathode system with its special membrane which is used to manufacture ECA. OPW is characterized by an outstanding antimicrobial activity killing viruses as well as bacteria, an unusually low pH of 2.7 or less, and oxidation-reduction potentials as high as 1050 mV or greater (Okuda et al, 1994), in contrast to that of tap water, that is 300-400 mV. Its dissolved chlorine content is 30-40 ppm and dissolved oxygen is 10-30 ppm. Bactericidal and demineralizing (Inoue et al, 1994) effects have recently been noted to occur in the tooth structure when OPW is used during dental treatment. The scientific basis for the development of the OPW is that microorganisms cannot survive in an aqueous environment with both low pH (less than 3) and high oxidation-reduction potential (greater than 0.9 mV). A suggested advantage of OPW is the absence of any toxicity and irritability caused by immediate loss of the high oxidation-reduction potential and low pH upon reacting to light and/or organic substances. This exempts dental personnel from concern about from tissue injury from periapical extrusion of highly acidic water.
    • Removal vs Retention 149 The research of Hata et al (1996) reported on the effectiveness of OPW as a root canal irrigant. They reported favorably on the cleaning ability of OPW (Super Miniwater, Janix Inc, Atsugi, Japan) to clean debris from the canal walls and also found that it was as effective as 5% NaOCl or 17% EDTA for opening and keeping patent the dentinal tubules. Hata et al (2001) also concluded that OPW by means of syringe following instrumentation with 5% NaOCl showed a similar effect to that of 15% EDTA irrigation for the removal of smear layer and debris. However, Serper et al (2001) found that OPW was less cytotoxic than other irrigants but did not effectively remove the smear layer and that treatment with EDTA followed by NaOCl efficiently removed the smear layer, but their cytotoxicity should be considered during endodontic therapy. ECA (ELECTROCHEMICALLY ACTIVATED WATER): Over the course of the past 28 years, Russian scientists have developed and refined the process of electrochemically activating water. ECA is the subject of more than 300 Russian and international patents and more than 20,000 units producing ECA are in operation in Russian hospitals today. It is claimed that it is harmless to humans, with patients drinking considerable quantities of ECA and open wounds being washed with it (Leonov, 1997; Bakhir, 1997). It has been tried as an irrigating solution due to the various disadvantages of NaOCl: 1) It is toxic to living tissues and periapical extrusion can cause post-operative pain, swelling and necrosis. 2) Because of its corrosive nature, ultrasonics unit are prone to breakdown. 3) Its taste is unacceptable to patients and the vapour can be an irritant to the eyes. 4) Does not effectively remove the smear layer (Berutti & Marini, 1996; Bertrand et al, 1999). ECA is produced from tap water and saline solution by special unit that houses a unique flow-through electrolytic module (FEM). The FEM contains the anode, a solid titanium cylinder and coated with ruthenium-oxide, iridium and platinum, and the cathode, a hollow cylinder into which the anode fits coaxially, is made from titanium coated with pyro-carbon and glass-carbon. These electrodes
    • Removal vs Retention 150 are separated by a ceramic membrane. The FEM is capable of producing types of solutions that have bactericidal and sporicidal activity, yet are odourless, safe to human tissue and essentially non-corrosive for most metal surfaces. The physical and chemical nature of ECA is not yet fully understood (Bakhir, 1997). The solution supposedly exists in a metastable or disequlibrious state for 48 hours after production and contains many free radicals and a variety of molecules and ions. After 48 hours the solution returns to the stable state, becoming inactive again (Marais, 2000). In the metastable state, the solution has a very high oxidation-reduction potential. Two types of ECA solution are produced: 1) Anolyte, with a high oxidation potential (400-1200 millivolts). The Anolyte solution has been termed Super-Oxidized Water (Selkon et al, 1999) or Oxidative Potential Water (Hata et al, 1996). The anolyte is considered to be antimicrobial. Depending on the type of FEM, the pH (pH 2-9) of anolyte varies: it may be acidic (anolyte), neutral (anolyte neutral) or alkaline (anolyte neutral cathodic) anolyte. Acidic anolyte was used initially but in recent years the neutral and alkaline solutions have been recommended for clinical application. It ahs been demonstrated that anolyte neutral and anolyte neutral cathodic with concentrations of active chloride up to 300 mg L-1 are non toxic when in contact with vital biological tissues. 2) Catholyte, an alkaline solution (pH 7-12) with a high reduction potential (-80 to -900 millivolts). Catholyte is reputed to have a strong cleaning or detergent effect. It also provides necrotic tissue debridement but is safe for vital tissues (Prilutskii & Bakhir, 1997). In a study done by Marais JT (2000), ECA produced cleaner root canal surfaces than did NaOCl, and removed the smear layer in large areas. It was seen that collagen fibres and fibrils became exposed suggesting that the dentin was decalcified to some extent, like an etchant would do. Yet the anolyte used was of
    • Removal vs Retention 151 a neutral pH and the catholyte of ph 9.8. The fact that such clean surfaces were achieved by this product is remarkable and important. Fig. 55 Group B: Surface of root canal walls cleaned by electro-chemically activated water. Note absence of smear layer, debris or bacteria and the open tubuli. Note the large tubuli measuring 10-20µm. (Bar indicates 50µm). Fig. 56 Group B: Higher magnification of small dentinal tubule showing inner structure consisting of collagen fibres. (Bar indicates 0.5µm) In a study done by Solovyeva & Dummer (2000), it was found that ECA solutions left a thinner smear layer with a smoother and more even surface. The combination of anolyte neutral cathodic (ANC) with catholyte resulted in more numerous open dentinal tubules throughout the whole length of canals in comparison to NaOCl. The ANC provided an increased antiseptic effect and an enhanced cleaning ability at lower concentrations of active chlorine compared to the acidic anolyte and anolyte neutral solutions because of its higher concentration of peroxides (Bakhir et al, 1999). It was thus concluded that ECA removed the smear layer from some surfaces of the canals.
    • Removal vs Retention 152 ULTRASONICS: After the introduction of ultrasonic devices, the use of ultrasound was investigated in Endodontics (Martin et al, 1980; Cunningham et al, 1982; Cunningham & Martin et al, 1982). A continuous flow of sodium hypochlorite solution activated by an ultrasound delivery system was used for the preparation and irrigation of the root canal. It was observed that this method produced smear free root canal surfaces (Cameron 1983, 19871 ; Griffiths and Stock 1986; Alacam 1987). This technique permits cleansing of irregularities in the canal wall and has the ability to exert its cleansing effect beyond the main root canal into an adjacent fin or the isthmuses. Thus ultrasonic irrigation must be considered superior to EDTA or the EDTA-/NaOCl combination, which tends to leave debris in a fin (Goldman et al, 1982). However, ultrasound does present some hazards if it is carelessly used. Debris may be forced through the apex, in turn damaging the periapical hard and soft tissues. It can cause a temperature rise in the tooth structure, which can be controlled with the use of low power and 30 sec applications of an irrigating solution (Cameron, 1987). The mechanism of action for debris removal was described as “acoustic streaming” by Ahmed et al 1987). Ahmad el al (1987b) claimed that their technique of modified ultrasonic instrumentation using 1% NaOCl removed the debris and smear layer more effectively than the technique recommended by Martin & Cunningham (1983). It was observed that the apical region of the canals showed less debris and smear layer than the coronal aspects, depending on the acoustic steaming, which was more intense in magnitude and velocity at the apical regions of the file. Acoustic streaming is maximized when the tips of the smaller instruments operate at high power and vibrate freely in a solution. Lumley et al (1992) recommended that only size 15 files be used to maximize the microstreaming for removal of debris. Cameron (1983) compared the effect of different ultrasonic irrigation periods on removing smear layer and found that a 5- min irrigation with 3% NaOCl produced smear-free canal walls as effectively as 3 days of exposure to
    • Removal vs Retention 153 5% NaOCl, while a 1-min irrigation was ineffective. Cameron (1988) showed that while concentrations of 2%-4% NaOCl in combination with ultrasonic energy, were able to remove the smear layer, lower concentrations of the solution were unsatisfactory. They recommended a 2% solution of NaOCl activated by an ultrasound delivery system for the final cleansing of root canal systems. Prati et al (1994) also achieved smear layer removal with ultrasonics. Cameron (1995) suggested that the most effective regime was irrigation with 1 ml EDTAC after each instrument size, followed by two 30 second exposures to ultrasound+EDTAC then four 30 second exposures to ultrasound + 4 per cent sodium hypochlorite. Guerisoli et al (2002) evaluated the use of ultrasonics to remove the smear layer and found it necessary to use 15% EDTAC with either distilled water or 1% NaOCl to achieve the desired result. In contrast to these results, it has also been found by other investigators (Cymerman et al, 1983; Baker et al, 1988; Goldberg et al, 1988; Ciucchi et al, 1989; Walker & Del Rio, 1989, 1991; Baumgartner & Cuenin, 1992; Abbott et al, 1991) that ultrasonic preparation was not able to remove the smear layer. There was reduction in the smear layer with NaOCl and endosonics but it was not completely removed (Cheung et al, 1993). Researchers who found beneficial cleaning effects of ultrasonics used the technique only for final irrigation of root canal after completion of instrumentation by hand (Alacam 1987; Ahmed et al 1987b; Cameron 1988). They exercised extreme care not to touch the ultrasonic file to the canal wall so as to allow free oscillation. Ahmad et al (1987) claimed that direct physical contact of the file with the canal walls throughout the ultrasonic instrumentation reduced acoustic steaming. This may be the reason for the contradictory results of the studies which showed that the use of ultrasonics did not remove the smear layer. LASERS: Takeda et al (1998, 1999) found that lasers can be used to vaporize tissues in the main canal, remove the smear layer, and eliminate the residual tissue in the
    • Removal vs Retention 154 apical portion of the root canals. Several investigators (Dederich et al, 1984; Onal et al, 1993; Moshonov et al, 1995) have reported that the effectiveness of lasers depends on many factors, including the power level, the duration of exposure, the absorption of light in the tissue, the geometry of the root canal, and the tip-to-target distance. The observable effects of laser irradiation (Nd:YAG) on the dentin of prepared canal walls ranged from no effects of disruption of the smeared layer to an actual melting and recrystallization of the dentin into a non-porous, glazed surface containing needle-like crystal formations in the non-porous dentin (Dederich et al, 1984; Tewfik et al, 1993). Nd:YAG lasers causes melting of dentin and closure of exposed dentinal tubules without dentin surface cracking (Lan & Liu, 1995). But, the depth to which Nd:YAG laser could work into the dentinal tubules is still unknown. The Nd:YAG laser can be used at the energy output of 30 mJ with 10 pulses for 2 min which can modify the dentin surface and occlude the openings of the dentinal tubule orifices. Such a dentin surface modification may be accepted in the future as a treatment modality, because melting and resolidification of the dentin and closure of the tubules without dentin surface cracking may be permanent and short lived (Lan & Liu, 1995; Tani & Kawada, 1987). This pattern of dentin disruption was observed in other studies with various lasers, including the carbon dioxide laser, the argon fluoride eximer laser and the argon laser. Harashima et al (1997) also found that Nd:YAG laser is useful to remove debris and smear layer and causes melting of internal structures on the instrumented root canal walls at the parameters of 2 W and 20 pps. Takahashi et al (1996) observed that after erbium-yttrium-aluminium- garnet (Er:YAG) laser irradiation, most of the debris and smear layer on canal walls were removed, and dentinal tubules were evident. Takeda et al (1998, 1999) using the Er:YAG laser, demonstrated optimal removal of the smear layer without the melting, charring, and recrystallization associated with other laser types. Kimura et al (2002) demonstrated the removal of the smear layer with an
    • Removal vs Retention 155 Er:YAG laser as well. Although they showed removal of the smear layer, the photomicrograph showed destruction of the peritubular dentin. Kumar et al (2002) found that the pulsed XeCl 308-nm excimer laser at a fluence of 0.4 J cm-2 , with an exposure time of 5 s uniformly occluded exposed smear layer covered dentine with no conspicuous variation in chemical structure. The main difficulty with laser removal of the smear layer continues to be the access to small canal spaces with the relatively large probes that are available for delivery of the laser beam. MODIFICATION OF SMEAR LAYER: After reviewing the agents for removal of smear layer, and because the advantages and disadvantages of smear layer are still controversial (Sen et al, 1995), a smear layer that can be modified to become more stable and resistant to microleakage may be more beneficial for the long-term success of endodontic therapy. Also that, when the smear layer is intentionally removed before any type of filling, there is a risk of reinfection of wide open dentinal tubules if the seal should fail. Sen & Buyukyilmaz (1998) evaluated the effect of 4% titanium tetrafluoride (TiF4) on root canal walls with or without a smear layer. It was seen that TiF4 solution modified the smear layer and produced a massive structure which was resistant to removal by EDTA and/or NaOCl solutions. It also indicated that this extremely stable structure may be advantageous in endodontics, because it can prevent further infection of root canal dentin by sealing off the tubules permanently, and can reduce microleakage by preventing further dissolution and disintegration of the smear layer. Further studies are required to investigate the chemistry and the effect of the modified smear layer on microleakage.
    • Removal vs Retention 156 Fig. 57 Scanning electron micrograph of the coating forming a tenacious layer on the root canal walls. Note that the dentinal tubules are densely occluded (X2000; bar=1-µm)