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A Comparative Study of Smear Layer Removal Using Different Salts of EDTA
JOURNAL OF ENDODONTICS Copyright © 2000 by The American Association of Endodontists
Printed in U.S.A. VOL. 26, NO. 12, DECEMBER 2000
A Comparative Study of Smear Layer Removal Using Different Salts of EDTA Michael S. O’Connell, DDS, Leslie A. Morgan, DMD, William J. Beeler, DMD, MS, and J. Craig Baumgartner, DDS, MS, PhD
Three solutions of EDTA—a 15% concentration of the alkaline salt, a 15% concentration of the acid salt, and a 25% concentration of the alkaline salt— were evaluated for smear layer removal in root canal systems. All solutions were adjusted to pH 7.1 using either NaOH or HCl. When the EDTA solutions were alternately used for root canal irrigation with 5.25% NaOCl, they completely removed the smear layer in the middle and coronal thirds of canal preparations, but were less effective in the apical third. None of the EDTA solutions by themselves were effective at completely removing the smear layer at any level. The alkaline tetrasodium salt, pH adjusted with HCl, is more cost effective and performed equally as well as the more commonly used disodium salt.
EDTA solutions affects their efficacy and calcium ion availability in several ways. As the pH increases, the availability of calcium ions from hydroxyapitite for chelation decreases. At the same time, a greater dissociation of the EDTA produces an increased attraction for calcium ions. Conversely, at lower pHs the calcium ions become more available for chelation, but the efficacy of EDTA decreases. The optimal pH for EDTA solutions seems to be between 6 –10 (15). Traditionally the disodium salt of EDTA has been used for root canal irrigation in a 15 to 17% saturated concentration. The trisodium or tetrasodium salts can be used to make solutions of higher concentration (16). The pH is the major factor in determining the ratio of ionized to nonionized forms of EDTA in solution as well as limiting the concentration that can be achieved. The use of solutions in higher concentrations might lead to increased demineralization properties, aiding in smear layer removal. The purpose of this study was to evaluate the removal of smear layer and patency of dentinal tubules after root canal irrigation with three different EDTA solutions. The efficacy of EDTA solutions used alternately with NaOCl was also evaluated.
It has been shown that an amorphous smear layer composed of inorganic and organic material is produced on root canal walls after instrumentation (1, 2). It is typically 1 to 2 m in thickness, but may be packed into dentinal tubules as far as 40 m (3). Removal of this layer prior to root canal obturation remains controversial (4, 5). It has been suggested that the smear layer may decrease the permeability of dentin and prevent bacterial penetration into underlying dentinal tubules (4). Others believe the smear layer may contain bacteria and prevent antimicrobial agents from gaining access to underlying contaminated dentinal tubules (6, 7). Some studies suggest that removal of the smear layer may enhance the seal of the root canal filling (8). NaOCl has been shown to be an effective agent in dissolving organic tissue (9), whereas chelating agents such as EDTA have been the irrigants of choice to demineralize dentin and aid in the removal of the inorganic component of the smear layer (1, 10 –12). Studies have shown that EDTA is incapable of completely removing the smear layer from instrumented root canal walls (11, 12). A final irrigation of the root canal system with NaOCl after EDTA irrigation seems to produce the cleanest walls (1, 11, 12). Seidberg and Schilder (13) and Patterson (14) suggest that it would be advantageous to use a clinically biocompatible agent to decalcify dentin to minimize any untoward effects. The pH of
MATERIALS AND METHODS Twenty-two matched pairs of single-rooted human teeth were used in this study. The teeth were stored in 0.9% saline after extraction. External root surfaces were scaled to remove soft tissue remnants. The crowns were removed at the level of the cementoenamel junction using a high-speed handpiece and carbide bur. Four of the matched pairs were pregrooved buccolingually along the entire root length; in four teeth the pulp was extirpated with an xx fine barbed broach (Kerr, Romulus, MI), whereas the pulpal tissue in the contralateral teeth was left intact. These teeth were split by applying force with wire cutters in the previously made grooves in the roots and then immersed in 2.5% gluteraldehyde solution for 24 h to fix the remaining tissue. These specimens were used as technique controls for scanning electron microscopic evaluation of the experimental teeth. The other 18 matched pairs were randomly divided into three groups of six pairs. The working length was determined 1 mm short of the major diameter, and patency was checked with an ISO #10 file. The teeth were pregrooved buccolingually and placed in a copper band, which was held in place with polyvinyl impression material (Reprosil, Caulk Dentsply, Milford, DE). Teeth were instrumented using a crown-down technique with rotary nickel739
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titanium GT files (Tulsa Dental, Tulsa, OK) starting with the 0.12 tapered file and followed sequentially by the next three files in the series. Profile series 29 (Tulsa Dental, Tulsa, OK), instruments (.04 taper, sizes #4 to #7) were then used sequentially to the working length to enlarge the apical preparation. Within each group, six teeth were irrigated after the use of each file with 3 ml of 5.25% NaOCl and 3 ml of an experimental EDTA solution. The three experimental solutions were: 15% disodium EDTA (pH 7.1; adjusted with NaOH), 15% tetrasodium EDTA (pH 7.1; adjusted with HCl), and 25% tetrasodium EDTA (pH 7.1; adjusted with HCl). All EDTA solutions are expressed in weight per unit volume concentrations. The solutions were delivered alternately using a Monoject syringe with a 27 gauge needle, such that 3 ml of NaOCl was used after the .12GT, .08GT, #4 Profile, and #6 Profile, and 3 ml of EDTA was used after the .10GT, .06GT, #5 Profile, and #7 Profile. After completing the instrumentation and irrigation regimen, each tooth was irrigated with 3 ml of NaOCl, followed by 3 ml of distilled water. The contralateral teeth within each group were subjected to the same instrumentation technique and irrigation quantities, but the solutions used were either saline alone (two teeth), NaOCl alone (two teeth), or each experimental EDTA solution alone (two teeth). These teeth received an extra 6 ml of each solution to standardize the amount of irrigant used. All teeth received a final 3 ml irrigation with distilled water to halt any chemical activity and were immediately fractured. Each fractured segment was immersed in 2.5% gluteraldehyde for 24 h to fix any remaining soft tissue. All teeth received 33 ml of irrigant including the final 3 ml of distilled water. Instrumentation times were ⬍10 min in all cases. After 24 h the teeth were transferred to distilled water for 24 h and then vacuum dried. Each specimen was sputter-coated with ⬃35 nm of gold palladium (Hummer VII, Anatech Ltd., Alexandria, VA) and examined using a scanning electron microscope (JEOL T-330A, JEOL Technics Co., Tokyo, Japan). The root canal walls were examined at the coronal, middle, and apical thirds for canal wall appearance, smear layer removal, and patency of the dentinal tubules. Representative photomicrographs were taken (⫻1000) at the junction of the middle and coronal thirds for comparison.
RESULTS Control specimens in which pulpal tissue was left in place showed numerous strands of tissue of a fibrous nature. The walls in which the pulp had been pulled free appeared smooth and had numerous visible dentinal tubules (Fig. 1A). Root canals in which the pulpal tissue had been extirpated showed fibrils consistent with predentin along the dentinal walls; tubule orifices could be seen along the lengths of the canals associated with this fibrilar network (Fig. 1B). No observable smear layer was present. Saline-irrigated root canals all exhibited a smear layer on instrumented surfaces with some residual areas of pulpal debris (Fig. 2A). All three levels of the canals exhibited a uniform amorphous smear layer along the entire instrumented canal wall. Dentinal tubule orifices were not visible. Uninstrumented surfaces were covered by predentin and remaining pulpal debris. NaOCl-irrigated root canals seemed similar to the saline-irrigated teeth on instrumented surfaces (Fig. 2B). A typical amorphous smear layer was seen covering the entire lengths of the instrumented canals, with the exception of areas where calcospher-
FIG 1. (A) Control tooth without pulpal extirpation (⫻1000). A large portion of the remaining pulp tissue is still visible. In the background some pulpal strands and a collection of interwoven fibers composing the predentin are clearly seen. (B) Control tooth after pulpal extirpation (⫻1000). Predentin and some remaining pulpal fibrils are seen attached to the walls of the canal and tubule orifices are clearly visible.
ites were visible. These areas seemed to have been uninstrumented. Little pulp tissue debris was evident throughout the canal space. Root canals irrigated with any of the three EDTA solutions seemed to have smooth instrumented walls that were often covered by an incompletely removed smear layer with some dentinal tubule openings visible (Fig. 2, C–E). In general the root canal walls irrigated with any EDTA solution seemed to be free of surface debris and contained a greater number of patent orifices compared with canals irrigated with saline or NaOCl, which were uniformly covered with a smear layer on instrumented surfaces. Root canals in which EDTA and NaOCl irrigants were alternated all exhibited open fields of dentinal tubules in the coronal and middle thirds, with very little remaining smear layer (Fig. 3, A, C, and E). A typical amorphous smear layer was evident in the apical third, regardless of the solution used (Fig. 3, B, D, and F). Some tubule orifices were visible where the smear layer appeared to be partially removed. Areas of eroded dentinal tubules were scattered along canal walls consistent with uninstrumented areas with corresponding fields of calcospherites. No differences could be detected between concentrations or types of EDTA solutions
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FIG 2. (A) Tooth irrigated with saline only (⫻1000). A typical amorphous smear layer with extensive debris is noted on the canal wall. No tubule openings are visible. (B) Tooth irrigated with NaOCl only (⫻1000). A typical smear layer is visible with less superficial debris apparent. No openings of dentinal tubules are visible. (C, D, and E) Teeth irrigated with 15% disodium, 15% tetrasodium, and 25% tetrasodium EDTA only (⫻1000). Less smear layer is present overall, and the openings of some dentinal tubules are present. No pulpal fibers are visible.
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FIG 3. (A, B) Tooth alternately irrigated with 15% disodium EDTA and 5.25% NaOCl (⫻1000). The middle third (A) of the canal shows a surface free of smear layer and pulpal fibers. The openings of the dentinal tubules are widened and clearly visible. The apical third (B) of the canal shows a smear layer present occluding the openings of many dentinal tubules. (C, D) Tooth alternately irrigated with 15% tetrasodium EDTA and 5.25% NaOCl (⫻1000). The middle third (C) is free of smear layer, and tubule orifices are clearly visible. The apical third (D) is covered with smear layer, which occludes the orifices of many dentinal tubules. (E, F) Tooth alternately irrigated with 25% tetrasodium EDTA and 5.25% NaOCl (⫻1000). The middle third (E) is free of smear layer and pulpal debris, whereas the apical third (F) shows a typical smear layer covering dentinal tubule openings.
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used. All seemed to provide adequate demineralization properties in the coronal two-thirds of the canals and were less effective in the apical third. DISCUSSION Nikiforuk and Sreenby (15) and Obst (17) found that chelation is maximized within a pH range of 6 to 10. The dissociation constants of the first, second, third, and fourth protons of EDTA are 2.0, 2.67, 6.16, and 10.26, respectively. Thus at higher pHs (above 10.3) the chelation efficiency of EDTA is greater due to a higher ratio of ionized to nonionized molecules in solution. Conversely, as the pH of the solution becomes more acidic, chelation becomes less efficient due to a decrease in the number of ionized molecules. A pilot study using 15, 25, and 50% concentrations of alkaline (pH 11.3) solutions of tetrasodium EDTA showed them to be even less effective in removing the smear layer than solutions prepared at a neutral pH. This lack of demineralization at alkaline pHs seems to be due to the solubility product constant of dentin and the lack of available calcium ions for chelation at higher pHs (15). The pH (7.3) of most commercial products results from 99% trisodium and chelates calcium on a mole per mole basis (18). At high pHs the excess number of hydroxyl groups will slow down the dissociation of hydroxyapatite, thus limiting the number of calcium ions available. At low or neutral pHs, the binding of calcium ions will tend to increase the dissociation of hydroxyapatite and their availability for chelation. Chelation will continue until the EDTA present in solution is depleted (18). The equation below illustrates the above description. ⫺ Ca10共PO4兲6共OH兲2 ^ 10Ca⫹2 ⫹ 6PO⫺3 4 ⫹ 2OH 共Dissociation of hydroxyapatite兲 ⫹ C10H13N2Na3O8共EDTANa3兲 2 ⫺ EDTANa-Ca ⫹ 2Na⫹⫹9Ca⫹2 ⫹ 6PO⫺3 4 ⫹ 2OH
No single irrigant is capable of removing both organic and inorganic material. Several studies have compared the efficacy of irrigating with 15 to 17% disodium EDTA with differing concentrations of NaOCl and their combined effect on smear layer removal. Goldman et al. (1) and Yamada et al. (12) reported that the most effective final irrigation to be 10 ml of EDTA followed by 10 ml of NaOCl. Baumgartner and Mader (11) used NaOCl as the final irrigant, but found alternating EDTA with NaOCl to be the most effective in eliminating the smear layer and producing clean root canal walls. These studies showed the importance and necessity of using a chelating agent to ensure the removal of the inorganic component of the smear layer, followed by a final irrigation using NaOCl to dissolve any remaining organic component. Baumgartner and Mader (11) examined the middle third of canals after preparation using sequentially larger files, #10 to #50. Yamada et al. (12) filed to at least a size #50 and used a highvolume flush of both irrigants to produce clean walls in the apical third. Neither Goldman et al. (1) nor Bystrom and Sundqvist (19) described their instrumentation technique. Bystrom and Sundqvist (19) showed that canal irrigation alternating EDTA and NaOCl produced better antimicrobial action than either solution used alone. Higher concentrations of EDTA solutions may exhibit increased antimicrobial action while maintaining effective demineralizing properties. A crown-down instrumentation technique may tend to allow removal of the majority of radicular pulp tissue early in the root canal preparation and increases the volume of irrigant in the canal. However
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rotary instrumentation may pack debris further into dentinal tubules thus making it more difficult to remove by irrigation. With nickeltitanium rotary systems it may be necessary to irrigate with higher final volumes or to allow irrigants to remain in the canals for longer times to ensure optimal canal cleanliness. The ease with which these solutions are made and the relative cost incurred with each are of interest. The disodium salt is approximately four times the cost of the tetrasodium salt, apparently due to the manufacturing process necessary in the formation of the disodium form. Both require pH adjustment to maximize the desired effects within the root canal system. It is this adjustment of pH that ultimately determines the ratio of salt forms that will predominate in a solution due to dissociation values (16). In conclusion, none of the EDTA solutions used alone were effective at completely removing the smear layer. All solutions of disodium and tetrasodium EDTA tested in combination with NaOCl were equally effective at removing the smear layer in the coronal and middle thirds, but were not as effective in the apical third. The tetrasodium salt, pH adjusted with HCl, is less expensive and just as effective as the more commonly used disodium EDTA. The authors thank Jerome D. Adey, MS, for his contribution during the scanning electron microscopic analysis portion of this project. Dr. O’Connell is an endodontic resident, Dr. Morgan was a former associate professor, Dr. Beeler is an assistant professor, and Dr. Baumgartner is professor and chairman, Department of Endodontology, Oregon Health Sciences University School of Dentistry, Portland, OR. Address requests for reprints to Dr. J. Craig Baumgartner, Department of Endodontology, Oregon Health Sciences University School of Dentistry, 611 S.W. Campus Drive, Portland, OR 97201. References 1. Goldman LB, Goldman M, Kronman JH, Lin PS. The efficacy of several endodontic irrigating solutions: a scanning electron microscopic study. Oral Surg 1981;52:199 –204. 2. McComb D, Smith DC. A preliminary scanning electron microscopic study of root canals after endodontic procedures. J Endodon 1975;1:238 – 42. 3. Mader C, Baumgartner JC, Peters D. Scanning electron microscopic investigation of the smeared layer on root canal walls. J Endodon 1984;10:477– 83. 4. Pashley DH, Michelich V, Kehl T. Dentin permeability: effects of smear layer removal. J Prosthet Dent 1981;46:531–7. 5. Madison S, Krell K. Comparison of ethylenediamine tetraacetic acid and sodium hypochlorite on the apical seal of endodontically treated teeth. J Endodon 1984;10:499 –503. 6. Chirnside LM. The bacteriological status of dentine around infected pulp canals. New Zealand Dent J 1958;54:173– 83. 7. Shovelton DS. The presence and distribution of microorganisms within nonvital teeth. Br Dent J 1964;117:101–7. 8. Economides N, Liolios E, Kolokuris I, Beltes P. Long-term evaluation of the influence of smear layer removal on the sealing ability of different sealers. J Endodon 1999;25:123–5. 9. Grossman LI, Meiman BW. Solution of pulp tissue by chemical agents. J Am Dent Assoc 1941;28:223. 10. Ostby BN. Chelation in root canal therapy. Sartryk Odontol Tidskr 1957;65:1–11. 11. Baumgartner JC, Mader C. A scanning electron microscopic evaluation of four root canal irrigation regimens. J Endodon 1987;13:147–57. 12. Yamada R, Armas A, Goldman M, Lin PS. A scanning electron microscopic comparison of a high volume final flush with several irrigating solutions. Part 3. J Endodon 1983;9:137– 42. 13. Seidberg BH, Schilder H. An evaluation of EDTA in endodontics. Oral Surg 1974;37:609 –20. 14. Patterson S. In vivo and in vitro studies of the effect of the disodium salt of EDTA on human dentin and its endodontic implications. Oral Surg 1963;16:83–103. 15. Nikiforuk G, Sreebny L. Demineralization of hard tissues by organic chelating agents. Science 1951;114:560. 16. Budvari S. The Merck index. 12th ed. Whitehouse Station: Merck and Co., Inc., 1996:593. 17. Obst JJ. Chelation and dentistry: conceptual orientation. NY J Dent 1962;30:331–5. 18. Dwyer FP, Mellor DP. Chelating agents and metal chelates. New York: Academic Press, 1964:95–141, 283–333. 19. Bystrom A, Sundqvist G. The antibacterial action of sodium hypochlorite and EDTA in 60 cases of endodontic therapy. Int Endod J 1985;18:35– 40.
Printed in U.S.A. VOL. 26, NO. 12, DECEMBER 2000
A Comparative Study of Smear Layer Removal Using Different Salts of EDTA Michael S. O’Connell, DDS, Leslie A. Morgan, DMD, William J. Beeler, DMD, MS, and J. Craig Baumgartner, DDS, MS, PhD
Three solutions of EDTA—a 15% concentration of the alkaline salt, a 15% concentration of the acid salt, and a 25% concentration of the alkaline salt— were evaluated for smear layer removal in root canal systems. All solutions were adjusted to pH 7.1 using either NaOH or HCl. When the EDTA solutions were alternately used for root canal irrigation with 5.25% NaOCl, they completely removed the smear layer in the middle and coronal thirds of canal preparations, but were less effective in the apical third. None of the EDTA solutions by themselves were effective at completely removing the smear layer at any level. The alkaline tetrasodium salt, pH adjusted with HCl, is more cost effective and performed equally as well as the more commonly used disodium salt.
EDTA solutions affects their efficacy and calcium ion availability in several ways. As the pH increases, the availability of calcium ions from hydroxyapitite for chelation decreases. At the same time, a greater dissociation of the EDTA produces an increased attraction for calcium ions. Conversely, at lower pHs the calcium ions become more available for chelation, but the efficacy of EDTA decreases. The optimal pH for EDTA solutions seems to be between 6 –10 (15). Traditionally the disodium salt of EDTA has been used for root canal irrigation in a 15 to 17% saturated concentration. The trisodium or tetrasodium salts can be used to make solutions of higher concentration (16). The pH is the major factor in determining the ratio of ionized to nonionized forms of EDTA in solution as well as limiting the concentration that can be achieved. The use of solutions in higher concentrations might lead to increased demineralization properties, aiding in smear layer removal. The purpose of this study was to evaluate the removal of smear layer and patency of dentinal tubules after root canal irrigation with three different EDTA solutions. The efficacy of EDTA solutions used alternately with NaOCl was also evaluated.
It has been shown that an amorphous smear layer composed of inorganic and organic material is produced on root canal walls after instrumentation (1, 2). It is typically 1 to 2 m in thickness, but may be packed into dentinal tubules as far as 40 m (3). Removal of this layer prior to root canal obturation remains controversial (4, 5). It has been suggested that the smear layer may decrease the permeability of dentin and prevent bacterial penetration into underlying dentinal tubules (4). Others believe the smear layer may contain bacteria and prevent antimicrobial agents from gaining access to underlying contaminated dentinal tubules (6, 7). Some studies suggest that removal of the smear layer may enhance the seal of the root canal filling (8). NaOCl has been shown to be an effective agent in dissolving organic tissue (9), whereas chelating agents such as EDTA have been the irrigants of choice to demineralize dentin and aid in the removal of the inorganic component of the smear layer (1, 10 –12). Studies have shown that EDTA is incapable of completely removing the smear layer from instrumented root canal walls (11, 12). A final irrigation of the root canal system with NaOCl after EDTA irrigation seems to produce the cleanest walls (1, 11, 12). Seidberg and Schilder (13) and Patterson (14) suggest that it would be advantageous to use a clinically biocompatible agent to decalcify dentin to minimize any untoward effects. The pH of
MATERIALS AND METHODS Twenty-two matched pairs of single-rooted human teeth were used in this study. The teeth were stored in 0.9% saline after extraction. External root surfaces were scaled to remove soft tissue remnants. The crowns were removed at the level of the cementoenamel junction using a high-speed handpiece and carbide bur. Four of the matched pairs were pregrooved buccolingually along the entire root length; in four teeth the pulp was extirpated with an xx fine barbed broach (Kerr, Romulus, MI), whereas the pulpal tissue in the contralateral teeth was left intact. These teeth were split by applying force with wire cutters in the previously made grooves in the roots and then immersed in 2.5% gluteraldehyde solution for 24 h to fix the remaining tissue. These specimens were used as technique controls for scanning electron microscopic evaluation of the experimental teeth. The other 18 matched pairs were randomly divided into three groups of six pairs. The working length was determined 1 mm short of the major diameter, and patency was checked with an ISO #10 file. The teeth were pregrooved buccolingually and placed in a copper band, which was held in place with polyvinyl impression material (Reprosil, Caulk Dentsply, Milford, DE). Teeth were instrumented using a crown-down technique with rotary nickel739
740
O’Connell et al.
Journal of Endodontics
titanium GT files (Tulsa Dental, Tulsa, OK) starting with the 0.12 tapered file and followed sequentially by the next three files in the series. Profile series 29 (Tulsa Dental, Tulsa, OK), instruments (.04 taper, sizes #4 to #7) were then used sequentially to the working length to enlarge the apical preparation. Within each group, six teeth were irrigated after the use of each file with 3 ml of 5.25% NaOCl and 3 ml of an experimental EDTA solution. The three experimental solutions were: 15% disodium EDTA (pH 7.1; adjusted with NaOH), 15% tetrasodium EDTA (pH 7.1; adjusted with HCl), and 25% tetrasodium EDTA (pH 7.1; adjusted with HCl). All EDTA solutions are expressed in weight per unit volume concentrations. The solutions were delivered alternately using a Monoject syringe with a 27 gauge needle, such that 3 ml of NaOCl was used after the .12GT, .08GT, #4 Profile, and #6 Profile, and 3 ml of EDTA was used after the .10GT, .06GT, #5 Profile, and #7 Profile. After completing the instrumentation and irrigation regimen, each tooth was irrigated with 3 ml of NaOCl, followed by 3 ml of distilled water. The contralateral teeth within each group were subjected to the same instrumentation technique and irrigation quantities, but the solutions used were either saline alone (two teeth), NaOCl alone (two teeth), or each experimental EDTA solution alone (two teeth). These teeth received an extra 6 ml of each solution to standardize the amount of irrigant used. All teeth received a final 3 ml irrigation with distilled water to halt any chemical activity and were immediately fractured. Each fractured segment was immersed in 2.5% gluteraldehyde for 24 h to fix any remaining soft tissue. All teeth received 33 ml of irrigant including the final 3 ml of distilled water. Instrumentation times were ⬍10 min in all cases. After 24 h the teeth were transferred to distilled water for 24 h and then vacuum dried. Each specimen was sputter-coated with ⬃35 nm of gold palladium (Hummer VII, Anatech Ltd., Alexandria, VA) and examined using a scanning electron microscope (JEOL T-330A, JEOL Technics Co., Tokyo, Japan). The root canal walls were examined at the coronal, middle, and apical thirds for canal wall appearance, smear layer removal, and patency of the dentinal tubules. Representative photomicrographs were taken (⫻1000) at the junction of the middle and coronal thirds for comparison.
RESULTS Control specimens in which pulpal tissue was left in place showed numerous strands of tissue of a fibrous nature. The walls in which the pulp had been pulled free appeared smooth and had numerous visible dentinal tubules (Fig. 1A). Root canals in which the pulpal tissue had been extirpated showed fibrils consistent with predentin along the dentinal walls; tubule orifices could be seen along the lengths of the canals associated with this fibrilar network (Fig. 1B). No observable smear layer was present. Saline-irrigated root canals all exhibited a smear layer on instrumented surfaces with some residual areas of pulpal debris (Fig. 2A). All three levels of the canals exhibited a uniform amorphous smear layer along the entire instrumented canal wall. Dentinal tubule orifices were not visible. Uninstrumented surfaces were covered by predentin and remaining pulpal debris. NaOCl-irrigated root canals seemed similar to the saline-irrigated teeth on instrumented surfaces (Fig. 2B). A typical amorphous smear layer was seen covering the entire lengths of the instrumented canals, with the exception of areas where calcospher-
FIG 1. (A) Control tooth without pulpal extirpation (⫻1000). A large portion of the remaining pulp tissue is still visible. In the background some pulpal strands and a collection of interwoven fibers composing the predentin are clearly seen. (B) Control tooth after pulpal extirpation (⫻1000). Predentin and some remaining pulpal fibrils are seen attached to the walls of the canal and tubule orifices are clearly visible.
ites were visible. These areas seemed to have been uninstrumented. Little pulp tissue debris was evident throughout the canal space. Root canals irrigated with any of the three EDTA solutions seemed to have smooth instrumented walls that were often covered by an incompletely removed smear layer with some dentinal tubule openings visible (Fig. 2, C–E). In general the root canal walls irrigated with any EDTA solution seemed to be free of surface debris and contained a greater number of patent orifices compared with canals irrigated with saline or NaOCl, which were uniformly covered with a smear layer on instrumented surfaces. Root canals in which EDTA and NaOCl irrigants were alternated all exhibited open fields of dentinal tubules in the coronal and middle thirds, with very little remaining smear layer (Fig. 3, A, C, and E). A typical amorphous smear layer was evident in the apical third, regardless of the solution used (Fig. 3, B, D, and F). Some tubule orifices were visible where the smear layer appeared to be partially removed. Areas of eroded dentinal tubules were scattered along canal walls consistent with uninstrumented areas with corresponding fields of calcospherites. No differences could be detected between concentrations or types of EDTA solutions
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FIG 2. (A) Tooth irrigated with saline only (⫻1000). A typical amorphous smear layer with extensive debris is noted on the canal wall. No tubule openings are visible. (B) Tooth irrigated with NaOCl only (⫻1000). A typical smear layer is visible with less superficial debris apparent. No openings of dentinal tubules are visible. (C, D, and E) Teeth irrigated with 15% disodium, 15% tetrasodium, and 25% tetrasodium EDTA only (⫻1000). Less smear layer is present overall, and the openings of some dentinal tubules are present. No pulpal fibers are visible.
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FIG 3. (A, B) Tooth alternately irrigated with 15% disodium EDTA and 5.25% NaOCl (⫻1000). The middle third (A) of the canal shows a surface free of smear layer and pulpal fibers. The openings of the dentinal tubules are widened and clearly visible. The apical third (B) of the canal shows a smear layer present occluding the openings of many dentinal tubules. (C, D) Tooth alternately irrigated with 15% tetrasodium EDTA and 5.25% NaOCl (⫻1000). The middle third (C) is free of smear layer, and tubule orifices are clearly visible. The apical third (D) is covered with smear layer, which occludes the orifices of many dentinal tubules. (E, F) Tooth alternately irrigated with 25% tetrasodium EDTA and 5.25% NaOCl (⫻1000). The middle third (E) is free of smear layer and pulpal debris, whereas the apical third (F) shows a typical smear layer covering dentinal tubule openings.
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used. All seemed to provide adequate demineralization properties in the coronal two-thirds of the canals and were less effective in the apical third. DISCUSSION Nikiforuk and Sreenby (15) and Obst (17) found that chelation is maximized within a pH range of 6 to 10. The dissociation constants of the first, second, third, and fourth protons of EDTA are 2.0, 2.67, 6.16, and 10.26, respectively. Thus at higher pHs (above 10.3) the chelation efficiency of EDTA is greater due to a higher ratio of ionized to nonionized molecules in solution. Conversely, as the pH of the solution becomes more acidic, chelation becomes less efficient due to a decrease in the number of ionized molecules. A pilot study using 15, 25, and 50% concentrations of alkaline (pH 11.3) solutions of tetrasodium EDTA showed them to be even less effective in removing the smear layer than solutions prepared at a neutral pH. This lack of demineralization at alkaline pHs seems to be due to the solubility product constant of dentin and the lack of available calcium ions for chelation at higher pHs (15). The pH (7.3) of most commercial products results from 99% trisodium and chelates calcium on a mole per mole basis (18). At high pHs the excess number of hydroxyl groups will slow down the dissociation of hydroxyapatite, thus limiting the number of calcium ions available. At low or neutral pHs, the binding of calcium ions will tend to increase the dissociation of hydroxyapatite and their availability for chelation. Chelation will continue until the EDTA present in solution is depleted (18). The equation below illustrates the above description. ⫺ Ca10共PO4兲6共OH兲2 ^ 10Ca⫹2 ⫹ 6PO⫺3 4 ⫹ 2OH 共Dissociation of hydroxyapatite兲 ⫹ C10H13N2Na3O8共EDTANa3兲 2 ⫺ EDTANa-Ca ⫹ 2Na⫹⫹9Ca⫹2 ⫹ 6PO⫺3 4 ⫹ 2OH
No single irrigant is capable of removing both organic and inorganic material. Several studies have compared the efficacy of irrigating with 15 to 17% disodium EDTA with differing concentrations of NaOCl and their combined effect on smear layer removal. Goldman et al. (1) and Yamada et al. (12) reported that the most effective final irrigation to be 10 ml of EDTA followed by 10 ml of NaOCl. Baumgartner and Mader (11) used NaOCl as the final irrigant, but found alternating EDTA with NaOCl to be the most effective in eliminating the smear layer and producing clean root canal walls. These studies showed the importance and necessity of using a chelating agent to ensure the removal of the inorganic component of the smear layer, followed by a final irrigation using NaOCl to dissolve any remaining organic component. Baumgartner and Mader (11) examined the middle third of canals after preparation using sequentially larger files, #10 to #50. Yamada et al. (12) filed to at least a size #50 and used a highvolume flush of both irrigants to produce clean walls in the apical third. Neither Goldman et al. (1) nor Bystrom and Sundqvist (19) described their instrumentation technique. Bystrom and Sundqvist (19) showed that canal irrigation alternating EDTA and NaOCl produced better antimicrobial action than either solution used alone. Higher concentrations of EDTA solutions may exhibit increased antimicrobial action while maintaining effective demineralizing properties. A crown-down instrumentation technique may tend to allow removal of the majority of radicular pulp tissue early in the root canal preparation and increases the volume of irrigant in the canal. However
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rotary instrumentation may pack debris further into dentinal tubules thus making it more difficult to remove by irrigation. With nickeltitanium rotary systems it may be necessary to irrigate with higher final volumes or to allow irrigants to remain in the canals for longer times to ensure optimal canal cleanliness. The ease with which these solutions are made and the relative cost incurred with each are of interest. The disodium salt is approximately four times the cost of the tetrasodium salt, apparently due to the manufacturing process necessary in the formation of the disodium form. Both require pH adjustment to maximize the desired effects within the root canal system. It is this adjustment of pH that ultimately determines the ratio of salt forms that will predominate in a solution due to dissociation values (16). In conclusion, none of the EDTA solutions used alone were effective at completely removing the smear layer. All solutions of disodium and tetrasodium EDTA tested in combination with NaOCl were equally effective at removing the smear layer in the coronal and middle thirds, but were not as effective in the apical third. The tetrasodium salt, pH adjusted with HCl, is less expensive and just as effective as the more commonly used disodium EDTA. The authors thank Jerome D. Adey, MS, for his contribution during the scanning electron microscopic analysis portion of this project. Dr. O’Connell is an endodontic resident, Dr. Morgan was a former associate professor, Dr. Beeler is an assistant professor, and Dr. Baumgartner is professor and chairman, Department of Endodontology, Oregon Health Sciences University School of Dentistry, Portland, OR. Address requests for reprints to Dr. J. Craig Baumgartner, Department of Endodontology, Oregon Health Sciences University School of Dentistry, 611 S.W. Campus Drive, Portland, OR 97201. References 1. Goldman LB, Goldman M, Kronman JH, Lin PS. The efficacy of several endodontic irrigating solutions: a scanning electron microscopic study. Oral Surg 1981;52:199 –204. 2. McComb D, Smith DC. A preliminary scanning electron microscopic study of root canals after endodontic procedures. J Endodon 1975;1:238 – 42. 3. Mader C, Baumgartner JC, Peters D. Scanning electron microscopic investigation of the smeared layer on root canal walls. J Endodon 1984;10:477– 83. 4. Pashley DH, Michelich V, Kehl T. Dentin permeability: effects of smear layer removal. J Prosthet Dent 1981;46:531–7. 5. Madison S, Krell K. Comparison of ethylenediamine tetraacetic acid and sodium hypochlorite on the apical seal of endodontically treated teeth. J Endodon 1984;10:499 –503. 6. Chirnside LM. The bacteriological status of dentine around infected pulp canals. New Zealand Dent J 1958;54:173– 83. 7. Shovelton DS. The presence and distribution of microorganisms within nonvital teeth. Br Dent J 1964;117:101–7. 8. Economides N, Liolios E, Kolokuris I, Beltes P. Long-term evaluation of the influence of smear layer removal on the sealing ability of different sealers. J Endodon 1999;25:123–5. 9. Grossman LI, Meiman BW. Solution of pulp tissue by chemical agents. J Am Dent Assoc 1941;28:223. 10. Ostby BN. Chelation in root canal therapy. Sartryk Odontol Tidskr 1957;65:1–11. 11. Baumgartner JC, Mader C. A scanning electron microscopic evaluation of four root canal irrigation regimens. J Endodon 1987;13:147–57. 12. Yamada R, Armas A, Goldman M, Lin PS. A scanning electron microscopic comparison of a high volume final flush with several irrigating solutions. Part 3. J Endodon 1983;9:137– 42. 13. Seidberg BH, Schilder H. An evaluation of EDTA in endodontics. Oral Surg 1974;37:609 –20. 14. Patterson S. In vivo and in vitro studies of the effect of the disodium salt of EDTA on human dentin and its endodontic implications. Oral Surg 1963;16:83–103. 15. Nikiforuk G, Sreebny L. Demineralization of hard tissues by organic chelating agents. Science 1951;114:560. 16. Budvari S. The Merck index. 12th ed. Whitehouse Station: Merck and Co., Inc., 1996:593. 17. Obst JJ. Chelation and dentistry: conceptual orientation. NY J Dent 1962;30:331–5. 18. Dwyer FP, Mellor DP. Chelating agents and metal chelates. New York: Academic Press, 1964:95–141, 283–333. 19. Bystrom A, Sundqvist G. The antibacterial action of sodium hypochlorite and EDTA in 60 cases of endodontic therapy. Int Endod J 1985;18:35– 40.