Improving endodontic success through coronal leakage prevention

Dr. Gregori M. Kurtzman demonstrates how to provide a sound coronal seal for positive treatment outcomes

Educational aims and objectives

The purpose of this article is to:
•    Improve the reader’s understanding of how coronal leakage can affect endodontic success.
•    Discuss methods to improve the quality of endodontic treatment and increase long-term success.

Expected outcomes

Correctly answering the quiz questions, worth 2 hours of CE, will demonstrate you can understand:
•    what coronal leakage can cause;
•    methods to improve coronal leakage;
•    how to improve the quality of the endodontic treatment provided.

Click here to take quiz.

Endodontic failure has been associated with coronal leakage within the canal system following obturation. The literature suggests that coronal leakage is far more likely a determinant of clinical success or failure than apical leakage.1  Recent advances in resin obturation materials have been shown to provide superior sealing of the canal system;  however, without addressing the coronal aspect of the tooth, endodontically, failure may occur. Studies confirm that a sound coronal seal is of paramount importance to the overall success of root canal treatment.2,3 Regardless of the obturation method, the best rule is: a properly cleaned, shaped, and obturated tooth should be permanently restored as soon as possible.4   
No matter what our intentions are following obturation of the canal system, patients may delay restoration of the tooth that has been treated. Financial and time constraints often influence when the final restoration is completed. Additionally, between visits, an adhesive material will prevent leakage and contamination of the canal.

Coronal leakage

Coronal leakage has been indicated in the literature as the major determinant of endodontic success or failure. No matter what we place in the canal, if the coronal portion of the tooth is not sealed with materials that bond to tooth structure and are resistant to dissolution by oral fluids, then, over time, endodontic failure may be inevitable.
It is not unusual to have a patient present with decay at the margin of a crown of a tooth that had prior endodontic therapy. Because the tooth was treated endodontically, sensitivity that may indicate a problem under the crown will not alert the patient to seek dental care. Coronal leakage for even a minimal amount of time may quickly lead to apical migration of bacteria. When the patient does arrive at the office,  coronal leakage already may have been ongoing for an extended period of time, complicating treatment or rendering the tooth non-restorable and necessitating extraction.

The literature indicates that significant coronal dye and bacterial leakage following exposure of sealed root canals to artificial and natural saliva leading to complete bacterial leakage may occur within 2 days.5 A supporting in-vitro study found that dye leakage can occur in as little as  3 days.6 It has been suggested that gutta percha does not offer an effective barrier to crown-down leakage when exposed to the oral environment.7 Additional studies using gutta percha and various sealers indicate that gutta percha will allow bacterial leakage, but use of an adhesive sealer can significantly slow or stop coronal-apical bacterial migration.8

The predominant bacteria found in root-filled teeth with coronal leakage and persistent apical periodontitis is the gram-positive facultative anaerobe, Staphylococcus. This is followed by the groups Streptococcus and Enterococcus; all normal salivary flora.9  Coronal leakage provides a constant source of microorganisms and nutrients that initiate and maintain periradicular inflammation and may well be the largest cause of failure in endodontic therapy.10

Endodontic obturation materials do not prevent coronal microleakage for an indefinite period of time.11 In a sample of 937 root-filled teeth that had not received restorative treatment during the previous year, the data showed that the technical standard of both coronal restoration and root filling were essential to periapical health.12 It is not uncommon for coronal leakage to occur following root canal treatment as a result of the presence of deficient composite resin fillings and secondary caries under restorations.13

Yet the endodontic materials utilized over the past 50 years have shown that they do not prevent coronal leakage when challenged. In yet another investigation, 45 root canals were cleaned, shaped, and then obturated with gutta percha and root canal sealer, using a lateral condensation technique. The coronal portions of the root-filling materials were placed in contact with Staphylococcus epidermidis and Proteus vulgaris. The number of days required for these bacteria to penetrate the entire root canals was determined. Over 50% of the root canals were completely contaminated after 19-day exposure to S. epidermidis. Fifty percent of the root canals were also totally contaminated when the coronal surfaces of their fillings were exposed to P. vulgaris for 42 days.14 When comparing AH-26® (Dentsply International) and other commonly used sealers after 45 days of exposure to the oral cavity, none of the sealers was capable of preventing leakage and coronal dye penetration.15 So we can see that the quality of both the coronal restoration and obturation material are essential to periapical health as none of the present-day root canal sealers may hermetically seal “the root canal wall—gutta-percha filling interface.” In this respect, the importance of perfectly sealing coronal restorations (both temporary and permanent) needs to be emphasized.16

Pre-endodontic therapy buildups (canal projection)

Coronal leakage is a major contributor to endodontic failure.17 A bonded core placed prior to disinfection and obturation of the canal system of the tooth can greatly diminish the leakage potential both during and after endodontic therapy.

Isolation of the pulp chamber can be a challenging task when minimal coronal structure remains, and endodontic therapy is required as part of the oral rehabilitation (Figure 1). Coronal reinforcement has traditionally been addressed following the endodontic phase, but a coronal bonded buildup can simplify the endodontic phase and strengthen the tooth, decreasing the possibility of further damage to the tooth due to the dam clamp or mastication before a full coverage restoration can be placed. The Canal Projector® (CMJ Engineering, Inc.) cone allows isolation of the individual canals by surrounding them with a resin buildup (Figure 2). Sealing the pulpal floor and area surrounding the canal orifices also will decrease coronal leakage potential during and following endodontic treatment.

Following identification of the canal orifices and caries removal, a Root Canal Projector cone is placed on a hand file and inserted into each canal. A dentin adhesive is placed on all exposed surfaces and light cured. This is followed by injection of a dual-cure buildup material around the projector cones. When set of the buildup material has been completed, the hand files and projectors can be removed leaving straight-line access into each individual canal. Visualization of the orifice is elevated to the occlusal plane instead of deep within the tooth, and a bonded seal coronally around each orifice is achieved. Should the restoring dentist wish to place posts into the tooth, post space preparation is simplified, and misdirection of the post preparation is minimized.

Coronal restoration (access sealing)

Microorganisms can penetrate through different temporary restorative materials and supposedly well-obturated root canals. The use of adhesive sealers may play an important role by minimizing coronal leakage. In addition, the importance of an immediate definitive coronal seal should be emphasized after obturation of the canal system.18-20

Seventy extracted single-rooted mandibular premolars were studied to determine the length of time needed for bacteria present in natural human saliva to penetrate through three commonly used temporary restorative materials and through the entire root canal system obturated with the lateral condensation technique. The average time for broth contamination of access cavities closed with gutta percha (7.85 days), Immediate Restorative Material (IRM) (12.95 days), and Cavit™G (3M™ ESPE™) (9.80 days) indicate that even in the short periods of time normally permitted between visits, complete leakage may result. IRM, a common temporary material, was shown to leak to a significantly higher degree than glass ionomers.21 Glass-ionomer cement due to its adhesive nature may prevent bacterial penetration to the periapex of root-filled teeth over a 1-month period as compared to IRM or Cavit temporary restorations.22 Another important consideration with regard to the temporary restoration’s ability to prevent coronal leakage is how the material behaves under mechanical load and thermocycling. Non-adhesive temporaries show an increased percentage of marginal breakdown and increased microleakage after thermocycling and loading. There was no significant improvement with increased thickness of the temporary material.23-25  When crowns were sealed with IRM, recontamination was detected within 13.5 days in the canals medicated with chlorhexidine, after 17.2 days in the group medicated with CaOH2, and after 11.9 days in the group medicated with both chlorhexidine and CaOH2. The group with no medication, but sealed with IRM, showed recontamination after 8.7 days. There were statistically significant differences between the teeth with or without coronal seal. The coronal seal delayed but did not prevent leakage of microorganisms.26 Other studies confirm that IRM started to leak after 10 days, whereas Cavit and Dyract® (Dentsply International) leaked after 2 weeks.27

The use of a resin-based temporary restorative material or glass ionomer over partially removed resin composite restorations could be beneficial in achieving better resistance to marginal leakage (Figure 3). Maintaining partially removed permanent restorations does not seem to cause a problem with achieving marginal seal.28 Glass ionomer provided a statistically better coronal seal then bonded composite or a bonded amalgam, preventing bacterial apical migration.29 This may be due to the glass ionomer’s ability to adhere to the scerlotic dentin found on the pulpal floor better then  adhesive resins. The key seems to be, lock out the coronal bacteria, and the apical area will heal (Figures 4 and 5).

Mineral Trioxide Aggregate (MTA), since its introduction a few years ago, has been advocated as a sealing material especially when perforation has occurred. However,  an investigation found mild inflammation was observed in 17% and 39% of the roots with and without an orifice plug, so respectively without development of severe inflammation, the sealing efficacy of MTA orifice plugs could not be determined.30

Should amalgam be the material of choice for the dentist, a bonded amalgam produced significantly less leakage than did the non-bonded amalgams. To prevent the reinfection of the endodontically treated molar, it may be preferable to restore the tooth immediately after obturation by employing a bonded amalgam coronal-radicular technique.31 Core buildup or access closure with adhesive materials has shown good long-term leakage resistance. The “sandwich” technique (glass ionomer base with overlaying composite) and the composite resin restorations allowed significantly less coronal leakage than glass ionomer cement restorations. This may be because the composite resin prevents salivary dissolution of the glass ionomer long term.32

Results indicate that the sealing ability of adhesive and flowable materials can decrease coronal leakage potential.33 Because of the risk of coronal microleakage, endodontically treated teeth should be restored as quickly as possible.34 It is more prudent to use a permanent restorative material for provisional restorations to prevent inadequate canal sealing and the resulting risk of fluid penetration.35 To minimize the potential of perforation when re-entering the tooth to place either a post or to retreat endodontically, placement of a contrasting colored resin over each orifice may be beneficial. This is followed by covering the entire pulpal floor with a tooth-colored flowable resin (Figures 6-8). These are available in a multitude of easily identifiable colored flowable composites. Available in pink (PermaFlo® Pink) or purple (PermaFlo® Purple) from Ultradent, dark red (Flow-It® dark gingival) from Pentron Clinical Technologies or dark blue from DenMat.

Coronal microleakage has received considerable attention as a factor related to failure of endodontic treatment, and much emphasis is placed on the quality of the final restoration. Intracanal posts are frequently used for the retention of coronal restorations. Many authors have examined coronal microleakage with respect to gutta-percha root fillings and coronal restorations, but few have investigated the coronal seal afforded by various post systems. The seal provided by a cemented post depends on the seal of the cement used. It appears that the dentin-bonding cements (adhesive resins and glass ionomers) have less microleakage than the traditional, non-dentin-bonding cements (i.e. zinc phosphates and polycarboxolates).36 Resin-supported polyethylene fiber and glass-fiber dowels showed the lowest coronal leakage when compared with stainless steel and zirconia dowels. This may be due to better adhesion of the luting agent to these resin impregnated posts than metal or ceramic posts, which do not allow adhesive penetration into the surface of the post. There were no significant differences between resin-supported polyethylene fiber and glass-fiber dowels at any time period. The initial leakage measurement in zirconia dowel and stainless-steel dowels were similar but became significantly different at 3 and 6 months. Resin-supported polyethylene fiber dowels and glass-fiber dowels tested exhibited less microleakage compared to zirconia dowel systems.37

Cleansing the canal (smear layers)

Coronal sealing ability is not the only factor to influence the seal of the canal and prevent apical leakage. How well the sealer adheres to the canal walls is also important. Smear layer can be a factor, which may prevent sealer penetration into the dentinal tubules. The frequency of bacterial penetration through teeth obturated with intact smear layer (70%) was significantly greater than that of teeth from which the smear layer had been removed (30%). Removal of the smear layer enhanced sealability as evidenced by increased resistance to bacterial penetration.38  The incidence of apical leakage was reduced in the absence of the smear layer, and the adaptation of gutta percha was improved no matter what obturation method was used later.39-41 However, regardless of the obturation technique (thermoplastized, lateral or vertical condensation, or single cone) when a non-adhesive sealer was used, leakage increased after 30 days.42

What is used to obturate the canals is important, however, the manner in which the canal was prepared prior to obturation also determines how well the canal is sealed when therapy is completed. Rotary instrumentation with NiTi files has shown less microleakage than hand instrument-prepared canals regardless of what was used to obturate the canal.43 The machining of the canal walls with NiTi rotary instruments provides smoother canal walls and shapes that are easier to obturate than can be achieved with stainless-steel files. The better the adaptation of the obturation material to the instrumented dentinal walls, the less leakage is to be expected along the entire root length. The better the canal walls are prepared, the more smear layer and organic debris is removed which is beneficial to root canal sealing.

Smear layer removal is best achieved by irrigating the canals with NaOCL (sodium hypochlorite) followed by 17% EDTA solution.44 Whereas, the NaOCL dissolves the organic component of the smear layer exposing the dentinal tubules lining the canal walls, EDTA, a chelating agent, dissolves the inorganic portion of the dentin opening the dentinal tubules. Alternating between the two irrigants as the instrumentation is being performed will permit removal of more organic debris further into the tubules, increasing resistance to bacterial penetration once the canal is obturated.45, 46

Obturation

The purpose of the obturation phase of a endodontic therapy is two-fold: to prevent microorganisms from re-entering the root canal system, and to isolate any microorganisms that may remain within the tooth from nutrients in tissue fluids. No matter how well we seal the canal, if the coronal portion of the tooth is not thoroughly sealed, then bacterial leakage may be a matter of time. Accessory canals may be present in the pulp chamber leading to the furcation area. This may be an additional source of leakage that often goes unaddressed either following obturation of the canals or during the restorative phase. Placement of a layer of resin-modified glass ionomer cement or adhesive resin to seal this area immediately following obturation can prevent leakage prior to final restoration of the tooth.47 But, it must always be remembered that success will only be achieved if the root canal system has been as thoroughly debrided as possible of infected material. Irrigation is key to removal of this smear layer lining the canal walls.

The obturation material is a two-pronged sword. What sealer is used is as important as which core material is placed within the canal. Gutta percha has limitations in resistance to coronal leakage that has been overcome with the newer resin alternatives. Although sealers can form close adhesion to the root canal wall, none is able to bond to the gutta-percha core material. Upon setting, shrinkage of the sealer allows the sealer to pull away from the gutta-percha core, leaving a microgap through which bacteria may pass.48  Several alternatives are available for core material selection.

Resilon™, a resin gutta-percha alternative that is bondable with methacrylic sealers such as Epiphany™ (Pentron Clinical Technologies) and RealSeal™ (SybronEndo) was introduced 3 years ago after extensive studies. The core material Resilon, is available in .02, .04 or .06 taper ISO-sized cones from Pentron Clinical Technologies or SybronEndo and as sized apical plugs (Lightspeed Technology Inc.).49, 50  Resilon showed significantly less leakage than gutta percha. In studies performed at University of North Carolina, the gutta percha group demonstrated leakage in 80% of specimens and was not dependent on obturation technique nor which sealer was used.51 Because of these limitations seen with gutta percha, the seal of a coronal restoration may be as important as the gutta-percha fill in preventing reinfection of the root canal. Studies have shown that leakage of bacteria with Resilon is significantly reduced compared with gutta percha. The significance of this is should the coronal aspect break down, the adhesive obturation material may slow down or prevent apical migration of bacteria allowing healing to occur (Figures 9 and 10).  As an additional benefit, when filling the canals with the new resin-based obturation material, an increase was observed in the in-vitro resistance to fracture of endodontically treated single-canal extracted teeth when compared with standard gutta-percha techniques. Resilon demonstrated a 25% increase in root strength than gutta-percha samples.52

Fiber obturators, an alternative core material, may be used when a post will be placed to strengthen the root and retain the coronal core. These allow obturation of the canal and placement of the post at the same step, assuring coronal seal.53, 54  Microbial leakage occurred more quickly in lateral and vertical condensation techniques compared with obturation with fiber obturation systems.55 Currently, two fiber obturator systems are commercially available: the FibreFill™ system (Pentron Clinical Technologies) that was introduced in 2001 and the recently available InnoEndo™ system (Heraeus Kulzer). Both systems use resin sealers, allowing formation of a monoblock across the root to both strengthen and seal the canal system.

Sealer selection is very important to prevent microleakage and permit a bond to the core material. Zinc oxide and eugenol (ZOE) sealers have been a mainstay in endodontic therapy for over 100 years. When exposed to coronal leakage, ZOE sealers demonstrated complete leakage by the second day. Results indicated that none of the ZOE formulations tested could predictably produce a fluid-tight seal even up to the fourth day.56

AH-26, an epoxy sealer originally introduced 40 years ago, was also unable to bond to gutta percha, leading to coronal leakage issues. Leakage with AH-26 was not dependent on obturation technique showing gross leakage increasing within the first 4 months following obturation when coronally challenged. Coronal leakage was significantly greater during the first 4 months57. Complete bacterial leakage with AH-26 may be seen in as few as 8.5 weeks should the coronal restoration permit leakage.58

Additionally, in-vitro studies found gutta percha and AH-26 or AH-26 Plus permitted leakage of both bacteria and fungi. Leakage in experimental teeth occurred between 14 and 87 days, with 47% of the samples showing leakage. AH-26 sealer permitted bacterial leakage in 45% and fungi leakage in 60% samples; whereas, the samples with AH Plus demonstrated bacterial leakage in 50% and fungi in 55% of the samples. There was no statistically significant difference in penetration of bacteria and fungi between the two versions of the sealer.59  Comparative studies looking at periapical inflammation between teeth treated with gutta percha with AH-26 sealer and Resilon with methacrylic sealer found statistically less inflammatory response with the Resilon-treated teeth. Mild inflammation was observed in 82% of roots filled with gutta percha and AH-26 sealer compared with 19% of Resilon-treated teeth. The monoblock provided by the Resilon system was associated with less apical periodontitis, which may be because of its superior resistance to coronal microleakage.60 As AH-26 is unable to bond to gutta percha, polymerization shrinkage of the epoxy resin can result in a microgap leading to the leakage reported in the literature (Figure 11). Alternatively, the bond reported between the methacrylic sealer (Epiphany or RealSeal) and Resilon is sufficient to prevent microgap formation as the sealer polymerizes (Figure 12).

Electrophoresis leakage studies recently completed at University of Maryland comparing gutta percha with AH-26 sealer and Resilon with Epiphany sealer found significant differences in leakage resistance. The gutta percha/AH-26 group demonstrated an average resistance of 404.6 microamps with 100% of the samples leaking compared to an average resistance of 27.7 microamps with 60% showing some leakage. The lower the value of resistance in microamps, the more resistant the specimen was to leakage.61 These results support other studies indicating that gutta percha and AH-26, when challenged, do not offer resistance to coronal leakage. Should the practitioner wish to continue using these materials, a permanent restoration needs to be placed at the appointment when endodontic therapy is completed.

Conclusion

Of 41 articles published between 1969 and 1999 (the majority from the 1990s), the literature suggests that the prognosis of root canal-treated teeth can be improved by sealing the canal and minimizing the leakage of oral fluids and bacteria into the periradicular areas as soon as possible after the completion of root canal therapy62.

Endodontic success is a multifactoral issue. Like a jigsaw puzzle, the full picture can only be seen when all the pieces are fit together. How the canals are instrumented is as important as what is used to obturate the canal system. This is also influenced by what is placed coronally, and when the coronal aspect is sealed. NiTi rotary instruments and an irrigation protocol that includes NaOCL and EDTA will maximize the sealing ability of glass ionomer or the newer methacrylic resin sealers. The last piece of the puzzle, sealing coronally, should be performed with adhesive permanent restorative materials immediately at the conclusion of the first endodontic appointment to prevent apical migration of bacteria and assure sealability of the canals.

References

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51.    Shipper G, Orstavik D, Teixeira FB, Trope M (2004). An evaluation of microbial leakage in roots filled with a thermoplastic synthetic polymer-based root canal filling material (Resilon). J Endod 30(5): 342-347.

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