Endod Dent Traumatol 1994: 10: 105-108Printed in Denmark . AU rights reserved

Copyright © Munksgaard 1994

Endodontics &Dental Traumatology

ISSN 0109-2502

Review article

Coronal leakage as a cause of failure in root-canal therapy: a reviewSaunders WP, Saunders EM. Coronal leakage as a cause offailure in root-canal therapy: a review. Endod Dent Traumatol1994; 10: 105-108. © Munksgaard, 1994.

Abstract - This paper reviews the evidence that coronal leakageof root canals may lead to failure of root-canal therapy. Thecauses of coronal leakage and methods by which this leakagemay be prevented are described.

W. p. Saunders\ E. M. Saunders^^Department ot Adult Dental Care, Glasgow DentalHospital and School, ^Department of ConservativeDentistry, Dundee Dental School, Scotland, UK

Key words: root canal treatment; failure; coronalleakage.

W P Saunders, Department of Adult DentalCare, Glasgow Dental Hospital and School,378 Sauchiehall Street, Glasgow G2 3JZ,Scotland.

Accepted October 13, 1993

Root-canal treatment can be difficult and time con-suming, especially in molars with multiple fine,curved root canals. In order to provide an environ-ment where healing can occur and be maintained,it is important that sufficient care is taken, not onlyduring the cleaning, shaping and obturation of thecanal system, but also when restoring the crown.

Apical leakage as a cause of failure

Failure of root-canal treatment can be attributed toa number of causes, but leakage through the rootfilling itself is thought to be a major factor. Strind-berg (1), in 1956, considered that the most commoncause of failure was leakage of tissue fluids apicallyaround inadequate root fillings. The University ofWashington School of Dentistry Study, undertakento evaluate treated endodontic cases and to deter-mine their rate of success, was reported by Ingle (2)in 1965. This study found that of 104 failed cases,66 were associated with a poor apical seal. Otherstudies (3, 4) have shown that prognosis for success-ful root-canal therapy was poorer when there were'̂oids apically between the root filling and the wall

of the canal. But why should this result in failure?t̂ seems very likely that necrotic debris and micro-

organisms cannot be completely eliminated from

the prepared root canal (5-7). If the canal space isnot sealed adequately then micro-organisms them-selves, or their toxins, can cause inflammation inthe periapical tissues (8).

Importance of coronal leakage in failure of root canaltreatment

Obturated root canals may be recontaminated bymicro-organisms in a number of ways:1) Delay in placing a coronal restoration following

root canal treatment. Although temporary re-storative materials such as Cavit G and re-inforced zinc oxide eugenol cements, such asKalzinol and IRM, have good sealing propertiesthey tend to dissolve slowly in the presence ofsaliva, and the seal may break down. If a tem-porary restoration is of inadequate thickness,leakage will occur (9).

2) Fracture of the coronal restoration and/or tooth.3) Preparation of post space for the provision of

a post-retained restoration when the remainingapical section of the root filling is of inadequtedensity and/or length.

The concept that one cause of failure of root-canal treatment may be the result of coronal leakageis not a new one. Marshall & Massler (10), in 1961,

Saunders & Saunders

were concerned about the role of the occlusal seal inroot-filled teeth. They wondered whether the overallseal of the root canal was altered if the seal wasbroken coronally. They also speculated on the prog-nosis of root-canal treatment if the quality of obtu-ration of the root canal was poor, but the coronalseal was good. They indertook a leakage study usinga radioactive tracer and showed that coronal leak-age occurred despite the presence of a coronaldressing.

Allison et al. (11) in 1979 made brief referenceto the possibility that a poor coronal seal mightcontribute to clinical failure.

In 1987 the importance of coronal leakage in theprognosis of root-canal treatment was readdressed.An in vitro leakage study (12) showed that after onlythree days exposure to artificial saliva there wasextensive coronal leakage of a tracer dye throughapparently sound root fillings. They considered thatleakage of this nature should be taken into accountas a potential aetiological factor in failure of root-canal treatment. Madison & Wilcox (13) confirmedthat exposure of root canals to the oral environmentallowed coronal leakage to take place, in some casesalong the whole length of the root canal.

Further studies have confirmed the importanceof coronal leakage as a possible cause of failure ofroot-canal treatment. Torabinejad et al. (14) foundthat 50% of single-rooted teeth, root filled usinglateral condensation of guttapercha and a sealercement, were contaminated with bacteria along thewhole length of the root after 19 days or 42 days,depending on the contaminating organism. Anotherstudy (15) assessed salivary penetration through ob-turated root canals. The results led to a recommen-dation that root fillings which had been contami-nated coronally for at least three months, should bere-done prior to placement of the definitive restora-tion. More recently, Khayat et al. (16) have shownthat root canals obturated with gutta-percha andRoth's sealer, using either lateral condensation orvertical condensation were contaminated apicallywith bacteria from saliva exposed to the coronalpart of the root canal only. All canals were contami-nated within 30 days of exposure.

Post space preparation and coronal leakage

Restoration of a root-filled tooth sometimes requiresthe use of an intra-canal post. During mechanicalpreparation of the post space it is possible that theroot filling may be twisted or vibrated, with disrup-tion of the seal (17). To avoid this problem therewas a vogue for placing sectional silver or gutta-percha cones in the apical portion of the root canal.It now seems that the advantages of leaving theapical portion of the root filling undisturbed is

outweighed by the fact that much of the. canal sys-tem is vulnerable to contamination from an inad-equate seal coronally. Three-dimensional obturationof the whole canal with gutta-percha will coat thewall of the root canal with sealer and may allowthe filling of lateral root canals.

Provided a minimum of 5 mm of sound apicalroot filling is left in situ (18, 19), studies have shownthat removal of laterally condensed gutta-perchadoes not affect the apical seal. This is the case ir-respective of whether the post space is preparedimmediately after obturation or is delayed (18, 20,2 1 ) .

Coronal leakage associated with molar teeth

Most studies of coronal leakage have involved theuse of single-rooted teeth. It is probably more im-portant, however, to seal the coronal part of theroot canal system in molars because accessory canalsmay be present in the floor of the pulp chamber(22). Gontamination through these canals may beresponsible for inflammatory changes taking placein the periodontal tissues of the furcation due todirect spread of micro-organisms and their toxinsfrom the pulp chamber (23, 24). Saunders & Saund-ers (25) showed that coronal leakage was a signifi-cant problem in root-filled molars. They demon-strated in a laboratory-based experiment, that thecommon practice of packing excess gutta-perchaand root canal sealer over the floor of the pulpchamber after completion of lateral condensation,did not provide an adequate coronal seal of theroot canals. It was recommended that excess gutta-percha should by removed level with the openingsof the root canals and the floor of the pulp chambersealed with a restorative material such as amalgamor glass polyalkenoate.

Other factors influencing coronal ieakage

When the root canal walls are instrumented a ten-acious layer of debris is formed which is known asthe smear layer (5). This layer cannot be removedwith canal irrigation techniques using sodium hypo-chlorite in a hand-held syringe. Therefore, thesmear layer is present in most root-filled teeth. Theeffect that the smear layer may have on the prog-nosis of root-canal therapy is unknown (26), butconceivably it might be broken down by bacterialtoxins (27). This would provide a path throughwhich leakage could take place. Hovland & Dum-sha (28) showed that most leakage occurs betweenthe root-canal sealer and the wall of the root canal-It is important, therefore, that this route for con-tamination is restricted as much as possible.

If the smear layer were to be removed then this

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Coronal leakage & root canal therapy

would expose patent dentinal tubules into which asealer may flow, thereby decreasing the possibilityof leakage taking place. The smear layer can beremoved with acids, such as citric acid, and chel-ating agents such as EDTA (ethylenediaminetetraacetic acid). Cameron (29) showed that sodiumhypochlorite, used with an ultrasonically poweredendodontic file, could also remove the smear layer.Smear-layer removal tends to be easier in the cor-onal than in the apical part of the canal (30). An-other way in which contamination could be re-stricted is by using a root-canal sealer that bondsehemically to the wall of the root canal. None ofthe more commonly used root-canal sealers adherechemically to tooth structure. A recent study useda glass ionomer lining material, Vitrebond, as aroot-canal sealer (31). In order to prolong the set,the cement was allowed to cure chemically. Whenthe smear layer was removed, the sealer flowed intothe dentinal tubules and coronal leakage was re-duced compared with Tubliseal, a commonly usedzinc-oxide eugenol-based, root-canal sealer. Glassionomer bonds to the inorganic part of dentine andthe use of such a material may have a place inendodontics. A proprietary glass ionomer root canalsealer, Ketac Endo, has recently become availableand early research reports are, on the whole, favour-able (32, 33).

Prevention of coronai leakage

It is essential, following root-canal treatment, thatthe canal system is protected from subsequent leak-age as much as possible. In the case of molars thefloor of the pulp chamber should be covered with alining of glass ionomer after removing excess gutta-percha and sealer.

The crown of the tooth should be restored ex-peditiously in such a way as to reduce leakage to aminimum. Coronal access cavities of teeth that can-not be restored immediately should be dressed witha proprietary temporary filling material, ensuringthat it is at least 3.5 mm thick to reduce leakage (9).If simple repair of an access cavity jeopardises thecoronal seal of an existing restoration, then the com-plete restoration should be replaced. Of course root-filled teeth have no pulpal sensory innervation and,therefore, leaking coronal restorations may go un-noticed by patients for many months. It is important,therefore, that regular reviews are made of root-filledteeth, not only to monitor the periradicular status,but to ensure that coronal leakage is not taking place.

Conclusions

Coronal leakage is an important cause of failure oftreatment. It is essential that due regard

be paid to the prevention of such leakage, bothduring and after root-canal therapy, by paying care-ful attention to the coronal restoration to the tooth.The use of chemically active, adhesive, root-canalsealers may, in future, play an important role inminimising coronal microleakage.

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