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Canal and isthmus morphology in mandibularincisors – An in vitro studyUma Ch.* Ramachandran S** Indira R*** and Shankar P****

ABSTRACT

The aim of the study was to evaluate the canal anatomy and the presenceof isthmus in mandibular incisors encountered during routine endodontictreatment and apical surgery.

Fifty randomly selected extracted mandibular incisors were examined toassess root canal anatomy and presence of the isthmus. A combinationmethod of radiography (mesiodistal and buccolingual views) and sectioningof apical 3mm (1, 2 and 3mm from the apex ) simulating a surgical resectionmethod was followed.

Radiographic study showed that 98% (49 teeth) had single portal of exit.One tooth had double portal of exit. Sectioning study showed that canalshapes varied from round to oval, long oval and ribbon shaped.

Surgical microscopy and ultrasonic root end preparation enables theclinician to have better visualization for root end resection. Greaterknowledge of canal anatomy in the apical 3rd of the root will help cliniciansin better preparation and sealing the root canal system in this critical area.

Keywords : Canal morphology, isthmus, mandibular incisors

* Post graduate student** Professor & Principal*** Professor & Head of the Department**** Assoc. Professor

Dept. of Conservative Dentistry & Endodontics.Ragas Dental College & HospitalChennai.

IntroductionFor the success of endodontic treatment,

awareness of normal configuration of pulpspace, together with aberrations of canalanatomy is critical. In reviewing the literature,one finds a divergence of opinion concerningthe anatomic configuration of the pulp cavity ofmandibular incisors1. One of the reasons forendodontic failure of lower incisors is thepresence of an undetected lingual canal or thepresence of an untreated isthmus2.

The prevalence of two canals in mandibularincisors has been reported to be from 11.5 –44.1%, although many merge into one canalin the apical 1-3 mm of the root3,4,5.

Bifurcation of the root canal in mandibularincisors may result in complications or inoperative failure during endodontic therapy4.Authors studying roots with two canalscommonly report an isthmus, fin or corridor,which may be present between the two canals.Green3 described this corridor as a “ribbonshaped passage”. He found this ribbon shapedpassage in 22% of mandibular central andlateral incisors. Non-surgical root canal therapyfailure has been attributed to an inability todebride this area adequately. In some rootswith two canals, periapical surgery is requiredto facilitate cleaning and sealing the root apex3.

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Endodontology, Vol. 16, 2004

Historically, a steep facial bevel is requiredduring surgical endodontics to visualize thecanal space –due to the lingual inclination ofmandibular incisors3. Root end resection mayexpose the second canal or an isthmus.Surgical microscopy and ultrasonic root endpreparation enables the clinician to have bettervisualization for root end resection. Greaterknowledge of canal anatomy in the apical thirdof the root will help the clinician in preparingthis area and sealing the root canal system3.

The purpose of this study was to evaluatethe canal anatomy and the presence of isthmusin mandibular incisors encountered duringroutine endodontic treatment and apicalsurgery.

Material and MethodsThe endodontic morphology of mandibular

incisors has been studied by manyinvestigators. Methods of study have included-

Replication techniques, Ground sectionsClearing techniques and Radiography. Thereappears to be a great disparity between differentstudies in the number of canals, canal exitsand variations in anatomy. These differencesmay be due to dissimilarities in examinationmethods6.

In the present study a combination methodof radiography (mesiodistal and buccolingualradiographs) and sectioning of apical 3 mm (at1, 2, & 3 mm from the apex), simulating a 200

surgical resection method (200 facial bevel) onmandibular incisors was followed3.

Fifty mandibular caries free incisors wererandomly collected and stored in 10% formalin.The teeth were placed in 5.25% NaOCI for fifteenminutes after which any remaining externaltissue debris was removed by scaling. Eachtooth was radiographed from both facial andproximal views using intra oral Kodak E speedsuper polysoft size 2 periapical films exposed

Fig. 1. Section at 1mm with single oval canal. Fig. 2. Section at 1mm with double canals(long oval) with Type I isthmus.

Fig. 3. Section at 3mm with double canals(round shaped) with Type I isthmus

Fig. 4. Section at 3mm with double canals(ribbon shaped) with Type I isthmus

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Table 1 : Radiographic interpretation (n-50)

Portal of ExitCanal Morphology (based on Vertucci’s classification)Type I Type II Type III Type IV Type V Type VI Type VII Type VIII

(1) (2-1) (1-2-1) (2) (1-2) (2-1-2) (1-2-1-2) (3) Single Double

22 (44%) 1 (2%) 26 (52%) – – – 1 (2%) – 49 1

with a dental x-ray unit (Satellec at 70 Kvp,8ma). The samples were mounted on the x-ray films with the help of wax in the requiredposition and exposed, such that the cone ofthe x-ray machine was at 900 angle, at 16-inchobject-source distance to the periapical film.All the periapical films were developed, fixedand dried by the manual method and mounted.

Radiographs were evaluated with the helpof a magnifying lens and X-ray viewer for thenumber of canals in the apical third and type ofcanal morphology. All the teeth were storedseparately until sectioning was done.Angulation of 200 was marked on a wax sheetand with the help of this, it was transferred onthe apical third of the root of each tooth at 1, 2,3 mm from the apex with a pencil. The rootportion of each tooth was embedded in selfcure clear acrylic resin and cut over the pencilmarkings visible through the resin at 1, 2, 3mm level from the apex with the help of adiamond disc mounted on a dental lathe. Thesectioned specimens were placed in 5.25%NaOCI for 45 minutes to remove organicmaterial remaining in the canal space andstored separately in vials containing saline.They were viewed under a stereomicroscopeat 16X magnification from the coronal aspect

Table 3: Type of Isthmus (n = 50)

Level of section Type of Isthmus No. of teethfrom apex with isthmus

I II III IV V

1 mm 1 - - - 2 47

2 mm - - - - 1 49

3 mm 4 - - - 5 41

and evaluated for number of canals, canalshape and presence of an isthmus.Photographs were made for recording andevaluation.

ResultsResults of the study are tabulated in tables

1, 2, 3, 4

Radiographic study showed that Type III(1-2-1) morphology was seen in majority ofteeth. Type I (1-1) was the next common seen.Tendency for bifurcation occurred at variouslevels .

Sectioning study showed that canal shapesvaried from round, oval, long oval and ribbonshaped at 1, 2 & 3 mm from apex.

Single Double

1 mm 49 1

2 mm 50 0

3 mm 46 4

No. of teethLevels of sectionfrom apex

Table 2: Sectioning study (n=50)

Uma Ch. et al. Canal and isthmus morphology...

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Endodontology, Vol. 16, 2004

Type I, Type V isthmus was seen at 1, 2 &3 mm sections.

DiscussionOne of the main reasons for endodontic

failure of lower incisors is the presence of anundetected lingual canal or the presence of anuntreated isthmus2. In terms of success oftreatment, awareness of normal configurationof the pulp space, together with aberrations ofcanal anatomy is critical. Mandibular incisorshave been reported presenting two canals withina single root. Rankine-Wilson and Henry in 1965observed that two canals were present in 40%of the incisors studied. These teeth presentedwith two foramina in 87% of the cases. Benjaminand Dowson in 1974 reported that 41% ofmandibular incisors contained two canals,whereas only 1.3% presented two foramina.7

In the present study, radiographs weretaken in both buccolingual and mesiodistaldirection, as it helps in three dimensional viewof a two dimensional object. Pucci and Reigh8

stated that root canals exhibit the largestnumber of variants in the buccolingual plane.

In this study, canal morphology wasdetermined based on Vertucci’s 9 classification.It was found that Type III (1-2-1) morphologywas seen in majority of the teeth (52%). Type I(1-1) was the next common seen in mandibularincisors (44%) .The present radiographic studyhas shown that Type III morphology iscommonly seen than usually thought. Tendencyfor bifurcation occurs in the middle third of theroot. Bifurcation of a single canal into twocanals occurred at various levels and thedistance of bifurcation varied.

The mesio-distal radiograph revealedgreater width of the pulp canal, that is nevervisualized by routine intra oral roentgenograms,which allows room for two separate canals orone wide canal with an island of dentin in themiddle. Ribbon shaped canals are commonenough to be considered normal and demandsspecial attention in cleaning and shaping10.

On examination of sectioned specimens,49 of 50 teeth examined had one single canaland only one had double canals at one mmlevel. At two mm level, all the teeth exhibitedone single canal. At three mm level of sectionfrom the apex, 46 teeth had single canal and 4teeth had double canals.

The type of isthmus was determined basedon Yeung Yi Hsu classification7, which is asfollows:

Two or Three canals with nonotable communication.

Two canals that posses adefinite connection betweenthe two main canals.

Three canals that are presentwith a definite connection.(incomplete "C" shapedcanals with three canals arealso included).

Canals extended into theisthmus area.

True connections or corriodorwas present throughout thesection.

Table 4: Shape of canals (n=50)

Level of section from apexCanal shape

Round Oval Long Oval Ribbon Shaped

1 mm 5 39 6 –2 mm 7 39 3 13 mm 3 37 4 6

Type I

Type II

Type III

Type IV

Type V

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The isthmus connection can be observedbetween any two root canal systems that occurwithin one root. The actual formation of theisthmus is embryonic in origin, through theepithelial root sheath. An isthmus is formedwhen an individual root projection is unable toclose itself off. Partial fusion of root projectionsresults in formation of two root canals with anisthmus formed in between, such as the mesialroot of the mandibular first molar. No fusionleads to a large ribbon shaped canal that alsoforms an isthmus throughout the entire root7.

In the present study, canals that were longoval and ribbon shaped were characterized ashaving Type V isthmus (a true connection orcorridor was present throughout the section).The canal shape did not remain constant. Itgenerally was more ribbon or long oval shapedin 3 mm sections. However in some teeth, theapical area of the root canal was larger thanthe more coronal area. Four long oval and fourribbon shaped canals were characterized ashaving Type V isthmus (combined 1mm, 2 mmand 3 mm level sections from root apex).

Cleaning the apical portions of canalshaving oval, long oval or ribbon shape is moredifficult. Generally, the canals were round oroval, closer to the apex and tended to elongateto a long oval or ribbon shape more coronally.Instrumenting the larger buccolingualdimension of the canal in these teeth isimpossible with pure rotary motion. Use ofNaOCl irrigation or ultrasonics is necessaryalong with mechanical instrumentation.

ConclusionFrom the present study, it can be concluded

that pulp space anatomy of mandibular incisorsshow high incidence of complexity whichincludes variations in canal configuration,number of canals and presence of isthmus.Additional radiographs taken at different angles

and correct interpretation of x rays andmodification of the access opening whererequired, will assist the clinician for a morethorough assessment.

Standard instrumentation techniquesalone cannot clean the narrow ribbon shapedcanals. Additional methods like use of NaOCland ultrasonics should be used to clean theseareas – not reachable by instruments. Anisthmus opened during surgery needs to beincluded in the root end preparation. Surgicaloperating microscope should be used in criticalcases as it provides better illumination withenhanced magnification for locating additionalcanals and identifying the isthmus in surgicalprocedures.

References1. Vertucci FJ. – Root canal anatomy of the mandibularanterior teeth. J. Am. Dent Assoc,1974; 89, 369-371.

2. Surgical Endodontics in Harty’s Endodontics inClinical Practice-4th ed. 1997 pp.179.

3. Mauger MJ, Schindler WG and Walker WA. Anevaluation of canal morphology at different levels ofroot resection in mandibular incisors. J. Endod 1998;24:607-609.

4. Madeira MC and Hetem S. Incidence of bifurcationsin mandibular incisors. Oral Surg. 1973:36;589-591.

5. Kartal N and Yanikoglu FC. Root canal morphologyof mandibular incisors. J. Endod 1992;18:62-564.

6. Walker RT. The root canal anatomy of mandibularincisors in a southern Chinese population. Int. Endod J1988;21: 218-223.

7. Yi-Hsu Y, Kim S. The resected root surface. Theissue of canal isthmuses. Dent Clin N Amer.1997;41:529-540.

8. Pineda F and Kuttler Y. Mesiodistal and buccolingualroentgenographic investigation of 7,275 root canals.Oral Surg. 1972; 33:101-110.

9. Caliskan MK, Pehlivan Y, Sepetcioglu F, Turkun Mand Tuncer SS. Root canal morphology of humanpermanent teeth in a Turkish population. J. Endod, 1995;21:200-204.

10. Weine FS. Endodontic Therapy 1996 5th edn.Harcourt Brace & Co. Asia Pvt. Ltd.

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