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Endodontics: Part 4Morphology of the root canal systemP. Carrotte1

Unless the practitioner is familiar with the morphology of the roots of all teeth, and the associated intricate root canalanatomy, effective debridement and obturation may be impossible. Recent research has improved knowledge andunderstanding of this intricate aspect of dental practice. After studying this part you should know in what percentage of eachtooth type you may expect unusual numbers of root canals and other anatomical variations.

● Practitioners must be aware that the main root canals in a tooth may only provide access tothe complexities of the root canal system, which must be fully cleaned of all microorganisms.

● Research has shown that the dental anatomy learned as a dental student may now be out ofdate.

● Knowledge of canal anatomy is essential in designing and executing access cavities that givestraight line access to the main root canals.

I N B R I E F

This part may seem at first sight the most boringin the book, yet it could be the most important inimproving clinical practice. Both undergraduatestudents and dentists on postgraduate coursesfrequently state that the reason they find rootcanal treatment so difficult, and the reason sur-veys frequently report inadequate treatmentstandards, is because they are working �blind�.Unless a surgical microscope is available it isimpossible to see down the root canal � to visual-ize exactly what the instruments are doing. Anunderstanding of the architecture of the rootcanal system is therefore an essential prerequisitefor successful root canal treatment (see Part 1,Fig. 1). As long ago as 1925, when Hess andZurcher first published their study,1 it becameclear that teeth had complicated root canal sys-tems rather than the simplified canals that hadbeen previously described.

Sadly their work, and many similar publica-tions, have largely been overlooked and den-tists still remain obsessed with the concept of a�root canal�, a hollow tube down a root whichhas to be cleaned and shaped, eventuallyappearing as a nice white line on the post-oper-ative radiograph. Undergraduate students learnthe number of canals in each tooth by rote.However, many teeth have more than onecanal, as described in this part. Where twocanals exist within the same root, for examplethe mesial root of a lower molar, lateral com-munication (anastomosis) in the form of fins oraccessory canals, occurs between them. Evenroots with a single canal will have lateral andaccessory canals leaving the main canal. Unlessthis concept of an entire root canal system isclearly understood, and a method of cleaningand shaping this system employed to addressthese anastomoses as well as the main canals,infection will remain in the root canal system,and the treatment may fail.

ROOT CANAL SYSTEMThe pulp chamber in the coronal part of a toothconsists of a single cavity with projections(pulp horns) into the cusps of the tooth (Fig. 1).With age, there is a reduction in the size of thechamber due to the formation of secondarydentine, which can be either physiological orpathological in origin. Reparative or tertiarydentine may be formed as a response to pulpalirritation and is irregular and less uniform instructure.

The entrances (orifices) to the root canals areto be found on the floor of the pulp chamber,usually below the centre of the cusp tips. Incross-section, the canals are ovoid, having theirgreatest diameter at the orifice or just below it.In longitudinal section, the canals are broaderbucco-lingually than in the mesiodistal plane.The canals taper towards the apex, followingthe external outline of the root. The narrowestpart of the canal is to be found at the �apicalconstriction�, which then opens out as the api-cal foramen and exits to one side between 0.5and 1.0 mm from the anatomical apex. Deposi-tion of secondary cementum may place the api-cal foramen as much as 2.0 mm from theanatomical apex. It must be realized, however,that the concept of a �single� root canal with a�single� apical foramen is mistaken. The rootcanal may end in a delta of small canals, andduring root canal treatment cleaning tech-niques should be employed to address thisclinical situation.

LATERAL AND ACCESSORY CANALSAs previously discussed, lateral canals formchannels of communication between the mainbody of the root canal and the periodontal liga-ment space. They arise anywhere along itslength, at right angles to the main canal. Theterm �accessory� is usually reserved for the small

4

ENDODONTICS1. The modern concept of

root canal treatment2. Diagnosis and treatment

planning3. Treatment of endodontic

emergencies4. Morphology of the root

canal system5. Basic instruments and

materials for root canaltreatment

6. Rubber dam and accesscavities

7. Preparing the root canal8. Filling the root canal

system9. Calcium hydroxide, root

resorption, endo-periolesions

10. Endodontic treatment forchildren

11. Surgical endodontics12. Endodontic problems

NOW AVAILABLE AS A BDJ BOOK

1*Clinical Lecturer, Department of AdultDental Care, Glasgow Dental Hospital andSchool, 378 Sauchiehall Street, Glasgow G2 3JZ*Correspondence to: Peter CarrotteEmail: p.carrotte@dental.gla.ac.uk

Refereed Paperdoi:10.1038/sj.bdj.4811711© British Dental Journal 2004; 197:379–383

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canals found in the apical few millimetres andforming the apical delta (Fig. 2). Both lateraland accessory canals develop due to a break in�Hertwig�s epithelial root sheath� or, duringdevelopment, the sheath grows around existingblood vessels. Their significance lies in their rel-atively high prevalence, Kasahara et al.2 finding60% of central incisors with accessory canals,and 45% with apical foramina distant from theactual tooth apex. Kramer found that the diam-eter of some lateral canals was often wider thanthe apical constriction.3 Lateral canals are

impossible to instrument and can only becleaned by effective irrigation with a suitableantimicrobial solution. Consequently, sealingsuch canals is only moderately successful.

The following descriptions of normal canalmorphology and access cavities are illustrated inFigure 3 (maxilliary) and Figure 4 (mandibular).

MAXILLARY CENTRAL INCISORSThese teeth almost always have one canal. Whenviewed on radiographs the canal appears to befairly straight and tapering, but labiopalatallythe canal will tend to curve either towards thelabial or palatal aspect at about the apical thirdlevel. One feature to note is the slight narrowingof the lumen at the cervical level, which immedi-ately opens up into the main body of the canal.The inverted-triangular shaped access cavity iscut with its base at the cingulum to give straightline access.

MAXILLARY LATERAL INCISORSimilar in shape to the central incisors, but frac-tionally shorter, the apical third tends to curvedistally and the canal is often very fine with thinwalls. Labiopalatally, the canal is similar to thecentral incisor, but there is often a narrowing ofthe canal at the apical third level. The root ismore palatally placed, an important point whenany periradicular surgical procedures are carriedout on this tooth. The access cavity is similar tothe central incisor.

MAXILLARY CANINEAs well as being the longest tooth in the mouth,its oval canal often seems very spacious duringinstrumentation. However, there is usually asudden narrowing at the apical 2�3 mm; thisleads to a danger of overinstrumentation if toolarge a file is used at this level. The length of thistooth can be difficult to determine on radi-ographs, as the apex tends to curve labially andthe tooth will appear to be shorter than it actual-ly is. The oval shape of the root canal is reflectedin the shape of the access cavity.

Fig. 3 The basic pulpcanal shape andsuggested access cavityopenings in themaxilliary teeth.

Fig. 4 The basic pulpcanal shape andsuggested accesscavity openings in themandibular teeth.

Fig. 1 The pulpchamber in the coronalpart of the toothconsists of a singlecavity with projections(pulp horns) into thecusps of the tooth.

Fig. 2 The small canals found in the apical few millimetres and forming theapical delta are seen here filled with sealer.

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MAXILLARY FIRST PREMOLARTypically, this tooth has two roots with twocanals. In many ways this is the most difficulttooth to treat, as it can have a complex canalsystem. Variations range from one to three roots,(Fig. 5), but there are nearly always at least twocanals present, even if they exit through a com-mon apical foramen. The roots of these teeth arevery delicate and at the apical third they maycurve quite sharply buccally, palatally, mesiallyor distally, so instrumentation needs to be car-ried out with great care (Fig. 6). In a small per-centage of cases the buccal root may subdivideinto two canals in the apical third, as shown inFigure 7. An oval access cavity is cut betweenthe cusp tips, being wider buccopalatally thanmesiodistally.

MAXILLARY SECOND PREMOLARIn 40% of cases, this tooth, which is similar inlength to the first premolar, has one root with asingle canal. Two canals may be found in about58% of cases.4 The configuration of the twocanals may vary with two separate canals andtwo exits, two canals and one common exit, onecanal dividing and having two exits. In onestudy,5 it was found that 59% of maxillary sec-ond premolars had accessory canals. As with thefirst maxillary premolar, the apical third of theroot may curve quite considerably, mainly to thedistal, sometimes buccally. The access cavity issimilar to the first premolar.

MAXILLARY FIRST MOLARThis tooth has three roots. The palatal root isthe longest, with an average length of 22 mm;the mesiobuccal and distobuccal roots areslightly shorter, at 21 mm average length. Thepercentage of mesiobuccal roots having twocanals reported in the literature has increasedsteadily as research techniques have devel-oped. In vitro studies have usually reported ahigher incidence than in vivo studies. Stropko,reporting an extensive in vivo study,6 foundsecond canals in 73% of cases before the useof an operating microscope, but 93% follow-ing its use. The canals of the mesiobuccal rootare often very fine and difficult to negotiate;consequently, more errors in instrumentationoccur in this tooth than in almost any other.Anastomosis between these two canals maytake the form of narrow canals or wide fins,both almost impossible to instrument. Thecurvature of the roots can be difficult to visu-alize from radiographs, and the secondmesiobuccal canal is nearly always superim-posed on the primary mesiobuccal canal. Thepalatal root has a tendency to curve towardsthe buccal and the apparent length on a radi-ograph will be shorter than its actual length.The access cavity represents the shape of thepulp chamber, enlarged slightly, and flared upon to the mesiobuccal aspect of the occlusalsurface to accommodate the angle of instru-ment approach when working at the back ofthe mouth.

MAXILLARY SECOND MOLARThis tooth is similar to the first maxillary molar,but slightly smaller and shorter, with straighterroots and thinner walls. Usually there are onlythree canals and the roots are sometimes fused.The access cavity is the same as for the firstmolar, modified further to accommodate theangle of approach.

MAXILLARY THIRD MOLARThe morphology of this tooth can vary consider-ably, ranging from a copy of the first or secondmaxillary molar to a canal system that is quitecomplex. They are best explored with a wideaccess cavity and direct vision of the individualcanal anatomy.

Fig. 5 Examples of upper premolars with three roots.

Fig. 6 The roots of upper firstpremolars are very delicate and maycurve quite sharply buccally,palatally, mesially or distally, soinstrumentation needs to be carriedout with great care.

Fig. 7 Cross-sectionstaken at differentlevels in a maxillaryfirst premolar showingthe division of thebuccal canal.

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MANDIBULAR CENTRAL AND LATERALINCISORSThe morphology of these two teeth is very simi-lar. The central incisor has an average length of20.5 mm and the lateral is a little longer withan average length of 21 mm. Over 40% of theseteeth have two canals, but only just over 1%have two separate foramina. Careful reading ofthe pre-operative radiograph may show achange in the radiodensity of the root canal,indicating division into two separate canals,and a correctly designed access cavity willfacilitate checking for a second canal. This isoval in shape, commencing above the cingu-lum and almost notching the lingual incisaledge.

MANDIBULAR CANINEThis tooth is similar to its opposite number,although not as long. On rare occasions, tworoots may exist and this can cause difficultywith instrumentation (Fig. 8). An oval accesscavity is again indicated.

MANDIBULAR FIRST PREMOLARThe canal configuration of this tooth can bequite complex. Vertucci7 has shown that the sin-gle canal normally found may divide into twocanals and two apical foramina in 25% of cases.It is the way in which the second canal branchesthat can cause difficulty with instrumentation.Occasionally, the canal terminates with anextensive delta, making obturation of the acces-sory canals even more challenging. As in theupper premolars, the access cavity is ovalbetween the cusp tips.

MANDIBULAR SECOND PREMOLARThis tooth is similar to the first premolar, exceptthat the incidence of a second canal is verymuch lower. One study stated this to be 12%.7

Another study revealed that only 2.5% had two

Table 1 Average root canal configurations

Tooth Average Length No. of roots No. of canals

Maxillary anteriors

Central incisor 22.5 mm 1 1

Lateral incisor 22.0 mm 1 1

Canine 26.5 mm 1 1

Maxillary premolar

First premolar 20.6 mm 2-3 1 (6.%)

2 (95%)

3 (1%)

Second premolar 21.5 mm 1-3 1 (75%)

2 (24%)

3 (1%)

Maxillary molars

First molar 20.8 mm 3 4 (93%)

3 (7%)

Second molar 20.0 mm 3 4 (37%)

3 (63%)

Third molar 17.0 mm 1-3

Mandibular anteriors

Central incisor 20.7 mm 1 1 (58%)

2 (42%)

Lateral incisor 20.7 mm 1-2 1 (58%)

2 (42%)

Canine 25.6 mm 1 1 (94%)

2(6%)

Mandibular premolars

First premolar 21.6 mm 1 1 (73%)

2 (27%)

Second premolar 22.3 mm 1 1 (85%)

2 (15%)

Mandibular molars

First molar 21.0 mm 2-3 3 (67%)

4 (33%)

Second molar 19.8 mm 2 2 (13%)

3 (79%)

4 (8%)

Third molar 18.5 mm 1-2

Fig. 8 Lower canines mayoccasionally be found with twoseparate roots.

Fig. 9 A lower secondpremolar with a severedistal curve at theapex.

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apical foramina.8 Consequently, it is a much eas-ier tooth to treat compared with the mandibularfirst premolar, unless the radiograph reveals asharp distal curve at the apex as shown in theextracted tooth at Figure 9.

MANDIBULAR FIRST MOLARThis is often the most heavily restored tooth inthe adult dentition and seems to be a frequentcandidate for root canal treatment. Generallythere are two roots and three canals: two canalsin the mesial root and one large oval canal dis-tally. According to Skidmore and Bjorndal,9 onethird of these molars have four canals. Occa-sionally, three roots are to be found: usually twodistal and one mesial (Fig. 10), rarely one distaland two mesial. Anastomoses occur betweenthe canals and accessory communication withthe furcation area is a frequent finding. Themesiobuccal canal tends to exhibit the greatestdegree of curvature. The access cavity onceagain represents the shape of the pulp chamber,enlarged slightly, and flared up on to themesiobuccal aspect of the occlusal surface toaccommodate the angle of instrument approachwhen working at the back of the mouth.

MANDIBULAR SECOND MOLARThis tooth is similar to the mandibular firstmolar, although a little more compact. The mesialcanals tend to lie much closer together, and theincidence of two canals distally is much less. Thistooth seems to be more susceptible to verticalfracture. Occasionally, the root canals may joinin a buccal fin giving a �C-shaped� canal, whichmay lead all the way to the apex.10 The accesscavity is similar to that of the first molar.

MANDIBULAR THIRD MOLARTogether with the maxillary third molar, thistooth displays some of the most irregularcanal configurations to be found in the adultdentition, as seen in Figure 11. However, themesial inclination of the tooth generallymakes access easier. The canal orifices are nottoo difficult to locate, but the degree of curva-ture of the apical half of the root canal systemis often pronounced. Added to this, the apex isfrequently poorly developed and lies close tothe inferior alveolar canal. A large access cav-ity allowing direct visualization of the floor ofthe pulp chamber enables the canal orifices tobe identified.

It should always be remembered that whilstthe above descriptions are the norm, occasional-ly other teeth may be encountered with unusualor even bizarre anatomy. This may only be dis-covered after the treatment has failed and thetooth has been extracted. Clues may sometimesbe found by careful examination of the radi-ographs, especially with the use of magnifica-tion, as described in Part 5.

1. Hess W, Zurcher E. The anatomy of the root canals ofthe teeth of the permanent dentition and the anatomyof the root canals of the deciduous dentition and thefirst permanent molars. London: Basle, Sons andDanielson, 1925.

2. Kasahara E,Yasuda E, Yamamoto A, Anzai M. Rootcanal systems of the maxillary central incisor. J Endod1990; 16: 158�161.

3. Kramer I R. The vascular architecture of the humandental pulp. Arch Oral Biol 1960; 2: 177�189.

4. Bellizi R, Hartwell G. Radiographic evaluation of rootcanal anatomy of in vivo endodontically treatedmaxillary premolars. J Endod 1985; 11: 37�39.

5. Vertucci F J, Seeling A, Gillis R. Root canalmorphology of the human maxillary second premolar.Oral Surg 1974; 38: 456-464.

6. Stropko J J. Canal morphology of maxillary molars:clinical observations of canal configurations. J Endod1999; 25: 446�450.

7. Zillich R, Dowson J. Root canal morphology of themandibular first and second premolars. Oral Surg1973; 36: 738�744.

8. Vertucci F J. Root canal morphology of mandibular premolars. J Am Dent Assoc 1978; 97: 47�50.

9. Skidmore A E, Bjorndal A M. Root canal morphologyof the human mandibular first molar. Oral Surg 1971;32: 778�784.

10. Melton D C, Krall K V, Fuller M W. Anatomical andhistological features of C-shaped canals in mandibularsecond molars. J Endod 1991; 17: 384�388.

Fig. 11 Thedevelopmentalanatomy of lower thirdmolars may be quitebizarre.

Fig. 10 Lower first molars may occasionally be foundwith two separate distal roots.

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