tics - problem solving in clinical practice 2002 cleaning & shaping

8/3/2019 tics - Problem Solving in Clinical Practice 2002 Cleaning & Shaping

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5 ROOT CANAL PREPARATION

CONTENTS Rationale Access Cavity Preparation Hand Instruments PreparationTechniques Hand Preparation Mechanical Preparation Techniques Further Reading

RATIONALE

MicrobiologyApical periodontitis is caused by microbialinfection of the root canal system. Successfultreatment is dependent on the control of thismicrobial infection. An understanding of themicrobiology of apical periodontitis is a pre-requisite for effective treatment.The General Microbial FloraWith the development of anaerobic culturingtechniques and sampling methods an insightinto the microbial flora of infected root canalshas become possible. Nowadays there aresophisticated techniques for identification ofbacteria that do not rely on culturing meth-ods. Indeed some bacteria that can be identi-fied by genetic techniques are non-cultivable.Apical periodontitis is typically a polymicro-bial infection dominated by obligately anaer-obic bacteria. Normally only a few species areisolated from anyone case. Porphyromonasendodontalis is a species that seems to predom-inate in infected root canals. Others include:Streptococcus, Enterococcus, Actinomyces,Lactobacillus, Peptostreptococcus, Eubacterium,Propionibacterium, Prevotella, Fusobacterium,Eikenella, Capnocytophaga and Wolinella.There are positive and negative interactionsbetween bacterial species as they compete inthe ecological niche of the root canal.Environmental conditions such as the amountof oxygen, availability of nutrients and hostdefence mechanisms will affect colonization.Bacteria infect the main canals and lateralcanals; they also may infect dentine by

spreading down dentinal tubules. The peri-radicular tissues are normally separated fromthe flora in the root canal by a dense wall ofpolymorphonuclear leukocytes, and it is rareto see bacteria in the periapical tissues unlessthere is acute apical periodontitis.

Rationale for Root Canal PreparationBecause the root canal systemof a tooth is oftenextremely complex it is difficult to disinfect itcompletely and quickly. Itmay be that the bestattempts of the operator merely reduce theresidual bacterial load to a non-pathogenicnumber, or change the resident flora suffi-ciently to allow periapical healing, but this hasnot been proved. These microbes and their by-products can be removed by a combination ofmechanical and chemicalmeans.Mechanical removal relies on the ability ofthe operator to remove infected pulp anddentine from the surfaces of the root canal byplaning the walls; infected material in thelumen of the root canal will be removed. Innearly all cases this is impossible to achieve,partly because the instruments cannot actu-ally contact all the internal surfaces and alsobecause attempting to remove all the infecteddentine would severely weaken the tooth.That is why chemically active irrigants areused to destroy colonies of micro-organisms.Instrumentation of the root canal is carriedout to produce a pathway for the delivery ofan antibacterial irrigant to all the ramifica-tions of the root canal system. It also makesspace for medicaments and the final rootcanal filling.


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Bacterial colonies may form themselvesintomultilayeredbiofilms, whichcan be diffi-cult to remove, as they adhere to the canalwall, while other microbes congregate in sus-pension. Irrigants must be delivered in suffi-cient volume and concentration tobe effectiveagainst these colonies. Sodium hypochloriteirrigant is inactivated by its action on organicmaterial, andmust be replenished.

ACCESS CAVITY PREPARAnONPreoperative RadiographA preoperative radiograph shouldnormally betaken using a paralleling device, as this pro-duces an image that is almost actual size. Theradiograph must show the entire tooth and atleast 2mmofbone surroundingthe root apices(Fig. 5.1). Images of root canals can becomesuperimposed onradiographs and sometimesitwill be necessaryto takemorethanoneviewfromdifferent angles to interpret the complexarrangementofmulti-rootedteeth.The preoperative radiograph(s) will allow

an assessment of basic anatomy: number ofroots, size of pulp chamber, fit of coronalrestoration, caries, pulp stones, curvature ofroot canals, likelihood of lateral canals,iatrogenic damage (perforations and frac-tured instruments). When a tooth has been

Figure 5.1A good-quality radiograph of the mandibular first molarwhich requires root canal treatment.

ENDODONTICS: PROBLEM-SOLVING IN CLINICAL PRACTICE

crowned it is very important to assess theposition of the pulp space in relation to theartificial crown, or else the access cavity maybemisdirected.

Why? A ims of A ccess Cavity PreparationThe access cavity should be designed to allowdebridement of the pulp chamber and enableroot canal instruments to be introduced intothe canals without bending-i.e. for straight-line access. The tooth must be caries-free andany restorations with deficient margins mustbe removed.All necrotic pulp remnants must be

removed from the pulp chamber, to preventinfected pulp material being pushed furtherinto the canals and causing recontaminationbetween visits. Necrotic material and break-down products may also be responsible forstainingdentine.There should be some degree of resistance

form to the completed access cavity; thisensures that temporary restorations are notdislodged between visits and a coronal seal isachieved.

How? Location of A ccess CavityHistorically access cavity designs have beenstandardized according to the type of tooth.Although the final cavity shapemayresemblethe standardized form, using modern tech-niques the underlying pulp chamber shoulddictate the final shape. Smallmodifications tothe shape may be needed to allow straight-lineaccessto all the root canals.

The Lid-off Approach to A ccess CavityPreparation (Fig. 5.2)1 . Estimatelength2. Penetrate to the pulp chamber3. Lift off the roofwith a bur in a pullingaction

4. Refinetheaccesscavity.


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ROO T C A NA L P R EP A RA TIO N

Preoperative cariousexposure Dome-ended fissurebur is used to penetrate

pulp chamber

Figure 5.2Access cavity preparat ion: ' lid-off technique'.

Estimating the DepthThe depth of the roof of the pulp chamber canbe estirnatod frorn the preoperative radio-graph(Fig. 5.3).Access is first made to this depth using a

tungsten carbide or diamond bur, F0557 orFG541, in a turbine (Fig. 5.4). Pre-measuringwill help prevent perforation of the pulp floorduring access cavity preparation. The bur isdirectedtowards the axis of the largest canal inmulti-rooted teeth, for example the palatal rootof maxillary rnolars and the distal root ofmandibular molars. Locating this canal firstmakes orientation easier and subsequent iden-tification of other canals rriore predictable.Further dentine can be removed with a long-shank low-speed round bur in a pulling action(Fig. 5.5). Theremainderofthe roof ofthe pulpcharnber can now be reIlloved using a nonend-cutting bur, FG332 safe-ended diamond orFO safe-ended TC Endo-Z (Maillefer,Ballaigues, Switzerland:seeFig. 5.4). Thenon-cutting end can be safely guided over the floorof the pulp chamber whilst removing dentinefromthewallsofthe cavity.To gain straight-line entry into some canalorifices it rriay be necessary to enlarge theaccess in specific areas: for example to gainaccess to the curved mesiobuccal canal of themaxillaryfirst molar tooth (Fig.5.6).

Roof of pulp chamberremoved with roundbur

Non end-cutting bur isused to 'lift lid' of pulpchamber and ref inecavity

Figure 5.3The depth of the pulp chamber can be estimated from apreoperative radiograph.

The orifices of the root canals can now belocated. Some teeth have avariable number ofroot canals, and the clinician should be awareof how many canals are likely to be found ineachtooth (Figs.5.7-5.12,Table5.1).

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82 ENDODONTICS: PROBLEM-SOLVING IN CLINICAL PRACTICE

Table 5.1 Number of roots and canals likely to be encountered in various types of tooth

Tooth Root Canals Comm entsMaxillary central incisor The orifice normally 11


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ROOT CANAL PREPARATION

Figure 5.4Burs for preparing an access cavity. The Jet Beaver dome-ended fissure bur (top: Beavers Dental, Morrisburg,Ontario, Canada) will cut through most restorations.Safe-ended diamond or Endo-Z burs are used to refinethe preparation.

Figure 5.5Round burs with different shank lengths (top: standardlength bur). Medium- and long-shanked burs are usefulin endodontics.

Figure 5.6The access cavity has been modified in order to gainstraight-line access to the mesiobuccal canal of this maxil-lary molar.

Figure 5.7A carious maxillary molar with pulpal exposure.

Figure 5.8The cavity outline is prepared with a diamond bur (541).

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Figure 5.9An Endo-Z non end-cutting tungsten carbide bur isinserted into the pulp chamber to remove the 'lid' of thepulp chamber.

Figure 5.10The access is refined with the Endo-Z bur.


Figure 5.11Remaining carious dentine is removed using a roundbur.

Figure 5.12The completed access cavity gives straight-line access toall the canals.


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The Pulp Floor MapThe floor of the pulp chamber can be read likea map. The dentine on the base of the cham-ber is generally darker (Figs. 5.13, 5.14) thanthat of the walls, and will normally indicatethe lateral extent of the pulp chamber. Thisjunction can be used to explore the extent ofthe pulp chamber.An endodontic probe (e.g. D016, Hu

Friedy, Chicago, IL, USA) is a double-endedlong probe designed for the exploration of thepulp floor and location of root canal orifices.A long-shanked excavator may also be helpfulfor removing small calcifications and obstruc-tions when locating canals (Fig. 5.15).

Troubleshooting Access Cavity PreparationCalc ificatio nsPulp stones and irritation dentine formed inresponse to caries and/or restorations maymake the location of root canal orifices diffi-cult. Special tips for ultrasonic handpieces(Fig. 5.16) are invaluable in this situation, asthey allow the precise removal of dentinefrom the pulp floor with minimal risk of per-foration. In the absence of a special tip apointed ultrasonic sealer tip can be used toremove pulp stones from the pulp chamber.

Figure 5.13The pulp floor is generally darker than the walls of thecavity.

Ultrasonic tips are best used with irrigant.Occasionally they may be used without, and aStropko irrigator (Obtura Corporation,Fenton, MO, USA: Fig. 5.17) is then useful for

Figure 5.14The access cavities in these maxillary molars have beenprepared conservatively but give good straight-lineaccess to the root canals.

Figure 5.15A typical preparation tray of instruments for endodontictreatment (left to right): college tweezers, flat plastic, ball-ended burnisher, amalgam plugger, excavator, DG16probe, Briault probe, periodontal probe, mirror, number14 clamp, Jet dome-ended fissure bur, Endo-Z bur, finebore tip for Stropko irrigator.

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Figure 5.16An ultrasonic tip for removal of dentine on the pulp floor.

Figure 5.17The Stropko irri

puffing away dentine chips. A solution of17%EDTA is excellentfor clearing awaythearea under exploration, as it r'errioves thesrrrear layer. Flood the pulp charnber withEDTA solution and allow it to stand for 1-2minutes. Dentine chips and other debris canthen be washed away with a syringe ofsodimnhypochlorite.

Sclerosed CanalsIllumination and magnification are vital forthe location of Sclerosed root canals. Theendodontist would use a surgical microscope(Fig.5.18),whilea generaldentalpractitionermighthave loupes(Fig. 5.19) anda headlightavailable. A thorough knowledge of the


Figure 5.18An operating microscope.

Figure 5.19Loupes give excel lent magnificat ion and i lluminat ion.


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anatomy ofthe pulp floor and the likely loca-tion ofthe canal orifices is essential.Chelatingagents such as EDTA are of little use in thelocationof sclerosedroot canals, asthe chelat-ing agent softens the dentine indiscriminatelyand may lead to the iatrogenic formation offalse canals and possibly perforation. If thepulp chamber is filledwith irrigant, bubblescan occasionally be seen appearing from thecanal orifice. Occasionally dyes, such asiodine inpotassium iodideormethyleneblue,have been used to demonstrate the locationofcanalorifices.Canalorifices tend tobe located at animag-

inarypoint directlyapical to the original loca-tion of the cusptip.Dentine needs to be removedvery carefully

when attempting to locate sclerosed canals.Ultrasonic tips such as the CT4 design arevery useful for precise removal of dentinefrom the floor of the pulp chamber. Long-shanked low-speed size 2 (ISO 010) roundburs canbeusedwith care.If the canal is completely sclerosed for sev-

eral millimetres apical to the pulp floor theninstruments should be advanced gradually,removing small increments of dentine. Itmaybenecessary to take radiographs to check thatthe ultrasonic tip or drill is inthe correct posi-tion inrelation to the root canal to avoidper-foration of the root. The operator can easilybecome disorientated.

Unusual AnatomyGoodradiographic technique should alert thepractitioner to unusual anatomy, such as C-shapedcanals. The C-shaped canal mayhavethe appearance of a fused root with very finecanals (Fig. 5.20). If confronted with a pulpchamber that looksunusual the dentine areason the pulp floor map should give some ideaof the location of root canals, and of the rela-tionship of the floor to surrounding toothstructure.

Angulation of the CrownIf the extracoronal restoration of the tooth isnot at the same angle as the long axis of theroot, or a tooth is severely tilted, then greatcare must be taken tomake the access cavitypreparation in the long axis of the tooth to

Figure 5.20A radiograph of a 'C'-shaped canal in a mandibular sec-ond molar.

avoid perforation. In teeth with particularlylong crowns it can also be difficult to locatethe root canal. Itmaybe appropriate in somerare instances tomake initial penetration ofthe pulp chamber without the rubber dam insitu. This allows correct angulation of thebur, as the operator is not distracted by theangulation of the crown. As soon as theaccess cavity is fully prepared the rubberdam should be applied. Itis important tomake sure that the rubber darn clamp ispositioned squarely on the tooth and per-pendicular to the long axis of the root, as itwill give a guide for access cavity prepara-tion. This is very important in incisor teeth,where an incorrectly placed clamp can leadto perforation.RestorationsUnless there are obvious signs of marginaldeficiencies or caries then full crown restora-tions can generally be retained, 'wi.ththeaccess cavity being cut through the restora-tion. Diamondburs arevery effective for cut-ting through porcelain restorations, while afine cross-cuttungsten carbidebur, e.g. thejet

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Area where second ----mesiobuccal canalcould be located

Distobuccal

Figure 5.21Location of the second mesiobuccal canal in the maxillary first molar.

Beaver bur, isparticularly useful for cuttingthroughmetal.PostsPosts shouldbe identified from the preopera-tive radiograph.When cuttingthe access cav-ity maximum post material should beretainedtomake its later removal easier. Corematerial may need to be removed fromaround a post to facilitate subsequentremoval ofthe post.The Location of 'Extra' CanalsThe second mesiobuccal canal of maxillary molars(Fig.5.21): There is a second mesiobuccalcanal in approximately 60% of maxillarymolars; it often lies under a lip of dentine onthemesialwall ofthe accesscavity(Figs.5.22,5.23). Location of the orifice canbe made byvisualizingapoint at the intersection betweena line running from the mesiobuccal to thepalatal canal and a perpendicular from thedistobuccal canal. The lip of dentine in thisarea can be removed using an ultrasonic CT4

Figure 5.22A lip of dentine has been removed using a round bur touncover the two mesiobuccal canals (arrowed) in thismaxillary molar.


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ROOT CANAL PREPARATION 89

tip or a size 2 round bur. There is often anisthmus between the main mesiobuccal canaland the second mesiobuccal (Fig. 5.24); thiscan be traced until the orifice is located. Thepulp floor map should be followed to avoid

Figure5.23Thelocationofthetwomesiobuccalcanals (arrowed)canclearlybe seen inthismaxillarymolarfollowingprepara-tion.

overzealous exploration in the incorrect direc-tion.Four canals inmandibular molars (Fig. 5.25):

Four canals are found in approximately 38%of mandibular molars. If the distal canal doesnot lie in the midline of the tooth, then a sec-ond distal canal should be suspected. Thecanals are often equidistant from the midline.

Figure5.24I nthiscase the lipofdentine has been removed usingultrasonicstorevealanisthmus(arrowed).

Distalcanalorificeslieequidistant tomidline

Figure5.25Locationofa second distalcanal inthe mandibularfirstmolar.


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Careful exploration with a CT4 ultrasonic tipor round bur should uncover the orifice. Itmay be necessary to take a radiograph from adifferent angle to confirm the presence of thesecond distal canal.

Two canals in mandibular incisors (Fig. 5.26):The incidence of two canals in lower incisorsmay be as high as 41 %. A common reason forfailure of root canal treatment in these teethoccurs when a second canal has not beenlocated and is consequently not cleaned.Canals may be missed owing to incorrectpositioning of the access cavity. If access isprepared too far lingually then it may beimpossible to locate a lingual canal. To gainentry into a lingual canal the access cavitymay sometimes need to be extended verynear to the incisal edge.Two canals in a mandibular premolar

(Fig. 5.27): The highest reported incidence oftwo canals in mandibular premolars is 11%.There are rarely two orifices. The lingualcanal normally proj ects from the wall of themain buccal canal at an acute angle. Itcanusually be located by running a fine (ISO 08or 10) file with a sharp bend in the tip alongthe lingual wall of the canal.

Incorrect access: placementtoo far l ingually preventsentry into lingual canal

Access should be placedmore incisally; straight-lineentry into buccal and l ingualcanals can then be achieved

Figure 5.26Location of the lingual canal in mandibular incisors.


+--+-The lingual canal can be extremelyfine and difficult to locateUse a sharp bend in the file tip andrun it down the lingual wallThe file often catches on the lingualcanal orifice

Figure 5.27Location of the lingual canal in mandibular premolars.

HAND INSTRUMENTSEndodontic research has shown how differentinstruments and materials behave within theconfined space of the root canal system dur-ing instrumentation. In parallel withadvances in hand instruments corne newpreparation techniques. These developmentscan occur so rapidly, however, that thechicken often precedes the egg!Instrumentation techniques where apical

preparation is carried out at the start of treat-ment have been superseded by crown-downtechniques, in which the coronal element ofthe root canal is prepared first. The actionswith which files can be used have beenanalysed, and now many modern methods ofinstrumentation use a balanced force motionas opposed to filing. With the introduction ofhighly flexible materials such as nickel-titanium alloys it has been possible to pro-duce instruments with tapers that are greaterthan the original stainless steel hand files,without losing flexibility; such instrumentsare invaluable for tapering the root canalpreparation predictably.As new instrument systems are produced,


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new preparation techniques evolve. The mor-phology of the root canal space is highly vari-able, however, and one preparation techniquecannot be applied to every situation. Itis per-haps more useful to develop and understandbasic concepts of root canal preparation.These can then be implemented to master thediversity of root canal systems with which thedentist may be faced. This approach alsoallows the clinician to modify his or her cur-rent technique as new instruments are pro-duced. Unfortunately no one system can be apanacea. An understanding of endodonticconcepts and a consequent adaptation of tech-niques with which the clinician is conversantwill avoid the unfortunate accumulation ofexpensive equipment that becomes redun-dant when it fails to deliver its promises!There are no secrets to effective root canalpreparation: just practice, patience and persis-tence.

Hand Files and ReamersFor the last forty years root canal instrumentshave been produced to international stan-dards. There are specifications for dimen-sions, fracture resistance, stiffness and colourcoding of endodontic files and reamers.Reamers have fewer turns per unit lengththan the equivalent-sized file.MaterialEndodontic files and reamers are mainlymanufactured from stainless steel, althoughcarbon steel, titanium and nickel-titanium arealso used. The different materials give theinstruments different properties, which inturn affect the way in which they should beused.

Tip DesignThe tip of the instrument can have variousshapes. Originally instrument tips were sharpand had a cutting action; but non-cutting(bullet-shaped) tips are now available thatallow the instrument to slide along the outercurvature of a root canal, allowing prepara-

tion to be centred on the original canal curva-ture.There is a non-linear increase in the diame-

ter of tip sizes between consecutive instru-ments. To address this, one manufacturerproduced intermediate sizes between 10 and30. Golden Mediums (Maillefer) are availablein ISO sizes 12, 17,22 and 27. Anothermethod of reducing the uneven 'jumps' indiameter between sizes has been to produceinstruments with a uniform increase in diam-eter between consecutively sized instruments.Series 29 (Dentsply, Weybridge, Surrey, UK)instruments have a 29% increase in tip sizebetween instruments.

Restoring ForceThis is the force produced by a file when itresists bending. The restoring force for anickel-titanium instrument is 3-4 times lessthan that for an equivalent-sized stainlesssteel file. The restoring force increases with

Table 5.2 ISO size and colour codingCOlOUT Size Tiip (mm)"Pink 06 0,06Grey 08 0,08Purple 10 0.10White 15 0.15Yellow 20 0.20R e d 2 5 O . 2 . . .' iBlue 30 03 0Green 35 0.35Black 4( ) O.4(}White 45 0.45Yellow 5Q 0,50Red 55 0.55Blue I lO 0.60Gre n 7{) 0.70Black 80 0.80White 90 0.90Yellow 100 l..oOR e d 110 1 .10rUll,e izo 1 20Green 130 1.30B!acrk 140 1.40"Nominal size at tip 1fit were net modified to bee,(orexampte, safe-ended.

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filediameter: i.e. files get stiffer astheir diam-eter increases.

Tape rThe standards for instruments specify thathand files have a 0.02 taper; i.e. the diameterof the instrument increases by 0.02 rnrnperrrrrri along the length of the instrument.However because nickel-titanium is super-elastic, it has been possible to create flexibleinstruments with larger tapers such as 0.04,0.06,0.08,0.10 and0.12.

In s tr um e n t De s ig nEndodontic files can be twisted from square,rhomboid or triangular stainless steel blanks,or machined. The standardized length of a fileor reamer blade is 16mm. Reamers normallyhave fewer flutes/blades per unit length thanan equivalent file, and are intended for use ina rotary action. Nickel-titanium instrumentsneed to bemachined using computer-assistedmanufacturing (CAM), as the rnaterral issuperelastic and cannot be twisted. Modernmanufacturing methods allow complex cross-sectional shapesto bemilled.K-file (e.g. Dentsply) instruments can be

manufactured by twisting a square or trian-gular blank or by machining. Files with a tri-angular cross-section are more flexible thanthe equivalent-sized filewith a square cross-section. Files with a triangular cross-sectiontend to have superior cutting characteristicsand are more flexible, and hence less likely totransport the canal during preparation.K-Flex files (Kerr, Romulus, MI, USA) are

produced from a blank that is rhomboid incross-section; this forrns both cutting andnon-cutting edges. The files are more flexiblethananequivalent-sizedK-file.Flexofile (Maillefer: Fig. 5.28) instruments

have a triangular cross-section and are manu-factured from flexible stainless steel.Flexofiles are more efficient at cutting andremoving dentine than an equivalent K-file,because the blade has a sharper angle andthere is more room for debris. The tip of thefile isnon-cutting. This is anadvantage whenpreparing curved canals, as the file is guidedalong the canal curvature, avoiding excessive


Figure 5.28A range of Flexofile instruments. The size 20 instrumentis being measured using a ruler on the Endoring.

cutting into the outer curve of the root canalor transportation. Any flexible, triangularcross-sectionedfile, suchasa Flexofile,canbeusedwith the balanced force action.Hedstroern (e.g. Dentsply: Fig. 5.29) filesare rriac'hirie.dfrorn a tapered cylindricalblock. In cross-section they have the appear-ance of a series of intersecting cones.Hedstroem files are highly efficient at remov-ing dentine on the outstroke when used in afiling motion, but have poor fracture resis-tance inrotation.

Figure 5.29Hedstroem files.


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In strum e nts fo r C o ro na l F la rin gThe following rotary instruments are used toprepare the coronal part of the root canalbefore the main part is prepared by handinstruments.

Gates-Glidden BursThese burs are used in a low-speed handpieceandare extremelyefficientat removingdentineinthe relatively straightparts ofthe root canal.They come in sixsizes (Fig. 5.30) (1-6, diame-ters 0.5-1.5mm) and two lengths. Penetratingtoo deep intocurvedcanals can result in iatro-genic damage, e.g. a strip perforation; for thisreasontheymustbeusedwithcare.

Nickel-Titanium Orifice OpenersThese files are used to flare the coronal aspectof the root canal. Although they aremanufac-tured from nickel-titanium the files arerelatively inflexible, as the cross-sectionaldiameter is larger than that of a standard file.Theuse of these instruments is therefore bestrestricted to the relatively straight parts of theroot canal to avoid stripperforation. Itis rec-ommended that they are used in a handpiecedriven by an electric motor at 150-300 rpm(Fig.5.31).

Figure 5.30Gates-Gl idden burs 1-6.

Figure 5.31The Profile orifice openers (Oentsply).

PREPARATION TECHNIQUESTheaimsofroot canal preparation are: To remove infected debris from the rootcanal system To shape the canal allowing thorough dis-infection "Withirrigants and intracanalmedication(Fig.5.32)

To provide a space for the placement of aroot canal filling. The filling materialshould ideally seal the entire root canalsystem from the periodontal tissues andoralcavity.

Termino logyCrown-down PreparationThe root canal system of a tooth can be pre-pared to give a tapered preparation in essen-tially two ways: apical to coronal or coronalto apical. Preparation of the coronal part ofthe root canal first has several significantadvantages(Fig. 5.33):1. The bulk of the infected material is foundin the pulp chamber and the coronal thirdof the root canal system. Removal of thismaterial early in preparation reduces thebacterial load considerably and prevents

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A


Region cleaned by irrigants Region cleaned by instruments

D Region cleaned!by irrigants Region cleaned"b y instruments

inoculation of the periapical tissues withbacteria extruded by hydraulic pressuresduring preparation. A file placed to theworking length before coronal flaring willact like a piston in a cylinder and forcematerial beyond the apex. Infected dentinechips and bacteria that are extruded willcausepostoperativediscomfort.

2. Early flaring of the coronal part of the rootcanalremoves dentineconstrictions, sothatsubsequent instruments do not bind shortoftheworkinglength.

3. If the working length is estimated follow-ing coronal preparation then there will belittle change in length during preparation.Over-preparation due to poor length

Figure 5.32Chemo-mechanical preparation.

control could be a cause of postoperativepain.4. Preparing the coronalpart of the root canalfirst enablesmore rapid penetration of irri-gants apically. If a pool of irrigant ismain-tained in the access cavity then this willbe guided into the root canal system.during preparation. Dentine chips will bekept in suspension, thereby avoidingblockages.Many procedural errors have been encoun-

tered when the apical portion of the rootcanal systemhas been prepared first, such aszip and elbowformation, instrument fracture,and an increased chance of postoperative


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A mandibular molar with unprepared canals andcarious exposure of the pulp.

An access cavity has been prepared and the cariesremoved. There is straight-line vision of the canals.

The lip of dentine is removed with Gates-Glidden bursduring coronal flaring or with special ultrasonic tips,giving better straight-line access.

The completed access cavity.

Figure 5.33Crown-down preparation.

The canal is divided into three parts: coronal straightpart, middle and apical.In the distal canal in this case the coronal and apicalparts converge.

Coronal f laring is carr ied out using f iles, Gates-Gliddenburs or orifice shapers.Following coronal preparation the root length isestimated using an apex locator and a radiograph.

Apical preparation is carried out with hand files orrotary instruments.

The preparation is completed, creating a gradual taper.This can be produced by stepping back with multipleinstruments or by using an instrument with greater taper.

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flare-up due to greater extrusion of infecteddebris. The outer curve of the apical part ofthe root canal is frequently over-preparedwith filingtechniques.

Procedural ErrorsTransportation, Elbows and ZipsTransportation results from the selectiveremoval of dentine fromthe root canalwall ina specified part. In cross-section the centralpoint of the canal will have moved laterally.Transportation can be carried out electively(duringcoronal flaringto straightenthe canal),or may occur as an iatrogenic error resultingfrom the incorrect use of hand instruments.Internal transportation is used to describe themovement of the canal system internally.External transportation occurswhen the canalis over-prepared and the apical foramen isenlarged or moved; often the foramenbecomesa tear-dropshape(Fig.5.34).Elbuws and zips are caused by the fileattempting to straighten in the root canal as it

is worked up and down. Filing produces acanal that takes on anhourglass shape, withthe narrowest cross-sectional area at theelbow and with the canal widening into thezipped region further apically. The resultantspace isdifficult to cleanandobturate.

Strip PerforationStrip perforation occurs in the middle part ofthe inner curve of a root canal if excessivedentine is removed during preparation (Fig.5.35). This may be a result of relatively largeand stiff files attempting to straightenwithinthe root canal, or of over-use of Gates-Gliddenbursor orificeshapers.

Movement of Files

Balanced ForceThis is amethod of preparing the root canalwith hand instruments in a rotary action (seelater).


Zp~------.:...,,----"canal transported asfill:!straightensFigure 5.34Zip and elbow.

Figure 5.35The circled area in the mesial canals is the position wherea strip perforation could occur by over-preparat ion.

FilingFilingconsistsof an apical-coronalmovementofthe filewhilst applyinglateral force againstthe wall of the canaL The file is moved1-2mm.Hedstroemfiles are extremely efficient in afilingaction, and cut onthe outstroke.Stainless steel files need to be precurved

when filing curved canals, as the inherentrestoring force in the file will attempt tostraighten it within the confines of the rootcanal and lead to a distorted canal shape


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kncrwrr as zipping. Nickel-titanilllll filesshould not be used in a filing motion. Eventhough the instruments arevery flexible therewill be a tendency to over-prepare the outercurve of the root canal inthe apical region, asthe files cannotbeprecurved.Circumferential filing involves workingfiles progressively around all the walls of the

canal. Itisused to ensure that the maximumarea of dentine is instrumented. Over-zealousfiling, however, can result in perforation.Even 'wtfi overlapping file strokes it isunlikely that all the surfaces of the canalwillactuallybe instrumented.Anticurvature filing involves filingprefer-

entially towards the outer curve of the rootcanal, away fromthe furcation, to avoid stripperforation. For example the buccal, mesialand lingual walls of the mesial canals of amandibular molar would be filed more thanthe distal wall during coronal flaring, withmore strokeson eachof thesewalls ina ratioof3:1.

Reciprocating ActionActions such as stem-winding, watch-winding and a quarter-turn pull combinerotary and filing actions. Reciprocatingactions rotate the file gently in a clockwiseand counter-clockwise motion of approxi-mately 45 degrees; the file can gradually beadvanced apically. Watch-winding actionsare particularly useful for exploring a tortu-ouscanalwithveryfinefiles(ISO 06-10).

Apical Flare (or Stepping Back)The apical part of the root canal is normallyflared to facilitate filling of the canal space.Traditionally, files have been used to instru-ment the canal at progressively shorterlengths (0.5 to 1.0rnrn) for concurrentincreases in file size. This will produce anincreased taper to the preparation. As stain-less steel files of large diameters (>ISO size35)become stiff, caremust be used topreventtransportation incurvedcanals.A muchmorepredictable method of flaring the apicalpreparation is by using a greater taperedinstrument. These are made of nickel-titanium andneed tobe used with a balanced

force technique. One single instrument takesthe place of a series of instruments used insteppingback.

RecapitulationRecapitulationsimplymeans repeating again:a smaller file is passed to the working lengthto ensure that length hasnot been lost duringpreparation and to encourage irrigantexchange in the apical ramifications of thecanal system. The use of files with greatertaper makes recapitulation largely unneces-sary.

Patency MaintenanceThe aimofpatency filing is toprevent leavinginfected material in the apical 0.5-1.0 rnrnbeyond the working length. Some endodon-tists advocatethat smallfiles ISO I0) shouldbe passed beyond the working length andtherefore potentially through the apex duringpreparation, to ensure that the canal is com-pletely patent. This procedure could result inextrusion of bacteria and infected dentinechips beyond the apex, and should thereforebe carried out 'wtfi extreme care. Smallinstruments are used to displace the infecteddebris intothe irrigant-filledcanal; largesizesand vigorous filing should be avoided, asthey could result inpostoperative pain. If thecanal is prepared to the zero reading on anapex locator then patency will normally bemaintained; there is therefore no need toinstrument beyond this.

Length EstimationThe lengthof the root canal can be estimatedby using an apex locator and a confirmatoryworking length radiograph. Itisnot appropri-ate to try to estimate the location of the con-striction ina root canal by tactile sense alone,as this too often leads to inaccurate measure-ments.The Apex LocatorThe apex locator (Fig. 5.36) is an electricaldevicethat allowsthe operator to estimatethecanal length, and with practice can be

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98

Figure 5.36Two popular apex locators; both work using multiplefrequencies and are extremely accurate.

extrernely accurate. Apex locators work byapplying an alternating current between twoelectrodes; one makes contact with the lip orcheek (the ground electrode), the other isattachedto a file inthe root canal (Fig. 5.37).The Irrrpeclarrce at the apical Forarrierr isap'prox.irnate ly equal to that between theperiodontal ligarnent and the oral rnucosa;this value isused to calibrate the instrument.The apex locator has a display showingthe zero reading that indicates when the filetip is at the apical foramen. Sorne rnodernapex locators measure the impedance at twoormore frequencies to improve the accuracyof the instrument. Sorne can be used effec-tively even in the presence of electrolytessuch as soclrurn hypochlorite and blood,although these are best avoided; the pulpfloor should always be dry toprevent short-circuiting.

Troubleshooting with an Apex LocatorNo reading Isthe unit switchedon? Are the leads all connected and is the liphookinplace? Are the batteries fully charged?


Figure 5.37The tip of the apex locator is kept in contact with the fileas it is advanced apical ly.

Short reading Is the file short-circuiting through ametal-licrestoration? Make sure the canals and pulp charnberarerelativelydry. Is it likely that there may be a perforationinthe root? Is there the likelihood of a large lateralcanal? Is there a communication betweencanals?-for instance, between rnesiobuc-cal canals of rnax.illary first rnolars, ormesialcanalsofmandibularmolars.

Long reading Is itpossiblethat the apicalregionhasbeendestroyedby chronic inflammatoryresorp-tion? (For instance, in cases with chronicapical periodontitis and large lesions; inthesecasestrya largerfile.)

Checkthe batterypower.

Radiographic Techniques for LengthEstimationApex locators should not take the place ofworking length estirnation radiographs, butmake anexcellent adjunct for accurate identi-


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fication of the root canal terminus. In this sit-uation, when used regularly an apex locatorwill help reduce the number of radiographsrequired for endodontic treatment. The apexof the tooth is more likely to be locatedaccurately first time by these means thanwithtactilemethods.Taking a good radiograph with the rubberdarn and clamp in situ can be difficult; withpractice, however, it becomes predictable.The Endoray (Dentsply) isparticularly usefulfor ensuring that the X-ray beam is correctlyalignedandnot conedoff (Fig. 5.38).The filmmust never be bent. Cutting down on repeatexposures reduces the radiation dose to thepatient. If coronal flaring is carried out at thestart of root canal preparation it should bepossible to use at least a size 15 file to esti-mate working length. The tip of such a filecan easilybeviewed onradiographs.Estimating the Working LengthIf the lengthradiograph shows that the file tipis more than 3mrn from the apical foramenthen the radiograph should be repeated(Fig. 5.39). On some occasions itmay alsobenecessary to take more than one workinglength radiograph atvarying angles. There isno significant difference in the ability toassess root length using digital radiographyor conventional film; however, there is a sig-nificant reduction in radiation dose and timewith digital radiography.

Figure 5.38The Endoray.

Figure 5.39A length-estimation radiograph. The three files in thepremolar were not clearly visible and a separate radi-ograph was taken.

Separating Root Canals in Length EstimationRadiographsMandibular incisors (Fig.5.40)Separating the buccal and lingual canals of

mandibular incisors can be difficult, unlessthe X-ray cone is rotated by a few degrees,either left or right, to separate the canals; butcare must be taken to ensure that the treatedtooth is centredon the film.Mandibular premolarsThe lingual canal of amandibular premolarcan oftenbe separated fromthemain (buccal)canal by aiming the beam from an anterior(andinferior)direction.Mandibular molars (Fig.5.41)Separating the buccal and lingual canals of

the mesial or distal roots of mandibularmolars can be achieved by rotating the beamto amore anteriorprojection.Maxillary premolarsSeparating buccal and lingual canals can be

achievedby aimingthe beamfromananteriordirection. If SpencerWells forceps areusedtohold the filmbehind the rubber darn, then it isnot necessaryto removetherubber damframe.Maxillary molarsTo separate the first and second mesiobuc-

cal canals the cone shouldbe aimed from thedistal aspect.

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Film

Mandibular incisor:moving cone left or rightseparates canals.

Buccalcanal

Beamdirection

Figure 5.40Mandibular incisors.

Mandibular molar: moving the.cum- anteriorly separates therout (;,1 n a I s,S imiJ< lr p roc e ss f u r premolars,

D i s to b IIea Icanal

BeamdirectionFigure 5.41Mandibular molars.

HAND PREPARATION RationaleThe root canal is divided into three parts: thecoronal 'straight' part of the root canal, theapical part, and a middle part termed theImerging' part.

It is exacting and time-consuming to learn anew technique and become conversant withit. Practice on extracted teeth is invaluablebefore embarking on the treatment of apatient.

Crown-down Preparation

Coronal PreparationThe coronal part of the root canal is preparedfirst. As it is relatively straight the instru-ments used to prepare it do not have to bevery flexible. A combination of files, Gates-Glidden burs and nickel-titanium orificeopeners can be used. The depth to whichinitial preparation is carried out should beestimated from the preoperative radiograph.

The principles of crown-down preparationwill be described. This allows the practitionerto develop a personalized technique that fol-lows the ideals of modern crown-downpreparation without unnecessary detail.


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The canal rnu.st be irrigated throughoutpreparation to remove debris, kill microbesand prevent blocking. Theuse of files beforeGates-Glidden burs can be used to transportthe orifice of a canal deliberately. Thisdecreases the initial curvature of the canaland improves straight-line access. The bursshould be used in a planing action, cuttingagainst the thickest wall of the root canal asthe drill is 'wtbdr-a'wrifr'orn the canal.Applying apical pressure and drilling into theroot canal shouldbe avoided, as this can leadto over-enlargement and ledging.

Apical PreparationThis will involve preparation around a curvein the vast rnajorrty of canals, and is theregion inwhich most iatrogenic errors haveoccurred in the past. Once the coronal part ofthe root canalhasbeen prepared, access to theapical part ismade easier. Instruments usedto prepare the apical part of the root canalsystem need to be thin and flexible, and aremade of materials such as flexible stainlesssteel ornickel-titanium.Before apical preparation is started anesti-

mate of the root canal length must be made,ideally using an apex locator and confirmedwith a radiograph.If coronal preparation hasbeen carried out,

then there will be less change inthe length ofthe preparation during instrumentation. Thelengths are marked on the file with a rubberstop to a clearly definable and easily rernern-

POWff PhasRotate fill!90o-120Qclockwise to engagedentine walls

Figure 5.42TheBalanced Forceaction.

bered reference point, which isbest recordedinthe patient's notes.The size of the apical preparation andmas-

ter apical filewill dependon:1. Thesizeof the natural canal.2. The obturation technique that is tobe used(with vertically compacted warm tech-niques the apical termination of the prepa-ration should be kept as small aspracticable).

3. Theoverall taper of the root canal.The apex isprepared to the working length

with a file of not less than ISO size 25, as thisallows delivery of irrigant to the most apicalpart of the root canal system. Irrigant can becarried apically and replenished by recapitu-lationwithfiles.

Using the Balanced Force Action for ApicalPreparationThe balanced force action when used withflexible files results in amore centred prepa-ration and less transportation than with a fil-ingtechnique.The technique (Roane et a11985)This instrumentation technique uses clock-wise/anticlockwise rotational rnoti on toremove dentine with flexible stainless steelfiles or nickel-titanium files (Fig. 5.42). Itisuseful for rapidly removing dentine in curvedcanals whilst maintaining curvature (files arenot precurved).

Control PhaseRotate file 120"-360" anti-clockwise whilst applyingapical pressure

I

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102

Place the file intothe canal until light resis-tance is rnet and turn it up to 90-120clockwise to engage the dentine walls (thepower phase).

Whilst applying light apical pressure (toprevent the file backing out of the canal)turn 120-360 anticlockwise. Listen for a'click' as a bite of dentine is removed (thecontrol phase).Repeat the clockwise /anticlockwisemotion until the file reaches the desiredlength. lfthe file does not appear tobe cut-ting, then it should be rotated clockwise toload the flutes with debris and removedfor cleaning. Check the irrstrurnerit fordamage; discard if evident.Some instruments (e.g. Greater Taper files:

Dentsply) are ground in the reverse directionand will therefore need to be used in theopposite manner.

Using Greater Taper Files for ApicalPreparationGreater Taper files are manufactured fromnickel-titanium and have to be used with areverse balanced force action (Fig. 5.43). Toprevent the tip binding and possibly fractur-ing it is important to prepare a pilot channel(glide path) for the instruments to follow. Thecanal is instrumented to the working lengthusing a balanced force action with Flexofilesto at least a size 25. Greater Taper files canthen be worked to the same length instead ofstepping backwith multiple instruments.The different tapers may be appropriate fordifferent situations (Table5.3).


Figure 5.43Greater Taper hand files: white 0.06, yellow 0.08, red 0.10,blue 0.12.

MergingThe apical and coronal preparation shouldmerge into one another in a gradual smoothtaper. This can often be achievedwith a singleGreater Taper instrument, as opposed to step-ping backwith multiple instruments.

Crown-down Preparation TechniquesThe following crown-down hand preparationtechniques are described: Stepdown Modified DoubleFlare

Table5.3Tapersappropriateforvariousrootcanalsituations

Multicanal premolars. mesial roots ofmandibular molars, buccal roots ofmax i li a r y-m. () l ar s,Palatal canal Ofmaxillary molars, distal roots of mandibular molars, ingle-rootedpremolcus, mandibular canines and maxillary anteriors.Large straight canals and canals with large terminal diamet r.

Colour Taper

White 0.06Yellow 0,0;8

R e d IU O

Blue 0.12

All thin 01' narrow canals.


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Stepdown Technique of Canal Preparation(A hand-filing technique with precurved files:Goerig et al (1982).)

Precurving FilesFiles can be easily and accurately precurvedin a file-bending device (Fig. 5.44). The curveis estimated from the radiograph, and byfeedback from small files that are placed inthe canal at the start of treatment.

Initial Coronal FlareInsert a size 15 Hedstroern file into thestraight part of the root canal (16 rum fromcusp tips in a typical molar). Pull back in a fil-ing motion and repeat, with more strokes onthe outer curved "Wallthan the inner "Wall,until the file is loose. Irrigate with hypochlo-rite. Repeat with sizes 20 and 25.Insert a Gates-Glidden bur size 1 into thestraight part of the canal; plane the outer wallgently in an apical-coronal direction. Repeatwith sizes 2 and 3 to shorter distances. Usecopious irrigation or waterspray to preventclogging the canals with dentine chips.

Apical PreparationInsert a fine Flexofile, such as size 10 or 15,0.5-1.0 rum short of the apical constriction.This is verified with an apex locator and radi-ograph. Files are precurved to conform to the

Figure 5.44A Flexobend for precurving files.

10 3

canal shape. Pull the file back in a filingmotion. Reinsert and repeat, working aroundthe canal walls until the file is loose. Irrigate.Repeat with the next largest file up to size 25.Apical Flare (Stepback)A Greater Taper instrument is selected andused with a reverse balanced force action toflare the apical preparation. Itis no longernecessary to use a sequence of instruments tostep back.

The Modified Double-Flare Technique(A technique that uses the balanced forceinstrumentation action: Saunders andSaunders (1992).)Coronal PreparationCheck the length of the straight section of thecoronal part of the root canal from a preoper-ative radiograph. Take a size 35 or 40 file withthe stop set at this length, coat with lubricantand instrument with balanced force action tothe full length of the straight part of the canal.If a size 35 is too tight then a smaller instru-ment will need to be used.Continue to prepare the straight part of

the canal "With hand files until a size 1Gates-Glidden bur will fit in the canal. A size40 instrument will create a canal with an ori-fice diameter of at least 0.5 rum and shouldprovide sufficient space. Irrigate bet"Weenfiles.Use Gates-Glidden burs 1,2 and 3 cutting

on outward stroke to flare the canal, beingcareful not to over-prepare. Use copious irri-gation with Gates-Glidden burs.Preparation of the Apical SectionA size 10 file should now pass just short ofthe full working length. Use a size 15 file toinstrument to the estimated working lengthusing balanced force action. Length is verifiedwith an apex locator and confirmed with aradiograph.The canal is then prepared to the workinglength with increasing file sizes to a masterapical file (MAF) of 35-40 using balancedforce. The size of the MAP will depend on thesize of the original canal and the degree ofcurvature. Fine, severely curved canals rnay


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10 4

only be prepared to the miriimum size (ISO25). Irrigatebetweeneachfile.Apical Flaring (Stepping Back)This can be achieved with a Greater Taperfile,as alreadydescribed.

Troubleshooting Preparation of RootCanalsTransportationPrecurving files reduces the restoring forcethat is appliedto the root canalwall, and con-sequently reduces the chance of transporta-tion. Usingthebalancedforce instnunentationtechniquewith non-end cutting, flexible fileswill produce less transportation of the canal.Ledges can also be created with Gates-

Glidden burs during coronal flaring; this canbe avoided if the bur isused toplane the wallof the root canal as it is withdrawn, ratherthanbeing forced apically as if drilling downthe rootcanal.

PerforationPerforation is the iatrogenic damage to thetooth or root canal wall that results in a con-nectionbeingmadewith the periodontal liga-mentororalcavity.Perforationcanbe avoidedby:

1. Using the pulp floor map to locate rootcanalorifices2. Gradually working up the series fromsmall files to larger sizes, always recapitu-latingwitha smallerfilebetweensizes

3. Using an apex locator and radiograph toconfirmroot canal length

4. MinimizingoveruseofGates-Gliddenburs,either too deep or too large, in curvedcanalswhere a stripperforation mayoccur.Try and direct the cutting action intothe bulkiest wall of dentine; this alsohelps straighten the first curvature of theroot canal and improves straight-lineaccess.

5. Restricting the use of 'orifice openers' tosmallsizes innarrowcanals

6. Using an anticurvature filing technique to


remove dentine selectively fromthe bulk-iestwall

7. NeverforcinginstrumentsorjumpingsizesS. Irrigating copiously; not only will thisdisinfect the root canal and dissolveorganicmaterial, but itwill alsokeep den-tine chips in suspension andprevent block-ing. If dentine chips are packed into theapical region of the root canal then thepreparation can become transported inter-nally. This could eventually lead toperfo-ration.

BlockageBlockagecanbeavoidedby:1. Using copious amounts of irrigant. Thiswill keep dentine chips suspended in theirrigant so that they can be flushed fromthe root canal system during preparation.Solutions such as EDTA are particularlyuseful, as they act as chelating agents,causing clumpingtogether ofparticles.

2. Keeping the pulp chamber flooded withirrigant during preparation; this allowsthe continuous transfer and replenish-ment of irrigant within the root canalsystem as each new file is introduced.Dentine chips are carried out into theaccess cavity in suspension for aspira-tion.

What To Do IfBlockage OccursIf a blockage occurs suddenly during rootcanal preparation, place a small amount ofEDTA lubricant (Fig. 5.45) on a fine pre-curvedfile (ISO 10)(Fig. 5.46) and introduceit into the root canal. Use a gentle watch-winding action to work loose the dentinechips of the blockage. When patency isregained, irrigate the canal with sodiumhypochlorite. This will flush out the dentinechips (effervescencemaypossibly help in theprocess). Whatever happens do not try toforce instrunrents through a blockage; thiswill simply compact the dentine chips furtherandmake the situationworse. If the blockageispersistent, endosonicsmayhelp to dislodgethe dentine chips. Ultrasonic irrigation sys-tems used at low power with full irrigantflowcansometimesdislodgeblockagesby the


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Figure 5.45File-eze is an EDTA-based lubricant.

Figure 5.46A sharp curve in a small file can be used to negotiate pastblockages.

action of acoustic rnicrostrearning around avibrating file. Another method to bypass aledge is to put a sharp curve at the tip of thefile sothat it overcomesthe defect(Fig.5.47).

Fractured InstrumentUnfortunately the occasional instrument mayfracture unexpectedly; but this should be arare occurrence. Fracture (or separation, as itis sometimes euphemistically called) is per-haps more frequent "Withnickel-titaniuminstruments. The risk of instrument fracturecanbereducedby:1. Always progressing through the sizes offiles in sequence, and not jumping sizes.Forcing an instrument will inevitably leadto fracture.

~.""'------T'--- S]1


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10 6

Wine-bottle Effect (Fig. 5.48)Not the consequence of drinking too much,but the shape that is created from overuse ofGates-Glidden burs to flare coronally! Thewine-bottle effect can make obturationdifficult,will increase the risk of stripperfora-tion andweakens the tooth.These problems will be avoided if Gates-

Glidden burs are used sequentially to planethe walls of the root canal, and larger sizesare used to progressively shorter distances, orthey are sustituted by orifice openers.

MECHANICAL PREPARATIONTECHNIQUESIntroduction

Rotary endodontic instruments manufacturedfromnickel-titaniumare3-4timesmoreflexiblethan equivalent flexible stainless steel instru-ments. The instruments have a greater taperthan standard instruments (0.02mmper mm),

Figure 5.45Over-use of Gates-Glidden burs in these mandibularincisors has resulted in a wine-bottle effect.


whileretainingflexibility.Modernmechanicallydriven instruments are designed for use in acontinuous rotary action at a slow speed(150-350 rpm). They have been shown to pro-ducelittletransportationof therootcanal.Thereare nowmany different systems available, butthebasicprinciplesfortheiruseare similar.

Basic Principles of Use

RotationThe use of a torque-controlled electric motorand a speed-reducing handpiece (Fig. 5.49) ismore reliable than a handpiece fitted to anairmotor. Itismuchmore difficult to controlthe speed of air-driven handpieces pre-dictably, and therefore their use can lead toinstrument fracture.

SpeedItis important that the instruments are usedat the manufacturer's reco=ended speed.Most rotary files should be rotated at speeds

Figure 5.49An electric motor and speed-reducing handpiece isessential for rotary preparation techniques.


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between 150 and 350 rpm for routine rootcanalpreparation.

Cutting ActionThe instruments should be advanced into thecanalwith a light apical-coronal action (peck-ing/pumping) inwaves of 8-10 seconds. Theyare then withdrawn, assessed for damage,cleaned of debris and reinserted. Files mustbe kept moving coronally-apically within thecanal; rotating an instrument at a stationaryposition close to or at full working lengthresults in alternating compression and straininthe file at the greatestpoint of itscurvature,and could eventually lead to instrument fail-ure. Theroot canals are flooded with irrigantor lubricant during instrumentation to pre-vent dentine chips compacting. Packing ofdentinechips aroundthe file can leadto bind-ingand instrument fracture.

Canal CurvatureNickel-titanium, because of its superelastic-ity, can be rotated in canals of sharp curva-ture; but care must be taken in severelycurvedcanals. Invery curvedcanals or canalswith sudden abrupt apical curvatures handinstrumentationispreferred for completionofpreparation.

Avoidance of Instrument FractureTo avoid fracture it is important to create apilot channel that the non-cutting tip of arotary instrument can follow. If such a chan-nel is not created the tip may bind and frac-ture.Preparation should be carried out in a

crown-downmanner, as this ensuresthat suc-cessive instruments arenot overworked.Nickel-titanium instruments can fracture

unpredictably, and may not show signs ofpermanent deformation before failing.Instruments should therefore be changed reg-ularly and should never be forced duringpreparation. Itis often recornrnencied thatnickel-titanium instruments should not beused inmore than5-10canals.

The InstrumentsTherearetwobasictypes:

For Coronal PreparationOrifice openers are usually relatively thick indiameter and are consequently not veryflexible.Theyaredesignedforcoronalflaring,and should onlybe used in the 'straight' partofa rootcanal.

ForApical PreparationTapered rotary files are available in tapers of0.04, 0.06, 0.08, 0.10 and 0.12. Profiles(Dentsply), Quantec (Analytic Endodontics,Glendora, CA, USA), Hero (Micro-mega,Geneva, Switzerland), K3 (Kerr, Bretton,Peterborough, UK) and Greater Taper filesare essentially used to flare rapidly a pre-prepared pilot channel in the apical part ofthe root canal. Instruments are used in acrown-downmanner(Figs.5.50-5.53).

Figure 5.50The Greater Taper rotary range of instruments: an acces-sory file for coronal flaring (left) and then tapers 0.06,0.08,0.10,0.12.

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1 0 8

Figure 5.51K3 rotary instruments. The first two on the left are usedfor coronal flaring; the others can be used apically.

Figure 5.52Profiles: orifice openers have three bands, 0.06 tapers twobands and 0.04 tapers a single band.

Figure 5.53A range of Profile 0.06 taper instruments.


Root Canal Preparation with RotaryInstrumentsThe aim of preparation is to produce a gradu-ally tapering shape in the bulk of the rootcanal system, with the narrowest diameterapically, the greatest diameter coronally and asmooth flow between the two. The apicalpreparation must be at least an ISO size 25 toallow good irrigant penetration.

Divide the canal mentally into three parts:Coronal (,Straight part of canal')Middle ('Merging')Apical ('Apical preparation')

Coronal PreparationNickel-titanium orifice openers are relativelyinflexible and are therefore only suitable foruse in the coronal 'straight part' part of theroot canal. This distance can be estimatedfrom the pre-treatment radiograph. The canalshould be irrigated between instrumentsusing sodium hypochlorite solution.The canal is first instrumented usingFlexofiles sizes 10-40 in a balanced forceaction within the coronal 'straight' part of theroot canal. This can be estimated from thepreoperative diagnostic radiograph. Orificeopeners are then used in sequence to flare thecanal rapidly to the same length. They can beused in ascending or descending order. Alarger instrument will make room for asmaller instrument, allowing deeper penetra-tion.Where the canal is already greater than anISO size 40, orifice openers can be used with-out the necessity of preparing a pilot channel.Apical PreparationFollowing coronal preparation the root canallength is measured using an apex locator(zero reading) and confirmed with a radio-graph. The working length should be 0-1 mmfrom the root canal terminus. A pilot channelis prepared using stainless steel hand instru-ments to the working length; this acts as aguide for the nickel-titanium instruments.


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Preparation of the Pilot ChannelThepilot channel isprepared using Flexofilessizes 15-25with the balanced force technique(see hand instrumentation). The canal is irri-gated with hypochlorite solution betweeninstruments, and in fine canals anEDTA-based lubricant may also be used toprevent binding of instrwnents and com-paction of any dentine chips that may havebeen created.The canal can now be flared to the desiredtaper. Nickel-titanium instruments are

rotated at 150-350 rpm in a speed-reducinghandpiece using an electric torque-controlledmotor. Instruments should be used sequen-tially. The master apical sizemust be at leastsize25 forgoodirrigant penetration.

Middle Section of the Canal: 'Merging'

If apical instrumentation has been completedwith a smaller-taper instrument (0.02, 0.04),the coronal and apical preparation mayneedto be merged by using an instrument with alarger taper. Thiswill create a smooth flowtothe preparation.During mechanical preparation the canal isthoroughly irrigated with sodium hypochlo-

rite solution. This can be carried out withhand syringes or endosonichandpieces.

1 0 9

FURTHER READING

Buchanan LS (2000). The standardized-taperroot canal preparation-Part 1. Concepts forvariably tapered shaping instruments.International Endodontic journal 33: 516-529.GoerigAC, MichelichRJ, SchultzHH(1982).Instrumentation ofroot canals inmolar usingthe step-down technique. Journal ofEndodontics8: 550-554.RoanelB, SabalaCL,DuncansonMG (1985).The 'balanced force' concept for instrumenta-tion of curved canals. Journal of Endodontics11:203-211.SaundersWP, Saunders EM (1992). Effect ofnon-cutting tipped instruments onthe qualityof root canal preparation using a modifieddouble-flared technique. Journal ofEndodontics18:32-36.Schilder H (1974). Cleaning and shaping theroot canal. Dental Clinics of North America 18:269-296.European Society of Endodontology (1994).Consensus report of the European Society ofEndodontology on quality guidelines forendodontic treatment. International Endodonticjournal 27:115-124.


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CONTENTS Why? Irrigants How? Medicaments Further Reading

Following thorough instrumentation of aninfectedroot canal therewillbe a significantlyreduced number of bacteria present; but it iswell documented that instrumentation alonecannot clean all the internal surfaces of theroot canal. Bacteria can be found on the rootcanal walls, within dentine tubules and in lat-eral canals. Antibacterial irrigants and inter-appointment medicaments are needed to killthe remainingmicro-organisms.A large series of follow up studies have

shown that, by using thorough mechanicalpreparation, irrigation with sodiumhypochlorite and dressing with calciumhydroxide, predictable disinfection can beachieved in almost 100% of root canals. Thisin turn has produced clinical and radio-graphic evidenceof healing apicalperiodonti-tis inover90%ofcases.Medicaments are also invaluable in pre-

venting the recolonization of the root canalbetween appointments. If the canal space isleft empty then the small number of bacteriaremaining canmultiplyto levels equivalent tothose that were initiallypresent.

IRRIGANTSIrrigants should: Be antimicrobial Have a lowsurfacetension Not be mutagenic, carcinogenic or overtlycytotoxic Possess tissue-dissolvingproperties

Remainactive followingstorage Be inexpensive.

Preparations Used as Irrigants

Sodium HypochloriteSodiumhypochlorite solution has been usedas an irrigant in endodontics for many years.Itis inexpensive, readily available, and highlyantimicrobial, and has valuable tissue-dissolving properties. A 0.5% solution killsbacteria. Sodium hypochlorite solutionsgreater than 1% will effectively dissolveorganic tissue. Solutions ranging in strengthfrom0.5% to 5.25% havebeenrecommendedfor use in endodontics (Fig. 6.1). Increasingthe concentration will increase the rate atwhich organic material is dissolved, andmayimprove its effectiveness as an antibacterialagent.Heating the solutionwill havea similareffect (Fig. 6.2). The tissue-dissolving abilityof sodium hypochlorite is affected by thearnotrnt of organic rnater ial present in thecanal, the fluid flow and the surface areaavailable.The volume of solution used is probably

more important than the concentration.Several millilitres should be exchanged fre-quently throughout the root canal s'ysternduring instrumentation. Frequent replenish-ment will improve the flushing action of theirrigant, which removes debris. Keeping thecanal systemfloodedwith freshsolutionat alltimes during preparation will improvedissolution of organic material and killing of

tics - problem solving in clinical practice 2002 cleaning & shaping

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