critical in rootcanal therapy.The root canalswithin curvedroots aresimilarlycurved, andwhen these arepreserved andcleaned thesystem exhibitsnatural flow.The greatestproblem lies inthe apicalportion of thecanal and thegreatest caremust be takento maintain thedirection ofthe curves inthis region.Properlyprepared rootcanals will

November 2003Irish Dentist

instrument fracture ortransportation by freeing upthe instrument in the canal.

Decreasing cross-sectionaldiameters at every pointapically and increasing ateach point as the access cavityis approachedThe second objective is acorollary of the first. Thisshape creates control at everylevel of the preparation. Itfacilitates the removal oforganic debris coronally bythe action of instruments andirrigating solutions. It allowsthe placement of condensinginstruments deep into theroot canal preparation to

14

Last issue I discussed theconcept of cleaning andshaping the root canal systemfrom a historical perspectiveand compared this with thegoals laid down in present-day endodontics. Schilderoutlined the modern conceptconcisely in his articleCleaning and Shaping the RootCanal in 1974 and this iswidely regarded as thedefinitive document on thesubject. Within the documenthe laid down strictmechanical and biologicalobjectives for attainingpredictably the goal ofsuccessful anatomically

generated endodontics. I willdiscuss the biologicalobjectives in a future issue.

Before explaining themechanical objectives andtheir fulfilment, it is assumedthat the operator hasprepared an adequate accesscavity. In other words, thereis straight line access to thebody of the canal.

The root canal preparationshould develop a continuouslytapering coneThis shape mimics the shapeof the canals before theyundergo calcifications andthe formation of secondarydentine. The canal shoulddevelop a continuouslytapering cone from the accesscavity to the apical foramen.This conical shape allowseffective cleansing of thecanal, as well as allowinghydraulic principles tooperate by the restricted flowprinciple. As flow isrestricted by the conicalform, compaction of warmgutta percha and sealergenerates vertical andhorizontal vectors of forcethat take the path of leastresistance, obturating andsealing the apical and lateralforamina. This shapeenhances the uses of reamersand files by allowing closecontact with the root canalsurface. Additionally,appropriate funnellingenhances irrigation, debrisremoval and tactility andreduces the potential for

CLINICAL

Explaining Schilders FiveMechanical Objectives

Dr Raphael Bellamy addresses each of Schilders Mechanical Objectives from a practical, clinical standpoint

Raphael Bellamy BDS (NUI)Cert. Endo. is a graduate ofUniversity College Cork andThe Goldman School ofDental Medicine in Boston,Massachusetts, where hecompleted his postgraduatestudies in endodontics. He iscurrently in private practicelimited to endodontics in Dublin

Decreasing cross sectional diameters at every

point apically

transmit realistic compactionpressures to the warm guttapercha. This potentiates thelikelihood of obturatingaccessory foramina in theapical one third of the system.The sole exception to thissecond principle is in the caseof internal resorption, whereadherence to it would greatlyweaken the remaining toothstructure.

In multiple planes, whichintroduces the concept offlowThe third objectiveencourages us to think in thethird dimension. As stated inmy previous artcle, this is

IR Nov Bellamy/clinical 30/9/03 12:18 pm Page 1

working length.Remember what Isaid about thinking inthree dimensions? Wemust continue to feelwith handinstruments for theribbons, flags andbanners that are thenorm of root canalsystems.

Keeping theforamen as small aspractical, not themuch misquotedsmall as possible,restricts the guttapercha in compactionand facilitatescompaction. Largevertical and

horizontal forces aresubsequently generated in thecorrect shape that will findand seal lateral anatomy.Some of the most difficultcases to obturate are thosewith enlarged apical openings.Although gutta percha doesexhibit excellent rheologicalproperties to facilitate theobturation of the open apex.

In cases of pulpal necrosis,sufficient enlargement musttake place to ensure thecleanliness of the region, butexcessive removal of apicaldentine and cementum shouldbe avoided. Clearly in a vital,early, pulpitic case the samewould not apply. Adherenceto the principle that theforamen should be as small aspractical places no maximumlimitation on its size since thisis determined by the clinicalsituation presented. The largeforaminal openings in cases ofapical resorption may still beas small as practical. Studies

the root in the adjacent bone.The number of root canalsthat have left the confines ofthe original canal but notperforated the root surfaceare far greater. Successinvolves the treatment of thenaturally existing root canal.Boring pathways in roots maymake for good radiographsbut it is not a successfultherapeutic measure. It is thiscommon and potentiallydisastrous outcome to whichthe fourth mechanical objectalludes.

Foramen transportationmost commonly takes twoforms, namely thedevelopment of an ellipticalor teardrop foramen and anoutright root perforation.Since apical foramina areusually found short and tothe side of the radiographicapex, repeated passages ofreamers and files tend tostraighten these delicatepassageways and actuallyenlarge the opening in the

November 2003Irish Dentist 15

maintain these fine apicalcurvatures, as should beevident radiographically inthe finished case.

Do not transport the foramenThe apical opening shouldremain it its original spatialrelationship both to the boneand to the root surface. Themovement or transportationof the apical opening is acommon error in root canalpreparation that leads all toofrequently to chronic rootcanal discomfort or outrightfailure in treatment. Nostraightening is permitted inthe apical few millimetres ofany canal without severe riskto the outcome of the case. Itis common to observe,during surgical endodontics,root canal fillings thatradiographically appeared tobe within the confines of theroot but which have inactuality left the root canalseveral millimetres short ofthe apex and have paralleled

Multiple planes that introduce the concept of flow

Perforation Tear or rip

direction away from thenatural curve of the canal. Asthe process develops ateardrop opening is created.It ought to be rememberedthat an inadequate accesscavity that restricts the shaftsof instruments encouragestransportation as well asrestrictions in the body ofthe canal.

Keep the foramen as small aspractical in all casesNumerous studies confirmthat although natural apicalforamina are not entirelyround, root canals do tend tobe rounder in their apicalone third. This anatomicalreality is of benefit to us,especially in the use ofperfectly round rotaryinstruments. However, weshould never fall into thecomfort zone of assuming wehave cleaned the apical areasimply because ourinstrument has reached the

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increase the risk oftearing, it alsoincreases thepotential formicroleakage.

In summary, thegoal is to produce athree dimensional,continuouslytapering,multiplaned conefrom access cavity toradiographicterminus whilepreserving foraminalposition and size.

It all sounds a bit dauntingdoesnt it? Well, no, notreally, because on closeanalysis each of themechanical objectives is acorollary of the one before.Believe me when I say thatthese objectives are the roadmap to predictableendodontics. Remember theburglar analogy I mentionedin my previous article? Wellthe mechanical and biologicalobjectives are the rules thathave to be adhered to if theburglar is to remainundetected. If the rules areviolated then we run the riskof awaking the sleeping giant.Next issue I will discuss thelittle known biologicalobjectives of cleaning andshaping.

ReferencesSchilder H (1974) Cleaning and

shaping the root canal. Dental

Clinics of North America 18(2)

West JD et al (1994) Cleaning and

shaping the root canal system. In:

Pathways of the Pulp, 6th Edition,

chapter 8

Green D (1956) Stereomicroscopic

study of the root apices of 400

maxillary and mandibular teeth.

OOO 9(11): 1224-1232

Kuttler Y (1955) Microscopic

investigation of root apices. OOO

50: 544-552

November 2003 17Irish Dentist

CLINICAL

by Green (1956) and Kuttler(1955) determined the extentof changes in the apicalforamen with increased age.However, the diameters wererarely greater than the 0.3mmto 0.5mm range in any teeth.I baulk at the conversations Ihear and cases I see that havebeen cleaned to and beyond0.6mm, and even 0.8mm! Itis surely no coincidence thatthese teeth are oftensymptomatic and weepcontinually, greatlycomplicating obturation.Once these sizes are attainedby cleaning and shaping, itbecomes very difficult indeedto generate a continuouslytapering cone to obturate.The goal is to clean but notto enlarge the foramen. If thediameter of a foramen isincreased from 0.2mm to0.4mm, the area of theforamen has increased byfour times! Not only does this

Violation of mechanical

objectives four and five (L to R)

Keep the foramen as small as

practical

Acknowledgement

All images provided for thisarticle are digital and capturedwith a Trophy RVG system,Trophy Radiologie SA, France

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