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Page 1: Mastering Endodontic Instrumentation - · PDF filecient use of rotary instruments, ... Although a dramatic reduction in time required to accomplish instrumen- ... Mastering Endodontic

MasteringEndodontic Instrumentation

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Mastering Endodontic Instrumentation

John T. McSpadden, D.D.S.

Publisher Goes Here

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Dedication/AcknowledgementsMastering Endodontic InstrumentationCopyright © 2006 John T. McSpadden, D.D.S Published by Publisher

All rights reserved. No part of this book may be reproduced (except for inclusion inreviews), disseminated or utilized in any form or by any means, electronic or mechanical,including photocopying, recording,or in any information storage and retrieval system,or theInternet/World Wide Web without written permission from the author or publisher.

For further information, please contact:

Contact Information

Book design by:Arbor Books, Inc.19 Spear Road, Suite 202 Ramsey, NJ 07446 www.arborbooks.com

Printed in Country

Mastering Endodontic InstrumentationJohn T. McSpadden, D.D.S

1.Title 2.Author 3. Genre

Library of Congress Control Number: XXXXXXXXXXXXX

ISBN: XXXXXXXXXXXXX

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The two primary goals for root canal instru-mentation are:

1. To provide a biological environmentthat is conducive to healing

2. To provide a canal shape that is con-formable to sealing.

At this stage of endodontic development,common to all instrumentation techniques isthe use of endodontic files.Although not uni-versally used, rotary instrumentation is gain-ing universal interest. The purpose of thisbook is to provide information gained fromextensive research to facilitate the most effi-cient use of rotary instruments, without thethreat of failure, while conforming to theclinician’s treatment ideals.

I am convinced the investment in timerequired for understanding the physics ofrotary instrumentation technology can savehundreds of hours,hundreds of mistakes andhundreds of thousands of dollars—benefits

rarely attainable.The greatest benefit, howev-er, of utilizing design concepts is enhancingthe quality of treatment while enjoying thepractice of excellence.

One should not, however, succumb to apredisposition that concepts resulting in areduction of time are necessarily a compro-mise. Just because threading a needle mayrequire multiple attempts and require moretime than being successful on the firstattempt does not mean that the extra timerenders it more worthy than immediate suc-cess; regardless of the time invested, theresult is a threaded needle.

Ask the question: If every single actionyou made during instrumentation resulted inthe greatest benefit possible in the most effi-cient manner, how would it change the qual-ity and profile of your practice? Most wouldagree that the ability to replace repetitious,unnecessary and counterproductive actions,with only the most effective actions, wouldbe true excellence. However, it certainly

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Someone once asked, “Which is worse, ignorance or apathy?” The answerwas, “I don’t know and I don’t care.”This book is not for them.This book isabout developing expertise and employing knowledge for those that aspireto become the best.

Although a dramatic reduction in time required to accomplish instrumen-tation may be the consequence of this understanding, it should be empha-sized that this is not the result of quickness or ergonomics. Rather, it is dueto increased control and the ability to anticipate the optimum approach aswell as eliminate the less than optimum, the unnecessary and sometimescounterproductive components of a technique.

Introduction:

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Unfortunately, I attempted to conveyinstructions by teaching conventional step-by-step techniques rather than the under-standing I had acquired and that clinicianscould have easily learned. I thought, and wastold, that dentists only wanted to know howrather than why. The reality, however, is asthe introduction of new products increasesand voices of advocates confuse thosechoices, coupled with the fact that productsbecome obsolete before they can be thor-oughly evaluated, the need for understand-ing the principles of all products, especiallyrotary instrumentation, becomes apparent.As scientific evaluations encompass only asmall portion of the total functionality ofinstrumentation, and as more instrumentsare described only in terms of their uniquefeatures, the need for consolidation ofinformation becomes indispensable for theendodontist who strives to exercise judg-ments and skills beyond those afforded bythe set of instructions as a beginner or thesatisfaction level of the consummate user. Abasic understanding of the scientific princi-ples of instrumentation needs to be the foun-dation of expertise rather than instructionsor recommendations that seem to lead tomultiple and temporary conclusions.

With understanding, there is no need torely on time consuming and costly trial anderror experience. It is easy to forget that itrequires ten years to have ten years of expe-rience. Neither is there a need to rely on theability to decipher conflicting explanationsof noted authorities. With understanding,improvements in the quality of care occurmore quickly and consistently.The need is tomake understanding accessible. The longerwe continue practicing without appreciating

the rudimentary principles and characteris-tics of instrumentation, the greater the gapbetween newer technologies and under-standing becomes and the less we use thefull potential technology has to offer. Thepurpose of this presentation is to provideand consolidate the principles necessary forunderstanding the design of instruments andfor developing the rationale necessary to for-mulate and use present and future instru-ments to their greatest benefit in relation tothe canal anatomy.

As one examines the principles of rotaryinstrumentation, the cookbook type tech-niques that were once beneficial for initiat-ing the use of rotary files in one’s practicebecome overly simplistic. The ability to dif-ferentiate between the attributes and limita-tions of instruments and techniques becomeapparent. Rather than espousing a populartechnique, understanding consigns only theappropriate technique that changes for everyanatomy and case history. It is important notto confuse the characteristics of instrumentswith the techniques with which they havebecome associated. The advantages and dis-advantages of techniques do not necessarilypertain to the instruments used. It is alsoimportant to understand that desired canalshapes can be prepared with virtually anyseries of instruments, but it is the risks andefficiency that varies from one instrumentto another.

With understanding, approaches to differ-ent cases become too diverse to fall withinany particular category other than canalanatomy. Even though the choices of instru-ments and techniques can become morenumerous and complex for cleaning and shap-ing canals, the solutions become less compli-

could not be done with the informationavailable. Most would also agree that there isno one source dedicated to this kind of infor-mation. Currently, accessible information ispractically limited to cookbook type instruc-tions with virtually no rationale of rotaryinstruments and techniques. Instructions fornew rotary files are generally comprised of afew concise technique recommendations,making the assumption that these will beadequate to prevent compromising endodon-tic treatment while attempting to convincethe practitioner to employ yet anotherunique design of instruments. Consequently,

endodontists often needlessly spend most oftheir chair time preparing root canals withthe accompaniment of uncertainty, medioc-rity or an arduous attempt at achieving theideal. Understanding essential informationprovides the means for expertise. The prob-lem is most will never study information, as itbecomes available, to know the difference.Some, however, who are willing to invest thetime required for understanding the dynam-ics of instrumentation will certainly savethousands of hours of chair time, significantlyincrease their income, and have the satisfac-tion that they are providing excellence fortheir patients.

Those who have incorporated rotaryinstrumentation into their practice under-standably looked for a simple system of files

and an easy technique that could be used asa routine. Many were attracted by claims thattechniques, having the fewest instruments,facilitated canal preparation. Most found afunctional comfort level quickly, with theirinitial choices, occasionally added their ownmodifications, and as experience wasacquired, ventured to use other instrumentsand techniques. Most eventually gravitatedtoward a level of satisfaction.

Lacking both the benefit of a teacher andany prescribed step-by-step technique, myinitial design and use of this new modalityfor canal preparation, required the continu-

ous attempt to thoroughly understand thephysics of rotary instrumentation. The“doing” required careful visualization of allthe consequences of actions before theywere performed. Ironically this exerciseproved uniquely advantageous over the reg-imented procedures most have had to fol-low and soon I developed the understand-ing for instrumenting complicatedanatomies with relative ease. The expedi-tious accomplishment of instrumentationcaused inexplicable guilt, but challengedmy ingrained notion so common to den-tists, that speed was a compromise. Thatnotion was replaced with the realizationthat expertise and efficiency are synony-mous and both are relative to the level of understanding.

John T. McSpadden, D.D.S

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Unaware of the benefits of the extraordinary, the ordinary benefits limitedthe development of the practice and the practitioner continued to operatewith the needless threat of failure and/or the unnecessary consumption oftime.This book is for those who want to progress beyond that point.

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vide significant predictability to be used as aguide for formulating techniques.You will notethat it is only after a thorough examination ofexisting instrumentation concepts, in the con-text of one of the most extensive researchprojects ever undertaken, that any parametersare recommended for using instruments cur-rently available and for designing prototypeinstruments for the future. Following those orany parameters should be consistent withyour understanding.Any inconsistency maymean either a lack of understanding, or

hopefully, and more importantly, may meanthat you have contributed in formulatingan advanced concept for a new instrumentor technique.

The hope for those reading this book isthat they will use the information presentedto visualize the actions of existing and futureendodontic files and be able to coordinatetheir characteristics with canal anatomies.The aspiration is to help in attaining expert-ise. The consequence would be advancingthe field of endodontics.

cated and expedient. As one broadens thescope of understanding, skill is enhanced in ascientific manner and success becomes morepredictable.The art of endodontics becomesthe science of endodontics and expertisebecomes the nature of the operator.

Since I receive royalties from several ofthe instruments discussed in this book, I amextremely sensitive to the fact that somemight view any evaluations under my direc-tion to be skewed by commercial interests. Ican only say that the motivation that prompt-ed me to seek ways to improve the quality oftreatment, ways that ultimately developedvirtually all innovations in our profession, isthe same motivation that has led me toadvance concepts for understanding. Eventhough every effort has been made to makeany findings of testing be the result of follow-ing solid scientific protocol that can be easi-ly duplicated, and all testing has been con-ducted by only using mechanical devicesthat operate independent of operator vari-ables or subjectivity, this book does not pre-tend to be an authoritative treatise to vali-date or invalidate the claims for instrumentsor techniques. Rather, the results of testingare presented as tools to promote under-standing, investigation and development. Asunderstanding is developed, any commercialinfluence of these or any other testingresults should become apparent regardlessof the source.

While reading this book, you may noticethat numerous popular recommendationsfor using rotary instrumentation will bechallenged and exposed as intuitive con-cepts. One primary purpose of this book isto instill a sense of curiosity for the reasonsof any concept. In fact, this book is a result

of other people’s curiosity and is organizedby asking questions that have at one time oranother have been asked of me. Theanswers are transcripts of those communi-cations or excerpts from lectures and are ina conversational mode for that reason.Addressing these questions is an exercise indetermining which procedures enable thedentist to operate with scientific pre-dictability for success.You may be interest-ed to know that some of the following pop-ular concepts are more intuitive, but arecounter to scientific evidence:

1. Specific speeds of rotation should notbe exceeded.

2. Complicated curvatures require slow-er speeds.

3. Use one continuous motion of fileinsertion until resistance is met.

4. Routinely follow the use of an instru-ment with another instrument havingthe same taper with a smaller tip size.

5. Routinely establish straight-lineaccess.

6. Routinely carry a .04 or even a .06taper file to working length.

7. A crown-down approach is alwayspreferable to a step-back approach.

8. One millimeter of file advancementinto the canal only results in one mil-limeter of additional engagement.

These and other concepts are often fol-lowed without question.The best use of thisbook is to use the questions as frameworks forexamination. Although research on endodon-tic instruments cannot result in absolutes,understanding the results of research will pro-

John T. McSpadden, D.D.S

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In 1977, Dr. McSpadden was exercising innovation. He was using mechanical instrumentation, the Dynatrac system,and mechanical obturation, the McSpadden Compactor System, both of which he invented. At that early date hewas routinely using the microscope for all practice procedures.

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With the introduction of nickel titanium,mechanical root canal preparation has quick-ly become a widely accepted modality inendodontics. The enhanced preparationresults and reduced preparation time ofrotary nickel titanium files have promptedthe rapid adoption of rotary instrumentation.Yet, in spite of added advantages and excel-lent canal cleaning and shaping ability, a lackof information has caused the formulation oftechniques that limited the comprehensivebenefits of rotary instrumentation. Eventhough instrumented canals may result inideal appearances, information for accom-plishing ideal instrumentation has not keptpace with the enhanced opportunities forefficiency,expertise,or the reduction of risks.

Particular canal shapes are often illustratedas being characteristic for certain file brands,

however, canal shapes are more dependenton the file dimensions, the sequence the filesare used and the depths to which they arecarried into the canal. Although a desiredcanal shape can be achieved with virtually allbrands of rotary nickel titanium files, varioustechniques have been proposed to achievethis shape. Too often the designs of thesetechniques are determined by marketingwhere product promotion prevails over sci-ence. Consequently, the practitioner oftenexperiences complications while conscien-tiously following instructions that disregardthe complexities of anatomy.

UUnnddeerrssttaannddiinngg tthhee rraammiiffiiccaattiioonnss ooff ffiilleeaanndd tteecchhnniiqquuee ddeessiiggnn rreellaattiivvee ttoo ccaannaallaannaattoommyy eennaabblleess tthhee ddeennttiisstt ttoo ccoonnssiisstteennttllyyaacchhiieevvee tthhee mmoosstt eexxppeeddiittiioouuss aanndd eexxcceelllleennttttrreeaattmmeenntt wwiitthh tthhee lleeaasstt rriisskkss.. This is not a

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The next level of development of rotary instruments is continuously beingintroduced. Design concepts for various new instruments are importantdepartures from previous designs. To fully comprehend the significance ofdesign principles for advanced developments, it is necessary to determinethe considerations by which all rotary endodontic files should be used andevaluated, then assess any new development in that context. This presen-tation offers those considerations and reviews the evolution of rotaryinstruments (assessing their advantages and limitations).Within this par-adigm will be an understanding of design concepts that enables the prac-titioner to maximize endodontic skills for any technique available todayand to most effectively use and evaluate advancements as they becomeavailable in the future. Armed with this knowledge, the practitioner gainsindependence from advocacy claims and the need for trial and error expe-rience. More importantly, a more rational approach will be offered in pro-viding expertise for treating their patients.

Mastering Instrumentation

Section I: Mastering the Concepts

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duced at the beginning of the 20th century.The first manual and mechanical rotary

files were formed from straight piano wirethat had flats ground on its sides and twist-ed to result in the configuration of files stillused today. Files were first mass-producedby Kerr Manufacturing Co. in the very early1900’s,hence the name K-type file or K-typereamer.Although the term file is commonlyused generically to describe all ground ortwisted endodontic instruments, morespecifically the term file is used to describean instrument used primarily during inser-tion and withdrawal motions for enlargingthe root canal, whereas a reamer is used pri-marily during rotation. K-type files andreamers were both originally manufacturedby the same process.Three or four equilat-eral flat surfaces were ground at increasingdepths on the sides of wire to form a

tapered pyramidal shape that was stabilizedon one end and rotated on its distal end toform the spiraled instrument. The numberof sides and spirals determined if the instru-ment was best suited for filing or reaming.Generally, a three-sided configuration, withfewer spirals, was used for reaming or rota-tion; a three- or four-sided configurationwith more spirals was used for filing orinsertion and withdrawing. Even though thetwisting method of file manufacturing hasgenerally been considered an outdatedmeans of fabricating files and has beenreplaced by computerized grindingprocesses for NiTi rotary files, newadvances for manipulating shape memoryalloys may offer economic and physicalproperty advantages for reconsidering thetwisting method of manufacturing for thefuture.

new concept. Frank Weine described as earlyas 1975 in the Journal of Endodontics(Weine, F. S.The effect of preparation proce-dures on original canal shape and on fora-men shape. Journal of Endodontics 1:8August 1975.), a techniquefor modifying files in orderto prevent transportingcurved canals. He advocat-ed using a diamond-surfacedfingernail file to remove theblades on one side of anendodontic file that wouldreciprocate against the outercanal wall between a cur-vature and apex to avoidzipping the canal, a designknown today as the safe-sided file.

Often, techniques aredesigned to avoid a failurethat has been experiencedin one particular procedure,even though the applicationcould be beneficial in othercircumstances.For example,we are often instructed bysome advocates never torotate a file more than 350rpm, yet in many circum-stances 1200 rpm can bemore than four times aseffective with less threat ofcomplications, and slowing the rotations canactually increase the threat. Consequently,without having the information needed tounderstand how to utilize the advantageswhile limiting the threat of failure, the practi-tioner frequently places limits on rotaryinstrumentation prematurely before expertise

and its most significant benefits are ever real-ized. The science for integrating anatomicalcanal complexities with instrumentation effi-ciency and effectiveness is the most oftenignored technique consideration.Wasted time

and needless difficulties aremost often the consequences.

By and large, basic rudi-mentary physics of root canalinstrumentation has been anelusive subject during the lastcentury, denying even theendodontist the understand-ing necessary to fully attaintheir potential expertise in

performing the task that often requires themajor portion of their time: root canal prepa-ration. Rotary instrumentation is certainlynot a new concept; it was introduced in thelate 19th century, as were the rubber dam,rubber dam clamps, and even solid core car-riers for gutta percha which were intro-

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BB

Fig. 1 Recommended tech-niques frequently are appro-priate in simple anatomies(A.), however, more com-plex anatomies (B.) requiregreater consideration for fileand technique design. (cour-tesy of P. Brown, PortolaValley, CA. and E. Herbranson,San Leandro, CA)

Fig. 2 A. A tapered pyramidal wire is used as a blank for forming a file.B. Each end of the blank is stabilized and one end is rotated to twist a spiraled shape on the file’s working surface.C. Multiple rotations result in the familiar spiraled shape of the endodontic file.

AA

AA

BB

CC

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2. Why Nickel-Titanium?

Manual stainless steel files provide excellentmanipulation control and sharp, long-lastingcutting surfaces. However, due to the inher-ent limited flexibility of stainless steel,prepa-ration of curved canals is often a problem formanual files, and the mechanical use withconventional designs and grades of stainlesssteel poses the likely threat of file breakageor canal transportation.

The significant advantage of a file madeof a nickel titanium alloy is its unique abilityto negotiate curvatures during continuousrotation without undergoing the permanentplastic deformation or failure that traditionalstainless steel files would incur. The firstseries of comparative tests demonstratingthe potential advantages of endodontic filesmade of nickel titanium over stainless steelwere conducted by Drs. Walia, Gerstein andBryant. The results of the tests were pub-lished in an article entitled “An InitialInvestigation of the Bending and theTorsional Properties of Nitinol Root CanalFiles,” (Journal of Endodontics, Volume 14,No.7, July 1988, at pages 346-351). In 1991,the first commercial nickel titanium manualand rotary files were introduced by NT Co.In1994, NT Co. also introduced the first seriesof nickel titanium rotary files having multiplenon-conventional tapers: the McXIM Series,which had six graduating tapers rangingfrom the conventional 0.02 taper to a 0.05taper file in order to reduce stress by limitingthe file’s engagement during the serialenlargement of rotary instrumentation.Based upon the initial success and recog-nized advantages, the use of nickel titaniumrotary files has proliferated and become

widely accepted by the profession.Nickel titanium is termed an exotic metal

because it does not conform to the normalrules of metallurgy. As a super-elastic metal,the application of stress does not result in theusual proportional strain other metals under-go. When stress is initially applied to nickeltitanium the result is proportional strain.However, the strain remains essentially thesame as the application of additional stressreaches a specific level forming what istermed llooaaddiinngg ppllaatteeaauu: during which thestrain remains essentially constant as the stressis applied. Eventually, of course, excessivestress causes the file to fail.

This unusual property of changing froman anticipated response to an unanticipatedresponse is the result of undergoing a molec-ular crystalline phase transformation. NiTican have three different forms: martensite,stress-induced martensite (superelastic), andaustenite.When the material is in its marten-site form, it is relatively soft and can be easilydeformed. Superelastic NiTi is highly elastic,while austenite NiTi is non-elastic and hard.External stresses transform the austenitic crys-talline form of nickel titanium into the stress-induced martensitic crystalline structure thatcan accommodate greater stress withoutincreasing the strain. Due to its unique crys-talline structure, a nickel titanium file hasshape memory or the ability to return to itsoriginal shape after being deformed. Simplyrestated, nickel titanium alloys were the first,and are currently the only readily availableeconomically feasible materials that have theflexibility and toughness necessary for rou-tine use as effective rotary endodontic files incurved canals. Other alternative materials arebeing investigated for the same purpose.

Dating from the late 19th century, theearliest endodontic instruments used forextirpating the pulp and enlarging the canalwere broaches or rasps.Still used today, theseinstruments are manufactured by hacking around tapered wire with a blade device toform sharp barbs that project out from itsside to form cutting or snagging surfaces.Although mostly used to engage and remove

soft tissue from the canal as manual instru-ments, these historic broach type instru-ments have the potential for becoming effec-tive rotary instruments. The evolutionarydevelopment for endodontic instrumentsseems to have some cyclic peculiarities andis far from over. Even the tapered pyramidaldesign originally used as blanks as describedabove is now being used as rotary NiTi files.

John T. McSpadden, D.D.S

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Fig. 4

The broach is formed by forcing a blade onto the surface of a tapered wire.

Fig. 3

A NiTi pyramidal wire can be twisted using a proprietary process.Prototype by Sybron Dental Specialties

1. What are the terms I need to know when comparing the physicalproperties of files?

The success of using instruments while preventing failure depends on how the material,designand technique relate to the forces exerted on the instruments.To fully understand how the filereacts to applied forces, terms have been defined to quantify the actions and reactions to theseforces.Common terms related to forces exerted on files have the following definitions:

1. Stress—The deforming force measured across a given area.

2. Stress concentration point—An abrupt change in the geometric shape of a file,such as a notch, will result in a higher stress at that point than along the sur-face of the file where the shape is more continuous.

3. Strain—The amount of deformation a file undergoes.

4. Elastic limit—A set quantity which represents the maximal strain, which, whenapplied to a file, allows the file to return to its original dimensions. The resid-ual internal forces after strain are removed and return to zero.

5. Elastic deformation—The reversible deformation that does not exceed the elastic limit.

6. Shape memory—The elastic limit is substantially higher than is typical of con-ventional metals.

7. Plastic deformation—Permanent bond displacement caused by exceeding the elas-

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3. Are nickel titanium files alwaysadvantageous over files ofstainless steel during rotaryinstrumentation?

The function and physical property require-ments of endodontic files are extremelyimportant and need to be matched to manu-facturing methods. Metallurgy of the specificmaterial should be understood to achieveoptimum properties for the application.

Stainless steels are a good case in point. Over100 alloys, of which many have only recentlybeen introduced,are included under the ban-ner of stainless steel. Endodontics, unfortu-nately, has been lacking in its investigation ofalloy selection and when we compare NiTifiles with stainless steel files, we do so with-in the narrow framework of older stainlesssteel alloys that have been used for files.Comparisons between the two metals maychange significantly in the future.

If all canals were straight, conventionalstainless steel files would have results asgood as, or better, than nickel titanium.Workhardened stainless steel files have more tor-sion strength and are able to maintain sharpedges longer. Of course, few canals areentirely straight and rarely can degree,radius, and direction of curvature be deter-mined prior to treatment.The minor curva-tures of most canal anatomies can causeexcessive stresses on conventional stainlesssteel files and result in unwanted canaltransportation or file failure. Nevertheless,the introduction of nickel titanium filesseemed to ignore the fact that nickel titani-

um offers no advantage for files having largediameters and tapers that lack any apprecia-ble flexibility.Accordingly, these instrumentshave become an unnecessary expense, onlybecause these larger files were a part of aseries of instruments.The advantage of stain-less steel rotary files of larger diameters andtapers to compliment the use of nickel tita-nium files is now recognized by many thathave become familiar with the attributesand limitations of nickel titanium. Stainlesssteel rotary files are being introduced foruse in lieu of nickel titanium files in largersizes and tapers that lack the flexibility.More advanced design developments thatreduce file stresses and modifications in themolecular structure of stainless steel arecontinuously causing reconsideration ofstainless steel as a viable NiTi alternative.

4. Are there other alloys that offeradvantages as rotary files?

Other alloys have been developed that aresuitable for rotary files and might have prop-erties that are advantageous over those ofnickel titanium. One problem is economics.

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J. McSpadden

V. Malagnino J. McSpadden J. McSpadden

Fig. 6 Although stainless steel files could negotiate abrupt apical curvatures (left image), canal transportation could easilyoccur due to the file’s lack of flexibility. Note the transportation that occurred in the apical curvature that occurred duringinstrumentation with stainless steel files. “Zipping” the apical foramen can be a consequence of file inflexibility.

Fig. 7 Early cases for using nickel titanium rotary files demonstrated the ability of preparing canal curvatures while maintain-ing the central axis of the canals.

Property NiTi Stainless Steel

Recovered Elongation 8% 0.8%

Biocompatibility Excellent Fair

Effective Modulus approx. 48 GigaPascal 193 GigaPascal

Torqueability Excellent Poor

Density 6.45 g/cm3 8.03 g/cm3

Magnetic No Yes

Ultimate Tensile Strength approx. 1,240 MegaPascal approx. 760 MegaPascal

Coefficient Thermal Expansion 6.6 to 11.0 x 10-6 cm/cm/deg.C 17 .3 x 10-6 cm/cm/deg.C

Resistivity 80 to 100 micro-ohm*cm 72 micro-ohm*cm

Comparison of Properties of NiTi and Conventional Stainless Steel

Table 5 (Breme HJ & Biehl V (1998) Metallic biomaterials. In: Black J & Hastings G (eds) Handbook of biomaterial properties,Chapman& Hall, London, p 135-213.)

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5. Why Rotary Instrumentation?

One benefit of mechanical rotation is theenhanced ability to collect and removedebris from the canal system. Hand instru-mentation can push debris laterally into theintricacies of the canal anatomy or even api-cally through the canal foramen when usingtechniques that commonly include inser-tions of files without rotation or rotations offiles in a counter-clockwise direction. In con-trast, continuous clockwise rotation will con-vey debris only in a coronal direction fromthe canal ramifications and apical foramen.

Mechanical rotation provides a more con-stant 360-degree engagement of the file tipin the canal that forces it to follow the canaland results in better control for maintainingthe central axis of the canal, reducing theincidence of ledging or perforating.The flex-ibility for following the canal allows us to bemore conservative in preserving tooth struc-ture while effectively cleaning and shapingthe canal.The most obvious benefit for con-tinuous rotation is the reduction in the timerequired for instrumenting the canal.The factthat a file, constantly rotating from 200 to2,000 rpm, produces results more rapidlythan hand instrumentation that has signifi-cantly slower and intermittent rotations,should come as no surprise.

6. Why do we need to know any-thing about instrument design?

Although radiographs portraying desiredcanal shapes are often used to illustrate thecapabilities of a particular type of file, thedesired canal shape can be attained with vir-tually any set of files provided they are usedproperly. How efficiently the shape can be

attained is another matter. The capabilitiesof files made of the same material are entire-ly dependent on design and can mean suc-cess or failure. No one aspect of file design isindicative of the file’s overall usefulness.Optimizing one design feature can compro-mise another benefit. Considerations fordesign effectiveness include the following:cutting ability, operational fatigue, stress con-centration points, operational torque, torqueat breakage, flexibility, screwing-in forces,ability to maintain the central axis of the canal,and tip mechanics. Successes of file designand, to a considerable extent, clinical successare determined by how efficiently these con-siderations address various canal anatomies.

Limitations of the initial nickel titaniumfile designs were largely due to an attempt toadapt the easily manufactured old hand filedesigns and technique concepts to thesenew rotary instruments. These old designsapplied to a new modality comprised thefirst generation of NiTi rotary instrumenta-tion. A second generation of designs, nowparticularly patterned for rotary instrumenta-tion, is being introduced that can substantial-ly advance treatment results. IInn uussiinngg aannyy ffiilleeddeessiiggnn,, uunnddeerrssttaannddiinngg tthhee rruuddiimmeennttaarryypphhyyssiiccss iinnvvoollvveedd iinn iittss uussee iiss iimmppeerraattiivvee ffoorrtthhee pprraaccttiittiioonneerr ttoo ttaakkee ffuullll aaddvvaannttaaggee ooff iittssbbeenneeffiittss.. RReeccooggnniittiioonn ooff iinnssttrruummeenntt ffeeaattuurreesstthhaatt iimmpprroovvee uusseeffuullnneessss oorr ppoossee ppoossssiibblleerriisskkss mmuusstt aallssoo bbee aacchhiieevveedd. This need isespecially important in employing a newfile design. Regardless of the design andtechnique, there are certain considerationsthat provide the understanding for usingrotary instrumentation to its fullest advan-tage. The practitioner must remember thatalthough any new introduction of rotary files

In order to be feasible, any other alloy usual-ly must have applications in addition torotary files that can help offset the cost ofproduction. Otherwise the costs can be pro-hibitive.Another problem is ignorance. Newmaterials and methods for altering the char-acteristics of existing materials are develop-ing at such a rapid pace that our awarenesssimply does not keep up.

One alloy having considerable potentialand economic feasibility is a nickel titaniumniobium alloy having a substantially higherloading plateau, making it tougher thaneither stainless steel or nickel titanium. It hassharper, more durable cutting edges, and can

be more resistance to breakage. Somewhatstiffer than the conventional NiTi alloys, butmore flexible than stainless steel, it is partic-ularly advantageous for rotary activation ofsmaller files. The flexibility is sufficient tonegotiate acute curvatures with minimumcanal transportation,yet stiff enough to with-stand the pressure desirable to feed it intosmall canals.

Other titanium alloys contain molybde-num and zirconium to increase stability,workability, or corrosion resistance. Onlytime will tell if the economic feasibility ofthese and other alloys will eventually pro-vide a better endodontic rotary file.

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Fig. 8 The FKG stainless steel rotary files are examples of rotary files available in sizes that NiTi files would lack any apprecia-ble flexibility or advantage.

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The taper is usually expressed as the amountthe file diameter increases each millimeteralong its working surface from the tip towardthe file handle. For example, a size 25 filewith a .02 taper would have a .27 mm diam-eter 1 mm from the tip, a .29 mm diameter 2mm from the tip, and a .31 mm diameter

3 mm from the tip. Some manufacturersexpress the taper in terms of percentage inwhich case the .02 taper becomes a 2%taper. Historically, as an ISO standard, a filewas fluted and tapered at 2% for 16 mm, butnow files incorporate a wide variation oflengths and tapers of working surface.

can represent significant improvements,some designs without merit will continue tobe introduced for marketing purposes andadvocated by clinicians who lack the com-plete comprehension for the ramifications ofuse. Any significant treatment advancementwill ultimately be predicated on each individ-ual practitioner’s understanding of designfunction. TThhee uullttiimmaattee ggooaall ffoorr aannyyoonnee uussiinnggrroottaarryy iinnssttrruummeennttaattiioonn iiss nnoott oonnllyy ttoo bbee aabblleettoo rreeccooggnniizzee tthhaatt ppiivvoottaall iinnssttaanntt jjuusstt bbeeffoorreeccoommpplliiccaattiioonnss ooccccuurr,, bbuutt ttoo rreeccooggnniizzee tthheemmoosstt aapppprroopprriiaattee aapppprrooaacchh ffoorr aacchhiieevviinnggssoolluuttiioonnss.. TThhaatt ggooaall ccaann oonnllyy bbee aaccccoomm--pplliisshheedd bbyy tthhoorroouugghhllyy uunnddeerrssttaannddiinngg tthheeffuunnccttiioonn ooff ddeessiiggnn..

7. Is an appropriate techniqueimportant?

CCaannaall aannaattoommyy,, ffiillee ddeessiiggnn aanndd ffiillee ddiimmeennssiioonnssddiiccttaattee tthhee aapppprroopprriiaattee uussee ooff aann iinnssttrruummeenntt..

Often techniques for particular files are theresult of subjective concepts recommendedfor the sake of simplicity.The capabilities ofthe files then become confused with thecapabilities of the inappropriately recom-mended technique with which they havebecome associated. HHooww wweellll aa ffiillee ppeerr--ffoorrmmss,, wwhhiillee ffoolllloowwiinngg aa ssppeecciiffiicc tteecchhnniiqquuee,,sshhoouulldd nnoott bbee tthhee mmeeaassuurree ooff tthhee eeffffeeccttiivvee--nneessss ooff aa ffiillee;; rraatthheerr,, hhooww wweellll tthhee ccaappaabbiillii--ttiieess ooff aa ffiillee ccaann aaddddrreessss tthhee rreeqquuiirreemmeennttss oofftthhee ccaannaall aannaattoommyy sshhoouulldd bbee tthhee mmeeaassuurreeooff iittss uusseeffuullnneessss.. Since canal anatomiesvary, techniques to effectively clean andenlarge the canal may include modifica-tions and may include different type instru-ments. Instrumentations involving morethan one type instrument or technique areknown as hybrid techniques.

8. What are the components of a file?

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Helix Angle(may change along working surface)

Taper (diameter increase/mm) Notch (curve orientation)

Measuring lines

Measuring stop

Taper

Fig. 9

Profile GT (13 mm)

K-3 Access handles can provide greater working and preventundue stress caused file distortion outside the canal.

Quantec (11.5 mm)

Fig. 11 MicroMega has used the uniqueapproach of combining the pinion gearof the hand-piece and the handle of thefile to increase access. The result is a han-dle that is 7.25 mm in length or less thanone half the length of a standard handle.

7.5mm

15.5mm

Stops may indicate size or taper

Numbers and Colored bandsindicate size and taper

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Standardized dimensions played an impor-tant role at the time they were instituted forproviding the needed consistency for handinstruments, but were soon seen as limita-tions for rotary instrumentation.As differentdimensions of rotary files are introduced, thecomplexities of identification cause confu-sion. Hopefully, the common components ofrotary files can eventually have standardizedidentifications for easier recognition.

The fflluuttee of the file is the groove in theworking surface used to collect soft tissueand dentine chips removed from the wall ofthe canal. (Figs. 12, 13 and 14) The effective-ness of the flute depends on its depth,width,configuration and surface finish.The surfacehaving the greatest diameter that follows thegroove (defined as where the flute and landintersect), as it rotates, forms the leading(cutting) edge, (Figs. 12, 13 and 14) or theblade of the file that forms and deflects chipsfrom the wall of the canal and severs or

snags soft tissue. Its effectiveness depends onits angle of incidence and sharpness. If thereis a surface that projects axially from the cen-tral axis as far as the cutting edge, betweenflutes, this surface is called the land (Figs. 13and 14) (sometimes called the marginalwidth).The land reduces the screwing-in ten-dency of the file, reduces transportation ofthe canal, decreases the propagation ofmicro-cracks on its circumference, gives sup-port to the cutting edge, and limits the depthof cut. Its position relative to the opposingcutting edge and its width determine itseffectiveness. In order to alleviate frictionalresistance or abrasion resulting from a land,some of the surface area of the land thatrotates against the canal wall may bereduced to form the relief (Fig.14).The anglethat the cutting edge makes with the longaxis of the file is called the helix angle (Figs.12,13,and 14) and serves to auger debris col-lected in the flute from the canal.

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Helix angle (angle of blade with long axis)

Cutting edgeFig. 12 ProTaper File Flute (extends from cutting edge to cutting edge)

Fig. 13 Profile

Fig. 14 Quantec file Cutting edge

Cutting Edge

Helix angle

LandFlute (separated by lands)Land

Flute Land Relief

Helix angle

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10. What is the differencebetween the rake angle andcutting angle?

If the file is sectioned perpendicular to itslong axis, the rraakkee aannggllee (Fig. 17-29) is theangle formed by the leading edge and theradius of the file. If the angle formed by theleading edge and the surface to be cut (its tan-gent) is obtuse,the rake angle is said to be ppooss--iittiivvee oorr ccuuttttiinngg.If the angle formed by the lead-ing edge and the surface to be cut is acute, therake angle is said to be nneeggaattiivvee oorr ssccrraappiinngg.However, the rake angle may not be the sameas the ccuuttttiinngg aannggllee (Figs. 17-29).The cuttingangle,eeffffeeccttiivvee rraakkee aannggllee,is a better indicationof the cutting ability of a file and is obtained bymeasuring the angle formed by the cutting

(leading) edge and the radius when the file issectioned perpendicular to the cutting edge.In some instances,as with some Quantec files,a file may have a blade with a negative rakeangle and a positive cutting angle. If the flutesof the file are symmetrical, the rake angle andcutting angle will be essentially the same.Onlywhen the flutes are asymmetrical are the cut-ting angle and rake angle different.Both anglesmay change as the file diameters change andmay be different for file sizes.

The ppiittcchh of the file is the distancebetween a point on the leading edge andthe corresponding point on the adjacentleading edge along the working surface, orit may be the distance between points with-in which the pattern is not repeated. Thesmaller the pitch or the shorter the distancebetween corresponding points, the morespirals the file will have and the greater thehelix angle will be. Most files have a vari-able pitch, one that changes along theworking surface, because the diameterincreases from the file tip towards the han-dle and the flute becomes proportionatelydeeper resulting in a the core taper that isdifferent from the external taper. Some

9. What is the core of a file?

The ccoorree (Fig. 15) is the cylindrical centerpart of the file having its circumference out-lined and bordered by the depth of theflutes. The flexibility and resistance to tor-sion is partially determined by the core diam-eter.The core taper and total external tapercan be different and the relative diameter of

the core, compared to the file’s total diame-ter, may vary along its working portion inorder to change the flexibility and resistanceto torsion. The importance of the ratio ofcore diameter to total diameter is often over-looked in predicting a file’s susceptibility tofailure and can be different for each file sizeof the same series.

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Fig.15 The central core circum-ference shown in cross-section ofthe K-3 file is determined by theboundaries of the depths of theflutes or is described as thelargest diameter of the cross-sec-tion that has not been ground.The core taper may be less thanthe file taper in order to propor-tionately increase the file’s flexi-bility toward the handle. A .04tapered file can have a .02tapered core, and the file wouldhave proportionally less cross-sectional mass toward the han-dle and greater flexibility towardthe handle than if the core taperand file taper were the same.

Fig. 16 Although the two files above have the same basic design and are of the same series, the ratio of the depth of the fluteto the external diameter differs significantly. The depth of flute of the small instrument is approximately the same as for thelarger instrument resulting in excess susceptibility to failure, whereas the larger instrument has adequate flexibility and ade-quate resistance to torsion failure.

Direction and Action of the Leading Edge(angle of incidence)

Negative angles result in a “scraping” action. Positive anglesresult in a cutting action. Although cutting actions can bemore efficient and require less force for enlarging a canal,a scraping action may have a smoother feel. The operatormay erroneously confuse smoothness with efficiency.However, if excessive pressure is applied to a cutting file,a larger chip may require more force to dislodge andexcessive torsion could be the result. (Arrows indicate thedirection of the blade motion).

Positive Cutting Angle (obtuse angle)

Fig. 17 Negative Cutting Angle (acute angle)

Size 25

Size 15

Both files have flutes witht the same depth.

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instruments,such as the Quantec and K-3 files,have asymmetrical cross-sectional designsin which case the pitch may be consideredto be the distance between points that thepattern is not repeated.

The cutting angles, helix angles, externaland core taper may vary along the workingsurface of the file and the ratios of these

quantities can vary between instruments ofthe same series.Any change of any of thesefeatures can influence the file’s effectivenessor its propensity for breakage as it progress-es into the canal space and can account forsome files to act uncharacteristically whencompared to files that have different dimen-sions in the same series.

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Fig. 18

rotation

cutting angle

debris

Fig. 19

cutting angle

debris

The ProTaper file utilizes a nega-tive angle of incidence to enlargethe canal. The surface of the fileblade meets the canal wall withan acute angle resulting in ascraping action. More pressure isrequired when enlarging thecanal in this manner

The K-3 file utilizes a slightly posi-tive angle of incidence to enlargethe canal. The file blade meets thecanal wall with an obtuse angleresulting in a cutting action. Lesspressure is usually required whenenlarging the canal in this man-ner. Excessive pressure can causeexcessive torsion by forming chipstoo large to be dislodged.

Section perpendicular to long axisdetermines rake angle

Section perpendicular to cutting edge determines cutting angle (effective rake angle)

Fig. 20

Fig. 21

Quantec File. Cutting edge

The cutting angle, effective rake angle, is a better indication for determining the cutting ability of a file than the rake angle,because it shows the actual angle of incidence.

Profile sectioned perpendicular to its cutting edge

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Fig.22 The Profile is sectioned perpendicular to its longaxis (left image) to illustrate the rake angle (red lineangle) of the leading edge in relation to the plane of thetooth surface to be prepared. When sectioned perpendi-cular to its leading edge (right image) the relationship ofthe cutting angle (red line angle) and the tooth surface tobe prepared have the same relationship as in the perpen-dicular to the long axis section. The rake angle and cut-ting angle are the same because the flutes are symmetri-cal on the Profile.

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Fig.24 The ProTaper file rake angles (red line angleof left image) and cutting angles (red line angle ofright image) have the same relationships to the sur-face to be prepared.

Fig. 25 The RaCe file’s rake angle (red line angle of leftimage) and cutting angle (red line angle of right image)have the same relationship to the surface to be prepared(red lines).

Fig. 23 The ProFile GT rake angle (red line angle of leftimage) and its cutting angle (red line angle of rightimage) have the same relationship to the surface to beprepared. The flutes are symmetrical on the Profile GT,and the rake angle and the cutting angles are the same.

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Fig. 26 The Hero file, with asymmetrical flutes, has a rakeangle (red line angle of left image ) that is different fromits cutting angle (red line angle of right image). The cuttingangle is less negative than the rake angle and a better indi-cation of its cutting ability. Fig. 28 The Quantec file, which has asymmetrical flutes,

has a negative rake angle (red line angle of left image) anda positive cutting angle (red line angle of right image).

Fig. 29 The K3 file can have a positive rake angle (red lineangle of left image) depending on the diameter sectionedbut has a definite positive cutting angle( red line angle ofright image).

Fig. 27 The M2 file has asymmetrical flutes that result ina difference in the rake angle (blue arrow of left image)and the cutting angle (blue arrow of right image). Therake angle is negative and the cutting angle is less nega-tive and can be slightly positive.

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12. Do cutting angles changealong the working surface ofa file?

If the flute design of a file has no radius (theflute is a flat surface) when viewed in itscross-section, the cutting angle will remainthe same along its working surface from D1to D max.Without a radius, the depth of theflute in its cross-section will be outlined as astraight line.The only files having that designare the K-type file and reamer, the RaCefile, the Sequence file, and the Liberator file.Although there are exceptions, as is the Herofile, any file that has a cross-sectional designwith an asymmetrical radius may likely havea cutting angle that changes along its working

surface.The flutes of these files usually occu-py proportionately less of the cross-sectionalarea at their tips than at their largest diame-ters for two reasons. One reason is the inten-tional design to provide a more rigid tip andthe other reason is the limitation of manufac-turing capabilities. Consequently, the cuttingangles near the tips would be less positivethan at their larger diameters and the tips ofthese files have comparatively less flexibilitybut more resistance to torsion stress. If oneattempts to mentally determine the cuttingaction of a file by viewing its cross-section, itis important to keep in mind that the cross-section design may change along the workingsurface and may be substantially different fromthe manufacturer’s representations.

11. Why are the rake angles andcutting angles the same onsome files and not on others?

All nickel-titanium files begin as round wires.When files are manufactured with convention-al grinding processes, the wire transverses agrinding wheel to form a flute (groove) in theside of the wire. If the wire is rotated, as it isfed across the grinding wheel, a spiraled fluteis formed having a helix angle (the angle of theflute with the long axis of the file), and theshape of the flute is formed by the shape andangulations of the grinding wheel. Positiverraakkee angles are difficult to accomplish due tothe size of the grinding wheel relative to thefile diameter. However, by adjusting angula-tions of the grinding wheel, positive ccuuttttiinngg

angles are more easily accomplished.Of all thecurrent spiraled instruments, positive rakeangles, of at least one blade, exist only on thelarger diameters of K3 files.However, it is con-ceivable that other H-type (Hedstrom) instru-ments could incorporate positive rake angles.

Files that have ssyymmmmeettrriiccaall flutes will nothave positive rake or cutting angles and bothof these angles will be essentially the same.Any positive cutting angle is the result of theflute having a smaller radius ((aassyymmmmeettrriiccaall))adjacent to its cutting edge as compared tothe radius of the remaining portion of theflute. By varying the depth and/or asymme-try of the flute, the cutting edge of the filecan be adjusted to become more or less pos-itive along its working length, in order toenhance its effectiveness.

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Fig. 30

Grinding angle for asymmetri-cal flute (sectioned perpendicu-lar to the cutting blade)

The grinding wheel during fabrica-tion of the K-3 file forms a positivecutting angle by grinding a smallerradius on one side of the flute(asymmetrical).

Fig. 31Although the ProFile and ProFile GT series of files are usually portrayed as having a U-file having a flute with a radius withneutral rake angles, diameters smaller than .6 mm have flutes that are essentially straight in cross section. The radius of theflute is limited by the radius of the grinding wheel during the manufacturing process

Profile GT sectioned at .90 mm diameterProfile GT sectioned at .35 mm. diameter

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to enlarge a canal with an efficient file. Onthe other hand, the less efficient file requiresmore time that results in more rotations andgreater fatigue,and/or more force that resultsin greater torsion.The additional fatigue andtorsion, of course, increase the possibilitiesof breakage.

One should also keep in mind that a filecannot transport unless it was at first whereit should be, and only the excessive time itremains in that position results in transporta-tion. OOnnccee aa ffiillee hhaass rroottaatteedd oonnee ttiimmee iinn oonneeppoossiittiioonn,, tthhee ccaannaall wwiillll bbee eennllaarrggeedd ttoo tthheeffiillee’’ss ddiiaammeetteerr aanndd ttoo aavvooiidd ttrraannssppoorrttaattiioonntthhee ffiillee sshhoouulldd nnoott rreemmaaiinn iinn tthhaatt ppoossiittiioonnoonnccee tthhee ccaannaall iiss eennllaarrggeedd.. Even very minordifferences in file design dimensions canaffect the cutting efficiency of files and theirpropensity for transporting canals.

14.What are the functions of lands?

Lands are the surfaces of files that extend asfar axially from the center as the cutting edgesthat define the file’s circumference.Lands areused to reduce screwing-in forces, support

the cutting edge, reduce transportation, andlimit the depth of cut in much the same man-ner that a safety razor functions.The surfaceof a land reduces the tendency of faults causedby stress or manufacturing imperfections inthe metal to propagate along its cutting edgeor circumference. Lands need not be verywide to function.

The force of abrasion is a direct result ofthe surface area of a land that rotates againstthe wall of the canal.Wide lands can result inexcessive abrasion forces that increase thetorque requirements for rotation. In addition,faster rotations of a file cause the lands to fur-ther limit the depth of cut,and wide lands onlarger files can prevent the blades from engag-ing an adequate depth into the canal. Widelands can be very useful in small diameter filesby adding rigidity and by enabling the file tonegotiate curvatures when canal enlargementis minimal.When lands are too wide for effec-tive canal enlargement, the files can be usedvery effectively for removing gutta perchafrom the canal and for circulating irrigationin the canal.

13. What is an aggressive file?

Efficiency is defined as the ratio of the workdone to the work equivalent of the energysupplied to it. An efficient file, a file havinggreater cutting ability, requires less time,torque and/or pressure to accomplish canalpreparation. The less pressure, torque andtime required, the more likely file failurecan be prevented.The concept is often con-fused, however, by describing a more effi-cient file as a more aggressive file, a termthat seems to be used with a negative conno-tation. Aggressive forces of the operator onan efficient file are unnecessary and can becounter-productive.For example, if one push-es with excessive pressure on an efficient filethe chips that are formed on the wall of thecanal can be larger than can be removedwithout requiring significantly more torquethan would have been required for formingand removing smaller chips with less pres-sure. Clinicians who change file systems and

begin working with more efficient files oftenhave a tendency to apply the same time orforce as was required with less efficient files.The excessive (aggressive) force on themore efficient file should be avoided andthe clinician will enhance the quality ofpreparation and reduce the threat of failureby learning to match the file’s efficiencywith the level of force required. Withoutthe benefit of efficiency data, cliniciansoften choose less efficient files because ofthe tactile sensations perceived. A file thatenlarges a canal with inefficient scrapingactions, for instance, can “feel” smootherthan a file that uses cutting actions. HHooww aanniinnssttrruummeenntt ffeeeellss dduurriinngg uussee iiss nnoott aa rreelliiaabblleeiinnddiiccaattiioonn ooff iittss eeffffiicciieennccyy..

The major concern for an efficient instru-ment is its ability to transport the canal. Itshould be remembered that ttiimmee as well asffoorrccee are functions of efficiency and lesstime will be required to transport as well as

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Fig. 32 The S1 and the SXProTaper files have the samecross-section design and max-imum diameter. However,slight dimensional differ-ences of the S1 enable it tohave greater cutting efficien-cy (generating less torsion)when engagement is limitedto the maximum diameters.(Refer to Chart 50)

LandFig. 33

The GPX instrument, Brasseler USA, is used for removing gutta percha from the canal. The friction of the wide land rotatingagainst gutta percha causes it to plasticize while the spirals auger it from the canal. The instrument is very effective for remov-ing gutta percha but is ineffective as a larger size file because the land occupies most of the working surface and keeps theleading edge from engaging into the canal surface.

Leading edge Flute

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AA rreecceessssiivvee surface that follows the bladeon H-type files gradually recedes from thefile’s outside diameter, provides support ofthe blade, and reduces propagation of cracksalong the blade in the same manner aslands, but lacks some of the effectiveness inavoiding canal transportation. However, theforce of abrasion is reduced. Rotary fileshaving this design include the three-flutedHero file (MicroMega) and the newly intro-duced two-fluted M2 file (Sweden Martina).The M2 file is essentially a modification of

the Dynatrac file and NT file design, havingpositive cutting angles but having fewer spi-rals. This modification is attributed to Dr.Vinio Malignino of Italy. Another modifica-tion is the LA Access file, which has a sur-face that at first gradually retreats from theblade, but becomes a flat recess or relief.This file is used primarily to intentionallytransport the canal orifice and utilizes thepiloted tip like the Dynatrac to minimizecanal transportation at the tip.This design isattributed to Dr. Steve Buchanan.

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Wide lands and a single flute

Dynatrac reciprocating file

Fig. 36 Dynatrac file. This file was the first multi-fluted H-type file to have a non-cutting pilot. Made of stainless steel, it wasused in a reciprocating handpiece to avoid fatigue.( Designed by J. McSpadden, 1977)

Hero

Fig. 37 Hero file. Even though this file has about the same pitch as the Dynatrac file, its three flutes result in a helix angle forless screwing-in forces during rotation.(Micro-Mega)

Fig. 38 M2 file. This 2-fluted H-type file has the same cross-section design as the Dynatrac file but has a longer pitch that ismore suitable for rotation.

M2

Fig. 39 LA Access file. Designed for preparing the canal access, this stainless steel file has much the same design as theQuantec file except it has no land following its cutting edge since it is not meant to negotiate curvatures.

LA Access file

Fig. 34 The Endomagic file (size 15) utilizes wide lands for smaller size files to facilitate negotiating curvatures.

Hero file cross section

Fig.35 Although not technically lands, since the surface does not extend axially from the center of the fileas far as the cutting edge, H-type files have surfaces that follow the blades that gradually retreat from thefile’s circumference.

RegRegrressivessive e surfsurface of ace of H-type fH-type f ileile

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16. Does the quality of manufac-turing make much difference?

Before different types of files are studied, itshould be stressed that the quality of manu-facturing is the most basic consideration fordetermining the success or failure of filesindependent of its composition or design.Less than ideal manufacturing quality con-trols result in the formation of micro-cracksand defects along the surface of a file.

Cracks can propagate to failure at a stresslevel lower than the stress ordinarily encoun-tered during instrumentation and other defectscan cause stress concentration points that

lead to file failure and jeopardize endodonticsuccess. It should be pointed out that consid-erably less force is required to propagate acrack than is required to form it. It is not sur-prising to find that fatigue cracks in files usu-ally start at geometrical irregularities on amacro- and micro-scale. If the defects are in aposition of high stress,failure can occur quick-ly.The area of highest stress is along the bladeor leading edge. Failure is the result of stressper unit area so a blade that is unsupportedby a land, such as a file having a triangularcross-section, will have greater forces forfailure than a blade supported by a land orregressing circumference.

15. Do the designs of files have tobe limited to a grindingprocess during manufacturing?

The capabilities for fabricating complex filedesigns have increased dramatically withcomputerized multi-axis grinding process-es. However, any process of grinding limitsthe shape and strength of files. The size ofthe grinding wheel limits the file’s shape,and cutting across the grain of the crys-talline structure of the wire limits its strength.The process of eelleeccttrriiccaall ddiisscchhaarrggee mmaacchhiinn--iinngg,, EEDDMM, is a promising alternative meansof manufacturing endodontic files. Theshape of the file is formed by electric sparkerosion of a wire. EDM manufacturing altersthe molecular structure on the file’s surfacepotentially strengthening the file withoutaffecting its flexibility.

Another promising method for manufac-turing nickel titanium files is using the pprroocceessssooff ttwwiissttiinngg that was used for fabricating steelfiles for decades but was initially thought to bean impractical method for nickel titanium.Residual stress and the problem of shapememory for nickel titanium can be avoidedduring this process by heat-treating before,during or after twisting.The helix angle can bevaried along the working surface by using acomputerized twisting process. The rationalefor using this manufacturing technique is that

work hardening the metal by twisting mightoccur as it does during the twisting process ofstainless steel files, and enhance its strengthwhile maintaining greater integrity of the crys-talline structure.Alternative manufacturingalso includes flute formation by forcibly push-ing a blade into a tapered wire. A ffuurrrroowwiinnggpprroocceessss forms the flute rather than beingground and the blade becomes projected fromthe shaft either with a continuous furrow orintermittently to form barbs. Barbed broachesare manufactured by this process. The fileshape can actually result from being pressedor impacted into the NiTi wire.

The molecular structure of conventionalmetals is organized into grains or crystals.The boundaries between the crystals are theareas of weakness where failure occurs whenundergoing excessive stress. Scientists havediscovered that if some alloys are cooled veryquickly during the process of casting,crystal-lization can be avoided. One of the mostunique developments for the potential forfabricating complicated file designs incorpo-rates this pprroocceessss ooff ccaassttiinngg and avoids manyof the limitations of grinding altogether.Theresulting metal has an amorphous non-crys-talline structure, the properties for accommo-dating stress are enhanced, and the micro-scopic irregularities caused by the grindingwheel that result in stress concentrationpoints are eliminated.

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Fig. 40 Liquidmetal cast file (the handle and shaft are cast as one piece). Casting allows the blades to be designed to spiralonly 180 degrees around the shaft in order to reduce canal wall engagement. It also allows each blade to have a differenthelix angle to avoid screwing-in forces. Continuous advancements in casting techniques can make it a viable manufacturingprocess in the near future.

Cast amorphous metal file

Fig. 41 Before and after surface treatment during manufacturing.

The formation of micro-cracks (shown on the file’s cutting edge in the left photo-graph) during initial production of files by FKG Dentaire SA was later eliminated by aspecial surface treatment process. (shown in the right photograph) and resulted inincreasing the resistance to torque failure by as much as 1000% in some samples. Fileswere compared rotating in a glass tube (inside diameter 2 mm, 90 degree curvature and8 mm radius) at a speed of 350 rpm until failure.

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18. How do we test designs?

To test the validity of claims for file designs,a computerized clinical simulator was con-structed to simultaneously measure torque,pressure and time, during the prescribed useof instruments, to determine efficiency andthe threat of file failure.The simulator com-puter provides the means for precisely dupli-cating motions (US Robotics) designed tosimulate clinical applications for comparingdifferent instruments. While eliminatingoperator variability and conforming to oper-ation recommendations, computer program-ming can control the preparation parametersfor the depth and the speed of file insertionand withdrawal, as well as the speed of filerotation. Not only can the stress of the forceof insertion and torsion of each individualfile size and taper be measured under differ-ent circumstances, but also the stresses,using different file sequences, can be record-ed in order to determine the least stressfuland most expeditious technique approaches.All measurements are plotted over time toillustrate when and how stress occurs.

Rather than measuring the over-all flexi-bility of the file, the simulator device can beused to measure dynamic flexibility, record-ing the resistance to bending as a rotating fileprogresses onto an inclined plane or simulat-ed curved canal. The measurement occursover time as different diameters and cross-section configurations of the file transverse acurvature.

The logged data help determine themethods for which each instrument may beused most effectively while minimizing thethreat of failure. The simulations can beapplied to different anatomies and techniquesolutions quickly become apparent, ratherthan having to rely on subjective and time-consuming trial and error experience thatlack the benefit of controls. An examinationof the results puts the manufacturer’s tech-nique recommendations in perspective, vali-dating or invalidating their claims. Identifyingtechnique enhancements and file designimprovements become more feasible. Theresults can be used to substantially enhanceefficiency and may be surprisingly differentfrom what has been recommended.

Design Considerations:

17. What are the most importantrelationships of the compo-nents of file designs andcanal anatomies that enableus to improve our technique?

Careful examination of technique and designconsiderations identifies the limitations andusefulness of existing instruments and facili-tates the development of a new generationof rotary instruments and techniques, oneunencumbered by traditional concepts. Afew all-important consequential relationshipsof different file designs and tooth anatomiesare useful in understanding how files func-tion.Although research on endodontic instru-ments cannot determine with absolute cer-tainty how files will react under all circum-stances, research can result in inferences hav-ing significant predictability that can be usedas considerations for instrument and tech-nique design.The following are some of theconsiderations and ramifications of designsthat are most important in formulating tech-niques in approaching difficult cases:

1. A file with a more efficient cuttingdesign requires less torque, pres-sure or time to accomplish root canalenlargement.

2. In a straight canal, the ability of a fileto withstand torsion is related to thesquare of its diameter.

3. In a curved canal, the ability of a file toresist fatigue has an inverse relation-ship with the square of its diameter.

4. The torque required to rotate a filevaries directly with the surface area ofthe file’s engagement in the canal.

5. Fatigue of a file increases with thenumber of rotations of the file in a cur-vature.

6. Fatigue of a file increases with thedegree of curvature of the canal.

7. To improve efficiency, the smaller thesurface area of a file engaged in thecanal, the greater the rotation speedshould be.

8. The more spirals a flute has per unitlength around the shaft of a groundfile, the less resistance to torsiondeformation there is, but the moreflexible the file is.

9. The fewer spirals a flute has per unitlength around the shaft of a ground file,the more it resists torsion deformation,but the more rigid it is.

10. The sharper the cutting blade of a file,the fewer spirals per unit length thefile should have.

11. The greater the number of flutes withsimilar helix angles, the greater tenden-cy a file has to screw into the canal andbecome bound.

12. Maximum engagement of a file occurswhen it progresses into the canal at arate that is equal to its feed rate, therate the file progresses into the canalwithout the application of positive ornegative pressure.

13. Less canal transportation occurs witha file having greater flexibility, anasymmetrical cross-section design,and/or a land.

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Section II. Mastering Instrument Designs

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19. What causes breakage?

In the most basic terms, the strength of a fileis due to the cohesive forces between atoms.As forces that tend to deform a file areincreasingly applied, the forces to separateatoms increase and their attractionsdecrease. Breakage occurs when the force ofseparation of the atoms exceeds the force ofattraction.

On a larger scale, the molecules of ametal are arranged in patterns denoting itscrystalline structure or grain, and the frac-ture of files usually can be characterized in

two ways. 1. One cause of fracture is accom-panied by an apparent deformation of a fileand the separation occurs as a result of slip-page between the planes of its crystallineboundaries, most often due to the excessiveforces of ttoorrssiioonn. 2. Another fracture mayoccur across the grain of the metal with lit-tle or no apparent deformation.This type offracture can be seen as a result of ffaattiigguueemost often caused from the excessive stressesof the repetitive compression and tension thatoccurs during rotation of a file around a cur-vature. Of course, most fractures are a combi-nation of different forces of separation.

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Fig. 42 The clinical simulator computer (B) includes two software programs. One program executes the motions of the rotaryhandpiece and the other is fully integrated with the hardware for the custom acquisition of data. The desired rotation speedof the file is adjusted by the handpiece control box (I). The handpiece is mounted on a stage (G) that is raised and lowered atrates and distances determined by the parameters of the particular program used to precisely reproduce selected clinical inser-tion-withdrawal movements of the rotating endodontic file. The file is inserted-withdrawn into, or from, a root canal or plas-tic practice block mounted on a bracket (C). The bracket is supported by a hinged stage (H) that is free to travel in the sameplane as the file to simulate the clinician’s resistance to any screwing-in forces that might result from the rotation of the file.The torsion exerted by the rotating file is measured by a torque transducer (E) and the pressure is measured by a pressuretransducer (F). The pressure and torque are simultaneously viewed on a screen display (A). The resulting measurements in realtime are based on graphical programming (B).

The most important information afforded by the simulator is not the means to just avoidbreakage, but to minimize stress on the file, data that can distance the clinician from thepossibility of failure while maximizing efficiency.Although the simulator can facilitate theformulation of technique design, it does not eliminate the need to understand the causesof file failure and the means for avoiding it.

Fig. 43 Irregularities in the surface of the leading edge of a file shown in image (A) act as stress concentration points forpotential torque or fatigue failure. The force to propagate the crack, shown in image (B), can be less than one half the amountof force that was required to form it. Examining the SEM images of the quality of manufacturing can provide valuable infor-mation for the probability for breakage.

AA BB

Fig. 44 The fracture across the grain of the metal of file (C) was probably the result of fatigue. Note the faults along the bladethat are particularly susceptible to stress concentration. The fracture resulting from slippage between the crystalline bound-aries in file (D) was probably the result of excessive twisting.

CC DD

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22. How do torque requirementsvary with the file diameters thatwe are likely to encounter incanals?

Smaller diameters of files are more likely tobreak with the application of torsion.However, binding of a small diameter canusually be detected and prevented iiff thatpart of the instrument that is likely tobecome bound is the only part that isengaged in the canal. When the differencebetween the largest and smallest diametersengaged is minimal, increases in torque areusually the result of increased applied pres-sure. IIff tthhee ttoorrqquuee aanndd pprreessssuurree rreeqquuiirreedd ffoorrrroottaattiinngg tthhee llaarrggeerr ddiiaammeetteerr ppoorrttiioonn ooff aa ffiilleeeexxcceeeeddss tthhee ttoorrqquuee rreeqquuiirreedd ttoo bbrreeaakk tthhee

ssmmaalllleerr ddiiaammeetteerr ppoorrttiioonn,, tthhee ffiillee iiss ppaarrttiiccuu--llaarrllyy vvuullnneerraabbllee wwhheenn eennggaaggiinngg tthhee llaarrggeerrddiiaammeetteerr ssiinnccee tthhee ssttrreessss oonn tthhee ssmmaalllleerrddiiaammeetteerr ccaannnnoott bbee ddeetteecctteedd..

Even establishing gglliiddee ppaatthhss (canals orsegments of canals enlarged to a diameterlarger than the tip of a subsequent file toallow its passive entrance into that portionof the canal) is no assurance that a small tipsize cannot be unknowingly pushed into,and become bound in, canal aberrationssuch as a fin, an anastomosis, a bifurcation,or auxiliary canal while the force necessaryfor engaging the larger diameter is applied.Glide paths are usually established withsmaller more flexible files that follow thepathways of least resistance, usually thatportion of the canal having the largest diam-

20. What is torsion?

Torsion is the axial force of being twistedwhen one part of a file rotates at a differentrate than another part.Any distortion of a filethat results from twisting, such as un-wind-ing, is caused by stress of torsion.When a fileresists rotation during hand instrumentationwith conventional .02 tapered files, exces-sive torque can usually be tactilely perceivedand file breakage can usually be avoided. Onthe other hand, even the use of torque limit-ing handpieces during rotary instrumenta-tion does not provide the means for adjust-ing to varying circumstances, such as curva-tures, the amount of file engagement, nor thediameters of the file that are engaged. Anyexcessive torque, as a result of these circum-stances, is not always avoided by presettorque limitations. On the other hand, thetorque limits can be set so low that file fail-ure would be difficult, but effective canalenlargement would also be limited.Understanding the factors that cause exces-

sive torque is the most reliable means foravoiding torsion failure.

21. What causes torsion stress?

Torsion stress on a file is primarily the result of:(1) the force of cutting, specifically, howeffectively a chip is formed and deflectedfrom the wall of the canal, (2) the force ofscrewing-in due to the spiraled blades thatbecome engaged in the wall of the canalwithout deflecting the chips that are formed,(3) the force of abrasion of the non-cuttingsurface of the file against the wall of thecanal, (4) the force of distortion resultingfrom rotating in curvatures, and (5) the forcethe debris exerts on the wall of the canal asit accumulates in the flutes. Incorporatingdesigns to reduce any of these forces increas-es the file’s efficiency and is one approach toadvance instrument design. Anotherapproach is to provide designs that canaccommodate greater forces, although theefficiency may remain unchanged.

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Fig. 45 Abrasion of rotating accumulated debris against the canal wall increases torsion stress. The file efficiency is decreasedwhen the debris prevents the cutting edge from engaging the canal wall. Limiting file engagement by using different filetapers, restricting the depth of file insertion, and frequent irrigation can minimize debris accumulation.

Debris filled flutes

A file with a larger diameter can resist more torsion stress than one with a smaller diameter.The relationship varies very closely with the square of the file radius.Therefore, a size .25mm diameter can resist as much as 50% more torque than a size .20 mm diameter havingthe same design,even though the difference in diameters is only .05 mm.The reason that thedescription direct relation between torque and radius squared is not used, is because thecomplicated variables in the crystalline structure of nickel titanium cause variations in thepatterns of breakage.

Profile GT distortedby torque

K-3 distortedby torque

Fig. 46 Stainless steel has an effective modulus greater than that of Nickel Titanium (Table 5). The significant advantage ofNiTi is its flexibility. Although NiTi’s super-elastic ability to be distorted without permanent deformation is often cited as anattribute, this quality is of little consequence for a file rotating over 200 rpm. Some NiTi files may continue to twist for threecomplete revolutions after becoming bound in the canal without failing, compared to less than one revolution for stainlesssteel files, but without warning, this may mean less than one second of reaction time for the operator. A more meaningfulrelationship may be the torsion force required to fracture the file relative to its flexibility. Most files having the same size andtaper usually distort in similar manners, however the force for distortion varies with file design.

Untwisted K-3 and Profile GT

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eter along its path. As larger tapered, lessflexible files that have smaller tip sizes thanthe established glide path are used, the filescan have a tendency to deviate from theglide path and can become bound in the

smaller canal aberrations. The file that ismost likely to follow the canal is one thatremains 360 degrees engaged, but that isassuming it has adequate flexibility andexcessive torsion is avoided.

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AA BB

Fig. 47 A glide path, a minimally enlarged pathway for subsequent files to follow (A), is established with a small flexible file.However, as a larger tapered file with small tip size (B) is used, its greater rigidity can force its tip into a fin or anastomosis ina curvature (C & D) and can become bound. The torque required for the larger diameter part of the file to function could besufficient to cause the bound tip to separate.

Bound file tipFin

Anastomosis

CC DD

Establishing a pathway might not preclude a file of a different size andtaper from following a different path than expected. The result can be atip that becomes bound when the necessary torque and pressure isapplied for a larger diameter and taper to function. The example at theleft illustrates several aberration of the canal system in which a small filetip could have become bound.

Fig. 48 J.T. McSpadden

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