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MasteringEndodontic Instrumentation

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Mastering Endodontic Instrumentation

John T. McSpadden, D.D.S.

Cloudland Institute

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Mastering Endodontic InstrumentationCopyright 2007 John T. McSpadden, D.D.S Published by Cloudland Institute

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:

Dr. John T. McSpaddenCloudland Institute 1428 Williams St. Suite GChattanooga,TN 37408

[email protected]

Book design by:Arbor Books, Inc.www.arborbooks.com

Printed in Canada

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

1.Title 2.Author 3.Textbook

Library of Congress Control Number: 2006909695

ISBN 10: 0-9791088-0-2ISBN 13: 978-0-9791088-0-8

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Acknowledgements

Perhaps nothing better typifies the generosity endodontics has been so fortunate to havehad in their leaders as their willingness to take a personal interest in the interest of others.I am fortunate to know that first hand. I was the recipient of that generosity when virtuallythe only reason they had for knowing me was that I was asking them questions. I know ofno one so indebted to so many internationally recognized endodontists as I am for havingbeen guided and inspired to pursue endodontic excellence.At the risk of being embarrassedfor not listing key individuals responsible for all the important events of my career, there aresome that have given particular personal support for my early endeavors.They are: StephenSchwartz, Barry Korzen, John Ingle, Dudley Glick, Al Frank, Richard Burns, Noah Chivian,Herbert Schilder, Frank Weine, Jeffery Hutter, Al Krakow, Vinio Malagnino, Jean MarieLaurichaise and Dan Even.

I am especially grateful to Dr. Melissa Marchesan from Brazil who spent weeks at my homeconducting research for hours on end, day in and day out, and to my son, John ThomasMcSpadden, for manning the computerized protocols.

Most of all, I am grateful, as always, to my wife, Jane, for her loving patience and supportwhile it must have seemed to her that I was writing an esoteric non-ending prescription forslumber. My children, Melinda, Matthew, John Thomas and Kathleen, make every endeavorworth while.

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Reviews Once in a great while, an individual has the courage, talent and expertise to provide the profession with a benchmark by which all other studies, anecdotal claims and personal bias must adhere. Dr. John T. McSpadden has provided the dental profession and the specialty of endodontics with such a benchmark. The text of this book is written for the most part in the first person allowing for personal interpretation of the experimental results based on hard data without being dogmatic. This style acknowledges that there may be other opinions relative to the information put forth in this text but Dr. McSpadden has set the bar high for those who may disagree. The essence of a work such as this is that Dr. McSpadden has dared to establish protocols and scientific methodology on which differences of opinion can be evaluated. This book should be mandatory reading for all dentists interested in the selection of an instrumentation technique along with an understanding of the safe and effective utilization of the instruments chosen. Stephen F. Schwartz, DDS, `MS Past Pres. AAE Vice Pres. ADA Houston TX Mastering Endodontic Instrumentation is a unique book covering a most important segment of the endodontic curriculum. Dr. McSpaddens book breaks down cleaning and shaping root canal systems into its most basic and scientific components.. a major goal of the author is to help the clinician truly understand why various approaches to cleaning and shaping work rather than limiting teaching to more conventional step-by-step instruction. New and experienced clinicians alike will benefit from understanding why techniques work rather than just how a system is reported to shape canals. Basic rules along with evidence of their validity are presented which apply to all endodontic shaping instruments.. Applying the basic principles presented should help clinicians with an interest in endodontics perform higher quality cases more efficiently. Van Himel, DDS Professor and Department Chair University of Tennessee Health Science Center College of Dentistry Memphis, Tn .."Mastering Endodontic Instrumentation" is a primer for anyone wishing to expand their knowledge and understanding of rotary instrumentation. Whether you are someone new to this genre of instrumentation or a seasoned endodontist or general practitioner, this book presents valuable information on how to elevate your cleaning and shaping techniques to a higher level. In addition, this presentation disseminates the most comprehensive approach to understanding the evolution, physics, limitations and underestimated benefits of rotary instrumentation I have ever seen. That makes this text a very important addition to your endodontic library. Marc Balson, DDS Past Pres. AAE Livingston NJ The book is a masterpiece of Socratic learning; a logical, progressive and systematic examination of how root canal anatomy dictates the selection and use of endodontic instruments. The text offers exhaustive assessments of materials, file systems, handpieces and design features and accompanies them with unsurpassed imagery. Research was the seminal force behind Mastering Endodontic Instrumentation. All the results are devoid of operator subjectivity; thus the content is evidence-based and incontrovertible Dr. McSpadden poses questions and then answers them and each logical thought progression takes you from one topic to the next as the assessment of all current systems, their strengths and their weaknesses, is defined in the most extraordinary detail. The scholarship evident will provide both the inexperienced and consummate practitioners with a means to exponentiate their expertise. In its elegance, lies its simplicity; the cogent and concise scientific explanations for instrumenting complex cases are unprecedented He has written the definitive text for present and future generations to understand and appreciate the enormity of his contributions to the endodontic discipline. Kenneth S. Serota, DDS, MMSc Founder, RxROOTS.com Mississauga ON .The high resolution photographs used in the book surpass anything I have seen in the dental literature. This book should be required reading for every dental student at both the graduate and under graduate levels. The information delivered in this book is truly evidence- based. L. Ronald Martin, DDS, MS Jackson, MS

John T. McSpadden, D.D.S

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ContentsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

SSeeccttiioonn II MMaasstteerriinngg tthhee CCoonncceeppttss Canal Anatomy and Instrumentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Brief History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Terms for Physical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Advantages of Nickel Titanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Other Alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Rotary Instrumentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Importance of Instrument Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Appropriate Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Components of a File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Functions of Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Relationship of File Design and Canal Anatomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

SSeeccttiioonn IIII MMaasstteerriinngg IInnssttrruummeenntt DDeessiiggnnssTesting Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37File Failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Torsion Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40Stress of Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51Fatigue and Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54File Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Canal Transportation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74Screwing-in Factor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76Function of File Tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82Rotation Speeds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84File Engagement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87File Tapers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90Canal Anatomy and File Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

SSeeccttiioonn IIIIII MMaasstteerriinngg tthhee TTeecchhnniiqquueeConsiderations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105File Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106Zones of the Canal for Preparation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

SSeeccttiioonn IIVV MMaasstteerriinngg FFuuttuurree DDeevveellooppmmeennttssDesign Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

SSeeccttiioonn VV MMaasstteerriinngg RReesseeaarrcchhInterpreting the Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

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

1. To provide a biological envi-ronment that is conducive tohealing.

2. To provide a canal shape thatis conformable to sealing.

At this stage of endodontic development,common to all instrumentation techniques isthe use of endodontic files. Although notuniversally used, rotary instrumentation isgaining universal interest. The purpose ofthis book is to provide information gainedfrom extensive research to facilitate the mostefficient use of rotary instruments, withoutthe threat of failure, while conforming to theclinicians 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 dollarsbenefits

rarely attainable.The greatest benefit,however,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 immediatesuccess; 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 qualityand profile of your practice? Most would agreethat the ability to replace repetitious, unnec-essary and counterproductive actions, withonly the most effective actions, would betrue excellence. However, it certainly could

1

Someone once asked, Which is worse, ignorance or apathy? The answerwas, I dont know and I dont 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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not be done with the information available.Most would also agree that there is no onesource dedicated to this kind of information.Currently, accessible information is practicallylimited to cookbook type instructions withvirtually no rationale of rotary instrumentsand techniques. Instructions for new rotaryfiles are generally comprised of a few con-cise technique recommendations, makingthe assumption that these will be adequateto prevent compromising endodontic treat-ment while attempting to convince the prac-titioner to employ yet another unique designof instruments. Consequently, endodontists

often needlessly spend most of their chairtime preparing root canals with the accompa-niment of uncertainty, mediocrity or an ardu-ous attempt at achieving the ideal.Understanding essential information providesthe means for expertise.The problem is mostwill never study information, as it becomesavailable, to know the difference. Some, how-ever, who are willing to invest the timerequired for understanding the dynamics ofinstrumentation will certainly save thousandsof hours of chair time, significantly increasetheir income, and have the satisfaction thatthey are providing excellence for theirpatients.

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,my ini-tial design and use of this new modality forcanal preparation, required the continuous

attempt to thoroughly understand thephysics of rotary instrumentation. Thedoing required careful visualization of allthe consequences of actions before theywere performed. Ironically this exerciseproved uniquely advantageous over theregimented procedures most have had tofollow and soon I developed the under-standing 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 ofunderstanding.


2

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.


Unfortunately, I attempted to conveyinstructions by teaching conventionalstep-by-step techniques rather than theunderstanding I had acquired and that clini-cians could have easily learned. I thought,and was told, that dentists only wanted toknow how rather than why.The reality, how-ever, is as the introduction of new productsincreases and voices of advocates confusethose choices, coupled with the fact thatproducts become obsolete before they canbe thoroughly evaluated, the need forunderstanding the principles of all prod-ucts, especially rotary instrumentation,becomes apparent. As scientific evaluationsencompass only a small portion of the totalfunctionality of instrumentation, and asmore instruments are described only interms of their unique features, the need forconsolidation of information becomes indis-pensable for the endodontist who strives toexercise judgments and skills beyond thoseafforded by the set of instructions as abeginner or the satisfaction level of the con-summate user.A basic understanding of thescientific principles of instrumentationneeds to be the foundation of expertiserather than instructions or recommendationsthat seem to lead to multiple and temporaryconclusions.

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 longer

we continue practicing without appreciatingthe rudimentary principles and characteristicsof 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 instrumentsto their greatest benefit in relation to the canalanatomy.

As one examines the principles of rotaryinstrumentation, the cookbook type tech-niques that were once beneficial for initiatingthe use of rotary files in ones practicebecome overly simplistic.The ability to differ-entiate between the attributes and limitationsof 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 within anyparticular category other than canal anatomy.Even though the choices of instruments andtechniques can become more numerous andcomplex for cleaning and shaping canals, the


3


solutions become less complicated and expe-dient. As one broadens the scope of under-standing, skill is enhanced in a scientific man-ner and success becomes more predictable.The art of endodontics becomes the scienceof endodontics and expertise becomes thenature 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 fol-lowing solid scientific protocol that can beeasily duplicated, and all testing has beenconducted by only using mechanicaldevices that operate independent of opera-tor variables or subjectivity, this book doesnot pretend to be an authoritative treatise tovalidate or invalidate the claims for instru-ments or techniques. Rather, the results oftesting are presented as tools to promoteunderstanding, investigation and develop-ment. As understanding is developed, anycommercial influence of these or any othertesting results should become apparentregardless of 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 reasons

of any concept. In fact, this book is a resultof other peoples 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 rotationshould not be exceeded.

2. Complicated curvatures requireslower speeds.

3. Use one continuous motionof file insertion until resist-ance is met.

4. Routinely follow the use ofan instrument with anotherinstrument having the sametaper with a smaller tip size.

5. Routinely establish straight-line access.

6. Routinely carry a .04 or evena .06 taper file to workinglength.

7. A crown-down approach isalways preferable to a step-back approach.

8. One millimeter of file advance-ment into the canal onlyresults in one millimeter ofadditional engagement.

These and other concepts are often fol-lowed without question.The best use of thisbook is to use the questions as frameworksfor examination. Although research onendodontic instruments cannot result inabsolutes, understanding the results of


4


research will provide significant predictabilityto be used as a guide for formulating tech-niques. You will note that it is only after athorough examination of existing instrumen-tation concepts, in the context of one of themost extensive research projects ever under-taken, that any parameters are recommendedfor using instruments currently available andfor designing prototype instruments for thefuture. Following those or any parametersshould be consistent with your understanding.Any inconsistency may mean either a lack of

understanding,or hopefully,and more impor-tantly, may mean that you have contributedin formulating an advanced concept for anew instrument or 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.


5

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.


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 ccaannaall aannaattoo--mmyy eennaabblleess tthhee ddeennttiisstt ttoo ccoonnssiisstteennttllyyaacchhiieevvee tthhee mmoosstt eexxppeeddiittiioouuss aanndd eexxcceelllleennttttrreeaattmmeenntt wwiitthh tthhee lleeaasstt rriisskkss.. This is not a

7

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 thepractitioner to maximize endodontic skills for any technique availabletoday and to most effectively use and evaluate advancements as theybecome available in the future. Armed with this knowledge, the practition-er gains independence from advocacy claims and the need for trial anderror experience. More importantly, a more rational approach will beoffered in providing expertise for treating their patients.

Mastering Instrumentation

Section I: Mastering the Concepts


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 canalpreparation. Rotary instrumentation is cer-tainly not a new concept; it was intro-duced in the late 19th century, as were therubber dam, rubber dam clamps, and evensolid core carriers for gutta percha which


8

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)

AA


were introduced at the beginning of the20th century.

The first manual and mechanical rotaryfiles 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 early1900s,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 increasing

depths on the sides of wire to form atapered 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 grinding process-es for NiTi rotary files, new advances formanipulating shape memory alloys mayoffer economic and physical propertyadvantages for reconsidering the twistingmethod of manufacturing for the future.


9

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 files working surface.C. Multiple rotations result in the familiar spiraled shape of the endodontic file.

AA

BB

CC


Dating from the late 19th century, the earliest endodontic instruments used for extirpat-ing the pulp and enlarging the canal were broaches or rasps. Still used today, these instru-ments are manufactured by hacking a round tapered wire with a blade device to form sharpbarbs that project out from its side to form cutting or snagging surfaces. Although mostlyused to engage and remove soft tissue from the canal as manual instruments, these historicbroach type instruments have the potential for becoming effective rotary instruments. Theevolutionary development for endodontic instruments seems to have some cyclic peculiari-ties and is far from over. Even the tapered pyramidal design originally used as blanks asdescribed above is now being used as rotary NiTi files.


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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 toknow when comparing thephysical properties of files?

The success of using instruments while pre-venting failure depends on how the material,design and technique relate to the forcesexerted on the instruments. To fully under-stand how the file reacts to applied forces,terms have been defined to quantify theactions and reactions to these forces. Commonterms related to forces exerted on files havethe following definitions:

1. StressThe deforming forcemeasured across a given area.

2. Stress concentration pointAnabrupt change in the geometricshape of a file, such as a notch,will result in a higher stress atthat point than along the sur-

face of the file where the shapeis more continuous.

3. StrainThe amount of defor-mation a file undergoes.

4. Elastic limitA set quantitywhich represents the maximalstrain, which, when applied to afile, allows the file to return to itsoriginal dimensions. The resid-ual internal forces after strainare removed and return to zero.

5. Elastic deformationThe reversibledeformation that does notexceed the elastic limit.

6. Shape memoryThe elasticlimit is substantially higher thanis typical of conventional metals.

7. Plastic deformationPermanentbond displacement caused byexceeding the elastic limit.

8. Plastic limitThe point at whichthe plastic deformed file breaks.


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,preparation of curved canals is often a prob-lem for manual files, and the mechanical usewith conventional designs and grades ofstainless steel poses the likely threat of filebreakage or 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, pages 346-351). In 1991, thefirst commercial nickel titanium manual androtary files were introduced by NT Co. In1994, NT Co. also introduced the first seriesof nickel titanium rotary files having multi-ple non-conventional tapers: the McXIMSeries, which had six graduating tapers rang-ing from the conventional 0.02 taper to a0.05 taper file in order to reduce stress bylimiting the files engagement during theserial enlargement of rotary instrumenta-tion. Based upon the initial success and rec-ognized advantages, the use of nickel titani-um rotary 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 austeniticcrystalline form of nickel titanium into thestress-induced martensitic crystalline struc-ture that can accommodate greater stresswithout increasing the strain. Due to itsunique crystalline structure, a nickel titaniumfile has shape memory or the ability to returnto its original shape after being deformed.Simply restated, nickel titanium alloys werethe first, and are currently the only readilyavailable economically feasible materials thathave the flexibility and toughness necessaryfor routine use as effective rotary endodonticfiles in curved canals. Other alternative materi-als are being investigated for the same purpose.


11


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.


12

J. McSpadden

Fig. 6 Although stainless steel files could negotiate abrupt apical curvatures (left image), canal transportation could easilyoccur due to the files 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.

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.)


Stainless steels are a good case in point. Over100 alloys, of which many have only recentlybeen introduced, are included under thebanner of stainless steel. Endodontics, unfor-tunately, has been lacking in its investigationof alloy selection and when we compareNiTi files with stainless steel files, we do sowithin the narrow framework of older stain-less steel 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 curvaturesof most canal anatomies can cause excessivestresses on conventional stainless steel filesand result in unwanted canal transportationor file failure. Nevertheless, the introductionof nickel titanium files seemed to ignore thefact that nickel titanium offers no advantage

for files having large diameters and tapersthat lack any appreciable flexibility.Accordingly, these instruments havebecome an unnecessary expense, onlybecause these larger files were a part of aseries of instruments. The advantage ofstainless steel rotary files of larger diame-ters and tapers to compliment the use ofnickel titanium files is now recognized bymany that have become familiar with theattributes and limitations of nickel titanium.Stainless steel rotary files are being intro-duced for use in lieu of nickel titanium filesin larger sizes and tapers that lack the flexi-bility. More advanced design developmentsthat reduce file stresses and modificationsin the molecular structure of stainless steelare continuously 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.


13

V. Malagnino J. McSpadden J. McSpadden

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.


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.


14

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


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. Incontrast, continuous clockwise rotation willconvey 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 entirelydependent on design and can mean successor failure. No one aspect of file design isindicative of the files 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 substantiallyadvance 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


15


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 practitioners 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 eeffffeecc--ttiivveenneessss ooff aa ffiillee;; rraatthheerr,, hhooww wweellll tthheeccaappaabbiilliittiieess ooff aa ffiillee ccaann aaddddrreessss tthheerreeqquuiirreemmeennttss ooff tthhee ccaannaall aannaattoommyy sshhoouullddbbee tthhee mmeeaassuurree ooff iittss uusseeffuullnneessss.. Sincecanal anatomies vary, techniques to effec-tively clean and enlarge the canal mayinclude modifications and may include dif-ferent type instruments. Instrumentationsinvolving more than one type instrument ortechnique are known as hybrid techniques.

8. What are the components of a file?


16

Helix Angle(may change along working surface)

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

Measuring lines

Measuring stop

Taper

Fig. 9


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.


17

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


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 llaanndd (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 rreelliieeff (Fig.14).The anglethat the cutting edge makes with the longaxis of the file is called the hheelliixx aannggllee (Figs.12,13,and 14) and serves to auger debris col-lected in the flute from the canal.


18

Helix angle (angle of blade with long axis)

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



19

Fig. 13 Profile

Fig. 14 Quantec file Cutting edge

Cutting Edge

Helix angle

LandFlute (separated by lands)Land

Flute Land Relief

Helix angle


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 torsionis partially determined by the core diameter.The core taper and total external taper canbe different and the relative diameter of the

core, compared to the files total diameter,may vary along its working portion in orderto change the flexibility and resistance to tor-sion. The importance of the ratio of corediameter to total diameter is often over-looked in predicting a files susceptibility tofailure and can be different for each file sizeof the same series.


20

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 files 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.

Size 25

Size 15

Both files have flutes witht the same depth.


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 beppoossiittiivvee oorr ccuuttttiinngg. If the angle formed by theleading edge and the surface to be cut isacute, the rake angle is said to be nneeggaattiivvee oorrssccrraappiinngg. However, the rake angle may not bethe same as the ccuuttttiinngg aannggllee (Figs.17-29).Thecutting angle, eeffffeeccttiivvee rraakkee aannggllee, is a betterindication of the cutting ability of a file and isobtained by measuring the angle formed by

the cutting (leading) edge and the radius whenthe file is sectioned perpendicular to the cut-ting edge. In some instances, as with someQuantec files,a file may have a blade with a neg-ative rake angle and a positive cutting angle. Ifthe flutes of the file are symmetrical, the rakeangle and cutting angle will be essentially thesame. Only when the flutes are asymmetricalare the cutting angle and rake angle different.Both angles may change as the file diameterschange and may 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


21

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)


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 files 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.


22

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.



23

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


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.


24

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.



25

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 files 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).



26

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. 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.



27

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).


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 the larg-er diameters of K3 files. However, it is conceiv-able that other H-type (Hedstrom) instrumentscould 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.


28

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).


12. Do cutting angles changealong the working surfaceof a 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 diametersfor two reasons. One reason is the intentionaldesign to provide a more rigid tip and theother reason is the limitation of manufactur-ing 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 manufacturers representations.


29

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


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 efficientfile as a more aggressive file, a term thatseems to be used with a negative connota-tion.Aggressive forces of the operator on anefficient 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 files 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


30

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)


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 tthheeffiilleess 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 files 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.


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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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Wide lands and a single flute

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

Fig.35 HHeerroo ffiillee ccrroossss sseeccttiioonn.. Although not technically lands, since the surface does not extend axiallyfrom the center of the file as far as the cutting edge, H-type files have surfaces that follow the blades thatgradually retreat from the files circumference.

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


AA rreecceessssiivvee surface that follows the bladeon H-type files gradually recedes from thefiles 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 Axxess file,which has a surfacethat at first gradually retreats from the blade,but becomes a flat recess or relief.This fileis used primarily to intentionally transportthe canal orifice and utilizes the piloted tiplike the Dynatrac to minimize canal trans-portation at the tip.This design is attributedto Dr. Steve Buchanan.


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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 Axxess 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


15. Do the designs of files haveto be 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 files 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 files 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.The resulting metal has an amorphous non-crystalline structure, the properties foraccommodating stress are enhanced, andthe microscopic irregularities caused bythe grinding wheel that result in stressconcentration points 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


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 notsurprising to find that fatigue cracks in filesusually start at geometrical irregularities ona macro- and micro-scale. If the defects are ina position of high stress, failure can occurquickly.The area of highest stress is along theblade or leading edge. Failure is the result ofstress per unit area so a blade that is unsup-ported by a land, such as a file having a tri-angular cross-section, will have greaterforces for failure than a blade supported by aland or regressing circumference.


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Fig. 41 Before and after surface treatment during manufacturing.

The formation of micro-cracks (shown on the files cutting edge in the left photograph)during initial production of files by FKG Dentaire SA was later eliminated by a special sur-face treatment process. (shown in the right photograph) and resulted in increasing theresistance to torque failure by as much as 1000% in some samples. Files were comparedrotating in a glass tube (inside diameter 2 mm, 90 degree curvature and 8 mm radius) at aspeed of 350 rpm until failure.


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 function.Although research on endodontic instrumentscannot determine with absolute certainty howfiles will react under all circumstances,research can result in inferences having signifi-cant predictability that can be used as consid-erations for instrument and techniquedesign.The following are some of the consid-erations and ramifications of designs that aremost important in formulating techniques inapproaching difficult cases:

1. A file with a more efficient cut-ting design requires lesstorque, pressure or time toaccomplish root canal enlarge-ment.

2. In a straight canal, the ability ofa file to withstand torsion isrelated to the square of itsdiameter.

3. In a curved canal, the ability ofa file to resist fatigue has aninverse relationship with thesquare of its diameter.

4. The torque required to rotate afile varies directly with the sur-face area of the files engage-ment in the canal.

5. Fatigue of a file increases withthe number of rotations of thefile in a curvature.

6. Fatigue of a file increases withthe degree of curvature of thecanal.

7. To improve efficiency, thesmaller the surface area of afile engaged in the canal, thegreater the rotation speedshould be.

8. The more spirals a flute has perunit length around the shaft ofa ground file, the less resist-ance to torsion deformationthere is, but the more flexiblethe file is.

9. The fewer spirals a flute has perunit length around the shaft of aground file, the more it resiststorsion deformation, but themore rigid it is.

10. The sharper the cutting bladeof a file, the fewer spirals perunit length the file shouldhave.

11. The greater the number offlutes with similar helix angles,the greater tendency a file hasto screw into the canal andbecome bound.

12. Maximum engagement of a fileoccurs when it progresses intothe canal at a rate that is equalto its feed rate, the rate the fileprogresses into the canal with-out the application of positiveor negative pressure.

13. Less canal transportationoccurs with a file havinggreater flexibility, an asym-metrical cross-section design,and/or a land.


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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 preciselyduplicating motions (US Robotics) designedto simulate clinical applications for compar-ing different instruments. While eliminatingoperator variability and conforming tooperation recommendations, computerprogramming can control the preparationparameters for the depth and the speed offile insertion and withdrawal, as well as thespeed of file rotation. Not only can the stressof the force of insertion and torsion of eachindividual file size and taper be measuredunder different circumstances, but also thestresses, using different file sequences, canbe recorded in order to determine the leaststressful and most expeditious techniqueapproaches.All measurements are plotted overtime to illustrate 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 manufacturers 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.

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Section II. Mastering Instrument Designs



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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 clinicians 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 affordedby the simulator is not the means to justavoid breakage,but to minimize stress on thefile, data that can distance the clinician fromthe possibility of failure while maximizing

efficiency. Although the simulator can facili-tate the formulation of technique design, itdoes not eliminate the need to understandthe causes of file failure and the means foravoiding it.


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 acurvature. Of course, most fractures are acombination of different forces of separation.


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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 manufact

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