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Instruments Used for Cleaning

and Shaping the Root Canal S


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The two primary goals of root canal

instrumentation are

(1) to provide a biologic environmentcontrol) conducive to healing and

(2) to develop a canal shape receptiv

obturation


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Recently, rotary instrumentation has g

considerable popularity and may be conjunction with hand instruments


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The information in the following sectio

facilitate the most efficient use of rotainstruments, minimizing the chance ofinstrument or procedural mishaps andthe clinician to achieve ideal treatme


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BOX 8-2

Classification of Instruments Used for C

and Shaping the Root Canal Space

Endodontic instruments for root canpreparation can be divided into six g


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Group I Manually-operated instruments, such as barbed broachH-type instruments

Group II Low-speed instruments with a latch-type attachment. Typicthis group are Gates-Glidden (GG) burs and Peeso reamertypically used in the coronal part of the canal and nevecanal curvature

Group III Engine -driven nickel-titanium rotary instruments. They consblade that can safely be operated in, and adapt itself to

canals. Most enginedriven instruments available today be

Group IV Engine-driven instruments that adapt themselves three-dimshape of the root canal. Like other nickel-titanium instrumto the shape of the root canal longitudinally but additionato the cross-section of the root canal. There is currently onlyin this group: the self-adjusting file (SAF; ReDent-Nova, R

Group V Engine-driven reciprocating instruments

Group VI Ultrasonic instruments


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GROUP I: MANUALLY OPERATEDINSTRUMENTS


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Manually operated instruments are

instruments that are generically calledefining endodontic instruments by f

Filesare instruments that enlarge canareciprocal insertion and withdrawal m

Reamerscut and enlarge canals withmotions. Before using either instrumenclinician must make sure the canal is p


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Files were first mass produced by the K

Manufacturing Co. of Romulus, Michigearly 1900s, hence the name K-type ffile) and K-type reamer (K-reamer).

K-files and K-reamers originally were

manufactured by the same process


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Three or four eflat surfaces wat increasing d

the sides of a wire, producinpyramidal sha

The wire then wstabilized on o

end, and the dwas rotated tospiral instrume


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The number of sides and the numb

spirals determine whether the instrubest suited for filing or reaming

Generally, a three-sided configurationfewer spirals is used for reaming; a thre

four-sided configuration with more spiused for filing


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Historically, root canal instruments w

manufactured from carbon steel. Subsequently, the use of stainless st

greatly improved the quality of instrum

More recently, the introduction of the

titanium (niTi) alloy in the manufactureendodontic instruments has resulted insignificant improvements in canal shabecause of its increased flexibility com

stainless steel


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Dating from the early to mid-19th cen

broaches and rasps were the earliestendodontic instruments used to extirppulp and enlarge the canal (fig. 8-12)


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Still used today, these instruments are

manufactured by hacking a round,wire with a blade to form sharp, projebarbs that cut or snag tissue. Specificahave been set for both the barbed br

and the rasp (AnSi/AdA standard noiSO/fdi standard no. 3630/1)


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A barbed broach does not cut or

dentin; this instrument is mostly usedengage and remove soft tissue fromcanal. it is also an excellent tool for recotton or paper points that have acc

become lodged in the root canal


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K-TYPE INSTRUMENTS


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The K-file and K-reamer are the oldest

instruments for cutting and machining dentin (fig. 8-1


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They are made from a stainless steel

is ground to a tapered square or trcross-section and then twisted to creaa file or a reamer. A file has more fluteComponents of a file) per length unit

reamer


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K-type instruments are useful for pene

and enlarging root canals. The instrument works prim

compressionand-release destruction dentin surrounding the canal.


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Generally, a reaming motion (i.e., consta

rotation) causes less transportation thamotion (reciprocating or watch-windinrotation). (Transportationis the excessive dentin from the outer wall of a curved capical segment. This procedural error caperforation of the root canal system or ato negotiate the canal space apical to transportation.) A stainless steel K-file caprecurved to a desired form to facilitateand minimize transportation


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Permanent deformation occurs when

become wound more tightly or openewidely (fig. 8-14).


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When such deformation occurs, the

instrument should no longer be used. instruments fracture during clockwise

after plastic deformation.


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This occurs when the instrument bec

bound while the force of rotation cinterestingly, although the force requifailure is the same in both directions ofailure occurs in the counterclockwiseat half the number of rotations require

failure in the clockwise direction. Theretype instruments should be operatecarefully when pressure is applied in acounterclockwise direction


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H-TYPE INSTRUMENTS


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An H-type instrument has spiral edg

arranged to allow cutting only duringstroke (fig. 8-15). An example is a HedAn H-type instrument is better for cuttiK-type instrument, because it has a mpositive rake angle (see Components

and a blade with a cutting rather thascraping angle.


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Bending a Hedstrm file results inpoints of greater stressconcentration than occurs withK-type instruments. Theseconcentration points can leadto the propagation of cracksand fatigue failure. Clinically,

fatigue happens without anyexternal physical signs of stress,such as the flute changesseen in K-type instruments (seefig. 8-14).


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Currently all H-type instruments are ground from a blank. Hedstrm files are formed by grinding a

continuous flute. Computer-assisted machining thas allowed the development of H-type instrumvery complex forms. This process, called multiaxisgrinding,allows adjustment of the rake angle, helixmultiple flutes, and tapers. H-files cut the canal wapulled or rotated clockwise; the file is relatively inewhen pushed or rotated counterclockwise. Becau

file generally has sharper edges than the K-file, ittendency to screw into the canal during rotation,if the instruments blades are nearly parallel. Awscrewing-in forces is important for avoiding instru


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INSTRUMENT DESIGNMODIFICATIONS


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K-files and H-files can be modified into numedesigns.

Often the instruments can be improved for meffective

instrumentation by changing the geometric dimusing

computerized multiaxis grinding machines. for

changing the cross-sectional geometry of a K-ty

instrument from square to rhomboid enhances the instrume

and rake angle. However, the possible geometrcomplicate adherence to iSO and AnSi standard


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TIP DESIGN


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Studies have shown that tipdesign can affect filecontrol, efficiency, andoutcome in the shaping ofroot canal systems. The tipof the original K-fileresembled a pyramid(see fig. 8-13). instrumenttips have been describedas cutting, noncutting,andpartially cutting,althoughno clear distinction existsamong the three types (fig.

8-16)


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The instrument tip has two functions: to gfile through the canal and to enlarge the ca

clinician who is unfamiliar with the tip design particular instrument is apt to do either of the(1) transport the canal (if the tip is capable othe canal and remains too long in one positioencounter excessive torsion and break the finoncutting tip is forced into a canal with a smdiameter than the tip). Transportation of taxis of the canal can occur by remaining tocurved canal with a tip that has efficient cut


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The angle and radius of its leading edgeproximity of the flute to its actual tip end

determines the cutting ability of a file tipability and file rigidity determine the protransport the canal. The clinician must kemind that as long as the file is engaged degrees, canal transportation is unlikely Only with overuse does the file begin to one side, resulting in transportation. Minstrumentation errors occur when the filloose in the canal, which gives it a propetransport the canal


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A good beginners rule is this: if the ca

smaller than the file, the prudent use ocutting tip is more efficient. if the canathan the tip, using a less-effective cutcan help prevent transportation (see16). Much has been written about th

importance of various sophisticated timodifications to prevent such ledgingscientific evidence exists that any onebetter than another


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METAL ALLOYS


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The development of nitinol, an equiatomic alloy composed titanium has proved a significant advancement in the manuendodontic instruments. niTi is called an exotic metalbecaus

conform to the typical rules of metallurgy. Because it is a supthe application of stress does not result in the usual proportioother metals such as stainless steel. When stress is initially appresult is proportional strain, but the strain remains essentially the application of additional stress reaches a specific leveis called a loading plateau. eventually, of course, applicatioresults in more strain, which increases until the file breakproperty is the result of a molecular crystalline phase transfostresses transform the austenitic crystalline form of niTi into a crystalline structure that can accommodate greater stress wthe strain. As a result of its unique crystalline structure, a niTi fisuperelasticity,or the ability to return to its original shape aftedeformed. Simply stated, niTi alloys currently are the only available affordable materials with the flexibility and toughnuse as effective rotary endodontic files in curved canals


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Attempts to improve the niTi alloy c

and recent reports indicate that newmay be five times more flexible than used alloys. Microscopic surface defconsidered a contributing factor for cpropagation and instrument fracture.attempts to improve surface charactelectropolishing, surface coatings, aimplantation have been used for this p

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