Guidelines and technique

The HERO 642® is simple, safe, versatile and economic as long as you respect the following instructions.

Catheterism or first penetration

• Its purpose is to open the first passage and control the depth and working length (WL).• It can be done with conventional hand instruments of the practitioner's choice.• In easy or average canals, a file No. 10 or 15 will often be enough to penetrate the canal and check the depth.• In difficult canals, it can be done after using a .06 HERO taper with the benefit of a wider access in the coronal area.

Canal Enlargement or the "crown-down" technique  

Canals are classified as easy, average or difficult, according to S.W. Schneider's curvature criteria (cf."A comparison of canal preparation in straight and curved canals", Oral Surgery, 1971 ; 32(2) : 271-275) and how hard their penetration can be.

• Easy canals : straight and curved < 5°• Canals of average difficulty : curved > 10° and < 25°• Difficult canals : curved > 25°

Easy canals / Blue sequence

.06 TAPER:

• Open pulp chamber, locate and explore orifice opening, determine working length (WL) by using conventional methods. • Place a .06 taper No 30 HERO in the selected reducing contra-angle. • Adjust the (black) rubber stop to correspond to 1/2 or 2/3 of the WL. Choose a rotation speed between 300 and 600 rpm end keep it constant. • Insert the instrument while rotating. • Proceed apically in a short in and out movement. • With normal pressure, as if writing with a sharp pencil. • Penetrate the canal until you reach 1/2. or 2/3 of its depth.

Frequent and abundant irrigation is recommended for best evacuation of debris. In some difficult cases, canal depth can be measured at this stag : debridement of the coronal half makes penetration of the apical third easier.

.04 TAPER:

• Change to a .04 taper No 30 HERO. • Adjust the (grey) rubber stop at WL minus 2 mm. • Use the same constant rotation speed. • Proceed the same way as before until WL minus 2 mm. • And combine with a circumferential filing.

Irrigate frequently and abundantly. In presence of difficulty (in particular when resistance is met), recapitulate with a hand file to control access and depth.

.02 TAPER:

• Change to .02 taper No 30 HERO. • Adjust the (white) rubber stop at WL. • Use the same constant rotation speed. • Proceed with the same quick pumping movement until WL. • Without excessive pressure. • And combine with a circumferential filing.

In the case of an easy canal, 3 instruments are used (= blue sequence, or HERO 642® No 30).Irrigate and dry the canal for obturation (all methods can be used).

Canals with average difficulty / Red sequence

• The "crown-down" method remains the same. • Begin with the instruments N° 25 and follow the red sequence.

• Use 5 instruments.

Difficult canals. Yellow sequence

• The "crown-down" method remains the same. • Begin with the instruments N° 20 and follow the yellow sequence. • Use 6 instruments.

• Whenever required by the morphology of a canal, .02 taper Nos 35, 30, 45 can be used to complete enlargement

of the apical area.

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Technique guide for Root canal instrumentation with LSXImportant points are highlighted in Red colorStep 1: Access, Flaring, Working Length, Canal PatencyGain access to the pulp chamber with a pear shaped diamond or carbide bur. Use a Kfile to explore the root canals, to whatever depth it goes without binding,LSX is not an instrument for coronal third instrumentation. Do coronal flaring withthe instrument of your choice. Here is one method to achieve straight line access, whichis very crucial for rotary instrumentation.1. Use H files # 15, 20 and 25 to do anti curvature filing.2. Irrigate and use Gates Glidden drills for the glide path (Fig 1)Obtain working length and Irrigate.Step 2: Instrument the Apical Part of the Canal and Determine WorkingWidth1. Instrument the root canal with K files # 15, 20, & 25. Do reaming first, then filingtill the file is loose in the canal, irrigate after each instrument.2. Flood the canal with EDTA (17% Solution). Start using LSX Rotary Instrumentsfor apical instrumentation. Attach # 20 LSX instrument to the EndoPal handpieceand then adjust the WL3. When the handpiece is not running, insert the tip of LSX into the canal orificeabout 2 to 3 mm and then start the handpiece4. See that the LSX is held in a straight position ( Importance of straight-line access)5. SLOWLY, steadily advance the instrument to the WL, and take it out. Don’t stayin the canal unduly.6. Change to the next size LSX. First few instruments may not do any cutting. Thereis no need to irrigate between change of LSX if the instrument has not done anycutting7. When you advance an instrument and it offers resistance before reaching the WL,this instrument will start cutting.28. When resistance is felt, briefly pause, and then proceed to Working Length. Thistechnique is simple: a) Advance to Resistance,b) Pause at Resistance,c) Proceed SLOWLY, steadily to Working Length.9. If you hear chatter while advancing the LSX apically, pause for a while, let theinstrument work on the area, and then SLOWLY advance. If you advance theLSX rapidly while there is chatter, it may separate or twist up.1) Chatter noise indicates present of septum. If there is a septum, the instrument

can go deeper because of the spade shape, but it will put a lot of strain while bringout and may separate the instrument2) If there is a sharp curvature, strain on the instrument can lead to separation10. Your Final Apical Size (FAS) is the instrument that encounters resistance 4mm(or more) from Working Length.

2 mm 3 mm 4mm3Step 3: Instrument the Apical and Middle thirdOnce you have reached the FAS, start stepping back by 2mm with the next largerinstrument.We have to now deal with the apical and middle thirds of the canal, which is on anaverage 4+4 mms. That means you have to work with the next 4 larger LSXs, eachstepping back by 2mm from the previous one.This stepping back is done to create an .02 taper, to facilitate obturation with .02 tapergutta percha. LSX is a non tapered instrument, so taper has to be created by steppingback with larger instruments. Irrigate, clear the canal thoroughly.NOTE: If you are using SimpliFill then you need to step back from WL minus 4mm.Step 4: RecapitulateUsing the FAS rotating in the handpiece, recapitulate to Working Length.Step 5: 13. Final irrigation.Irrigate with NaOCL, then EDTA, then NaOCL again to clear the dentinal tubules.Finally, irrigate with Chlorhexidine.The canal is now ready for obturation.For updates and latest in rotary endo keep visiting www.healthmantra.com/rotary/---------------------------

STEP 1: ACCESS PULP CHAMBERUse the appropriate diamond or carbide burs to create a conservative access to the pulp chamber. Begin by cutting the

outline, then penetrate deeper. Orifice locations generally follow the anatomy of the occlusal cusps on molars.

STEP 2: LOCATE CANAL ORIFICESUse hand files or an Endodontic Explorer to locate the orifices. Once you have found them, instrument with a #10 hand file

to mid-root. Do not attempt to go to Working Length yet.

STEP 3: MOVE MESIAL CANAL ORIFICESWith Hedstrom files sizes 15, 20, and 25 move the mesial canal orifices and coronal third of the mesial canal walls away

from the furcation by forcefully directing the Hedstrom blade mesially. The dentin at the orifices and walls is cut away with a pulling motion. This mesially directed filing is called “anticurvature filing”.

STEP 4: FLARE CORONALLYUsing Gates Gliddens and light pecking motion, flare the coronal walls of the canal,

no more than 4mm deep into the canal.

STEP 5: COMPLETE THE STRAIGHTLINE ACCESSRemove any remaining projections of the chamber roof with a long, tapered, high speed diamond, leaving smooth,

continuous access walls from chamber floor to the top of the crown. Then, make a “path” from each mesial canal orifice to the top of the crown by placing the tip of the diamond (not rotating) into the canal orifice, raising the tip just slightly above

the orifice, activating the handpiece and tilting the diamond vertically.Do not move the tip. Use the diamond the same way for the distal (Mandibular) and

palatal canals (Maxillary) except that the diamond is tilted parallel to the coronal part of the canal rather than vertically.

STEP 6: MODIFY THE ACCESS FOR ROTARY INSTRUMENTSModify the path to the orifices in order to make entry easier and reduce the possibility of a separation during

instrumentation. Position the handpiece at each orifice as described in Step 5. The modification to Step 5 entails further tilting of the long tapered diamond beyond the vertical towards the mesio-lingual line angle. Modify the mesio-lingual “path”

by tilting the diamond until the handpiece can be withdrawn without hitting the upper teeth.For the MB “path” tilt the diamond until the hand piece can be withdrawn without hitting the lower teeth. Your StraightLine

access is now complete.

-What is the GOAL of RCT ?

Shaping the canal to receive proper obturation to achieve hermetic seal. This is

done by use of instruments.

Cleaning the canal to make it bacteria free. This is done by instrumentation and

irrigation. Some areas of canal are so small that irrigants can not reach unless

those areas are enlarged at least to size 30.

Avoid excessive destruction of coronal portion by using large tapered instruments

so that after restoration tooth can serve in mouth without being weakened.

Revolutions in Endodontic instruments- A review‘Revathi M “Rao CVN and “*‘Lakshminarayanan L

ABSTRACTGreparation of root canal system is one of the most important proceduresin endodontic treatment.There has been a constant quest for quicker, saferand more efficient method for cleaning and shaping of root canals. Use ofautomated Ni-Ti instruments was a logical development to improve theefficiency of the treatment. Separation of instruments while preparing rootcanals is something that has plagued all practitioners. Therefore, anevaluation of effect of speed and torque on the rotary Ni-Ti instruments isof value to the clinician.The purpose of this paper is to discuss the behavioural properties of Ni-Ti,importance of speed and torque and the necessity for its understandingfor effective, safe and successful treatment.2-. . .Key words - Ni-Ti instruments, speed and torque, endodontic instruments,IntroductionThe technical demands and level ofprecision required for successful performanceof endodontic procedures have traditionallybeen achieved by careful manipulation of handinstruments within the root canal space andby strict adherence to the biologic and surgicalprinciples, essential for disinfection andhealing. To improve the speed and efficiencyof the treatment, recently there has been aresurgence of mechanized or automatedsystem for both preparation and sealing of rootcanal system.The purpose of this paper is to discussthe behavioural properties of Ni-Ti rotaryinstruments, importance of speed and torqueand the necessity for its understanding duringPost Gracbate S t u d e n t .* Lecturer.** Prof. & HeadDept. of Conservative Dentistry & EndodonticsS a v e e t h a D e n t a l c o l l e g e & Hospitals,C h e n n a i .

endodontic preparation for effecetive, safe and .successful treatment outcome.Clinical evidence demonstrate that rootcanal systems can be cleaned and shaped andobturated in three dimensions with a highdegree of predictability approaching 100%success. Three major elements determine thepredictability of successful endodontics. Thefirst is the knowledge, the second is the skilland the third is the desire. Almost 30 yearsago, Schilder introduced the concept “cleaningand shaping” of root canals. In fact, most ofthe obturation the problems are really problemsof cleaning and shaping. The secret to

successful endodontics is proper shaping.What is the modern meaning of cleaningand shaping? Cleaning refers to removal of allcontents of the root canal system before andduring shaping. Shaping refers to a specificcavity form with five design objectives:1. Develop a continuously tapering conicalform in the root canal preparation.432. Make the canal narrower apically, with thenarrowest cross-sectional diameter at itsterminus.3. Make the preparation in multiple planes.4. Never transport the foramen.5. Keep the apical foramen as small aspractical.Ni-Ti in EndodonticsThe shaping of curved canals presents,aconsiderable problem for practitioners whenstainless steel instruments are used. There isa tendency for all preparation techniques totransport the prepared canal from its originalaxis.Deviation from the original curvature canlead to procedural errors, such as ledgeformation, zipping, stripping and perforations.As a consequence, new endodonticinstruments and techniques have beenintroduced which help to minimize these risksParameters including radius of curvature,angle of curvature, instrument size and thepoint of maximal instrument flexure were allfound to have a significant effect on the numberof cycles to failure and location of breakage.Radius of curvature and angle ofcurvatureal = 60°rl = 5mma2 = 60°r2=2mmRe .

A B More flexible Ni-Ti instruments have beenFig. 1 Radius of Curvature and Angle of Curvature developed and found to be efficient. The superEndodontology, Vol. 13, 2001

Fig 1 describes canal geometry using twoparameters. Radius of curvature (r) and theangle of curvature are determined on the sametooth. This teeth accurately depict the 60degree angle (al=a2).*Angle of curvature (a) is determined bythe angle formed by the lines that intersect atthe circle’s center. These two lines areperpendicular to the lines drawn along the longaxes of the coronal and apical portions of theroot canal space. Points cand dare the pointswhere in the canal deviates from the straight

lines and begin or end the curved portion ofthe root canal space. The angle (a) is taken tobe the angle formed by the arc in degreesbetween points cand d.The arc lies on a circlewhose size is specified by it’s radius, and thecircle’s radius is taken to be the radius ofcurvature of the canal in that. local area. Thecircle’s radius is the radius (rl and r2) of thecurved portion of the root canal space anddefines how abruptly the canal curves. Thecanal geometry of these two teeth differs onlyin the radius of curvature (rl and r2), whereasthe angle of curvature (al and a2) equals 60degrees. A represents a sweeping canalcurvature having a 5 mm radius of curvature(rl). B represents an abrupt canal curvaturehaving a 2 mm radius of curvature (r2).Clinically, the radius of curvature and angle ofcurvature of any root canal space could bemeasured using this technique with the aid ofa circle guage.*Of the two canal shape parameters, angleand radius of curvature, radius of curvaturewas the most significant factor of canal shapeaffecting the number of cycles to failure ofNi-Ti engine driven rotary instruments. As theradius of curvature decreases, instrumentstress and strain increases, and the fatiguelife decreases, which may contribute toinstrument breakage and canal transportationin clinical situations. 24 47 *’ &,,is* ,. ,, ,..

Revathi M et a/. .,_

elasticity of Ni-Ti alloy allows these instrumentsto flex far more than stainless steel instrumentsbefore exceeding their elastic limit, allowingeasier instrumentation of curved canals whileminimizing canal transportation. With thedevelopment of Ni-Ti alloy which offers a tough,modulling, corrosion resistant instrument withmechanical memory, these instruments gainedsignificant popularity and are routinely includedin current endodontic armamentarium. Theautomated use of Ni-Ti endodontic files was alogical development to increase the efficiencyof clinical treatment.Metallurgy of Ni-TiAfter years of relative inactivity with regardto improvements in metals and alloys used tomake endodontic instruments, we now haveinstruments made from Nickel-Titanium. (Ni-Ti) which is a remarkable alloy. Theinstruments made of Ni-Ti offer possibilitiesfor improving the speed and efficiency of

treatment, as well as achieving greaterprecision and accuracy. This alloy exhibitssuper elastic behaviour, allowing it to return toits original shape upon unloading followingsubstantial deformation. By contrast, othermetals such as stainless steel sustain plasticdeformation leading to permanent shapechange when deformed similarly. The superelastic property of Ni-Ti has been known for30 years, and was discovered by chance byBuchler and Wang while searching fornonmagnetic , salt-resisting, water proof alloysfor naval use. Alloys that show super elasticityundergo a stress-induced martensitictransformation from a parent structure whichis austenite. Upon release of the stress, thestructure reverts back to austenite, recoveringits original shape in the process. Deformationsinvolving as much as 10 percent strain can becompletely recovered in these metals, .ascompared to a maximum of one percent inconventional alloys.3Nickel-Titanium also known as “Nitinol”(Ni-Ti Naval Ordinance Laboratory) in the., i . Revolutions in endodontic..

United States, has been manufactured inShanghai, China since 1979 as “Nitialloy”-56% of Nickel and 44% titanium. The firstinvestigation of nickel titanium in endodontics wasreported in 1988 by Walia, Brantley, and Gerstein.Shape memory alloys, such as Ni-Ti,undergo a phase transformation in their crystalstructure when cooled from the stronger, hightemperature form (austenite) to the weaker,low temperature form (martensite). Thisinherent phase transformation is the basis forthe unique properties of these alloys, in particularshape memory effect and super elasticity.This latter property is important forendodontic use. Ni-Ti alloys can show asuperelastic behaviour if deformed at atemperature which is slightly above theirtransformation temperatures. This effect iscaused by the stress-induced formation ofsome martensite above its normaltemperature. Because it has been formedabove its normal temperature, the martensitereverts to undeformed austenite as soon asthe stress is removed. This process elicits aspringy, rubber like elasticity from the alloy.Thetypical loading and unloading behaviour ofsuper elastic Ni-Ti (stress-strain curve) whensubjected to tensile stress is shown in theFig 2. The super elastic behaviour is typicallyrepresented by the martensitic yield plateauwithin which the stress remains approximately

constant until the martensite finish(Mf)STRAINF i g . 2 T y p i c a l loadmg a n d unloadmg behavlour o fsuperelastic Ni-Ti (Stress-strain-curve) when subjectedto tensile stress

45. L..',, ' '.<i':,. ,.aEndodontology, Vol. 13, 2001

transformation stress, a value which is slightlylower than the elastic limit, is reached.This plateau is clinically useful, becauseit allows easy and efficient instrumentdeformation without significantly increasing theapplied load. This explains why Ni-Tiinstruments require a certain amount of torqueand rotation to overcome the linear elasticresponse of the initial structure and reach themartensite start clinical stress (Ms).The figurealso explains why Ni-Ti rotary instrumentsshould be operated with constant speed andtorque(constant load) when the martensitestart clinical stress is reached, to maximizeefficiency and minimize iatrogenic errors.Andreasen and Morrow“ have demonstratedthat stainless steel wires undergo a muchlarger change in force compared to the changein force of Ni-Ti wires when deflected anequivalent amount(spring rate).Clinically, this means that Ni-Ti is moreflexible, requires less force to undergo achange in deflection(i.e. when negotiating acurved canal) and consequently requires lowrecovery loads, thus reducing the tendency ofstraightening the root canals.Martensite is the more deformable, lowertemperature phase present in Ni-Ti, which isable to absorb upto 8% recoverable strain.Upon minimal further deformation there is asmall linear elastic response upto the elasticlimit (E), caused by the elastic deformation of1) Torque: x = F. r- sin(+)or: x = F,,,,,. rFig. 3 Forces acting on Rachet and its effect on torque

the self-accommodated martensitic product,in which a small amount of slip and dislocationmotion is apparent. Further deformation resultsin plastic deformation and final failure. In clinicalpractice, plastic deformation of Ni-Ti rotaryinstruments should be avoided, because it mayeasily lead to fracture. As shown in figure 2the range of deformation allowed by the plasticfield is twice as small as that allowed by theelastic field.Extensive tension testing of Ni-Ti wires hasbeen done in the last few decades.Researchers have found that compression,

torsion and flexural loading of Ni-Ti wires resultin similar constitutive behaviour to thatobserved in tension. However, the criticalstress in torsion is much smaller than thestress observed in tension or compression,while the recovery strains are much greater5.Importance of Speed and TorqueSpeedSpeed refers not only to revolutions perminute but also to the surface feet per unitthat the tool has with the work to be cut.‘jIn endodontics speed varies from 150-40,000 rpm Greater the speed, more thecutting efficiency, but at higher speed, thereare more disadvantages such as : 71) loss of tactile sensation2) breakage of instruments precededby flute distortion3) change in anatomical curvature ofcanal4) loss of controlTorqueTorque (also called a moment) is the termused about forces that act in a rotationalmanner. Examples of torque or momentapplication are turning a dial, flipping a light switch,drilling a hole or tightening a screw or bolt.As shown in the Fig 3 of a ratchet, a torqueis created by a vertical force applied at the46Rev&hi M et al.

end of the handle. The force, F, applied to theratchet causes a tendency to rotate about point0, the force can be broken down into twocomponents: a radial component , Frad,parallel to the ratchet handle that does notcontribute to the torque and the distance frompoint 0 to the point of action F is described bythe direction vector, r. The moment arm, I isthe perpendicular distance between point 0and the line of action of F.If we were to shorten the moment arm byapplying the force closer to the head of theratchet, the magnitude of the torque woulddecrease, even if the force remained the same.Thus, if we change the effective length of thehandle, we change the torque.According to Marzouk, torque is the abilityof the handpiece to withstand lateral pressureon the revolving tool without decreasing itsspeed or reducing its cutting efficiency.‘jTorque is dependent upon the type ofbearing used and the amount of energysupplied to the handpiece.47

Importance of Torque- during Cleaningand ShapingTorque is another parameter that might* influence the incidence of instrument locking,deformation, and separation.Theoretically, aninstrument used with a high torque is veryactive and the incidense of instrument lockingand consequently deformation and separationwould tend to increase, whereas a low torquewould reduce the cutting efficiency of theinstrument, and instrument progression in thecanal would be difficult. The operator wouldthen tend to force the instrument and mayencourage instrument locking, deformationand separation.A variety of speeds for different rotaryinstrumentation have been recommended bythe manufacturers. Conventional endodonticmotors to recent motors use a wide range ofspeed of 150 rpm - 40,00Orpm, which are. . / s : .. Revolutions in endodontic..

either controlled by electrical or air-driveninstructions handpieces. Depending of themanufacture, and the condition of thehandpieces (i.e. old or new), each singlehandpiece has a different degree ofeffectiveness, which results in different torquelosses, which are very difficult to define. Hence,possibility of calibrating the handpieces is animportant issue, which every endodontist mustbe aware of, while choosing an appropriatehandpiece, according to the required speedand torque.Role of Handpiece IA handpiece is a device for holding rotatinginstrument, transmitting power to them and forpositioning them intraorally.Both speed and torque in a handpiece canbe modified by the incorporation of gearsystems. Operative procedures involving rotaryinstruments can be optimized by correctselection of handpieces and correspondinggear ratios. Handpieces can incorporategearing systems of various types but gearingis limited by the need to maintain the driveconcentrically through the handpiece and head.A common method of gearing a handpieceis the use of an epicyclic ball-race gear system.This is usually located in the shank of thehandpiece. The epicyclic ball race can be usedto either increase or decrease the speed ofrotation from the drive spindle depending uponwhich way around it is mounted.e.fThe basic design is a modification of’a ball- .race bearing. If the outer ring of an ordinary

bearing is held stationary whilst the inner ring *is turned, the cage separating the balls turn ata much reduced speed. The speed reductionis proportional to the relative diameters of theinner and outer rings. In the shank of thehandpiece the cage unit is extended and isattached to either the drive shaft or the drivenshaft depending on whether a speed reductionor increase is needed.The great advantage of using a ball racegearing system is that it is very smooth andrelatively quiet in operation. Surprisingly hightorque can be transmitted without the ballbearingsslipping. Two units can be usedserially where larger changes in speed arerequired.8Full miniaturized epicyclic gear boxes withtoothed gears are now being used in sometop range handpieces. Provided that these aremanufactured from strong alloys and aredesigned effectively, they can provide excellentand powerful transmission of torque.Such boxes are particularly appropriate forspeed increasing handpieces. Reductionhandpieces reduce the speed of the drivewhilst increasing the torque. Electronic controlsystems can be used to maintain the speedof the motor against the effects of increasingload during cutting.eTorque control motors allow the setting oftorque generated by the motor. In low torquecontrol motors, torque values set on the motorare supposed to be less than the value oftorque at deformation and at separation of therotary instruments. Where as in high torquecontrol motors, the torque values are relativelyhigh compared to the torque at deformationand at separation of the rotary instruments.During root canal preparation all theinstruments are subjected to different levelsI of torque. If the level of the torque is equal orgreater than the torque at deformation or atseparation, the instrument will either deformor separate. Theoretically, with low torquecontrol motors, the motor will stop rotating andcan even reverse the direction of rotation whenthe instrument is subjected to torque levelsequal to the torque values set on the motor.Thus instrument failure could be avoided. Withhigh torque control motors, the instrumenttorque at deformation and separation wouldbe reached before the relatively high torqueset on the motor. Consequently, the instrumentwould deform and separate.Endodontology, Vol. 13, 2001

The main problem with Ni-Ti rotary

instrumentation techniques probably isinstrument failure. lntracanal instrumentfracture is an iatrogenic error which canseriously jeoparidize the success of root canaltherapy. Preutt et al has shown that thecontinous cycle of tensile and compressiveforces to which engine driven instruments aresubjected, produce a very destructive form ofloading.’ Moreover, mechanical stress onNi-Ti rotary instruments is proportional withthe motor torque. Hence torque control is animportant factor, to reduce the risk of Ni-Tifracture. If there is no torque control, then onceload is applied to rotating instrument, it willstop rotating.5Slow speed, low-torque(right-torque)motorsThe high stress is not clinically importantin straight canals where the resistance todentin removal is low. On the contrary, incurved or calcified canals, the resistance ishigh and the instrument may become blockednear the tip. In these situations, the high torqueprovided by the motor might immediately leadto fracture of the blocked instrument, especiallysince the clinician usually has no time to stopor retract the instrument.The use of slow-speed high torque Ni-Tirotary instrumentations has lead to manyiatrogenic errors. Ideally it should, now bechanged to slow-speed low-torque orpreferably right-torque motors, since eachinstrument has a specific ideal (right) torque.The values are usually low for the smaller andless tapered instruments, and high for thebigger and more tapered ones.To minimize the risk of intracanal breakagethe instruments should be operated in a rangebetween the martensite start clinical stressvalues and the martensite finish clinical stressvalues, which is a safe and efficient load.However, this range is small and difficult todetermine. With good approximation, it can bedefined to be slightly lower than the limit of48Revathi M etel.

* ..,‘.Revolutions in endodontic..

elasticity. The elastic and fracture limits ofNi-Ti rotary instruments are obviouslydependent on design, dimensions and taper.This means that the right torque value for eachindividual instrument must be calculated by themanufacturers to obtain optimum cuttingperformance while minimizing risks of failure.Moreover, motors must have a very precise,

fine- adjusted control of torque values, in orderto take advantage of these concepts of notexceeding the limit of elasticity andconsequently avoiding plastic deformation andintracanal breakage.The latest development with regard to torquecontrol is the incorporation of gear systemswithin the handpiece that regulates torquedepending on the size of the rotary instrument.(Endoflash-Kavo, Anthogy Ni-Ti control-Dentsply). This obviates the need for torquecontrol motors. IFuture developments in Ni-Ti filesSeveral areas offer exciting researchpossibilities to further enhance theperformance of Ni-Ti files. These researchpossibilities include:Conventional endodontic motors are not ableto allow precise, low-torque settings fordifferent reasons. A step-motor with computercontrolledelectronics, which allows fineadjustment of the torque values for each andevery instrument of different brands, ispresently available as prototype(Endostepper,SET, Emmering, etc.). The maximum torquevalues for the individual instruments can beadjusted and programmed such that the elasticlimit is not exceeded. All data for eachinstrument (including operating speed, limit ofelasticity, maximum torque and angle of rightleftmotion) are stored in the computermemory. If the motor is loaded right up to theinstrument-specific limit-torque, the motorstops momentarily and attempts to start again.If the externally required torque(determined byanatomic complexities and hardness of dentin)is so high that the motor cannot startautomatically, by means of a pedal function,the motor executes a precisely defined leftrightmotion, which succeeds in safely freeingthe blocked instrument. Once the instrumentis released, the motor rotates in the usual,programmed direction.. Use of ion implantation and thermalnitridation to provide harder and wearresistant cutting edges in the file.g. Investigation of failure models in Ni-Ti files ’to develop mathematical models toaccurately predict the life expectancy ofthese files during use.Optimization of flexibility, bending andtorsional strength of files without sacrificingcutting ability, by using modern mechanicsand analysis methods: for example, finiteelement analysis.

Conclusion IThis article is aimed at providing acomprehensive review of rotary endodonticswith emphasis on the behavioural propertiesof Ni-Ti and its mode of application.Furthermore, the use of right speed and torqueare stressed for controlled instrumentation,Finally, the authors feel that it would be unwiseto attempt Ni-Ti rotary endodontics withoutcomplete understanding of physical andmechanical properties of Ni-Ti Instruments.This safety mechanism was developed toreduce the risk of instrument fracture.The mainadvantage of this motor was that it dramaticallyincreased tactile and mental awareness ofrotary instrumentation.This was a fundamentalstep in reducing the risk of instrument fractureto a minimum.5, 2001